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Über dieses Buch

Dynamic Behavior of Materials, Volume 1 of the Proceedings of the 2016 SEM Annual Conference& Exposition on Experimental and Applied Mechanics, the first volume of ten from the Conference, brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on fundamental and applied aspects of Experimental Mechanics, including papers on:

Quantitative Visualization

Fracture & Fragmentation

Dynamic Behavior of Low Impedance Materials

Shock & Blast

Dynamic Behavior of Composites

Novel Testing Techniques

Hybrid Experimental & Computational Methods

Dynamic Behavior of Geo-materials

General Material Behavior

Inhaltsverzeichnis

Frontmatter

Chapter 1. Atomistic Simulation of a Two-Dimensional Polymer Tougher Than Graphene

A graphene/polyethylene hybrid 2D polymer, “graphylene”, exhibits a higher theoretical fracture toughness than graphene, while remaining 2× stiffer and 9× stronger than Kevlar®, per mass. Within the base structure of graphylene, the sp3-bonded polyethylene linkages provide compliance for ductile fracture, while the benzene rings contribute to high stiffness and strength. Combining stiff and compliant units to achieve enhanced mechanical performance demonstrates the potential of designing 2D materials at the molecular level.

Emil Sandoz-Rosado, Todd D. Beaudet, Radhakrishnan Balu, Eric D. Wetzel

Chapter 2. Transverse Compression Response of Ultra-High Molecular Weight Polyethylene Single Fibers

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.

Subramani Sockalingam, John W. Gillespie, Michael Keefe, Dan Casem, Tusit Weerasooriya

Chapter 3. Morphology and Mechanics of the Young Minipig Cranium

The Göttingen 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.

Stephen Alexander, C. Allan Gunnarsson, Ann Mae DiLeonardi, Tusit Weerasooriya

Chapter 4. Dynamic Characterization of Nitronic 30, 40 and 50 Series Stainless Steels by Numerical Analysis

Nitronic® stainless steels are austenitic, high-strength, corrosion-resistant products that provide higher-performance alternatives to many conventional 300 and 400 Series stainless steels. They provide excellent mechanical properties at sub-zero and elevated temperatures, impact resistance at low temperatures and, superb resistance to high- temperature oxidation while they retain good weldability and fabricability. The microstructure of Nitronic® 30, 40 and 50 series of 25 and 35 % cold worked, thermally aged at 750 C for 8 h, and annealed coupons was characterized by metallography, X-ray diffraction (XRD) and scanning electron microscopy. The coupons etching of each series needed specific attention and it resulted in an improved procedure. Nitronic® 50 steels remain non-magnetic even after extensive cold work. Nitronic 50 metallographic analysis showed the existence of cementite and enlarged grain boundaries after thermal ageing. The austenite, martensite and ε phases were recognized by XRD. The dynamic behavior of these alloys was studied by modeling and simulation using the commercial software ANSYS/AUTODYN and was compared to simulation results of projectile made of 4340 steel on known materials models. The correlation of the microstructure to the alloy properties and the dynamic behavior of these alloys will be presented in detail.

C. G. Fountzoulas, E. M. Klier, J. E. Catalano

Chapter 5. Mechanical Response of T800/F3900 Composite at Various Strain Rates

Composite materials are becoming more and more popular in the automotive and aerospace industries. Many models exist that can describe the plastic deformation and failure response of composites, however, a new orthotropic material model (MAT_213 in LS-DYNA) has been developed to improve the predictive accuracy of numerical simulations of dynamic structural events involving composite materials. A test series was devised to calibrate this model for T800/F3900, a strengthened epoxy carbon-fiber reinforced polymer. In this paper, the mechanical response of T800/F3900 is studied experimentally under different loading conditions. Tests include in plane uniaxial compression, in plane transverse tension, compression and out of plane tension, compression at various strain rates. High rate tension and compression tests are conducted using the split Hopkinson bar technique, while static tests are conducted on a hydraulic load frame. In plane and out of plane deformations are studied using Digital Image Correlation (DIC).

Peiyu Yang, Jeremy D. Seidt, Amos Gilat

Chapter 6. Full-Field Temperature and Strain Measurement in Dynamic Tension Tests on SS 304

The thermomechanical response of 304-stainless steel tension specimens to a range of strain rates from 7 × 10−3 s−1 to 2600 s − 1 was investigated. Quasi-static tests (7 × 10−3 to 0.8 s−1) were completed on a hydraulic load frame, intermediate tests (200 s−1) were performed with a modified pressure bar, and high strain rate tests (2600 s−1) on a split Hopkinson pressure bar. Full-field infrared thermography and strain measurements were recorded during each test. Infrared measurements were taken using the Telops FAST-IR 1000 infrared camera at rates up to 30,0000 frames per second. 2D-DIC was used to compute strain from simultaneously recorded visible images taken at rates up to 90,000 frames per second. Max temperatures of 290 °C were recorded in the necking region of a uniaxial specimen at a strain rate of 2600 s−1. These measurements can be used to investigate the transition of isothermal deformation to adiabatic deformation and to determine the portion of plastic work converted to heat at each strain rate.

