Experimental Investigation of the Tensile-Tensile Fatigue Behaviour of Carbon/Epoxy Plain Weave Laminates

In order to develop safe designs of aerospace composite structures it is important to study the effect of manufacturing flaws such as delamination on the fatigue behaviour of composites. The objective of the present study is to determine the fatigue behaviour of carbon/epoxy plain weave laminates containing artificial flaws under tensile loading. The standard test coupons contain a Teflon insert of pre-determined size, shape and location, which simulates a common manufacturing defect. The coupons were subjected to tensile load-controlled cyclic loading with a stress ratio R = 0.1 and frequency f = 7 Hz. A failure onset criterion was proposed to determine the delamination propagation threshold under cyclic tensile loading. The methodology used for this purpose is based on the changes in the test coupon's compliance which was measured using a high-speed video-extensometer. The propagation of delamination was verified using the ultrasonic imaging (C-Scan) technique and validated by optical microscopy observations of the coupons' cross-sections. A S-N curve corresponding to the delamination onset was established permitting to determine the fatigue life of the specimen versus the maximum applied stress.