A new method to measure critical strain in composite materials - combining the Euler-Fresnel spiral with acoustic emission to assess crack positions

摘要

Critical strain is a material constant and a critical parameter for many applications. It is not depending on temperature if no phase transition is involved. For composites only few publications on critical strain are known due to the fact that critical strain and thereof resulting micro cracks are not considered to cause fatal failure. New applications and combinations of composites with dissimilar top layers have recently shown unforeseen damage modes which can be directly related to the critical strain of those materials. In this paper, the Euler-Fresnel spiral is introduced as an elegant, simple, but discriminating and therefore reliable method to measure critical strains of composite materials at different temperatures and environments. As micro cracks, caused by exceeding critical strain values, are difficult to detect, the initiation of the micro cracks is detected by means of multiple accelerometers, measuring the acoustic emission of the crack initiations. In this manner both the initiation and the position of the micro-cracks can be estimated.