Dynamic Behavior of Materials, Volume 1

River Rapids Conference Proceedings of the Society for Experimental Mechanics Series Dynamic Behavior of Materials, Volume 1 Tom Proulx Proceedings of the 2011 Annual Conference on Experimental and Applied Mechanics River Publishers

Conference Proceedings of the Society for Experimental Mechanics Series

River Publishers Tom Proulx Editor and Applied Mechanics Dynamic Behavior of Materials, Volume 1 Proceedings of the 2011 Annual Conference on Experimental

Published, sold and distributed by: River Publishers Broagervej 10 9260 Gistrup Denmark www.riverpublishers.com ISBN 978-87-7004-853-8 (eBook) Conference Proceedings of the Society for Experimental Mechanics An imprint of River Publishers © The Society for Experimental Mechanics, Inc. 2011 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, or reproduction in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Preface Dynamic Behavior of Materials represents one of eight volumes of technical papers presented at the Society for Experimental Mechanics Annual Conference & Exposition on Experimental and Applied Mechanics, held at Uncasville, Connecticut, June 13-16, 2011. The full set of proceedings also includes volumes on Mechanics of Biological Systems and Materials, Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials, MEMS and Nanotechnology; Optical Measurements, Modeling and, Metrology; Experimental and Applied Mechanics, Thermomechanics and Infra-Red Imaging, and Engineering Applications of Residual Stress. Each collection presents early findings from experimental and computational investigations on an important area within Experimental Mechanics. The Dynamic Behavior of Materials conference track was organized by: Vijay Chalivendra, University of Massachusetts Dartmouth; Bo Song, Sandia National Laboratories; Daniel Casem, U.S. Army Research Laboratory This Volume represents an ever growing area of broad interest to the SEM community, as evidenced by the increased number of papers and attendance in recent years. This track was initiated in 2005 and reflects our efforts to bring together researchers interested in the dynamic behavior of materials and structures, and provide a forum to facilitate technical interaction and exchange. The Sessions within this track are organized to cover the wide range of experimental research being conducted in this area by scientists around the world. The following general technical research areas are included:  Composite Materials  Dynamic Failure and Fracture  Dynamic Materials Response  Novel Testing Techniques  Low Impedance Materials

vi  Metallic Materials  Response of Brittle Materials  Shock and Blast Loading  Optical Techniques for Imaging High Strain Rate Material Response  Simulation & Modeling of Dynamic Response & Failure  Dynamic Response of Transparent Materials The contributed papers span numerous technical divisions within SEM. It is our hope that these topics will be of interest to the dynamic behavior of materials community as well as the traditional mechanics of materials community. The track organizers thank the authors, presenters, organizers and session chairs for their participation and contribution to this track. We are grateful to the SEM TD chairs who cosponsored and organized sessions in this track (e.g., Composite Materials, Optical Techniques for Imaging High Strain Rate Events). The SEM support staff is also acknowledged for their devoted efforts in accommodating the large number of submissions this year. The opinions expressed herein are those of the individual authors and not necessarily those of the Society for Experimental Mechanics, Inc. Bethel, Connecticut Dr. Thomas Proulx Society for Experimental Mechanics, Inc

Contents 1 Punch Response of Gels at Different Loading Rates 1 2 A Kolsky Torsion Bar Technique for Characterization of Dynamic Shear Response of Soft Materials 11 X. Nie, W. Chen, R. Prabhu, J.M. Caruthers, Purdue University; T. Weerasooriya, U.S. Army Research Laboratory 3 Loading Rate Effect on the Tensile Failure of Concrete and Its Constituents Using Diametrical Compression and Direct Tension 13 S. Weckert, Defence Science and Technology Organisation; T. Weerasooriya, C.A. Gunnarsson, U.S. Army Research Laboratory 4 Influence of Strain-rate and Confining Pressure on the Shear Strength of Concrete 29 5 Dynamic Tensile Properties of Steel Fiber Reinforced Concrete 37 R. Chen, National University of Defense Technology; Y. Liu, X. Guo, Beijing Institute of Technology; K. Xia, University of Toronto; F. Lu, National University of Defense Technology 6 Effect of Liquid Environment on Dynamic Constitutive Response of Reinforced Gels 43 S. Padamati, V.B. Chalivendra, A. Agrawal, P.D. Calvert, University of Massachusetts Dartmouth 7 Ballistic Gelatin Characterization and Constitutive Modeling 51 D.S. Cronin, University of Waterloo 8 Strain Rate Response of Cross-linked Polymer Epoxies Under Uni-axial Compression 57 S. Whittie, P. Moy, A. Schoch, J. Lenhart, T. Weerasooriya, U.S. Army Research Laboratory 9 Strength and Failure Energy for Adhesive Interfaces as a Function of Loading Rate 67 Purdue University 10 77 U.H. Bankar, A. Rajesh, P. Venkitanarayanan, Indian Institute of Technology Kanpur 11 Stress Variations and Particle Movements During Penetration Into Granular Materials 85 H. Park, W.W. Chen, Purdue University 12 Sand Particle Breakage Under High-pressure and High-rate Loading 93 Md.E. Kabir, W. Chen, Purdue University M. Foster, P. Moy, R. Mrozek, J. Lenhart, T. Weerasooriya, U.S. Army Research Laboratory T. Weerasooriya, C.A. Gunnarsson, R. Jensen, U.S. Army Research Laboratory; W. Chen, P. Forquin, Université Paul Verlaine-Metz Fracture in Layered Plates Having Property Mismatch Across the Crack Front

