The effect of fiber pre-stress on residual stresses and the onset of damage in symmetric laminates

Stresses and deformations caused by fiber pre-stress are analyzed in symmetric elastic laminates that are loaded by uniform in-plane tractions, temperature changes, and uniform eigenstrains induced by various physical processes in symmetrically located pairs of plies. The pre-stress is applied to all fibers of selected symmetric pairs of plies prior to matrix cure, and is released after cooling to room temperature. The effect of the pre-stress on residual stress distribution is illustrated by failure maps constructed in overall stress planes of ply strength branches, derived from the computed local stresses and the maximum stress criterion. Examples are presented for 9-ply (0̄/90)2s and (0/±45/90/0̄)s S-glass/epoxy laminates, both in the stress-free state, and after −150°C cooling from the curing temperature, with different magnitudes of pre-stress applied either to all plies or to the 0° plies. Releasing the pre-stress is shown to be equivalent to applying a sustained compression in the axial direction of each ply. The ply strength branches and initial failure envelopes, therefore translate in the overall stress space in the direction and by the magnitude of the overall stresses that are equivalent to the applied pre-stress. In systems with compliant polymer matrices, fairly small fiber stress changes are needed to support large translations of the ply strength branches. The implication is that matrix damage can be retarded or avoided altogether by moderate pre-stress magnitudes that are within the operating capacity of existing filament-winding fiber or placement devices.