Controlling Thermal Stresses in Composites by Means of Fiber Prestress

A new method of applying a prestress to the fibers in composite laminates prior to and during cure is described, and several [0 ^P /90] _T laminates produced at varying prestress levels are discussed. Experimental measurements described in this paper confirm that fiber prestressing can reduce or eliminate residual thermal stresses. This is evidenced by a reduction in the thermal warpage of unsymmetric [0 ^P /90] _T laminates. An existing thermal analysis of unsymmetric laminates (developed by Hyer) is modified slightly in this paper to account for fiber prestressing, and is then used to predict out-of-plane deformations of the [0 ^P /90] _T panels. Measurements compare well with theory. Specimens machined from the prestressed panels were also subjected to tensile loadings and inspected for matrix cracks in the 90° ply. It was found that fiber prestressing greatly reduced matrix cracking in [0 ^P /90] _T specimens, but had little effect on [0/90 ^P ] _T specimens.