Showing papers in "Experimental Mechanics in 2007"

An Experimental Investigation of Deformation and Fracture of Nacre–Mother of Pearl

Abstract: Nacre, also known as mother-of-pearl, is a hard biological composite found in the inside layer of many shells such as oyster or abalone. It is composed of microscopic ceramic tablets arranged in layers and tightly stacked to form a three-dimensional brick wall structure, where the mortar is a thin layer of biopolymers (20–30 nm). Although mostly made of a brittle ceramic, the structure of nacre is so well designed that its toughness is several order of magnitudes larger that the ceramic it is made of. How the microstructure of nacre controls its mechanical performance has been the focus of numerous studies over the past two decades, because such understanding may inspire novel composite designs though biomimetics. This paper presents in detail uniaxial tension experiment performed on miniature nacre specimens. Large inelastic deformations were observed in hydrated condition, which were explained by sliding of the tablets on one another and progressive locking generated by their microscopic waviness. Fracture experiments were also performed, and for the first time the full crack resistance curve was established for nacre. A rising resistance curve is an indication of the robustness and damage tolerance of that material. These measurements are then discussed and correlated with toughening extrinsic mechanisms operating at the microscale. Moreover, specific features of the microstructure and their relevance to associated toughening mechanisms were identified. These features and mechanisms, critical to the robustness of the shell, were finely tuned over millions of years of evolution. Hence, they are expected to serve as a basis to establish guidelines for the design of novel man-made composites.

Scanning Electron Microscopy for Quantitative Small and Large Deformation Measurements Part I: SEM Imaging at Magnifications from 200 to 10,000

Abstract: A series of baseline displacement measurements have been obtained using 2D Digital Image Correlation (2D-DIC) and images from Scanning Electron Microscopes (SEM). Direct correlation of subsets from a reference image to subsets in a series of uncorrected images is used to identify the presence of non-stationary step-changes in the measured displacements. Using image time integration and recently developed approaches to correct residual drift and spatial distortions in recorded images, results clearly indicate that the corrected SEM images can be used to extract deformations with displacement accuracy of ±0.02 pixels (1 nm at magnification of 10,000) and mean value strain measurements that are consistent with independent estimates and have point-to-point strain variability of ±1.5 × 10−4.

Micro- and Nanoscale Deformation Measurement of Surface and Internal Planes via Digital Image Correlation

Abstract: The digital image correlation (DIC) tech- nique is successfully applied across multiple length scales through the generation of a suitable speckle pattern at each size scale. For microscale measure- ments, a random speckle pattern of paint is created with a fine point airbrush. Nanoscale displacement resolution is achieved with a speckle pattern formed by solution deposition of fluorescent silica nanoparticles. When excited, the particles fluoresce and form a speckle pattern that can be imaged with an optical microscope. Displacements are measured on the surface and on an interior plane of transparent polymer samples with the different speckle patterns. Rigid body translation calibrations and uniaxial ten- sion experiments establish a surface displacement resolution of 1 mm over a 5� 6 mm scale field of view for the airbrushed samples and 17 nm over a 100� 100 mm scale field of view for samples with the fluorescent nanoparticle speckle. To demonstrate the capabilities of the method, we characterize the internal deforma- tion fields generated around silica microspheres em- bedded in an elastomer under tensile loading. The DIC technique enables measurement of complex deformation fields with nanoscale precision over relatively large areas, making it of particular relevance to materials that possess multiple length scales.

