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The characterisation of a new composite material system, particularly for application to fields such as aerospace structures, is a time-consuming process costing potentially millions of dollars. This is partially due to the vast number of tests conducted with a single mode of loading in a qualification scheme. In fact, testing of materials has consistently been driven to a purity of loading mode, so that material response is easily understood and modelled. However, the costs associated with this approach provide a large barrier to the qualification of new materials. Collaborative research undertaken by the US Naval Research Laboratory (NRL) has been focused on the development of a testing system to address this barrier, utilizing a more comprehensive understanding of mechanical response of composite panels.
The NRL system involves a mechanical testing machine capable of applying loading to a specimen in any combination of six degrees of freedom (DoF). Other aspects of the system include an optical system for full-field strain and displacement measurement on two surfaces, and an automated specimen loading mechanism for continuous specimen testing.
The purpose of the system is to generate a massive database of material response data from a single specimen geometry tested under multi-axial loading. This database can then enable the characterisation of the constitutive behaviour of complex, anisotropic materials in general and composite materials in particular. This is in contrast to the standard approach of using uni-axial testing for material characterisation that then requires extrapolation for multi-axial conditions.
The NRL research is led by Dr John Michopoulos, Head of the Computational Multiphysics Systems Laboratory of the Center of Computational Materials Science in the Materials Science Division of NRL. According to Dr Michopoulos, the use of automated robotic systems can “influence drastically” the certification and qualification of high performance composite materials.
To date, the NRL robotic system has gone through functional verification and has been used to generate its first multi-axial characterisation database from 1152 carbon-fibre reinforced polymer (CFRP) specimens. This has demonstrated the rapid and continuous testing capabilities of the system, with loading rates of up to 26 specimens per hour recorded.
Analysis predictions of dissipated energy in double-notch characterization specimens (left) and open hole tension specimens (right). Loading directions on top and bottom edges shown.
The work at NRL is part of a broader collaborative research project partly funded by the US Office of Naval Research (ONR) involving CRC-ACS and its members RMIT University, University of New South Wales (UNSW) and Defence Science and Technology Organisation (DSTO), working with NRL, Massachusetts Institute of Technology (MIT), USDA Forest Products Laboratory and Virginia Polytechnic University in the USA.
CRC-ACS involvement includes utilising the data generated in the robotic test machine as part of analysis methodologies for capturing the mechanical response of composite materials. This includes PhD researchers at RMIT and UNSW who use characterisation of the damage energy dissipated under multi-axial loading to model non-linear material behaviour.
CRC-ACS is also involved in design and manufacture of test specimens at a range of length scales, including the specimens used by NRL for the first characterisation database. Open hole, ply drop and stiffened panels are also being tested by CRC-ACS to investigate composite material behaviour and for use as experimental validation.
The approach taken by NRL and its team is still some way from commercial application in the aerospace sector, however initial test outcomes are validating the value of this approach to materials characterisation. With further development and success, this approach may remove one of the most significant barriers in materials qualification for aerospace, and encourage the rapid development of new generation materials with quick introduction into service.
More information: http://crc-acs.com.au/