Showing papers in "Experimental Mechanics in 2018"

Digital Volume Correlation: Review of Progress and Challenges

Abstract: 3D imaging has become popular for analyzing material microstructures. When time lapse series of 3D pictures are acquired during a single experiment, it is possible to measure displacement fields via digital volume correlation (DVC), thereby leading to 4D results. Such 4D analyses have been performed for almost two decades. The present paper aims at reviewing the achievements of and challenges faced by such measurement technique. Ex-situ and in-situ experiments are discussed. A general and unified DVC framework is introduced. Various sources of measurement bias and uncertainties are analyzed. The current challenges are studied and some propositions are given to address them.

DIC Challenge: Developing Images and Guidelines for Evaluating Accuracy and Resolution of 2D Analyses

Abstract: With the rapid spread in use of Digital Image Correlation (DIC) globally, it is important there be some standard methods of verifying and validating DIC codes. To this end, the DIC Challenge board was formed and is maintained under the auspices of the Society for Experimental Mechanics (SEM) and the international DIC society (iDICs). The goal of the DIC Board and the 2D–DIC Challenge is to supply a set of well-vetted sample images and a set of analysis guidelines for standardized reporting of 2D–DIC results from these sample images, as well as for comparing the inherent accuracy of different approaches and for providing users with a means of assessing their proper implementation. This document will outline the goals of the challenge, describe the image sets that are available, and give a comparison between 12 commercial and academic 2D–DIC codes using two of the challenge image sets.

Disintegration of Meatball Electrodes for LiNi x Mn y Co z O 2 Cathode Materials

Abstract: Mechanical degradation of Li-ion batteries caused by the repetitive swelling and shrinking of electrodes upon electrochemical cycles is now well recognized. Structural disintegration of the state-of-art cathode materials of a hierarchical structure is relatively less studied. We track the microstructural evolution of different marked regimes in LiNi x Mn y Co z O2 (NMC) electrodes after lithiation cycles. Decohesion of primary particles constitutes the major mechanical degradation in the NMC materials, which results in the loss of connectivity of the conductive network and impedance increase. We find that the structural disintegration is largely dependent on the charging rate – slow charging causes more damage, and is relatively insensitive to the cyclic voltage window. We use finite element modeling to study the evolution of Li concentration and stresses in a NMC secondary particle and employ the cohesive zone model to simulate the interfacial fracture between primary particles. We reveal that microcracks accumulate and propagate during the cyclic lithiation and delithiation at a slow charging rate.

Experimental investigation of the coupled magneto-mechanical response in magnetorheological elastomers

Abstract: Magnetorheological elastomers (MREs) are materials made of a soft elastomer matrix filled with magnetizable particles. These flexible composites that deform in response to an externally applied magnetic field are of special interest in advanced engineering applications such as actuators, artificial muscles or shape control. However, no systematic characterization of their coupled response has been undertaken so far, thus limiting the efficient design of MRE-based devices. In this study, we propose a framework—relying on both specially designed samples and a dedicated experimental setup—to characterize experimentally the coupled magneto-mechanical response of MREs since magnetization within the sample is nearly uniform and structural-dependent effects are minimized. The influence of particle content and arrangement within the composite are particularly studied and the corresponding experimental results give some insight into the underlying microstructural mechanisms that are responsible for the macroscopic deformation of MREs under combined magnetic and mechanical loading conditions. Such data is crucial for the design of new MRE composite materials in which the microstructure is optimized (to have the largest coupling effect with minimal energy input).