Three-dimensional Full-field Measurements of Large Deformations in Soft Materials Using Confocal Microscopy and Digital Volume Correlation

Abstract: A three-dimensional (3-D) full-field measurement technique was developed for measuring large deformations in optically transparent soft materials. The technique utilizes a digital volume correlation (DVC) algorithm to track motions of subvolumes within 3-D images obtained using fluorescence confocal microscopy. In order to extend the strain measurement capability to the large deformation regime (>5%), a stretch-correlation algorithm was developed and implemented into the Fast Fourier Transform (FFT)-based DVC algorithm. The stretch-correlation algorithm uses a logarithmic coordinate transformation to convert the stretch-correlation problem into a translational correlation problem under the assumption of small rotation and shear. Estimates of the measurement precision are provided by stationary and translation tests. The proposed measurement technique was used to measure large deformations in a transparent agarose gel sample embedded with fluorescent particles under uniaxial compression. The technique was also employed to measure non-uniform deformation fields near a hard spherical inclusion under far-field uniaxial compression. Introduction of the stretch-correlation algorithm greatly improved the strain measurement accuracy by providing better precision especially under large deformation. Also, the deconvolution of confocal images improved the accuracy of the measurement in the direction of the optical axis. These results shows that the proposed technique is well-suited for investigating cell-matrix mechanical interactions as well as for obtaining local constitutive properties of soft biological materials including tissues in 3-D.

Scanning Electron Microscopy for Quantitative Small and Large Deformation Measurements Part II: Experimental Validation for Magnifications from 200 to 10,000

Abstract: A combination of drift distortion removal and spatial distortion removal are performed to correct Scanning Electron Microscope (SEM) images at both ×200 and ×10,000 magnification. Using multiple, time-spaced images and in-plane rigid body motions to extract the relative displacement field throughout the imaging process, results from numerical simulations clearly demonstrate that the correction procedures successfully remove both drift and spatial distortions with errors on the order of ±0.02 pixels. A series of 2D translation and tensile loading experiments are performed in an SEM for magnifications at ×200 and ×10,000, where both the drift and spatial distortion removal methods described above are applied to correct the digital images and improve the accuracy of measurements obtained using 2D-DIC. Results from translation and loading experiments indicate that (a) the fully corrected displacement components have nearly random variability with standard deviation of 0.02 pixels (≈25 nm at ×200 and ≈0.5 nm at ×10,000) in each displacement component and (b) the measured strain fields are unbiased and in excellent agreement with expected results, with a spatial resolution of 43 pixels (≈54 μm at ×200 and ≈1.1 μm at ×10,000) and a standard deviation on the order of 6 × 10−5 for each component.

Assessment of High Speed Imaging Systems for 2D and 3D Deformation Measurements: Methodology Development and Validation

Abstract: Ultra high-speed and moderate speed image acquisition platforms have been characterized, with special emphasis on the variability and accuracy of the measurements obtained when employed in either 2D or 3D computer vision systems for deformation and shape measurements. Specifically, the type of image distortions present in both single channel cameras (HS-CMOS) and multi-channel image intensified cameras (UHS-ICCD) are quantified as part of the overall study, and their effect on the accuracy of experimental measurements obtained using digital image correlation have been determined. Results indicate that established methods for noise suppression and recently developed models for distortion correction can be used effectively in situations where the primary intensity noise components are characterized by minimal cross-talk and stationary spatial distortions. Baseline uniaxial tension experiments demonstrate that image correlation measurements using high speed imaging systems are unbiased and consistent with independent deformation measurements over the same length scale, with point-to-point strain variations that are similar to results obtained from translation experiments. In this study, the point-to-point variability in strain using the image intensified system is on the order of 0.001, whereas the non-intensified system had variability of 0.0001. Results confirm that high speed imaging systems can be utilized for full field two and three-dimensional measurements using digital image correlation methods.

Calibration of Stress-triaxiality Dependent Crack Formation Criteria: A New Hybrid Experimental–Numerical Method

Abstract: A new experimental technique has been developed to investigate the onset of fracture in metals at low and intermediate stress triaxialities. The gage section of a flat specimen has been designed such that cracks are most likely to initiate within the specimen center, remote from the specimen boundaries. Along with the specimen, a biaxial testing device has been built to apply a well-defined displacement field to the specimen shoulders. The stress state within the specimen is adjusted by changing the biaxial loading angle. Using this new experimental technique, the crack initiation in metals can be studied experimentally for stress triaxialities ranging from 0.0 to 0.6. The stress and strain fields within the specimen gage section are determined from finite element analysis. The reliability of the computational model of the test set-up has been verified by comparing the simulation results with laser speckle-interferometric displacement measurements during testing. Sample experiments have been performed on the Al-7Si-Mg gravity die casting alloy. A three-step hybrid experimental–numerical calibration procedure has been proposed and applied to determine a phenomenological crack formation criterion for the Al-7Si-Mg alloy.

