Application of image processing to study fracture surface of API X70 steel under Charpy impact test

The Charpy impact test is an experimental method for determination of materials dynamic properties at different temperatures to investigate the ductile to brittle transition behavior of tested materials. The percentages of ductile and brittle fractures can be evaluated based on fracture area of Charpy specimen (according to API E23 standard) by visual techniques which do not provide exact percentages of these fractures. In this study, a method is proposed to calculate the exact percentage of ductile fractures using image processing, which makes it possible to quantitatively examine different parts of the fracture surface with high accuracy. All steps of image processing are described for eleven Charpy standard specimens of API X70 steel, tested at temperatures between +20 to -80 °C with a temperature increment of 10 °C. In this research, converting a qualitative image of fracture surface to a quantitative matrix is described for the first time. Prediction of the shape of ductile and brittle parts of the fracture surface at temperatures between +20 and -80 °C is one of the results of this study. The percentages of ductile fractures using image processing for temperatures of +20, 0, -20, -40, -40, -60 and -80 °C were obtained as 100, 100, 86, 53, 36 and 0, respectively. The transition temperature was -45 °C for this steel, corresponding of 50% ductile fracture.