The effect of the loading rate on elastic-plastic fracture toughness, which defines stable crack initiation, has not been clear; therefore, the test method of elastic-plastic fracture toughness under rapid loading (rapid-loading fracture toughness JId) has not yet been established in ASTM Test Method for JIc, a Measure of Fracture Toughness (E 813-81).
The authors studied the JId test method by using a high-speed servohydraulic testing machine, by which the rapid-loading fracture toughness test could be done in accordance with ASTM Test Method for Plane-Strain Fracture Toughness of Metallic Materials (E 399-83 ANNEX A7). We evaluated the applicability of the unloading compliance method to determine the stable crack initiation under rapid loading by comparing the results with the multiple specimen method.
We also applied the electric potential method, which is expected to be able to detect crack growth continuously to determine the stable crack initiation. Results are as follows. 1. The unloading compliance method is applicable to rapid-loading tests.JId measured by the unloading compliance method is the same as that obtained by multiple specimen method. 2. JId based on the electric potential method is about the same as that of the unloading compliance method. 3. JId depends on loading rate. Transition temperature between the brittle-ductile region tends to be higher as the loading rate increases.