Jarrod L. Smith, Veli-Tapani Kuokkala, Jeremy D. Seidt, Amos Gilat

Chapter 7. Dynamic Fracture Response of a Synthetic Cortical Bone Simulant

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.

Thomas Plaisted, Allan Gunnarsson, Brett Sanborn, Tusit Weerasooriya

Chapter 8. Fracture Response of Cross-Linked Epoxy Resins at High Loading Rate as a Function of Glass Transition Temperature

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.

John A. O’Neill, C. Allan Gunnarsson, Paul Moy, Kevin A. Masser, Joseph L. Lenhart, Tusit Weerasooriya

Chapter 9. Measurement of Dynamic Response Parameters of an Underdamped System

The dynamic behavior of a system is highly influenced by the energy dissipation due to its damping. To clearly understand dynamic behavior, an accurate measurement of damping is very important. One of the most commonly used methods for evaluating damping coefficients is the measurement of decay of free vibrations. The purpose of this study is to introduce a new experimental technique, a pluck test coupled with High speed Digital Image Correlation (DIC) [1] technique, to analyze the dynamic response of underdamped systems. It is further validated comparing the results from this test with those of obtained from the existing techniques such as using laser vibrometer and by using a general purpose single-axis accelerometer. Using the pluck test, an initial displacement is introduced into the system and the response with time is measured using DIC. High speed DIC employs two synchronized high speed digital cameras. Images captured at a high frame rate by high speed cameras are analyzed using 3D DIC software and therefore, a full field dynamic response of the test subject is captured The parameters such as natural frequency and damping ratio are obtained from the measured time history of the displacement using logarithmic decrement method [2]. The test vehicles used in this study to implement this technique are different liquid crystal display (LCD) modules used in commercially available smart phones and Corning® Gorilla® Glass that is used as cover glass in the same.

Charandeep Singh, Satish Chaparala, S. B. Park

Chapter 10. Dynamic Penetration and Bifurcation of a Crack at an Interface in a Transparent Bi-Layer: Effect of Impact Velocity

Dynamic fracture behavior of layered PMMA sheets is studied using transmission-mode Digital Gradient Sensing (DGS) technique. DGS is a relatively new optical method that exploits elasto-optic effects exhibited by transparent solids allowing a direct quantification of two orthogonal in-plane stress gradients simultaneously and hence crack tip parameters when used to study fracture mechanics problems. The current work builds on authors’ previous two reports on this topic. Interfacial trapping, bifurcation and mixed-mode penetration into the second layer of a dynamically growing mode-I crack in the first layer encountering a normally oriented interface in a bi-layered configuration was reported in the first report [1]. In the second, the role of the location of a weak interface relative to the initial crack tip within the given geometry of the specimen was studied and interfacial penetration vs. bifurcation mechanisms was demonstrated and analyzed [2]. The current work focuses on the effect of impact velocity and the resulting loading rate on crack branching/penetration phenomenon when the mode-I crack encounters a normally oriented interface. In this ongoing work, a select location of interface relative to the initial crack tip is re-examined by varying the impact velocity. Using DGS for visualization and quantification, fracture mechanisms associated with crack growth are explained for the bi-layered system.

Balamurugan M. Sundaram, Hareesh V. Tippur

Chapter 11. Influence of Loading Rate on Fracture Strength of Individual Sand Particles

Dynamic loading on granular materials, such as impact, blast, or projectile penetration, can impose large inter-particle forces to cause significant particle fracture within individual particles. Extensive research has been conducted at different strain rates on granular media mass, but very little has been published to investigate the influence of strain or loading rate on individual particles. Therefore, a gap in the knowledge base is present since comprehensive multi-scale modeling of granular material begins at the micro (particle) scale. In this paper, individual natural sand particles are compressed to fracture at loading rates of 0.2 mm/min, 2.25 m/s, and 14.5 m/s using quasi-static unconfined compression and unconfined mini-Kolsky bar techniques. Fracture loads are compared for various “types” of particles within the natural sand, and compared to conventional quasi-static failure definitions for particles. Particles exhibited loading rate dependence when comparing Weibull characteristic tensile strength with loading rate.

