Showing papers in "Experimental Mechanics in 2016"
TL;DR: In this paper, digital image correlation (DIC) is used to measure the strain at the sub-grain level in a polycrystalline nickel-base superalloy where plasticity is localized into physical slip bands.
Abstract: Damage during loading of polycrystalline metallic alloys is localized at or below the scale of individual grains. Quantitative assessment of the heterogeneous strain fields at the grain scale is necessary to understand the relationship between microstructure and elastic and plastic deformation. In the present study, digital image correlation (DIC) is used to measure the strains at the sub-grain level in a polycrystalline nickel-base superalloy where plasticity is localized into physical slip bands. Parameters to minimize noise given a set speckle pattern (introduced by chemical etching) when performing DIC in a scanning electron microscope (SEM) were adapted for measurements in both plastic and elastic regimes. A methodology for the optimization of the SEM and DIC parameters necessary for the minimization of the variability in strain measurements at high spatial resolutions is presented. The implications for detecting the early stages of damage development are discussed.
140 citations
TL;DR: In this paper, the effect of various process and design parameters on the dimensional accuracy and compressive strength of cellular lattice structures fabricated using selective laser melting (SLM) was investigated.
Abstract: This paper investigates the effect of designs and process parameters on the dimensional accuracy and compressive behavior of cellular lattice structures fabricated using selective laser melting (SLM). Two unit cell types, square pyramid and truncated cube & octahedron from the Computer Aided System for Tissue Scaffolds (CASTS), an in-house developed library system were used. Powder adhesions occur on the struts of the lattice structures. The thickness of powder adhesion on the struts decreases with an increase in laser power or laser scan speed. The elastic constant in compression of the lattice structures increases with an increase in relative density, and ranged from 7.93 ± 2.73 MPa to 7.36 ± 0.26 GPa. Analysis of Variance (ANOVA) is also carried out to determine the significance of various process and design parameters on the dimensional accuracy and compressive strength of the lattice structures. The processing parameters, such as laser power and laser scan speed have no significant effect on the elastic constant but have a significant effect on the powder adhesion on the struts, which in turn, affects the dimensional accuracy. However, geometrical design parameters such as unit cell type and strut diameter have significant effects on the elastic constant but not dimensional accuracy of the lattice structures.
113 citations
TL;DR: In this article, the effects of roughness and fractality on the normal contact stiffness of rough surfaces were investigated by considering samples of isotropically roughened aluminium and found that the observed contact stiffness is a power-law function of the normal force with the exponent of this relationship closely correlated to surfaces' values of fractal dimension.
Abstract: The effects of roughness and fractality on the normal contact stiffness of rough surfaces were investigated by considering samples of isotropically roughened aluminium. Surface features of samples were altered by polishing and by five surface mechanical treatments using different sized particles. Surface topology was characterised by interferometry-based profilometry and electron microscopy. Subsequently, the normal contact stiffness was evaluated through flat-tipped diamond nanoindentation tests employing the partial unloading method to isolate elastic deformation. Three indenter tips of various sizes were utilised in order to gain results across a wide range of stress levels. We focus on establishing relationships between interfacial stiffness and roughness descriptors, combined with the effects of the fractal dimension of surfaces over various length scales. The experimental results show that the observed contact stiffness is a power-law function of the normal force with the exponent of this relationship closely correlated to surfaces’ values of fractal dimension, yielding corresponding correlation coefficients above 90 %. A relatively weak correlation coefficient of 60 % was found between the exponent and surfaces’ RMS roughness values. The RMS roughness mainly contributes to the magnitude of the contact stiffness, when surfaces have similar fractal structures at a given loading, with a correlation coefficient of −95 %. These findings from this work can be served as the experimental basis for modelling contact stiffness on various rough surfaces.
59 citations
TL;DR: In this paper, an efficient integer-pixel search scheme with combination of an improved particle swarm optimization (PSO) algorithm and the block-based gradient descent search (BBGDS) algorithm has been proposed.
Abstract: The algorithms for Digital image correlation (DIC) in subpixel determination have been well developed regarding the accuracy and efficiency. In this paper, an efficient integer-pixel search scheme with combination of an improved particle swarm optimization (PSO) algorithm and the block-based gradient descent search (BBGDS) algorithm has been proposed. Incorporated with the inverse compositional Gauss-Newton (IC-GN) algorithm for subpixel registration and the parallel computing technology, a real-time DIC algorithm for displacement or strain measurement for dynamic tests has been achieved. Numerical simulation and the experimental results showed that the proposed method could reach a rate of 60 fps (frames per second) for strain measurement under cyclic loading with equivalent accuracy compared with the conventional DIC algorithm.
