Effect of specimen geometry on ductile initiation CTODi using a direct method

This paper presents two approaches for use in defining the minimum size requirements for obtaining valid CTODi (crack tip opening displacement at ductile fracture initiation) measurements. The macroscopic approach uses the microcrack resistance curve (r-curve) to establish the validity of the CTODi criterion for ductile fracture initiation by reducing the specimen dimensions down to 1 mm for specimen thickness, 0.8 mm for the length of both crack and uncracked ligament, and 4 × 4 mm for cross section of specimen, using a side grooved specimen. A second method uses a microscopic examination of the strain disturibution around crack tip and indicates the validity of the macroscopic method. The present work also considers the discrepancies in size requirements between the direct method and other existing methods such as the clip gauge method.