Andrew Druckrey, Dan Casem, Khalid Alshibli, Emily Huskins

Chapter 12. Arrested Compression Tests on Two Types of Sand

Silica sand and quartz sand were subjected to uniaxial loading and unloading at rates of 0.1/s and 0.0001/s. The particle size distribution was measured, and found to be significantly altered when peak strains were 10 % or greater. The loading modulus for silica sand was bilinear, and suggestive of elastic-plastic behavior, where the plastic part is due to void closure. On unloading, the modulus is close to the loading “elastic” value. Coral sand is softer than silica sand on loading, and the modulus is almost constant and much less than for silica sand. Both types of sand are recovered with a higher density than can be obtained with the starting particle mix. This suggests particles have crushed and filled some of the voids. Indeed, reduction of mean particle size is verified from post-test analysis. Coral sand, which has the greater reduction in void content, also exhibits increased particle breakup.

Eduardo Suescun-Florez, Stephan Bless, Magued Iskander, Camilo Daza

Chapter 13. Composite Plate Response to Shock Wave Loading

The mechanical response and characterization of composite materials under transient dynamic loading caused by shock or blast wave impingement is not well understood. Air blast is associated with a fast-traveling, high-pressure shock front followed by a lower pressure expansion wave. The timescales associated with the shock front are typically 103 faster than those associated with the expansion waves which follow. A new split-view Time-Resolved Stereo Digital Image Correlation system has been developed capable of measuring time dependent information of three component displacement vectors on two dimensional surfaces in a shock tube facility where transient aerodynamic loads on material specimens develop over the short time associated with the shock wave reflection time scales. To validate the techniques we embedded strain gauges in a S-2 glass epoxy 12.7 mm thick composite test specimen during in-house using vacuum assisted resin transfer molding fabrication. High-frequency-response, semiconductor, strain gauges were used in various combinations and locations in order to measure the transient strain rate during the impingement of the shock wave. In addition to strain gauges PZT transducers were also embedded which helped measure the frequency response of the test plate. In-house fabricated composite plates were tested under three different boundary conditions: clamped, quasi-simply supported, and bolted. Different displacements and deformation patterns were observed in each of these cases.

Douglas Jahnke, Vahid Azadeh Ranjbar, Yiannis Andreopoulos

Chapter 14. Initial Experimental Validation of an Eulerian Method for Modeling Composites

Impact loading response of unidirectional and plain weave fiber reinforced polymer composite materials is typically modeled using a Lagrangian method such as the finite element method. However, these methods often lack a coupled equation of state. In anisotropic materials, the pressure (equation of state) and deviatoric (strength) portions of the stress tensor are coupled: a shear stress can produce a volumetric response and a volumetric strain can produce a shear stress. High-velocity impacts of composite materials instigate a coupled pressure and stress response, so an equation of state is important in determining the non-uniform stress response of the material. A new composite model, which couples the pressure response to the constitutive response of the material, has been implemented in an Eulerian large deformation, strong shock wave, solid mechanics code. Experiments of steel projectiles perforating composite targets were numerically simulated to begin to validate this new composite model. This paper will discuss the coupled equation of state and strength response, and compare the results of these experiments with the results predicted by the model.

Christopher S. Meyer, Christopher T. Key, Bazle Z. (Gama) Haque, John W. Gillespie

Chapter 15. Characterization of High Strain Rate Dependency of 3D CFRP Materials

New composite materials are increasingly used in aviation to reduce the mass of structures. Aeronautic structures have to be designed with respect to a broad range of mechanical loadings during their operational life. These loadings are considered in the design by numerous cases, from low up to high speeds. The motivation of the presented work is to establish and characterize the high strain rate dependency of the linear behavior of composites materials. More specifically, new generations of 3D carbon/epoxy composite materials are of interest because of their high mechanical performances, which require specific experimental developments to be done. Due to the large size of their textile Unit Cell and carbon fiber high strength and stiffness, unusual dynamic test capabilities are required, which leads to revisit the test protocols, specimens definition, instrumentation and exploitation techniques. The experimental method described in this work is applied to analyze the strain rate sensitivity of the mechanical behavior of such a 3D woven composite material. The experiments are done with a servo-hydraulic testing machine (ONERA) in a strain rates range varying between 10−4 and 10 s−1. The linear mechanical behavior of the material in the warp, weft and 45∘ orientations is characterized. These tests, together with the new experimental protocol, permit to accurately reveal and measure the material behavior strain rate sensitivity, which proved to be large in the 45∘ direction.

