An Assessment of Buffer Strips for Improving Damage Tolerance of Composite Laminates

Abstract

Graphite/epoxy panels with buffer strips were tested in tension to measure their residual strength with crack-like damage. Panels were made with [45/0/-45/90]2 S and [45/0/-45/0]2 S layups. The buffer strips were parallel to the loading direction. They were made by replacing narrow strips of the 0° graphite plies with strips of either 0° S-Glass/epoxy or Kevlar-90/epoxy on either a one-for-one or a two-for-one basis. In a third case, 0° graphite/epoxy was used as the buffer material and thin, perforated Mylar strips were placed between the 0° plies and the cross-plies to weaken the interfaces and thus to isolate the 0° plies. Some panels were made with buffer strips of different width and spacings. The buffer strips arrested the cracks and increased the residual strengths significantly over those of plain laminates without buffer strips. A shear-lag type stress analysis correctly predicted the effects of layup, buffer materi al, buffer strip width and spacing, and the number of plies of buffer material.