AE analysis of mode II interlaminarfracture toughness of 3D reinforced CFRP

Abstract

The industrial application of composite materials is increasing, showing its viability to replace other materials have been gradually growing. Particularly, composite materials reinforced with carbon fibres have a good ratio of resistance/weight, being an advantage for many applications since Carbon Fibre Reinforced Polymers (CFRP) allows reducing weight and keeping structural resistance. Resistance to delamination damage is a key factor for composite materials, since it deals with material safety during service life. Fibre orientation in composite materials is designed to reinforce resistance in specific directions of severe solicitation. Nonetheless, out-of-plane loadings are inherent solicitations that occurs in service life and fibre orientation are not intended to reinforce thought thickness direction, making out-of-plane loadings a critical factor. Reinforcing composite materials through thickness, or 3D, has the potential to increase the material out-of-plane resistance and thus permitting a higher accuracy levels in terms of structural design. There are different ways to apply 3D reinforcement on composite materials, like Z-pinning, Stitching and Tufting. Non-destructive testing (NDT) are efficient methods of reassuring safety of components, NDT allows detection of damage and flaws in early stages and monitoring of a component. NDT methods are particular useful for composite materials for avoiding parts containing flaws, indicating repairs to increase lifespan and for lifespan estimation. Acoustic emission is one technique of NDT (non-destructive testing) that allows monitoring of damage evolution in real time, since it detects elastic waves emitted by energy release caused by changes in internal structure, for NDT purposes it means damage. This paper monitors and evaluates the signs generated during the mode II interlaminar fracture toughness test, end-notched flexure (ENF), monitored by acoustic emission comparing the effect of different though thickness reinforcements and a non-reinforced sample. The testing procedure and damage development are analysed regarding acoustic emission features and its behaviour. The results show that acoustic emission has a strong relationship with stages of testing, and samples resistance induced by the reinforcement. The data acquired from the ENF tests in synergy with acoustic emission provided damage characterization and differentiation, reaffirmed by fractography analysis, especially in terms of final stage of damage and 3D reinforcement effects on it.