N. Tran, J. Berthe, M. Brieu, G. Portemont, J. Schneider

Chapter 16. High-Strain Rate Compressive Behavior of a Clay Under Uniaxial Strain State

Dynamic compressive behavior of a clay obtained from Boulder, Colorado was investigated on a 24.4 m split Hopkinson pressure bar. The as-received clay was first pulverized into powders, which were subsequently dried in oven at 105 °C for 2 weeks, and then sieved to collect powers with dimensions smaller than #50 (0.3 mm) using ASTM E-11 standard sieves. The sorted clay was compressed inside a hardened steel tube, by a tungsten carbide rod at one end in contact with the incident bar, and a brass rod at the other end in contact with the transmission bar. This assembly was subjected to repetitive shaking to consolidate the soil to attain a desired bulk mass density; it is then placed between the incident steel bar and the transmission aluminum bar for dynamic compression. Through measurements of both axial and transverse responses of the cylindrical clay specimens under confinement, both volumetric and deviatoric responses were determined under the 3D stress state involved. Both dry and moist clay specimens were characterized at high strain rates. The effect of moisture content on the compressive behavior was investigated. The compressibility as a function of axial stress was determined.

Huiyang Luo, Zhenxing Hu, Tingge Xu, Hongbing Lu

Chapter 17. Mesoscopic Modelling of Ultra-High Performance Fiber Reinforced Concrete Under Dynamic Loading

For the two last decades numerous research works have been developed on the mechanical behavior of UHPFRC (Ultra-High Performance Fiber Reinforced Concrete) under quasi-static and dynamic loadings. However fibers orientation remains a major problem as during the implementation of fibers in concrete structure a random distribution and orientation of fibers is difficult to achieve. In this study four UHPFRC have been tested in dynamic tension and numerically simulated. The first concrete is made without any fibers. A random orientation of fibers is considered in the second concrete. Fibers oriented parallel or orthogonally to the loading direction (tensile loading) are considered in the two last cases. The four sets of concrete have been subjected to dynamic tensile loading by means of spalling tests. Next, a mesoscopic numerical simulation has been developed by considering a biphasic model: the concrete matrix is modelled by applying the DFH (Denoual-Forquin-Hild) anisotropic damage model to 3D finite-elements. In addition two nodes-finite elements are introduced in the 3D mesh to simulate numerically the presence of fibers by considering three orientations: fibers randomly distributed, parallel or orthogonal to the loading direction. As observed in the experiments, a small influence of fibers is observed regarding the peak-stress whereas a strong influence of fibers orientation is noted regarding the post-peak tensile response of UHPFRC.

P. Forquin, J. L. Zinszner, B. Lukic

Chapter 18. Comparison of Failure Mechanisms Due to Shock Propagation in Forged, Layered, and Additive Manufactured Titanium Alloy

The objective of this paper is to propose experimental techniques for studying the behavior of titanium alloy, Ti-6Al-4 V (Grade 5), under shock loading. Single-layer and multi-layered stacks of forged titanium, and additive manufactured (AM) titanium plates were considered. In these experiments, target materials were subjected to ballistic impact using a two-stage light gas gun. A Photonic Doppler Velocimetry (PDV) diagnostics system was used to measure free-surface velocity on the back of each target. The experimental measurements were used to describe the behavior of these materials under shock loading. In addition to velocity measurements, physical damage and spall crack formation were monitored.

Melissa Matthes, Brendan O’Toole, Mohamed Trabia, Shawoon Roy, Richard Jennings, Eric Bodenchak, Matthew Boswell, Thomas Graves, Robert Hixson, Edward Daykin, Cameron Hawkins, Zach Fussell, Austin Daykin, Michael Heika

Chapter 19. Instrumented Penetration of Metal Alloys During High-Velocity Impacts

A methodology is presented for characterizing the failure behavior of metallic targets due to high-velocity and hypervelocity impacts. Time-resolved sub-scale terminal ballistic experiments were performed at approximately 1.2 km/s to assess the feasibility of using high-speed optical imaging, photon Doppler velocimetry, and high-speed 3D digital image correlation for measuring back face deformation. Spherical copper impactors were fired into aluminum alloy targets with thickness equal to one half the impactor diameter. The approach has implications for determining the susceptibility of metallic targets to different failure modes including bulk plastic deformation resulting in tensile failure, cratering, plugging, spallation and adiabatic shear band formation. Results will be used to assist in validation of large-scale computational models used to model ballistic impact.

P. Jannotti, B. Schuster, R. Doney, T. Walter, D. Andrews

Chapter 20. Confined Underwater Implosions Using 3D Digital Image Correlation

This study experimentally investigates fluid structure interactions occurring during confined implosions using high-speed digital image correlation (DIC). Aluminum tubular specimens are placed inside a confining cylindrical structure with one end open to a pressurized environment. These specimens are exposed to hydrostatic pressure, which is slowly increased until they collapse onto themselves. The implosion event is viewed through an acrylic window on the confining structure. Full field deformation and velocities are captured with DIC and are synchronized with the pressure history. Experiments show that implosion inside a confining structure leads to extremely high oscillating water hammer effects. Both peak structural velocities and hammer impulses increase linearly with increasing collapse pressure.

