Thermomechanics & Infrared Imaging, Inverse Problem Methodologies and Mechanics of Additive & Advanced Manufactured Materials, Volume 6

The maximum hole depth that ensures acceptable results when performing IHD of isotropic materials is well established and a final hole depth is specified in the ASTM standard. Limited investigation has, however, been conducted on the equivalent maximum hole depth in FRP laminates. The least-squares solution of the series expansion method benefits from an abundance of measured strain data, however, a depth is reached where the insensitive data at greater depths negatively impact on the stability of the least-squares solution. This not only leads to higher uncertainty in stress at greater depths but potentially at shallower depths also. A similar effect can occur in the case of the regularized fit of the integral method. It is therefore important to determine the maximum hole depth before the accuracy of the calculated residual stress distribution reduces to unacceptable levels. The effect of final hole depth on the accuracy of calculated residual stresses is investigated for a GFRP laminate of [0₂/90₂]_s construction using separate series expansion in each ply and also the regularized integral method. The same investigation is also performed on a GFRP laminate of [0₂/+ 45/−45]_s construction using series expansion only. Results indicate that the calculated stress distribution is dependent on the depth to which strain data is incorporated into the analysis and, in the case of series expansion, also on the relative position of the bottom of the hole within the deepest drilled fibre orientation. Neither computational method produces reliable results beyond a hole depth of about 1 mm which corresponds well with the limit specified by ASTM E837 for metallic materials.

The fatigue behaviour and intrinsic dissipations are studied in the present research by using different approaches: a thermography-based one leading to the use of a second harmonic amplitude temperature component as a damage parameter and the one using the mechanical energy input related to the energy dissipated during fatigue processes.

Moreover, the relation between second amplitude harmonics of the heat-converted energy and of instantaneous power density is investigated for C45 steel undergoing fatigue tests by using stepwise loading sequences at two stress ratios. The analysis allows to assess the mathematical relation between second amplitude harmonics of instantaneous power density and temperature. Once these calibration coefficients are assessed the area under the hysteresis loop can be finally determined.

It will be shown that the relation between second amplitude harmonics of the heat converted energy and instantaneous power density is independent from the damage level and cycles run, but depends on material only.

The approach leads to a local analysis and does not require specific loading conditions because no hypothesis is made on the percentage of energy converted in heating. Furthermore, the analysis does not require material stabilization.

The proposed procedure involves the estimation of the area under the hysteresis loop at any loading condition by simply assessing second harmonic temperature variations. The verification of the procedure is presented on extra samples.

The fatigue crack propagation behaviour of material fabricated by the SLM process is investigated by using several complementary experimental techniques aiming at addressing the problems of early identification of crack initiation and growth, the transition between the fatigue crack growth regimes, and the assessment of parameters capable of describing the plastic work at the crack tip. We provide a report on the preliminary study based on the thermographic data reflecting the fracture mechanics behaviour of a titanium-based additive manufactured material.

This paper shows that it is possible to non-destructively evaluate hot-spots or critical stress points on the surface of structural components by using an infrared (IR) quasi-static methodology. The quasi-static method to measure the material fatigue limit was applied to a four-point bending beam case. Temperature measurements were taken at the surface while uniaxial and pure bending test specimens were quasi-statically loaded under monotonic conditions. Experimental observations relate the stress-strain-temperature measurements to the onset of yielding at the observed points of the uniaxial test specimens and strain-temperature measurements of the bending test specimens. This work aims to use the temperature monitoring of structures under quasi-static increased loading to predict how far from the material yield strength onset the stress or strain states of the observed points are located, providing information about the component’s current situation.