Aerodynamic Coefficients and Deformation Measurements on Flexible Micro Air Vehicle Wings

Abstract: This paper documents the elastic deformations and corresponding aerodynamic coefficients of flexible wings used for micro air vehicles (MAVs). These low-aspect ratio wings, developed and fabricated at the University of Florida, incorporate an elastic latex membrane skin covering a thin carbon fiber skeleton. The wings were tested in a unique low-speed wind tunnel facility integrating a visual image correlation (VIC) system with a six-component strain gauge sting balance. Model characteristics are presented, along with the appropriate specimen preparation techniques and wind tunnel instrumentation. The static response characteristics, including full-field displacements and plane strain measurements, for three distinct MAV wing designs are presented. The full-field deformation results show how passive wing flexibility preferably affects aerodynamic performance when compared to a rigid model of similar geometry.

Development of Patterns for Digital Image Correlation Measurements at Reduced Length Scales

Abstract: Methods for patterning metal thin films at the microscale and nanoscale by applying the patterns to metallic and polymeric materials for use in shape and deformation measurements in a scanning electron microsope (SEM) or other high magnification imaging system are described. In one approach, thin films of metallic materials (e.g., Au, Ag, Cu, and Cr) are applied to a variety of substrates. The coated samples are then placed into a reaction vessel, where the specimens are heated and exposed to a nitrogen atmosphere saturated with selected volatile chemicals. This process results in nano-scale remodeling of the metallic films, thereby affording high contrast random patterns with different morphologies. In a second approach, thin films of metallic materials, including gold and silver, also have been applied using a simplified UV photolithographic method requiring a minimum amount of laboratory preparation. Using selected substrates, both methods have been used successfully to transfer patterns onto polymeric and metallic materials ranging from 50–500 nanometers with chemical vapor rearrangement and 2 to 20 microns with UV photolithography, providing a pattern that can be used with digital image correlation to quantify both the surface profile and also surface deformations at reduced length scales.

Strain Measurements of Silicon Dioxide Microspecimens by Digital Imaging Processing

Abstract: Silicon dioxide thin film is a common component in electronic devices and in MEMS, but its mechanical properties have rarely been studied. Techniques have been adapted and developed to conduct tensile tests on 1.0 μm thick silicon dioxide specimens that are 100, 150, and 200 μm wide and either 1 or 2 mm long. One end of the specimen remains fastened to the substrate, and the other is glued to a silicon carbide fiber attached to a 30 g load cell mounted on a piezoelectric translation stage. Strain is measured by digital imaging of two gold lines applied to the gage section of the transparent specimen. Twenty-five tests yield a Young’s modulus of 60.1 ± 3.4 GPa and a fracture strength of 364 ± 57 MPa.

Modulus, Fracture Strength, and Brittle vs. Plastic Response of the Outer Shell of Arc-grown Multi-walled Carbon Nanotubes

Abstract: The fracture strengths and elastic moduli of arc-grown multi-walled carbon nanotubes (MWCNTs) were measured by tensile loading inside of a scanning electron microscope (SEM). Eighteen tensile tests were performed on 14 MWCNTs with three of them being tested multiple times (3×, 2×, and 2×, respectively). All the MWCNTs fractured in the “sword-in-sheath” mode. The diameters of the MWCNTs were measured in a transmission electron microscope (TEM), and the outer diameter with an assumed 0.34 nm shell thickness was used to convert measured load-displacement data to stress and strain values. An unusual yielding before fracture was observed in two tensile loading experiments. The 18 outer shell fracture strength values ranged from 10 to 66 GPa, and the 18 Young's modulus values, obtained from a linear fit of the stress–strain data, ranged from 620 to 1,200 GPa, with a mean of 940 GPa. The possible influence of stress concentration at the clamps is discussed.

