Use of the acoustic emission method in detecting the fracture process in specimens made of composite materials

Abstract

Experimental results on the localization of acoustic emission (AE) signals obtained in stresscycling tests of specimens made from the Organit-10T composite material (CM) are considered. The technique developed for collecting prompt information on the changes in the shape of the AE amplitude distribution is analyzed. This distribution characterizes the flow of arriving signals and allows one to determine the beginning of destruction for CM specimens.Structural strength is determined by the mechanical properties of a material an test object (TO) is made of and by real operational conditions. For most products, fracture under alternating stress is caused by fatigue [1]. Fatigue fracture of a TO occurs when a crack nucleates and grows in a material under stress. The process of crack propagation determines the durability of a material. When the durability is high, the crack propagation velocity is small and the crack can be detected by non-destructive testing (NDT) methods [1]. The development of high-durability composite materials (CM) is one of the main objectives of materials science.State-of-the-art CMs have rather high strength and toughness characteristics. The possibility of generation of numerous defects in CM structures necessitates the development of NDT methods. The acousticemission technique is the most efficient among these methods. However, the practical application of this method for strength testing of CM products is limited by certain unsolved problems [2][3][4][5]. So far, no techniques have been designed for the reliable localization of a fracture area or for collecting prompt information on the changes in characteristics of the AE signal flow and relating these changes to the structure of the material. In the majority of available studies that describe the use of the AE method in the testing of CM products, the data obtained are either presented in the form of amplitude distributions (AD) of the signals or the number of signals N counted that have arrived during time t from the start of loading [2-7].As it has been noted in [6,7], AE signals carry information on the sizes of cracks, their propagation velocity, the strain in their nucleation sites, and the elastic properties of the TO material. It is difficult to obtain quantitative information about these parameters due to the fact that the acoustic signals detected by acoustic-emission transducers (AET) are considerably distorted because of the superposition of different types of waves reflected by the TO boundaries and by the AETs themselves.The objective of this work is to design a method for AE testing of the fracture process during stresscycling tests of CM specimens.To estimate strength properties and operational life, studies of specimens made of the Organit-10T material were carried out under cycling stress testing. The material was treated in a technological chain standard for manufacturing an aircraft wing flap. The specimens consisted of the SVM material with UP-22-27 as the binding agent. Local strengthening was performed by carbon-fibe...

A procedure for locating acoustic-emission signals during static testing of carbon composite samples

“…Defects in objects that are made from composition materials (CMs) occur in both the course of their production and operation; this necessitates the improvement of the methods and tools for nondestructive testing (NDT) [1][2][3][4]. To perm quality control of composites, the ultrasound, acoustic emission (AE), X ray, radiographic, thermal imaging, thermographic, optical, and several other methods are used [5][6][7][8][9].…”

“…This complicates the determination of the signal arrival time at piezoantenna sensors; for this reason, the location that is performed by the step out times (SOTs) of AE signals is complicated and leads to substantial dispersion of the defect coordinates [6][7][8]. Therefore, studies that are related to the development of procedures for the AE testing of objects that are made from CMs based on CFRP that allow a decrease in the dispersion of the defect coordinates during their location are topical [1,4,[6][7][8][9]. The aim of this work is to develop a procedure for the location of AE signal sources during the devel opment of defects in samples made from CFRP under static loading.…”