viii 13 Experimental and Numerical Study of Wave Propagation in Granular Media 95 14 Communication of Stresses by Chains of Grains in High-Speed Particulate Media Impacts 99 W.L. Cooper, Air Force Research Laboratory 15 Effects of Thermal Treated on the Dynamic Facture Properties Using a Semi-circular Bend Technique 109 T.B. Yin, University of Toronto/Central South University; X.B. Li, Central South University; K.W. Xia, S. Huang, University of Toronto 16 Networks (t-IPNs) 117 K.C. Jajam, S.A. Bird, M.L. Auad, H.V. Tippur, Auburn University 17 Dynamic Ring-on-Ring Equibiaxial Flexural Strength of Borosilicate Glass 123 18 125 Purdue University 19 135 J.L. Li, National University of Defense and Technology/Chinese Academy of Engineering and Physics; F.Y. Lu, R. Chen, J.G. Qin, P.D. Zhao, L.G. Lan, S.M. Jing, National University of Defense and Technology 20 Dynamic Compressive Properties of A PBX Analog as a Function of Temperature and Strain Rate 141 J. Qin, Y. Lin, F. Lu, National University of Defense Technology; Zh. Zhou, Beijing Institute of Technology; R. Chen, J. Li, National University of Defense Technology 21 Dynamic Response of Shock Loaded Architectural Glass Panels 147 P. Kumar, A. Shukla, University of Rhode Island 22 A Dynamic Punch Method to Quantify the Dynamic Shear Strength of Brittle Solids 157 S. Huang, K. Xia, F. Dai, University of Toronto 23 A Sensored Projectile Impact on a Composite Sandwich Panel 165 M. Mordasky, W. Chen, Purdue University 24 Cut Resistance and Fracture Toughness of High Performance Fibers 167 U.S. Army Research Laboratory 25 Kolsky Tension Bar Techniques for Dynamic Characterization of Alloys 175 B. Song, H. Jin, B.R. Antoun, Sandia National Laboratories 26 Prediction of Dynamic Forces in Fire Service Escape Scenarios 179 M. Obstalecki, J. Chaussidon, P. Kurath, G.P. Horn, University of Illinois at Urbana-Champaign 27 187 J.D. Seidt, T.A. Matrka, A. Gilat, G.B. McDonald, The Ohio State University Development and Characterization of a PU-PMMA Transparent Interpenetrating Polymer Stress-strain Response of PMMA as a Function of Strain-rate and Temperature X. Nie, W. Chen, Purdue University J.B. Mayo, Jr., Tuskegee University/U.S. Army Research Laboratory; E.D. Wetzel, A. Spadoni, C. Daraio, California Institute of Technology P. Moy, C.A. Gunnarsson, T. Weerasooriya, U.S. Army Research Laboratory; W. Chen, T. On, K.J. Smith, P.H. Geubelle, J. Lambros, University of Illinois at Urbana-Champaign; Dynamic Behavior of Three PBXs with Different Temperatures Tensile Behavior of Kevlar 49 Woven Fabrics over a Wide Range of Strain Rates

ix 28 The Effect of Loading Rate on the Tensile Behavior of Single Zylon Fiber 195 29 Statistical Analysis of Fiber Gripping Effects on Kolsky bar Test 205 J.H. Kim, N.A. Heckert, S.D. Leigh, H. Kobayashi, W.G. McDonough, R.L. Rhorer, K.D. Rice, G.A. Holmes. National Institute of Standards and Technology 30 Perpendicular Yarn Pull-out Behavior Under Dynamic Loading 211 J. Hong, Purdue University; J. Lim, Hyundai Motor Company; W.W. Chen, Purdue University 31 Dynamic Response of Homogeneous and Functionally Graded Foams When Subjected to Transient Loading by a Square Punch 213 32 217 K.A. Dannemann, R.P. Bigger, S. Chocron, Southwest Research Institute; K. Nahshon, Naval Surface Warfare Center Carderock Division 33 Ultra High Speed Full-field Strain Measurements on Spalling Tests on Concrete Materials 221 34 Contact Mechanics of Impacting Slender Rods: Measurement and Analysis 229 A. Sanders, I. Tibbitts, D. Kakarla, University of Utah; S. Siskey, J. Ochoa, K. Ong, Exponent, Inc.; R. Brannon, University of Utah 35 Solenoid Actuated, Rail Mounted, Aircraft Payload Release Mechanisms 237 C.L. Reynolds, Dynetics, Inc.; J.A. Gilbert, University of Alabama in Huntsville 36 Finite Element Modeling of Ballistic Impact on Kevlar 49 Fabrics 249 D. Zhu, McGill University; B. Mobasher, S.D. Rajan, Arizona State University 37 Optimal Pulse Shapes for SHPB Tests on Soft Materials 259 M. Scheidler, J. Fitzpatrick, R. Kraft, U.S. Army Research Laboratory 38 Dynamic Tensile Characterization of Foam Materials 269 B. Song, H. Jin, W.-Y. Lu, Sandia National Laboratories 39 On Measuring the High Frequency Response of Soft Viscoelastic Materials at Finite Strains 273 S. Teller, R. Clifton, T. Jiao, Brown University 40 The Blast Response of Sandwich Composites With a Graded Core: Equivalent Core Layer Mass vs. Equivalent Core Layer Thickness 281 N. Gardner, A. Shukla, University of Rhode Island 41 Face-sheets and the Sandwich Composite 289 S. Gupta, A. Shukla, University of Rhode Island 42 Influence of Texture and Temperature on the Dynamic-tensile-extrusion Response of High-purity Zirconium 297 D.T. Martinez, C.P. Trujillo, E.K. Cerreta, J.D. Montalvo, J.P. Escobedo-Diaz, Los Alamos National Laboratory; V. Webster, Case Western Reserve University; G.T. Gray, III., Los Alamos National Laboratory Dynamic Strain Measurement of Welded Tensile Specimens Using Digital Image Correlation Effects of High and Low Temperature on the Dynamic Performance of the Core Material, C. Periasamy, H. Tippur, Auburn University C.A. Gunnarsson, T. Weerasooriya, P. Moy, Army Research Laboratory F. Pierron, Arts et Métiers ParisTech; P. Forquin, Paul Verlaine University