Coupling Effect of State-of-Health and State-of-Charge on the Mechanical Integrity of Lithium-Ion Batteries

Abstract: Two governing factors that influence the electrochemical behaviors of lithium-ion batteries (LIBs), namely, state of charge (SOC) and state of health (SOH), are constantly interchanged, thus hindering the understanding of the mechanical integrity of LIBs. This study investigates the electrochemical failure of LIBs with various SOHs and SOCs subjected to abusive mechanical loading. Comprehensive experiments on LiNi0.8CoO15Al0.05O2 (NCA) LIB show that SOH reduction leads to structural stiffness and that the change trend varies with SOC value. Low SOH, however, may mitigate this phenomenon. Electrochemical failure strain at short circuit has no relationship with SOC or SOH, whereas failure stress increases with the increase of SOC value. Experiments on three types of batteries, namely, NCA, LiCoO2 (LCO), and LiFePO4 (LFP) batteries, indicate that their mechanical behaviors share similar SOH-dependency properties. SOH also significantly influences failure stress, temperature increase, and stiffness, whereas its effect on failure strain is minimal. Results may provide valuable insights for the fundamental understanding of the electrochemically and mechanically coupled integrity of LIBs and establish a solid foundation for LIB crash-safety design in electric vehicles.

High-Resolution Deformation Mapping Across Large Fields of View Using Scanning Electron Microscopy and Digital Image Correlation

Abstract: This paper details the creation of experimental and computational frameworks to capture high-resolution, microscale deformation mechanisms and their relation to microstructure over large (mm-scale) fields of view Scanning electron microscopy with custom automation and external beam control was used to capture 209 low-distortion micrographs of 360 μm × 360 μm each, that were individually correlated using digital image correlation to obtain displacement/strain fields with a spatial resolution of 044 μm Displacement and strain fields, as well as secondary electron images, were subsequently stitched to create a 57 mm × 34 mm field of view containing 100 million (7678 × 13,004) data points This approach was demonstrated on Mg WE43 under uniaxial compression, where effective strain was shown to be relatively constant with respect to distance from the grain boundary, and a noticeable increase in the effective strain was found with an increase in the basal Schmid factor The ability to obtain high-resolution deformations over statistically relevant fields of view enables large data analytics to examine interactions between microstructure, microscale strain localizations, and macroscopic properties

Experimental Characterization of the Shear Properties of 3D–Printed ABS and Polycarbonate Parts

Abstract: As 3D printing has increased in popularity, it has become more important to understand the properties and characteristics of the parts being produced. The material properties of 3D printed parts are not equivalent to the bulk properties of the materials used to print them. The nature of 3D printing, with varying raster and build orientations, results in anisotropy of the printed parts. This paper presents the experimental techniques and results for the mechanical characterization of acrylonitrile butadiene styrene (ABS) and polycarbonate (PC) 3D printed parts to determine the anisotropy of their shear properties. Because of the anisotropy, tensile material properties cannot be used to determine shear properties. Iosipescu shear specimens were manufactured at various print raster ([+45/−45], [+30/−60], [+15/−75], and [0/90]) and build orientations (flat, on-edge, and up-right) to determine the directional properties of both ABS and PC samples. Ten samples of each raster were subjected to loading in a universal testing machine while making use of 2D digital image correlation (DIC) to measure full-field strain on both sides of the specimen. Results for both the ABS and PC materials show that there were measurable differences in the modulus of rigidity between build orientations and significant differences in shear strength. The ABS samples exhibited strong anisotropy as a function of build orientation while the PC samples exhibited strong anisotropy as a function of raster orientation.

A q-Factor-Based Digital Image Correlation Algorithm (qDIC) for Resolving Finite Deformations with Degenerate Speckle Patterns

