Residual Hygrothermal Stresses. Data and Analysis

Composite materials are cured at elevated temperatures that may range between 175 and 190°C for epoxy-based materials and could exceed 450°C for polyimide resin components. Cooling down to room temperature gives rise to residual thermal stresses on both the laminate and individual ply levels. While the laminate-level residual stresses were discussed in Chap. 2, the ply-level stresses arise due to the disparity in thermal expansion coefficients between fiber and resin. The evaluation of those stresses is hampered by the paucity of data on the coefficient of the transverse expansion of fibers. Nevertheless, it was possible to demonstrate the 

The above figures demonstrate that both sets of residual stresses tend to counteract each other, which is due to the fact that cool-down is associated with shrinkage, whereas the uptake of fluid causes expansion. 

Nevertheless, a straight forward superposition of the profiles drawn in Figs. 6.16a, b would fail to represent realistic circumstances due to the huge disparity, of about six orders of magnitude, between the coefficients of thermal and moisture diffusion. In fact, the exceedingly slow process of moisture diffusion would leave major portions of the composite ply in the dry, or nearly dry, state for long times and the non-uniformity in moisture profiles would modify the distribution of residual stress on the ply level, as shown in Fig. 5.7. 

Another aspect of residual stresses stems from their sign reversal upon drying caused by viscoelastic relaxation that was already alluded to in Chap. 2 and sketched in Fig. 2.3. For the case of fluid ingress alone, this is demonstrated by the following example (Weitsman 1979). 

Consider a thin adhesive layer confined between rigid adherends exposed to moisture at X = 0. In view of the imposed constraints, moisture-induced swelling 

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