Statistical Analysis of the Effect of Intensity Pattern Noise on the Displacement Measurement Precision of Digital Image Correlation Using Self-correlated Images

Abstract: This paper discusses the effect of intensity pattern noise on the displacement measurement precision of digital image correlation (DIC) Through mathematical deduction, a formula is presented to estimate the displacement measurement error caused by intensity pattern noise The resulting formula synthetically reflects the effects of the variance of noise, the intensity variance and the subset size on the displacement measurement precision To verify the correctness of the resulting formula, two experiments are done The first one is a real self-correlation experiment, and aims to analyze the effect of the subset size, while the second is a numerical self-correlation experiment, and is to analyze to the effect of the different noise levels The experimental results are in good agreement with the theoretical predictions

Elastic Properties and Representative Volume Element of Polycrystalline Silicon for MEMS

Abstract: A nanoscale mechanical deformation measurement method was employed to obtain the Young’s modulus and Poisson’s ratio of polycrystalline silicon for Microelectromechanical Systems (MEMS) from different facilities, and to assess the scale at which these effective properties are valid in MEMS design. The method, based on in situ Atomic Force Microscope (AFM) imaging and Digital Image Correlation (DIC) analysis, employed 2–2.5 μm thick freestanding specimens with surface measurement areas varying between 1×2 and 5×15 μm2. The effective mechanical properties were quite invariant with respect to the fabrication facility: the Poisson’s ratio of polycrystalline silicon from the Multi-user MEMS Processes (MUMPs) and from Sandia’s Ultra planar four layer Multilevel MEMS Technology (SUMMiT-IV) was 0.22±0.02, while the elastic moduli for MUMPs and SUMMiT-IV polysilicon were 164±7 and 155±6 GPa, respectively. The AFM/DIC method was used to determine the size of the material domain whose mechanical behavior could be described by the isotropic constants. For SUMMiT polysilicon with columnar grains and 650 nm average grain size, it was found that a 10×10-μm2 specimen area, on average containing 15×15 columnar grains, was a representative volume element. However, the axial displacement fields in 4×4 or 2×2 μm2 areas could be highly inhomogeneous and the effective behavior of these specimen domains could deviate significantly from that described by isotropy. As a consequence, the isotropic material constants are applicable to MEMS components comprised of 15×15 or more grains, corresponding to specimen areas equal to 10×10 μm2 for SUMMiT and 5×5 μm2 for MUMPs, and do not provide an accurate description of the mechanics of smaller MEMS components.

Use of 3-D Digital Image Correlation to Characterize the Mechanical Behavior of a Fiber Reinforced Refractory Castable

Abstract: Refractory castables exhibit very low fracture strain levels when subjected to tension or bending. The main objective of this work is to show that 3-D digital image correlation (3-D DIC) allows such low strain levels to be measured. Compared to mechanical extensometer measurements, 3-D DIC makes it possible to reach similar strain resolution levels and to avoid the problem of position dependance related to the heterogeneous nature of the strain and to strain localization phenomena. First, the 3-D DIC method and the experimental set-up are presented. Secondly, an analysis of the 3-D DIC method is performed in order to evaluate the resolution, the standard uncertainty and the spatial resolution for both displacement and strain measurements. An optimized compromise between strain spatial resolution and standard uncertainty is reached for the configuration of the experimental bending test. Finally, the macroscopic mechanical behavior of a fiber reinforced refractory castable (FRRC) is studied using mechanical extensometry and 3-D DIC in the case of tensile and four-point bending tests. It is shown that similar results are obtained with both methods. Furthermore, in the case of bending tests on damaged castable, 3-D DIC results demonstrate the ability to determine Young’s modulus from heterogeneous strain fields better than by using classical beam deflection measurements.