x 43 Modeling and DIC Measurements of Dynamic Compression Tests of a Soft Tissue Simulant 307 S.P. Mates, R. Rhorer, A. Forster, National Institute of Standards and Technology; R.K. Everett, K.E. Simmonds, A. Bagchi, Naval Research Laboratory 44 Measurement of R-values at Intermediate Strain Rates Using a Digital Speckle Extensometry 317 J. Huh, Y.J. Kim, H. Huh, Korea Advanced Institute of Science Technology 45 Study of Strain Energy in Deformed Insect Wings 323 H. Wan, H. Dong, Y. Ren, Wright State University 46 Experimental Study of Cable Vibration Damping 329 A. Maji, Y. Qiu, University of New Mexico 47 Dynamic Thermo-mechanical Response of Austenite Containing Steels 337 V.-T. Kuokkala, Tampere University of Technology; S. Curtze, Tampere University of Technology/Oxford Instruments Nano Analysis; M. Isakov, M. Hokka, Tampere University of Technology 48 Investigation into the Spall Strength of Cast Iron 343 49 Development of Brick and Mortar Material Parameters for Numerical Simulations 351 C.S. Meyer, U.S. Army Research Laboratory 50 Electrical Behavior of Carbon Nanotube Reinforced Epoxy Under Compression 361 N. Heeder, A. Shukla, University of Rhode Island; V. Chalivendra, University of Massachusetts Dartmouth; S. Yang, K. Park, University of Rhode Island 51 Effect of Curvature on Shock Loading Response of Aluminum Panels 369 P. Kumar, University of Rhode Island; J. LeBlanc, Naval Undersea Warfare Center; A. Shukla, University of Rhode Island 52 Deformation Measurements and Simulations of Blast Loaded Plates 375 K. Spranghers, Vrije Universiteit Brussel; D. Lecompte, Royal Military Academy; H. Sol, Vrije Universiteit Brussel; J. Vantomme, Royal Military Academy 53 The Blast Response of Sandwich Composites With Bi-axial In-plane Compressive Loading 383 E. Wang, University of Illinois Urbana-Champaign; A. Shukla, University of Rhode Island 54 Dynamic Response of Porcine Articular Cartilage and Meniscus under Shock Loading 393 Y.-C. Juang, L. Tsai, National Kaohsiung University of Applied Sciences; H.R. Lin, Southern Taiwan University 55 Dynamic Response of Beams Under Transverse Impact Loadings 399 D. Goldar, Sharda University 56 Constitutive Model Parameter Study for Armor Steel and Tungsten Alloys S.J. Schraml, U.S. Army Research Laboratory 57 A Scaled Model Describing the Rate-dependent Compressive Failure of Brittle Materials J. Kimberley,G. Hu, K.T. Ramesh, Johns Hopkins University 58 Experimental Verification of Negative Phase Velocity in Layered Media A.V. Amirkhizi, S. Nemat-Nasser, University of California, San Diego G. Plume, C.-E. Rousseau, University of Rhode Island 409 419 423

xi 59 Gas Gun Impact Analysis on Adhesives in Sandwich Composite Panels M. Mordasky, W. Chen, Purdue University 60 Damage Analysis of Projectile Impacted Laminar Composites B.S. Nashed, J.M. Rice, Y.K. Kim, V.B. Chalivendra, University of Massachusetts Dartmouth 61 Rate Sensitivity in Pure Ni Under Dynamic Compression K.N. Jonnalagadda, Indian Institute of Technology Bombay 62 Temperature Effect on Drop-weight Impact of Woven Composites 443 Y. Budhoo, Vaughn College of Aeronautics and Technology; B. Liaw, F. Delale, The City College of New York 63 Dynamic Mode-II Characterization of a Woven Glass Composite 455 W.-Y. Lu, B. Song, H. Jin, Sandia National Laboratories 64 Rate Dependent Material Properties of an OFHC Copper Film 459 J.S. Kim, Korea Railroad Research Institute; H. Huh, Korea Advanced Institute of Science and Technology 65 Zirconium: Probing the Role of Texture Using Dynamic-tensile-extrusion 467 C.P. Trujillo, J.P. Escobedo-Diaz, G.T. Gray, III., E.K. Cerreta, D.T. Martinez, Los Alamos National Laboratory 425 427 439

Punch Response of Gels at Different Loading Rates Mark Foster mark.foster12@us.army.mil Paul Moy paul.moy@us.army.mil Randy Mrozek randy.mrozek@us.army.mil Joe Lenhart joseph.lenhart1@us.army.mil Tusit Weerasooriya tusit.weerasooriya@us.army.mil Army Research Laboratory Weapons and Materials Research Directorate Bldg 4600 Deer Creek Loop Aberdeen Proving Ground, MD 21005-5069 ABSTRACT Synthetic soft polymer gels have many advantages over protein-based gels that are derived from animal collagen and bones such as stability at room temperature and prolonged shelf life. In addition, the ability to tailor the formulations and processes of synthetic gel to control mechanical properties both isotropically or anisotropically is another essential feature in order for gels to mimic the spectrum of biological tissues. However, it is impractical to physically characterize all aspects of every gel available. To do so would require production of a significant amount of material to accommodate all the varying tests needed for a comprehensive study. A novel punch test was developed as a simple solution to obtain mechanical responses at different loading rates without the production of a large amount of sample material. The gels used in this effort are 10% and 20% ballistic gelatin, the commercially marketed PermaGel™, and triblock copolymer gels. The experimental setup is discussed, and the results are presented and compared to a previous study that discussed the tensile behavior of these soft materials. INTRODUCTION Traditionally, ballistic gelatin is used extensively to examine the penetration depth of firearms. While useful as a base material for qualitative comparison, the applications of this biologically derived gelatin become very limited when used as a tissue simulant for quantitative testing. Next generation armor and protection systems require an understanding of injury mechanisms that has not yet been realized [1-3]. However, ballistic gelatins inconsistent viscoelastic properties, high sensitivity to temperature, and aging effects all prove detrimental to producing data to validate proposed material models [4]. Synthetic gelatins are therefore a promising solution because networks of cross-links and polymer chains can be tuned to resemble the mechanics of biological tissues. Unfortunately, such a wide range of controllable properties necessitates laborious and repetitive testing for full characterization. T. Proulx (ed.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society for Experimental Mechanics Series 99, DOI 10.1007/978-1-4614-0216-9_1, © The Society for Experimental Mechanics, Inc. 2011 1

