Crack Arrest and Residual Strength of Composite Panels with Softening Strips

Three materials, E‐glass, Kevlar, and S‐glass, are assessed in terms of their effectiveness as softening strips in the fail‐safe design of composite structures. The softening effect is assessed in terms of its influence on the failure behavior of AS/3501‐6 graphite‐epoxy laminates consisting of 16 plies in a lay‐up of $[\pm 45\text{,}{O}_2{]}_{\mathrm{2S}}\text{.}$ Softening strips are created in the basic material by locally replacing 0°‐ply fibers with the candidate softening materials. Initial damage in the panels is introduced by saw‐cutting' or as a result of an impact load in the geometric center of the specimen. In some cases, the specimens are fatigue‐cycled prior to the introduction of the initial damage; in other cases, the cyclic loads are applied subsequent to the saw‐cutting or impact loading. Moisture effects on strength degradation and the crack‐arrest capability of the softening strips are included. The experimental results indicate that the S‐glass softening material has a greater beneficial effect than Kevlar or E‐glass on the fail‐safe characteristics of the basic material. Finally, analytical correlations of some test results are presented.