On the Effect of Residual Stresses on the Transverse Cracking in Cross-ply Carbon-polyetherimide Laminates Proefschrift

Two themes form the basis of this study on fibre reinforced laminated composites: namely 'thermal residual stress' and 'transverse cracking'. Thermal residual stresses are inherent to any composite structure and are internal stress within the structure that form during manufacture. Their formation arises from the difference in coefficient of thermal expansion of the fibre and the matrix. These stresses will act on both a fibre-matrix scale and on a ply-to-ply scale within a laminate. The presence of residual stress reduces the strength potential of composites. Transverse cracks are formed in laminates, which comprise plies of different fibre orientation. Transverse cracks run parallel to the fibres, through the thickness of an individual ply and are induced by a stress field acting transverse to the fibres. These cracks are recognised as being the first damage mechanism to occur in composite structures. Although the formation of transverse cracks is not generally directly responsible for the complete fracture of a composite structure, transverse cracks can serve to catalyse the development of further damage mechanisms. In certain composite systems, the level of thermal residual stress in a direction transverse to the fibres can approach the stress at which transverse cracking occurs. This is the case for the unidirectional carbon-fibre/polyetherimide system formed as a cross-ply laminate. The carbon-fibre/polyetherimide system combines a large difference in coefficient of thermal expansion between the fibre and the matrix with a large temperature differential during initial process cooling. This combination leads to large residual stresses in the laminate after its formation. The high level of residual stress that can be obtained with this system makes it ideal for the study of the effect of residual stresses on the formation of transverse cracking. This study investigates the role played by the residual stresses in the formation of transverse cracks in composite laminates. Limiting stress and energy release failure criteria are examined for the prediction of transverse crack formation in transversally loaded cross-ply carbon-fibre reinforced polyetherimide laminates The level of residual stress in the laminates considered is varied in different ways. The experimental and theoretical study concentrates initially on transverse crack formation in beam specimens under 3-point bending. A final section extends the study to the crack formation in point loaded laminate plates, where specimen edge effects are eliminated. The thesis is divided into 4 chapters, each representing a separate article dealing with a different aspect of the study. In chapter 2, the …