Presentation on theme: "Materials that fight back R. Critchley, I. Corni, K. Stokes, J. Wharton, F. Walsh, R. Wood 22 April 2010 – TriboUK – Imperial College London."— Presentation transcript:
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
Outline EPSRC and DSTL call: Study the properties of new materials under high rate dynamic loading to enhance their damage tolerance. Proposals from the following areas: Nanomaterials or novel materials approaches to improve the resistance of materials to high rate mechanical loading, wear and impact. Modelling of the response of materials to high rate loading, including high rate materials property development for the input to such models. Predicting and improving the high rate deformation performance of polymer composite materials and structures. Novel ceramic materials, composite materials and steels- highlighted in the recent Defence Technology Strategy - that address these issues. Bio-inspired approaches to improving high rate materials response. The development of new techniques and the use of existing ones are both of interest.
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)
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 (W 3 C and B 13 C 2 ) and PVD (Ti-V-Zr and Ti-V-Zr nitride): CVD is a high temperature process (1000ºC for B 13 C 2 ) 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 21 st Century Defence Needs, 2003.
Vent gastropod shell Ortiz et al. PNAS 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, 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 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 Poissons ratio Auxetic materials have a negative Poissons ratio - grow fatter when stretched Evans and Alderson, Advanced Materials, Poissons 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 1980s
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, x25 x10
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 Auxetic Evans and Alderson, Advanced Materials,
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 Increase in strain rate
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
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.