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Ultrahigh molecular weight polyethylene (UHMWPE) film composite materials such as Tensylon® HSBD30A are useful for ballistic applications, but quantification of their interlaminar shear behavior is necessary to inform design considerations. This study investigates a double lap shear specimen to determine mode II interlaminar shear properties. Average shear stresses calculated from measured loads and relative displacements calculated from full-field displacement data obtained via digital image correlation are used to produce an approximate traction-separation law. For a representative specimen, interfacial shear strength is calculated to be 3.18 MPa, mode II critical energy release rate is 465 J/m2, and interfacial stiffness is 36.8 MPa/mm. The double lap shear specimen is found to be appropriate for approximating these values, but the potential for complex loading states caused by non-simultaneous crack growth necessitates further investigation into alternative specimen configurations.

Bone specimens were collected from the frontal and parietal bones of 4 adult, human skulls. The microstructure was characterized using microcomputed tomography (micro- CT) at about 6-μm resolution to map the change of porosity as a function of the depth, P(d), from the inner surface nearest to the brain to the outer surface nearest to the skin. A quantifiable method was developed using the measured P(d) to objectively distinguish between the three layers of the skull: the outer table, diploe , and inner table. The thickness and average porosity of each of the layers were then calculated from the measured porosity distributions, and a Gaussian function was fit to the P(d) curves. Morphological parameters were compared between the two bone types (frontal and parietal), while accounting for skull-to-skull variability. Parietal bones generally had a larger diploe accompanied by a thinner inner table. The arrangement of the porous vesicular structure within the outer table was also obtained with micro-CT scans with longer scan times, using enhanced parameters for higher resolution and lower noise in the images. From these scans, the porous structure of the bone appeared to be randomly arranged in the transverse plane, compared to the porous structure of the human femur, which is aligned in the loading direction.

The primary objective of this research is to investigate, through fundamental experiments, the dynamic multiaxial deformation, failure and strength degradation mechanisms that govern individual ballistic fiber failure. Predicting ballistic impact performance of armor systems requires an understanding of fiber failure under complex multiaxial loading conditions. This study examines the failure behavior of ultrahigh molecular weight polyethylene (UHMWPE) Dyneema® SK76 single fibers under dynamic transverse impact as a function of varying loading rates and projectile geometry. A novel single fiber transverse impact experiment is developed by modifying the Kolsky bar to characterize failure of fibers to create the foundation for a failure model. Experiments are performed with load cells at the fiber ends and with high speed imaging for determining average stresses and strains. Post-test microscopy imaging of failure surfaces are compared to determine the dominant fiber failure modes for each experimental group.

This article investigates the failure of ultra-high molecular weight polyethylene Dyneema® SK76 single fibers widely used in protective armor applications. Indenter geometry and the associated stre...

Mechanical compression tests were performed on an economical Ti–6Al–4V alloy over a range of strain-rates and temperatures. Low rate experiments (0.001–0.1/s) were performed with a servo-hydraulic load frame and high rate experiments (1000-80,000/s) were performed with the Kolsky bar (Split Hopkinson pressure bar). Emphasis is placed on the large strain, high-rate, and high temperature behavior of the material in an effort to develop a predictive capability for adiabatic shear bands. Quasi-isothermal experiments were performed with the Kolsky bar to determine the large strain response at elevated rates, and bars with small diameters (1.59 mm and 794 µm, instrumented optically) were used to study the response at the higher strain-rates. Experiments were also conducted at temperatures ranging from 81 to 673 K. Two constitutive models are used to represent the data. The first is the Zerilli-Armstrong recovery strain model and the second is a modified Johnson–Cook model which uses the recovery strain term from the Zerilli–Armstrong model. In both cases, the recovery strain feature is critical for capturing the instability that precedes localization.

Cellular and tissue-scale indent/impact thresholds for different mechanisms of functional impairments to the brain would be the preferred method to predict head injuries, but a comprehensive understanding of the dominant possible injury mechanisms under multiaxial stress-states and rates is currently not available. Until then, skull fracture could serve as an indication of head injury. Therefore the ability to predict the initiation of skull fracture through finite element simulation can serve as an in silico tool for assessing the effectiveness of various head protection scenarios. For this objective, skull fracture initiation was represented with a microstructurally-inspired, mechanism-based (MIMB) failure surface assuming three different dominant mechanisms of skull failure: each element, with deformation and failure properties selected based on its microstructure, was allowed to fail either in tension, compression, or shear, corresponding to clinical linear, depressed or penetrating shear-plug failure (fracture), respectively. Microstructure-inspired a priori values for the initiation threshold of each mechanism, obtained previously from uniaxial and simple-shear experiments, were iterated and optimized for the predicted load-displacement to represent that of the corresponding indentation experiment. Element-level failure enabled the visualization of the evolution of fracture by different mechanisms. The final crack pattern at the time of macroscopic (clinically-identifiable) injury was compared between the simulation and experiment obtained through 3D tomography. Even though the timing was slightly different, the simulated prediction represented remarkably well the experimental crack pattern before the appearance of the catastrophic unstable fast crack in the experiment, thus validating the implemented hybrid-experimental-modeling-computational (HEMC) concept as a tool to predict skull fracture initiation.

This paper investigates the high strain rate transverse compression behavior of Kevlar® KM2 and ultra high molecular weight polyethylene Dyneema® SK76 single fibers widely used in protective components under ballistic and blast loading conditions. The micron scale fibers are compressed at strain rates in the range of 10,000–90,000 s−1 in a small (283 μm) diameter Kolsky bar with optical instrumentation. The nominal stress–strain response of single fibers exhibits nonlinear inelastic behavior under high rate transverse compression. The nonlinearity is due to both geometric and material behavior. The contact area growth at high rates is found to be smaller than at quasi-static loading leading to a stiffer material response at higher rates. The fiber material constitutive behavior is determined by removing the geometric nonlinearity due to the growing contact area. Atomic force microscopy analysis of the compressed fibers indicates less degree of fibrillation at high strain rates compared to quasi-static loading indicating that fibril properties and inter-fibrillar interactions could be strain rate dependent.

