Apparent volumetric shrinkage study of RTM6 resin during the curing process and its effect on the residual stresses in a composite

A comprehensive characterization of the volumetric shrinkage of a commercially important aerospace resin (RTM6) during the various stages of the curing process was studied. The apparent volumetric shrinkage, evaluated from density measurements at room temperature, was correlated with the progress of epoxide conversion. During the entire curing process, the apparent volume shrinkage was found to be less than 3% and occurred before vitrification. A slight re-expansion of the resin, attributed to self-antiplasticization effects, was observed during postcuring at 1808C. It was concluded that residual stresses were not generated due to chemical cross-linking during curing but rather from thermal contraction occurring during the cooling stage after cure. A photo-elastic method was used to characterize residual stresses during cooling in a deliberately engineered resin rich hole of a carbon fiber/RTM6 composite. The residual stress was found to reach approximately 28 MPa, which is in good agreement with the value calculated from the shrinkage and elastic moduli. It is proposed that this simple method can be provide insights useful to the design and materials selection processes by measuring and localizing residual stresses from resin during curing and or thermal cycling.