Proceedings of the 14th International Conference on the Technology of Plasticity - Current Trends in the Technology of Plasticity

This study entailed an evaluation of the features observed in the retrofitted-accelerometer signal obtained for scrap float detection in stamping. The author employed a machine-learning technique with retrofitted accelerometers. Three accelerometers (hereafter we call them sensors A, B, and C) were mounted on three different locations on the side surface of the stripper plate of a three-holes-stamping die set. A1050 aluminum sheets of thickness 0.8 mm were used as blank materials. Six features in the accelerometer signal were determined for the detection. Half of them were associated with the downward journey of the press slide, and another three were associated with its return journey. Next, the author applied a machine learning technique using the Mahalanobis-Taguchi system. For sensor C, unknown and unexpected events unrelated to the scrap floating occurred two times to cause false detections of normal samples without scraps. By contrast, the unit spaces which contain normal samples for sensors A and B were completely separated from the error samples with scraps. Moreover, the author conducted the S/N ratio analysis to investigate the influential features on the detection of the error samples for each sensor. Depending on the above, the author conducted machine learning with selected three features for each sensor. As a result, for sensors A and B, normal and error samples were separated as completely as the detection with six features. For sensor C, false positives for two normal samples when six features were used were now also correctly detected to be normal.

Near-isothermal forging (NIF) is an effective method for forming P/M nickel-based superalloys. Prediction and control of microstructure evolution during NIF has long been a great challenge. This paper established a mathematical model coupled with dynamic recrystallization kinetics and grain size evolution for a novel P/M nickel-based superalloy. Then the microstructure evolution of the superalloy during the NIF process was simulated by embedding the model into the DEFORM finite element platform. The effect of initial billet temperature, die temperature, the height-to-diameter ratio of the billet, and the forging strain rate on grain size distribution was further discussed. To tailor the ideal deformed microstructure, the average grain size and its standard deviation were chosen as the optimization target. Artificial neural network was adopted to build the relationship between processing parameters and microstructure. The optimized NIF processing window was further determined by the genetic algorithm. Our work provides helpful guidance for controlling the microstructure in the industrial production of high-performance P/M superalloy parts.

Manufacturing systems are often characterized by a complex dynamic behavior. In this context, machine learning (ML) offers great potential for a better understanding of such systems to reduce operating costs while at the same time ensure a reliable process control. However, the training procedure including the training data generation to set up the ML model is time-consuming and cost intensive. Especially, the consideration of faulty process states to avoid unbalanced data sets is associated with a high technical effort due to their low likelihood of occurrence in a running production. Therefore, this study proposes a simulation-based data augmentation method which integrates synthetic data into the training procedure of ML models to estimate wear in high-speed forming processes inline. The synthetic data is generated by a rudimentary simulation of the punch phase during a blanking process, which allows a parameterized variation of the abrasive wear states. To quantify the potential of the approach, the performance of the ML model based on synthetic data is compared with a reference ML model trained with experimentally acquired force signals. The proposed method significantly reduces the effort required to create a training data set, and only requires an initial set of time series representing a single wear state to validate the simulation model. At the same time, the augmented ML model estimates the current wear state during the blanking process with a mean absolute deviation of 15 µm from the target wear state described by the cutting edge radii of the tool, considering sensorial acquired force signals as a test input.

Freeform bending is a kinematically-controlled bending process that can be used to achieve complex 3D geometries. Geometry control of the bent part is particularly challenging because a geometric error that has already occurred, for example due to material fluctuations, can only be detected after a certain amount of time and after it has left the bending unit. In order to still meet various quality criteria on the part geometry, the error can be compensated for at a later point in the bending process and at a different location in the part by adjusting the amplitudes, thus minimizing a global quality measure. However, this requires a global consideration of the amplitudes and their effect on the resulting geometry. The aim of this paper is to train a neural network based controller that is able to improve the global geometry of the bent component for different optimization criteria with respect to material variations. This is done by training a data-based surrogate model that is fast enough to perform inverse optimisation of the global part geometry. This surrogate model is trained with simulation data and provides the training data to train a neural network that makes the control decisions in the process.

This paper presents a novel deep learning-based platform for addressing shape distortion in sheet metal stamping (e.g., springback, thermal distortion) by tool compensation. Conventional approaches to tool compensation involve computationally expensive Finite Element (FE) simulations to update tool geometries. In contrast, the proposed platform uses a generator network to create 3D tool geometries and an evaluator network to predict the resulting shape distortion and post-stamping thinning. The generated tool geometries are iteratively updated by a gradient-based optimisation technique in the direction of minimising shape distortion in the resulting component. The platform is demonstrated on a cold stamped U-channel component case study, which experiences severe shape distortion in the form of springback. The optimisation problem was formulated to find the optimum tool geometry that enables a desired U-channel geometry to be formed after springback by tool compensation, while meeting a maximum thinning constraint. The platform successfully optimised the tool geometry to compensate for springback in this setting, showcasing its effectiveness in improving manufacturing outcomes and product quality. The presented approach offers a superior method for addressing shape distortion in stamping processes, as compared to conventional FE simulation iterations or trial-and-error methods. This approach can efficiently and effectively compensate for arbitrarily complex tool geometries without requiring extensive process expertise.

Bulk metal forming techniques provide advantageous production routes in the modern manufacturing industry. Although the scientific literature has offered continues advances in the process design, the well-known bulk forming methods, including extrusion, forging, drawing and rolling, still require update and optimisation. In this study, the attention was pointed out on porthole die extrusion employed for production of cross-sectional hollow profiles. Coupled statistical and machine learning techniques were implemented with the aim to predict the output of the process and to allow a performant design of the porthole die before its manufacturing.Specifically, a dataset was considered identifying 12 geometric variables, each one varied on three levels, starting by a specific case study, where a standard porthole die is employed to extrude circular section profiles. Then, employing the process data extracted by literature, a statistical analysis was preliminary carried out to highlight the relationships among the most significant variables that can affect the outcome modelling. Subsequently, a set of models based on machine learning techniques was trained and tested to well generalize the behaviour of the process, changing the investigated factors. The considered methodologies were implemented on R-Studio software.The aim of the study is the rapid control of any alteration of the standard processing conditions, giving flexibility to the work, ensuring the correct design of the porthole die in a shorter time and saving money. All the details will be provided in the manuscript.

The demand for aluminum alloy extrusion is increasing and the requirements for the accuracy and geometric complexity of extrusion products are becoming more stringent. However, the traditional extrusion die design often relies on the designer's experience, therefore, a lot of die refinement works are still needed. To avoid this situation, deep learning technology, namely deep convolution neural network (DCNN), is applied to obtain suitable design parameters for extrusion die. In this study, DCNN in extrusion die design is mainly divided into two parts: suggesting to porthole structure at the upper die for hollow extrusion and determining the bearing length for balancing the metal flow in hollow extrusion process. First, the cross-sectional shapes of the products from the over 100 extrusion literatures are used as training data set to establish four neural network training models which belong to image classification. Second, the training label for the object detection model is built by assigning a complexity factor for the die bearing length of each feature on the product profile. Next, the design parameters provided by DCNN models are used to design the extrusion die as a validation test. Finally, a finite element analysis based on the obtained die design is conducted to verify the flow balance of extrusion process.

A titanium gear with the industrial grade I was fabricated by fine blanking the titanium plate in cold. The carbon supersaturated (CS-) high-speed steel punch was prepared by the low temperature plasma carburizing system for this single-shot fine-blanking. The free-carbon tribofilm was in situ formed onto the interface of CS-punch to titanium work so that no adhesive wear occurred even during severe shearing process. SEM-EDX was utilized for analysis on the contact interface between fine blanking punch and titanium work. This analysis proved that the free-carbon tribofilm was formed on the sheared interface. The titanium gears were continuously punched out by using CS-punch. Their dimensional accuracy was evaluated by the coordinate measuring system to analyze the gear-grade balancing at the blanked titanium gears.

The amorphous electrical steel sheets were expected to be used for motor cores because of their high strength and low iron loss even in high frequency electromagnetic field. However, their high strength without ductility caused various difficulties in piercing them. In addition to reduce the punch and die wear in piercing, their shearing performance must be improved to minimize the affected damages induced at the vicinity of punched out holes. In the present paper, the shearing behavior in piercing the amorphous electrical steel sheet stack was investigated by using the micro-/nano-textured punches. In particular, the effect of nano-texturing orientation and the punch-to-die clearance on this piercing behavior was discussed for improvement of the sheared blank quality. In the piercing experiments, five sheets were stacked into a specimen. The affected damages induced at the vicinity of pierced holes were analyzed by SEM (Scanning Electron Microscopy) and three dimensional profilometer. The punch side surfaces were also observed by SEM to analyze the adhesive behavior of debris particles from the specimen work on them. As a result, it was confirmed that the micro-/nano-textured punch improves the quality of the sheared surface when shearing amorphous electromagnetic steel sheets, resulting in lower shear load, and the tool adhesion was reduced while the hole diameter variation was suppressed. A shearing mechanism by the micro-/nano-textured punch was discussed to develop the piercing process with less affected damages and less iron loss.

