3.9 EDGE/END EFFECTS

This section reviews the influences that edges, ends, and surfaces have on the dimensional stability of composite materials. While there are many effects associated with edges and boundaries, such as loss of strength [e.g., 60, 62, 73], we are primarily concerned with their influence on dimensional stability. We consider first the physical extent of these effects (decay length), then their causes (interlaminar shear and 3D stresses, etc), the effects of these stresses on microcracking and delamination tendencies, the actual material distortions at the edges, and finally their effects on other sources of instability, such as the hygrothermal expansion coefficients.

Edges, corners, or free surfaces possess a different stress state than the bulk material. This is to be expected; since the restraints on the material are less near a surface than in the bulk. We may also consider a boundary layer or decay length in the vicinity of a free edge, whose dimensions are similar to the thickness of the laminate [16]. Studies of the end effects in the torsion or tension of cylinders, or in the bending of beams, suggest that end effects die away within a length about equal to the maximum cross-sectional dimension (St. Venant's principle). This is usually true for isotropic materials but with highly anisotropic composites, the decay length can be considerably greater than the maximum cross-sectional dimension [89-92], and this is especially true of sandwich structures [91, 93]. Horgan [91] summarizes the situation with sandwich structures where the Young's modulus of...