Showing papers in "Experimental Mechanics in 2019"
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TL;DR: In this paper, the effect of filling pattern on tensile and flexural strength and modulus of the parts printed via fused deposition modeling (FDM), 3D printer was investigated.
Abstract: This experimental study investigates the effect of filling pattern on tensile and flexural strength and modulus of the parts printed via fused deposition modeling (FDM), 3D printer. The main downside of the printed products, with an FDM 3D printer, is the low strength compared to the conventional processes such as injection molding and machining. The issue stems from the low strength of thermoplastic materials and the weak bonding between deposited rasters and layers. Selection of proper filling pattern and infill percentage could highly influence the final mechanical properties of the printed products that were experimentally explored in this research work. Concentric, rectilinear, hilbert curve, and honeycomb patterns and filling percentage of 20, 50 and 100 were the variable parameters to print the parts. The results indicate that concentric pattern yields the most desirable tensile and flexural tensile properties, at all filling percentages, apparently due to the alignment of deposited rasters with the loading direction. Hilbert curve pattern also yielded a dramatic increase in the properties, at 100% filling. The dramatic increase could be mainly attributed to the promotion of strong bonding between the rasters and layers, caused by maintaining a high temperature of rasters at short travelling distances of nozzle for the hilbert curve pattern. Scanning electron microscopy (SEM) examination revealed the strong bonding between rasters and sound microstructures (less flaws and voids) for concentric and hilbert curve pattern at a high filling percentage of 100. Besides, SEM examination revealed large voids in honeycomb pattern, deemed to be responsible for its lower strength and modulus, especially at the filling percentage of 100.
138 citations
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TL;DR: In this paper, a direct write additive manufacturing (AM) process for carbon fiber composites can simultaneously achieve a high degree of fiber alignment and a low degree of porosity, obtaining 90% of the theoretical tensile modulus and 66% of a fully aligned composite.
Abstract: Despite the promise of additive manufacturing (AM) to bring unprecedented agility and design freedom to manufactured components, structural applications remain largely out of reach due to material restrictions – notably the lack of a mature AM process for reinforced thermoset composites. AM is also hindered by process-induced defects such as porosity and unfavorable microstructure. This research shows that a direct write AM process for epoxy / chopped carbon fiber composites can simultaneously achieve a high degree of fiber alignment and low degree of porosity, obtaining 90% of the theoretical tensile modulus and 66% of the theoretical tensile strength for a fully aligned composite. These values exceed those of compression molded properties for the same material. Transverse properties of AM samples were roughly half of the longitudinal properties but showed no statistically significant difference from the matrix material, suggesting that the process may not adversely affect microstructure. The addition of only 5.5 vol% carbon fiber more than doubled the strength and stiffness of the neat epoxy, and more than tripled the properties of ABS thermoplastic while achieving a higher glass transition temperature. Flexural properties show similar trends. SEM and CT imaging shows that fiber orientation is largely maintained in the print direction and cross-section micrographs show there is sufficient local material flow during deposition to achieve low porosity.
61 citations
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TL;DR: In this article, the effects of internal pores on the tensile behavior of austenitic stainless steel 316L manufactured with laser powder bed fusion (L-PBF) additive manufacturing were investigated.
Abstract: In this study, the effects of internal pores on the tensile behavior of austenitic stainless steel 316L manufactured with laser powder bed fusion (L-PBF) additive manufacturing (AM) were investigated. Both fully-dense samples and samples with intentional internal pores of varying diameters were fabricated. For each sample with a pore, the internal pore was deliberately fabricated in the center of the cylindrical tensile sample during AM processing. By varying the diameter of the 180 μm-tall initial penny-shaped pores, from 150 to 4800 μm within 6 mm gauge diameter cylindrical samples, the impact of lack-of-fusion, commonly present in AM, as well as the impact of well-defined pores in general, on tensile mechanical properties was studied. To link the pore size and morphology to the mechanical properties, the sizes of the initial pores were evaluated using non-destructive Archimedes measurements, 2D X-ray radiography, 3D X-ray computed tomography, and destructive 2D optical microscopy. Samples with and without the single, penny-shaped pore were subjected to uniaxial tension to evaluate the defect size dependent mechanical properties. The intentional pore began to impact ultimate tensile strength when the pore diameter was 2400 μm, or 16% of the cross-sectional sample area. Elongation to failure was significantly affected when the pore diameter was 1800 μm or 9% of the cross-sectional sample area. This shows that 316L stainless steel manufactured by additive manufacturing is defect-tolerant under uniaxial tension loading.
58 citations
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TL;DR: In this article, the influence of the mesostructure on the overall mechanical behavior of the parts synthesized via fused filament fabrication is investigated by performing mechanical testing on the printed parts.
