The initiation toughness (JIc) and the resistance to stable crack growth (JR-curve) of AISI Type 303 stainless steel were characterized using side-grooved compact-type specimens. The test material was produced by a hot extrusion process which resulted in long cylindrical MnS inclusions aligned in the longitudinal direction. In addition, several interrupted tests were conducted in which CT specimens were loaded monotonically to different levels of stable crack growth. These specimens were sectioned at mid-thickness for detailed metallography to observe the microscopic aspects of the ductile crack growth process and the evolution of damage ahead of the crack tip. Following is a summary of the results and the conclusions from the study.
Stable crack growth or ductile tearing occurs by nucleation, growth, and coalescence of voids which nucleate at the inclusions. Void growth appears to be most dominant during stable crack growth. At JIc the void closest to the initial crack tip coalesces with the crack. During this period, growth of voids further away from the crack tip also occurs. Thus the increment in J required to continue the crack growth process decreases progressively until a steady-state dJR/da is obtained.
Based on the observations of this study and the void growth model of Rice and Tracey, a simple model relating the microstructural parameters such as inclusion spacing and size to the stable crack growth resistance of the material is proposed. The model is capable of predicting the JR-curve. The predicted JR-curve for the test material agreed with the observed behavior.