Abstract: Digital image correlation (DIC) has become a widely utilized non-contact, full-field displacement measurement technique for obtaining accurate material kinematics. Despite the significant advances made to date, high resolution reconstruction of finite deformations for images with intrinsically low quality speckle patterns or poor signal-to-noise content has not been fully addressed. In particular, large image distortions imposed by materials undergoing finite deformations create significant challenges for most classical DIC approaches. To address this issue, this paper describes a new open source DIC algorithm (qDIC) that incorporates cross-correlation quality factors (q-factors), which are specifically designed to assess the quality of the reconstructed displacement estimate during the motion reconstruction process. A q-factor provides a robust assessment of the uniqueness and sharpness of the cross-correlation peak, and thus a quantitative estimate of the subset-based displacement measure per given image subset and level of applied deformation. We show that the incorporation of energy- and entropy-based q-factor metrics leads to substantially improved displacement predictions, lower noise floor, and reduced decorrelation even at significant levels of image distortion or poor speckle quality. Furthermore, we show that q-factors can be utilized as a quantitative metric for constructing a hybrid incremental-cumulative displacement correlation scheme for accurately resolving very large homogeneous and inhomogeneous deformations, even in the presence of significant image data loss.

Distortion of Digital Image Correlation (DIC) Displacements and Strains from Heat Waves

Abstract: “Heat waves” is a colloquial term used to describe convective currents in air formed when different objects in an area are at different temperatures. In the context of Digital Image Correlation (DIC) and other optical-based image processing techniques, imaging an object of interest through heat waves can significantly distort the apparent location and shape of the object. There are many potential heat sources in DIC experiments, including but not limited to lights, cameras, hot ovens, and sunlight, yet error caused by heat waves is often overlooked. This paper first briefly presents three practical situations in which heat waves contributed significant error to DIC measurements to motivate the investigation of heat waves in more detail. Then the theoretical background of how light is refracted through heat waves is presented, and the effects of heat waves on displacements and strains computed from DIC are characterized in detail. Finally, different filtering methods are investigated to reduce the displacement and strain errors caused by imaging through heat waves. The overarching conclusions from this work are that errors caused by heat waves are significantly higher than typical noise floors for DIC measurements, and that the errors are difficult to filter because the temporal and spatial frequencies of the errors are in the same range as those of typical signals of interest. Therefore, eliminating or mitigating the effects of heat sources in a DIC experiment is the best solution to minimizing errors caused by heat waves.

Evaluation of the Stress Equilibrium Condition in Axially Constrained Triaxial SHPB Tests

Abstract: Over the years many improvements and modifications have been made to the split Hopkinson pressure bar (SHPB). For example, the triaxial SHPB has been developed and used to study various materials. The dynamic stress equilibrium condition for conventional SHPB tests has been well studied in the literature. However, the stress equilibrium in the axially constraint triaxial SHPB tests requires careful examination because the wave propagation is affected by the axial pre-load stress. In this study, we analyzed the stress equilibrium condition in such cases using stress wave analysis and numerical simulations. Based on the study, we offer methods and suggestions for performing valid stress equilibrium assessment in axially constrained triaxial SHPB tests.

Leveraging Full-Field Measurement from 3D Digital Image Correlation for Structural Identification

Abstract: Within the domain of structural health monitoring (SHM) measurement techniques have primarily relied on discrete sensing strategies using sensors physically attached to the structural system of interest. These sensors have proven effective in describing both global and local phenomena, but are limited to providing discrete response measurements of these systems. With the introduction of novel imaging tools and image analysis techniques, such as digital image correlation (DIC), the ability to measure the full-field response of these systems provides a novel approach to refining structural identification (St-ID) approaches used in SHM. This paper explores this proposed concept through a case study on a series of structural test specimens analyzed using 3D digital image correlation (3D-DIC) for St-ID. Finite element model updating (FEMU) was used as the technique for the structural identification. For the identification process, ABAQUS was interfaced with MATLAB to converge on the optimal unknown/uncertain system parameters of the experimental setup. 3D-DIC results provided a rich full-field dataset for the identification process, which was compared against measurements derived from traditional physical in-place sensors typically used in SHM. In this work a Hybrid Genetic Algorithm (HGA), which combines the genetic algorithm as a global optimization and a gradient-based method as a local optimization, was used for the FEMU based on 3D-DIC results of structural specimen subjected to variable loading. To minimize the error between the full field 3D-DIC measurements and FEA model updating results, an objective function was introduced that included the full-field contributions of strains and deformation response. The evolution of this objective function illustrated satisfactory convergence of the identified parameters and the excellent agreement of the experimental and numerical strain and displacement responses after the model updating process confirmed the success of the proposed approach. The results of this study highlight the advantage of this hybrid approach and provide the foundation for effective deployment of the proposed strategy for large-scale structural systems.