Much prior work has been performed on soft tissue stimulants. Methods have been established in constrained and unconstrained compression at a variety of strain rates [2,5,6]. Often this work is to measure a shear modulus for use in various constitutive models. While existing models can be useful at determining material response at lower rates of strain, a fully nonlinear model is required at higher rates [7]. Wu et al. [6] also demonstrated that friction can cause a non-uniform deformation state in unconfined compression tests which contradicts assumptions in traditional models. In a study on the effects of temperature, aging time, and strain rate to the penetration depth of ballistic gel, Cronin and Falzon [4] found that tensile strain dominates over shear strain during failure. This also contradicts many current models of tissues that are based on shear failure criteria. Microscale indentation and rheology tests have been performed, proving that viscoelastic and hyperelastic models are applicable at small (physiological) deformations, but large strain behavior of nonlinear tissues warrants further analysis [8-11]. Other methods of modeling human tissue behavior involve porcine, bovine, rat or mouse tissue. Snedeker et al. [8] performed impact experiments on porcine and human kidneys, and found large differences in stress to failure. Fracture tear tests have also found basic energy dissipation and tear resistance values [12]. Moy et al. [13,14] have used digital image correlation techniques to measure strain field qualities in notched gelatin tensile specimens. This technique provided a measurement of the maximum tensile strain and energy required for a crack to propagate through gelatin. This interest led to the development of a simple punch penetration test to screen the multitude of possible gelatin formulations into a smaller group of gels that resemble the mechanical behaviors of ordnance gelatins. Various gelatin materials were subjected to a constant-rate displacement by a 6.35 mm hemispherical penetrator tip. These materials included synthetic polymer gels, ballistic gels, and Permagel™, a commercially available ballistic gel replacement. Then puncture data was compared to gelatin fracture tear data from Moy et al. [13] to ensure that the basic puncture test can be used as a screening process to find suitable tissue surrogates without an indepth investigation into each material. Materials Three different synthetic polymer gels were made from different concentrations of Poly(styrene-b-ethylene-cobutylene-b-styrene) (SEBS) G1652 as-received from Kraton Polymers (Houston, TX, USA) and mineral oil asreceived from Aldrich Chemical (Milwaukee, WI, USA). These three gels were mixed in sheets with concentrations of 70, 80, and 90 percent mineral oil to SEBS polymer. Ballistic gelatin was made from Bloom 250 Type A ordnance gelatin mix as-received from GELITA USA (Sioux City, IA). Each batch was mixed in water according to manufacturer directions, poured into a sheet, refrigerated at 3.89°C, and tested the following day. The two types of ballistic gel typically used for bullet penetration testing are either 10 or 20 percent by mass gelatin, and both are accounted for in this work [4]. Permagel™ is a transparent material designed to have similar properties to 10% ballistic gelatin, but without its inherent disadvantages. The material was used as-delivered from USALCO (Browns Mills, NJ) and was cut from a large base block, molded into sheets and allowed to cool before testing. The group of gelatins tested is included in Table 1. 2

Table 1: Collection of Materials Tested Material Concentration Source Mineral Oil/SEBS polymer 70/30 Aldrich Chemical/ Kraton Polymers 80/20 90/10 Permagel N/A USALCO Bloom 250A Ordnance Gel (by weight to water) 10% GELITA, USA 20% Test Methods A 4mm thick specimen was bonded to one side of a standard 7/16 inch washer, excess material was trimmed and the specimen was tested to full failure with a 6.35 mm hemispherical indenter. All specimens were inspected for bubbles or debris prior to gluing. An elevated acrylic table fixture was used to provide clearance for full specimen failure and give ample space for observation. A custom 45° triangle mirror fixture was then designed to fit underneath the elevated fixture to observe the specimen during the indentation. Figure 1 shows the test fixture. After the specimens fully adhered to the washer, the entire sample was placed on the top of the fixture centered over the middle 12.7 mm diameter hole in the acrylic plate. While the washer did not provide clamping pressure by any means, it did give an open space on the top gelatin surface to fill with lubricant. Friction between the acrylic and the gelatin provided ample resistance to prevent any slippage during the experiment. Figure 1: Punch Test Fixture with 45° Mirror An Instron 8871 servo-hydraulic load frame was used to control and directly measure the load and displacement required to indent and to puncture the gel specimens. Three different displacement rates were addressed: 12.7, 127 and 1270 mm/min. These gave a wide range of material characteristics while remaining within the capabilities of the load frame. The overall machine setup is in Figure 2. Lubrication was used in order to minimize friction between the gel and indenter. For ballistics gel an olive oil was used, but silicone oil of similar viscosity was used for the Permagel™ and synthetic polymer gels, to ensure the lubricant did not affect the gel material being tested. Prior to each experiment, the indenter was lowered until it just contacted the gelatin surface. 3

Figure 2: Machine Setup with Accompanied Gel Punch Fixture RESULTS AND DISCUSSION Without lubrication, friction between the indenter and the gelatin was quite evident in the preliminary testing. It was observed that when oil was omitted specimens would exhibit a “cork” type failure characteristic of a shearing failure instead of the desired Mode I tearing failure. Load and displacement data obtained for all experiments at each extension rate can be seen in Figures 3a through c. Figures 4a to 4c provides a magnified view of the gels at the lower load range. This data includes all testing, and it is clear from the amount of overlap that the test is inherently repeatable. The gelatins displayed a close agreement between certain pairs of material, such as the close agreement between 80/20 and Permagel™ for all three rates. However, ballistic gels do not exhibit the relaxation portion of the load-displacement curve that the other 4 materials expressed around 8 mm displacement. This can clearly be seen in the 1270 mm/min data where the 20% ballistic gel deviates from 70/30 data curve at around 7.5 mm extension. Note that prior to this point there is a close agreement between the 20% ballistic gel and 70/30 curves. The ballistic gels also show lower overall extensions than the other materials. When the 10% ballistic and Permagel™ gelatins are compared across the three rates studied, the load response converges as the displacement rate increases. This compels the idea that Permagel™ could be used as a replacement for the 10% ballistic gelatin, but only at higher rates. 4