Warpage tends to occur when the stamped leadframe strip is soldered with silicon-dies and baked during the packaging process. This causes offset or fall-off of the silicon-dies near the both ends of the leadframe strip. The length of the strip must be shortened to avoid the excessive warpage, and therefore the production yield is reduced. The situation worsens when the leadframe is not carefully stamped. In this paper, FE analysis was used to simulate the hole punching process as well as the baking process for a simplified leadframe unit attached with a silicon-die. The parameters investigated include the punch/die clearance, the blank-holder force, and the shoulder radii of both the punch and the die. The result shows that the main cause of the excessive warpage in baking is by the worn-off of the punch-shoulder radius.

A numerical model is proposed to predict the evolution of the tool temperature during a dry milling process using the finite volume method. This approach considers the secondary shear zone (tool-chip interface) and two distinct tertiary shear zones (tool-workpiece interfaces). Heat fluxes are calculated using experimentally measured cutting forces and linear velocity fields at the tool-chip and tool-workpiece interfaces, while considering the temperature dependent thermal properties of the tool’s substrate and coating. The simulated temperature results are compared to those measured in a previous experiment and the influence of the different interfaces on the evolution of the temperature field is studied.

In the blanking process of medium-thick aluminum alloy plate, the forming quality of the cutting surfaces is hard to meet the requirement. According to the sheet thickness, material, and geometry of the parts, the corresponding finite element mold was established, and the Deform-3D was used to simulate the blanking process of medium-thick aluminum alloy plate. The effect of blanking clearance and blanking speed on the cutting surface quality was studied. The results indicated that the increased blanking speed is beneficial to the acceleration of the shear deformation and fracture process of blanking material. As the blanking clearance decreases, the burr and tear at the edge of the blanking material are reduced. In addition, the properly applied blank holder force can prevent the transverse flow of the material in the shear zone. Based on the theoretical simulation of the material forming defects, the process parameters were optimized, the workpiece forming accuracy was improved, and the numerical simulation was verified by experimental experiment.

With the increasing demand for component quality and shorter cycle times at the same time, the demands on the production processes are increasing. This also affects the separating processes such as shear cutting. Due to the increasing use of high-strength steels, conventional shear cutting processes are reaching their limits. One way to meet the challenges is to increase the cutting speed. At speeds above 3 $$\frac{m}{s}$$ m s , this is called high-speed impact cutting. A new machine concept with linear motors was developed to investigate the influences on different materials. With the help of this test bench, it is possible to flexibly adjust the cutting speeds and monitor the process. This enables the determination of the correlation between process parameters and the shear cutting result.

In this work, tensile tests were performed on H340 and DP1000 steel sheets at various strain rates and temperatures. The displacement, deformation and local strain fields of the specimens were measured using digital image correlation (DIC) techniques. Meanwhile, the temperature fields of specimen gauge section were measured with a high-speed thermal camera in the uniaxial tensile tests at different strain rates. Experimental results show that the strain rate and adiabatic temperature have significant effects on the deformation and fracture behavior of the investigated steels. Therefore, a user-defined plasticity and damage mechanics model was developed and calibrated based on the comprehensive effects of stress state, strain rate and temperature. The proposed rate-dependent model has been proven to be successful in predicting the deformation and fracture behavior of the investigated steel sheets at both laboratory and structure scales.

This paper presents a rate-dependent extension of Jia-Zang ductile fracture model with five parameters considering void nucleation, growth and coalescence. A series of ductile fracture tests is designed and conducted on aluminum alloy (6016-T6) specimens, which allows wide ranges for stress state and strain rate (0.1–100/s) under room temperature. A hybrid experiment-simulation method is employed to investigate the effects of stress state and strain rate on the material ductility. The results indicate an increasing equivalent plastic strain to fracture for each specimen as the strain rate increases from 0.1/s to 100/s. In addition, several nonlinear empirical functions are used to extend the original model parameters with strain rate dependent integrated. Subsequently, an uncoupled ductile fracture model considering strain rate dependent is developed, which can work from quasi-static to dynamic scenarios.

To address the increasing demand for lightweight and high-performance materials, metal/carbon fiber hybrid laminates that contain a metal sheet substrate laminated with a layer of carbon fiber reinforced polymer (CFRP) have been developed. Recent work has identified roll forming as a potential high volume manufacturing process for such carbon fiber reinforced metal hybrid (CFRMH) components [1]. However, some forming issues including fiber buckling and kinking have been observed that may influence component performance in application. The major deformation mode in roll forming is simple bending.This work investigates the formability and related failure modes of a steel/CF hybrid with different layup sequences in bending deformation. For this pure bending tests are performed followed by damage behavior analysis with microscope observation. A finite element (FE) model is developed and achieved a good agreement with the experimental results. The results suggest that the formability and bending strength of steel/CF are a function of the fiber orientation. The FEA model accurately reflects the main trends but underestimates the bending strength of some fiber orientation conditions.

In this paper, a numerical methodology using cycle jump algorithm is developed to predict the low cycle fatigue residual life of mechanical structures subjected to initial state conditions. A fully coupled model that accounting for isotropic and two kinematic mixed hardening and coupling with fatigue damages are developed in order to accurately predict the residual fatigue life. Obviously, the time-based algorithms used for solving fatigue problems are confronted to the huge size of the CPU time needed. The cycle jump algorithm that adapt the jump according to evolution of the mechanical fields constitutes an alternative solution that avoids the calculation of the total loading cycles. The model parameters have been identified in the case of DP600 steel specimen subject to symmetric tension-compression loading path. An application is presented to evaluate the accuracy and the efficiency of the fatigue numerical methodology: a plate structure tensile operation is firstly simulated followed by the numerical simulation of fatigue test accounting for all the residual state due to the previous metal deformation.

Steel fibers applied as concrete reinforcement like in Ultra High Performance Concrete reduce the wall thickness and thus the amount of concrete [1]. The process chain notch rolling and cyclic bending presents the foundation of an ecologically and economically efficient approach for production of steel wire fibers. For fundamental perception of the characteristics and interdependencies of the innovative production method, analysis by experimental model processes is required. Previous research determined influence of various process parameters on crack propagation [2], which is further analyzed in this study. As testing material, a DP600 sheet metal (t0 = 0.8 mm) is applied. Cyclic bending tests of wire strip with staggered amounts of cycles and varying bending angles are performed and evaluated based on load curves and micrographs. The analysis supplies identification of stages of crack propagation, which allows recommendations for process design regarding controllability and geometry of fracture. For supporting the analysis, stamping and cyclic bending is numerically implemented in the simulation software LS DYNA to picture the present strain stage applied during bending. The process evaluation together with the built simulation model can later be used as reference for a suitable numerical modeling of crack propagation that will enable a more flexible analysis of the process chain and parameter effects by simulation.

This paper introduces a new test based on the double-action radial extrusion concept to characterize bulk formability of highly ductile metals. The test is carried out in a multidirectional tool system and the methodology involves determination of the strain loading paths up to fracture through combination of digital image correlation and force-time evolutions. Strain loading paths together with finite element modelling and fractography analysis by means of scanning electron microscopy allows characterizing the crack opening modes. Results in as-deposited wire additive manufactured aluminum specimens show that material flow at the radially extruded flanges gives rise to crack opening by out-of-plane shear and tension. The new test is adequate to determine the fracture limits of highly ductile metals, which cannot (or are very difficult to) be obtained by conventional upset bulk formability tests.

Material formability under hot stamping conditions can be characterised by using an innovative Gleeble-based biaxial testing method with cruciform samples. However, due to the temperature gradient in the cruciform samples, the determined material forming limits have significant uncertainties, therefore compromising the accuracy of the formability testing method. In this paper, 2D remote measurements of the temperature field over the gauge area of cruciform samples are presented. A manganese-doped fluorogermanate phosphor - Mg4FGeO5.5:Mn, mixed with a flame-resistant chemical binder is sprayed over cruciform samples, machined from aluminium alloy 6082, forming a thin paint that is later optically interrogated. This resulting coating emits temperature-dependent luminescence (peak spectral emission shift) when excited by UV light that is later exploited for thermometry purposes. The region of the gauge area of the cruciform sample is defined based on the measured temperature uniformity. The effect of temperature gradients in the gauge area on the determined forming limit curve is evaluated at elevated temperatures. Accurate online determination of the instantaneous temperature field during deformation will allow for improved process temperature control, leading to determination of characterised material formability with a lower uncertainty.

