⊙ Residual stresses in composite materials tensile stress close to the surface.Thus it significantly slows down the growth of a crack as it reaches the region of compressive stress.The second reason is that with the extension of the crack,the stiffness or compliance of the area changes, resulting in the release of the previously trapped-in load.An example of the first case would be a surface crack,which is exposed to uni-axial tension.Such a crack will enlarge and infiltrate a plate,as the width of the crack reaches a size of nearly four times its thickness(Raju and Newman,1979).However,a crack loaded by the same outer pressure from above,with a considerable residual stress beneath its surface,will extend more quickly on the surface,with a width to depth ratio greater than ten.Consequently,the crack takes longer to penetrate the object, despite the fact that the component continues to lose strength with the further extension of the crack upon the surface.For a pressurized vessel under such circumstances,any leakage taking place may be a sign of a quickly growing crack (Finnie et al.,1990). It is usual for parts to become deformed upon welding or heat-treatment during manufacturing processes,and the understanding of this is an indicator of the extent to which engineers are experienced (Prime,1999a b).Analyses of the fundamental mechanics and measurement of residual stresses under different circumstances have substantially increased our knowledge about residual stresses throughout the past century,allowing us to assess and boost the integrity of current components.The behavior of materials and components is crucially affected by the existence of residual stresses. Residual stresses can lead to defects in composite structures,such as fiber waviness,cracking,delamination,warpage,dimensional instability and spring-in (Stamatopoulos,2011).Fiber waviness in uni-directional materials occurs when the fibers deviate from the average direction of the laminate,creating a pattern that can usually be mathematically represented by a sine wave (Fig.1.1).Fiber 0.1mm 1.1 Micrograph of a composite laminate showing fiber waviness (Parlevliet et al.,2007). Woodhead Publishing Limited,20148 Residual stresses in composite materials © Woodhead Publishing Limited, 2014 tensile stress close to the surface. Thus it signifi cantly slows down the growth of a crack as it reaches the region of compressive stress. The second reason is that with the extension of the crack, the stiffness or compliance of the area changes, resulting in the release of the previously trapped- in load. An example of the fi rst case would be a surface crack, which is exposed to uni- axial tension. Such a crack will enlarge and infi ltrate a plate, as the width of the crack reaches a size of nearly four times its thickness (Raju and Newman, 1979). However, a crack loaded by the same outer pressure from above, with a considerable residual stress beneath its surface, will extend more quickly on the surface, with a width to depth ratio greater than ten. Consequently, the crack takes longer to penetrate the object, despite the fact that the component continues to lose strength with the further extension of the crack upon the surface. For a pressurized vessel under such circumstances, any leakage taking place may be a sign of a quickly growing crack (Finnie et al. , 1990). It is usual for parts to become deformed upon welding or heat- treatment during manufacturing processes, and the understanding of this is an indicator of the extent to which engineers are experienced (Prime, 1999a,b). Analyses of the fundamental mechanics and measurement of residual stresses under different circumstances have substantially increased our knowledge about residual stresses throughout the past century, allowing us to assess and boost the integrity of current components. The behavior of materials and components is crucially affected by the existence of residual stresses. Residual stresses can lead to defects in composite structures, such as fi ber waviness, cracking, delamination, warpage, dimensional instability and spring- in (Stamatopoulos, 2011). Fiber waviness in uni- directional materials occurs when the fi bers deviate from the average direction of the laminate, creating a pattern that can usually be mathematically represented by a sine wave ( Fig. 1.1 ). Fiber 1.1 Micrograph of a composite laminate showing fi ber waviness (Parlevliet et al. , 2007)