Impact and fracture

This chapter considers the impact and fracture behaviour of unidirectional, woven and mat composites. Impact behaviour is a difficult area for the designer and the results given here can only act as an approximate guide to material behaviour and the effects of specimen geometry and edge conditions. 

The impact and fracture behaviour of composites is complex. Thermosets do not undergo plastic deformation, composites are not necessarily described by linear elastic fracture mechanics (LEFM) and it is very difficult, if not impossible at the present time, to predict the initiation and progression of failure in complicated structures from the behaviour of unidirectional materials. Because of the latter much of the work on fracture has involved studies of plied or laminated specimens. 

There are several approaches to impact and fracture failure and it is useful to allude to some of them before giving specific data. 

One way of rating materials is based on the elastic energy that can be stored, in a unit volume, for a particular mode of stressing. For the tensile case this is proportional to 

In practical applications the designer or analyst may combine a failure criterion and knowledge of known defects and fracture energies to predict how and where failure is initiated and the subsequent progress of damage in the material. Such analyses usually have to be augmented by empirical data to model the complete failure process. 

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Bader M.G., Bailey J.E. and Bell 1. (1973). The effect of fiber-matrix interface strength on the impact and fracture properties of carbon fiber-reinforced epoxy resin composites. J. Phys. D Appi. Phvs. 6, 572-586. 

The compression, impact, and fracture properties of many cured unsaturated polyester resin formulations have been studied and reviewed.The compressive strengths of the rigid cured polyesters are of the order 15,000-30,000 psi. Cyclic glycols and specialty fumarate resins tend to raise this value, while aliphatic saturated acids lower the compressive strength. For aliphatic acids, the impact strength is proportional to the acid chain length. 

Irradiation-induced embrittlement, modifications of impact, and fracture toughness properties... 

Sreekumar et al. (2008) have reported the effect of fiber content in polyester composites and have reported 40% volume fraction to show maximum tensile strength. Sapuan et al. (2006) investigated the mechanical properties of woven banana fiber reinforced with epoxy composites. Tensile strength. Flexure, impact and fracture surface study of woven pseudo stem banana fiber reinforced with epoxy composites was reported and the chemical modification of kenaf fiber was carried out by Edeerozey et al. (2007). Anuar et al. (2011) studied the thermal... 

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