Detection of subsurface damage in composite structures is essential for assessing performance and identifying regions for repair. Thermoelastic stress analysis (TSA) is a full-field infrared imaging technique which uses the thermoelastic response of a material to infer the stress state on the surface of a component under cyclic loading. Similarly, digital image correlation (DIC) is a full-field surface measurement technique which employs white light imaging to obtain surface displacements and hence strains. This study combines TSA and DIC to identify subsurface damage evolution and the source of thermoelastic response under cyclic loading. The non-adiabatic thermoelastic response at low loading frequencies is used to reveal damage in multidirectional coupons of carbon fibre reinforced polymer (CFRP). The paper considers the effect of loading frequency and shows that increasing the loading frequency prevents internal heat diffusion and as a result achieves adiabatic conditions. It is shown that the heat diffusion that occurs at lower loading frequencies allows the thermoelastic response from the subsurface plies in the CFRP laminate to influence surface response. Hence, appropriate selection of loading frequency results in either surface or combined surface and subsurface thermoelastic responses. Conversely, DIC relies on the kinematics of the surface and therefore is independent of heat diffusion effects. Additionally, both composite components’ stacking sequence and ply fibre orientation perform an essential role in the thermoelastic emission since the mechanical and thermal properties’ directionality influences the result. The main purpose of the paper is to study the damage evolution of the CFRP laminate at different loading frequencies.

One of the most used manufacturing processes for producing mechanical components with complex geometries in a short time is additive manufacturing (AM). For this reason, thanks to the application of these kind of processes, it is possible to simulate the presence of simulated defects with a particular shape inside the produced material, that represents a powerful tool in the field of non-destructive controls to calibrate the technique and establish the limit in a quantitative way. To do this, without AM, the investigation concerns defects like flat bottom holes, with a geometry and a behavior very far from a real defect.

This work is focused on the application of active thermography as non-destructive technique to assess the quality of AISI 316L metal samples produced by means of laser powder bed fusion (L-PBF) process. It is known that one of the most widespread and common defects in steel materials is porosity, diffuse and localized, due to sudden and unexpected changes in process conditions. From a thermal point of view, this type of defect is very far from the one normally simulated with non-destructive techniques, that are flat bottom holes or delaminations. More in general, sub-superficial defects, like porosities, affect the quality of the signal and the detection limit. For this reason, a complete experimental plan has been carried out to print sub-superficial spheres of different size (depth and diameter) in different specimens, for a total of about 150 defects, including replications at fixed nominal aspect ratio and shape factor. In particular, these spheres have non-melted powder of the same material inside.

A reflection set-up with two flash lamps and a MWIR-cooled sensor has been used to perform different tests and to define the limits and the advantages of the proposed technique. To improve the quality of the thermal signal and the signal-to-noise ratio, post-processing algorithms have been used to process the raw thermal data. The study is a preliminary research activity aimed to calibrate the technique for the quantitative estimation of porosity in steel components.

In the last few years, “metamaterials” have become very popular in materials’ mechanics. The most interesting aspect of such structures is that their mechanical response is strongly linked to the geometrical configuration rather than their chemical composition.

The Miura Ori is a particular origami technique that consists of folding a sheet to realize a lattice of periodic cells: the leading parameters of the cells are the folding angles and the length of the edges. The literature demonstrates that a careful selection of these parameters consents to achieving an auxetic response of the structure, i.e., the global Poisson coefficient is negative.

In this work, the authors want to investigate the Miura Ori auxetic behavior experimentally. Because sharp edges characterize the Miura Ori cell, the authors opted to produce the specimens with a FFF 3D printing technology and analyze the displacement field with Digital Image Correlation.

The results show a strong correlation between the Poisson ratio and specimen thickness, thus allowing for significant negative values. In addition, DIC results are in good agreement with numerical simulations.

There is an increasing interest in the application of additive manufacturing techniques such as material extrusion to highly filled granular composites in the form of pastes. This category of material is generally understood to include cements, various foodstuffs, and uncured energetic materials such as polymer-bonded explosives and composite propellants. Pastes generally comprise a volume fraction of solid particles in excess of 0.5 dispersed in a liquid carrier. At these volume fractions of solid, particle-particle interactions such as lubricated contacts and jamming are understood to greatly affect the bulk rheology of the material, giving rise to complex properties such as yield stress and pseudoplasticity. As such, accurately modeling the behavior of these materials, which is an important part of the design and optimization of an additive manufacturing process, can be extremely difficult.