Figure 3: Load vs. Extension for all Gel Materials Tested at (a) 12.7 mm/min, (b) 127 mm/min, and (c) 1270 mm/min Displacement Rate 5

0 0.5 1 1.5 2 2.5 3 3.5 4 0 5 10 15 20 80/20 90/10 Permagel 10% Ballistic Force (N) Extension (mm) 0 1 2 3 4 5 6 0 5 10 15 20 25 80/20 90/10 Permagel 10% Ballistic Force (N) Extension (mm) (4a) (4b) 0 2 4 6 8 10 12 0 5 101520253035 80/20 90/10 Permagel 10% Ballistic Force (N) Extension (mm) (4c) Figure 4: Expanded Load vs. Extension for Punch Experiments at the Lower Load Region at (a) 12.7 mm/min, (b) 127 mm/min, and (c) 1270 mm/min Displacement Rate The maximum displacement to failure was compared in Figure 5 across the three displacement rates of 12.7, 127, and 1270 mm/min. Two distinct trends are clear. Most interesting is the distinct difference in trend between the mineral oil and the Permagel™/ballistic gel. The synthetic mineral oil gels exhibit a much larger stretch ratio than the ballistic gelatins or Permagel™ at a higher rate of displacement, which suggests a different strengthening mechanism may occur at these higher displacement velocities. This could easily be attributed to the structure of polymer chains and amount of crosslinking in the mineral oil gelatins which do not occur in the others. As the manufacturer claims Permagel™ is similar to the ballistic gels but requires a larger overall extension to fully penetrate, which is shown from the data at all rates of displacement. 6

0 10 20 30 40 50 60 10 100 1000 70/30 80/20 90/10 Permagel 10% Ballistic 20% Ballistic Maximum Extension (mm) Displacement Rate (mm/min) Figure 5: Maximum Extension as a Function of Displacement Rate for all Gel Punch Experiments A similar analysis was performed with the maximum load to failure in Figure 6. The 80/20 formulation ratio of mineral oil to SEBS polymer closely agrees with the Permagel™ at all displacement rates studied. Unlike the 70/30 gelatin which gave a much larger penetration resistance. Despite a similarity in extension to failure between the 70/30 and Permagel™ at lower displacement rates, the punch test showed a vast difference in stiffness of the 70/30 gelatin. While miniscule, the loads of the 90/10 polymer gel resemble the 10% ballistic gel across all displacement rates. 0 5 10 15 20 25 30 10 100 1000 70/30 80/20 90/10 Permagel 10% Ballistic 20% Ballistic Maximum Load (N) Displacement Rate (mm/min) Figure 6: Maximum Load as a Function of Displacement Rate for all Gel Punch Experiments 7

Each load-extension curve was integrated to determine the overall energy required to fully puncture the gelatin. These values were then correlated to a prior work by Moy et al. which tested the fracture characteristics of both ballistic gel concentrations, and Permagel™ [13]. A Mode I test was used with a dogbone shaped specimen that had a small pre-crack. The crack propagation was then inspected using high speed cameras with DIC to acquire surface strains near and around the crack tip. The load-extension curve was also integrated to obtain fracture energy values. This comparison is shown in Figure 7 a, b, and c. It is important to note the difference in scaling between the two data sets, for they are not in perfect alignment in any case. However, considering the differences between the two test methods, the results are quite similar in trend. It is very easy to see the agreement in the 20% ballistic gel for example, where the fracture energy is close to a factor of 5x higher than the punch energy. This comparison validates the punch method presented here as a simple screening process as it is a much less demanding test. 0 20 40 60 80 100 120 140 160 0 30 60 90 120 150 180 210 240 1 10 100 1000 104 Punch Energy (mJ) Fracture Energy (mJ) Punch Energy (mJ) Fracture Energy (mJ) Displacement Rate (mm/min) Figure 7a: Comparison between Punch Energies and Fracture Energies for Permagel™ [13] 8

0 6 12 18 24 30 0 40 80 120 160 200 1 10 100 1000 104 Punch Energy (mJ) Fracture Energy (mJ) Punch Energy (mJ) Fracture Energy (mJ) Displacement Rate (mm/min) Figure 7b: Comparison between Punch Energies and Fracture Energies for 10% Ballistic Gel [13] 0 40 80 120 160 200 0 200 400 600 800 1000 1 10 100 1000 104 Punch Energy (mJ) Fracture Energy (mJ) Punch Energy (mJ) Fracture Energy (mJ) Displacement Rate (mm/min) Figure 7c: Comparison between Punch Energies and Fracture Energies for 20% Ballistic Gel [13] CONCLUSIONS A significant number of synthetic gelatins offers multiple solutions for potential replacement of ballistic gelatin. More importantly, polymer gels can be tailored to have characteristics to actual biological tissues. To adequately examine the load response of every gel available would be very demanding. In an effort to find a suitable tissue surrogate to validate theoretical models for next-generation armor and protection systems, several gelatin materials were compared in a simplified puncture test. Ballistic gels, mineral oil gels, and the commercially available Permagel™ were punctured to full failure using a hemispherical indenter across extension rates of 12.7, 127 and 1270 mm/min. Similar initial load responses were seen in the 10% ballistic, 80/20 mineral oil/SEBS, and 9

