Experimental and Applied Mechanics, Volume 4

A tension or compression load was applied onto the Ti rod to test the fracture strength and fatigue life of Ti-cement interface under static and fatigue loadings, respectively. These tests are referred as static and fatigue in this study. A customized holder for the cement is required for the static and fatigue experiments, since the typical wedge, pneumatic, or hydraulic gripper are not suitable for static and fatigue tests on the fracture tests of bi-material samples. The objectives of this study are (1) to evaluate the effect of cement thickness on the fracture strength and fatigue life on Ti-cement union by finite element analysis; (2) to evaluate the effect of plastic cement holder and aluminum cement holder on fracture strength and fatigue life on Ti-cement union by experiment and finite element analysis. Ti-cement union model with 0.22 and 0.11 in. cement, Ti-cement-holder union with plastic and aluminum holders were created and validated using ANSYS in this study to develop a suitable specimen holder for static and fatigue tests. Experimental static tests of Ti-cement with both plastic and aluminum specimen holders were conducted as well. The result clearly showed that both plastic and aluminum holders can be used for static test whereas aluminum holder required much larger fracture load compared to the fracture load on plastic holder. Plastic holder is not suitable for fatigue test, because fatigue test required a stronger and more rigid holder such as aluminum.

Copper-21%zinc-6%aluminum (Cu21Zn6Al) alloys were produced through induction casting and processed using various heat treatment methods to investigate their microstructure and mechanical behavior. Some samples were homogenized for different durations, heated for some duration, and quenched by four types of routes. The other samples were heated for different durations and quenched by a two-step quenching. One sample was aluminized to study the aluminization process of the alloy. The processed alloys were studied using optical microscope and scanning electron microscope to explore the microstructure. A lamellar microstructure was observed. The Vickers hardness data were obtained using a microhardness tester. The results show that the grain size increases with the increase of the homogenization duration; the sample quenched to ice directly experienced the fastest cooling rate and thus has the smallest grain size compared with the other three types of quenching routes; the sample experienced longer homogenization duration has lower hardness value and the lowering rate of hardness value levels off with the increase of homogenization duration; the increase of heating temperature from 900 to 950 °C resulted in the decrease of hardness value; and the aluminization process realized an aluminized protection layer on the sample surface.

Conventional coiled tubing experiences a set of bending and straightening cycles during deployment into and out of a well bore. The severe bending strains that must be imposed on the tubing can be up to 3 % and are therefore well above the yield strength of the material. This leads to fatigue failure at extremely short lives.

However, during deep water well interventions coiled tubing is also subjected to high cycle fatigue. After exiting the sheave, the tubing is deployed through open water with a clump weight and suspended vertically at water depths up to 3800 m. This happens while fluid is being pumped at pressures that can reach 68 MPa. The ocean waves impose pitch and roll on the vessel. This causes wrapping and unwrapping motion of the tubing on and off the sheave, which induces stress/strain fluctuations at the tangent point and can lead to high cycle fatigue damage accumulation.

To understand the stress state at the critical location, detailed finite element analyses were conducted using sophisticated incremental plasticity and contact elements to quantify the influence of the angular displacement magnitude, the axial force, and the internal pressure on the stress range at the critical location. Four cases were investigated: low pressure/medium force, high pressure/medium force, low pressure/high force, and high pressure/high force. When comparing the maximum principal strain ranges at small pitching angle (≤2°), it has been found that at a fixed internal pressure, the same strain strange is depicted for both axial forces. At higher pitching angles, however, the strain range increases with increasing axial force. In addition, axial forces tend to have a greater effect on the maximum principal strain range at a smaller pressure.

Three analytical-experimental hybrid approaches for determining the range of stress intensity factors (SIF) and the pseudo-SIFs of fatigue cracks on the presence of crack closure, crack tip plasticity and blunting are presented and evaluated. These approaches use the Digital Image Correlation (DIC) technique to measure cyclic-varying displacement fields near the crack tip. The first uses displacement data of points located on the cracked component surface near and along the crack faces, finding the parallel and orthogonal displacements of symmetrical points with relation to the crack faces to determine the crack opening displacements (COD), which are placed in the crack displacement field equations to obtain the SIF values. The second determines coefficients of generalized Westergaard displacement field functions by an over-deterministic nonlinear Least Square scheme that allows the accurate localization of the crack tip coordinates. The third computes the J-integral along an arbitrary but elastic contour path placed around the crack tip to determine the SIF, using experimentally determined displacement gradients, the calculated stresses and the calculated energy densities for points along the path. The three approaches are applied to the case of cracks propagating in disk-shaped compact-tension DC(T) specimens subjected to mode I cyclic loads, considering the non-linear effects mentioned above.