Effect of Particle Morphology, Compaction, and Confinement on the High Strain Rate Behavior of Sand

Abstract: The effect of grain shape, size distribution, intergranular friction, confinement, and initial compaction state on the high strain rate compressive mechanical response of sand is quantified using Long Split Hopkinson Pressure Bar (LSHPB) experiments, generating up to 1.1 ms long load pulses. This allowed the dynamic characterisation of different types of sand until full compaction (lowest initial void ratio) at different strain rates. The effect of the grain morphology and size on the dynamic compressive mechanical response of sand is assessed by conducting experiments on three types of sand: Ottawa Sand with quasi-spherical grains, Euroquartz Siligran with subangular grains and Q-Rok with polyhedral grain shape are considered in this study. The adoption of rigid (Ti64) and deformable (Latex) sand containers allowed for quasi-uniaxial strain and quasi-uniaxial stress conditions to be achieved respectively. Additionally, the effect of intergranular friction was studied, for the first time in literature, by employing polymer coated Euroquartz sand. Appropriate procedures for the preparation of samples at different representative initial consolidation states are utilized to achieve realistic range of naturally occurring formations of granular assembly from loose to dense state. The results identify material and confining sample state parameters which have significant effect on the mechanical response of sand at high strain rates and their interdependency for future integration into rate dependent constitutive models.

Residual Stress Measurement in Composite Laminates Using Incremental Hole-Drilling with Power Series

Abstract: Current methods for incremental hole-drilling in composite laminates have not been successfully applied in laminates of arbitrary construction or where significant variation of residual stress exists within a single ply. This work presents a method to overcome these limitations. Series expansion is applied to each ply orientation separately so that the discontinuities in the residual stresses at ply interfaces can be correctly captured. Temperature variations described by power series are used to set up eigenstrains and consequent stresses which vary in the through-thickness direction. The calibration coefficients at each incremental hole depth are calculated through the use of finite element modelling. The inverse solution employs a least-squares approach which makes the resulting solution insensitive to measurement uncertainty. Robust uncertainties in the residual stress distributions are determined using Monte Carlo simulation. The residual stress distribution is found from that combination of series orders in the different ply orientations that has the lowest RMS uncertainty, selected only from those combinations that have converged. The method is demonstrated on a GFRP laminate of [02/+45/−45]s construction where it is found that transverse cracking of the plies at the inner surface of the hole may have impacted on the accuracy of the results.

Strain Evolution in Lithium Manganese Oxide Electrodes

Abstract: Lithium manganese oxide, LiMn2O4 (LMO) is a promising cathode material, but is hampered by significant capacity fade due to instability of the electrode-electrolyte interface, manganese dissolution into the electrolyte and subsequent mechanical degradation of the electrode. In this work, electrochemically-induced strains in composite LMO electrodes are measured using the digital image correlation (DIC) technique and compared with electrochemical impedance spectroscopy (EIS) measurements of surface resistance for different scan rates. Distinct, irreversible strain variations are observed during the first delithiation cycle. The changes in strain and surface resistance are highly sensitive to the electrochemical changes occurring during the first cycle and correlate with prior reports of the removal of the native surface layer and the formation of cathode-electrolyte interface layer on the electrode surface. A large capacity fade is observed with increasing cycle number at high scan rates. Interestingly, the total capacity fade scales proportionately to the strain generated after each lithiation and delithiation cycle. The simultaneous reduction in capacity and strain is attributed to chemo-mechanical degradation of the electrode. The in situ strain measurements provide new insight into the electrochemical-induced volumetric changes in LMO electrodes with progressing cycling and may provide guidance for materials-based strategies to reduce strain and capacity fade.

