The technique of acoustic emission has two broad applications areas. The first is nondestructive evaluation. The second is as a tool in studies or research which are not fundamentally directed towards acoustic emission. It is this second application with which we are concerned here. Acoustic emission is a very useful tool in this role because of its high sensitivity, real-time capability, volume-monitoring approach, and sensitivity to any process or mechanism which generates sound waves. This paper presents a comprehensive review of areas where acoustic emission (AE) has been used for materials studies on composite materials. The following fields, among others, will be covered: (1) time-dependent composite properties, (2) impact studies, (3) correlation of AE with stress level, (4) application to matrix cure studies, (5) relationship of AE-detected damage to other measures of damage, (6) studies of the effects of matrix material, (7) application to differences in second phase, (8) interface studies, (9) AE and dimensional stability, (10) AE applied to orientation studies, and (11) environmental effects. This review will emphasize the roles that AE can play as a tool for the materials scientist: (1) discovery of damage mechanisms, (2) characterization of damage progression with increasing time or stress, (3) optimization of fabrication variables, and (4) reduction in the numbers of test specimens required in various studies.

A review: Acoustic emission, a tool for composite-materials studies

A review of the current acoustic emission literature relating to fiber reinforced composite materials is presented. Summary tables which assist m the prompt delineation of the achievements in this research area are developed. Because of the qualitative character of much of the current literature, suggestions to develop quantitative AE standards are strongly recommended

Acoustic Emission Monitoring of Fiber Composite Materials and Structures

Acoustic emission analysis is used to investigate microscopic damage mechanisms and damage progress in unidirectional glass and carbon fibre reinforced composites. Under static loading the influence of fibre orientation on damage initiation and propagation is determined. A novel polyurethane matrix system significantly enhances material performance in terms of crack initiation load levels, crack growth, damage tolerance and off-axis tensile strength. Hysteresis measurements during stepwise increasing dynamic load tests highlight the effect of fibre–matrix-adhesion and resin fracture toughness in unidirectional 0° fibre reinforced composites. Acoustic detection of beginning fibre breakage correlates with a significant increase of loss work per cycle.

Acoustic emission analysis for characterisation of damage mechanisms in fibre reinforced thermosetting polyurethane and epoxy

Fully integrated monitoring systems have shown promise in improving confidence in composite materials while reducing lifecycle costs. A distributed optical fibre sensor is embedded in a fibre reinforced composite laminate, to give three sensing regions at different levels through-the-thickness of the plate. This study follows the resin infusion process during fabrication of the composite, monitoring the development of strain in-situ and in real time, and to gain better understanding of the resin rheology during curing. Piezoelectric wafer active sensors and electrical strain gauges are bonded to the plate after fabrication. This is followed by progressive loading/unloading cycles of mechanical four point bending. The strain values obtained from the optical fibre are in good agreement with strain data collected by surface mounted strain gauges, while the sensing regions clearly indicate the development of compressive, neutral, and tensile strain. Acoustic emission event detection suggests the formation of matrix (resin) cracks, with measured damage event amplitudes in agreement with values reported in published literature on the subject. The Felicity ratio for each subsequent loading cycle is calculated to track the progression of damage in the material. The methodology developed here can be used to follow the full life cycle of a composite structure, from manufacture to end-of-life.

Early Damage Detection in Composites during Fabrication and Mechanical Testing

Nondestructive testing (NDT) of fibre-reinforced composite materials

Acoustic emission was monitored during the axial loading of unidirectional fibre composite tensile specimens. The material consisted of strong, brittle fibres (E glass) embedded in a viscoelastic matrix (epoxy). It was found that when the load was held constant the acoustic emission output continued, but at a decreasing rate with time at load. As the load level was increased, the acoustic emission output at load continued for a longer period. It is suggested that the acoustic emission under constant load is a result of fibre fracture which continues after loading ceases because of the viscoelastic nature of the matrix which allows stress redistribution with time. The experimental results from acoustic emission are compared with computer calculations for fibre fracture based on theoretical considerations. Good agreement is noted between the theoretical and experimental results.

J. Baruch

Papers

Determining the load-time history of fibre composite materials by acoustic emission