Monoblock tubular shafts (MTS) are manufactured using seamless hollow tubes by radial and axial forming operations. The load path of such forming operations strongly influences the damage evolution in the produced parts. Heat treatment of cold formed parts has also shown to influence the damage level or the porosity in the metal. After cold forming operations, the shafts are solution annealed and then quenched in order to harden the MTS profiles. Metallographic investigations have shown the generation of newer voids along the external contour of the gears. The current work aims at clarifying the damage evolution mechanism in the cold forming and then quenching operation and its correlation with the temperature induced phase changes. Numerical simulations were used to study the triaxiality during the forming operation. The results show the presence of a positive triaxiality along the tooth flank surface and the inner contour of the hollow shafts. Metallographic investigations and electron backscatter diffraction (EBSD) are performed to determine the individual metallic phases before and after the quenching operation. Using simulation, the effect of such thermal shock on the residual stresses and the damage evolution are investigated. Characterizing the morphology of voids and non-metallic inclusions enhances the scope of the process design, which allows manufacturing of shafts with lower damage and an increased product life of the MTS profiles.

Generally, oblique penetration is more frequent than normal penetration when a penetrator attacks to the target. The penetrator experiences complex loading states and can be easily deformed when oblique penetration occurs on concrete target. So, it is essential to investigate and acquire the survivability of the penetrator in the design phase. In this study, structural survivability evaluation with various hardening models of the penetrator material was carried out through a precise numerical analysis of a test case. We obtained the dynamic material properties of the penetrator material (AISI4340 steel) and approximated with isotropic and kinematic hardening models to describe complex loading states during oblique penetration. In addition, dynamic fracture properties are also applied to the numerical analysis. For target material, concrete material constants in good agreement with test database were obtained. Finally, we confirmed the numerical analysis method suggested in this paper is possible to evaluate the survivability of penetrating warheads and showed the effect of material hardening model on the impact fracture.

The damage introduced into components during forming operations is often critical for the component performance in service. Because these performance properties depend strongly on the local stress state, it is important to consider this while evaluating material or components produced under specific processing chains. In this paper, we present an approach to correlate the process parameters during cold rolling with the crash impact toughness of DP800 steel, using a straightforward approach. Already hot-rolled S355 steel is cold-rolled with varying pass reductions and then heat treated to DP800 steel. In order to account for a relevant stress state, numerical simulations of a crash box are used. The critical stress state is extracted and assigned to an impact tensile specimen by varying the side notch of the sample. The damage quantification using density- and SEM-measurements shows a clear difference between the individual material states, but there is no fully identifiable correlation between damage and energy absorption in the impact tensile tests. It is assumed that other parameters influenced by rolling (e.g., strain hardening) have a greater influence than the damage. Further studies will be conducted to clarify under which loading conditions the damage is more likely to have a strong correlation with the performance capability.

In formability evaluation, it is important to accurately predict the onset timing and morphology of fracture in a workpiece from a physical point of view. The authors have developed a material model based on the stress rate direction-dependent constitutive equation, which is a non-normality law model, and applied it to forming analyses. The authors have also developed a forming limit prediction method based on the three-dimensional local bifurcation theory, which can theoretically predict the forming limits of arbitrary sheet and bulk metals. In this study, these methods are applied to evaluate the formability of thin and ultra-thin metallic sheets. The proposed model was applied to a square cylinder drawing analysis, and the accurate evaluation of the local necking allowed us to capture the transition of the fracture zone in response to changes in the amount of corner cut. In addition, appropriate forming parameters for ultra-thin sheets were also investigated. These analyses demonstrated the validity and usefulness of the proposed stress rate direction-dependent constitutive model.

In bipolar plate production, extreme thin foil materials are becoming increasingly important due to the trend towards high-energy dense fuel cells. For a better of the material behavior and component failures, finite element simulations are used. In order to achieve an expressive numerical representation of the forming process, the behavior of material failure in sheet metal forming is described by forming limit curves (FLC). However, especially for thin metal foils, proven testing methods such as the Nakazima test are not applicable because the specimens start wrinkling or fail outside the defect zone specified in the norm. While there are alternative testing methods for the detection of the pure tension area of the FLC, there is no applicable testing method for the evaluation of the forming limit in the tensile-compression zone. Therefore, in this paper simulations as well as physical tests were carried out to define a suitable specimen geometry for the characterization of stainless steel foil (1.4404) with a thickness of 0.1 mm using a scaled Nakazima set up. The simulation results showed that by decreasing the parallel web length as well as the fillet radius the equivalent strain maximum is shifted towards the specimen center. This observation is supported by the physical tests where necking occurred in the specimen center. Additionally to the position of failure, first investigations in physical testing showed maximum strain ratios of $${\upvarepsilon }_{1}$$ ε 1 = 0.23 in major strain and $${\upvarepsilon }_{2}$$ ε 2 = −0.095 in minor strain. The strain ratio therefore represents the uniaxial tension area.

Elastomer rollers are added to the conventional roll forming process to produce V-sections with reduced damage and increased product performance. This performance of high strength steel profiles during service-life is affected by ductile damage, which can be defined as the nucleation, growth and coalescence of voids on a microscopic level. Damage evolution during forming can be controlled by the stress state. In this work, the stress state during roll forming of a DP800 sheet to a V-section is altered by compressive stress superposition using elastomer rollers. Those are placed between steel rollers creating contact in the otherwise contact-free main forming zone at the outer fiber. The influence on the stress state during forming is investigated numerically, as it depends on the properties of the elastomer. For validation, parts are manufactured in an experimental setup of a five-stage roll forming process with and without elastomer rollers. To determine the influence on damage evolution, the void area fractions in the outer fiber of the bent parts are examined by SEM images. The effect of changing the damage evolution on the product performance of the V-sections is shown by stiffness tests and the analysis of the remaining formability after roll forming.

In this work the latest developments on the damage to fracture transition modeling framework of FORGE® are presented. In [8] & [9] the CIPFAR algorithm (Crack Initiation and propagation using the Phase Field and Adaptive Remeshing) was introduced, leading to a shift in the mesh management paradigm to introduce real cracks from damage fields and for ductile forming simulations. In this work we review the generalized framework enabling to: (1) Simulate damage initiation and propagation as a field state variable following any user-specified material law. This state variable is then used within a Phase-Field approach that serves as a proxy variable to identify the crack location within the continuous mechanics framework. (2) Introduce actual crack discontinuities in the finite-elements mesh by intersection of the phase-field’s gradient with the current mesh. Coupled with an automatic mesh adaptation technique this approach creates a robust framework for complex forming scenarios involving fracture.Further improvements are also introduced in this work: (a) The phase-field framework introduced new numerical parameters, such as a characteristic crack length, that can complexity the model adjustment and its industrial use, here we propose an automated approach to reduce the numerical adjustments. (b) As the crack insertion lays on top of the phase-field gradient computation, and the latter on a field recovery strategy, we also present a tensor filtering strategy enabling a better description of the phase-field gradient required for crack intersection. (c) An alternative automatic isotropic remeshing strategy is also introduced.

This paper describes a numerical modelling approach to study the behaviour of a solar receiver tube subjected to cyclic thermomechanical loading. The Lagamine finite element (FE) code was utilized along with a Chaboche type material law and a Lemaitre's unified damage model to simulate the material behaviour under fatigue, creep, and corrosion. The cycle jump procedure is evaluated, as it is a method for efficient computation of long-term evolution of material behaviour under cyclic loading. The procedure involves computing a number of cycles in the FE code, then extrapolating the results from these cycles over a number of cycles. This alternating process is repeated until the end of the computation. A parametric study was achieved to assess the effects of different strategies within the cycle jump. It was observed that the strong evolution of the material behaviour for the first cycles (around 100 cycles) of the computation prevented the use of the cycle jump during that period. Also, a sufficient number of cycles (minimum 4) must be computed with the FE code between the jumps to ensure reaching a stable solution. With the optimum parameters, the cycle jump permitted to significantly decrease the computation time (factor 10), while having a limited impact on the accuracy of the results (lower than 1%).

In recent years, there has been increasing interest in the mechanical characterization of polymers, including their formability and failure within the forming limit diagram (FLD), as incremental sheet forming (ISF) technologies have been applied to these materials. This interest is connected with the experimental verification that, as with metallic sheet, ISF of polymers can achieve levels of principal strains well above the material's forming limit curve (FLC), allowing for stable deformation up to the fracture forming limit (FFL). However, unlike metallic sheet, there is no systematic procedure for constructing the FLD for polymeric materials. Therefore, the aim of this experimental work is to establish a critical procedure for determining the FLD for polymeric materials at necking (FLC) and fracture (FFL). To achieve this, methodologies widely validated for sheet metals have been adapted and applied to two polymeric materials, ultra-high-molecular-weight polyethylene (UHMWPE) and polyether ether ketone (PEEK).

