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Abstract
This thesis investigates the methodology for implementing the Onset Theory, previously known as the Strain Invariant Failure Theory (SIFT), proposed by researchers at Boeing Research & Technology for failure of fibre reinforced composite structures. Implementation of the Onset Theory involves off-axis coupon tests and subsequent finite element analyses of these tests. However, modelling the response of the off-axis tensile tests is challenging due to stress concentrations caused by the shear-extension coupling effect, especially for the cases where the fibre angle varies from 5deg to 30deg. Furthermore, using the continuum models requires strain amplification factors to determine the strains in the resin and fibres which takes time and effort. New modelling approaches have been proposed in this thesis to overcome these issues. Applications to date have focussed on material systems comprising high modulus fibres and glassy resins typical of the materials used in highly loaded structure in the aircraft.
The work for this thesis had two parts. First an investigation of the testing of uni-directional off-axis specimens used to define the critical values for the strain invariants was conducted. This work led to the proposal to implement high aspect ratio tabbed specimens. The second part investigated the numerical modelling required to determine the critical values from the strain to failure of the unidirectional specimens. A micromechanical modelling approach has been developed incorporating the unit cell geometry that can be extended to the full geometry of the unidirectional specimen. A micromechanical sub-modelling approach has also been proposed. This is a multiscale modelling process that combines a shell or layered solid model of a laminate with a local 3D submodel of fibre and matrix. Both micromechanical modelling and sub-modelling approaches enable direct de-homogenised strain fields without the need of strain enhancement factors. These strain fields allow for a prediction and visualisation of the onset of damage within both fibre and matrix phases as well as type of deformation (dilatational or distortional).
The original work in this thesis includes the recommendation for the higher aspect ratio specimens to test for the critical invariants, the recommendation for tabbing of the specimens to reduce grip damage and the new finite element micromechanical approaches for the analysis of the test specimens. In addition the Onset Theory has been applied to a new material system comprising a toughened resin and low modulus fibre. This material system is typical of the materials used in aircraft structures susceptible to impact damage such as flaps and ailerons. This application has identified new challenges to the development team working on the Onset Theory and some extensions to the theory are proposed in this thesis. Finally the theory has been applied to a multidirectional [0/0/90/90/90/90/0/0] specimen and use to predict damage initiation.