Permagel™ at each rate despite the ballistic gelatins lower extension and maximum load to failure. Through a comparison with previous mode I gel fracture data, the punch technique provides an easy screening method to classify a gelatin as a possible tissue simulant. REFERENCES 1. Song, B., Ge, Y., Chen, W., and Weerasooriya, T. Radial Inertia Effects in Kolsky Bar Testing of ExtraSoft Specimens. Experimental Mechanics, 47, pp. 659-670. 2007. 2. Saraf, H., Ramesh, K.T., Lennon, A.M., Merkle, A.C., and Roberts, J.C. Mechanical Properties of Soft Human Tissues Under Dynamic Loading. Journal of Biomechanics, 40, pp. 1960-1967. 2007. 3. Van Sligtenhorst, C., Cronin, D. S., and Brodland, G. W. High Strain Rate Compressive Properties of Bovine Muscle Tissue determined using a split Hopkinson bar apparatus. Journal of Biomechanics, 39, pp 1852-1858. 2006. 4. Cronin, D.S., and Falzon, C. Characterization of 10% Ballistic Gelatin to Evaluate Temperature, Aging and Strain Rate Effects. Proceedings of the 2010 SEM Annual Conference, Indianapolis, IN. 2010. 5. Kwon, J., and Subhash, G., Compressive Strain Rate Sensitivity of Ballistic Gelatin. Journal of Biomechanics, 43. pp 420-425. 2010. 6. Wu, J.Z., Dong, R.G., and Schopper, A.W. Analysis of Effects of Friction on the Deformation Behavior of Soft Tissues in Unconfined Compression Tests. Journal of Biomechanics, 37, pp 147-155. 2004. 7. Cronin, D.S., and Falzon, C. Dynamic Characterization and Simulation of Ballistic Gelatin. Proceedings of the 2009 SEM Annual Conference. Albuquerque, N.M. 2009. 8. Snedeker, J.G., Barbezat, M., Niederer, P., Schmidlin, F.R., and Farshad, M. Strain Energy Density as a Rupture Criterion for the Kidney: Impact Tests on Porcine Organs, Finite Element Simulation, and a Baseline Comparison Between Human and Porcine Tissues. Journal of Biomechanics, 38, pp 993-1001. 2005. 9. Lin, D.C., Shreiber, D. I., Dimitriadis, E. K., and Horkay, F. Spherical Identation of Soft Matter Beyond the Hertzian Regime: Numerical and Experimental Validation of Hyperelastic Models. Biomechanics and Modeling in Mechanobiology, 8 (5), pp 345-358, 2009. 10. Wu, J.Z., Dong, R.G., Smutz, W.P., and Schopper, A.W. Nonlinear and Viscoelastic Characteristics of Skin under Compression; Experiment and Analysis. Biomedical Materials and Engineering. 13 (4), pp 373-385, 2003. 11. Clark, A.H., Richardson, R.K., Ross-Murphy, S.B., and Stubbs, J.M. Structural and Mechanical Properties of Agar/Gelatin Co-gels. Small Deformation Studies. Macromolecules. 16, pp 1367-1374, 1983. 12. Furukawa, H., Kuwabara, R., Tanaka, Y., Kurokawa, T., Na, Y., Osada, Y., and Gong, J.P. Tear Velocity Dependence of High-Strength Double Network Gels in Comparison with Fast and Slow Relaxation Modes Observed by Scanning Microscopic Light Scattering. Macromolecules. 41, pp 7173-7178, 2008. 13. Moy, P. Gunnarsson, C. A. and Weerasooriya, T. Tensile Deformation and Fracture of Ballistic Gelatin as a Function of Loading Rate. Proceedings of the 2009 SEM Annual Conference. Albuquerque, NM. 2009. 14. Moy, P. Foster, M. Gunnarsson, C. A. and Weerasooriya, T. Loading Rate Effect on Tensile Failure Behavior of Gelatins under Mode I. Proceedings of the 2010 SEM Annual Conference. Indianapolis, IN. 2010. 10

A Kolsky Torsion Bar Technique for Characterization of Dynamic Shear Response of Soft Materials Xu Nie1*, Weinong Chen1, Rasika Prabhu2, James M. Caruthers2, Tusit Weerasooriya3 1AAE&MSE schools, Purdue University. 2ChE school, Purdue University. 3Army Research Laboratory * Corresponding author: Xu Nie, 701 W. Stadium Ave. West Lafayette, IN 47907-2045 Email: xnie@purdue.edu ABSTRACT A novel Kolsky torsion bar technique is developed and successfully utilized to characterize the high strain rate shear response of a rate-independent end-linked polydimethylsiloxane (PDMS) gel rubber with a shear modulus of ~10 KPa. The results show that the specimen deforms uniformly under constant strain rate and the measured dynamic shear modulus well follows the trend determined by dynamic mechanical analysis (DMA) at lower strain rates. Contrastive Kolsky compression bar experiments are also performed on the same gel material with annular specimens. The dynamic moduli obtained from compression experiments, however, are an order of magnitude higher than those predicted by the torsional technique, due to the pressure caused by the radial inertia and end constraints. INTRODUCTION Characterization of dynamic response of soft biological tissues has seen a tremendous rise in the past decade. Among all the published non-oscillatory high rate results, dynamic uniaxial compression/tension has generated the most popular group of data [1], and its experimental conditions have also been extensively investigated [2]. There are two major issues associated with the axial loading conditions when the strain rate is high: 1. Dynamic stress (or force) equilibrium across the specimen length, and 2. Radial inertia induced pressure by strain rate and strain acceleration. A preliminary solution to minimize the inertia effect is to punch a hole in the center of the specimen, for which the pressure was greatly reduced by creating a stress-free inner surface. However, for materials as soft as human brain tissues whose elastic moduli are typically in the range of 0.1-10 KPa, even the reduced pressure in an annular sample can be sufficiently high to overshadow the intrinsic material response. To separate the pressure from the intrinsic mechanics response of soft materials, a pure shear loading condition is desired. In this paper, we present a newly developed desktop Kolsky torsion bar technique for the characterization of high rate shear mechanical properties of soft materials. The effectiveness of this torsion bar technique was demonstrated by our calibration experiments on the end-linked polydimethylsiloxane (PDMS) gel rubber. EXPERIMENTS AND RESULTS A typical oscilloscope record of the modified Kolsky torsion bar experiment is shown in Fig. 1. The trace noted as “incident bar signal” is measured by the strain gages mounted on the incident bar, while the other trace is taken from the torque sensor which connects to the external ring adapter. Since the gel material under investigation has extremely low wave impedance compared to that of the incident bar, most of the incident wave is reflected back. Consequently, the reflected pulse would not see any noticeable difference, both in shape and amplitude, from the incident pulse. We used the incident wave to calculate the shear strains in the T. Proulx (ed.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society for Experimental Mechanics Series 99, 11 DOI 10.1007/978-1-4614-0216-9_2, © The Society for Experimental Mechanics, Inc. 2011

