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

Critical strain is the strain level corresponding to the very first micro-crack in a composite layer. Current detection methods for such micro-cracks are limited to strain gage measurements, digital image correlation techniques or the Bergen ellipse [9]. With the Bergen ellipse a well-defined but non-linearly increasing strain can be applied to a strip of composite material to induce micro-cracks. In this paper a combined approach is described to measure the critical strain at which the first micro-crack occurs. The combined approach consists of 2 elements. First the Euler-Fresnel spiral is introduced as an elegant alternative to the Bergen ellipse to introduce a well-defined and linearly increasing strain to a strip of composite material secondly, since micro-cracks, caused by exceeding the critical strain values, are difficult to detect, multiple accelerometers on the composite strip are used to detect the micro-cracks by measuring the acoustic emission of the crack initiations. By measuring the time of arrival of the acoustic emission waves at the two ends of the strip, the positions of the micro-crack initiations can be determined. Using the geometrically defined relationship between strain and position on the Euler-Fresnel spiral, the strain-levels at crack initiations can be estimated. An important advantage of the approach described in this paper is that all micro-cracks are detected instantaneously during the measurement procedure. This includes the subsurface cracks that cannot be detected by the penetrant method. In this paper the proposed measuring method for critical strain in composites was successfully demonstrated.