Acoustic Emission – the Sentry Function

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  • Topic: Fracture mechanics, Materials science, Composite materials
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  • Published : March 20, 2012
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ACOUSTIC EMISSION – THE SENTRY FUNCTION
Giangiacomo Minak
Alma Mater Studiorum – Università di Bologna, Mechanical Engineering Department, Faculty of Engineering, viale Risorgimento 2, 40136 Bologna, Italy Corresponding author: giangiacomo.minak@unibo.it

1. Introduction
The acoustic emission (AE) technique has been used for decades to detect damage onset and propagation in different kind of materials [1]. The more complex microstructure of the material, the more information can be derived from the AE signal [2]. Solid mechanics experimentalist are familiar with the acoustic emission produced by the material during the loading phase, which sometimes can be heard simply by naked ears. In fact, during a material test or in general when a component is subject to external loads, a rapid stress redistribution can occur due to permanent and irreversible phenomena, caused by damage mechanisms. During this redistribution, part of the strain energy stored in the material is released in the form of heat and of elastic waves that propagate in the material until they reach the free surface. These transient elastic waves are commonly detected as acoustic waves. Some acoustic emission can be also produced by mechanisms different from damage (such as sliding and friction of two surfaces in contact) and this must be taken into account. The elastic waves propagating at the component surfaces are detected by means of piezoelectric devices that convert the mechanical signal into an electrical one. Even if the AE physical principle is very simple and immediate, the use of this technique is not so straightforward because the acoustic wave propagation in solids is quite complicated. Multiple waves that propagate with different velocities, reflection, refraction, dispersion, and attenuation, may affect the measured signal. Nevertheless some advantages with respect to other non destructive testing techniques can be found in the possibility to monitor a large volume of material by means of few sensors able to locate the damage by triangulation and to make it continuous during real life service. In reality, the acoustic emission is produced within the material itself once loaded at a level that

produces some form of damage. In this sense, it is not strictly a non destructive testing method since it is based on passive monitoring of acoustic energy released by the material or structure itself while under load. Mechanical information and AE information can be analyzed separately to determine damage in the structure. However, when one is taken into account and the other is omitted a comprehensive damage characterization cannot be taken out. In this paper some possible application of a recently defined function [3,4] called Sentry (SF), to the damage identification and residual strength determination in the case of composite laminates, is shown.

2. Definition of the Sentry Function
In order to perform a deeper analysis of the laminate behavior, a function that combines both the mechanical and acoustic energy information is employed. This function is expressed in terms of the logarithm of the ratio between the strain energy (Es) and the acoustic energy (Ea), where x is the test driving variable (usually displacement or strain).

The function f(x) is divided into five distinct areas: an increasing function PI(x), a sudden drop function PII(x), a constant function PIII(x) a decreasing function PIV(x) and a some times a Bottom-up function BU(x). Each region represents a specific stage in the damage process (Fig. 1). The sentry function, type PI, represents the strain energy storing phase when it is increasing. During the test the ability of the material to store energy reaches its limits and the AE cumulative energy significantly increases due to damage progression hence the slope of the PI(x) function decreases. During the damage process when a major failure takes place in the material the stored

mechanical energy is suddenly released...
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