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    UNIVERSITY OF CINCINNATI

    Date: ___________________

    I, _________________________________________________________,

    hereby submit this work as part of the requirements for the degree of:

    in:

    It is entitled:

    This work and its defense approved by:

    Chair: _______________________________  

    _______________________________

     

    _______________________________ 

    _______________________________

     

    _______________________________

     

    08/03/2006

    Gaurav Nilakantan

    Master of Science

    Mechanical Engineering

    Design and Development of an Energy Absorbing Seat and Ballistic

    Fabric Material Model to reduce Crew Injury caused by Acceleration

    from Mine/IED blast

    Dr. Ala Tabiei

    Dr. Jay Kim

    Dr. David Thompson

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    DESIGN AND DEVELOPMENT OF AN ENERGY ABSORBING SEAT AND

    BALLISTIC FABRIC MATERIAL MODEL TO REDUCE CREW INJURY

    CAUSED BY ACCELERATION FROM MINE/IED BLAST

     A Thesis submitted to the

    Division of Research and Advanced Studies

    of the University of Cincinnati

    in partial fulfillment of the

    requirements for the degree of

    MASTER OF SCIENCE

    in the Department of Mechanical, Industrial and Nuclear Engineering

    of the College of Engineering

    2006

    by

    Gaurav Nilakantan

    Bachelor of Engineering (B.E.)

     Visveswaraiah Technological University, India, 2003

    Committee Chair: Dr. Ala Tabiei

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     Abstract

     Anti tank (AT) mines pose a serious threat to the occupants of armored vehicles.

    High acceleration pulses and impact forces are transmitted to the occupant

    through vehicle-occupant contact interfaces, such as the floor and seat, posing

    the risk of moderate injury to fatality.

    The use of an energy absorbing seat in conjunction with vehicle armor plating

    greatly improves occupant survivability during such an explosion. The axial

    crushing of aluminum tubes over a steel rail constitutes the principal energy

    absorption mechanism. Concepts to further reduce the shock pulse transmitted

    to the occupant are introduced during the study, such as the use of a foam

    cushion and an inflatable airbag cushion.

    The explicit non-linear finite element software LS-DYNA© is used to perform all

    numerical simulations. Vertical drop testing of the seat structure with the

    occupant are performed for comparison with experimental data after which

    simulations are run, that utilize input acceleration pulses comparable to a mine

    blast under an armored vehicle. The occupant is modeled using a 5th percentile

    HYBRID III dummy. Data such as lumbar load, neck moments, hip and knee

    moments, and head and torso accelerations are collected for comparison with

    known injury threshold values to assess injury.

    Numerical simulations are also conducted of the impact of a dummy’s feet by

    a rigid wall whose upward motion is comparable to an armored vehicle’s reaction to a mine blast directly underneath it. A 50 th  percentile HYBRID III

    dummy is used in various seated positions. The input pulses that control the

    motion of the rigid wall are varied in a step wise manner to determine the effect

    on extent of injury. Data such as hip and knee moment, femoral force and foot

    acceleration are collected from the dummy and compared to injury threshold

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     values from various references. By numerically simulating the mine blast under a

     vehicle, the significant cost of conducting destructive full scale tests can be

    avoided.

     A simple numerical formulation is presented, to predict the deceleration

    response during dynamic axial crushing of cylindrical tubes. The formulation

    uses an energy balance approach and is coded in the high level language

    MATLAB©. It can track the histories of plastic work, kinetic energy, and dynamic

    crushing load during the crushing process, and finally yields the peak

    acceleration magnitude, which can then be calibrated and used for injury

    assessment and survivability studies by comparing with allowable values for

    human occupants. Further, the geometric and material properties of the tube

    can be varied to study its response during the dynamic axial crushing.

    The impact resistance of high strength fabrics makes them desirable in

    applications such as protective clothing for military and law enforcement

    personnel, protective layering in turbine fragment containment, armor plating of

     vehicles, and other similar applications involving protection resistance against a

    high velocity projectile. Such fabrics, especially Kevlar©, Zylon©, and Spectra©,

    can be used in the energy absorbing seat as a cushion cover for the high

    density foam, to prevent tearing by unexpected shrapnel during an explosion

    underneath the armored vehicle. The protective fabric can also be used as a

    protective vest for the dummy occupant and as a liner inside the vehicle hull. A

    material model has been developed to realistically simulate ballistic impact of

    loose woven fabrics with elastic crimped fibers. It is based upon a

    micromechanical approach that includes the architecture of the fabric and the

    phenomenon of fiber reorientation, and excludes strain rate sensitivity as the

     yarns are simplified as elastic members. The material model is implemented as

    a FORTRAN© subroutine and integrated into the explicit, non-linear dynamic

    finite element code LSDYNA© as a user-defined material model (UMAT). Results

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    of axial fabric tests run in LSDYNA© using this material model agree well with

    other models. This justifies the use of a simplistic, computationally inexpensive

    material model to realistically simulate ballistic impact.

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     Acknowledgements

    I am indebted to my advisor Prof. Ala Tabiei for giving me a chance to work with

    him on all his fascinating research, for believing in me and constantly guiding

    and encouraging me.

    I express my utmost gratitude to my parents S. Nilakantan and Nirmala

    Nilakantan for all that they have done for me, for all their love, support, sacrifice,

    and encouragement.

    I sincerely thank committee members, Prof. Jay Kim, and Prof. David Thompson

    for their presence on my committee and their suggestions.

    I am also grateful to the University of Las Vegas-Nevada for funding part of this

    research, as well as the Ohio Supercomputing Center for their high-speed

    computing support.

    I thank my colleague Srinivasa Vedagiri Aminijikarai for all his technical advice

    and support.

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    Contents

    a. List of Figures……………………………………………………………………………....  i

    b. List of Tables………………………………………………………………………………..   vi

    1. Introduction 1

    1.1 Background…………………………………………………………………………..  1

    1.2 Literature Review……………………………………………………………………..  2

    1.2.1 Energy Absorbing Seat…………………………………………………..  2

    1.2.2 Foot Impact during IED/Mine blast……………………………………..  4

    1.2.3 Human Injury Criteria…………………………………………………….  5

    1.2.4 Dynamic Axial Crushing of Circular Tubes…………………………….  13

    1.2.5 Ballistic Impact of Dry Woven Fabrics………………………………….  14

    1.3 Scope of Work………………………………………………………………………..  26

    1.4 Outline of Thesis………………………………………………………………………  27

    2. Energy Absorbing Seat 29

    2.1 Preliminary Design……………………………………………………………………  29

    2.2 Dynamic Axial Crushing of the Aluminum Tubes…………………………………  32

    2.2.1 Techniques to reduce the initial crushing load of a tube…………...  36

    2.3 Additional energy absorbing elements…………………………………………… 39

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    2.3.1 Low Density Foam Cushion……………………………………………..  39

    2.3.2 Airbag Cushion…………………………………………………………..  42

    2.4 Shock Pulses Applied to the Structure……………………………………………..  44

    2.4.1 Impact After Free Fall……………………………………………………  44

    2.4.2 Mine Blast…………………………………………………………………  45

    2.5 Fil