EXPERIMENTAL JUSTIFICATION FOR PROPOSED CHANGES TO THE MEASUREMENT OFK-IC USING ASTM-E-399

During the past five years considerable effort has been devoted to dev eloping a fracture toughness measurement standard that combines the li near elastic stress intensity factor at the onset of crack extension, K-Ic, and the elastic-plastic fracture toughness measurement quantitie s, namely, J(Ic), the J resistance curve (J-R curve), and the correspo nding crack tip opening displacement resistance curve (CTOD-R curve). The objective of this effort was to allow the engineer to start with o ne specimen geometry, conduct a specified test, develop either a K-Q, or a J(Q)-R curve, and then, after application of the relevant size re quirements, obtain a valid fracture toughness result. At present, diff erent specimens are required by E 399 and E 1737 (the combination and replacement of E 813 and E 1152), and if the engineer uses the E 399 s pecimen geometry and test procedure and subsequently finds that the me asured K-Q fails the E 399 size requirements, new specimens must be ma chined and tested according to E 1737 to obtain valid fracture toughne ss measurements. This paper describes experimental tests that have bee n conducted in parallel with the development of the new ASTM Standard E 1820-97, a combined fracture toughness test standard that is intende d to address this problem. These experimental results show that the st andard J integral specimen, incorporating load line crack mouth openin g displacement measurements and side grooves, can be used to obtain K- Ic as presently defined by ASTM E 399. The use of larger a/W ranges is also supported by these results as long as the basic size requirement s of E 399 are met and applied to both the specimen crack length and r emaining ligament as well as to the specimen gross thickness.