Helio Matos, Sachin Gupta, James M. LeBlanc, Arun Shukla

Chapter 21. Response of Composite Cylinders Subjected to Near Field Underwater Explosions

Experiments were conducted on woven E-glass/epoxy roll wrapped cylinders in three configurations; base composite, and base composite with a thin (100 % composite thickness) and thick (200 % composite thickness) polyurea coating. Each cylinder configuration was subjected to near-field UNDEX loading at charge standoff distances of 2.5 cm and 5.1 cm inside of a large diameter test tank. Results show that the application of a polyurea coating is effective for reducing damage in the cylinders.

E. Gauch, J. LeBlanc, C. Shillings, A. Shukla

Chapter 22. Microstructural Effects on the Spall Properties of 5083 Aluminum: Equal-Channel Angular Extrusion (ECAE) Plus Cold Rolling

5083 Aluminum alloy is a light weight and strain-hardened material used in high strain-rate applications such as those experienced under shock loading. Symmetric real-time (in-situ) and end-state (recovery) plate impact experiments were conducted to study the spall response and the effects of microstructure on the spall properties of both 5083-H321 and 5083-ECAE + 30 % cold-rolled (CR) aluminum alloys shock loaded to approximately 1.46 GPa (0.2 km/s) and 2.96 GPa (0.4 km/s). The results show that mechanically processing the 5083-H321 aluminum by ECAE, followed by subsequent rolling significantly increases the Hugoniot Elastic Limit (HEL) by 78 %. However, this significant increase in HEL was at the expense of spall strength. The spall strength of the 5083-ECAE + 30 % CR aluminum dropped by 37 % and 23 % when compared to their 5083-H321 aluminum counterpart at shock stresses of approximately 1.46 GPa (0.2 km/s) and 2.96 GPa (0.4 km/s) respectively. This reduction in spall strength is attributed to the re-alignment of the manganese (Mn)-rich inter-metallic second phase particles during mechanical processing (i.e., ECAE and subsequent cold rolling) which are consequently more conducive to spallation.

C. L. Williams, T. Sano, T. R. Walter, L. J. Kecskes

Chapter 23. Experimental Study of the Dynamic Fragmentation in Transparent Ceramic Subjected to Projectile Impact

Transparent ceramics, such as monocrystalline sapphire, are very interesting materials for many applications including armor systems. Indeed, these technical ceramics present a very high compressive strength, a high Hugoniot Elastic Limit and a low density. However, due to their brittleness and low tensile strength, a fragmentation of the ceramic target occurs under ballistic impact. In the present study, the fragmentation process in a transparent ceramic subjected to projectile impact is investigated. To achieve this goal, edge-on impact tests have been performed at various impact speeds. The use of an ultra-high speed camera at a frame-rate set to one million frames per second allows visualizing “in real time” the whole fragmentation process resulting from the initiation and propagation of multiple cracks in the targets. Depending on the impact speed, cracks initiate from the impacted edge, the rear edge or from the two lateral surfaces of the targets. It is also observed that cracks propagate following specific directions related to crystallographic planes. Finally, the impact velocity and the orientation of the crystal are seen to play a major role on the final cracking pattern of the target.

P. Forquin, J. L. Zinszner

Chapter 24. Instrumented Projectiles for Dynamic Testing

Drop weight impact testers (DWIT) are commonly used in experiments for characterizing material properties and structural performance. With large mass, DWIT can provide large impact energy. However, the impact velocity involved in DWIT is usually low due to the fact that the impact velocity is proportional to the square-root of dropping height, i.e. v = (2gh)1/2. Even if the impact velocity can be increased, the wave propagation involved in impact may become entangled, resulting in distorted wave for dynamic analysis. This study investigates a mechanical technique to reduce the effect due to wave propagation and its application to dynamic impact measurements.

Guojing Li, Dahsin Liu, Dan Schleh

Chapter 25. NIST Mini-Kolsky Bar: Historical Review

The Society for Experimental Mechanics (SEM) has sponsored a series of technical paper sessions titled “Novel Testing Techniques” at their annual meetings. These sessions were organized by the Dynamic Behavior of Materials Technical Division of SEM and started in 2008. One of the novel techniques that we first learned about by attending SEM was the use of a small-size Kolsky bar system especially designed for the testing of polymer-single fibers. The Mini-Kolsky Bar was added to the National Institute of Standards and Technology (NIST) Kolsky Bar Laboratory based, in part, on the work presented at SEM conferences. A number of informal discussions at the annual SEM conferences added to the understanding and design details as we constructed our first small tension Kolsky bar. Subsequent developments of the NIST Mini-Kolsky bar, including improved gripping techniques were presented and discussed at SEM conferences. This paper reviews some of the work presented in the SEM’s Novel Testing Techniques sessions and discusses the history of additional follow-on work precipitated by the original papers.

