Interlaminar Fracture Toughness Evaluation in Glass/Epoxy Composites Using Acoustic Emission and Finite Element Methods

© 2014, ASM International. Delamination is one of the most common modes of failure in laminated composites and it leads to the loss of structural strength and stiffness. In this paper, mode I, mode II, and mixed of these pure modes were investigated using mechanical data, Finite Element Method (FEM) and Acoustic Emission (AE) signals. Experimental data were obtained from insitu monitoring of glass/epoxy laminated composites with different lay-ups when subjected to different modes of failure. The main objective was to investigate the behavior of delamination propagation and to evaluate the critical value of the strain energy which is required for onset of the delamination (GC). For the identification of interlaminar fracture toughness of the specimens, four methods were used: (a) ASTM standard methods, (b) FEM analysis, (c) AE method, and (d) sentry function method which is a function of mechanical and AE behaviors of the specimens. The results showed that the GC values obtained by the sentry function method and FEM analysis were in a close agreement with the results of nonlinearity methods which is recommended in the ASTM standards. It was also found that the specimens under different loading conditions and various lay-up have different GC values. These differences are related to different stress components distribution in the specimens which induce various damage mechanisms. Accordingly, stress components distribution obtained from FEM analyses were in agreement with SEM observations of the damaged surfaces of the specimens

An integrated approach based on acoustic emission and mechanical information to evaluate the delamination fracture toughness at mode I in composite laminate

his paper addresses a new method based on the combination of mechanical behavior and acoustic emission (AE) information of composite materials during mode I delamination. The method is based on a special purpose function, called sentry function, which is defined as the logarithm of the ratio between mechanical energy and acoustic energy (f=Ln(Es/Ea)). The sentry function is used to study the delamination process and to evaluate the delamination fracture toughness in mode I. The relationship between cumulative fracture toughness energy release rate (GI) and the integral of the sentry function during crack propagation showed a transition point with two sensitive regions below and above it. This behavior can be followed to obtain the critical strain energy release rate value (GIc). Results obtained by means of the sentry function are compared with results obtained by a methodology proposed by other author

Monitoring the initiation and growth of delamination in composite materials using acoustic emission under quasi-static three-point bending test

Control and optimization of machining processes are important issues affecting the development of productivity. Monitoring systems have become indispensable in the evaluation of materials during machining. In this paper, the path toward the delamination-free drilling of glass/epoxy composite material is established using some novel methods based on acoustic emission features. Acoustic emission monitoring with three techniques, sentry function, acoustic emission energy distribution, and acoustic emission count distribution, are developed to detect and realize the critical force at the onset of delamination process on glass/epoxy composite materials. The three-point bending test simulates thrust force, the most effective factor in delamination, throughout the process of drilling without backup plate. Sentry function is regarded as a new method based on the combination of AE information and mechanical behavior of composite materials. The sentry function was used to study the initiation and growth of delamination process. Two types of specimen with different layups, woven [0, 90]s and unidirectional [0]s, leading to different levels of damage evolution, were studied. Results show that AE parameters and sentry function method are useful tools for the examination of initiation and the growth of delamination during drilling process and can help to avoid delamination damage while drilling