K_Id-Values Deduced from Shear Force Measurements on Double Cantilever Beam Specimens

Time-varying shear force measuring techniques have been used to investigate the dynamic critical stress-intensity factor versus crack propagation velocity curve. The product of the shear force at the loading end times the square root of the loading time on a rapidly wedged double cantilever beam specimen is uniquely related to the critical bending moment at the crack tip. Static compliance measurements on side-grooved specimens were incorporated into a Bernoulli-Euler beam model for calibration purposes and to eliminate the inappropriate built-in beam assumption. The compliance calibration shows a crack length shift from a measured crack length to a beam model length at a fixed compliance value. This shift does not affect the magnitude of the calculated critical bending moment at the crack tip when the load and the load-point displacement are measured quantities. The effective crack length is calculated from the beam model length with the length shift correction. The K_Id-values (calculated from the critical bending moment) versus crack velocity have been investigated at several test temperatures for a low-carbon steel. K_Id-values show a generally decreasing trend when crack velocity increases. K_Ic at fast fracture initiation is larger than the corresponding K_Id-value for all tests recorded.