A procedure for determining crack opening load from differential displacement data

In this study, a numerical differentiation method is proposed to determine the crack opening load from differential displacement data. The derivatives were calculated using a linear least squares fit. The crack opening point is determined from the third derivative of the differential displacement data. Calculating the derivative by using a straight fit line effectively reduces the scatter of the measured crack opening results. The proposed method was applied in practice to constant amplitude loading tests and the measured crack opening results were compared with those of the ASTM method. The proposed method gives stable and consistent results and the measured crack opening values are relatively reasonable.