specimen so that a better-quality signal can be used directly from the oscilloscope reading. The stress-strain curves of five different samples loaded at shear strain rate of ~1000s-1 are displayed in Fig. 2. Although some discrepancies were found on the five measured stress-strain curves, all of them exhibited linear elasticity when the strain is beyond 8%. In order to compare the shear modulus with those obtained from DMA tests, and thus evaluate the validity of our Kolsky torsion bar experiment, the tangential of these stress-strain curves in Fig. 2 were measured in the strain range from 8% to the maximum strain on each curve. As mentioned before, the purpose of conducting current dynamic torsional experiments on soft materials is to directly acquire their shear constitutive properties, which in the past were mostly inferred from the dynamic compression results. To compare the measured modulus value from uniaxial compression experiments with those obtained from DMA and Kolsky torsion bar experiments, dynamic compressive experiments on this same PDMS gel were also conducted at comparable strain rates. The results are plotted in Fig. 3. The dynamic shear elastic modulus of gel measured with torsion bar technique follows the trend of DMA test results, while the same material exhibited much higher modulus value (about an order of magnitude) when it was under dynamic compression. Such a large discrepancy between the two dynamic testing techniques and the analysis of the discrepancy reveal that the Kolsky torsion bar experiment is necessary to characterize the shear behavior of extra soft materials under high strain rate loading conditions. Fig. 1 The original signals of torsional experiments Fig. 2 Shear stress-strain curve of PDMS at 1000/s by Kolsky torsion bar technique Fig. 3 Comparison of shear modulus obtained by different testing techniques at different shear rates REFERENCE: [1] Bo Song, Weinong Chen, Yun Ge and Tusit Weerasooriya, “Dynamic and quasi-static compressive response of porcine muscle”, Journal of Biomechanics, 40, 2999-3005, 2007 [2] Bo Song and Weinong Chen, “Dynamic stress equilibration in split Hopkinson pressure bar tests on soft materials”, Experimental Mechanics, 44, 300-312, 2004 12

Loading Rate Effect on the Tensile Failure of Concrete and Its Constituents using Diametrical Compression and Direct Tension Samuel Weckert1 samuel.weckert@dsto.defence.gov.au Tusit Weerasooriya2 tusit.weerasooriya@us.army.mil C. Allan Gunnarson2 allan.gunnarson@us.army.mil 1Defence Science and Technology Organisation Edinburgh, South Australia, 5111 2Army Research Laboratory Weapons and Materials Research Directorate Bldg 4600 Deer Creek Loop Aberdeen Proving Ground, MD 21005-5069 ABSTRACT The loading rate effect on the tensile failure strength of concrete and its constituent materials has been investigated. Concrete is inherently weaker in tension than compression so tensile failure represents the dominant failure mode. Understanding the failure characteristics of concrete, particularly at high loading rate, is important for developing modeling capabilities, in particular for predicting spallation damage and fragmentation. Several concretes, and their constituents, have been investigated at different loading rates to understand the tensile failure behavior as a function of loading rate. In this paper, experimental procedures that were used are discussed, and results from two different tensile testing methods, direct tension and diametric compression (Brazilian/split-tension), are presented for several of these materials. INTRODUCTION Tensile failure is a vulnerable failure mode for concrete as it is much weaker in tension compared with other modes of failure such as compression. Typically the tensile strength is an order of magnitude less than the compressive strength. High strain rate tensile testing of concrete is important for weapons effects problems, such as penetration and explosive loading, where the loading rates are very high and tensile failure can occur as spallation damage in a target. Materials can behave differently at high strain rates, so material characterization in this regime is important for developing accurate material models for simulations. Direct tension experiments produce a nominally uniaxial tensile stress state, however, it can be difficult to implement because of issues associated with gripping the sample. This is particularly the case for brittle 1This work was undertaken as a collaborative effort between Australia and the U.S. while Sam Weckert was on a 6 month attachment at ARL in the High Rate Mechanics and Failure branch under the Scientists and Engineers Exchange Program in 2010. T. Proulx (ed.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society for Experimental Mechanics Series 99, 13 DOI 10.1007/978-1-4614-0216-9_3, © The Society for Experimental Mechanics, Inc. 2011