Influence of Molecular Conformation on the Constitutive Response of Polyethylene: A Comparison of HDPE, UHMWPE, and PEX

Abstract: The current work presents the characterization and comparison of the mechanical response of three different industrial forms of polyethylene. Specifically, high-density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), and cross-linked polyethylene (PEX) were tested in compression as a function of temperature (−75 to 100°C) and strain-rate (10−4 to 2,600 s−1). The responses of UHMWPE and PEX are very similar, whereas HDPE exhibits some differences. The HDPE samples display a significantly higher yield stress followed by a flat flow behavior. Conversely UHMWPE and PEX both exhibit significant strain hardening after yield. The temperature and strain-rate dependence are captured by simple linear and logarithmic fits over the full range of conditions investigated. The yield behavior is presented in terms of an empirical mapping function that is extended to analytically solve for the mapping constant. The power-law dependence on strain-rate observed in some polymers is explained using this mapping function.

Cure-dependent viscoelastic poisson's ratio of epoxy

Abstract: The evolution of the lateral contraction ratio of two commercial (high and low temperature cure) epoxy resins is studied in uniaxial tension using moire interferometry The ratio of transverse to axial strains evolves from an elastic value of about 040 to a rubbery plateau value of 049 at long times Furthermore, the data indicate that the contraction ratio follows time-temperature superposition with a shift function indistinguishable from other axial viscoelastic functions The lateral contraction behavior at several cure states past gelation was measured and a model is proposed to describe the cure dependence

Radial Inertia Effects in Kolsky Bar Testing of Extra-soft Specimens

Abstract: Impact responses of extra-soft materials, such as ballistic gelatins and biological tissues, are increasingly in demand. The Kolsky bar is a widely used device to characterize high-rate behavior of materials. When a Kolsky bar is used to determine the dynamic compressive response of an extra-soft specimen, a spike-like feature often appears in the initial portion of the measured stress history. It is important to distinguish whether this spike is an experimental artifact or an intrinsic material response. In this research, we examined this phenomenon using experimental, numerical and analytical methods. The results indicate that the spike is the extra stress from specimen radial inertia during the acceleration stage of the axial deformation. Based on this understanding, remedies in both specimen geometry and loading pulse to minimize the artifact are proposed and verified, and thus capture the intrinsic dynamic behavior of the specimen material.

Experimental Techniques for the Mechanical Characterization of One-Dimensional Nanostructures

Abstract: New materials and nanostructures with superior electro-mechanical properties are emerging in the development of novel devices Engineering application of these materials and nanostructures requires accurate mechanical characterization, which in turn requires development of novel experimental techniques In this paper, we review some of the existing experimental techniques suitable to investigate the mechanics of one-dimensional (1D) nanostructures Particular emphasis is placed on techniques that allow comparison of quantities measured in the tests with predictions arising from multiscale computer simulations on a one to one basis We begin with an overview of major challenges in the mechanical characterization of 1D nanostructures, followed by a discussion of two distinct types of experimental techniques: nanoindentation/atomic force microscopy (AFM) and in-situ electron microscopy testing We highlight a recently developed in-situ transmission and scanning electron microscopy testing technique, for investigating the mechanics of thin films and 1D nanostructures, based on microelectromechanical systems (MEMS) technology We finally present the coupled field (electro and mechanical) characterization of a NEMS bistable switch in-situ a scanning electron microscope (SEM)

Measurements of Two Independent Viscoelastic Functions by Nanoindentation

Abstract: Current nanoindentation measurement techniques normally assume that one material function (such as the Poisson's function) is a constant, and measures just one material function, such as the creep compliance in shear. For materials with significant viscoelastic effects and unknown viscoelastic functions, assuming a constant for one material function is not satisfactory. Accurate measurements require simultaneously determining two independent material functions. This paper provides a method to use nanoindentation to measure both bulk and shear relaxation functions. Two different nanoindenter tips, namely Berkovich and spherical indenters, are used for nanoindentation on polymers. Any two independent viscoelastic functions, such as bulk relaxation modulus and shear relaxation modulus, have different representations in the load–displacement curves obtained with these two indenters so that the two independent viscoelastic functions can be separated and determined. Two polymers, poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) were used in nanoindentation. Nanoindentation measurements were conducted on PVAc above glass transition temperature (Tg) and on PMMA below Tg. Both shear and bulk relaxation functions determined from nanoindentation were found in a reasonably good agreement with data obtained from conventional tests, providing validation of the method presented. The new method can be applied in measurements of two independent viscoelastic functions at sub-micron scale of very small amounts of materials such as polymeric films on a substrate, heterogeneous materials such as bones, tissues, and nanocomposites.