The goal of this research is to understand the effect of fiber meso/nanostructure on the macroscopic quasi-static transverse compression response of ultra-high molecular weight polyethylene (UHMWPE) Dyneema SK76 fibers. These fibers exhibit nonlinear inelastic behavior with a small elastic limit and negligible elastic recovery upon unloading. Finite element model predictions of the experiment, using a continuum nonlinear inelastic constitutive description agree reasonably well with experimental force-displacement, but under-predict the contact area. The apparent fiber cross-sectional area is found to increase up to a maximum of 1.83 times the original area at 46% nominal strain. SEM and AFM images of the meso/nanostructure of the compressed fibers indicate the apparent area growth is due to fibrillation. This fibrillation results in the deformation of a fibril network causing non-uniform fibril nesting and nucleation of new nanoscale voids between fibrils. A comparison of UHMWPE and Kevlar KM2 fiber transverse compressive response is also discussed.

Strategically located Fiber Bragg Grating (FBG) Sensors have been proposed as an in situ method to increase the signal to noise ratio (SNR) for metallic and composite components. This paper presents a systematic study that investigates the viability of FBG Sensors under high strain rate loading by initially measuring 1D-strains in a compression Hopkinson bar experiment, followed by 2D full-field strain-tensor in impact and blast experiments on plates. Specifically, high strain rates from commercialized FBG Sensors are compared to traditional resistive and semi-conductor based strain gages under various levels of 1D high strain rate loading. In the projectile-plate impact experiments, full-field back-surface strain measured using FBG Sensor arrays are compared with that measured from 3D surface Digital Image Correlation (3D-sDIC) strain measuring technique. Finally, strains in welded steel plates subjected to high explosive discharge are monitored with mounted FBG Sensors on the back surface. From this study, potential improvements in the SNR of FBG Sensors are recommended, and the survivability of these sensors under more complex, dynamic loading is evaluated.

Full thickness skull specimens were extracted from the human crania, with both the inner and outer surfaces intact. The BVF-morphology (bone volume fraction) of these specimens had been previously characterized in detail and reported, with high-resolution micro-computed tomography at ~5 μm resolution. A subset of these specimens was loaded in the direction normal to the outer surface in quasi-static compression. In contrast to many previous mechanical characterization studies of skulls, following two additional procedures were used in this study. (1) Fresh skull specimens were used, which were stored refrigerated before mechanical loading, instead of using embalmed or dried specimens. (2) Furthermore, using digital image correlation, non-contact full-field inhomogeneous strain measurements were made using the speckled specimen surfaces and the compression platens, also avoiding possible errors in strain measurements from machine compliance and due to irregularities in the loading surfaces of the specimen. The averaged far-field compressive mechanical response was obtained from these local full-field measurements on the composite bone specimens. Assuming a layered structure for the skull bone, using the local averaged full-field strain measurements of each layer, a power law was used to represent the relationship between initial mechanical response and the averaged BVF of the layers. Using the measured porosity maps of the rest of the non-compressed specimens, this relationship was used to predict the modulus-depth dependency of the skull bone and the variabilities associated with the structure. The mechanical properties and density as a function of the normalized thickness of the skull are presented for use in finite element simulations to model the skull with the desired degrees of complexities, also based on the region of action, depending on the goals of the computer simulation of the impact: either as a single homogenous layer, three-layer sandwich, multilayer heterogeneous or continuous elemental structure. In addition, a power law was derived relating the compressive failure strength and bone volume fraction (BVF) for the skull bone.

A shear-punch test (SPT) experimental method was developed to address the lack of shear deformation and failure response data for the human skull as a function of local bone microarchitecture. Improved understanding of skull deformation and fracture under varying stress-states helps implement mechanism-based, multi-axial material models for finite element analysis for optimizing protection strategies. Shear-punch coupons (N = 47 specimens) were extracted from right-parietal and frontal bones of three fresh-frozen-thawed human skulls. The specimens were kept as full through-thickness or segmented into the three skull constituent layers: the inner and outer cortical tables and the middle porous diploe. Micro-computed x-ray tomography (μCT) before and after SPT provided the bone volume fraction (BVF) as a function of depth for correlation to shear mechanisms in the punched volumes. Digital image correlation was used to track displacement of the punch above the upper die to minimize compliance error. Five full-thickness specimens were subjected to partial indentation loading to investigate the process of damage development as a function of BVF and depth. It was determined that BVF dominates the shear yield and ultimate strength of human skull bone, but the imposed uniaxial loading rate (0.001 and 0.1 s−1) did not have as strong a contribution (p = 0.181–0.806 > 0.05) for the shear yield and ultimate strength of the skull bone layer specimens. Shear yield and ultimate strength data were highly correlated to power law relationships of BVF (R2 = 0.917–0.949). Full-thickness and partial loaded SPT experiments indicate the diploe primarily dictates the shear strength of the intact structure.

This paper presents a novel experimental method for impacting microscale single fibers under dynamic multiaxial loading conditions. Experimental setup is developed by modifying a 0.25-inch diameter Hopkinson bar to directly impact fibers. Using this setup, ultrahigh molecular weight polyethylene (UHMWPE) Dyneema® SK76 single fibers, with average diameter 17 µm, are transversely impacted with indenter radii of 200 (blunt), 20 (sharp), and 2 (razor) µm at velocities of 10 and 20 m/s, corresponding to nominal strain rates of 4000–6300 s−1. Compared to high strain rate (1156 s−1) uniaxial tensile loading, significant reductions in failure strains are measured for transverse impact with blunt (34%), sharp (39%) and razor (61%) indenters. The reduction in tensile properties is attributed to strain rate and multiaxial stress-states induced by impactor geometries; while all three geometries induced transverse compression, sharp and razor induced a greater degree of transverse shear, observed by failure surfaces.