A Novel Image-based Ultrasonic Test to Map Material Mechanical Properties at High Strain-rates

Abstract: An innovative identification strategy based on high power ultrasonic loading together with both infrared thermography and ultra-high speed imaging is presented in this article. It was shown to be able to characterize the visco-elastic behaviour of a polymer specimen (PMMA) from a single sample over a range of temperatures and strain-rates. The paper focuses on moderate strain-rates, i.e. from 10 to 200 s−1, and temperatures ranging from room to the material glass transition temperature, i.e. 110∘C. The main originality lies in the fact that contrary to conventional Dynamic Mechanical Thermal Analysis (DMTA), no frequency or temperature sweep is required since the experiment is designed to simultaneously produce both a heterogeneous strain-rate state and a heterogeneous temperature state allowing a local and multi-parametric identification. This article is seminal in nature and the test presented here has good potential to tackle a range of other types of high strain-rate testing situations.

Development of Optimal Multiscale Patterns for Digital Image Correlation via Local Grayscale Variation

Abstract: In many applications of digital image correlation (DIC), it is advantageous to have measurements at multiple scales. Because it is rare to have natural features that can be used for DIC at multiple magnifications, an appropriately multiscale DIC pattern is needed. This work develops a multiscale DIC pattern that (1) contains features appropriate for both high and low magnification, (2) does not need to know the location of high magnification a priori, and (3) does not require specialized DIC equipment beyond what is necessary to achieve the two magnifications. The pattern is developed based on an optimization framework that minimizes expected DIC error while constraining sub-regions of the pattern to biased average grayscale values. The inclusion of local grayscale biases in the pattern has the effect of introducing resolvable features at a length scale much larger than the speckles of which the pattern is composed. Numerical and physical experiments were performed to illustrate the functionality and utility of the designed patterns. Notable among the findings is the trade off between DIC accuracy at the two scales and how it is controlled by grayscale bias.

Evaluation of Volume Deformation from Surface DIC Measurement

Abstract: Stereo-DIC allows to track with a high accuracy the shape change and the surface displacement field of objects during deformation processes. When multiple camera arrangements are used, the shape and deformation measurement can be performed over the whole surface of the object. We submit that, in the case of intact specimens, with no internal defects and/or discontinuities, such boundary information can be used to estimate the internal displacement field by using proper interpolation functions. This calculation could serve, for instance, to evaluate the strain localization that occurs in metal specimens subjected to plastic deformation, hence allowing to get a better insight in the necking initiation and fracture propagation processes. In this paper, an interpolation method based on Bezier curves is developed and tested using simulated and real experiments on specimens with flat and cylindrical geometries. In particular, the deformation behaviour in the necking zone was investigated in the case of highly ductile and anisotropic materials. Numerical models were used to validate the method while the application to two real experiments demonstrated its feasibility in practical cases. The applicability of the method to more complex loading cases (e.g., bending, torsion, mixed-loads) or different initial shapes (e.g., curved beams, notches) will be investigated in future studies.

Quantitative In Situ Studies of Dynamic Fracture in Brittle Solids Using Dynamic X-ray Phase Contrast Imaging

Abstract: We demonstrate the use of X-ray phase contrast imaging with sub-microsecond temporal resolution to obtain quantitative visualization of dynamic fracture processes in brittle solids. We examine an amorphous solid (fused silica), a ceramic single crystal (single-crystal quartz), and a polycrystalline ceramic (boron carbide), in the form of single-edge notched specimens loaded using a three-point apparatus at nominal strain rates up to $\sim $ 800 s−1. We observe that the crack tip speed for boron carbide is relatively independent of mode I stress intensity factor rate ( $\dot {K}_{\mathrm {I}}$ ) for these rates of loading, while that of fused silica and single-crystal quartz increases with $\dot {K}_{\mathrm {I}}$ . Further, for the amorphous and single crystal cases, we observe the development of a crack tip instability in the form of crack branching as the crack tip speed approaches 45% of the Rayleigh wave speed. This suggests that strain-rate-dependent mechanisms contribute to crack branching. Such mechanisms may, in turn, affect the macroscopic fracture properties of these materials.