Abstract: A comprehensive understanding of process–structure–property relationship of 3D printed parts is currently limited. In the present study, we investigate the influence of the mesostructure on the overall mechanical behavior of the parts synthesized via fused filament fabrication. In particular, characterization of anisotropic behavior is carefully studied by performing mechanical testing on the printed parts. The printed parts are treated as laminates and are characterized using laminate mechanics. Test coupons of thick layered and also thin layered unidirectional as well as bidirectional laminates are printed with polymeric material for tensile and bending tests. Test results revealed that the process parameters govern the mesostructure and therefore the material behavior of the parts. Mechanical behavior of the bidirectional printed laminates is studied in detail. The properties are significantly influenced by the layer thickness and layup order of the printed parts. Mechanical behavior of the printed parts can be characterized using laminate theory. The effect of lamina layup and layer thickness on the flexural properties of the laminates is significant. Furthermore, the first ply failure theory is employed for the finite element failure analysis of the printed parts. The results provide insights in the relationship between mesostructure–mechanical properties of the printed parts.
41 citations
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TL;DR: In this article, the authors report on their dynamic full-field measurements of displacement, velocities, strains and strain rates associated with the spontaneous propagation of shear ruptures in the laboratory earthquake setup.
Abstract: Producing dynamic ruptures in the laboratory allows us to study fundamental characteristics of interface dynamics. Our laboratory earthquake experimental setup has been successfully used to reproduce a number of dynamic rupture phenomena, including supershear transition, bimaterial effect, and pulse-like rupture propagation. However, previous diagnostics, based on photoelasticity and laser velocimeters, were not able to quantify the full-field behavior of dynamic ruptures and, as a consequence, many key rupture features remained obscure. Here we report on our dynamic full-field measurements of displacement, velocities, strains and strain rates associated with the spontaneous propagation of shear ruptures in the laboratory earthquake setup. These measurements are obtained by combining ultrahigh-speed photography with the digital image correlation (DIC) method, enhanced to capture displacement discontinuities. Images of dynamic shear ruptures are taken at 1-2 million frames/s over several sizes of the field of view and analyzed with DIC to produce a sequence of evolving full-field maps. The imaging area size is selected to either capture the rupture features in the far field or to focus on near-field structures, at an enhanced spatial resolution. Simultaneous velocimeter measurements on selected experiments verify the accuracy of the DIC measurements. Owing to the increased ability of our measurements to resolve the characteristic field structures of shear ruptures, we have recently been able to observe rupture dynamics at an unprecedented level of detail, including the formation of pressure and shear shock fronts in viscoelastic materials and the evolution of dynamic friction.
36 citations
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TL;DR: In this article, the compressive response of an open cell polyurethane foam currently used as liner in the advanced combat helmet is examined across strain rates, where a traditional load frame is used to investigate the quasi-static behavior, and two different modifications of a conventional split-Hopkinson bar configuration are used to probe the dynamic response.
Abstract: Polymeric foams are used for impact protection due to their ability to absorb large amounts of strain energy. In this work, the compressive response of an open cell polyurethane foam currently used as liner in the advanced combat helmet is examined across strain rates. A traditional load frame is used to investigate the quasi-static behavior, and two different modifications of a conventional Kolsky (split-Hopkinson) bar configuration are used to probe the dynamic response. A unique, independent method not relying on strain gage signals is presented that leverages high-speed full-field imaging to track the velocity on each side of the sample-bar interface and used to extract the dynamic stress-strain response; the results are compared against traditional strain gage measurements. X-ray tomography is used to examine the global morphological characteristics of the foam. The foam is found to be strongly rate dependent, where the characteristic properties vary logarithmically with strain rate. An analytical expression is presented to describe the rate dependency that collapses all stress-strain curves on a master curve. Full-field kinematic data from digital image correlation taken during loading is used to extract a nonlinear Poisson’s ratio as a function of strain, which is found to be strain rate insensitive. A tangent Poisson function is used to explore the foam’s auxetic behavior. These findings provide insight on physically-based constitutive modeling of foams, crucial to predictive brain injury simulations, as well as motivate the need to probe local heterogenous behavior across strain rates moving forward.
36 citations
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TL;DR: This paper proposes a new method, the augmented-Lagrangian digital image correlation (ALDIC), that combines the advantages of both the local (fast) and global (compatible) methods and demonstrates that ALDIC has higher accuracy and behaves more robustly compared to both local subset DIC and global DIC.
Abstract: Digital image correlation (DIC) is a powerful experimental technique for measuring full-field displacement and strain. The basic idea of the method is to compare images of an object decorated with a speckle pattern before and after deformation, and thereby to compute the displacement and strain fields. Local subset DIC and finite element-based global DIC are two widely used image matching methods. However there are some drawbacks to these methods. In local subset DIC, the computed displacement field may not be compatible, and the deformation gradient may be noisy, especially when the subset size is small. Global DIC incorporates displacement compatibility, but can be computationally expensive. In this paper, we propose a new method, the augmented-Lagrangian digital image correlation (ALDIC), that combines the advantages of both the local (fast) and global (compatible) methods. We demonstrate that ALDIC has higher accuracy and behaves more robustly compared to both local subset DIC and global DIC.
