Mehdi Yasaee

Damage-tolerant composite structures by Z-pinning

Abstract This chapter presents a focused update and the addition of new information to published reviews of the manufacture and performance of Z-pinned composite structures, concentrating on the mechanisms of enhancement of delamination damage resistance. Special emphasis is placed on the effect of manufacturing routes in terms of generation of the mesostructure of the composite. Effects of manufacturing defects, such as Z-pin misalignment, on the apparent toughness of the composite are explored in experimental and modeling terms, detailing the use of single Z-pin coupons to generate data and to validate models. The strong relationship between the mesostructure of the Z-pinned composite and its performance in both in-plane and out-of-plane properties is highlighted in relation to the differences in dominant failure mechanisms between control and Z-pinned structural elements. The chapter concludes with an update on recent advances in modeling of the effects of load mixity on the apparent toughness of Z-pinned composites.

Control of Compressive Fatigue Delamination Propagation of Impact Damaged Composites Using Discrete Thermoplastic Interleaves

A delamination damage control and management concept is demonstrated showing how, through selective placement of discrete thermoplastic film interleaves, it is possible to manipulate the formation of impact damage and control its subsequent propagation during compressive fatigue cyclic loading. Under cyclic loading the gradual growth of impact damaged delamination was shown to be arrested by the discretely embedded interleaved strips. This technique then resulted in the growth of delamination away from the arrested sites effectively moving the damage zones and thus confirming the concept of delamination damage control. By managing the propagation of delamination, significant improvements to the compression after impact fatigue life of the composite were achieved. The proposed method is cost-effective and can be implemented with minimal disturbance to the global laminated composite properties. With an optimised design approach to managing delamination damage, safe ‘damage tolerant’ composite structures can be realised with significant weight saving benefits.

Drawdown prepreg coating method using epoxy terminated butadiene nitrile rubber to improve fracture toughness of glass epoxy composites

Laminates of fibre-reinforced prepreg have excellent in-plane mechanical properties, but have inadequate performance in the through thickness direction. Here, we address this issue by application of epoxy-terminated butadiene nitrile (ETBN) liquid rubber between the prepreg laminae using an automatic draw bar coating technique. Test results reveal that by adding ETBN in small quantities in the range of 9.33–61.33 g/m2, the interlaminar critical energy release rates (GIc and GIIc) are improved by up to 122% in mode-I and 49% in mode-II. Moreover, this finding is further supported by the dynamic mechanical analysis thermograms that clearly indicate that coating has not altered the Tg of ETBN-coated samples. Scanning electron microscopic analysis of fracture surfaces showed that rubber particles formed micro cavitations in the epoxy, causing localised rubber rich regions. These resin-rich regions require more energy to fracture, resulting in increased toughness of the glass epoxy prepreg systems.

Dynamic Mode II Delamination in Through Thickness Reinforced Composites

Through thickness reinforcement (TTR) technologies have been shown to provide effective delamination resistance for laminated composite materials. The addition of this reinforcement allows for the design of highly damage tolerant composite structures, specifically when subjected to impact events. The aim of this investigation was to understand the delamination resistance of Z-pinned composites when subjected to increasing strain rates.