Cure path dependency of mode I fracture toughness in thermoplastic particle interleaf toughened prepreg laminates

Chris Hunt,James Kratz,Ivana K Partridge

doi:10.1016/j.compositesa.2016.04.016

Chris Hunt, James Kratz + Show 1 more

Open Access

https://doi.org/10.1016/j.compositesa.2016.04.016

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Abstract

The effect of cure cycle on fracture behaviour of a commercial thermoplastic particle interleaved prepreg system was investigated. Laminates were manufactured at 700kPa in an autoclave using eight different thermal cycles that included both raising the cure temperature above the standard 180°C cure cycle and incorporating an intermediate dwell stage between 150 and 170°C prior to reaching the 180°C cure temperature. Double cantilever beam tests were conducted on specimens from the cured laminates. The stick–slip crack behaviour, observed in samples manufactured using the standard cure cycle, changed to stable crack growth when processing deviated by 10°C. The mode I fracture toughness values were reduced by 11–22% when incorporating an intermediate dwell stage before the final cure temperature. Scanning electron microscopy inspection of the fracture surfaces showed differences between samples made by standard cure cycles and those made using process deviations.

Highlights

Interleaving or interlayer toughening is a common technology used to improved delamination resistance of advanced composite materials
As well as increasing the plastic zone ahead of the crack tip, this may involve a number of supplementary toughening mechanisms including particle bridging, crack pinning, crack path deflection and microcracking [5,6]
Mode I delamination fracture toughness of an interleaf toughened prepreg was investigated in laminates manufactured with a variety of cure cycles, some of which were outside the manufacturers’ recommended 180 °C cure cycle

Summary

Introduction

Interleaving or interlayer toughening is a common technology used to improved delamination resistance of advanced composite materials. Curing reactions of thermosetting resins are exothermic, and when combined with the low through-thickness thermal conductivity of prepregs, thermal gradients and local temperature overshoots can arise within a part while curing [9] This phenomenon is prevalent in the manufacturing of thick composite parts, where high temperatures can lead to polymer degradation and the generation of residual stresses [10]. The influence of the manufacturing process on interlaminar properties of an interleaf particle toughened prepreg was investigated by Zhang and Fox [17] They found that for laminates cured using the quickstep process, mode I fracture toughness was 2.6 times higher than those cured in an autoclave. The cure cycles were chosen to investigate the effects of temperature overshoots and typical cure cycle modifications used to avoid temperature overshoots, including additional temperature dwell stages and lower ramp rates

Laminate manufacturing

Test procedure

Results and discussion

Microstructure observations

Unconstrained plastic zone

Conclusion