36 citations
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TL;DR: In this article, the authors present a comprehensive experimental study into the loading acting on, and subsequent deformation of, targets subjected to near-field explosive detonations, and show that initial plate velocity profiles are directly proportional to the imparted impulse distribution, and that spatial variations in loading as a result of surface instabilities in the expanding detonation product cloud are significant enough to influence the transient displacement profile of a blast loaded plate.
Abstract: The shock wave generated from a high explosive detonation can cause significant damage to any objects that it encounters, particularly those objects located close to the source of the explosion. Understanding blast wave development and accurately quantifying its effect on structural systems remains a considerable challenge to the scientific community. This paper presents a comprehensive experimental study into the loading acting on, and subsequent deformation of, targets subjected to near-field explosive detonations. Two experimental test series were conducted at the University of Sheffield (UoS), UK, and the University of Cape Town (UCT), South Africa, where blast load distributions using Hopkinson pressure bars and dynamic target deflections using digital image correlation were measured respectively. It is shown through conservation of momentum and Hopkinson-Cranz scaling that initial plate velocity profiles are directly proportional to the imparted impulse distribution, and that spatial variations in loading as a result of surface instabilities in the expanding detonation product cloud are significant enough to influence the transient displacement profile of a blast loaded plate.
35 citations
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TL;DR: In this article, a set of experiments on 3D printed pantographic unit cells were performed to parametrically investigate their response when undergoing tensile, compression and shear loading with the aim of studying the role of each parameter in the resultant mechanical behavior of the sample, and providing a benchmark for the mathematical models developed to describe pantographic structures.
Abstract: Pantographic metamaterials are known for their ability to have large deformation while remaining in the elastic regime. We have performed a set of experiments on 3D printed pantographic unit cells to parametrically investigate their response when undergoing tensile, compression, and shear loading with the aim of i) studying the role of each parameter in the resultant mechanical behavior of the sample, and ii) providing a benchmark for the mathematical models developed to describe pantographic structures. Results show the existence of local extrema in the space of the geometrical parameters, suggesting the use of optimization techniques to find optimal geometrical parameters resulting in desired functionalities. We have also performed tensile relaxation tests on the samples, with the results indicating the complexity of the dynamic behavior and the existence of multiple relaxation characteristic times. Such results can be used to for calibrating mathematical models describing pantographic structures under dynamic loadings.
34 citations
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TL;DR: A unified general framework to correct for the three dominant types of SEM artifacts, i.e. spatial distortion, drift distortion and scan line shifts is proposed and the potential of the framework is tested by a number of virtual experiments.
Abstract: The combination of digital image correlation (DIC) and scanning electron microscopy (SEM) enables to extract high resolution full field displacement data, based on the high spatial resolution of SEM and the sub-pixel accuracy of DIC. However, SEM images may exhibit a considerable amount of imaging artifacts, which may seriously compromise the accuracy of the displacements and strains measured from these images. The current study proposes a unified general framework to correct for the three dominant types of SEM artifacts, i.e. spatial distortion, drift distortion and scan line shifts. The artifact fields are measured alongside the mechanical deformations to minimize the artifact induced errors in the latter. To this purpose, Integrated DIC (IDIC) is extended with a series of hierarchical mapping functions that describe the interaction of the imaging process with the mechanics. A new IDIC formulation based on these mapping functions is derived and the potential of the framework is tested by a number of virtual experiments. The effect of noise in the images and different regularization options for the artifact fields are studied. The error in the mechanical displacement fields measured for noise levels up to 5% is within the usual DIC accuracy range for all the cases studied, while it is more than 4 pixels if artifacts are ignored. A validation on real SEM images at three different magnifications confirms that all three distortion fields are accurately captured. The results of all virtual and real experiments demonstrate the accuracy of the methodology proposed, as well as its robustness in terms of convergence.
33 citations
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TL;DR: By minimizing the V-shaped models of theoretically predicted total errors, optimal subvolume size and the best shape function can be identified as inputs for self-adaptive DVC analysis at each calculation point.
Abstract: Digital volume correlation (DVC) has evolved into a powerful tool for quantifying full-field internal deformation. In existing subvolume-based local DVC, subvolume size and shape function are two key user-defined parameters closely related to the DVC measurement errors. In routine implementation, the user must define fixed subvolume size and shape function according to prior experience and intuition, which cannot ensure accurate measurements, particularly for unknown complex heterogeneous deformation fields. Self-adaptive selection of optimal subvolume size and the best shape function is therefore highly desirable to realize full-automatic and quality DVC measurements. In this work, we first establish theoretical error models that relate total displacement errors to subvolume sizes and shape functions. By minimizing the V-shaped models of theoretically predicted total errors, optimal subvolume size and the best shape function can be identified as inputs for self-adaptive DVC analysis at each calculation point. The accuracy advantage of the presented self-adaptive DVC approach over classic one using fixed subvolume size and shape function is demonstrated through numerically simulated three-point bending tests.