This study investigates forming methods that enable high formability in forming in-plane bent parts using finite element analysis (FEA). In-plane bent parts are conventionally formed by in-plane bending of strip-metal. However, in in-plane bending, if the cross-sectional ratio is high and bend radius is small, irregular deformation is likely to occur. Authors have proposed “Bending and compression method (BCM)”, in which material is bent and compressed perpendicular to the bending plane. In this proposal method, it might be effective to use a non-circular cross-section of raw material for increasing the cross-sectional ratio and reducing the bend radius. If the cross-sectional shape is optimized, it is possible to achieve even higher cross-sectional ratio and smaller bend radius. Therefore, in this study, the effect of initial cross-sectional shape on formability was investigated in straight and bend material by FEA. This study clarified the effects of the initial cross-sectional shape and compression method on the cross-sectional area ratio and bend radius in “BCM”.

Digital transformation in metal forming puts forward more and more opportunities for product quality control. The dimensional accuracy of products formed by stretch bending – one of the most widely used technologies for shaping aluminium extruded profiles – is influenced by both upstream and in-process uncertainties. In this paper, the influence of material property and friction-induced uncertainties on the dimensional accuracy in an advanced flexible stretch bending process was investigated with the aim to improve dimensional capabilities. A series of carefully controlled experiments and finite element (FE) simulations were conducted using AA6082-T4 rectangular, hollow profiles. Several loading paths with different pre-stretch strain levels prior to bending were designed and tested for a given geometrical tool configuration. Different levels of flow stress were considered to explore the impact of extrusion- and ageing-induced history-caused property variation on dimensional accuracy. In addition, both lubricated and unlubricated conditions were tested to simulate the influence of friction variation. The final global bend shape (springback) was selected as the key characteristic to analyse the impact of mechanical property and friction variations. Finally, a guideline for minimizing dimensional uncertainty was developed to provide improved process control of formed products.

The large-diameter and thin-walled aluminum alloy tube has superiority in terms of weight reduction and high transmission efficiency which has been widely used in the aerospace field. However, it is a tough issue to deform a desirable bent tube with such extreme specification and small bending radius. In recent years, aluminum alloy materials have been found to show strong enhancement in both strength and ductility when deforms at cryogenic temperature (CT), which provide the cryogenic forming potential for the hard-to-bend aluminum alloy tubes. In this work, tube formability at room temperature (RT) and CT was explored. The anisotropic characterization of the thin-walled tube was realized by combining experiment and viscoplastic self-consistent (VPSC) model. The overall mechanical properties at CT are significantly improved compared to those at RT. Furthermore, a finite element model of cryogenic bending of the thin-walled 6061-O aluminum alloy tube was constructed. The results provide evidence from two aspects of wrinkling and wall thickness reduction that the thin-walled aluminum alloy tube difficult to form at RT can achieve better formability when bent at CT. The average wrinkle height decreases first from 1.182 mm at RT to 0.201 mm at −60 ℃ with 83.0% reduction, and then increases to 0.425 mm at −180 ℃. The average thickness reduction rate decreases monotonically with temperature decreasing, and the drop is fastest at −60 ℃ of 15.4% reduction. Cracks no longer appear in cryogenic bending. In terms of the effect on the two defects of wrinkling and wall thickness reduction, −60 ℃ is the temperature at which the best forming properties are obtained.

A new method to reduce warping and springback for kinematic bending of arbitrary profile geometries, which are frequently used in transportation applications, is presented. The warping occurs naturally in such processes due to a misalignment of the force initiation axis and the shear center of the profile. Warping is commonly prevented by using restrictive tools, which in turn reduce the process flexibility. Springback is usually compensated through overbending. However, if the regular forming process is designed close to the forming limit, overbending might not be an option. The proposed solution involves a partial heating of the profile. In this case the base of a U-shaped profile is heated. As a result, the position of the shear center is shifted in the direction of, or, at best, to align with the force initiation axis. At the same time, the heating helps to reduce springback due to thermal softening. Three-roll-bending experiments on U-profiles made of S500MC are used to describe the warping and springback behavior in the unloaded state after bending. With these experiments an FEM model is validated. The model is used to investigate bending moments and shear stresses in the profile during bending. Results indicate that by partial heating, warping can be reduced by up to 76% and springback by up to 48% compared to the values for bending at room temperature.

Press forming of metal sheets has been widely used in industry. Press forming processes clamp sheet-metal between upper and lower dies, and deform them into diversified shapes by plastic deformation [1]. V-bending is a representative method of the press forming process and is widely used in the metal working field because it enables the use of general-purpose dies to form products of various thicknesses and shapes [2]. However, the stress concentrates at the bottom of the V-groove, and it sometimes causes a crack to develop, which may result in die cracking. The cracking may occur due to fatigue in normal operation or incorrect operation which applies excessive load over specification. This study employed a special die geometry to suppress the cracking of the V-bending die. The special die geometry was obtained through three-step FEM analysis. First, topology optimization was conducted and suggested that two-holes might be effective for stress relief. Second, a parametric study by iterative s-FEM was used to find the position of the holes. Third, a further parametric study by FEM determined the detailed hole geometry. Finally, experiments verified the effect of the two-holes on stress relief.

Rotary draw bending (RDB) is one of the most commonly used bending processes for metal tubes in present era. Over the years, efforts have been in progress to identify the process parameters of RDB process so that the quality of the bent portions of metal tubes can be improved. It has been demonstrated in recent studies that the wrinkle reduction in the bent portions of tubes can be achieved by selecting the directly influencing process parameters e.g. adjustment of the lateral displacement of pressure die and adjustment of relative collet speed. The contact forces in a RDB process are the forces applied by the forming tools during a bending operation. In this research the effect of contact forces on the quality of bent tubes is investigated. The directly influencing process parameters are used as the building blocks to examine the effect of contact forces on tubes quality in terms of ovality and wall thickness. The methodology of research encompasses simulating a stainless steel tube (1.4307) in the FE-solver software PAMSTAMP. The experiment is conducted practically on a specially designed tube bending machine and the FE-simulation results are compared with the experimental results. Two test cases are studied using identical directly influencing process parameters. A difference of contact forces in both the cases is incorporated by variation in pulling back time of the mandrel. A comparison of the test cases with different contact forces shows improved quality of tubes in terms of ovality. This study provides an insight about the role of contact forces of forming tools in refinement of process parameters in a RDB process.

In newly developed tube bending process, fillers are generally used to introduce internal pressure in active or passive way, which can reduce the risk of wrinkling and collapse at the outer and inner wall of tube, respectively. Therefore, it is of great significance to study the internal pressure distribution provided by fillers in tube bending process. In this paper, the mechanical reaction of granular filler and its interaction mechanism with tube during the granular media filler assisted push-bending (GMF-PB) process were studied. Based on the theory of discrete element method (DEM), the discrete element model of the bending of granular filler was established. The discrete distribution of contact force at the interface between granular filler and bending tube was obtained, then equivalent contact pressure can be calculated by using equivalent pressure model. The influencing factors for the distribution of internal pressure of the granular filler were investigated. The interaction mechanism between internal pressure distribution of granular filler and wrinkling and collapse of tube during GMF-PB process was explored. This paper provides a reliable foundation for analyzing the influence of mechanical reaction of granular filler on the wrinkling defects of bent tubes.

Laser-assisted robotic roller forming (LRRF) is a robot-based forming process that bends metal sheets by a universal forming roller with the synchronous aid of laser heating. Previous investigations have proved that LRRF has the ability to form ultrahigh strength steels to channel sections with a sharp bending radius. In this study, LRRF was applied to form dual-phase steels to hat-shaped beams by sequential flanging, and the bending performance of the thin-walled structures was compared with that formed without laser heating by means of three-point bending tests. The experimental results show that the LRRF process is capable of promoting the peak force, mean force, and apparent stiffness of thin-walled structures by 14.5%, 12.7%, and 140%, respectively. Smaller bending radii and laser-induced heat-affected zones (HAZ) were observed in the hat-shaped beams formed by LRRF. To account for the improvements in bending performance, finite element simulation considering bending radius and laser-induced HAZ was performed. It is found that the small bending radius has a greater influence on the bending performance, especially the apparent stiffness of thin-walled structures formed by LRRF, in comparison to the laser-induced HAZ. The findings of this research provide an excellent solution for forming advanced high-strength steels to thin-walled structures with sharp bending radii and the synergistic enhancement of bending properties via the LRRF process.

Curved aluminium alloy extrusion profiles have been widely used in many engineering applications due to their high-strength and high-stiffness to weight ratios. The curved extrusion profiles could be formed in several forming processes, such as roll-bending, stretch-bending and extrusion bending. This paper presents the review and analysis of their manufacturing processes and includes two parts. The first part is the review of manufacturing methods for producing curved extrusion components. The current cold bending processes can be grouped into three typical operations according to their characteristics, i.e., 2-point bending (cantilever bending), 3-point bending and stretch-bending. Key features of the operations, the main advantages, disadvantages and typical defects in the bending processes are summarised and analysed. The second part is the stress and strain analysis for the cold stretch-bending and pure bending of extrusion profiles. Particularly, the stress-strain distributions through the thickness of extrusion profiles, together with the neutral axis changes, are graphically illustrated and analysed. Further stress-strain analysis has been carried out on the “inner most” and “outer most” bending profile surfaces against the bending angle. Lastly, suggestions have been given on the future research directions, and the selection of the current existing processes for manufacturing defect free components.