51 citations
TL;DR: Detailed theoretical error analyses of the IC-GN algorithm reveal that it outperforms the classic FA-NR algorithm not only in terms of computational efficiency, but also in respect of subpixel registration accuracy and noise-proof performance, and is strongly recommended as a standard subpixels registration algorithm for practical DIC applications.
Abstract: The two major subpixel registration algorithms, currently being used in subset-based digital image correlation, are the classic Newton-Raphson (FA-NR) algorithm with forward additive mapping strategy and the recently introduced inverse compositional Gauss-Newton (IC-GN) algorithm. Although the equivalence of these two algorithms has been proved in existing studies, practical implementations of the two subpixel registration algorithms do involve differences, and therefore lead to different performance. In the present work, detailed theoretical error analyses of the two algorithms are performed. Based on the simple sum of squared difference criterion and the practical first-order shape function, analytic formulae that can quantify both the bias error (systematic error) and the variability (random error) in the displacements measured by IC-GN and FA-NR algorithms with various interpolation methods (i.e., cubic convolution interpolation, cubic polynomial interpolation, cubic B-spline interpolation and quintic B-spline interpolation) are derived. It is shown that, compared with FA-NR algorithm, IC-GN algorithm leads to reduced bias error in displacement estimation by eliminating noise-induced bias error, and gives rise on the average to smaller random errors in displacement estimation in the cases of high noise levels or using small subsets. Numerical tests with precisely controlled subpixel displacements confirm the correctness of the theoretical derivations. The results reveal that IC-GN algorithm outperforms the classic FA-NR algorithm not only in terms of computational efficiency, but also in respect of subpixel registration accuracy and noise-proof performance, and is strongly recommended as a standard subpixel registration algorithm for practical DIC applications instead of FA-NR algorithm.
50 citations
TL;DR: In this paper, a novel biaxial testing system for use in a Gleeble testing machine has been adopted to generate forming limit diagrams (FLDs) for sheet metals at different linear strain paths.
Abstract: Conventional experimental approaches used to generate forming limit diagrams (FLDs) for sheet metals at different linear strain paths are not applicable to hot stamping and cold die quenching processes because cooling occurs prior to deformation and consistent values of heating rate, cooling rate, deformation temperature and strain rate are not easy to obtain. A novel biaxial testing system for use in a Gleeble testing machine has been adopted to generate forming limits of sheet metals, including aluminium alloys, magnesium alloys and boron steel, under practical hot stamping conditions in which heating and cooling occur. For example, the soaking temperature is about 900 °C and the deformation temperature range is 550–850 °C for boron steel [1] and the soaking temperature is about 535 °C and the deformation temperature range is 370–510 °C for AA6082 [2]. Resistance heating and air cooling were introduced in this pioneering system and the thermal analysis of different heating and cooling strategies was investigated based on a type of cruciform specimen. FE models with a UAMP subroutine were used to predict temperature fields on a specimen in ABAQUS 6.12. Digital image correlation (DIC) system was used to record strain fields of a specimen by capturing images throughout the deformation history and its post-processing software ARAMIS was used to determine forming limits according to ISO standards embedded in the software. Heating and cooling strategies were determined after the analysis. Preliminary results of forming limit curves at the designated temperatures are presented in order to verify the feasibility of this new method.
49 citations
TL;DR: In this article, an integrated infrared thermography and 3D digital image correlation (TDIC) technique was developed which allows for simultaneous measurement of spatial and temporal distributions of temperatures and displacements, and the advantages of evaluating linked full-field temperatures and strain measurements through the TDIC technique are demonstrated through measurements obtained on an E-glass/vinyl ester/balsa wood sandwich composite subjected to simultaneous one-sided heating and compressive loading.
Abstract: An integrated infrared thermography and 3-D digital image correlation (TDIC) technique has been developed which allows for simultaneous measurement of spatial and temporal distributions of temperatures and displacements. For this, a novel technique was developed to calibrate the IR thermal cameras with a stereo-vision digital image correlation (DIC) system using the standard pin-hole stereo calibration model. This method fuses thermal and displacement information and compensates for the difference in camera resolutions. Several high temperature black and white paints were evaluated to determine their characteristics including the temperature-dependent emissivity of each paint, the mixed emissivity of both paints in the speckle pattern, and optical thickness. The advantages of evaluating linked full-field temperatures and strain measurements through the TDIC technique are demonstrated through measurements obtained on an E-glass/vinyl ester/balsa wood sandwich composite subjected to simultaneous one-sided heating and compressive loading.
