LARGE-DISPLACEMENT LIGHTWEIGHT ARMOR A Thesis presented to the Faculty of California Polytechnic State University, San Luis Obispo In Partial Fulfillment of the Requirements for the Degree Master of Science in Mechanical Engineering by Eric Christopher Clough December 2013 © 2013 Eric Christopher Clough ALL RIGHTS RESERVED ii COMMITTEE MEMBERSHIP TITLE: Large-displacement Lightweight Armor AUTHOR: Eric Christopher Clough DATE SUBMITTED: December 2013 COMMITTEE CHAIR: Tom Mackin, PhD Professor of Mechanical Engineering COMMITTEE MEMBER: Tom Mase, PhD Professor of Mechanical Engineering COMMITTEE MEMBER: Jacob Hundley, PhD Member of the Technical Staff, HRL Laboratories iii Abstract Large-displacement Lightweight Armor Randomly entangled fibers forming loosely bound nonwoven structures are evaluated for use in lightweight armor applications. These materials sacrifice volumetric efficiency in order to realize a reduction in mass versus traditional armor materials, while maintaining equivalent ballistic performance. The primary material characterized, polyester fiberfill, is shown to have improved ballistic performance over control samples of monolithic polyester as well as 1095 steel sheets. The response of fiberfill is investigated at a variety of strain rates, from quasistatic to ballistic, under compression, tension, and shear deformation to elucidate mechanisms at work during ballistic defeat. Fiberfill’s primary mechanisms during loading are fiber reorientation, fiber unfurling, and frictional sliding. Frictional sliding, coupled with high macroscopic strain to failure, is thought to be the source of the high specific ballistic performance in fiberfill materials. The proposed armor is tested for penetration resistance against spherical and cylindrical 7.62 mm projectiles fired from a gas gun. A constitutive model incorporating the relevant deformation mechanisms of texture evolution and progressive damage is developed and implemented in Abaqus explicit in order to expedite further research on ballistic nonwoven fabrics. iv Acknowledgments I would like to first thank my advisor, Professor Mackin, for the good talks about engineering, science, and philosophy. Without your input and direction none of this thesis would have ever come together. I also want to acknowledge Professor Mase for his input and guidance on the modeling and continuum mechanics sections. I want to thank Jake Hundley for taking time out of his busy schedule to teach me the ins and outs of user subroutines and damage mechanics, and for the many discussions on mechanics. I want to acknowledge Thijs van Loon and all of the hard work and enthusiasm that he put in to running experiments, not to mention all of the excellent Matlab scripts he contributed, all of the Subway sandwich breaks he forced me to take, and all of the great insights he had on the project. A huge thanks to the folks at UCSB: Oshin Nazarian, Pete Maxwell, Kirk Fields, and Professor Frank Zok for the tremendous amount of help and guidance in completing ballistic experiments on their light gas gun. I want to thank the entire grad-lab crew. Your late night/early morning antics, and dedication to working hard, but still having fun went a tremendous way in keeping my spirits up throughout the course of this thesis. Thanks to everyone that helped in some way to get all of the equipment, software, and test fixtures I needed to use up and running. Thank you Josh Smith and Dustin Draper for your help in machining test fixtures, and thanks to Nash Anderson for the SEM work. Most importantly, I want to thank my family for teaching me my love of science, engineering, and hard work. Thank you for supporting me throughout my life. None of this would be possible without you. v Table of Contents List of Tables .................................................................................................................... ix List of Figures .................................................................................................................... x 1 Introduction ............................................................................................................... 1 1.1 Background ...................................................................................................................... 1 1.2 Motivation ........................................................................................................................ 1 1.3 Thesis Organization ......................................................................................................... 2 2 Armor Materials and Defeat Mechanisms .............................................................. 4 2.1 Ballistic Defeat: ............................................................................................................... 4 2.2 Mechanics of Ballistic Impact.......................................................................................... 6 2.3 Monolithic Armor Plates .................................................................................................. 7 2.3.1 Metals ....................................................................................................................... 7 2.3.2 Ceramics .................................................................................................................. 8 2.3.3 Polymers .................................................................................................................. 8 2.3.4 Failure modes ........................................................................................................... 9 2.4 Composite Armor........................................................................................................... 11 2.4.1 Elastic stress wave analysis in a fiber .................................................................... 12 2.5 Ballistic Felts ................................................................................................................. 18 2.6 Large Displacement Armor ............................................................................................ 21 3 Preliminary Ballistic Testing .................................................................................. 23 3.1 Experimental Procedure ................................................................................................. 23 vi 3.2 Results ............................................................................................................................ 24 4 Mechanical Testing and Micromechanics ............................................................. 28 4.1 Compression .................................................................................................................. 28 4.1.1 Micromechanics of compression ........................................................................... 28 4.1.2 Compression experimental setup and procedure .................................................... 29 4.1.3 Compression Results .............................................................................................. 32 4.2 Drop Tests ...................................................................................................................... 36 4.2.1 Micromechanics of drop test experiments ............................................................. 36 4.2.2 Drop test experimental setup and procedure .......................................................... 36 4.2.3 Drop test results ..................................................................................................... 39 4.3 Tension ........................................................................................................................... 41 4.3.1 Micromechanics of fiberfill tension ....................................................................... 41 4.3.2 Tension experimental setup and procedure ............................................................ 41 4.3.3 Tension results ....................................................................................................... 44 4.4 Shear .............................................................................................................................. 51 4.4.1 Micromechanics of fiberfill Shear ......................................................................... 51 4.4.2 Shear experimental setup and procedure................................................................ 51 4.4.3 Shear results ........................................................................................................... 52 5 Gas Gun tests ........................................................................................................... 55 5.1 Experimental procedure and test fixture ........................................................................ 55 5.2 Results ............................................................................................................................ 59 vii 6 Numerical Analysis .................................................................................................. 69 6.1 Models Considered ........................................................................................................ 70 6.2 Fiberfill Continuum Model ............................................................................................ 72 6.2.1 Progressive Damage Model ................................................................................... 73 6.2.2 Abaqus implementation ......................................................................................... 76 6.2.3 Model