Abstract
This paper describes a new method for measuring microscopic strain distribution in carbon fiber reinforced plastics (CFRP). This method aims to evaluate the effect of heterogeneity, e.g. voids caused by different pressure conditions in curing process, on transverse crack initiation. Local Area Gray Level Thresholding (LAGT) method is a new feature-based matching method that uses gray level difference between fibers and matrix in scanning electron microscope (SEM) pictures. This paper also proposes improved LAGT method adjusting gray level in local evaluating area. LAGT method and improved LAGT method were compared to existing methods, such as Phase-Only Correlation (POC) and Zero-mean Normalized Cross Correlation (ZNCC). Among these 4 methods, improved LAGT is less influenced by the noise from optical system and its measurement accuracy is better even if gray level gradient changes locally in evaluating area. The microscopic strain distribution in 90º layers under transverse tensile loading were obtained by fiber location search based on image analysis using the improved LAGT method, and the measured strains corresponded well with the strains calculated by beam theory and laminate theory. Applying the above-mentioned method to the laminates including voids, it became possible to evaluate the effect of void on microscopic strain distribution experimentally.
