Fig. 4: Diagram showing mode II fracture toughness "in-plane shear" mode testing Fig. 6: Mode II fracture toughness - GIIprop plotted against GIIc These laminates were also subjected to mode I and II fracture toughness measurements using a methodology similar to that described above. Fig. 5: Mode I fracture toughness - GIprop plotted against GIc whereby a bending moment is applied to an end-notched flexure sample as represented diagrammatically in figure 4. The studies by Kuwata and Hogg [6, 7] encompassed various material and laminate types but also included a UD carbon fibre-epoxy laminate (similar to the laminates used by Sampson et al. [8]) with a 13 g/m2 polyester (PET) veil interleave. A simplified summary of the mode I and II fracture toughness results of CFRP laminates with nonwoven interleaves reported by Kuwata and Hogg [6, 7] and Sampson et al. [8] is shown in figures 5 and 6 respectively. Control laminates, which did not contain nonwoven interleaves but were identical to the interleaved laminates in all other respects, were also included to aid the interpretation of results. It can been seen clearly from figures 5 and 6 that all fracture toughness measurements are improved by the addition of a lightweight thermoplastic interleave (Control Laminate 1 is compared to the PET veil laminate and Control Laminate 2 is compared to both the PPS and PEEK veil laminate types). The degree of improvement as a percentage is tabulated below. Tab. 1: Comparison between the different laminates Fibre type Areal weight GIC Improvement vs control GI PROP Improvement vs control GIIC Improvement vs control GIIPROP Improvement vs control Polyether ether ketone (PEEK)8 11 g/m² 102% 62% 334% 334% Polyphenylene sulphide (PPS)8 7 g/m² 61% 21% 199% 59% PPS8 10 g/m² 98% 26% 246% 189% PPS8 15 g/m² 133% 40% 322% 320% PPS8 20 g/m² 163% 61% 350% 430% Polyester (PET)6 & 7 13 g/m² 56% 15% 56% 48% No103 March 2016 / jec composites magazine 83