Strain rate dependence of mode II delamination resistance in through thickness reinforced laminated composites

Abstract

A thorough experimental procedure is presented in which the mode II delamination resistance of a laminated fibre reinforced plastic (FRP) composite with and without Zpins is characterised when subjected to increasing strain rates. Standard three-point End Notched Flexure (3ENF) specimens were subjected to increasing displacement loading rates from quasi-static (~0m/s) to high velocity impact (5m/s) using a range of test equipment including drop weight impact tower and a Modified Hopkinson Bar apparatus for dynamic three-point bending tests. The procedure outlined uses compliance based approach to calculate the fracture toughness which was shown to produce acceptable values of GIIC for all loading rates. Using detailed high resolution imaging relationships between delamination velocities, apparent fracture toughness, longitudinal and shear strain rates were measured and compared. Confirming behaviours observed in literature, the thermosetting brittle epoxy composite showed minor increase in GIIC with increase in strain rate. However, the Z-pinned specimens showed a significant increase in the apparent GIIC with loading rate. This highlights the need to consider the strain rate dependency of the Zpinned laminates when designing Z-pinned structures undergoing impact.

Coupon scale Z-pinned IM7/8552 delamination tests under dynamic loading

“…The TTR effectiveness in resisting crack propagation is not significant until the shear displacement reach about 0.1 mm, therefore , the crack had propagated for a considerable distance before considerable delamination retardation effect of the Z-pin kicked in. The effectiveness of Z-pins in dynamic shear delamination observed in the tests presented here was much lower than what previously reported [16]. This could be due to the fact that no precracking from the PTFE film [30] was performed.…”

“…However, despite the fact that Z-pinning has been employed for improving impact-damage resistance, the resulting dynamic effects are still not fully understood. Recently, ENF tests have been conducted at displacement rates up to 5m/s [16]. It was reported that the delamination resistance at the moment of crack initiation was significantly improved with Z-pinning, and this effect became more pronounced as the loading rate was increased.…”

“…The delamination of Z-pinned composite laminates has been characterised using DCB and ENF samples under quasi-static regimes [12,13,15], with dynamic ENF tests reported in [16].…”

Dynamic bridging mechanisms of through-thickness reinforced composite laminates in mixed mode delamination

“…The Z-pins were split in quasi-static tests, while no significant splitting was found in dynamic test samples. With the increase in mixed mode ratio, the Z-pins were ruptured at a position closer to the mid-plane, leading to lower pull-out lengths [29]. The energy dissipation during the failure of Z-pins decreased with mode mix ratio, mainly due to the pin rupture and reduced energy dissipation during frictional pull-out stage.…”

Dynamic inter-fibre failure of unidirectional composite laminates with through-thickness reinforcement

“…Many composite structures may be threatened by impact loading [20], and comprehensive understanding about the dynamic property of Z-pinned laminates is vital for the design and analysis of such structures [12,13,21]. To the authors' knowledge, there is no research reported addressing the dynamic in-plane failure process of Z-pinned composites yet.…”