Materials that fight back R. Critchley, I. Corni, K. Stokes, J. Wharton, F. Walsh, R. Wood 22 April 2010 – TriboUK – Imperial College London.

Materials that fight back

R. Critchley, I. Corni, K. Stokes, J. Wharton, F. Walsh, R. Wood

22 April 2010 – TriboUK – Imperial College London

Outline• Current problems

• Proposed solution

• The outer hierarchical coating

• Inner auxetic foam

• Where the project is heading

http://hermes52.deviantart.com/art/U-S-Army-Soldier-34508852

Body armour: problems and requirements• Lightweight (now the body armour is 13 kg)

• Flexible wear and allowing mobility • Low cost

• Stop projectile penetration

• Diffuse the impact energy to reduce behind armour blunt trauma (BABT)

http://cn1.kaboodle.com/hi/img/2/0/0/c0/e/AAAAAhTx1z0AAAAAAMDr-A.jpg

http://www.computerweekly.com/PhotoGalleries/235075/757_30_Hi-tech-body-armour.JPG

How it works

Hybrid Sandwich Structure

Outer layer: a laminate composite structure to diffuse the energy of the impact and to resist to deeper penetrations

Armour substrate

Inner layer: auxetic foam structure acting as small airbags to protect the body from BABT

Schematic structure

1mm

5 mm

10 mm

Outer hierarchical

coating

The outer hierarchical coating

• It is well-known that biological materials present optimized structure and excellent mechanical properties.

• The challenge is to deposit a hierarchical hard and tough ceramic-metal multilayered coating that mimics nature (e.g. nacre (mother of pearl), mollusc shell and ancient fish armour).

• The initial coatings have been deposited by CVD (W3C and B13C2) and PVD (Ti-V-Zr and Ti-V-Zr nitride):

• CVD is a high temperature process (1000ºC for B13C2) and therefore just few substrate are compatible.

• PVD coatings can be deposited on CVD films but the opposite is not always true – e.g. we experienced decomposition of the PVD films in the atmosphere needed for CVD deposition.

Nacre (mother of pearl)

• Nacre is a ceramic laminate composite made of aragonite tablet layers separated by thin layers of an organic material (polymer).

• Nacre resists impact by dissipating the impact energy through nano and micron cracks, plastic deformation and elastic responses.

Barthelat, et al. Experimental Mechanics 2007

Materials Research to Meet 21st Century Defence Needs, 2003.

Vent gastropod shell

Ortiz et al. PNAS 2010.

• The hardness and Young modulus reported to be higher in the two external layers (with the inner layer presenting the higher value) and lower in the middle layer (ML).

• The outer layer (OL) behaves as a sacrificial layer and helps to dissipate the impact energy through localized micro-cracking and delamination.

Ancient fish armour

Each layer had a specific deformation and energy dissipation mechanism:

•The stiff outer layer transferred the energy of an applied load to the layers below

•The stratified isopedine layer hinders deeper penetrations and prevents catastrophic cracking through micro-cracking in the sub-layers.

Bruet et al. Nature

Materials, 2008.

Young modulus and hardness increase (inside to outside of the armour)

Improved ballistic performance of coated plates

Ozsahin and Tolun, Materials & Design 2010; Ozsahin and Tolun, Materials & Design, In Press.

• Özsahin and Tolun studied the ballistic performance against a high velocity (> 350 m/s) impact 9 mm bullet of aluminium alloy plates with and without 0.762 mm thick cobalt-molybdenum-chromium or Zirconia plasma spray coatings.

• Penetration depth on the front face of the plate and the bulging on the rear face of the plate were compared for plates with and without coatings.

• The coatings improved the ballistic resistance of the plates with an increase in non-perforating projectile velocity and a decrease in penetration depth and bulging.

Inner auxetic foam

Auxetic Materials – What are they ?

• Conventional materials have a positive Poisson’s ratio

• Auxetic materials have a negative Poisson’s ratio - grow fatter when stretched

Evans and Alderson, Advanced Materials, 2000.

Poisson’s Ratio = _ Change in X Change in Y

Auxetic Materials - Background• Auxetic materials have been known for

approximately 100 years

• Field only started to be studied in 1987 by Rod Lakes

• The term auxetic was coined by Ken Evans - derived from the Greek word auxetikos which means ‘that tends to increase’

• Surge in interest since the late 1980’s

http://www.imeche.org/NR/rdonlyres/4CB11B44-DB9A-44D0-B94C-662AE7A93740/517/meshauxeticfoam94.png

Auxetic Materials – How they work• Fabricated through a compression, heating and

cooling process

• Foam cell structure determines the overall properties

Conventional Cell Structure Auxetic Random Rib Structure

Evans and Alderson, Advanced Materials, 2000.

x25x10

Conventional Foam Auxetic Foam

x5 x5

Alicona InfiniteFocus optical 3D measurement device

Scanning Electron Microscopy (SEM)

Auxetic Materials – Importance to the high strain project

• Auxetic foam to act as ‘smart airbags’

• Act behind sandwich structure

• Auxetic materials are popular for their increase in impact resistance and energy absorption:

- Reduce BABT - Reduce / prevent backface signature injury

Non-auxetic

AuxeticEvans and Alderson, Advanced Materials, 2000.

http://www.turtleskin.com/Body-Armor/Body-Armor-Blunt-Trauma-Protection.aspx

Testing Methods• Indentation test

• Falling drop impact test

• High energy particle erosion test (350 m/s)

• Ballistic test

Other Testing Methods:

• Compressive test

• Tensile test

• Shear test

• 3 point bending test

• Quasi-static cyclic loading

http://www.mpmtechnologies.com/images/BatteryImpactTest_med.jpghttp://english.ymhiten.com.tw/image/E-003a.jpghttp://www.worldoftest.com/img/products/Q250.jpg

http://www.tc.umn.edu/~seong001/web%20image/7R009r1.jpg

Incre

ase in

str

ain

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te

Where project is heading

• Select a standard polymer foam – most likely polyurethane

• Determine optimum conditions for the selected foam

• Characterise the foam

• Test the foam in a number of different situations

http://reasonsoflife.files.wordpress.com/2008/08/success2.png?w=317&h=320

Acknowledgements The author would

like to acknowledge EPSRC and DSTL for providing funding and opportunity for this project, and my project supervisors for all their direction and support.

THE END

Any Questions ?

http://croaky.deviantart.com/art/Universal-Soldier-19082158

Understanding the threat - Bullets

http://www.faqs.org/photo-dict/photofiles/list/2895/3890bullets.jpghttp://www.autosafety.com/