Composite Materials Practice Test

Curing residual stresses come from the mismatch between how the matrix and the fibers shrink or expand during cure and cooling, and they are set by how the part is processed. The key factors shaping these stresses are the cure temperature, how fast you cool, and the boundary constraints from the layup. Higher cure temperatures create a larger temperature difference from the end of cure to room temperature. Since the resin and fibers respond differently to temperature and have different shrinkage behaviors, a bigger temperature swing leads to a greater differential strain that becomes locked in as residual stress when the laminate solidifies and is restrained by the surrounding material. Cooling rate matters because it controls how much time the material has to relax stresses before the network becomes rigid. Slower cooling allows more viscoelastic relaxation and rebalancing of strains, reducing residual stresses, while rapid cooling traps more of the differential strain as stress in the laminate. Layup constraints, including the stacking sequence, fiber directions, and tooling boundaries, dictate how the differential strains are constrained within the laminate. Boundaries that resist transverse or in-plane movement convert those differential strains into residual stresses, especially where plies with different CTEs are bonded together and cannot freely contract. Why the other ideas don’t fit: humidity during service isn’t a curing factor and won’t create curing residuals. Resin viscosity at room temperature isn’t controlling the cure-stage mismatch that locks in residual stresses. Making plies thicker doesn’t eliminate residual stresses and can actually influence them through greater constraint and different stress distribution.