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TL;DR: In this article, an experimental characterization of lattice structures that are based on cubic cells fabricated through selective laser melting (SLM) and electron beam melting (EBM) is carried out, and three main failure modes of the lattice are identified, depending on the response of ductile/brittle material and the direction of crack propagation.
Abstract: This study carries out an experimental characterization of lattice structures that are based on cubic cells fabricated through selective laser melting (SLM) and electron beam melting (EBM). The lattice failure under compressive load is studied as a function of the process typology, material properties, and dimensional parameters of the unit cell. The bulk material is first characterized to evaluate the process stability. Three main failure modes of the lattice are identified, depending on the response of ductile/brittle material and the direction of crack propagation. The relationship between lattice geometrical parameters and mechanical strength is observed. The results of the modeling and experiments are suitable to validate the design of lightweight components built with AM processes. The structural performances related to geometrical features, material properties and technological constraints are well explained for further applications in structural design. The equivalent Young’s module of lattice samples with different cell size has been measured and compared with numerical simulations based on the homogenization method.
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TL;DR: In this article, nanoindentation was used to measure the in-situ mechanical behavior of individual phases in a cathode composite electrode LiNixMnyCozO2 and evaluate the influence of electrolyte soaking on the elastic modulus, hardness, and volume change of the conductive matrix with different degrees of porosity.
Abstract: The state-of-the-art cathode materials experience severe structural and mechanical degradation over lithiation cycles albeit their small deformation. It has been a great challenge to characterize the mechanical behavior of composite electrodes in-situ and in real-time because of their environmental sensitivity and intricate microscopic heterogeneity. We use nanoindentation to measure the in-situ mechanical behavior of individual phases in a cathode composite electrode LiNixMnyCozO2. We focus on the understanding of the mechanical properties of the constituents in dry and wet conditions. We evaluate the influence of electrolyte soaking on the elastic modulus, hardness, and volume change of the conductive matrix with different degrees of porosity. More interestingly, we measure the modulus, hardness, and fracture strength of agglomerated active particles and sintered pellets, and compare their mechanical properties in the dry and liquid environment. We show that the electrolyte enhances the fracture strength of NMC agglomerated particles. The increase in interfacial strength may be a result of the additional capillary force between primary particles. Results offer mechanistic understanding of the complex behavior of composite electrodes and will feed chemomechanical models on Li-ion batteries.
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TL;DR: In this article, a laser-launched micro-flyer was used for spall strength measurement of AZ31B Mg alloy thin foils, a material system with potential applications as a lightweight protection material.
Abstract: We describe a laser-launched micro-flyer apparatus designed for spall strength measurement. The launcher uses a single pulse from a pulsed laser that is stretched in time to nominally 20 nanoseconds using an optical ring cavity, while inexpensive multi-lens arrays are used to spatially homogenize the beam. The velocimetry technique that we developed for the experiment provides the required sub-nanosecond time resolution. We demonstrate the capability of the apparatus to interrogate the spall strength of AZ31B Mg alloy thin foils, a material system with potential applications as a lightweight protection material. Numerical simulations and fractography are very useful to determine the quality of the experimental data and help to interpret our results. The simulations and fractography analyses of the experiments suggest that the short shock pulse duration in the experiment causes incipient spallation. The short pulse also likely introduces stochasticity to the measured spall strength through limited activation of failure mechanisms within the samples. The shocked AZ31B Mg alloy has spall strengths that are greater than previously reported figures for fine grained Mg alloys, likely because the laser based system achieves higher strain rates than in prior work on this material.
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TL;DR: In this paper, an innovative chiral stent design with auxetic properties is proposed, and an amplified stent sample is fabricated with SLS additive manufacturing technique, which can be tailored through adjusting the unit cell design parameters, such as: struct numbers along circumferential directions, ligament lengths, and node radius.
Abstract: The mechanical properties of the stent are of key importance to the mechanical integrity and performance reliability of stent-plaque-artery system, and an ideal stent should have good bending compliance, axial deformation stability, hoop strength and stiffness, larger radial expandable ability, etc. In this paper, innovative chiral stent designs with auxetic properties are proposed, and amplified stent sample is fabricated with SLS additive manufacturing technique. Firstly, through combining micro-CT tomography and image-based finite element analysis, the mechanical properties of as fabricated SLS stent are explored; Secondly, two series of stent samples are fabricated with SLS additive manufacturing techniques, and in-situ compression experiments are performed for studying the deformation mechanisms and auxetic mechanical behaviors of stents. Finally, effects of geometrical parameters on the tensile mechanical performance of these stents are studied with finite element analysis. The proposed chiral stent exhibits auxetic behaviors, and can be tailored through adjusting the unit cell design parameters, such as: struct numbers along circumferential directions, ligament lengths, and node radius.
