Methodology analysis for evaluation of drilling-induced damage in composites

Drilling operations in composite materials introduce damage along the periphery of the hole that depends on the process parameters. Damage assessment requires image processing using either different algorithms or threshold levels for segmentation in order to be able to measure and calculate the delamination factors. The aim of this work is to carry out a systematic analysis on the image processing methodologies and algorithms to clarify and quantify their influence on the levels of delamination factors in carbon fibre-reinforced polymers (CFRP). Several tests were carried out using different types and shapes of carbide drills. The results show that both the algorithms and threshold levels for segmentation have a strong influence on the final results. Contrary to the usual procedure in the literature, the threshold level cannot be a predefined standard constant because its optimum value depends on the image histogram. The delamination factor and the adjusted delamination factor were found to be influenced by the threshold level in such a way that its increase causes an exponential decay in both factors. This clearly evidences the need to standardize the image processing methodologies so that different drilling results can be comparable.