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 symmet ric elastic laminates that are loaded by uniform inplane tractions, tempera ture changes, and uniform eigenstrains induced by various physical processe s 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 r eleased 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 l ocal stresses and the maximum stress criterion. Examples are presented for 9-ply ((0) over bar/90)(2s) and (0/ +/- 45/90/(0) over bar)(S) S-glass/epox y laminates, both in the stress-free state, and after -150 degrees C coolin g from the curing temperature, with different magnitudes of pre-stress appl ied either to all plies or to the 0 degrees 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 envel opes, 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 strengt h branches. The implication is that matrix damage can be retarded or avoide d altogether by moderate pie-stress magnitudes that are within the operatin g capacity of existing filament-winding fiber or placement devices. (C) 200 0 Elsevier Science Ltd. All rights reserved.