Superelasticity in a New BioImplant Material: Ni-rich 55NiTi Alloy

Abstract: With the drive towards minimally invasive procedures, the medical industry is looking towards ‘avant-garde’ materials, with 50NiTi currently being the prime choice for many critical components/applications This paper examines a new Ni-rich NiTi alloy that exhibits superelasticity (SE) and shape memory (SM) properties Superelastic (SE) properties of 55NiTi* [all compositions are quoted in atomic% throughout the paper The reader should note the following conversions: 50NiTi (at%)≈55NiTi (wt%) and 55NiTi (at%)≈60NiTi (wt%)] are studied here as a function of heat-treatment between 400–800°C, and compared with the corresponding response of 50NiTi*, with an aim to develop and optimize thermal treatment procedures to maximize recoverable elastic strains While optimal tuning of the SE properties in 50NiTi necessitates cold working in conjunction with specific heat treatment/aging, 55NiTi does not require cold work to achieve its optimal SE behavior Moreover, it can be heat treated to produce strong, stable SE and SM response from the same ingot, with transformation temperatures being a strong function of heat treatment The main difference between the two alloys is that Ni–Ti alloys with Ni content greater than 506 at% are sensitive to heat treatment; aging in these materials leads to precipitation of several metastable phases The initial work focuses on SE properties relevant to biomedical use, such as: plateau stress, recoverable strains and strength, as a function of heat treatment and microstructure

Methods for Examining the Fatigue and Fracture Behavior of Hard Tissues

Abstract: An understanding of the fatigue and fracture behavior of hard tissues (e.g., bone and tissues of the human tooth) is critical to the maintenance of physical and oral health. Recent studies suggest that there are a number of mechanisms contributing to crack extension and crack arrest in these materials, and that they appear to be a function of moisture and age of the tissue. An understanding of these processes can provide new ideas that are relevant to the design of multi-functional engineering materials. As a result, we have adopted the use of microscopic Digital Image Correlation (DIC) to examine the mechanisms of crack growth resistance and near-tip displacement distribution for cracks in human dentin that are subjected to opening mode loads. We have also developed a special compact tension (CT) specimen that permits evaluation of crack extension within small portions of tissue under both quasi-static and fatigue loads. The specimen embodies a selected portion of hard tissue within a resin composite restorative and enables an examination of diseased tissue, or portion with specific physiology, that would otherwise be impossible to evaluate. In this paper we describe application of these experimental methods and present some recent results concerning fatigue crack growth and stable crack extension in dentin and across the dentin-enamel-junction (DEJ) of human teeth.

Capillary Adhesion Between Elastically Hard Rough Surfaces

Abstract: The adhesion versus vapor pressure (p/p s) trend between two elastically hard rough surfaces is modeled and compared with experimental results. The experimental samples were hydrophilic surface-micromachined cantilevers, in which the nanometer-scale surface roughness is on the order of the Kelvin radius. The experimental results indicated that adhesion increases exponentially from p/p s=0.3 to 0.95, with values from 1 mJ/m2 to 50 mJ/m2. Using the Kelvin equation to determine the force-displacement curves, the mechanics of a wetted rough interface are treated in two ways. First, the characteristics of a surface with rigid asperities of uniform height are derived. At low p/p s, menisci surrounding individual asperities do not interact. Beyond a transition value, [p/p s]tr, a given meniscus grows beyond the asperity it is associated with, and liquid fills the interface. Capillary adhesion in each realm is found according to the integrated work of adhesion. Second, a more general approach allowing an arbitrary height distribution of Hertzian asperities subject to capillary forces is justified and developed. To compare with experimental results, a Gaussian height distribution is first assumed but significantly underestimates the measured adhesion. This is because equilibrium is found far into the Gaussian tail, where asperities likely do not exist. It is shown that by bounding the tail to more likely limits, the measured adhesion trend is more closely followed but is still not satisfactorily matched by the model. The uniform summit height model fits the data very well with a single free parameter. These results can be rationalized if the upper and lower surfaces are geometrically correlated.