The strength of building is increased by using rebars. The bent rebar is obtained with a bending machine. In the bending machine, there are the fulcrum roller, circular roller, and rebar receiver. The rebar was placed in the contact with rebar receiver. Then the circular roller rotates around the central fulcrum roller to a set angle(θF). Thereafter, the circular roller returns to the initial position. Then the bent shape of the rebar is influenced by the diameter of the fulcrum roller. The diameter of the fulcrum roller is determined by the trial results and the experience. In this paper, the effect of diameter of the fulcrum roller on shape of rebar in bending is reported. The experimental material is SD345 (JIS G3112) and yield stress 398 MPa. The bending deformation of rebar was photographed with a camera. The following conclusions were obtained from the experimental results. An interior angle formed by two linear parts of bent rebar was decreased with increasing the diameter of fulcrum roller. The length of area with constant radius of curvature of bent rebar was increased with increasing the diameter of fulcrum roller.

In industrial applications, established forming processes for series production are designed for the manufacture of annual lot sizes larger than 100,000 pieces. 3D swivel bending was developed for the economical production of smaller batches. As an extension to conventional swivel bending, variable cross-section geometries with curved bending edges and surfaces can be produced with a minimum of tools. To further increase flexibility of the process, variable shape-flexible tool concepts are developed. Investigations are carried out on the deformation behavior when using segmental tools, and calculation options for predicting the maximum degrees of deformation are derived. Based on these findings, different concepts for the design of a form-flexible tool for 3D swivel bending are developed. The concept developed makes use of many slim segments that can be moved transversely to the longitudinal direction of the bending axis. In this way, the tool setup can be scaled to handle variable component geometries, materials, and batch sizes. To ensure scalability to variable geometries, general calculation steps were developed for dimensioning the components, and design precautions were also taken to enable the forming of a higher-strength material. The characteristic feature for the form-flexible 3D swivel bending tool is the mapping of variable contours. To ensure that certain geometries can be produced reproducibly and accurately, the segments must be aligned precisely. For this purpose, a configurator was developed that can be adapted to the various bending tools and takes over the alignment of the segments under computer control.

Inconsistent behavior and premature failure are common when bending Ultra-High strength steels (UHSS). Practicality limits laboratory testing to smaller samples, which may not replicate the conditions of production or result in plane strain loading, and larger volumes of material used in manufacturing may increase the likelihood that material inhomogeneities will cause failure.A rig was developed, capable of replicating the size and scale of production. Tests have been undertaken on a commercial grade of UHSS at 5 forming radius/thickness ratios with strains measured using digital image techniques. Results show that punch separation occurs on specimens that ultimately fail, causing bend severity to increase unexpectedly. This occurs at consistent bending punch displacements during the bending process, with initial bending strains matching until a deviation point. Clear visual separation was observed to occur more frequently when the line along which the material was bent was parallel to its rolling direction than when it was perpendicular. Upcoming investigations will focus on behavior at this deviation point to try and better understand this separation. Understanding of this phenomenon may increase suitable applications for UHSS when bend forming is required.

The warm V-bending and hydrogen embrittlement properties of the ultrahigh-strength transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel sheets were investigated to apply for the automotive structural parts manufactured by cold- or warm-press forming. The V-bending tests were carried out at a forming speed of 1 mm/min and a forming temperature (T, °C) of 25 and 100 ℃ using a hydraulic servo type universal testing machine with a 88-degree V-punch and a V-die using V-bend specimens with dimensions of 5 mm width, 50 mm length and 1.2 mm thickness without and with hydrogen. Hydrogen charging was conducted by means of cathodic charging using a 3 wt% NaCl + 3 g/L NH4SCN solution at a current density of 10 A/m2 for 48 h before V-bending.The main results were as follows.(1) The 1100-MPa-grade TBF375 steel with a chemical composition of 0.2C-1.5Si-1.5Mn (mass%) enabled to conduct the V-bending at the forming temperature of 25 ℃ with hydrogen charging. This is considered that the hydrogen embrittlement crack propagation was suppressed owing to the finely and uniformly dispersed retained austenite (γR) although a large amount of retained austenite transformed into martensite at the outside of the V-bending part during V-bending.(2) At T = 100 ℃, the TBF steels were able to perform the 90-degree warm V-bending, considering the springback, by the moderate strain-induced martensitic transformation of γR as the TRIP effect at the plastic deformation region of outside of the specimen owing to the increase in the stability of γR in comparison with that at T = 25 ℃.

The thermo-elastic effect describes the cooling of a classical material under tensile load. In the tensile test, this cooling is superimposed with a temperature increase due to dissipation when plasticity sets in. This can be exploited from a measurement point of view and can be used for a physically motivated identification of the onset of yielding. In this study, different evaluation routines for the identification of the onset of yielding from the temperature signal are analyzed systematically. For this purpose, an analytical model is first used to generate evaluation data that provide a unique evaluation reference. To mimic sensor signals from real experiments, the artificially generated data is manipulated applying typical measurement noise, artifacts and other disturbance variables. Based on this data set, five approaches for identifying the onset of yielding are contrasted. The methods investigated are one based on the temperature minimum itself, a line-fit-method applied to the first derivative of the temperature signal, the second derivative of the temperature signal by means of finite differences, the second derivative of the temperature signal by means of regularization, and the solution of an inverse problem.

Aluminum metal matrix composites (AMMCs) reinforced with B4C particles (Al-B4C) exhibit excellent properties, rendering them a promising material for use in aerospace, automotive, electronic packaging, and military applications. However, the creation of intricate parts from Al-B4C is hindered by the instability of the mechanical properties and a dearth of research on the forming technology of this material. To surmount these issues, a novel simulation and prediction method was developed based on the crystal plasticity finite element-cohesive zone model (CPFE-CZ). In this work, the CPFE method was employed to model the mechanical response of the Al matrix, while the cohesive zone (CZ) model was utilized to describe the separation of matrix and reinforcement particles through the implementation of the bi-linear traction separation law and QUADS criterion. The proposed CPFE-CZ method was utilized to investigate the effect of various factors, including microstructure morphology, grain orientation, and size of matrix grains, as well as the volume fraction and size of particles, on the deformation behavior of Al-B4C. This research fills a gap in the exploration of the deformation mechanisms of AMMCs and presents a novel computational method that will allow for a better understanding of the deformation and damage mechanisms of similar material types.

The mechanical properties of materials are fundamentally derived from the microscopic structures and their behavior, and nanoindentation testing is widely used in fundamental studies of microscopic plasticity in materials. However, it is difficult to capture the plastic deformation phenomena moment by moment in experiments. For this issue, molecular dynamics (MD) simulation is a useful tool for visualizing atomistic behavior in materials. To cultivate a better understanding the elementary process of microscopic plastic deformation, in this study, we conducted nanoindentation MD simulation for body-centered cubic (BCC) iron (Fe) and hexagonal close-packed (HCP) magnesium (Mg) and visualized the initiation of plastic deformation, that is, dislocation nucleation. Furthermore, displacement burst events, called “pop-in,” are observed in nanoindentation and well-known as catastrophic events which result from dislocation nucleation. Here, we also predicted the temperature dependency of first pop-in load based on nanoindentation MD simulation for HCP Mg. Compared to the prediction result for BCC Fe in our previous study, the temperature dependence for Mg is much smaller than that for Fe.

A microstructure based constitutive analytical model considering the temperature effects on creep-ageing behavior of aluminium alloy has been developed. The evolution of precipitates for age hardening, as well as their interactions with dislocation evolution during creep have been considered and related equations have been proposed based on the nucleation and growth theory of precipitates for artificial ageing. These microstructures are then related to the yield strength and creep strain evolutions to update the strengthening and dislocation-based creep models, with which the temperature effects can be explicitly included. The analytical model has been successfully utilized to predict the microstructures (precipitation and dislocation) and macro properties (creep strain and yield strength) during creep ageing under different temperatures of an Al-Zn-Mg alloy. Furthermore, the capability of the developed model to predict the detailed creep and age hardening behaviour with complex multi-step heat treatment histories for creep-ageing processes has been presented and discussed with the same set of material constants. The analytical model developed in this study could be used to help the process design to achieve the concurrent optimization of accurate shape and high strength in the formed alloys.