Stress Corrosion Cracking (SCC) is a cracking process observed in metals under tensile stress and corrosive environment. It has been reported as one of the major failure modes in high strength steel pipelines over few decades. This study aims to characterize sub-surface damage, which leads to near surface mechanical property changes during the initiation stage of SCC. The quasi-static and dynamic nano indentation technique, were utilized to resolve the mechanical property variation in the near-surface region. The measurements indicated significant softening and modulus reduction near the grain boundaries. Such findings could further elucidate the underlying deformation mechanisms during the early stage of SCC as a function of degradation level.

At elevated temperature (550 °C) intergranular creep cracks are prone to develop in thermally and environmentally aged 316 stainless steel. To improve the understanding of mechanisms responsible of creep cracking, some micromechanical experiments have been conducted. Single Edge Notched Tensile specimens (SENT) made of 316H have been machined with a desired ratio of a/W = 0.15 with a the crack length and W the width specimen. After a fine polishing preparation, the samples are then thermally aged using an oxidizing treatment at 600 °C in a rich-carbon environment during 2000 h. It has been assumed that it induces carbide precipitation at grain boundaries (Cr₂₃C₆) leading to a loss of corrosion resistance and to a material embrittlement. In parallel, the oxide (Fe₃O₄) layer grows at the surface up to 50 μm. Its random structure is particularly convenient as a surface marker for the Digital Image Correlation (DIC). A tensile device placed in the SEM chamber is used to apply cycles of loading/unloading to the specimen combining this with a Scanning Electron Microscope (SEM) images acquisitions of the crack tip vicinity. Complementary finite element (FE) simulations of intergranular cracks in bicrystals have been performed and used as reference fields to develop an identification procedure of the crack tip position. It relies on kinematic measurements using a local approach and on projections using Linear Elastic Fracture Mechanic (LEFM) expressions. Experimental evidences have been obtained of plasticity developing ahead of the crack when a local load is applied. A quantification of biais due to the model errors when plasticity is taken into account is done as well as an assessment of the robustness of the procedure. We propose a comparison between FE modeling and full-fields measurements resulting from an in-situ tensile test in a pre-cracked sample.

We have developed a novel specimen for studying crack paths in glass. Under certain conditions, the specimen reaches a state where the crack must select between multiple paths satisfying the K _II = 0 condition. This path selection is a simple but difficult benchmark case for both analytical and numerical methods of predicting crack propagation. We document the development of the specimen, using an uncracked and instrumented test case to study the effect of adhesive choice and validate the accuracy of both a simple beam theory model and a finite element model. In addition, we present preliminary fracture test results and provide a comparison to the path predicted by two numerical methods (mesh restructuring and XFEM).

This paper proposed the construction methodology of spot weld failure model for a crash simulation based on the experiments and the simulations which is called a hybrid method. The test procedure was designed to obtain the failure load of a spot weld under combined loading conditions. The failure surface is determined based on bending moment, normal force and shear force. Those components acting on the spot welded part are obtained from finite element analysis results, which are defined by the real experimental conditions. The proposed failure surface was constructed based on Wung [Wung, Exp Mech 41, 107–113, 2001; Wung et al., Exp Mech 41, 100–106, 2001] model except torsion term. It was found that the failure surface of mild steel was expressed as a function of previous researches, however failure surface of high strength steels and advanced high strength steels were different shape from previous research results. The proposed failure criterion is well-estimated for specimen levels such as cross tension, loading angle of 30°, loading angle of 45°, coach-peel and lap-shear tests.

Modern plasticity models contain numerous parameters that can be difficult and time consuming to fit using current methods. Additional experiments are seldom conducted to validate the model for experimental conditions outside those used in the fitting procedure. To increase the accuracy and validity of these advanced constitutive models, software and testing methodology have been developed to seamlessly integrate experimentation, parameter identification, and model validation in real-time over a range of multiaxial stress conditions, using an axial/torsional test machine. Experimental data is reduced and finite element simulations are conducted in parallel with the test based on experimental strain conditions. Optimization methods reconcile the experiment and simulation through changes to the plasticity model parameters. Excursions into less-traveled portions of the multiaxial stress space can be predicted, and then executed experimentally, to identify deficiencies in the model. Most notably, the software can autonomously redirect the experiment to increase the robustness of the plasticity model where further deficiencies are identified, thus providing closed loop control of the experiment. This novel process yields a calibrated plasticity model upon test completion that has been fit and more importantly validated, and can be used directly in finite element simulations of more complex geometries.

