Development of failure models and criteria for laminated composites

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The potential of composite materials as structural parts is currently hindered by our lack of ability to both understand and predict failure initiation and propagation. Developments in this area can lead to faster and more economical design, as well as lighter and more efficient structures. This project will contribute to the capability of predicting failure of laminated composites consisting of unidirectional plies, by focusing on (i) experimental characterisation of each failure mode, (ii) analytical interpretation of the failure mechanisms and (iii) development of numerical simulation tools for failure propagation.The experimental investigation into each failure mode will be particularly detailed, focusing on the qualitative and quantitative description of the sequence of events leading to each failure mode and their interaction. These events take place initially at a micromechanical scale, and then progressively grow to cause structural failure. An example of this is fibre compressive kinking. The kink bands observed in failed composites are the outcome of a sequence of events which include matrix cracking, fibre-bending failure and eventually fibre micro-buckling, and are dependent on fibre misalignments and matrix nonlinear behaviour in shear. Within these events, matrix cracking for instance, is itself the result of the growth and coalescence of matrix micro-cracks. The experimental investigation aims at producing extensive, detailed, univocal information on these processes. It will require design of test rigs, and will make use of intensive instrumentation (e.g. acoustic emission, photogrammetry and strain gauges) as well as optical and scanning-electron microscopy.The experimental findings will be the basis for the development of analytical models describing the sequence of physical events leading to failure and their interaction. These analytical models will form a physical, mechanist, interpretation of each failure mode. They can be understood as a physical theory for each failure mode, translating the observed events into mathematical expressions involving material (e.g. elastic, strength, toughness) and geometric (e.g. fibre diameter, typical fibre misalignments magnitude and distribution, typical matrix micro-cracks size and distribution) properties. The outcome of these models will be a set of equations, which will be expressed as failure criteria, for direct use in design.For the accurate analysis of complex structures, numerical models have to be considered. For this reason, an advanced numerical failure model including the previous failure criteria will be developed, to be used within commercial finite elements software. In order to accurately model failure propagation and avoid spurious mesh dependency, the numerical model will be based on a to-be-developed smeared-crack methodology appropriate for the variety of issues of failure in laminated composites. These have to do with the multiple failure modes composites can exhibit and how each failure mode affects the material response. For instance, matrix cracking will result in the shear components of the traction vector on the fracture plane being reduced to zero, as well as the normal component if positive; the computational model should be able to reproduce this accurately, as well as correctly accounting for the fracture energy of the process. Finally, the numerical model will be validated against experimental data obtained for this effect during the project as well as published in the literature.