45 citations
TL;DR: In this paper, a load stage and X-ray microscopy (TXRM) is used to follow the initiation and propagation of damage during quasi-static mechanical testing of natural, crystalline, and metallic materials.
Abstract: Whether it be the mechanical response of biomaterials or the crack propagation pathways within metal alloys, observing how damage occurs (both spatially and temporally) is critical to understanding materials behavior. Here, nanoscale transmission X-ray microscopy (TXRM) is used to follow the initiation and propagation of damage during quasi-static mechanical testing of natural, crystalline, and metallic materials. The coupling of a novel load stage and TXRM for in situ mechanical testing enables both radiographic (2D) and tomographic (3D) characterization. With an imaging resolution down to 50 nm during uniaxial nanoindentation, compression, or tension, TXRM is ideally suited for the characterization of materials degradation. Several applications are demonstrated including nanoindentation of dentin, compression of a single crystal of high explosive, and tensile testing of both beetle cuticle and Al-Cu alloy. These experiments highlight the capability of the new experimental fixture to provide enhanced insight on material performance through four dimensional (3D + time) observation and analysis.
42 citations
TL;DR: In this paper, a generalized theoretical prediction of displacement resolution is validated in an experimental mechanics laboratory environment, where the model of the sensor noise is used to account for sub-pixel displacements.
Abstract: This paper focuses on one of the metrological properties of DIC, namely displacement resolution. More specifically, the study aims to validate, in the environment of an experimental mechanics laboratory, a recent generalized theoretical prediction of displacement resolution. Indeed, usual predictive formulas available in the literature neither take into account sub-pixel displacement, nor have been validated in an experimental mechanics laboratory environment, nor are applicable to all types of DIC (Global as well as Local). Here, the formula used to account for sub-pixel displacements is first recalled, and an accurate model of the sensor noise is introduced. The hypotheses required for the elaboration of this prediction are clearly stated. The formula is then validated using experimental data. Since rigid body motion between the specimen and the camera impairs the experimental data, and since sensor noise is signal-dependent, particular tools need to be introduced in order to ensure the consistency between the observed image noise and the model on which prediction hypotheses are based. Pre-processing tools introduced for another full-field measurement approach, namely the Grid Method, are employed to address these issues.
41 citations
TL;DR: An experimental study with corresponding numerical simulations has been conducted to evaluate the response of E-Glass / Epoxy composite plates, including polyurea coating effects, subjected to near field underwater explosion (UNDEX) loading.
Abstract: An experimental study with corresponding numerical simulations has been conducted to evaluate the response of E-Glass / Epoxy composite plates, including polyurea coating effects, subjected to near field underwater explosion (UNDEX) loading. Experiments are performed in a water filled blast tank in which the including transient plate response during the UNDEX loading is measured utilizing high speed photography coupled with Digital Image Correlation. The experimental results show that the transient response of the plate is improved through the use of a thicker plate or through the application of a polyurea coating, although there is a weight penalty associated with the additional material which should be considered. Corresponding computational models of the experiments have been conducted with the commercial finite element code LS-Dyna. The simulations are shown to have a high level of correlation to the experimental data.
39 citations
TL;DR: The experimental study demonstrates the superiority of the GPU-based parallel computing for acceleration of DVC over the multi-core CPU-based one, in particular on a PC level computer.
Abstract: A sub-voxel digital volume correlation (DVC) method combining the 3D inverse compositional Gauss-Newton (ICGN) algorithm with the 3D fast Fourier transform-based cross correlation (FFT-CC) algorithm is proposed to eliminate path-dependence in current iterative DVC methods caused by the initial guess transfer scheme. The proposed path-independent DVC method is implemented on NVIDIA compute unified device architecture (CUDA) for GPU devices. Powered by parallel computing technology, the proposed DVC method achieves a significant improvement in computation speed on a common desktop computer equipped with a low-end graphics card containing 1536 CUDA cores, i.e., up to 23.3 times faster than the sequential implementation and 3.7 times faster than the multithreaded implementation of the same DVC method running on a 6-core CPU. This speedup, which has no compromise with resolution, accuracy and precision, benefits from the coarse-grained parallelism that the points of interest (POIs) are processed simultaneously and also from the fine-grained parallelism that the calculation at each POI is performed with multiple threads in GPU. The experimental study demonstrates the superiority of the GPU-based parallel computing for acceleration of DVC over the multi-core CPU-based one, in particular on a PC level computer.