Numerical studies aimed at evaluating head injury due to externally applied loading can be made more biofidelic by incorporating nonlinear mechanism-based and microstructurally-inspired material models representing the mechanical response and fracture (failure or injury) of the human skull bone. Thus, incorporation of these mechanism-based models would increase the ability of simulations of mechanical impact to identify more realistic fracture-based injuries at clinical relevancy, such as linear (tensile), depressed (compressive), or penetration (shear). One of the challenges for accurate modeling of the mechanical response of the human skull is the intricate location dependent heterogeneous mesostructural arrangement of bone within the structure of the skull. Recently, a power-law relationship between the localized bone volume fraction (BVF) and modulus (E) within the human skull was developed based on quasi-static compression experiments. However, the parameters of the power-law were optimized and obtained using approximations which were not experimentally or computationally validated for the actual heterogeneous 3D bone structure. Here, a hybrid experimental-modeling-computational (HEMC) based concept was used to develop a microstructurally compatible detailed meso-scale finite element (FE) model of the heterogeneous microstructure of one of the human skull bone coupons previously used to derive the E-BVF relationship. Finite elements were mapped to the corresponding regions from microcomputed tomography images, and the BVF of each element was identified. Then, element-specific moduli were calculated from the E-BVF power relationship. The goal of the simulations was twofold: to assess the assumptions used to derive the E-BVF relationship from the linear regime of the experimental response, and also to model the subsequent deviation from linearity. Using the E-BVF relationship, the 3D simulation was able to match the experimentally measured global modulus to within 3%. After validating the E-BVF power law using the initial linear response, to develop and validate failure models, the following steps were completed. The subsequent deviation of the mechanical response from its initial linearity was assumed to be due to failure of elements either by compression or tension. Elemental microstructure-specific compressive and tensile failure thresholds (σf) for each element were modeled by BVF (fBV) power functional relationships of the form: [Formula: see text] MPa. The initial leading coefficients (σf,0) for compression and tension were derived from prior reported experimental work. Through incorporating element-level failure and then iterating the leading coefficients, the simulation was able to represent the nonlinearity of the stress-strain curve and its catastrophic failure in the experiment. Evolution of the measured non-uniform full-strain-fields on two surfaces of the coupon, showing the localized regions of failure, was compared between experiment and simulation, and was approximately similar, thus validating the developed HEMC procedure and failure models. The simulation methodology developed here allowed for identification of failure location within the skull coupon specimen, thereby providing a tool to predict the localized failure (fracture or injury) initiation within the human skull in FE simulations at larger length scales.

Under extreme environments, such as a blast or impact event, the human body is subjected to high-rate loading, which can result in damage such as torn tissues and broken bones. The ability to numerically simulate these events would help improve the design of protective gear by iterating different configurations of protective equipment to reduce injuries. Computer codes capable of simulating these events require accurate rate-dependent material models representing the material deformation and failure (or injury) to properly predict the response of human body during simulation. Therefore, the high-rate material response must be measured to allow for simulation of high-rate events. This study seeks to quantify the high-rate mechanical response of human femoral cortical bone for use in high fidelity human anatomical models. Cortical bone compression specimens were extracted from the longitudinal and transverse directions relative to the long axis of the femur from three male donors, ages 36, 43, and 50. The compressive behavior of the cortical bone was studied at quasi-static (0.001/s), intermediate (1/s), and dynamic (~1000/s) strain rates using a split-Hopkinson pressure bar to determine the strain rate dependency and anisotropic effect on the strength of bone. The results indicate that cortical bone is anisotropic and stronger in the longitudinal direction compared to the transverse direction. The human cortical bone compressive response was also rate dependent in both directions, demonstrating significant increase in strength with increase in strain rate. Additionally, as the strain rate increased from intermediate to dynamic, a decrease in the elongation at transverse orientation was observed, which would indicate the bone becomes more brittle.

In recent decades, traumatic brain injury has been at the forefront of public health and military injury research. Improved understanding of skull deformation and fracture are required to advance the development of numerical simulations and surrogate materials aimed at providing critical feedback for future protective equipment design. Numerical simulations of skull fracture require material properties and identification of fracture mechanisms for compressive, tensile, and shear loading; skull–bone failure mechanisms are complex and stress-state dependent. A knowledge gap exists on the mechanical response of human cranial bone subjected to pure shear loading. A shear-punch experimental method was designed to understand the shear response of human skull as a function of through-thickness for cadaveric calvaria specimens. The compliance of the shear-punch system was first tested on a polyurethane foam trabecular bone simulant and a stereolithographic additively manufactured cortical bone surrogate, and their shear strength values were compared to known values of human bone. The sandwich-like structure of human cranial bone consists of dense outer and inner tables separated by a highly porous central diploe layer. Force data obtained from the shear-punch experiments was analyzed as a function of normalized penetration depth to assess the shear strength compared to the microstructure. Preliminary results show 1% offset yield and ultimate shear strengths of human calvaria to be 17.7 ± 4.9 MPa and 25.5 ± 5.1 MPa, respectively.

The Gottingen minipig has been used as a surrogate in impact experiments designed to better understand the mechanisms by which mechanical loading induces traumatic brain injury (TBI). However, the relationship between mechanical response and structural morphology of the minipig cranium must be understood relative to the human skull in order to accurately scale any quantitative results, such as injury thresholds, from non-human TBI experiments to the human anatomy. In this study, bone specimens were dissected from the crania of adolescent Gottingen minipigs. These specimens were small cubes that contained the entire thickness of the skull. The microstructure of these skull specimens was quantified at the micron-length scale using micro-computed tomography (micro-CT). The skull was found to be highly porous near the skin-side surface and became less porous nearer the brain-side surface. The skull specimens were then loaded in quasi-static compression to obtain their mechanical response. The surface strain distribution on the specimen face was measured during loading using digital image correlation (DIC). The 2-D strain field formed a gradient of iso-strain bands along the thickness (depth) dimension from the skin-most to brain-most sides of the skull. The variation of the minipig microstructure along the thickness differed significantly from that of the adult human skull; thus the mechanical load transmission through the minipig skull is expected to be quite different from that of the human skull. The objective was to develop the methodology of relating the microstructure, as quantified by the bone volume fraction (BVF), to the mechanical response. The specimen was modeled by discretizing the depth dimension into a series of layers, which enabled the calibration of a power law relating the depth-dependent BVF to the depth-varying modulus. The relationship was used to predict moduli values for the adolescent minipig skull to provide updated, biofidelic parameters for finite element simulations at varying levels of complexity. Moreover, the methodology outlined in this paper can be applied to other skulls with different structural variations, such as the human.

