Introduction

AET is considered quite unique among the non-destructive testing methods, what starts with the question of whether this method should be classified as completely non-destructive or not, since fracture of the material is necessary for testing. In contrast to other NDT methods, however, AET is usually applied during loading, while most others are applied before or after loading of a structure. Following these arguments, and according to the way in which the signals are recorded, AET is correctly described as nondestructive. The statement is certainly true if a material is tested under a working load without any additional load. On the other hand, AE is often used to detect a failure at a very early stage, long before a structure completely fails.