TL;DR: In this paper, a modified approach for extracting stress intensity factors (SIFs) is implemented for propagating crack-tips under mixed-mode conditions using ultrahigh-speed photography.
Abstract: The dynamic fracture behavior of layered architectures is experimentally studied. Specifically, crack penetration, trapping, and branching at an interface are examined. A newly introduced optical technique called Digital Gradient Sensing (DGS) that quantifies elasto-optic effects due to a non-uniform state of stress is extended to perform full-field measurements during the fracture event using ultrahigh-speed photography. By exploiting the richness of two simultaneously measured orthogonal stress gradient fields, a modified approach for extracting stress intensity factors (SIFs) is implemented for propagating crack-tips under mixed-mode conditions. The method is first calibrated using a quasi-static experiment complemented by finite element simulations before implementing it for studying dynamic mixed-mode fracture mechanics of layered configurations. The layered systems considered consist of two PMMA sheets bonded using an acrylic adhesive with the interface oriented normally to the initial crack propagation direction. Interfaces are characterized as ‘strong’ and ‘weak’ by their crack initiation toughness. The dynamic fracture of monolithic PMMA sheet is also studied in the same configuration for comparison. The crack growth and fracture parameter histories of propagating cracks are evaluated. The interface is shown to drastically perturb crack growth behavior resulting in higher dissipation of fracture energy by exciting crack trapping, branching, and mixed-mode growth mechanisms.
TL;DR: In this paper, a new method combining micro-X-ray computed tomography (μXCT) and volumetric digital image correlation (V-DIC) in conjunction with in-situ mechanical testing was used to probe 3D deformation behavior in a friction stir blind rivet (FSBR) joint of carbon fiber reinforced polymer (CFRP) composite.
Abstract: A new method combining micro-X-ray computed tomography (μXCT) and volumetric digital image correlation (V-DIC) in conjunction with in-situ mechanical testing was used to probe three-dimensional (3D) deformation behavior in a friction stir blind rivet (FSBR) joint of carbon fiber reinforced polymer (CFRP) composite. Intrinsic microstructural features such as fibers, pores and metal inclusions enabled accurate volumetric strain calculation of dense fiber reinforced polymer composites using V-DIC without the need for high-contrast additives. Deformation calculated with V-DIC was employed to determine variation of local mechanical properties within the FSBR altered stir-zone microstructure. Unique deformation mechanisms such as internal interfacial shear and microstructure-dependent local buckling were observed in situ. The obtained 3D microscale strain maps revealed that the deformation behavior in joint-affected zones was fundamentally different from that of the bulk composite. Combined μXCT and V-DIC were shown to be effective for understanding 3D microscale deformations in composites.
TL;DR: In this paper, a controlled elasto-plastic strain gradient was created in a stainless steel single crystal by four point bending deformation, and the corresponding elastic strain field was probed, with an intragranular spatial resolution, in-situ by Laue microdiffraction and ex-situate by High Resolution EBSD.
Abstract: Determining the accuracy of elastic strain measurements in plastically deformed alloys is an experimental challenge. To develop a novel cross-validation procedure, a controlled elasto-plastic strain gradient was created in a stainless steel single crystal by four point bending deformation. The corresponding elastic strain field was probed, with an intragranular spatial resolution, in-situ by Laue microdiffraction and ex-situ by High Resolution EBSD. Good agreement is found for the two independent measurements and the predictions of a mechanical model, at plastic strains below 0.5 %. The accuracy of the measurements is estimated at 3.2 × 10 − 4 .
TL;DR: In this article, the authors used stereo digital image correlation (DIC) in the blue spectrum to measure surface displacements and strains of Hastelloy-X plates to validate numerical simulations of combined thermoacoustic loading.
Abstract: The mechanical behavior of solids in combined high-temperature and vibratory environments, such as those experienced during hypersonic flight, are historically not well explored. In this work on Hastelloy-X plates, elevated temperatures were achieved by induction heating and periodic vibratory loading was applied using a shaker. Surface displacements and strains were measured using stereo digital image correlation (DIC) in the blue spectrum to alleviate issues associated with thermal radiation. Through the use of image decomposition techniques the resultant high-quality experimental data were used to validate numerical simulations of combined thermoacoustic loading. The simulations were based on the deformed shape and the corresponding temperature distributions measured experimentally as well as taking into account the thermal dependence of Hastelloy-X mechanical properties.
