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Title: Mixed mode intra-laminar fracture in carbon/epoxy composites
Author: Vrellos, Nikolaos
ISNI:       0000 0004 2684 3778
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2009
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Matrix crack growth in the off-axis plies of a range of CFRP laminates has been investigated under different loadings conditions. Mechanical property data for the IM7/8552 carbon fibre/epoxy composite laminae have been measured experimentally, including the shear modulus of the material and the intra-laminar fracture toughness of a unidirectional laminate. Off-axis ply crack initiation and propagation has been investigated for coupons with the following lay-ups: (02/θ4)s with θ = 450, 600, 750 or 900; (0/90)4s coupons and coupons cut from quasi-isotropic laminates. For the unbalanced (02/θ4)s, angled end tabs have been designed and manufactured in order to ensure uniform displacements of the specimens near the grip. For the (02/θ4)s specimens, both un-notched and notched coupons have been tested. The un-notched coupons had carefully polished edges, whereas the notched specimens had a machined-in defect (a notch) parallel to the direction of the fibres in the off-axis ply. Matrix crack development has been studied for these specimens under quasi-static and fatigue loading. Edge microscopy and penetrant enhanced Xradiography have been used to monitor the crack development. The stress state at the crack tip for crack initiation and crack propagation has been calculated based on measurements of the longitudinal strain for the onset of cracking. The results show that under quasi-static loading, the transverse normal stress appears to be the controlling stress for crack initiation and propagation in the (02/θ4)s coupons with θ = 600, 750 or 900. However, this is not true for the 450 plies, for which there appears to be a significant degree of interaction between the transverse normal stress and the shear stress components. Data from other multi-layer angleply laminates are consistent with the data from the (02/θ4)s laminates when effects due to ply thickness and neighbouring ply interactions are considered. A fracture mechanics model has been used to explain some of these of observations. For fatigue crack propagation, the crack growth rates were independent of crack length suggesting that the governing fracture mechanics parameter is independent of crack length. The crack growth rate has been related to the maximum value of an approximate stress intensity factor, which depends on the transverse normal stress, for all individual cracks propagated within the (02/θ4) coupons. The results plotted in log-log form indicate that a Paris law is applicable for fatigue crack growth.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available