3.7 THICKNESS EFFECTS

Thick composites present unique problems in manufacturing, design, analysis, and testing. More importantly the prediction of their dimensional stability behavior is coupled to many of these considerations. We can define "thick" composites from the limitations of classical laminate plate theory, which requires h << a or b. A laminate that is thicker than about 0.1 inch or 2.5 mm should be considered from this viewpoint. Thick composites are often used in aircraft, such as in wing root sections where the thickness may exceed 10 mm (80 plies) [74]. They are being promoted for naval, automotive, missile and aircraft applications and special manufacturing procedures are being developed [191].

In manufacturing, there are at least five problem areas [191]: (1) internal thermal spiking (see also Section 9.7), (2) non-uniform cures, (3) non-uniform consolidation, (4) lengthy process cycle times, and (5) residual stresses [56, 57]. Besides residual stresses, composite dimensional behavior is affected by increased thickness coupled with strength [56-61], optimal stacking sequences [60-66], microcracking [59, 65, 67], and delamination tendencies [68-73]. For example, microcrack crack density decreases with increasing layer thickness [65]. Control of ply orientations, ply thickness, and fiber volume fractions becomes more difficult with total laminate thickness [74]. In the case of a coated fiber in a continuous matrix subjected to thermo-mechanical loadings, it was found that the maximum stress occurs in the coating, and that it decreases as the fiber volume fraction and coating thickness increase [76].

Analytically, 3D failure criteria and finite...