R. L. Rhorer, J. H. Kim, S. P. Mates

Chapter 26. A General Approach to Evaluate the Dynamic Fracture Toughness of Materials

In this paper a combined experimental and numerical approach is proposed to evaluate the dynamic fracture toughness of materials. A circular tube specimen, made of Aluminum alloy, 7050-T7651, having a spiral crack on its outer surface is used to demonstrate the technique. A torsional Hopkinson bar is used to generate a dynamic torsion pulse, which in turn creates predominantly a tensile load along the crack line of the specimen. The torque applied on the sample is obtained from strain gages attached on the bars using one dimensional wave equation. Commercial FE package, ABAQUS, is used to simulate the dynamic fracture parameters. In this case the subspace projection method and standard implicit integration in ABAQUS with time increment are used, assuming the system is linear. The angle of twist associated to the torque measured on the specimen is used as input in the model. The dynamic stress intensity factor is determined using minimum strain energy theory. Digital image correlation is used to measure the deformation field near the crack tip. The measured strain/displacement fields are used to determine the exact time at which the crack initiated. The result show that, all the three classical modes of fracture are existing, but mode I is at least one order magnitude higher than the others. Also the dynamic SIF of the Aluminum alloy 7075-T6571 is higher than the quasi-static SIF (i.e KIcd=1.36KIc), The value obtained in this experiment is in well agreement with the values documented in the literature.

Ali Fahad Fahem, Addis Kidane

Chapter 27. Which One Has More Influence on Fracture Strength of Ceramics: Pressure or Strain Rate?

Both strain rate and confinement pressure have been known to have a strong influence on the failure strength of ceramics. However, the debate about which one has more influence has never been resolved. This manuscript aims to conclusively prove that confinement pressure has significantly higher influence than strain rate on the compressive strength of a ceramic. Normalized shear stress versus hydrostatic pressure plot on a variety of ceramics shows that the quasi-static and dynamic strength of brittle solids is a strong function of applied pressure and not the strain rate. The plots also revealed that despite the differences in material properties, test methods, and strain rate the data on failure strength fall in a narrow range and therefore a unified model, that extends the traditional Mohr-Coulomb criteria by adding an exponential term, can capture the overall deformation behavior of structural ceramics at high pressures.

M. Shafiq, G. Subhash

Chapter 28. Dynamic Strength and Fragmentation Experiments on Brittle Materials Using Theta-Specimens

Characterization of the strength and fragmentation response of brittle materials poses unique challenges related to specimen gripping and alignment. These challenges are often exacerbated when the characterization is to be conducted at elevated strain rates. Tensile strength of brittle samples are often characterized using the Brazilian disk testing geometry. While this ameliorates issues related to specimen alignment, the stress field in the specimen is not uniform, complicating the analysis of the test results. The theta specimen geometry was designed specifically to provide a uniform state of uniaxial stress in the specimen gage section when the exterior of the sample is subjected to compressive loading. Here we evaluate the use of the theta specimen geometry with a compressive Kolsky bar to measure the dynamic tensile strength and fragmentation response of a brittle polymer, Poly(Methyl methacrylate). Finite element simulations are used to investigate the effect of geometry and loading pulse shape on the ability to establish a state of uniaxial stress in the gage section. Particular attention is given the excitation of lateral vibrations in the gage section, which would perturb the desired uniaxial stress state.

Jamie Kimberley, Antonio Garcia

Chapter 29. DTEM In Situ Mechanical Testing: Defects Motion at High Strain Rates

Defect nucleation and motion during high strain rate experiments has not been observed in situ at the nanoscale in metals. However, imaging dislocation and twin nucleation and propagation will enhance our understanding and ability to predict dynamic behavior and spall strength. In the experiments described here we use the Dynamic TEM at the Lawrence Livermore National Laboratory which is capable of recording pictures with a 20-ns time resolution in movie mode (a short multi-frames experiment), and we developed a new TEM holder capable of deforming samples at strain rates ranging from quasistatic to 104 s−1. The holder uses two piezoelectric actuators that bend rapidly to load samples and TEM specimens with small gauge sections to obtain high strain rates. The TEM specimens and their narrow gauge sections are machined from bulk specimens using a femtosecond laser. The 50-μm wide gauge sections are ion milled to create electron transparent areas. We present high strain rate in situ mechanical test results for copper specimens.