Ballistic impact onto fiber-based armor systems induce high strain rate (HSR) multiaxial loading including axial tension, axial compression, transverse compression and transverse shear. Fiber failure during impact is expected to occur under multiaxial loading conditions. The transverse compressive deformation induced in the fibers during impact is significant enough to cause permanent deformation (shear cutting and compressive fibrillation) at the sub-micron length scales. However, the influence of high strain rate transverse damage from compression and/or shear on the tensile strength of fibers is not well understood. In this study, ultrahigh molecular weight polyethyelene (UHMWPE) Dyneema SK76 single fibers are compressed at HSR loading conditions in a unique small (283 μm) diameter Kolsky bar. Subsequently, the compressed fibers are subjected to axial tension at quasi-static and HSR loading to understand the influence of transverse compression.

Blunt impacts to the head can transfer energy to the skull and brain, which can cause injuries such as skull fracture, and traumatic brain injury (TBI). The presence of a cranial fracture has been shown to be associated with a higher incidence of intracranial lesions, neurological deficit, and poorer outcome. Therefore, it is important to understand the effect of blunt impact on skull deformation and fracture. In this study, human skull specimens were subjected to blunt loading at high rate in a controlled setting to quantify the deformation, loading, and fracture mechanisms that occur. The microstructure of the specimens were characterized using high-resolution CT (Northstar XRD 1620, at 25 μm resolution). After scanning, the skull specimens were loaded using a high-rate indentation technique. Digital image correlation was used to quantify the skull deformation. Here, the experimental setup and preliminary results are discussed.

Blast and impact events regularly cause damage to human tissues. Efforts to improve protective gear are made through numerical simulation of these events where human tissues are exposed to high-rate loading conditions. Accurate simulation results can only be obtained if constitutive models are used that are based on precisely carried out experimental studies. Experimental studies on bone are challenging because of the relatively brittle nature of bone as well as the importance of the bone being in a hydrated state prior to experiments to avoid changing the mechanical properties. Past studies have utilized strain gages which require a period of drying time to bond strain gages to the surface of the bone. In this study, rate dependent fracture and compressive responses of wet human femur bone are investigated with in situ quantitative visualization. The fracture properties of cortical bone are studied transverse to the longitudinal axis of the bone up to high stress intensity factor rates, and the rate dependent compressive response is investigated in both longitudinal and transverse directions. The rate dependent nature of the fracture response, and the compressive behavior of human cortical bone over a range of rates from 0.001–1000 s-1 is discussed with the aid of quantitative visualization.

An accurate understanding of fracture in human bone under complex loading scenarios is critical to predicting fracture risk. Cortical bone, or dense compact bone, is subject to complex loading due to the inherent multi-axial loading conditions, which are also influenced by the anisotropy of the microstructure. When determining critical fracture parameters, bone is traditionally idealized as isotropic. This paper presents a method to examine rate-dependent mode mixity associated with cortical bone crack initiation. Four-point bend experiments have been conducted on cortical femoral bone samples from three human donors at quasi-static (slow), intermediate, and dynamic loading rates. Digital image correlation was used to obtain full-field displacement maps at the crack tip during the experiments. An over-deterministic least squares method is presented and used to evaluate Mode I (opening) and Mode II (shear) stress intensity factors (SIF) for fracture initiation at slow (10 $$^{-2}$$ MPa-m $$^{1/2}$$ s $$^{-1}$$ ), intermediate (15 MPa-m $$^{1/2}$$ s $$^{-1}$$ ), and high ( $$4.5^{4}\,\hbox {MPa-m}^{1/2}\,\hbox {s}^{-1}$$ ) stress intensity factor rates. Results show that under dynamic loading, the critical SIF in Mode I assuming material anisotropy is approximately 50 % lower than fracture toughness assuming isotropy. Additionally, critical Mode I and II SIFs had the lowest values at the highest rate of loading examined, decreasing to one third of the values under quasi-static loading. Crack growth in the low and intermediate SIF rates appears to be Mode II dominant, and shows a transition to completely mixed-mode at the high rate of loading. These results suggest that the conventional assumption of isotropy is a conservative estimate at low and intermediate rates, but overestimates fracture strength at dynamic rates.

Ballistic impact onto fiber-based personnel protection armor systems induce multiaxial loading in the affected impact zone that includes axial tension, axial compression, transverse compression and transverse shear. The influence of the transverse compression response of ballistic fibers at high rates of loading is not well understood. In this study, high strain rate transverse compression response of aramid Kevlar KM2 and ultra-high molecular weight polyethylene (UHMWPE) Dyneema SK76 single fibers are investigated. Micron scale single fibers are transversely compressed at high loading rates in a small diameter Kolsky bar with optical interferometry to measure strains in the bars. The fibers exhibit a nonlinear inelastic behavior in transverse compression. Comparison of high rate and quasi-static response indicate a smaller contact area growth and a stiffer material response at higher rates of loading for both types of fibers.

