Buffer Strips in Composites at Elevated Temperature

Abstract

The present investigation evaluated the buffer strip concept at elevated temperature. Accordingly, graphite/polyimide buffer strip panels were tested at room and elevated (177 ± 3 °C) temperatures. One layup was used: [45/0/45/90]_2S. The buffer strip material was 0° S-glass/polyimide. The buffer strips were made by replacing narrow strips of the 0° graphite plies with strips of the 0 °S-glass on either a one-for-one or a two-for-one basis. All panels had the same buffer strip spacing and width. Slits were cut in the center of each panel to represent damage. Each panel was loaded in tension until it failed. During the tests, panels were radiographed and crack-opening displacements were recorded to indicate fracture onset, fracture arrest, and the extent of damage in the buffer strip after crack arrest.

At the elevated temperature, the buffer strips increased the panel strength by at least 40 percent compared to panels without buffer strips. The buffer strip panels tested at 177 °C exhibited stable crack growth until the crack grew into the buffer strip. Com pared to similar panels tested at room temperature, the buffer strip panels tested at 177 °C had lower residual strengths but higher failure strains. The panels with two-for- one buffer strip replacement had slightly lower strengths (by 5 percent) than panels with one-for-one replacement. A shear-lag type stress analysis correctly predicted remote failing strains for buffer strip panels tested at 177°C and at room temperature.