A serious disadvantage of composite materials. in general. is their weakness transverse to the fibre direction. In filament wound composite vessels such cracking can lead to the weepage of the vessel contents. The aim of this work was to identify the micro-mechanisms involved in transverse cracking and other, more general. composite failure modes. In this way. a more thorough understanding of the nature of fibre-resin interactions and the role of the matrix in laminate deformation behaviour has been reached. Sections of filament wound pipe tested in either a 2:1 Hoop/Axial stress ratio or a uniaxial Hoop stress were examined m lc roscop lce lly , Weepage at low fibre stress levels was possible by the interaction of many transverse cracks in each lamina. Where transverse cracks did not form prior to failure. weepage was possible by the interaction of inter- and intra-lamina cracks. The change in cracking modes found in the pipe wall, as the in-plane stress condition varied. agreed well with theoretical predictions. Matrix flexibilization improved the biaxial weepage performance of the pipes by inhibiting transverse cracking. Transverse failure was by resin crazing. in regions of closely packed fibres, rather than by fibre debonding. The cri terion for crack suppression being a non-elastic matrix stress-strain response. The propagation of transverse cracks through uni-directional composites was studied under the microscope. In certain cases, cracking was foun d to be controllable, gradual fibre-resin separation was followed by massive non-linear deformation of the inter-fibre resin ligaments. The roles of fibre volume fraction and matrix type were also examined Failure modes in finite width, angle-plied, filament wound rings were studied using the "Split 0 Test". Three failure modes were identified depending on the fibre angle. Below 40 degrees failure was predominantly by transverse cracking and was virtually width independent. Above 50 degrees, failure was by the catastrophic unpeeling of the laminations from the edges, this gave rise to highly width dependent failure stresses. Between these limits, width dependent, mixed mode failure was found. The micro-mechanisms of failure were followed by the examination of pre-pol ished ring edges, after loading, and by pulling very thin polished sections of rings under the microscope. Microscopic resin non-linear deformation, particularly that of the thin interlamina resin layer, was a feature of all the mechanisms observed. It was the ability of this layer to sustain high shear strains which enabled large-scale laminate non-linearity to take place, especially, in the angular range for mixed mode failure between 45 and 50 degrees
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:581980 |
| Date | January 1981 |
| Creators | Jones, M. L. C. |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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