The development of a multi-axial failure criterion for trabecular skull bone has many clinical and biological implications. This failure criterion would allow for modeling of bone under daily loading scenarios that typically are multi-axial in nature. Some yield criteria have been developed to evaluate the failure of trabecular bone, but there is a little consensus among them. To help gain deeper understanding of multi-axial failure response of trabecular skull bone, we developed 30 microstructural finite element models of porous porcine skull bone and subjected them to multi-axial displacement loading simulations that spanned three-dimensional (3D) stress and strain space. High-resolution microcomputed tomography (microCT) scans of porcine trabecular bone were obtained and used to develop the meshes used for finite element simulations. In total, 376 unique multi-axial loading cases were simulated for each of the 30 microstructure models. Then, results from the total of 11,280 simulations (approximately 135,360 central processing unit-hours) were used to develop a mathematical expression, which describes the average three-dimensional yield surface in strain space. Our results indicate that the yield strain of porcine trabecular bone under multi-axial loading is nearly isotropic and despite a spread of yielding points between the 30 different microstructures, no significant relationship between the yield strain and bone volume fraction is observed. The proposed yield equation has simple format and it can be implemented into a macroscopic model for the prediction of failure of whole bones.

Ultra-high-molecular-weight polyethylene (UHMWPE) fibers such as Dyneema and Spectra are seeing more use in lightweight armor applications due to higher tensile strength and lower density compared with aramid fibers such as Kevlar and Twaron. Numerical modeling is used to design more effective fiber-based composite armor. For accurate simulation of ballistic impacts, material response such as tensile stress-strain of the composite constituents must be studied under experimental conditions similar to ballistic events. UHMWPE fibers are difficult to grip using adhesive methods typically used for other fibers due to low surface energy. Based on previous studies, the ability to grip UHMWPE fibers using traditional adhesive methods depends on fiber diameter and is limited to smaller diameter fibers that could affect reported stress values. To avoid diameter restrictions and surface energy problems, a direct gripping method has been used to characterize Dyneema SK76 single fibers at strain rates of 0.001 s-1, 1 s-1, and 1000 s-1. In this report, the dependence of fiber diameter and gage length on failure strength is discussed as well as success rate of failures in the gage section with this gripping technique. A comparison of the tensile properties with previous studies is also explored.

Understanding Kevlar's mechanical response at strain rates relevant to impact events is essential for the development of numerical simulations of impact events. The strength and orientation of the individual filaments may become compromised as a result of crimping, weaving, or finishing processes required to weave the Kevlar yarn into the fabric used for protective equipment. To elucidate and quantify any damage to the fibers as a result of the weaving or post treatment finishing process, single fibers were extracted from the warp and weft directions of plain woven, hydrophobically treated Kevlar cloth. The strength of these fibers was measured over a wide range of strain rates and compared with fibers extracted from an unwoven yarn. The tensile response was also measured from single fibers subjected to varying levels of shear strain. The tensile strength of the fibers was evaluated at 0.001 s−1, 1 s−1, and approximately 1000 s−1 using a Bose Electroforce test setup and a Hopkinson tension bar modified for fiber experiments. A wide range of gage lengths was investigated to find the effect of defect distribution on the tensile strength of the woven fibers. The results show that fibers taken from the weft direction of the woven fabric decreased in strength 3%–8% compared with the unwoven fiber. The warp fibers were a minimum of 20% weaker than unwoven and weft fibers at all loading rates. Twisted fibers retained 93% of untwisted tensile strength up to a shear strain of about 0.10–0.15. Measured Young's modulus as a function of strain rate and tensile strength as a function of gage length in relation to defect distribution are also presented in this paper.

This experimental study systematically investigates the dynamic response of bovine brain tissue under uniaxial strain and pure shear loading conditions. The combination of such stress states and strain rates are representative in the brain under blast loading in which both bulk and shear deformation may be involved. The dynamic uniaxial strain experiments (bulk response) were conducted on a modified Kolsky compression bar with an aluminum confinement collar. Bovine brain samples were sealed in the collar and loaded by the Kolsky bars so that the specimen only deformed in the axial direction. The resultant stress-strain curves reflected the pressure–volume relations from which the bulk modulus were obtained. The dynamic shear response was determined by using a recently developed Kolsky torsion bar technique for tissue materials characterization. Quasi-static torsion experiments were also conducted on an MTS system to construct a full set of shear stress-strain curves over a wide range of shear strain rates (0.01–700 s−1). The experimental results show that the dynamic bulk modulus of brain is in the range of 1.68–2.33 GPa which is close to the low rate values reported in the literature, while the shear responses show significant rate sensitivity over the tested strain rate range.

This work characterizes the fracture response of a composite material designed to mimic the response of human cortical bone. We have identified additive manufacturing, more generally known as 3-D printing, as a means of reproducing the curvature, variation in thickness, and gradient in porosity characteristic of the human bone between the cortical and trabecular regions. As the base material for developing bone surrogates via additive manufacturing, we evaluate a photocurable polymer with a high loading of ceramic particulate reinforcement that is compatible with stereolithographic additive (SLA) manufacturing. Specimens were printed in two orientations to measure fracture response perpendicular and parallel to the direction of deposition of the layer-by-layer manufacturing process. Mode I fracture behavior of the material was measured in four point bending configuration at high rate via modified split Hopkinson pressure bar for both orientations. In this paper, the fracture behavior of the bone simulant are presented and are compared to the mode I fracture behavior of human cortical bone perpendicular to the long axis of the human femur characterized under the same conditions.

This work reports on the experimental quasi static transverse compression response of ultra-high molecular weight polyethylene (UHMWPE) Dyneema SK76 single fibers. The experimental nominal stress-strain response of single fibers exhibits nonlinear inelastic behavior under transverse compression with negligible strain recovery during unloading. Scanning electron microscopy (SEM) reveals the presence of significant voids along the length of the virgin and compressed fibers. The inelastic behavior is attributed to the microstructural damage within the fiber. The compressed fiber cross sectional area is found to increase to a maximum of 1.83 times the original area at 46 % applied nominal strains. The true stress strain behavior is determined by removing the geometric nonlinearity due to the growing contact area. The transverse compression experiments serve as validation experiments for fibril-length scale models.