A Theoretical Model for Estimating the Peak Cutting Force of Conical Picks

Abstract: The peak cutting force (PCF) estimation plays an important role in the design of cutting tools for mining excavators. In most of the existing theoretical models for cutting force prediction, the PCF is often modeled as the force on the cutting tool at the moment when the rock fragment is formed. However, according to the theory of fracture mechanics, the PCF is supposed to occur during the crack initiation phase. Consequently, this paper attempts to add to the existing literature by proposing a fracture mechanics-based theoretical model for PCF prediction. The proposed PCF prediction model distinguishes itself from existing models by determining the PCF during initiation of the rock crack. The PCF is determined using the energy and stress criteria of Griffith’s fracture mechanics theory. In this new model, the PCF is positively related to the fracture toughness of the rock and the cutting depth. The experimental results demonstrated the validity of the proposed model. The proposed model performs well in predicting the PCF in terms of reliability and accuracy. Besides, the PCF prediction capability of the proposed model was compared with those of the other rock cutting models existing in the literature.

Specimen Size and Strain Rate Effects on the Compressive Behavior of Concrete

Abstract: Three high-performance concrete (HPC) materials with different specimen geometries were characterized using Kolsky compression bar techniques to study the strain rate and specimen size effects on their uniaxial compressive strength. A large-diameter Kolsky bar and recently established annular pulse shaping technique were used to achieve dynamic stress equilibrium and constant strain-rate deformation in the experiments. A complimentary effort was conducted using a 19-mm-diameter Kolsky compression bar to understand the strain rate and specimen size effects on failure strength and dynamic increase factor (DIF) for concrete. It was found that, for all three concrete materials investigated, the failure strength is highly dependent on the specimen geometry, however such a relationship is not apparent for the DIF. The DIF observed in this study shows significantly lower values compared to historical data, which may indicate the importance of well-controlled dynamic testing conditions on the accuracy and validity of experimental results for concrete materials.

Static and Modal Analysis of Low Porosity Thin Metallic Auxetic Structures Using Speckle Interferometry and Digital Image Correlation

Abstract: This study presents an evaluation of the static and dynamic mechanical behavior of low porosity, ductile two-dimensional auxetic metamaterials. The full in-plane displacement fields and the eigenmodes of different geometric structures were investigated and compared with finite element simulations using speckle interferometry and digital image correlation. The results showed strong agreement, validating the theoretical approach used and establishing a method for testing and quantitatively assessing the performance of negative Poisson ratio structures, and metamaterials in general, for different purposes and fields. The findings of this study also increase our knowledge of elastic instabilities in metallic auxetic structures, with further applications in several engineering fields that can benefit from combining the qualities of ductile materials with additional features typical of these smart structures.

Digital Image Correlation and Color Cameras

Abstract: Digital Image Correlation (DIC) is a well-known experimental technique. It works by constructing a (surjective) mapping of pixel intensity from reference to target image, where the mapping parameters are identified using a Least Squares approach. Because it makes use of the luminance component of the image, Digital Image Correlation is usually implemented by assuming monochrome cameras. In this work, we will discuss its implementation when color cameras are used, focusing on pitfalls and potential advantages of this solution. Since most cameras implement color acquisition using a Color Filter Array (CFA), much of the article will focus on this technology. However, we will not limit ourselves to this aspect and will show that Three-CCD cameras can provide significant advantages over both CFA and monochrome cameras.

