MODE-II DELAMINATION TOUGHNESS OF CARBON-FIBER EPOXY COMPOSITES WITH CHOPPED KEVLAR FIBER REINFORCEMENT

Fibre bridging over cracks is an important aspect of reinforcement in fibre composites. In view of this, it is clear that there is an inherent structural weakness in continuous-fibre laminates, i.e., their tendency to delamination as a consequence of the lack of fibre bridging. To overcome this structural flaw, chopped Kevlar fibres are spread between continuous fibre layers. It is intended to see how the mode II delamination toughness G(II), is affected by those Kevlar fibres lying within the fracture plane. Three different carbon-fibre/epoxy composite laminates are processed and tested: the normal laminate without the Kevlar fibre reinforcement, and the other two with chopped Kevlar fibres of lengths 5 to 7 and 13 to 15 mm, respectively. Experimental results obtained from end-notched flexure specimens show that the delamination toughness, G(II), has been at least doubled, although only 1.7 mg cm-2 of Kevlar fibres are used in the mid-plane where delamination occurred. Detailed scanning electron microscopy photographs show that extensive fibre bridging, pull-out and fracture of the chopped Kevlar fibres have occurred during delamination. Also, because of the influence of the Kevlar fibres, more carbon fibres have been involved in the delamination process, leading to additional toughness increase, which is a very beneficial side effect and an important factor for the toughness improvement. The more pronounced crack growth resistance (R) curve behaviour observation in G(II) clearly shows the effectiveness of the interlaminar reinforcement. In comparison with the interleaf method suitable for 'selective toughening', one potential advantage of the current technique is that the whole laminate structure can be reinforced without affecting its bulk strength, stiffness and overall fibre composite volume ratio.