An accurate flow curve over a large strain range is necessary and important for highly accurate numerical analysis of the load and deformation behavior during cold forging. A method using the upsettability test is widely used to obtain such a flow curve. This method converts the load–reduction in height relationship into a flow curve by using two calibration curves for the reduction in height–average equivalent strain relationship and reduction in height–restraint factor relationship. We explored the effect of work hardening on the calibration curves to improve the accuracy of load prediction. The work hardening of materials was described by the modified Voce hardening law. Finite element method (FEM) analyses were performed by assuming various material constants of the modified Voce hardening law, and the effect of the work hardening behavior on the calibration curves was investigated and formulated. We found that the work hardening behavior affects the calibration curve for the average equivalent strain and not the calibration curve for the restraint factor. Moreover, both relationships were formulated by polynomials. We proposed a new flow curve determination method that combines an optimization method with the formulated relationships. This method converted the load–reduction in height relationship into an accurate flow curve after automatically selecting a calibration curve for an appropriate average equivalent strain for various steels according to their work hardening behaviors. By applying the flow curve acquired by the proposed method to the FEM analysis of the compression test, the load prediction accuracy improved.

A series of high-strain rate tests has been carried out involving high-speed, kinematic and thermal metrology. The concomitant measurement of strain, stress and self-heating allowed the estimation of the inelastic heat fraction and further increase in inelastic stored energy. A thermo-viscoplastic constitutive model has then been developed within the irreversible thermodynamics framework by considering state variables and driving forces different from those usually considered. The experiment-based and physics-motivated approach proposed allows for deriving loading-path dependent self-heating consistently from the constitutive model, without the need for considering any arbitrary inelastic heat fraction.

The hot deformation behavior of bi-metallic ring blanks has an important influence on the hot ring rolling. In this paper, the influences of strain rate and temperature on flow characteristic of centrifugal casting 40Cr/Q345B bi-metallic ring blank was investigated by hot compression tests, which were carried out on the Gleeble-3500 thermo-mechanical simulator over the strain rate range of 0.005–1 s−1 and temperature range of 950–1200 °C. The true stress-strain curves of 40Cr/Q345B bimetal all show typical dynamic recrystallization (DRX) behavior, and dynamic recovery (DRV) and DRX are the main softening mechanisms. The peak stress of 40Cr/Q345B bimetal is always higher than 40Cr and approached to Q345B. The constitutive model was established using the Estrin and Mecking (EM) method in the work hardening and dynamic recovery (WH-DRV) phase, coupled with the Avrami equation for describing the softening behavior of DRX phase. Characteristic stress and DRV rate as functions of lnZ were fitted by third-order polynomials to improve the accuracy of the constitutive equation. The correlation coefficients (R) and average absolute relative error (AARE) between the experimental and the stress predicted by established constitutive equation were 0.9976 and 3.088%, respectively. The constitutive model has high precision and can be used to simulate the hot rolling of 40Cr/Q345B bi-metallic ring blank.

Woven fabric composites are increasingly used in the automotive industry because of their high specific strength, stiffness, and energy absorption capabilities. Material modelling of woven fabric materials is essential to accurately predict their dynamic behaviors. Furthermore, during crash events, the material can reach deformation with strain rates of up to 500 1/s. Therefore, it is necessary to measure the material’s strain rate-dependent properties and incorporate the strain rate effects into the material model for explicit dynamic simulations. Therefore, a VUMAT in Abaqus was developed to simulate the rate dependency and nonlinear shear behaviors of woven fabric materials.

A rolled AISI D2 tool steel was selected as the starting material in this study. Reheating and rapid cooling experiments and microstructural observation of AISI D2 tool steel were conducted. On the basis of the results of microstructural observation and elemental analysis, the microstructural evolution behaviors of AISI D2 tool steel during partial remelting was investigated. The semisolid slurry with homogeneous spherical morphology was obtained when the reheating temperature and isothermal holding time were 1300 °C and 20 s, respectively. In consideration of geometrical morphology and volume fraction of solid particles in semisolid slurry, A physical constitutive equation describing remelting behavior of AISI D2 tool steel during partial remelting was established. In this physical constitutive equation, a truncated octahedron was used as the basic unit. The volume fractions of liquid phase in AISI D2 tool steel reheated to different target temperatures were calculated by used this established physical constitutive equation. The difference between results obtained from high temperature differential scanning calorimetry (HDSC) analysis and physical constitutive equation were caused by the dissolution of residual carbides and the transformation of liquid film to solid phase particles during rapid cooling of AISI D2 tool steel.

Belonging to the 3rd generation of Advanced High Strength Steels (AHSS), Quenching and Partitioning (Q&P)-steels exhibit an excellent combination of high strength and good ductility while having a similar alloying concept as the already existing 1st and 2nd generation AHSS, such as DP- and TRIP-steels. To widen the understanding of mechanical response to high-speed loading, a broad range of strain rates from 0.0001/s up to 1000/s was studied for two grades of Q&P-steels, namely QP980 and QP1180. Special attention is given to the hardening behavior, which is influenced by a rate-dependent TRIP effect. The strain rate effect on the microstructure evolution is also investigated to enable the connection of the rate-dependent TRIP effect with the respective mechanical response.

Material processing induces preferential arrangements of the grains which in turn results in anisotropy in the macroscopic plastic properties. Improvement of the finite-element predictions of the geometry of the final part (e.g. shape, thickness reduction) necessitates accurate modeling of the plastic anisotropy (see [1]). In this paper, we present finite-element (FE) simulations of deep-drawing process in which we account for both the anisotropy in the plastic deformation of the constituent grains and the initial texture of the material. Specifically, an elastoplastic anisotropic constitutive model recently developed [2] is used to model the crystal level behavior. This crystal model is defined for any stress state and fulfills the symmetry requirements associate with crystal lattice. In the FE simulations, a polycrystalline aggregate is associated with each FE integration point. The FE code imposes the computed macroscopic velocity gradient on the polycrystal. The orientation and the hardening of the individual grains, which depend on the deformation history of the element, are updated, and the macroscopic stress for use in the solution of the continuum equilibrium equations is obtained from the stresses in each grain, which in turn were calculated by solving the full-set of coupled equations governing the elasto-plastic single crystal behavior (i.e. elastic response, the crystal yield condition expressed in terms of anisotropic invariants (see [3]), flow rule, consistency-condition) using a fully-implicit backward Euler method. Illustrative examples presented demonstrate the predictive capabilities of our model to describe the behavior of strongly textured materials for the highly non-linear applications.

Full-field deformation measurement is used for many approaches in material characterization. To the best of our knowledge, optical measurement of the entire deformation field is not possible, regardless of the number of cameras used in the measurement system. There are always losses near the edges, so some assumptions are required to close these spots and restore the full-field measurement. The reason for these losses is the well-known problems of the subset-based DIC technique related to the sharp edges of samples cut from sheet materials. We believe that measuring the entire full-field is not even necessary for simultaneous calibration of the normalized yield locus curve and the normalized flow curve. The joint normalization of yield locus, flow curve and elastic modulus of a material with its initial yield stress results in the same deformation field as the original material model in a finite element simulation. The deformation field is thus purely related to the material parameters normalized in this way. This fact helps in simultaneous calibration of the normalized flow curve and the yield locus by using only a selected part of the deformation field without considering the stress distribution and force measurements. The obvious proof is given that the problem breaks down to pure geometry. This is followed by the details of the approach. Finally, a validation study with virtual experiments is presented.

The accurate prediction of the material behavior of sheet metals under deformation is crucial for the design and optimization of sheet metal forming processes, which are widely used in many industrial applications. The conventional characterization of orthotropic flow behavior up to failure requires about 60 individual tests, making sample preparation, test execution and evaluation time-consuming and expensive. To overcome this issue, inverse identification strategies are employed using optical measurement systems that provide a transient record of the deformation field on the entire specimen. The comparison of such measured deformation fields with the corresponding simulation results, provides information about the accuracy of the underlying material model and the chosen parameters, namely, modulus of elasticity, formulation and parameters of the yield locus and yield curve [1]. The success of such inverse parameter identification strategies relies on the variety of the strain paths covering the whole possible deformation space occurring in real components during forming and crash. In this paper, we present a study on the optimization of the yield locus of the Barlat 2000 yield criterion by means of Full-Field Calibration and a new specimen geometry. The specimen geometry was designed to enable more representative strain field under complex loading conditions. The experimental data obtained from these tensile tests were used to calibrate the Barlat 2000 yield criterion. The results show that the Full-Field Calibration significantly improves the accuracy of the Barlat 2000 yield criterion in predicting the material behavior of the sheet metal under complex loading conditions.

Around 20% of powders are scattered as waste outside the deposition chamber during spray forming of AA7055. To recycle these valuable powders, a recovery and a hot deformation processing routes are developed. Firstly, strict screening of powder is established in which the chemical compositions and microstructure are measured. Cold isostatic pressing followed by hot extrusion and T76 or T6 heat treatment is developed to compact the powders into a solid bar shape which could be used as billets. The hyperbolic sinusoidal constitutive equation is obtained using Gleeble hot compression test of the densified material. The analysis of mechanical properties shows the samples after T76 heat treatment possess slightly lower strength but enhanced toughness than that of sprayed samples. Thus, further deformation processing like forging is needed to improve compactness and mechanical properties of these billets.

