Rotation Point and K^JC Estimations for Miniature CT-Specimens Based on Off-Load Line Displacement

In fracture toughness tests, the energy absorbed by the specimen is calculated from load versus load point displacement data. If tests are made with small CT-specimens, direct measurement of load point displacement is often not possible due to small specimen dimensions; subsequently, specimen displacement is measured on the front face of the specimen. The measured displacement is transferred into load point displacement with the help of a geometric rotation point concept. The measurements were performed with miniature CT-specimens of dimensions B = 4 mm, W = 8 mm having the integral clip seated on the specimen front face. The location of the rotation point was concluded from the measured initial compliance by applying the rotation point values of r = 0.5 and r = 0.13 and by comparing the calculated crack lengths with the measured crack lengths. The value of r = 0.5 was observed to be too high and the value r = 0.13 produced the measured crack lengths accurately. A linear combination of elastic and plastic deformations is proposed to be used for calculating the location of the rotation point outside the elastic loading range. The load line load–displacement curves were derived and the measured K^JC-values were calculated with the assumptions r = 0.5 and with the linear combination described in the paper. The linear combination believed to be more relevant than the value r = 0.5 resulted in lower K^JC-values than r = 0.5. However, the difference was only about 1.6 % in K^JC-values, which changes the T⁰ values approximately by 1°C. Hence, the measured K^JC-values are not sensitive to the exact location of the rotation point used in the analysis.