Application of DIC to Static and Dynamic Testing of Agglomerated Cork Material

Abstract: In this work, experimental compression tests have been performed on parallelepiped specimens cut from an agglomerated cork slab. The tests have been performed both using a quasi-static testing machine and a polymeric Split Hopkinson Bar, in order to assess the sensitivity of the material to the strain rate. A standard and a high-speed digital camera have been used to collect frames of the samples during the tests. 2D DIC analyses have been conducted on the pictures of lateral faces of the specimens in order to evaluate the actual strain distributions, which showed a significant heterogeneity within each sample. Moreover, the DIC analyses on the dynamic tests have been used for evaluating the local accelerations and to compute the inertia stresses. The latter may affect the global response that can be measured by following the standard Hopkinson bar procedures, and are responsible for the fluctuations in the force histories observed in the tests at highest strain rates.

State-of-Charge and Deformation-Rate Dependent Mechanical Behavior of Electrochemical Cells

Abstract: The state-of-charge and deformation-rate dependent mechanical behavior of cylindrical lithium-ion battery cells was investigated. The research revealed that both state of charge and deformation rates affected the stiffness of the battery cells. Battery mechanical failure load was only weakly dependent on the state of charge. For the deformation-rate dependency on the mechanical integrity of battery cells, the battery mechanical failure load was either decreased significantly at high state of charge or decreased slightly at low state of charge as deformation rate increased. For the correlation between mechanical integrity and electrical failure, the displacement at the battery mechanical failure load coincided with a voltage drop. However, at high state of charge, premature and incomplete voltage drops were observed before the definite final voltage drop. No such premature voltage drop was observed in low state-of-charge specimens. The results of this research may be used as a reference for the design of impact and damage tolerant electric vehicle battery systems.

Dynamic Response of Aluminum 5083 During Taylor Impact Using Digital Image Correlation

Abstract: Aluminum 5083 was tested using forward Taylor impact experiments for impact velocities ranging from 75 to 393 m/s, using stereo digital image correlation (DIC) at 320,000 Hz as the primary data collection method. The Taylor impact experiment is a simple method for evaluating the response of materials at high strain and strain rate, historically by using relationships between the material sample geometric profile after the experiment and yield stress. Initially, this limited Taylor impact experiments to qualitative studies, since the time-variance of the stress-strain state complicates analysis. A benefit of using DIC is that a single experiment sample provides quantitative information about the material deformation response at a wide range of strains and strain rates. DIC was used in this experimental program to measure the strain field, strain rate, and profile of the samples during impact. Peak strains approaching 80% and strain rates approaching 105/s were measured at the impact-end of the material samples at higher velocities (320 m/s and higher). Numerical simulations of the experiments were conducted using LS-DYNA and showed good time-resolved agreement with the experiment result strain and strain rate profiles from DIC. Highlights of the DIC data, including strain fields, time histories, and comparisons between experiments will be presented along with comparisons of the DIC data to the numerical analysis.

Evaluation of Conformal and Non-Conformal Contact Parameters Using Digital Photoelasticity

Abstract: Experimental studies to exploit photoelastic data of conformal geometries to extract contact parameters are non-existent because closed-form stress field equations were not available until recently. In this paper, the explicit equations recently reported in the literature for a flat punch with rounded edges are generalized so that a single set of equations can be used for a flat punch with rounded edges and Hertzian contacts with arbitrary radii of curvatures. The generality of the governing equations is verified by plotting isochromatics for conformal and non-conformal contact situations. A generic method to evaluate unknown contact parameters from the whole-field isochromatic data for conformal and non-conformal geometries is implemented. The methodology is initially verified using theoretically generated isochromatic data and is then used to experimentally evaluate two contact situations. In view of high-fringe gradient zones, the suitability of various digital photoelastic methods is compared. A novel four-step phase shifting technique is proposed in which isochromatic and isoclinic data can be evaluated using the minimum number of images.