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TL;DR: In this article, multi-layered Au, Ti, and Ag sputtered coatings are reconfigured in a NaCl solution to quickly form DIC-appropriate speckle patterns.
Abstract: Digital image correlation of scanning electron microscope images is a powerful technique for measuring full-field deformation at microstructural length scales. A major challenge in applying this technique is the fabrication of speckle patterns small enough to facilitate full-field measurements with high spatial resolutions and at high magnifications. Current approaches are inconsistent, damaging to the substrate, or highly substrate dependent, which requires researchers to recalibrate or develop new patterning approaches when changing materials systems. Here, multi-layered Au, Ti, and Ag sputtered coatings are reconfigured in a NaCl solution to quickly form DIC-appropriate speckle patterns. Our proposed technique is shown to be substrate independent, as demonstrated on neat epoxy, Ti-6Al-4V titanium alloy, and tetragonal zirconia polycrystal samples, and allows for controllable particle distributions by varying the sputtered Ag layer thickness. Patterns produced by the proposed technique enable the use of correlation window (subset) sizes smaller than 1 μm, small enough to capture highly localized deformation gradients at material discontinuities areas. Capabilities of this method in characterizing highly heterogeneous deformation conditions at sub-micron scales are demonstrated by measuring localized deformations in a single fiber model composite system.
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TL;DR: The results suggest that the best use of strength distribution information for MEMS manufacturers and designers might be in estimation of the strength threshold.
Abstract: A clear relationship between the population of brittle-fracture controlling flaws generated in a manufactured material and the distribution of strengths in a group of selected components is established. Assumptions regarding the strength-flaw size relationship, the volume of the components, and the number in the group, are clarified and the contracting effects of component volume and truncating effects of group number on component strength empirical distribution functions highlighted. A simple analytical example is used to demonstrate the forward prediction of population → distribution and the more important reverse procedure of empirical strength distribution → underlying flaw population. Three experimental examples are given of the application of the relationships to state-of-the-art micro- and nano-scale strength distributions to experimentally determine flaw populations: two on etched microelectromechanical systems (MEMS) structures and one on native and oxidized silicon nanowires. In all examples, the minimum threshold strength and conjugate maximum flaw size are very well estimated and the complete flaw population, including the minimum flaw size, are very poorly estimated, although etching, bimodal, and oxidation effects were clearly discernible. The results suggest that the best use of strength distribution information for MEMS manufacturers and designers might be in estimation of the strength threshold.
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TL;DR: The proposed H-DVC method gave an access to high-resolution details, which indeed are not observable using classical DVC method, which allows a better evaluation of the distribution of localization phenomena in volumes under loading.
Abstract: This study reports on Digital Volume Correlation and its limitation in the case of fracture mechanics. Due to its sensitivity, detecting the crack opening in sub pixel level is extremely difficult and in-turn it does not provide an accurate estimation of the stress intensity factors. To address these limitations an improved DVC method was proposed to solve the uncertainty problems in the vicinity of cracks. The method (H-DVC) was developed using classical minimization process, including Heaviside functions in the kinematical field representation. Initial simulation has been performed for opening and sliding modes using classical DVC and proposed H-DVC. From these tests, crack detection limit has be evaluated to a jump of 0.1 voxels. A direct comparison of performances of DVC and H-DVC has been carried out on a fractured polymer sample to detect the kinematics discontinuity and to highlight the significant contribution of this novel approach. Furthermore, the local Crack Opening Displacement and local Stress Intensity Factor (KI) are calculated for mode-I loading (opening mode activated) condition. Parallelized computation of the proposed H-DVC method gave an access to high-resolution details, which indeed are not observable using classical DVC method. This allows a better evaluation of the distribution of localization phenomena in volumes under loading.
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TL;DR: In this article, the authors present an experimental method based on the propagation of highly nonlinear solitary waves (HNSWs) to determine the internal pressure of tennis balls and to estimate rubber degradation.
Abstract: Rubber is a material present in many commodities, including tennis balls. The characteristics of tennis balls are specified by the International Tennis Federation and are evaluated using standard tests that are too cumbersome to be staged easily and quickly. In this article we present an experimental method based on the propagation of highly nonlinear solitary waves (HNSWs) to determine the internal pressure of tennis balls and to estimate rubber degradation. HNSWs are compact waves that can form and travel in a closely-packed assembly of systematically arranged particles that generally interact according to the Hertz contact law. In the study presented here, we developed a model that predicts the internal pressure of tennis balls and estimates rubber degradation by observing the waves propagating within a chain in contact with the ball to be estimated. The model was validated experimentally, by testing 18 identical balls played over a few weeks period. We found that the dynamic interaction between the waves and the rubber can successfully detect changes in internal pressure and bouncing characteristics and that these changes were barely detected using a conventional rebound test. In the future, the findings of this study may be expanded to characterize any rubber of any shape.
