Unfolding the effects of tuft density on compression after impact properties in unidirectional carbon/epoxy composite laminates

Abstract

Post impact compressive residual strength of epoxy-based composite laminates is studied in the presence of through-thickness reinforcement (TTR). The tufting technique is used to introduce TTR in compact carbon unidirectional (UD) composites. Composite laminates are tufted using Kevlar thread with different tuft density by varying tuft pitch and spacing. The specimens are impacted with low-velocity impact to achieve barely visible impact damage (BVID). The damaged area is quantified using the ultrasonic C-scan method. Tufted specimens exhibited a significant reduction in the damaged area. The reduction in damage area is about 26% to 51% depending upon the tuft density. Residual strength post impact is determined by compression after impact (CAI) test. Upon increasing the tuft density up to 0.56%, the damaged area decreases by 51%, and CAI strength increases by 43%. Above this tuft density, damage area slightly decreases and CAI strength remains almost the same. The improvement in CAI strength is due to an increase in apparent interlaminar strength attributed to the enhancement in bridging effect due to TTR. Different failure modes such as delamination, fiber crushing, kink-band formation, etc. are observed in both the untufted and tufted specimens.