Intralaboratory Repeatability of Residual Stress Determined by the Slitting Method

Abstract: This paper presents repeated slitting method measurements of the residual stress versus depth profile through the thickness of identically prepared samples, which were made to assess repeatability of the method. Measurements were made in five 17.8 mm thick blocks cut from a single plate of 316L stainless steel which had been uniformly laser peened to induce a deep residual stress field. Typical slitting method techniques were employed with a single metallic foil strain gage on the back face of the coupon and incremental cutting by wire EDM. Measured residual stress profiles were analyzed to assess variability of residual stress as a function of depth from the surface. The average depth profile had a maximum magnitude of −668 MPa at the peened surface. The maximum variability also occurred at the surface and had a standard deviation of 15 MPa and an absolute maximum deviation of 26 MPa. Since measured residual stress exceeded yield strength of the untreated plate, microhardness versus depth profiling and elastic–plastic finite element analysis were combined to bound measurement error from inelastic deformation.

An Alternative Mechanism of Non-contact Anterior Cruciate Ligament Injury During Jump-landing: In-vitro Simulation

Abstract: In the United States, an estimated 100,000 anterior cruciate ligament (ACL) injuries occur every year. Despite decades of research, to this date, the mechanism or mechanisms of non-contact ACL injuries are not well understood. This is primarily because trials cannot be conducted on live subjects to understand the injury mechanism, and it is difficult to instrument a live human knee to measure the response of tissues during dynamic activities. In this paper, we present a dynamic knee injury simulator capable of in-vitro modeling of the ACL injury during jump-landing activity. This system was used to simulate jump-landing on cadaveric knees and to successfully test which conditions would result in isolated ACL injury. A restricted flexion of the hip (a hip that flexes minimally or not at all during landing), combined with low quadriceps and hamstring force levels during landing were found to be conducive to ACL injury. Elevated levels of quadriceps force prevented the injury from occurring even under restricted hip flexion conditions. The measured strain rates in the ACL tissue during injury causing activities were over 250%/s.

Residual Stress Analysis of Orthotropic Materials by the Through-hole Drilling Method

Abstract: The present study deals with the development and the application of the through-hole drilling method for the residual stress analysis in orthotropic materials. Through a systematic theoretical study of the stress field present on orthotropic plates with a circular hole, the relationships between the relaxed strains measured by a rectangular strain gauge rosette and the Cartesian components of the unknown residual stresses are obtained. The theoretical formulas of each influence coefficient allow the user an easy application of the method to the analysis of uniform-residual stresses on a generic homogeneous orthotropic material. Furthermore, to extend the method to the analysis of the residual stresses on orthotropic laminates, caused by initial in-plane loadings, an alternative formulation is implemented. The accuracy of the proposed method has been assessed through 3D numerical simulations and experimental tests carried out on unidirectional, cross-ply and angle-ply laminates.

Determination of Early Flow Stress for Ductile Specimens at High Strain Rates by Using a SHPB

Abstract: In a dynamic experiment to obtain the high-rate stress–strain response of a ductile specimen, it takes a finite amount of time for the strain rate in the specimen to increase from zero to a desired level. The strain in the specimen accumulates during this strain-rate ramping time. If the desired strain rate is high, the specimen may yield before the desired rate is attained. In this case, the strain rates at yielding and early plastic flow are lower than the desired value, leading to inaccurate determination of the yield strength. Through experimentation and analysis, we examined the validity and accuracy of the flow stresses for ductile materials in a split Hopkinson pressure (SHPB) bar experiment. The upper strain-rate limit for determining the dynamic yield strength of ductile materials with a SHPB is identified.