Measurement of stresses in structures such as bridges, buildings, pipelines and railways is difficult because the loads cannot easily be manipulated to allow direct measurements. This paper focuses on the development of a method that combines the hole-drilling technique with Digital Image Correlation (DIC) to evaluate these difficult-to-measure structural stresses. The hole-drilling technique works by relating local displacements caused by the removal of a small amount of stressed material to the original stresses within the drilled hole. Adaptation of this method to measure structural stresses requires scaling up the hole size and modifying the calculation approach to measure deeper into a material. DIC provides a robust means to measure full-field displacements that can easily be scaled to different hole sizes and corrected for measurement artifacts. There are two primary areas of investigation: the adaptation of the DIC/hole-drilling method to measure structural stresses and the development of a correction method to remove coexisting stresses such as residual and machining stresses from the measurement. Experimental measurements are made to demonstrate the measurement method on different structure types including the example practical problem of measuring thermally induced stresses in railroad tracks.

In the field of material characterization, the use of Digital Image Correlation (DIC) is coupled with an inverse methodology, such as the Virtual Fields Method (VFM), is getting an increasing interest in the recent years. That allows identifying material properties by performing tests on specimens with heterogeneous stress-strain fields. Respects to standard techniques, VFM consents to reduce the number of tests required to identify complex constitutive models. However, that methodology is sensitive to the adopted experimental set-up. The specimen geometry and the parameters used in the DIC settings can influence remarkably the identification results. Therefore, a preliminary study is necessary to determine the best experimental set-up to apply. To this purpose a simulating procedure, which reproduces numerically the whole measurement chain, was developed. The simulator includes most of the experimental uncertainties typically present in DIC measurements, moreover is able to look at different specimen geometries. The simulator was used here to optimize the geometry of specimens to be used to identify the elasto-plastic behavior of isotropic materials.

Flow of granular material during processing, handling and transportation strongly influences the quality of the final product and its cost, that is why it is important to measure flow properties of granular materials. Flowability of granular materials depends on the characteristics of the material and on the conditions at which flow is occurring. In this paper a new methodology is introduced to measure friction between granular materials under pressure induced with uniaxial compression. Apparatus also allows analysis of conditions at which granular material starts to flow when exposed to uniaxial compressive load, i.e., zero-rate flowability. We call the apparatus the Granular Friction Analyzer (GFA).

The concept of the GFA was tested by measuring four different materials with different average particle sizes. It was observed that as the particle size decreases so does its zero-rate flowability. This is in agreement with powder literature. Therefore, it can be concluded that in general the GFA method can be a very useful tool to study friction between granular materials and conditions at which the granular material flow initiates, i.e. zero-rate flowability of powders under pressure.

Tensile and shear experiments were performed on AA7075-T6 sheet at strain rates ranging from quasi-static (0.001 s⁻¹) to high (1000 s⁻¹) in three sheet orientations (0°, 45° and 90°) with respect to the rolling direction. Digital image correlation (DIC) techniques were employed to measure the strains in the experiments. The AA7075-T6 alloy showed mild rate sensitivity over the range of strain rates tested. The level of plastic anisotropy was characterized and was shown to be rate-insensitive. The quasi-static experimental data was used to calibrate the eight-parameter Barlat YLD2000 anisotropic yield criterion to describe the anisotropic behaviour of the sheet material. The quasi-static hardening behaviour to large strains was also experimentally determined by converting the shear stress to an equivalent uniaxial stress and fit using a Hockett–Sherby model. The calibrated anisotropic yield criterion was able to capture the material anisotropy providing good agreement with the experiment data.

A polymeric foam commonly used in composite sandwich structures was characterized under multi-axial loading at strain rates varying from quasi-static to dynamic. Tests were conducted under uniaxial compression, tension, pure shear and combinations of normal and shear stresses. Quasi-static and intermediate strain rate tests were conducted in a servo-hydraulic testing machine. High strain rate tests were conducted using a split Hopkinson pressure bar (Kolsky bar) system made of polycarbonate bars having an impedance compatible to that of the foam material. The typical compressive stress-strain behavior of the polymeric foam exhibits a linear elastic region up to a yield point, a nonlinear elastic-plastic region up to an initial peak or “critical stress” corresponding to collapse initiation of the cells, followed by strain softening up to a local minimum (plateau or saddle point stress) and finally, a strain hardening region up to densification of the foam. The characteristic stresses of the stress-strain behavior vary linearly with the logarithm of strain rate. A general three-dimensional elastic-viscoplastic model, formulated in strain space, was proposed. The model expresses the multi-axial state of stress in terms of an effective stress, incorporates strain rate effects and includes the large deformation region. Stress-strain curves obtained under multi-axial loading at different strain rates were used to develop and validate the elastic-viscoplastic constitutive model. Excellent agreement was shown between model predictions and experimental results.