materials, such as concrete, where it is not possible to use conventional grips or threaded joints. High strength adhesives can be used to grip normal concrete; however they are not strong enough for testing the new generation of high strength concretes. Notched specimens can be used to reduce the cross-sectional area [1], however, this creates a stress concentration which leads to an under prediction of the tensile strength. Dog-bone specimen geometries are also possible, however these are difficult implement in brittle materials. The diametric compression test, also known as the Brazilian or Split-tension test, offers an alternate test method to indirectly obtain the material tensile response. This test induces tensile stresses within the specimen by point diametric compression of a disc shaped sample. This permits the use of simpler compression testing apparatus to obtain tensile material response data. Other tensile test techniques also include three or four point bend tests and high rate specific spallation experiments [2]. This report presents the tensile strength for various concretes and their constituent materials (mortar and aggregate) using the diametric compression technique. Tests are conducted at high loading rate using a Split Hopkinson Pressure Bar (SHPB) apparatus and intermediate and low loading rates using an Instron hydraulic test machine for comparison. This allows an investigation of the loading rate effects for each of these materials. Direct tension tests at high loading rate have also been conducted for several of the concrete materials to allow a comparison of tensile strengths from the direct tension and diametric compression test methodologies. MATERIALS The tensile strengths of five different materials were investigated in this study: 1. SAM35 concrete: a 3500psi (~24MPa) minimum quasi-static unconfined compressive strength concrete produced by the US Engineering and Research Development Center (ERDC) [3]. It contains small limestone aggregate components up to approximately 8mm in size. 2. Mortar: prepared from a commercially available mix - Drypack basic mortar sand and cement, Adelaide Brighton Cement Limited, Australia. 3. Granite: charcoal black granite, Starrett, True Stone Tech Division, MN, US. 4. Ultra High Performance Concrete (UHPC): a reactive powder concrete reinforced with steel fibers of length 12.7mm and diameter 0.2mm, randomly distributed through the concrete at 6.2% by weight [4], samples obtained from Australia. 5. Alcatraz concrete: obtained from a tourist commercial vendor at the Alcatraz prison in San Francisco, California. The SAM-35 represents a common concrete mix and the mortar and granite are representative of typical concrete constituents (however, the SAM-35 contains limestone aggregate not granite). The UHPC is a new generation high strength concrete which was tested to evaluate its enhanced characteristics. The diametric compression tests used disc shaped specimens with a nominal thickness of ¼” (6.35mm). The mortar, UHPC and Alcatraz concrete specimens had a nominal diameter of 20mm, whereas the SAM-35 and granite specimens had a nominal diameter of 1” (25.4mm). Direct tension tests at high loading rate were also performed on the mortar and SAM35 concrete using cylindrical samples with ¾” diameter and ¾” length. The size of the specimen geometry was dictated by the available experimental equipment and was fairly small relative to the size of the aggregrate components in the concrete and the steel fiber distribution for the UHPC. In addition, the material heterogeneity is further amplified in the diametric compression test where only a portion of the sample is in tension. Consequently it is expected that there will be considerable scatter in the results and so a minimum of five tests were conducted for each material at each loading rate. EXPERIMENTAL METHODOLOGY Diametric compression test technique The diametric compression (Brazilian/split tension) test uses a circular disc sample, which is point loaded at diametrically opposite points in compression. This test methodology produces a biaxial stress state where a tensile stress is induced perpendicular to the compressive stress along the loading axis and the material fails in tension. The stress state produced with this loading condition is discussed in detail in [5]. The diametric 14

compression test methodology gives the tensile failure strength of the material, however, no pre or post-peak stress-strain response can be obtained. A problem with the point loading used in this test methodology, particularly for brittle materials, is that the sample is subject to high stress concentrations at the external load contact points. Thus failure may initiate at these contact points rather than in the induced tensile region in the bulk of the specimen, which invalidates the test. To reduce the stress concentrations at the contact points, wooden bearing strips are recommended for distributing the load in quasi-static diametric compression tests [6,7], however, these are not suitable for high rate tests because of the reflections of stress waves and material impedance effects. For high rate diametric compression tests, other researchers have suggested several techniques to overcome this problem, both with the objective of spreading the load over a small area at the sample sides to reduce the stress concentrations. The first method is to cut flat areas onto the sides of the sample at loading points [8]. The second method is to maintain a circular disc shaped sample, but use concave curved input/output bars for loading the sample [9], as shown in figure 1. It is this second method, which was adopted at all loading rates for the tests presented here. Figure 1: Curved input/output bars for loading the disc specimen Figure 2 shows the normal equation used to obtain the tensile strength, σ, for a diametric compression test. Figure 2: Compressive loading and tensile stress diagram/equation for diametric compression setup where P is the axial compressive load, D is the specimen diameter and t is the specimen thickness. A modification to this equation is used in [9] to account for the spreading of the load at the contact points. However, the modified equation introduces a contact width parameter between the sample and the curved loading platens, which is difficult to measure. Thus the modified equation is not used for the work presented here. For a contact width of 2.5mm for a 25mm diameter sample, the stress is only reduced by 4% by the modified equation, so the implication of ignoring this correction factor is relatively small. Low and intermediate loading rate experiments The low and intermediate rate diametric compression tests were performed using a 5000lb Instron hydraulic test machine. The tests were conducted with a constant compressive displacement rate of 0.001mm/s for the low rate and 1mm/s for the intermediate rate experiments. Instrumentation for these tests included the load cell and displacement transducer in the test machine and high-speed video to record the material loading and failure process. High loading rate experiments 15

The high rate diametric compression tests were performed on a compression Split-Hopkinson Pressure Bar (SHPB) with 1¼” diameter aluminum input and output bars. Background on the SHPB and test methodology is provided in [10]. The SHPB input and output bars were instrumented with semiconductor strain gauges. The semiconductor gauges have a much higher sensitivity compared with traditional metal foil strain gauges and are essential for measuring the small strains associated with testing concrete in tension. A comparison of the signals from the two gauge types is shown in figure 3, which illustrates the noise reduction using the higher sensitivity semiconductor gauges. Figure 3: Comparison of metal foil and semiconductor strain gauges - voltage signal (left); and strain signal (right) A 24” long striker bar, accelerated by compressed nitrogen, was used to impact the input bar to produce the compressive incident pulse. This was used to load the sample at a compressive displacement rate of approximately 1000mm/s. The striker bar had a flat impact end, however a small amount of silicon grease on the impact face was used for shaping the incident pulse. This has the effect of damping high frequency components (ringing) associated with the impact and increases the pulse rise time to load the specimen more gradually. Ramping the load in this way is critical for allowing time for the stress to equilibrate, through multiple wave reverberations, in the sample. This is particularly important for brittle materials, which only undergo minimal strain before failure and equilibrium needs to be achieved before this time for a valid fracture strength to be reported. Pulse-shaping of the incident pulse for SHPB experiments is discussed further in [11]. Figure 4 shows a detail of the high loading rate experimental setup. Figure 4: High rate loading experimental setup 16

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