Characterization and prediction of post-impact fatigue damage in stitched composites.

摘要

The post-impact fatigue response of stitched carbon/epoxy composite materials was characterized and modeled in this investigation. The five material configurations evaluated were composed of uni-woven AS4 graphite lamina arranged in a (45/0/ {dollar}-{dollar} 45/90) {dollar}sb{lcub}rm 6S{rcub}{dollar} preform. Four preforms were reinforced by a modified lock stitch process, and each material was fabricated with a specific stitch row spacing of either 1/4, 3/16, or 1/8 inches. An unstitched baseline laminate was the fifth configuration. A brittle resin system was used to consolidate three stitched and the unstitched preforms, while a toughened resin system was used in a second preform which was also stitched with 1/4-inch rows. Short-block compression and compression-after-impact tests were conducted on every material to establish their baseline performances. All the impacts were performed with a 10-lb drop-weight impacter with a 1/2-inch hemispherical tup set to deliver a uniform 1500-in.-lb/in. energy level. Impacted fatigue specimens were tested at a 4-Hz frequency with a tension-compression stress ratio of R = {dollar}-{dollar}5. Dye-enhanced radiographic techniques and extensometers were used to monitor both the post-impact condition and the fatigue damage propagation. Destructive sectioning and pyrolysis methods were also used to quantify the damage states.; Statistical analysis of the undamaged compressive properties shows no strength penalty associated with the stitching process. However, stitching reinforcement is found to have statistically significant affects on the elastic moduli and ultimate compressive strains. Lower elastic moduli and higher compression strains are found in the stitched materials compared to the unstitched baseline. All the post-impact fatigue coupons display stable damage growth. While circular delaminations elongate into ellipses in the unstitched samples, the stitched materials develop narrow damage zones transverse to the loading direction. These narrow zones are characterized by extensive regions of transverse shear failure surfaces. Critical damage zone length is predicted using a point-stress failure criterion based on an open-hole stress distribution. The propagation of these narrow zones is empirically predicted with a non-dimensional parameter, P, which allows the post-impact fatigue data to be correlated with an exponential damage growth model.