Thomas Voisin, Michael D. Grapes, Yong Zhang, Nicholas J. Lorenzo, Jonathan P. Ligda, Brian E. Schuster, Melissa K. Santala, Tian Li, Geoffrey H. Campbell, Timothy P. Weihs

Chapter 30. High-Strain-Rate Deformation of Ti-6Al-4V Through Compression Kolsky Bar at High Temperatures

In this paper, we present our first results from the study of the constitutive response of a popular Titanium alloy, Ti-6Al-4V, using a variation of the compression Kolsky Bar technique that employs electrical pulses to achieve high temperatures. Experiments are conducted at temperatures ranging from room temperature to 1000 °C at a strain rate of about 2200 s−1 and a heating rate of about 1500 °C/s. The dynamic stress-strain results demonstrate significant thermal softening in the alloy that could be described by Johnson-Cook equation with m = 0.8 up to 650 °C. Above 650 °C the rate of change in the flow stresses was faster, which is attributed to allotropic transformation that results in a change in the phase fractions of the hcp and bcc phases present in the alloy. Evidence of transformation is observed in the microstructure of post-compression specimens, which showed an acicular morphology formed from the high temperature bcc phase on quenching.

S. Gangireddy, S. P. Mates

Chapter 31. Parametric Study of the Formation of Cone Cracks in Brittle Materials

Brittle materials such as ceramics and oxide glasses are widely employed because they possess a variety of useful properties including hardness, strength, wear resistance and/or transparency. Normal and oblique impacts of spherical projectiles on brittle materials are similar to the classical Hertzian sphere indentation problem, yet different in significant ways. In the case of oblique impacts, these differences can result in the formation of unique damage patterns, including cone cracks, which are not axisymmetric, and they are distinct from partial cone cracks formed by sliding indentation. Some selected results of oblique sphere impacts on brittle targets are shown and discussed, identifying unique features. A parametric study of the factors contributing to the differences between normal and oblique impacts is reported. The effects of normal and tangential velocity, friction, and softening are investigated as factors influencing the peak principal stresses produced by oblique impacts. Friction and projectile softening were found to have a significant impact on peak maximum principal stress values. An obliquely impacting projectile’s lateral motion and its interaction with the sides of the cone crack were also found to have a significant effect on the stress field and the shape of the resulting cone crack.

Brady Aydelotte, Phillip Jannotti, Mark Andrews, Brian Schuster

Chapter 32. Shockless Characterization of Ceramics

The dynamic response of brittle materials like ceramics is usually studied by means of flyer plate impact experiments. In this work, original tests has been carried out on several ceramics (alumina and silicon carbides) using a high pulsed power generator named GEPI. This electromagnetic device allows obtaining a ramp loading, spread over 500 ns. This particularity is a great advantage to get data on brittle fragmentation through spalling tests in which strain-rate can be accurately determined. Several samples have been recovered partially damaged, giving an interesting insight into the fragmentation process. The ramp loading has also been used to study the compressive response of ceramics thanks to lagrangian analysis. Data have been gathered up to more than 15 GPa on a silicon carbide.

J. L. Zinszner, B. Erzar, P. Forquin

Chapter 33. Dynamic Hyper Elastic Behavior of Compression Shock Loaded Vibration Dampers

A variety of rubber dampers are available, these having been designed over the years through approximate methods of analyses and based on the experience drawn from their operation over the years. Behavior of rubbers, polymers and elastomers is highly non-linear posing difficulties in the analyses. Although the behavior of rubber blends have been researched in the past decades, its behavior in high strain range has attracted the attention of researchers only in the recent past. In the present work, the characterization of natural rubber and high performance SB rubber under severe dynamic compression loading is carried out based on the material response curves provided by recent researchers using material models. With a view to examining the feasibility of using these rubber formulations, these material models are then applied to the case of dynamically compression-loaded dampers in the high strain rate regimes. The discrepancies in the results obtained by utilizing the different material models are discussed in detail.

V. B. S. Rajendra Prasad, G. Venkata Rao

Chapter 34. Specimen Size Effect on Stress-Strain Response of Foams Under Direct-Impact

Stress-strain response of rigid closed-cell polymeric foam under direct impact loading conditions is investigated, focusing on the specimen size effects. Cylindrical specimens with two different length-to-diameter ratios are impacted at different projectile velocities. Stereovision high speed photography in conjunction with 3D digital image correlation is utilized to study the full-field deformation of specimens under impact loading. A non-parametric analysis is then conducted to extract the local stress-strain curves within specimens. The proposed analytical method takes into account the concurrent influences of inertia stresses and material compressibility. The inertia stress within the area of interest is evaluated using the full-field distributions of acceleration and density. The calculated inertia stress is then superimposed with the stress measured at the boundary to enable the determination of full-field stress distribution over the entire gauge area. The results obtained in this work confirm that the effects of inertia stresses become more pronounced as specimen length-to-diameter ratio increases; whereas the degree of strain and strain rate variability is also elevated.