The Gottingen miniature pig is a useful surrogate to understand mechanisms of traumatic brain injury (TBI) in the human. However, the mechanical response of the minipig skull has not been previously reported. In this study, cranial samples were extracted from the skulls of adolescent minipigs (six months of age, average weight of 13.8 kg). The microstructure was first characterized using high-resolution μCT. A highly gradient structure was observed, with the bone volume fraction (BVF) almost doubling in the through-thickness (depth) dimension. These specimens were then mechanically loaded in quasi-static compression. The surface strain distribution along the loading direction was measured during the experiments using digital image correlation (DIC). Depth-dependent moduli were derived from the measured DIC strains rather than machine displacement, due to the large gradient in morphology. An elasticity-morphology relationship from literature was extended to represent the modulus variation in the functionally gradient skull structure (BVF), by calibrating the relationship with the experimentally derived local moduli. The model enables the prediction of local moduli based solely on the morphological parameter BVF measured with μCT, and also provided an estimation of the modulus of the bony phase of the cranium.

The failure behavior of cross-linked polymer epoxies with different glass transition temperatures (Tg) was investigated under Mode I fracture at high loading rate using a novel experimental method with in situ observation of the fracture process. By varying the monomer choices, the properties of the epoxies can be tailored to achieve greater resistance to cracking and higher impact toughness. For these experiments, a unique four-point bending specimen was used. High rate experiments were conducted on a modified split Hopkinson pressure bar with pulse-shaping. High speed digital imaging was used to visualize failure initiation. The images were also used with digital image correlation to optically measure the crack opening displacement and crack propagation velocity. The experimental results were used to calculate the energy required to initiate fracture at high loading rate. The results indicate that the critical energy required to initiate fracture at high loading rate was higher for epoxies with lower Tg values, up to an optimum Tg. This dependence of critical energy on the Tg of the epoxy was similar to that which has been previously measured for the epoxy’s impact resistance. In this paper, the experimental methods and results are discussed.

During a ballistic impact, Ultra High Molecular Weight Polyethylene (UHMWPE) fibers are subjected to high temperatures and high strain-rates. Their tensile strength increases with increasing strain-rate and decreases with increasing temperature. To understand the impact of both factors simultaneously, a single fiber heater has been fabricated to heat UHMWPE fibers up to the melting temperature (~148 °C) to measure the change in mechanical properties as a function of temperature and strain-rate. Custom grips have been fabricated for use with the single fiber heater and performed well across all strain rates and temperatures in this study. 251 tensile tests have been conducted on 10-mm gage length UHMWPE single fibers at temperature-strain-rate combinations spanning five strain-rates between 10−3and 550 s−1 and 11 temperatures from 20 to 148 °C. A non-failure boundary is created by temperature-strain-rate combinations where fibers can be strained to 25 % without mechanically failing. This occurs at 75 °C for 10−3 s−1, 100 °C for 10−2 s−1, 130 °C for 10−1 s−1, 148 °C for 100 s−1, and fail regardless of temperature at 550 s−1. It is estimated that for similar mechanical response, an increase in temperature of 25–30 °C is equivalent to lowering the strain-rate by one decade for strain-rates between 10−3 and 10−1 s−1. At 550 s−1 strain-rate, there was minor change in the strain-to-failure from 20 to 145 °C indicating strain-rate is the dominant factor.

Modeling and simulation of traumatic brain injury (TBI) resulted from collision or blast loading requires characterization of mechanical response over a wide range of loading rates under valid testing conditions. In this study, mechanical response of fresh bovine brain tissue was studied using the two modified Kolsky bar techniques. Radial deformation behavior of annular specimens, which are typically used to characterize the dynamic uniaxial compressive response of biological tissues, was examined using a modified Kolsky bar and a high speed camera to collect images while the specimen deforms at an axial strain rate of 2000s(-1). The high-speed images revealed inhomogeneous specimen deformation possibly brought about by radial inertia and causing a multi-axial stress state. To acquire valid stress-strain results that can be used to produce constitutive behavior of the soft materials, a novel torsion technique was developed to obtain pure shear response at dynamic loading rates. Experimental results show clear differences in the material response using the two methods. These results indicate that the previously demonstrated annular specimen geometry aimed at reducing inertia induced stress components for high rate soft materials uniaxial-compressive testing may still possess a significant component of radial inertia induced radial stress which consequently caused the observed inhomogeneous deformation in brain tissue test samples. Language: en

Uniaxial tensile experiments were performed on pig skin to investigate the tensile stress–strain response at both quasi-static and dynamic rates of deformation. A Kolsky tension bar, also called a split Hopkinson tension bar (SHTB), was modified to conduct the dynamic experiments. Semiconductor strain gages were used to measure the low levels of the transmitted signal from pig skin. A pulse shaper technique was used for generating a suitable incident pulse to ensure stress equilibrium and approximate constant strain rate in the specimen of a thin skin sheet wrapped around the ends of the bars for minimizing radial inertia. In order to investigate the strain-rate effect over a wide range of strain rates, quasi-static tests were also performed. The experimental results show that pig skin exhibits rate-sensitive, orthotropic, and non-linear behavior. The response along the spine direction is stiffer at lower rate but is less rate sensitive than the perpendicular direction. An Ogden model with two material constants is adopted to describe the rate-sensitive tensile behavior of the pig skin.

This study aims to experimentally determine the strain rate effects on the compressive stress–strain behavior of bovine liver tissues. Fresh liver tissues were used to make specimens for mechanical loading. Experiments at quasi-static strain rates were conducted at 0.01 and 0.1 s −1 . Intermediate-rate experiments were performed at 1, 10, and 100 s −1 . High strain rate (1000, 2000, and 3000 s −1 ) experiments were conducted using a Kolsky bar modified for soft material characterization. A hollow transmission bar with semi-conductor strain gages was used to sense the weak forces from the soft specimens. Quartz-crystal force transducers were used to monitor valid testing conditions on the tissue specimens. The experiment results show that the compressive stress–strain response of the liver tissue is non-linear and highly rate-sensitive, especially when the strain rate is in the Kolsky bar range. The tissue stiffens significantly with increasing strain rate. The responses from liver tissues along and perpendicular to the liver surface were consistent, indicating isotropic behavior.

