Large-displacement Lightweight Armor

150 Pages·2014·6.18 MB·English

by  Eric C. Clough

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

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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

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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.
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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.
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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
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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
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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 Results ........................................................................................................ 79
7 Conclusion ................................................................................................................ 86
References ........................................................................................................................ 88
Appendix A. Complete drop test results ..................................................................... 100
Appendix B. JS Model Derivation ............................................................................... 103
B.1 Texture tensor structural analysis ................................................................................ 103
B.2 JS Elastic Model .......................................................................................................... 105
B.3 Inelastic extension of the JS model .............................................................................. 108
Appendix C. JS VUMAT FORTRAN Code ............................................................... 118

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List of Tables
Table 2.1 NIJ 0108.01 ...................................................................................................................... 5
Table 2.2 Fiber Comparison .......................................................................................................... 17
Table 2.3 Areal density comparison of Nylon ballistic felt with metals of equivalent ballistic
performance versus a 5.56mm FSP [1]. ......................................................................... 21
Table 3.1 Ammunition details (Manufacturer's Data) ................................................................... 23
Table 4.1 Strain rates and curve fit parameters for compressions samples.................................... 33
Table 4.2 Viscoelastic relaxation fit parameters ............................................................................ 35
Table 4.3 Parameters for drop test experiments ............................................................................. 37
Table 4.4 Parameters for tension experiments ............................................................................... 43
Table 4.5 Parameters for shear experiments .................................................................................. 51
Table 5.1 Ballistic penetration tests ............................................................................................... 61
Table 6.1 Parameters used in model results (unless otherwise noted) ........................................... 79

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List of Figures
Figure 2.1 Plate armor failure modes [19] ..................................................................................... 10
Figure 2.2 Fiber Reinforced Composite ......................................................................................... 12
Figure 2.3 Longitudinal dynamic deformation of a single fiber. ................................................... 13
Figure 2.4 Transverse impact of an individual fiber [14] .............................................................. 14
Figure 2.5 Linear-Log plot of strain energy behind the elastic wave front versus specific fiber
modulus for an impact velocity of 400 m/s. ................................................................. 17
Figure 2.6 Fiber ballistic properties. Contours of constant c shown [33] to [38]. ...................... 18
Figure 3.1 Schematic of the second ballistic experimental setup. ................................................. 24
Figure 3.2 "Cocoon" structures surrounding defeated projectiles................................................. 25
Figure 3.3 Defeated projectile being extracted from a “cocoon.” The tail on this projectile has
already been removed. ................................................................................................. 25
Figure 3.4 "Tail" structure following projectile path. .................................................................... 25
Figure 3.5 "Tail" emanating from the side of a defeated projectile. .............................................. 25
Figure 3.6 Fibers from the projectile affected region showing melting and adhesion. .................. 26
Figure 3.7 Fibers from a region unaffected by projectiles. ............................................................ 26
Figure 3.8 Close up of fiber from the affected region showing inter-fiber friction wear and
plastic deformation. ...................................................................................................... 26
Figure 3.9 Pristine fibers from unaffected region. ......................................................................... 26
Figure 3.10 Results of the exit velocity experiments, values shown are averages of multiple
shots fired into the same sample. ................................................................................ 27
Figure 4.1 Compression test fixture mounted to servo hydraulic test frame ................................. 30
Figure 4.2 Example of toe correction procedure for compression data ......................................... 31
Figure 4.3 Compressive stress versus volume fraction (all data shown). Fit to the Toll van-
Wyk equation shown in black. ..................................................................................... 34
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