Behrad Koohbor, Addis Kidane, Wei-Yang Lu, Ronak Patel

Chapter 35. Texture Evolution of a Fine-Grained Mg Alloy at Dynamic Strain Rates

AMX602 (Mg-6%Al-0.5%Mn-2%Ca) is a high strength Mg alloy that was manufactured by the spinning water atomization process (SWAP) and extruded into a plate geometry. The processing produces an alloy with a weak rolling texture (for Mg) and grains between 0.5 and 5 μm. Quasi-static and dynamic compression experiments were carried out to probe the material’s mechanical behavior in the three processing directions. The tested plate showed deformation mechanism induced anisotropy consistent with what has been observed for other Mg alloys. The texture evolution was measured after loading in the three directions using X-ray diffraction at the Cornell High Energy Synchrotron Source (CHESS). A computationally efficient crystal plasticity model that demarcates twinning, basal slip, and non-basal slip mechanisms was utilized to predict the texture evolution and compared to the experimental texture. The model was able to predict the reorientation of grains associated with twin dominated yielding in the extrusion and transverse directions, and strengthening of the texture associated with slip dominated deformation in the normal direction.

Christopher S. Meredith, Jeffrey T. Lloyd

Chapter 36. Failure Processes Governing High Rate Impact Resistance of Epoxy Resins Filled with Core Shell Rubber Nanoparticles

Epoxy resins are classically toughened by rubber additives, but the effectiveness of rubber toughening tends to diminish with increasing strain rate, decreasing temperature, and decreasing matrix ductility. In this study we demonstrate that low loadings of 100–200 nm core-shell rubber (CSR) particulate additives can improve high strain rate (104–105 s−1) impact resistance by nearly 200 % for epoxy resins with glass transition temperatures T g in a range between 60 and 110 °C, without large reductions in T g or stiffness. Size and surface chemistry of the CSR particles influence the ballistic response, with 200 nm diameter, weakly bound, poorly dispersed CSR particles providing the greatest toughening performance at low filler loadings and high rates. Impact resistance for a systematic series of CSR modified epoxies covers a transition from brittle to tough behavior, where the failure mechanism changes with effective fracture resistance. For brittle resins, failure is dominated by initiation of Hertzian cone fracture which depends strongly on fracture toughness K IC , while for tough resins, failure is dominated by plastic yield at the impact site and is independent of fracture toughness above a minimum K IC value of approximately 1.2–1.5 MPa-m1/2. Interestingly, quasistatic mechanical properties are reasonably effective qualitative predictors of high rate impact resistance, suggesting that the toughening mechanisms of CSR particles are similar over the rates studied here. The insights gained from this study are valuable for design of next generation adhesives, polymers, and polymer composite matrices for lightweight protective applications.

Erich D. Bain, Daniel B. Knorr, Adam D. Richardson, Kevin A. Masser, Jian Yu, Joseph L. Lenhart

Chapter 37. Ballistic Response of Polydicyclopentadiene vs. Epoxy Resins and Effects of Crosslinking

The ballistic performance of polydicyclopentadiene (pDCPD) was investigated and compared to two epoxy resins that a have similar glass transition temperature (Tg) to pDCPD. The ballistic performance of these materials (at an effective stain rate of 104–105 s−1) was characterized by determining the kinetic energy of the projectile where there is a 50 % probability that the projectile will penetrate a witness foil behind the sample (KE50). The ballistic performance of pDCPD showed a 300–400 % improvement over the structural epoxy resins. Typical, highly crosslinked epoxy networks become brittle at low temperatures, but pDCPD has a superior ballistic performance over a broad temperature range from (−55 to 75 °C), despite having a glass transition temperature of 142 °C, which characteristic of structural resins. pDCPD also exhibited a room temperature glassy storage modulus of 1.7 GPa, making pDCPD a potential structural resin that can overcome the structural vs. energy dissipation trade-off that commonly exists with some conventional crosslinked polymers. Quasi-static measurements of pDCPD when compared to epoxy resins suggested that the performance of pDCPD relates to higher fracture toughness and lower yield stress relative to typical epoxies, while molecular dynamics simulations comparing pDCPD to epoxy resins suggest that the performance of pDCPD is due to the lack of strong non-covalent interactions and the facile formation of nanoscale voids.

Tyler R. Long, Daniel B. Knorr, Kevin A. Masser, Robert M. Elder, Timothy W. Sirk, Mark D. Hindenlang, Jian H. Yu, Adam D. Richardson, Steven E. Boyd, William A. Spurgeon, Joseph L. Lenhart
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