TL;DR: In this article, an experimental technique to simultaneously measure the full-field temperature and deformation of composite material subjected to flame heating at high temperature is developed using the technique of image processing.
Abstract: In this work an experimental technique to simultaneously measure the full-field temperature and deformation of composite material subjected to flame heating at high temperature is developed using the technique of image processing. The testing stage is integrated with an oxy-propane flame torch for flame heating, a CCD camera for image recording, a synchronized blue light source for light compensation and an infrared pyrometer for temperature calibration and comparison. The principle of the synchronous measurement of temperature and deformation field is demonstrated and discussed. Experiment on carbon fiber reinforced silicon carbide (C/SiC) composite was conducted to validate this method. The temperature was calculated using an improved two-color method while the displacement field and strain field were calculated using the digital image processing method. Results show that the proposed method is applicable for synchronous measurement of temperature and displacement by using one camera, and the mutual interference between the radiation and reflected light can also be effectively eliminated.
TL;DR: In this paper, a combined numerical and experimental methodology for determining the stress-strain curve of metallic materials from the measurements of force and displacement obtained in the axial compression of cylindrical test specimens with friction between the specimens and the platens is presented.
Abstract: This manuscript presents a combined numerical and experimental methodology for determining the stress-strain curve of metallic materials from the measurements of force and displacement obtained in the axial compression of cylindrical test specimens with friction between the specimens and the platens The methodology is based on minimizing the error between the average surface pressure obtained from the experimental measurements of the force and displacement and that obtained from the slab method of analysis of metal plasticity Three different friction models based on Coulomb friction, the constant friction model or combined friction models are utilized Experimental results obtained from cylindrical and Rastegaev test specimens with different lubricants combined with the experimental determination of friction by means of ring compression tests allows compensating the effect of friction in the determination of the material flow curve Comparison with the flow curves determined without friction compensation shows the viability of the proposed methodology The proposed methodology is a simple and effective alternative to other solutions available in the literature and the pseudo-code supplied in the Appendix is provided for those readers interested in utilizing the associated numerical algorithm for determining the stress-strain curves of metallic materials
TL;DR: In this paper, a shear-tension specimen (STS) was designed, evaluated and tested quasi-statically and dynamically, and the obtained stress-strain curves and ductility were validated numerically.
Abstract: A new shear-tension specimen (STS) is designed, evaluated and tested quasi-statically and dynamically. The specimen consists of a long cylinder having an inclined gauge section created by two diametrically opposed semi-circular slots which are machined at 45° with respect to the longitudinal axis. The geometry imposes stress condition within the gauge section which correspond to a Lode parameter of ~ −0.5, between pure shear (0) and uniaxial tension (−1). It thus provides a wider span of loading conditions for a material. A thorough numerical study reveals that the stresses and strains within the gauge are rather uniform, and the average Mises stress and plastic strain on the mid-section of the gauge represent the material true stress–strain characteristics. The data reduction technique to determine the stresses and strains is presented. Quasi-static and dynamic tests at strain rate of 104 1/s were carried out on specimens made of 1020 cold-rolled steel. No necking or softening was observed with this specimen, and the fracture location was always well within the gauge. The obtained stress–strain curves and ductility were validated numerically. The STS is a new specimen to study the combined influence of tension and shear on the mechanical characteristics of a material.
TL;DR: In this paper, the dynamic tensile stress-strain behavior of an EPDM rubber was characterized at quasi-static (<0.01 ǫ s−1), medium and high strain rates.
Abstract: The dynamic tensile stress-strain behaviour of an EPDM rubber was characterized at quasi-static (<0.01 s−1), medium and high strain rates (100–600 s−1). The quasi-static experiments were conducted by a simple uniaxial tensile test; the medium and high strain-rate tests were performed using drop-weight and gas-gun apparatuses. In these dynamic tests, high speed imaging and digital image correlation were used to measure dynamic displacement fields in the specimen. The dynamic stress state is not in equilibrium, which is a usual requirement for a conventional dynamic experimental analysis. Instead, the non-equilibrium deformation was analysed by the Virtual Fields Method (VFM) using inertial forces, clearly generated due to the non-equilibrium state, as a virtual load cell. The linear VFM associated with a linear isotropic model was applied to the drop-weight test data, in these experiments specimens were subjected to various static pre-stretches before dynamic loading was applied. For the gas gun experiments, in which the dynamic strain and experimental durations were larger, the nonlinear VFM was developed to include the one-term Ogden hyperelastic model so that the long deformation history could be analysed. The material parameters identified by these two techniques were used to reconstruct uniaxial true stress-strain curves which showed a clear and consistent rate dependency.