Woven fabric is an increasingly important component of many defense and commercial systems, including deployable structures, restraint systems, numerous forms of protective armor, and a variety of structural applications where it serves as the reinforcement phase of composite materials. With the prevalence of these systems and the desire to explore new applications, a comprehensive, computationally efficient model for the deformation of woven fabrics is needed. However, modeling woven fabrics is difficult due, in particular, to the need to simulate the response both at the scale of the entire fabric and at the meso-level, the scale of the yarns that compose the weave. Here, we present finite elements for the simulation of the three-dimensional, high-rate deformation of woven fabric. We employ a continuum-level modeling technique that, through the use of an appropriate unit cell, captures the evolution of the mesostructure of the fabric without explicitly modeling every yarn. Displacement degrees of freedom and degrees of freedom representing the change in crimp amplitude of each yarn family fully determine the deformed geometry of the mesostructure of the fabric, which in turn provides, through the constitutive relations, the internal nodal forces. In order to verify the accuracy of the elements, instrumented ballistic impact experiments with projectile velocities of 22–550 m/s were conducted on single layers of Kevlar ® fabric. Simulations of the experiments demonstrate that the finite elements are capable of efficiently simulating large, complex structures.

Determination of dynamic tensile response of soft materials has been a challenge because of experimental difficulties. Split Hopkinson tension bar (SHTB) is a commonly used device for the characterization of high-rate tensile behavior of engineering materials. However, when the specimen is soft, it is challenging to design the necessary grips, to measure the weak transmitted signals, and for the specimen to achieve dynamic stress equilibrium. In this work, we modified the SHTB on the loading pulse, the equilibrium-monitoring system, and the specimen geometry. The results obtained using this modified device to characterize a soft rubber indicate that the specimen deforms under dynamic stress equilibrium at a nearly constant strain rate. Axial and radial inertia effects commonly encountered in dynamic characterization of soft materials are also minimized.

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.

Skull fracture can be a complex process involving various types of bone microstructure. Finite element analysis of the microscopic architecture in the bone allows for a controlled evaluation of the stress wave interactions, micro-crack growth, propagation and eventual coalescence of trabecular fracture. In this paper, the microstructure and mechanics of small-volume sections of a 6-month-old Gottingen Minipig skull were analyzed. MicroCT scans were used to generate finite element models. Various computational methods were investigated for modeling the intricacies contained within the porous microstructure of the trabecular bone. Pores were explicitly meshed in one method, whereas in the second, a mesh was created from a microCT image-informed mapping algorithm that mapped the trabecular porosity from an image stack to a solid volume mesh of the model. From here, all models were subject to uniaxial compression simulations. The output of the simulations allowed for a detailed understanding of the failure mechanics of the skull structure and allowed for comparison between the methods. Fracture typically occurs in the weakest areas where the bone is highly porous and forms a fracture surface throughout the material, which causes the bone to collapse upon itself.Copyright © 2015 by ASME

It is well known that amorphous polymers, such as polycarbonate (PC), will exhibit adiabatic heating due to the large plastic work that occurs when undergoing significant plastic deformation. However, the extent of adiabatic heating has not been investigated, with respect to strain rate, with full-field temperature measurements performed on the specimen during deformation. In this study, American Society for Testing and Materials tensile dog-bone PC specimens were used to investigate the rate-dependent mechanical response from quasi-static to intermediate (5/s) strain rates using a traditional servo-hydraulic load frame. To determine the variations in yield and post-yield response at different locations of the gage area of the specimen, digital image correlation was used to measure the full-field surface strains. In addition, an InSb thermal camera was used concurrently to measure the full-field temperature distribution in the gage area during the deformation. The material experienced nonuniform temperature increases as high as 30 C and showed significant rate-sensitive mechanical response.

The mechanical response of living tissue is important to understanding the injury-risk associated with impact events. Often, ballistic gelatin or synthetic materials are developed to serve as tissue surrogates in mechanical testing. Unfortunately, current materials are not optimal and present several experimental challenges. Bulk measurement techniques, such as compression and shear testing geometries, do not fully represent the stress states and rate of loading experienced in an actual impact event. Indentation testing induces deviatoric stress states as well as strain rates not typically available to bulk measurement equipment. In this work, a ballistic gelatin and two styrene-isoprene triblock copolymer gels are tested and compared using both macroscale and microscale measurements. A methodology is presented to conduct instrumented indentation experiments on materials with a modulus far below 1 MPa. The synthetic triblock copolymer gels were much easier to test than the ballistic gelatin. Compared to ballistic gelatin, both copolymer gels were found to have a greater degree of thermal stability. All of the materials exhibit strain-rate dependence, although the magnitude of dependence was a function of the loading rate and testing method.

The procedure for determining quasi-static fracture toughness of ceramics has been standardized. To expand the loading rate into the dynamic region, the dynamic equilibrium over the entire specimen needs to be satisfied to interpret the crack tip loading state with the far-field loading conditions. Furthermore, to determine the loading-rate effects, the loading rate at the crack tip should be nearly constant during an experiment. A new four-point bending experimental technique, based on a split Hopkinson pressure bar, has been developed to determine the dynamic fracture toughness of ceramics at high rates under valid conditions, which is demonstrated through the determination of the dynamic fracture toughness as a function of loading rate for a silicon carbide (SiC–N).

An aluminum split Hopkinson pressure bar is instrumented with quartz force transducers and used to test low impedance materials. Two transducers are used, one at the interface between the specimen and the incident bar and the other at the interface between the specimen and the transmitter bar. It is shown that the stress measured by the incident bar gage often contains a substantial acceleration component, i.e., a significant portion of the signal recorded by the gage is due to its own inertia and not representative of the stress within the sample. Attempts are made to actively compensate for this with measurements of the acceleration of the gage. This is done in three ways: (i) by differentiation of the interface velocity, as determined by a standard strain gage analysis; (ii) by a more direct determination of acceleration, using a measurement of the strain gradient within the bar; (iii) by adding a compensation crystal and mass to the gage to remove the inertial component from the output. It is shown that all three techniques successfully mitigate inertial effects.

