The effect of defects on the performance of post-buckled CFRP stringer-stiffened panels

Abstract This paper describes the testing and failure analysis of wing relevant skin-stringer panels containing defects. Five panels containing two types of defect; embedded defects (representative of inclusions introduced during fabrication) and impact damage (representative of the damage from dropped tools), were tested under monotonic compression loading. The defects were located at two sites; within the bay between stringers and beneath the stringer foot. All the panels buckled at approximately the same applied strain (−2780 μe) and buckling mode; three-half waves in each bay. The undamaged panel failed at an applied strain of −6261 μe and the failure was attributed to skin/stiffener debonding. The presence of defects led to a reduction in the strength ranging from 7% (bay impact) to 29% (foot impact). Fractographic evidence showed that the effect of the defects on the panel strength was related to how they influenced skin/stiffener debonding. For foot defects, impact was worse than an embedded defect, whilst for bay damage this effect was reversed. The rate and extent of damage growth differed with defect type; the results indicate that modelling impact damage as a single plane embedded defect is of limited validity and may be non-conservative.