Experimental investigations of the extrusion of pastes on the laboratory scale have been carried out using bespoke apparatus at the University of Cambridge with the aim of optimizing additive manufacturing processes. As part of this work, a novel approach to visualizing the flow of pastes through ram extruders has been developed using colored tracer formulations of glass spheres in thermally cured silicone resin. Following extrusion experiments, in which material can be extruded at a defined rate while monitoring the overall pressure, the material can be cured to a rubbery solid then sectioned. A custom application can then be used to analyze the sections, producing quantitative information about the material extension and static zones preceding the nozzle entry as well as flow through the nozzle.

This paper describes a series of paste extrusion experiments designed to investigate the relationship between the dimensions of the extension zone, the volume fraction of solid in the formulation, and the extrusion pressure. By directly measuring the dimensions of the extension and static zones across a range of conditions and comparing them with measured extrusion pressures, the mechanism associated with steady-state extrusion can be elucidated along with various unstable regimes. This understanding of the extrusion mechanism and how it changes as a function of solid volume fraction will provide critical input for the modeling of additive manufacturing of granular composites via material extrusion, supporting the safe, efficient, and rapid development of the technology.

Voronoi cellular structures (VCSs) are multifunctional materials that can be customized to specific design requirements by suitable density gradation. By locally changing the density, their response can be modulated, and therefore, they can be readily adapted to tailored requirements. Despite these merits, their fabrication is a challenge due to the complexities in the topology of their structure. Conventional manufacturing methods are inadequate to produce the designs dictated by specialized requirements. In this study, the technique for the fabrication of 3D VCSs with spatially varying density is developed using additive manufacturing. Open-celled VCSs are additively manufactured using photopolymer jetting technology that allows the creation of complex parts with high accuracy. The data processing required for generating 3D Voronoi models and managing their complex geometrical features is performed using Python scripts. Uniform-density and density-graded VCSs are fabricated by controlling the cell size, which directly influences their local density. Their dynamic response under the impact of a rigid mass is experimentally determined using a drop tower setup. The compression in the specimen is measured using digital image correlation with the help of a high-speed camera. It is observed that the deformation behavior of VCSs can be tailored by local density variation.

Ni-based superalloy IN718 is a popular choice for metal additive manufacturing (AM), and modified IN718 with the minor addition of Y₂O₃ is of interest to develop improved microstructure and mechanical properties. In this research, we studied the tensile mechanical property of IN718 with 0.5 wt.% Y₂O₃ addition. A novel mechano-chemical bonding (MCB) method was used to produce oxide dispersion strengthened (ODS) IN718 powders suitable for AM applications. SEM, EDX, and TEM were applied for microstructure analyses of both as-deposited and heat-treated coupons. The results revealed adding Y₂O₃ benefits both strength and ductility of as-printed ODS IN718, while pristine IN718 showed anisotropic ductility. Improved strength and reduced ductility were found in heat-treated IN718 and ODS IN718 due to the high density of strengthening phases and grain growth, while ODS IN718 with 0.5 wt.% Y₂O₃ showed slightly lower strength and better ductility compared with IN718 owing to the higher grain structure stability and lower density of γ′. EDS results revealed that high density of Al, Ti-rich particles formed in ODS IN718 by consuming γ′ forming elements. Detailed correlation between microstructure and mechanical properties was discussed.

Continuous welded rails (CWR) are track segments welded together and widely used in many rail networks worldwide. Compared to mechanically jointed rails, CWR provide smoother ride to the passengers, require less maintenance, increase the life cycle of the tracks, and can be traveled at higher speeds. However, CWR are prone to buckling during the warm seasons as the rise in the steel temperature induces excessive compressive loads. To prevent such instability issues, axial stress or the so-called rail neutral temperature are determined in order to take proper safety measures. This article presents some results of in-situ measurements relative to a nondestructive evaluation method to determine axial stress and rail neutral temperature (i.e., the temperature at which the axial stress is zero). The technique builds upon a parametric modal analysis of the track under varying boundary conditions and axial stresses, using a finite element model tested in the field. The modal analysis results were used to create a database which was then fed into a machine learning algorithm to predict the axial stress. The estimated neutral temperatures by this technique appeared to be in good agreement with the recorded measurements from strain gages bonded (after a cumbersome distressing procedure) on the CWR.