TL;DR: In this paper, a novel direct contact resistance measurement apparatus is proposed that enhances the previously reported extrapolation based erroneous contact resistance evaluation methods, which exhibits sufficient performance to evaluate high efficiency thermoelectric modules.
Abstract: In developing intermediate temperature (300–700 °C) thermoelectric modules with high conversion efficiencies exceeding 10 %, the evaluation of the electrical contact resistance between the thermoelectric material and metallic electrode is a critical issue. In this work, a novel direct contact resistance measurement apparatus is proposed that enhances the previously reported extrapolation based erroneous contact resistance evaluation methods. The accuracy and resolution of this apparatus are investigated in detail, and the proposed novel contact resistance measurement exhibits sufficient performance to evaluate high efficiency thermoelectric modules. The presence of the Peltier effect in the direct current-induced contact resistance measurements is verified experimentally using the proposed apparatus. Two modified measurement parameters, i.e., the pulse shape input current and heat dissipating metallic block, are proposed and their effects in suppressing the unintended Peltier effect are discussed in detail.
TL;DR: In this paper, the use of orthogonal linear polarizers on light source(s) and camera(s), as an effective method for improving optical DIC measurements, is characterized through quantitative and statistical comparisons.
Abstract: Digital image correlation (DIC) is a surface deformation measurement technique for which accuracy and precision are sensitive to image quality. This work presents cross polarization, the use of orthogonal linear polarizers on light source(s) and camera(s), as an effective method for improving optical DIC measurements. The benefits of cross polarization are characterized through quantitative and statistical comparisons from two experiments: rigid body translation of a flat sample and uniaxial tension of a superelastic shape-memory alloy (SMA). In both experiments, cross polarization eliminated saturated pixels that degrade DIC measurements, and increased image contrast, which enabled higher spatial precision by using smaller subsets. Subset sizes are usually optimized for correlation confidence interval (typically with subsets of 21×21 px or larger), but can be decreased to achieve the highest possible spatial precision at the expense of increased correlation confidence intervals. Smaller subset sizes (such as 9×9 px) require better images to maintain correlation within error thresholds. By comparing DIC results from a uniaxial SMA tension test with unpolarized and cross-polarized images, we show that for 9×9 px subsets, the loss of valid DIC data points was reduced almost ten-fold with cross polarization. The only disadvantage we see to cross polarization is the decrease in specimen illumination due to transmission losses through the polarizers, which can easily be accommodated with sufficiently intense light sources. With the installation of relatively inexpensive linear polarizing filters, an optimum optical DIC setup can provide even better DIC measurements by delivering images without saturated pixels and with higher contrast for increased DIC spatial precision.
TL;DR: In this paper, a new procedure of thermal signal processing is investigated to obtain the corrected thermoelastic data through a new approach based on revised thermo-elastic theory.
Abstract: Thermoelastic Stress Analysis (TSA) is based on the thermoelastic effect, well described by a linear relationship between change in body temperature and state of stress in the presence of local adiabatic conditions. In TSA material properties are usually considered constant and a peak to peak variation of the state of stress provides a linearly correlated peak to peak temperature variation. For titanium and aluminium alloys thermoelastic properties of materials are not constant and, in fact, the second order effect due to mean stress on thermoelastic signal is not negligible any more. If neglected for these kind of materials, this second order effect could lead to an error that can be higher than 20 %. In this work a new procedure of thermal signal processing is investigated to obtain the corrected thermoelastic data through a new approach based on revised thermoelastic theory.
TL;DR: In this paper, a mixed numerical-experimental approach is adopted where a finite-element updating scheme is used to simulate the swelling of an earlywood spruce fiber going from the experimental fiber geometry at 47% relative humidity to the predicted geometry of the fiber in the wet state at 80% humidity at equilibrium conditions.
