CCMC Armor Ceramics Subgroup Meeting

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CCMC Armor Ceramics Sub-group Meeting October
16, 2007 Some Thoughts on Armor Ceramics Jim
McCauley
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PROGRAM EVOLUTION PROCESS STEPS

• Examine Future Force attributes and
  characteristics and concept of operations
• Identify Current and Force Operational
  Capability Gaps
• Merge into underpinning technology shortfalls
• Obtain performance requirements and constraints
  for all systems
• Translate performance requirements and
  constraints into property requirements (Figure of
  Merit) This is the key step
• Identify research objectives
• Identify and prioritize related tasks/projects
  and timelines
• Prioritize required continuing underpinning
  science and engineering

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Future Force attributes, characteristics and
concept of operations for Light Weight Army
Ground Vehicles

• Responsiveness
• Deployability
• Survivability
• Agility
• Sustainability

Advanced Materials and Design
Bottom Line Reduce Weight Maintain/Enhance
Performance
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Steps 2 and 3 Future Force Operating Capability
Gaps Current Capability Gap Soldier protection
in counter-insurgency environment Future
Capability Gap - Enhanced Soldier
Protection Enhanced Platform/Group
Protection Merged Gap - Lighter, more effective
ballistic protection Technology
Shortfalls/Requirements Lightweight Soldier
Protection Lightweight Platform Ballistic
Protection
Ceramics, light metals and composites
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Selected Material Choices

• Densities (g/cc)
• Steel 7.9
• TiB2 4.6
• Ti 4.5
• AlN 3.3
• SiC 3.2
• Al 2.7
• B4C 2.5
• Organic matrix composites 1.5 2.0
• Mg 1.7

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STEP 5 FIGURES OF MERIT DETERMINATION Materials
Science and Engineering Approach
This is the obvious sticking point in the
process. It is the reason why researchers revert
to bubble up projects or ballistic testing of
commercial materials, rather than strategically
focused 6.1 and 6.2 work. Of course, one can
obfuscate the lack of connections with power
point cartoons, but that will not help decision
makers understand what we are doing.
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Step 5 Translate performance requirements and
constraints into property requirements (Figures
of Merit) Very Complex and Controversial

• SUMMARY OF THIS STEP
• Determine state-of-the-art quantitative and
  qualitative (plus anecdotal) armor ceramics,
  dynamic testing, modeling, ballistics, etc.
• Identify key aspects of ballistic stress
  environment
• Identification of stages (separable parts) in
  ballistic event each stage has its own property
  requirements and figure of merit thickness
  dependent
• Systematic relationship of relevant bulk
  mechanical properties to stages
• Systematic relationship of intrinsic material
  properties (crystallography, single crystal
  elastic constants) to relevant bulk mechanical
  properties
• Systematic relationship of extrinsic material
  characteristics (phases, microstructure, defects)
  to relevant bulk mechanical properties
• Ashby materials down select focus on density
  at this point

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Simplified Differences between Quasi-static and
Ballistic Mechanical Stress Environments on
Armor Ceramics
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Recent Breakthroughs

• Importance of dwell and how to measure
  transitional velocity (pressure)
• Boron carbide shock amorphization
• Apparent importance of effective plasticity in
  armor ceramics as it influences dwell and
  possible relation to twinning and stacking faults
• High frequency ultrasonic testing
• More ..

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Translate Projectile Velocity into Impact
Pressure

• Much Better Scientifically Based Parameter than
  Velocity
• Normalizes changes in projectile density
• Better to understand mechanisms and material
  transitions

• Impact pressure, not velocity
• P (vProj)(ZprojZtarg)/(ZprojZtarg)
• Z acoustic impedance vProj projectile
  velocity
• Impedance Z ?cl (?E)½
• cl longitudinal sound velocity

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Defects
Massive Failure in Ballistic Environment Cracks/Da
mage Nucleate at Defects Grow
Coalesce Massive Failure

• Stressed volume is large Large number of
  defects activated
• Shock wave activates defects and conditions
  microstructure

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Defect Size Variability
Strength Variability
Tail of Distribution Inverse Power
Law Probability Defect Density Function f (c) a
Kc-n
Weibull Variability of Strength Probability of
Failure Pf 1- exp -V (s/so)m
s a 1/ vc m 2n s1/s2 (N2/N1)m N number of
defects
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• Bottom line on Defects
• Minimize them by processing controls, including
  final stage HIP
• Utilize confinement in armor package
• Screen (quality control) by rapid go/no-go NDT
• If defects are inevitable or too costly to
  remove or do QC incorporate inverse power law
  defect distributions and related Weibull modulus
  in performance design codes

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Crystallographically/Directionally Controlled
Mechanisms and Properties

• Anisostropic elasticity Boron carbide -
  maximum Youngs modulus along c is 523 GPa
  minimum Youngs modulus is 64.4 GPa (McClellan
  et al. 2001) Mica ? 164 and 45 GPa
• Twinning/stacking faults resolved shear stress
• Amorphization

Strongly suggests that texturing (controlled
crystallographic directions) in armor ceramics
could be a very important additional material
control
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• Total Effective Plasticity in Compression
• Not Toughness Non-linear Yield Before Failure
• Not really metal-like but some dislocation
  motion
• Grain boundary mechanisms grain size and GB
  constituent dependent
• Sliding
• Viscous flow
• Micro-cracking
• Transgranular mechanisms grain size dependent
• Twinning/stacking faults
• Slip
• Brittle ductile transitions (e.g. AlN)
• Multiphase addition of plastic phase/s
• Strain rate and defects effects

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Deformation and Damage Mechanisms Hypothesis
Hardness/Compressive Strength
Intragranular Plasticity
Total Effective Plasticity
Ballistic Sweet Spot?
Grain Boundary Plasticity
Grain Size
For improved armor ceramic ballistic
performance want to minimize defects and increase
hardness and effective plasticity
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Nano-structured Ceramics

• Background
• Strong evidence that single phase or multi-phase
  nano-structured (lt 0.5 µm) ceramics have
  significantly improved ballistic performance
  ballistic sweet spot
• Nano-structured composite ceramic with a plastic
  deforming material (AlN) may be even better.

• Challenge
• Powders available, but probably much less pure
• Processing route to nano-structure not so clear
• Metastable powders plus HIP Rutgers
• Microwave processing
• Spark Plasma Sintering
• Two step sintering (I.W. Chen)
• Japanese superplastic multiphase assemblies

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Deformation and Failure from the Bottom up
(Nano/Micro) or Top Down (Macro)

• Nano and micro-mechanisms
• deformational twinning and stacking fault
  plasticity
• grain boundary microcracking and/or sliding
• Shock amorphization
• Defects effects on reduced strength and
  concomitant plasticity
• Macro-mechanisms
• Hertzian mechanisms and influence of defects
• Shock attenuation
• Back stiffening

Dwell
Penetration
Optimum Solution will involve a tradeoff between
dwell and penetration that is, if dwell is
maximized at the expense of low penetration
resistance then there is no gain in total
ballistic performance, and this will change for
thin and thick plates.
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Comparison of FEA Analysis with EoI Experimental
Results on AlONat 8.7 µs and about 400m/s
Edge-on Impact (EoI) at EMI Strassburger et al.
Crossed Polarizers
Plane Light Shadowgraph
8.7 µs
MCOE
von Mises stress
S11 stress - Principal axis
ABAQUS Explicit , Fully 3-D
From Kovalchick, Ramesh, JHU and McCauley
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Krell and Strassburger, Daytona Beach Conference,
2007.
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Transparent Gem-quality SiC
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50 years ago How did we do??