Several open-cell flexible foams, including aged polyurethane foams, were mechanically characterized over a temperature range of −40 to 20 °C. Quasi-static compression was performed to obtain the stress-strain behavior of the foams. The stress-strain relation is nonlinear, but typically there is a small range of linear behavior initially. Compressive cyclic loading at different amplitudes and frequencies of interest (20–60 Hz) were applied to measure foam’s hysteresis properties, i.e. stiffness and energy dissipation. The cyclic characterization includes foams with different amount of pre-strains, some are beyond the initial linear range as occurred in many applications.

Flexible open celled foams are commonly used for energy absorption in packaging. Over time polymers can suffer from aging by becoming stiffer and more brittle. This change in stiffness can affect the foam’s performance in a low velocity impact event. In this study, the compressive properties of new open-cell flexible polyurethane foam were compared to those obtained from aged open-cell polyurethane foam that had been in service for approximately 25 years. The foams tested had densities of 10 and 15 pcf. These low density foams provided a significant challenge to machine cylindrical compression specimens that were 1 “in height and 1” in diameter. Details of the machining process are discussed. The compressive properties obtained for both aged and new foams included testing at various strain rates (0.05. 0.10, 5 s⁻¹) and temperatures (−54, RT, 74 °C). Results show that aging of flexible polyurethane foam does not have much of an effect on its compressive properties.

Responsive hydrogels are a class of shape memory materials that undergo a large elastic volumetric change when interacting with a stimulus and can return to their original shape when that stimulus is removed. Important to their use in these device applications are the fundamental mechanical properties of these materials. The pH-sensitive 2-hydroxyethyl methacrylate, 2-(dimethylamino) ethyl metha-crylate (HEMA-DMAEMA) hydrogel is a linear-viscoelastic polymer that is used in microfluidics because it can be easily polymerized within these devices and their special stimuli responsive capabilities making them ideal candidates for sensors and actuators. In the experiments described in this paper the stress relaxation due to a step strain is studied. The rise time for our experiments was 0.3 s and the step-strains ranged from 3 to 7 % strain. Relaxation was recorded over three decades of time (1, 10, 100 and 1000 s). It was found that within this range the HEMA-DMAEMA hydrogel displayed linear-viscoelastic behavior.

The present study focuses on the experimental investigation of the mechanical response of recycled asphalt mixtures (RAP) using a full-field measurement technique: the grid method. Four hot mixture asphalt (HMA) specimens containing 0, 20, 40, and 100 % of RAP were prepared. Aggregates of different colors were selected in order to distinguish RAP and virgin aggregates. Compression tests were then carried out during which the displacement and strain fields were measured. It was found that the 100 % RAP specimen exhibits highly concentrated strain distributions around the aggregates. Strain is more homogeneously distributed over the specimen surface of virgin mixes. The analysis of the local behavior of RAP showed a local stiffening of the binder along the border of the RAP aggregates. The obtained results highlight the difference in behavior between specimens both at the micro- and the macro-scales.

The mechanical behavior, of Earlywood (EW) and Latewood (LW) of Abies Alba Mills (white fir of Massif Central) during drying under mechanical loading, is investigated. The grid method is used to measure the strain field and distinguish the heterogeneities at rings scale. Images are recorded by a Sensicam CCD camera. A 50-kN Zwick testing machine is used to apply the load. The tested specimen is first conditioned to a moisture content of 57.42 %. It is then tested and regularly removed in order to measure the moisture content. A tracking system ensuring constant positioning of the sample with respect to the camera has also been developed. Before loading, images have been taken to highlight the impact of moisture conditioning. Images are also grabbed during the loading steps and between them after totally unloading. All the strain maps are determined through a reference image (zero strain) taken before water conditioning of the specimen. During the experiments, strain gradients clearly appear between EW and LW, highlighting the variability of the mechanical properties at the rings scale. The effect of moisture content changes in the mechanical behavior of wood is also shown.