The dynamic fracture of sheet metal usually occurs during the high speed forming and automobile crashing processes. The electromagnetically impacted Nakazima test was adopted with the hemispherical punch under the pressure of drive plate, and the drive plate was loaded by the Lorentz force to obtain the impact energy. Different geometries of specimen were designed to investigate the dynamic fractures of AA5182-O aluminum alloy sheet over various strain paths of tension-compression and biaxial tension. The limit strains at fracture were determined by the numerical simulation of electromagnetically impacted Nakazima test. The result showed that the dynamic fracture forming limit curve is higher than the quasi-static fracture forming limit curve. Compared with the Rice-Tracey and Oyane-Sato fracture models, the Johnson-Cook fracture model was validated to be more accurate to characterize the quasi-static fracture locus. Hence, the strain-rate-sensitive parameter of Johnson-Cook fracture model was subsequentially determined by minimizing the discrepancy between the experimental and predicted dynamic fracture locus.

The effect of plastic deformation prior to peak age-hardening on mechanical properties of an Al-Mg-Si-Cu aluminium alloy was investigated. The alloy was prepared by DC casting and then hot extruded into flat strips of 3 mm in thickness, which were immediately quenched after extrusion to obtain a supersaturated solid solution state. The extruded and quenched flat strips were either left naturally-aging for a controlled short period of time or pre-aged before deformation. Plastic deformation was performed at room temperature by cold rolling or stretching to a strain of 4% or 8%. Hardness measurements and tensile testing were performed to obtain the mechanical properties of the alloy under different processing conditions. Scanning electron microscopy techniques, including electron backscattered diffraction (EBSD), were conducted to characterize microstructural features such as the dislocation density and crystallographic texture. Experimental results showed that the combination of pre-ageing and deformation by stretching resulted in a significant increase in the mechanical properties after the final peak ageing treatment. The interactions between dislocations and pre-ageing induced solute clusters and fine precipitates during deformation are considered to have increased the dislocation density in the material and promoted heterogeneous nucleation during final age hardening treatment.

A strain-rate potential (SRP) can be used in place of the classical stress potential to describe the response of plastically deforming solids. Orthotropic potentials are typically developed by extending existing isotropic criteria. For mathematical convenience, the construction of orthotropic SRPs using the linear transformation approach often involves a constrain on the fourth-order orthotropic tensor such that the transformed stress (and therefore strain-rate) tensors are deviatoric. This constrain is not however necessary to ensure plastic incompressibility and in fact reduces the number of independent orthotropic coefficients from nine to six, impairing the flexibility of the criterion. In this work, a new definition is proposed for the fourth-order orthotropic strain-rate transformation tensor with no constrains on the original form of the stress transformation tensor counterpart, such that the number of independent orthotropic coefficients is maximum. Based on this idea, as a proof of concept, a more general orthotropic SRP is construct, dual of a quadratic stress potential with a single linear transformation for materials with the same response in tension and compression.

The influences of tensile and compressive stresses on stress relaxation behavior and mechanical properties of 2219 aluminum alloy have been systematically studied with different initial stress levels at 165℃. Under the same initial stress level and ageing time, tensile stress relaxation is greater than that in compression. The stress relaxation difference between them mainly originates from the primary relaxation stage, while the steady-state relaxation stage is basically identical. With the increase of stress level, the yield strength of tensile-stress-relaxation aged sample augments gradually, and the compressive stress sample fluctuates slightly. The corresponding deformation and strengthening mechanisms are revealed through transmission electron microscopy observation and analysis. When the initial stress is 100MPa, the sample in compression has larger diameter and volume fraction of the mixed GP I and GP II precipitates with unobvious stress- oriented effect. These microstructural features in compression lead to higher yield strength and smaller stress relaxation rate than the tensile sample. Nevertheless, at the high initial stress of 170MPa, massive dislocations and main strengthening θ’precipitates in tensile sample result in stronger yield strength and rapider stress relaxation as compared with compressive sample.

The phenomenon of asymmetry exists in yield stress, hardening evolution, and damage accumulation under tension and compression loading paths for Magnesium (Mg) alloys. In order to simulate accurately the plastic deformation and damage occurrence during Mg sheet metal forming processes, advanced constitutive model accounting for asymmetry is highly required. This paper aims to develop a thermodynamically consistent constitutive model to treat the asymmetry in magnesium alloys. The proposed model is based on the fully coupled constitutive equations accounting for combined isotropic and kinematic hardenings, in which the hardening parameters are evolving with the plastic strain to capture the hardening asymmetry. A carefully parametric study of the new parameters is conducted to show the predictive capabilities of the proposed model. Through comparison between the simulation results and experimental observations, the proposed model could reproduce the stress-strain curves of Mg alloys under tensile and compressive loading conditions with high accuracy.

Accurate material characterization is important to structural analysis. To obtain better prediction accuracy, complex material constitutive models are used widely, resulting the requirement of more test data for parameter identification. DIC (Digital Image Correlation) becomes more and more popular, providing full-field strain history with an optical method that could not be obtained by conventional test methods.This paper compared the full-field calibration (FFC) of 3rd generation AHSS QP980 based on DIC data and the traditional curve matching calibration (CMC) based on force-displacement data using LS-OPT. Barlat89 yield criterion and combined Swift and Hockett-Sherby hardening model were used to better characterize the anisotropy and hardening behavior of QP980 material. The standard uniaxial tensile, R5, R20 and R80 notched tensile tests were carried out to calibrate all needed parameters.In terms of the force-displacement curve, the difference between FFC and CMC is small, with a deviation of 8.9%. On the other hand, in terms of strain field, the difference is significantly different. Compared with CMC, max error of local strain decreases 41% in FFC. Simulated result of FFC correlates better to experiment than that of CMC.

The hot deformation behaviors of GH4698 superalloy are studied by the hot compression tests performed over a range of strain rates (0.001–10 $$s^{ - 1}$$ s - 1 ) and deformation temperatures (1020–1200 $$^\circ {\text{C}}$$ ∘ C ). The results show that the flow stress is significantly affected by deformation temperature and strain rate. As the deformation temperature increases or the strain rate decreases, the flow stress decreases. In the work hardening-dynamic recovery stage, a physically-based constitutive equation for flow stress is obtained from the stress-dislocation relation. In the subsequent dynamic recrystallization stage, the flow stress is predicted by employing the dynamic recrystallization kinetics in the constitutive model. The stress-strain curves of the studied superalloy predicted by the established models are in good agreement with measured data, which indicates that the developed physically-based constitutive model enjoy the high accuracy in characterizing the hot deformation behaviors of GH4698 superalloy.

Recent studies highlight the importance of improving the description of the material behavior, particularly for non-proportional loading paths. This work discusses the differences in the stress and strain ratios predicted by different yield criteria, when applied to describe the plastic behavior of an aluminum alloy, submitted to different non-proportional biaxial loading conditions. Reloading with or without unloading is analyzed to highlight the importance of the unloading stage on the stress paths, since the stress state cannot change abruptly for a material undergoing plastic deformation. Although the yield criteria adopted seem to have a small impact on the stress state predicted for each force-controlled loading condition, the influence on the strain path is non-negligible.

In dual-phase steels, microstructural characteristics such as phase volume fraction and phase contrast tend to have opposite effects on the ductility measured in hole expansion capacity (HEC) testing as compared to the forming limit curve (FLC). This has lead to a number of paradoxical observations in the literature, in which microstructures which were optimized for ductility in terms of the FLC turned out to perform poorly on HEC and vice versa. This study systematically analyzes the issue by means of microstructural simulations. Artificial, highly idealized two-phase microstructures are constructed with have the same nominal strength, but which achieve this strength by different combinations of martensite volume fraction and hardness. They are subjected to pure shear deformation and based on the computed response their hardening curve and damage resistance are predicted; furthermore, the point of necking in plane-strain tension is predicted based on a Considère-like criterion. If the latter is taken as representative of the FLC and the strain to (local) failure due to damage of the HEC, the paradoxical trend discussed above is reproduced. It may furthermore be traced to the distinct hardening behavior of martensite, with a rapid hardening at low strain levels followed by early saturation.

In the case of thick high strength steels, 3D anisotropic yield criteria are required to accurately capture the plastic material response. Material testing 2.0 aims at extracting material behaviour from an information-rich mechanical experiment with aid of inverse methods and full-field measuring techniques. In this context, inverse identification of 3D anisotropy through a single experiment is particularly challenging because of the large amount of anisotropy parameters. The difficulty lies in the specimen design that ideally ensures a simultaneous activation of all sought anisotropy parameters with sufficient sensitivity towards the observed strain field. In this contribution, we scrutinize the effectiveness of a specimen designed by shape optimization to inversely identify a 3D anisotropic yield function. This is done via a so-called Digital Virtual Twin (DVT) mimicking the actual measurement including the Digital Image Correlation (DIC). Given that the synthetic data is generated with a known material model used by the FE model, the identification accuracy of the Finite Element Model Updating (FEMU) approach can be objectively assessed. The novelty here is that the synthetic data consists of the data captured by two stereo-DIC (sDIC) systems. Moreover, the FEMU approach accounts for the multi-sDIC data sets. The DVT-results show that the adopted specimen design enables to simultaneously identify the sought anisotropy parameters. The proposed methodology is experimentally validated on S700MC steel with a thickness of 12 mm.

