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Title: Pseudo-ductility of thin ply angle-ply laminates
Author: Fuller, Jonathan
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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Composite materials usage is limited by the sudden, brittle failure they often exhibit. It has been shown in this work that it is possible to remove this inherent limitation and enlarge the design space by using thin ply angle-ply laminates that exhibit a non-linear 'pseudo-ductile' stress-strain response. Characterisation of the spread tow thin ply carbon-epoxy prepreg material (ply thickness of 0.03 mm) was conducted in order to allow an experimental study of the potential for pseudo-ductility. It has been shown that highly non-linear stress-strain responses can be achieved with angle-ply laminates, whilst suppressing the damage that normally causes their premature failure . Several orientations between 15°- 45° were investigated in a [±e5 ls layup. For all angles, micrographs and X-ray computed tomography showed that delamination and matrix cracking were suppressed, allowing pseudoductile strains to develop. Significant fibre rotations were shown to take place, permitted by matrix plasticity, leading to a stiffening at high strains. Analytical modelling that incorporates matrix plasticity and reorientation of the fibres into classical laminate analysis has been developed. The approach was successfully validated against experimental results, capturing the main characteristics of the stress-strain curve, such as the initial largely linear, 'yield ' and stiffening regions. Further analysis identified a fibre angle (±26°) that exhibited strength in excess of 900 MPa, strain to failure greater than 3.5% and pseudo-ductile strain of 1.2%. Inclusion of unidirectional plies at the mid-plane of the laminate led to experimental pseudo-ductile metal-like tensile stress-strain responses with 700 MPa 'yield' stress and 2.2% pseudo-ductile strain using [±265 /O]8 lam-' inates. Enhancements to the analytical model allowed predictions of other [±θm/On]8. layups, which were validated with experimental tests. Postfailure specimen analysis greatly improved the understanding of the pseudoductile stress-strain behaviour. A gradual failure was identified, caused by multiple fibre fractures in the 0° plies that, in some cases, led to dispersed delaminations at the 0/ -θ interfaces. A preliminary study of the compressive behaviour of thin ply angle-ply laminates has also been conducted, via four-point bending of a honeycomb sandwich beam. Layups of [±266]8 and [±306]8 showed significantly less 11011- linearity than the same configurations loaded in tension. This is likely to be have been due to the relatively low shear 'yield' stress of the material. These tests , however, have shown potential for thin ply angle-ply laminates to exhibit pseudo-ductility under compression.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available