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TL;DR: In this paper, a miniaturized cruciform-shaped sample geometry which allows reaching high plastic strain under equibiaxial loading with reduced thickness at the test section is presented, which results in excellent surface quality that can be used for electron backscatter diffraction and high-resolution digital image correlation (HRDIC) investigations.
Abstract: In this study, a miniaturized cruciform-shaped sample geometry which allows reaching high plastic strain under equibiaxial loading with reduced thickness at the test section is presented. The thinning method results in excellent surface quality that can be used for electron backscatter diffraction (EBSD) and high-resolution digital image correlation (HRDIC) investigations. The new cruciform geometry is used to study the slip activity in metastable austenitic stainless steel 304 during uniaxial and equibiaxial deformation using HRDIC. The results are discussed with respect to the Schmid law and the effect of multiple slip activity on the nucleation of martensitic transformations.
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TL;DR: In this article, the authors examined the case of checkerboard pattern and found that the noise level observed in displacement and strain maps is significantly lower with a checkboard than with a classic 2D grid.
Abstract: The performance of white-light full-field measurement methods strongly depends on the nature of the pattern used to mark the surface on which displacements and strains are measured. Finding optimized patterns is therefore a topical question. The aim of this study is to examine the case of the checkerboard pattern. It is first shown that this periodic pattern can be processed with a Fourier-based technique such as LSA. Experiments are then carried out to compare the noise level in displacement and strain maps obtained by processing classic 2D grid and checkerboard images. The conclusion is that the noise level observed in displacement and strain maps is significantly lower with a checkerboard than with a classic 2D grid. A notched specimen is finally tested to illustrate that very low strain levels can be measured with checkerboard patterns.
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TL;DR: In this paper, the authors demonstrate transverse velocity detection using a modified PDV system where they subtract the measured normal velocity history from a concurrent velocity history measured at a canted angle to the target surface.
Abstract: Pressure-shear plate impact experiments generate normal and transverse particle velocities during high strain rate deformations. Traditionally, freespace lenscoupled tabletop laser interferometry techniques are used together with diffraction gratings to interrogate the evolving velocity vector at the back face of the target plate. Recently, fiberoptic velocimetry (photon Doppler velocimetry or PDV) has become commonplace for measuring normal particle velocities above 200m/sec. In this work, we demonstrate transverse velocity detection using a modified PDV system where we subtract the measured normal velocity history from a concurrent velocity history measured at a canted angle to the target surface to obtain the transverse velocity component. This modified system is time-multiplexed to reduce the number of components, and uses an erbium doped fiber amplifier (EDFA) to boost the angled signal intensity while maintaining low noise. The system operates as a heterodyne interferometer, but features a frequency upshifted reference leg to improve data analysis at the particle velocities expected in the experiment. We demonstrate by direct comparison that this inexpensive and simple approach is as effective as traditional grating methods.
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TL;DR: In this paper, the simple shear response of soft polymers under large deformation (>50%) and strain rates spanning 10−3 − 103−s−1 is characterized by developing quasi-static and split-Hopkinson pressure bar based single-pulse dynamic simple deformation experiments rooted in continuum mechanics fundamentals.
Abstract: The simple shear response of soft polymers under large deformation (>50%) and strain rates spanning 10−3 – 103 s−1 is characterized by developing quasi-static and split-Hopkinson pressure bar based single-pulse dynamic simple shear experiments rooted in continuum mechanics fundamentals. Cross-linked polydimethylsiloxane (PDMS) is chosen as a model material. By examining the evolution of stress, strain and strain rate, the latter two parameters measured using two-dimensional digital image correlation (DIC), it is demonstrated that dynamic simple shear deformation consists of four distinct stages: momentum diffusion, inertia effect, steady-state material response, and strain rate decay. By isolating the unsteady and steady-state deformation stages, inertia-free material response is captured under a uniform strain rate. It is shown that the shear response of PDMS is nearly linear with a weakly rate-sensitive shear modulus in the investigated strain rate range. Further, by analyzing the DIC strain-field and comparing the kinematic experimental results with those predicted by classical continuum mechanics, it is demonstrated that the proposed experiments not only achieve a nearly theoretical simple shear state that is uniform across the specimen, but also allow for post-test validation of individual experiments based on these criteria.
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TL;DR: In this paper, a modified split Hopkinson pressure bar with a copper pulse shaper was used to conduct a three-point bend impact experiment to characterize the dynamic fracture initiation toughness and crack dynamics of 3D printed specimens.