Abstract: Wood cells constitute the main building block in engineered wood-based materials, whose delimiting property frequently is moisture induced swelling. The hygroexpansion properties of wood cells, technically known as fibers, are used as input in predictive micromechanical models aimed for materials design. Values presented in the literature largely depend on the microfibrillar angle, the geometry of the fiber and limiting modelling assumptions. Synchrotron X-ray micro-computed tomography has recently prompted means for detailed measurements of the geometry of unconstrained individual fibers undergoing moisture-induced swelling, which makes it possible to directly quantify the hygroexpansion properties of the cell wall. In addition to a well-defined three-dimensional geometry, the present approach also accounts for large deformations and the fact that cell-wall stiffness depends on the presence of moisture. A mixed numerical-experimental approach is adopted where a finite-element updating scheme is used to simulate the swelling of an earlywood spruce fiber going from the experimental fiber geometry at 47 % relative humidity to the predicted geometry of the fiber in the wet state at 80 % relative humidity at equilibrium conditions. The hygroexpansion coefficients are identified by comparing the predicted and the experimental three-dimensional fiber geometry in the wet state. The obtained values are 0.17 strain per change in relative humidity transverse to the microfibrils in the cell wall, and 0.014 along the microfibrils.
TL;DR: In this article, a novel biaxial tensile test apparatus utilizing miniature cruciform samples was introduced for in-plane and in-situ strain measurements at the microstructure scale.
Abstract: Small-scale and multi-axial testing of sheet metals, particularly of lightweight alloys and advanced steels are becoming important as these materials exhibit forming behavior sensitive to their unique microstructural features and strain paths. As an alternative to large-scale standard tests, in this paper we introduce a novel biaxial tensile test apparatus utilizing miniature cruciform samples. The compact and portable apparatus includes a custom-built optical microscope and high-resolution digital image correlation (DIC) equipment for in-plane and in-situ strain measurements at the microstructure scale. The small strain and premature fracture problems common to the cruciform tests are solved by optimizing the sample design and by meticulously controlling the manufacturing steps and surface finish. Strain analyses reveal a key mechanism responsible for large strains and fracture at the center. This mechanism suppresses the local neck formation and allows uniform deformation under equibiaxial conditions until fracture. When normalized with the strain hardening exponent of the sample material (Al 6061-T6), the effective strain value before fracture, $$ \overline{\varepsilon}/n \sim 3 $$
, surpass the reported values for similar materials tested by cruciform and standard methods.
TL;DR: In this paper, a method for inferring inter-particle forces in opaque granular materials during dynamic experiments is presented, linking experimental measurements of particle kinematics and volume-averaged strains to forces, which provides a new tool for quantitatively studying force transmission and its relation to macroscopic behavior.
Abstract: Inter-particle force transmission in granular media plays an important role in the macroscopic static and dynamic behavior of these materials This paper presents a method for inferring inter-particle forces in opaque granular materials during dynamic experiments By linking experimental measurements of particle kinematics and volume-averaged strains to forces, the method provides a new tool for quantitatively studying force transmission and its relation to macroscopic behavior We provide an experimental validation of the method, comparing inter-particle forces measured in a simple impact test on two-dimensional rubber grains to a finite-element simulation We also provide an application of the method, using it to study inter-particle forces during impact of an intruder on a granular bed We discuss the current challenges for applying the method to both model materials and real geologic materials
TL;DR: An effective method has been developed to correct the commonly seen refraction induced distortions in the optical coherence tomography reconstructions, based on Fermat’s principle, and the 3D deformation field was determined by performing digital volume correlation on these corrected 3D reconstructions.
Abstract: The study of vertebrate eye cornea is an interdisciplinary subject and the research on its mechanical properties has significant importance in ophthalmology. The measurement of depth-resolved 3D full-field deformation behaviour of cornea under changing intraocular pressure is a useful method to study the local corneal mechanical properties. In this work, optical coherence tomography was adopted to reconstruct the internal structure of a porcine cornea inflated from 15 to 18.75 mmHg (close to the physical porcine intraocular pressure) in the form of 3D image sequences. An effective method has been developed to correct the commonly seen refraction induced distortions in the optical coherence tomography reconstructions, based on Fermat’s principle. The 3D deformation field was then determined by performing digital volume correlation on these corrected 3D reconstructions. A simple finite element model of the inflation test was developed and the predicted values were compared against digital volume correlation results, showing good overall agreement.
TL;DR: In this article, a full-scale 100 kg TNT equivalent air blast testing at a 15 m stand-off distance was performed on glass-fibre reinforced polymer (GFRP) sandwich panels with polyvinyl chloride (PVC); polymethacrylimid (PMI); and styrene acrylonitrile (SAN) foam cores, all possessing the same thickness and density.