Kevlar® KM2 fiber is a transversely isotropic material. Its tensile stress-strain response in the axial direction is linear and elastic until failure. However, the overall deformation in the transverse directions is nonlinear and nonelastic, although it can be treated linearly and elastically in infinitesimal strain range. For a linear, elastic, and transversely isotropic material, five material constants are needed to describe its stress-strain response. In this paper, stress-strain behavior obtained from experiments on a single Kevlar KM2 fiber are presented and discussed. The effects of loading rate and the influence of axial loading on transverse and transverse loading on axial stress-strain responses are also discussed.

A new experimental setup is developed to investigate the transverse mechanical properties of Kevlar ® KM2 fibers, which has been widely used in ballistic impact applications. Experimental results for large deformation reveal that the Kevlar ® KM2 fibers possess nonlinear, pseudo-elastic transverse mechanical properties. A phenomenon similar to the Mullins effect (stress softening) in rubbers exists for the Kevlar ® KM2 fibers. Large transverse deformation does not significantly reduce the longitudinal tensile load-bearing capacity of the fibers. In addition, longitudinal tensile loads stiffen the fibers' transverse nominal stress–strain behaviors at large transverse deformation. Loading rates have insignificant effects on their transverse mechanical properties even in the finite deformation range. An analytical relationship between transverse compressive force and displacement is derived at infinitesimal strain level. This relation is used to estimate the transverse elastic modulus of the Kevlar ® KM2 fibers, which is 1.34 ± 0.35 GPa.

A pulse-shaped split Hopkinson pressure bar (SHPB) was employed to determine the dynamic compressive mechanical responses and failure behaviors of a S-2 glass/SC15 composite along two perpendicular directions under valid dynamic testing conditions. The loading pulses in SHPB experiments were precisely controlled to ensure that the composite specimen deforms at a nearly constant strain rate under dynamically equilibrated stress during dynamic compression. Quasi-static experiments were conducted with an MTS and an Instron to study material rate sensitivity over a wider range. The compressive stress-strain behaviors along both directions were found to be strain-rate sensitive, but with different strain-rate sensitivities. A compressive constitutive model with strain-rate and damage effects was modified to accurately describe both quasi-static and dynamic compressive stress-strain behaviors of the composite material along the two perpendicular directions.

It is well known that amorphous polymers, such as polycarbonate (PC), will exhibit adiabatic heating due to the large plastic work that occurs when undergoing significant plastic deformation. However the extent of adiabatic heating has not been investigated with respect to strain rate, with full-field temperature measurements performed on the specimen during deformation. In this study, ASTM tensile dog bone PC specimens were used to investigate the rate-dependent mechanical response from quasi-static to intermediate (~10 s−1) strain rates using a traditional servo-hydraulic load frame. To determine the variations in yield and post-yield response at different locations of the gage area of the specimen, digital image correlation was used to measure the full-field surface strains. In addition, an InSb thermal camera was used concurrently to measure the full-field temperature distribution in the gage area during the deformation. The material experienced non-uniform heating as high as 50–70 °C, and showed significant rate sensitive mechanical response. In this paper, the experimental techniques and results are discussed.

The dynamic behavior of materials is an extremely active and increasingly relevant area of research, but with many significant scientific challenges due to the extreme time-scales of events, limited by the ability to visualize. Its applications range from traditional ballistics to the design and optimization of next-generation transportation and communication systems, critical areas of energy and the environment through geological resource recovery, and planetary and celestial formation processes. These application areas require an in-depth understanding of material or structural response as a function of dynamic loading, with distinct emphases on extreme conditions such as high strain rates, impact, blast, penetration, and shock loading. In order to successfully understand a dynamic event and its associated mechanisms, it is not only sufficient to visualize the event, but also necessary to quantify the governing parameters through in situ visualization. The transition from exclusively qualitative imaging to the addition of the quantitative aspect of visualization enhances dynamic experimental mechanics research, often by obtaining full-field information from various types of imaging such as optical, thermal, and X-ray, to name a few. Current work in this area has begun to leverage novel experimental configurations and measurement methodologies due to recent advances in spatial and/or temporal resolution of high-speed and ultra-high-speed cameras, microscopes and sensors, consequently transforming the dynamic behavior research landscape. In order to continue to embrace and articulate these advancements, and increase the number of publications inExperimental Mechanics on this relevant topic, beginning in 2014 the SEM Dynamic Behavior Technical Division has conducted three successful sessions with fourteen papers on the topic of Quantitative Visualization. Based on the interest shown by the experimental mechanics community and the importance of the concept of Quantitative Visualization, the Experimental Mechanics editor suggested the creation of this special issue. As such, the following issue includes nine papers focusing on the Quantitative Visualization of Dynamic Material Behavior. Breakthroughs in understanding dynamic material behavior is mostly limited by the availability of experimental methods to conduct investigations to understand the associated mechanisms of deformation and failure in material under dynamic loading. Novel dynamic loading methods with in situ quantitative visualization of deformation and failure help us to overcome formidable challenges and observe these underlying mechanisms and develop associated governing equations. A novel modification of a classical Kolsky (split-Hopkinson) bar system is used to conduct dynamic compression and fourpoint bend experiments on human femoral cortical bone, and is presented in the paper by Sanborn et al. In this paper, they use full-field digital image correlation (DIC) at dynamic loading rates to quantify the deformation response of small transversely isotropic specimens. Another full-field optical technique using a Kolsky bar maps light intensity from the surface of in situ shear evolution in fiberglass composites with varying resin binders, and is presented in the paper by Lamberson et al. A notably novel contribution to the biomedical field is * L. Lamberson les@drexel.edu