During the hot straightening process, the sheet undergoes multiple tensile and compressive deformation cycles. In this paper, cyclic tensile tests and microstructure observation under different strain amplitude, temperature, and strain rates were carried out. The macroscopic deformation behavior and microstructure evolution of X70 pipeline steel under cyclic tension and compression load were investigated, then, macroscopic mechanical properties and microstructure evolution of sheet steel in the process of hot straightening were deeply understood. The experimental results show that the X70 pipeline steel has the characteristics of cyclic hardening. Compared with temperature and strain rate, the hysteresis loop was dominantly influenced by strain amplitude. There is little difference between the peak value and the trough value of the circulating stress in the hysteresis loop. Hot straightening has little effect on the microstructure of the material. The study shows that the amount of deformation has a great influence on the deformation behavior of materials during the process of hot straightening, and the Bauschinger effect of materials is not obvious. The material deformation in the process of hot straightening belongs to the macroscopic deformation behavior.

Friction is part of almost any forming process, thus, friction modelling is mandatory for process modelling. Since frictional stresses are difficult to measure, typically laboratory experiments are used, where a dimension sensitive to frictional stresses is measured instead. The conical tube-upsetting test, an advancement of ring compression test, is such a laboratory experiment. Here, the change in geometry is measured to inversely determine friction parameters. The process parameters, such as temperatures, strain rate and contact pressure should be as close as possible to the forming process that is modelled with the laboratory experiment. Especially in hot bulk metal forming, normal stresses greater than yield stress of the work piece can occur, which has significant influence on the frictional conditions. However, previous work has shown that normal stresses in conical tube-upsetting test are lesser than the yield stress of the workpiece material, which restricts the experiments application regarding hot bulk metal forming. Thus, in this work the specimen geometry of conical tube-upsetting test is investigated by means of FE-simulations with respect to the occurring normal stresses between workpiece and die. Additionally, the influence of altered geometries regarding the sensitivity towards occurring friction is taken into account. The results suggest that especially an increased thickness of the specimen in the bottom area leads to greater normal stresses. At the same time however, the sensitivity towards friction conditions decreases.

A testing method to determine the flow stress of wire material and the friction coefficient of solid lubricant coating proposed by authors is improved using the simulations. Finite element analyses with the elastic tools are compared with those with the rigid tools to develop the method to eliminate the effect of the elastic deformation of the tools on the punch stroke. The modification method of the parameters of the flow curve is developed to accurately predict the flow stresses for large strain range from 0 to 3 despite of the maximum strain in the specimen during testing is approximately 0.5. The temperature increase during testing by the friction between the specimen and tools and plastic work of the specimen is simulated and then change of the friction coefficient which is a function of the temperature at the coating is evaluated to determine the appropriate testing speed to minimize the predictive error of the friction coefficient.

The high strength steel zinc-iron coated (GA) sheet with the dual functions of barrier protection and cathodic protection has broad application prospects in the field of boron steel hot stamping. The essential issue for the industrial application of the zinc-iron coating sheet for hot stamping is how to achieve the optimal balance between the microstructure, morphology, and protective performance of the coating and the properties of the steel by adjusting the heat treatment process. In this paper, a novel step-type rapid heating process path is presented that can achieve short-time alloying of the coating and less Zn loss while ensuring sufficient austenitization of the matrix. The heating process is divided into two stages. The first stage is called the pre-oxidation process, which is used to promote the formation of a continuous oxide layer on the surface of the coating, thereby effectively inhibiting the loss of zinc at high temperature, and ultimately increasing the effective Zn content of the coating. The second stage is the austenitizing process to ensure the complete austenitization of the matrix. The experimental results show that the average thickness of the fully alloyed coating is increased by 23.3%-28.7% compared with the direct rapid heating process, and the average Zn content is increased by 14.3%-17.2% compared with the radiation heating process. The effect of the rapid heating process on the deformation and cracking of the coating was then investigated. The test results show that when the holding time is 20 s, the deformation temperature to avoid premature fracture of the coating sheet is lower than 750 ℃. When the dwell time is extended to 50 s, the critical forming temperature is increased to 780–820 ℃.

An accurate knowledge of the value of young’s modulus is essential for design studies, for finite elements and modeling calculations and for giving reliable values for constitutive laws. Several ways to measure the elastic coefficients exist: Direct measurement by a tensile test; Measurement by an impulse excitation technique (resonant or damping frequency); Ultrasonic measurement. This paper proposes a comparative study of these different methods associated to other topics related to sample properties and manufacturing process. From this study, the best characterization methods selected are those by vibration.

The present work investigates the influence of two different processing routes on mechanical behavior of Mg-Zn-Nd-Y-Zr (ZEWK2000) Mg sheets. Hot-rolled sheets, with dimensions of 200 × 200 × 1.8 mm3, were processed at 225 ℃ by a two equal channel angular pressing (ECAP) passes following the route DC2 with a rotation of 90° around the normal direction (ND) of the sheet. Shear deformation applied by ECAP weakens the texture and promotes a broad angular distribution of basal planes in the pressing direction. The resulting crystallographic texture promotes a change of the balance of active deformation mechanisms, i.e. slip of dislocations and twinning, during subsequent uniaxial deformation. To assess the effect of texture changes on the activation of slip systems, a systematic analysis of the slip traces after uniaxial tension of ECAP processed samples at room temperature was carried out. The analysis reveals the profuse activation of additional slip systems than the common basal <a> slip and pyramidal II <c+a>, where non-basal slip such as prismatic  <a>, pyramidal <a> and pyramidal I<c +a> played a major role than expected in the rolled sheet. The relative activity of deformation mechanisms was significantly changed in the ECAP processed sheet, where extension twinning, basal <a>, pyramidal I <c+a> and pyramidal II <c+a> slip dominated the deformation. Overall, the activation of extension twinning in the ECAP processed sheet is beneficial for the reduction of anisotropy and the improvement of formability of the processed samples.

When compared to the simple shear test, the in-plane torsion test allows for large strains to be achieved without edge effects, specimen tearing from the jaws or buckling. The proposed device offers full optical access to the specimen and enables the use of 2D Digital Image Correlation. In this way, we can observe the effects of the material anisotropy on the strain all along the circumference of the specimen up to large strains. A series of radial grooves machined on the specimen’s inner clamped surface enables the transmission of large torques necessary for high-strength steels. The paper illustrates the potential of this test for the study of plasticity with a focus on anisotropy, large deformations, and cyclic testing. Two steel sheets are considered, a deep drawing low carbon steel DC01 and a stainless steel AISI304. The paper aims to establish a direct relation between Hills’ anisotropy model parameters identified from the standard uniaxial tests and the angular evolution of the effective strain along the shear gage section of the in-plane torsion specimen.

The Mg-7Gd-4Y-2Zn-0.4Zr scraps were solid state recycled through hot pressing sintering (HPS) and spark plasma sintering (SPS). Rotary extrusion with different parameters were conducted on the homogenized, hot-pressed sintered and spark plasma sintered alloy. The mechanical properties and microstructure evolution of the alloy fabricated by different processes were investigated. At 475 ℃ and rotational velocity of 1.2 rad/s, the homogenized alloy after rotary extrusion has the best mechanical properties. At 450 ℃ and rotational velocity of 2.4 rad/s, the hot-pressed sintered and spark plasma sintered alloy after rotary extrusion demonstrate better mechanical properties. With the increase of the extrusion temperature and rotational velocity, the mechanical properties of the alloy are improved. However, excessively high temperature and rotational velocity will cause the broken of bonding interface. The bonding interface of the alloy recycled by SPS is more stable than that of the alloy recycled by HPS, which is conducive to the improvement of mechanical properties of the recycled alloy.

Resistance heating (RH) method has been proposed to be used for assisting the high-temperature tensile testing of metal tubes with small diameters. Digital image correlation (DIC) with laser speckles was adopted to measure the displacement and true strain of the deformed material. Magnesium alloy tubes of ZM21 with an outer diameter of 6 mm and gauge length of 10 mm were used. Tensile tests were conducted at both room and high temperatures under a nominal strain rate of 0.01 s−1. The displacement and nominal strains measured by DIC were compared with that measured by a non-contacting extensometer. True stress-strain curves and full-field strain were obtained. As results, the temperature of the tube increased to 400 ℃ and became stable within 50 s. The maximum temperature difference within the gauge was 24 ℃. The good agreement of the nominal strains calculated by DIC and the extensometer validated the accuracy of the displacement measured by this system. The true stress-strain curves simply calculated from the nominal stress and strain have been confirmed inaccurate to express the flow stress of the material. The measurement of full-field strain has been achieved.