Stress and Moisture Effects on Thin Film Buckling Delamination

Abstract: Deposition processes control the properties of thin films; they can also introduce high residual stresses, which can be relieved by delamination and fracture. Tungsten films with high 1–2 GPa compressive residual stresses were sputter deposited on top of thin (below 100 nm) copper and diamond-like carbon (DLC) films. Highly stressed films store large amounts of strain energy. When the strain energy release rate exceeds the films' interfacial toughness, delamination occurs. Compressive residual stresses cause film buckling and debonding, forming open channels. Profiles of the buckling delaminations were used to calculate the films' interfacial toughness and then were compared to the adhesion results obtained from the superlayer indentation test. Tests were conducted in both dry and wet environments and a significant drop in film adhesion, up to 100 times was noticed due to the presence of moisture at the film/substrate interface.

Miniaturized Compression Test at Very High Strain Rates by Direct Impact

Abstract: A modified miniaturized version of the Direct Impact Compression Test (DICT) technique is described in this paper. The method permits determination of the rate-sensitive plastic properties of materials up to strain rate ∼105 s−1. Miniaturization of the experimental setup with specimen dimensions: diameter d S = 2.0 mm and thickness l S = 1.0 mm, Hopkinson bar diameter 5.2 mm, with application of a novel optical arrangement in measurement of specimen strain, makes possible compression tests at strain rates from ∼103 s−1 to ∼105 s−1. In order to estimate the rate sensitivity of a low-alloy construction steel, quasi-static, Split Hopkinson Pressure Bar (SHPB) and DICT tests have been performed at room temperature within the rate spectrum ranging from 5*10−4 s−1 to 5*104 s−1. Adiabatic heating and friction effects are analyzed and the final true stress versus true strain curves at different strain rates are corrected to a constant temperature and zero friction. The results have been analyzed in the form of true stress versus the logarithm of strain rate and they show two regions of a constant rate sensitivity $$ \beta = {\left( {{\Delta \sigma } \mathord{\left/ {\vphantom {{\Delta \sigma } {\log {\mathop \varepsilon \limits^ \cdot }}}} \right. \kern- ulldelimiterspace} {\log {\mathop \varepsilon \limits^ \cdot }}} \right)}_{\varepsilon } $$ : relatively low up to the strain rate threshold ∼50 s−1, and relatively high above the threshold, up to strain rate ∼4.5*104 s−1.

Measurement of the Dynamic Bulk and Shear Response of Soft Human Tissues

Abstract: Traditionally, Kolsky bars are used to study the dynamic response of hard materials in uniaxial compression, tension or torsion. We present modifications to the technique that allow loading of a soft tissue specimen in (a) hydrostatic compression and (b) simple shear. The first modification is designed to determine the pressure vs. volume behavior of each material, and thence to extract a measure of the dynamic compressibility or equivalently of the bulk modulus. The second modification is designed to develop the shear stress versus shear strain behavior for a near-simple shear experiment. The critically important questions of the proper acquisition of human tissue samples and protocols for appropriate experimentation have also been addressed. The experimental techniques and the results are discussed in detail and the results compared to finite element simulations. We present examples of the dynamic response of typical tissue simulants as well as human liver and stomach tissues.

Colour Adaptation in Three Fringe Photoelasticity

Abstract: Three fringe photoelasticity (TFP) can give the total fringe order from a single colour isochromatic fringe field by suitably comparing the colour with a calibration specimen. The fringe order evaluation can be erroneous when the materials for the calibration specimen and the application specimen are different. This is because of the colour variation between the two materials. This is conventionally handled by preparing individual calibration tables for each application. A new methodology to tune the calibration table obtained for a single material to accommodate the tint variation in TFP is proposed for the use of different specimen materials. Discontinuities in fringe order variation are smoothed using the refined TFP (RTFP) procedure. The elegance of the new methodology for solving a multi-material system is bought out by solving the problem of a bi-material Brazilian disc. The results obtained are compared with the phase shifting technique.