Abstract: Sandwich composites are of interest in marine applications due to their high strength-to-weight ratio and tailorable mechanical properties, but their resistance to air blast loading is not well understood. Full-scale 100 kg TNT equivalent air blast testing at a 15 m stand-off distance was performed on glass-fibre reinforced polymer (GFRP) sandwich panels with polyvinyl chloride (PVC); polymethacrylimid (PMI); and styrene acrylonitrile (SAN) foam cores, all possessing the same thickness and density. Further testing was performed to assess the blast resistance of a sandwich panel containing a stepwise graded density SAN foam core, increasing in density away from the blast facing side. Finally a sandwich panel containing compliant polypropylene (PP) fibres within the GFRP front face-sheet, was subjected to blast loading with the intention of preventing front face-sheet cracking during blast. Measurements of the sandwich panel responses were made using high-speed digital image correlation (DIC), and post-blast damage was assessed by sectioning the sandwich panels and mapping the damage observed. It was concluded that all cores are effective in improving blast tolerance and that the SAN core was the most blast tolerant out of the three foam polymer types, with the DIC results showing a lower deflection measured during blast, and post-blast visual inspections showing less damage suffered. By grading the density of the core it was found that through thickness crack propagation was mitigated, as well as damage in the higher density foam layers, thus resulting in a smoother back face-sheet deflection profile. By incorporating compliant PP fibres into the front face-sheet, cracking was prevented in the GFRP, despite damage being present in the core and the interfaces between the core and face-sheets.
TL;DR: In this paper, a method for residual stress analysis of ceramic coatings by applying a laser for quasi non-destructive material removal and measuring the 3D displacement around the machined area by means of high-resolution digital holography is described.
Abstract: In this paper, a method for residual stress analysis of ceramic coatings by applying a laser for quasi non-destructive material removal and measuring the 3D displacement around the machined area by means of high-resolution digital holography is described. The residual stresses are retrieved by numerical calculations using the finite element method (FEM) from the measured 3D displacements, the profile of the machined hole and the material parameters of the coating and substrate. Experimental results on thermal spray coatings together with discussion of the difficulties, work in progress, potential of the method, and comparative measurements by the hole-drilling method are presented.
TL;DR: The rate dependent fracture and compressive responses of wet human femur bone are investigated with in situ quantitative visualization, and the fracture properties of cortical bone are studied transverse to the longitudinal axis of the bone up to high stress intensity factor rates.
Abstract: Blast and impact events regularly cause damage to human tissues. Efforts to improve protective gear are made through numerical simulation of these events where human tissues are exposed to high-rate loading conditions. Accurate simulation results can only be obtained if constitutive models are used that are based on precisely carried out experimental studies. Experimental studies on bone are challenging because of the relatively brittle nature of bone as well as the importance of the bone being in a hydrated state prior to experiments to avoid changing the mechanical properties. Past studies have utilized strain gages which require a period of drying time to bond strain gages to the surface of the bone. In this study, rate dependent fracture and compressive responses of wet human femur bone are investigated with in situ quantitative visualization. The fracture properties of cortical bone are studied transverse to the longitudinal axis of the bone up to high stress intensity factor rates, and the rate dependent compressive response is investigated in both longitudinal and transverse directions. The rate dependent nature of the fracture response, and the compressive behavior of human cortical bone over a range of rates from 0.001–1000 s-1 is discussed with the aid of quantitative visualization.
TL;DR: In this paper, the potential of the Electro-Mechanical Impedance (EMI) method to provide an estimation of axial stress in bar-like structures was examined, and analytical models were proposed to relate such admittance signatures to uniaxial applied stress, as a function of relevant parameters of the EMI monitoring system.
Abstract: In-situ stress determination in structures without a reference value is a challenging experimental mechanics task, especially if it requires a non-destructive approach. One potential application of this task is the management of longitudinal loads in railroad structures. This paper examines the potential of the Electro-Mechanical Impedance (EMI) method to provide an estimation of axial stress in bar-like structures. The EMI method is completely non-destructive, as it simply involves bonding a piezoelectric element on the host structure and measuring the electrical admittance signature of the PZT-structure assembly in selected frequency bands. Analytical models are proposed to relate such admittance signatures to uniaxial applied stress, as a function of relevant parameters of the EMI monitoring system. These models compare favorably to experimental results obtained from an aluminum bar and a steel bar, where appreciable sensitivities of the EMI admittance terms are found under varying stress levels.