Abstract: An experimental study is performed to investigate the dynamic fracture of additive manufactured Acrylonitrile Butadiene Styrene (ABS). A single edge notched bending (SENB) specimen with three orientations, namely horizontal builds with 45°/−45° (H45), 0°/90° (H90) raster orientations, and vertical builds with layers perpendicular to the pre-crack (V0) are considered for this study. In addition, a novel toughening mechanism is explored by changing the surface topology to deflect the crack paths. A modified split Hopkinson pressure bar with a copper pulse shaper (to increase the raising time of incident loading pulse) is used to conduct a three-point bend impact experiment to characterize the dynamic fracture initiation toughness and crack dynamics of 3D printed specimens. Real-time crack initiation and propagation is captured by using a high-speed video camera. Using the load history diagram, accurate fracture initiation load is found to determine dynamic fracture initiation toughness. Fracture initiation toughness is increased by 138% for a V0 specimen configuration compared to H90. Three different sized circular patterns (with diameters of 1, 1.75 and 2.5 mm) and a square pattern (with a length of 1.53 mm) are considered to observe the effect of surface topology on the dynamic fracture initiation toughness. Introducing surface pattern to the specimen increases the fracture toughness by 58% as compared to specimens without surface pattern. Surface pattern also exhibits two steps of crack growth and decreases the initial crack propagation velocity significantly for all three orientations. Additionally, higher fracture initiation toughness is achieved with the increase in the size of the pattern and the change of the pattern shape.
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TL;DR: In this paper, the activity of grain boundary sliding (GBS) and the relationship between GBS and slip transmission at grain boundaries was investigated by the characterization of full-field strain and microstructural information in an experimental system of high-purity (99.99%) columnar aluminum subjected to uniaxial tension at 190°C.
Abstract: Despite its significance in polycrystalline materials, there have been few experimental investigations of the activity of grain boundary sliding (GBS) and the relationship between GBS and slip transmission at grain boundaries. The present work addresses this knowledge gap by the characterization of full-field strain and microstructural information in an experimental system of high-purity (99.99%) columnar aluminum subjected to uniaxial tension at 190 °C. High-resolution, full-gage strain fields were characterized on an unloaded specimen by distortion-corrected and stitched scanning electron microscope-enabled digital image correlation (SEM-DIC). Alignment between the lower-resolution electron backscatter diffraction (EBSD) and higher-resolution strain fields was significantly improved by clustering of strain data within an EBSD-defined boundary mantle. Grain boundary sliding was investigated at select boundaries, and it was determined that GBS magnitude profiles can have large gradients along a single boundary and vary significantly between boundaries. Using a geometric compatibility factor (m′) to quantify favorability of slip transmission, the two grain boundaries that exhibited the largest average GBS magnitude experienced contiguous slip on moderately well aligned slip systems, although the exact nature of this slip activity, whether transmission or nucleation, remains under investigation.
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TL;DR: An experimental method quantifies local mechanical properties in a fibrous network at the scale of a cell, while also accounting for inherent nonlinearity of the collagen network.
Abstract: The extracellular matrix provides macroscale structural support to tissues as well as microscale mechanical cues, like stiffness, to the resident cells. As those cues modulate gene expression, proliferation, differentiation, and motility, quantifying the stiffness that cells sense is crucial to understanding cell behavior. Whereas the macroscopic modulus of a collagen network can be measured in uniform extension or shear, quantifying the local stiffness sensed by a cell remains a challenge due to the inhomogeneous and nonlinear nature of the fiber network at the scale of the cell. To address this challenge, we designed an experimental method to measure the modulus of a network of collagen fibers at this scale. We used spherical particles of an active hydrogel (poly N-isopropylacrylamide) that contract when heated, thereby applying local forces to the collagen matrix and mimicking the contractile forces of a cell. After measuring the particles’ bulk modulus and contraction in networks of collagen fibers, we applied a nonlinear model for fibrous materials to compute the modulus of the local region surrounding each particle. We found the modulus at this length scale to be highly heterogeneous, with modulus varying by a factor of 3. In addition, at different values of applied strain, we observed both strain stiffening and strain softening, indicating nonlinearity of the collagen network. Thus, this experimental method quantifies local mechanical properties in a fibrous network at the scale of a cell, while also accounting for inherent nonlinearity.
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TL;DR: In this article, a triaxial split Hopkinson pressure bar (SHPB) system is used to measure the dynamic fracture toughness of rocks under five hydrostatic pressures, and an empirical formula is proposed to describe the influences of the loading rate and the hydrostatic pressure on the rock dynamic fracture strength.