A Space-Time PGD-DIC Algorithm:

Abstract: The aim of this study is to develop a new regularized Digital Image Correlation (DIC) method for time dependent measurements. The correlation problem is written as a minimization problem over the space-time domain in a general formulation including 2D-DIC and Stereo DIC (SDIC). The unknown time-resolved displacement field is found as a sum of products of space and time functions, similarly to the Proper Generalized Decomposition in computational mechanics. It is shown that the space fields are less sensitive to noise as time regularity acts as a physical regularization of the space fields. The proposed method is illustrated by vibration measurement under harmonic excitation in 2D-DIC and SDIC.

Damage Monitoring of Composite Components under Vibration Fatigue using Scanning Laser Doppler Vibrometer

Abstract: This research article proposes experimental methods for monitoring damage propagation of Carbon Fibre Reinforced Plastic (CFRP) components under vibration fatigue. High cycle fatigue (HCF) behaviour of composites is often not as dramatic as that of metal alloys, where any crack initiation might rapidly lead to failure. Instead, composites HCF behaviour is often a prolonged state of continuous degradation of the resin-to-fibre bonding. Furthermore, the interlaminar contact conditions at any opening delamination cause complex dynamic responses, which can be nonlinear and temperature dependent. Vibration fatigue is generally caused by large deformations which can be suitably measured by non-contact measurement systems. Hence, this paper explores the application of the scanning laser vibrometer to monitor damage propagation of CFRP components under vibration fatigue loading. The objective is to show how this measurement system can systematically implement a set of experimental methods aimed at monitoring the dynamic properties during the crack propagation caused by fatigue. The set of experimental techniques are autonomously executed by a custom-made control panel. A thermal camera is also part of the measurement chain and provides qualitative information about location of temperature hot spots and damage evolution.

Effect of Moisture Content on High Strain Rate Compressibility and Particle Breakage in Loose Sand

Abstract: Soil-filled gabion structures are widely used to protect against the effects of blast and fragmentation. It is known that moisture content significantly affects the capability of such structures, but the behaviour of partially-saturated soils is not well characterised at the strain rates and stresses experienced in these events. In particular, little data is available for loose soils, whose compaction behaviour can have a substantial impact on structural stability and ballistic performance. This paper describes the use of split Hopkinson pressure bar experiments to characterise the pre- and post-saturation compressibility of a loose quartz sand at moisture contents of up to 15.0%. In contrast to dense soils, increases in moisture content between 0.0% and 7.5% led to a decrease in the stiffness of the sand. Above 7.5% moisture content, specimens reached full saturation during the experiment: the additional water had no further effect on the pre-saturation stiffness, but post-saturation behaviour was dominated by the stiffness of the pore water. Full saturation occurred at lower dry densities as moisture content increased, leading to a decrease in particle breakage.

Plasticity Effects in the Hole-Drilling Residual Stress Measurement in Peened Surfaces

Abstract: The incremental hole-drilling technique (IHD) is a widely established and accepted technique to determine residual stresses in peened surfaces. However, high residual stresses can lead to local yielding, due to the stress concentration around the drilled hole, affecting the standard residual stress evaluation, which is based on linear elastic equations. This so-called plasticity effect can be quantified by means of a plasticity factor, which measures the residual stress magnitude with respect to the approximate onset of plasticity. The observed resultant overestimation of IHD residual stresses depends on various factors, such as the residual stress state, the stress gradients and the material’s strain hardening. In peened surfaces, equibiaxial stresses are often found. For this case, the combined effect of the local yielding and stress gradients is numerically and experimentally analyzed in detail in this work. In addition, a new plasticity factor is proposed for the evaluation of the onset of yielding around drilled holes in peened surface layers. This new factor is able to explain the agreement and disagreement found between the IHD residual stresses and those determined by X-ray diffraction in shot-peened steel surfaces.