Abstract: Dynamic fracture failure of rocks subjected to static hydrostatic pressure is commonly encountered in deep underground rock engineering. The static fracture behavior of rocks under hydrostatic stress has been well studied in the literature. However, it is desirable to investigate the dynamic fracture failure of rocks under various hydrostatic pressures. In this study, a triaxial split Hopkinson pressure bar (SHPB) system is used to measure the dynamic fracture toughness of rocks under five hydrostatic pressures. The results show that dynamic fracture toughness under a certain hydrostatic pressure enhances with the increase of the loading rate, and the dynamic fracture toughness at the similar loading rate increases with the hydrostatic pressure due to the closure of microcracks in rocks. An empirical formula is proposed to describe the influences of the loading rate and the hydrostatic pressure on the rock dynamic fracture toughness.
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TL;DR: The present procedure can be considered as robust to noise, in the sense that off-the-shelf deconvolution algorithms do not converge in the presence of classic levels of noise observed in strain maps, which means that the proposed procedure improves the compromise between spatial resolution and measurement resolution.
Abstract: Digital Image Correlation (DIC) and Localized Spectrum Analysis (LSA) are two techniques available to extract displacement fields from images of deformed surfaces marked with contrasted patterns. Both techniques consist in minimizing the optical residual. DIC performs this minimization iteratively in the real domain on random patterns such as speckles. LSA performs this minimization nearly straightforwardly in the Fourier domain on periodic patterns such as grids or checkerboards. The particular case of local DIC performed pixelwise is considered here. In this case and regardless of noise, local DIC and LSA both provide displacement fields equal to the actual one convolved by a kernel known a priori. The kernel corresponds indeed to the Savitzky-Golay filter in local DIC, and to the analysis window of the windowed Fourier transform used in LSA. Convolution reduces the noise level, but it also causes actual details in displacement and strain maps to be returned with a damped amplitude, thus with a systematic error. In this paper, a deconvolution method is proposed to retrieve the actual displacement and strain fields from their counterparts given by local DIC or LSA. The proposed algorithm can be considered as an extension of Van Cittert deconvolution, based on the small strain assumption. It is demonstrated that it allows enhancing fine details in displacement and strain maps, while improving the spatial resolution. Even though noise is amplified after deconvolution, the present procedure can be considered as robust to noise, in the sense that off-the-shelf deconvolution algorithms do not converge in the presence of classic levels of noise observed in strain maps. The sum of the random and systematic errors is also lower after deconvolution, which means that the proposed procedure improves the compromise between spatial resolution and measurement resolution. Numerical and real examples considering deformed speckle images (for DIC) and checkerboard images (for LSA) illustrate the efficiency of the proposed approach.
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TL;DR: In this paper, the authors used Digital Image Correlation (DIC) to measure the deformation field in tension loaded samples containing a central hole, a circular edge notch and a sharp crack.
Abstract: Hydrogels are a class of soft, highly deformable materials formed by swelling a network of polymer chains in water. With mechanical properties that mimic biological materials, hydrogels are often proposed for load bearing biomedical or other applications in which their deformation and failure properties will be important. To study the failure of such materials a means for the measurement of deformation fields beyond simple uniaxial tension tests is required. As a non-contact, full-field deformation measurement method, Digital Image Correlation (DIC) is a good candidate for such studies. The application of DIC to hydrogels is studied here with the goal of establishing the accuracy of DIC when applied to hydrogels in the presence of large strains and large strain gradients. Experimental details such as how to form a durable speckle pattern on a material that is 90% water are discussed. DIC is used to measure the strain field in tension loaded samples containing a central hole, a circular edge notch and a sharp crack. Using a nonlinear, large deformation constitutive model, these experiments are modeled using the finite element method (FEM). Excellent agreement between FEM and DIC results for all three geometries shows that the DIC measurements are accurate up to strains of over 10, even in the presence of very high strain gradients near a crack tip. The method is then applied to verify a theoretical prediction that the deformation field in a cracked sample under relaxation loading, i.e. constant applied boundary displacement, is stationary in time even as the stress relaxes by a factor of three.
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TL;DR: In this article, the authors review how the interactions of graphene and other 2D materials with their growth and any target substrates have been characterized and make distinctions between direct and indirect methods of extracting the traction-separation relations, which are the continuum representation of the functional form of the interactions between the 2D material and the substrate of interest.
Abstract: Here we review how the interactions of graphene and other 2D materials with their growth and any target substrates have been characterized. Quantifying such interactions is particularly useful for modeling the transfer of the 2D materials to other substrates. It should also help model the assembly of structures made of 2D materials. Distinction is made between direct and indirect methods of extracting the traction-separation relations, which are the continuum representation of the functional form of the interactions between the 2D material and the substrate of interest. Salient features of traction-separation relations include the energy, strength and range of the interaction being considered. Adhesion and separation energies have been the hallmark of linearly elastic fracture mechanics characterizations in the past. The additional information on the strength and range of interactions provided by the measured traction-separation relations allows closer reference to the force fields associated with them and help with the identification of mechanisms. It should also spur theoretical developments to account for some of the interesting features that are being observed.