Fracture toughness investigations using holographic interferometry

Cracks are one of the most important defects influencing the failure of technical constructions. In order to determine the potential danger of existing cracks, expressed in terms of the stress intensity factor K and established in fracture toughness tests, it is necessary to know the extent of plastic deformation which develops in front of the crack tip when a construction is subjected to applied stress. From the range of available measuring methods, holographic interferometry was found to be very suitable for solving this measurement problem. The formation of the plastic zone was observed as a function of applied loading. Plastic straining of the specimen was achieved in three-point bending in a 60t tensile testing machine remote from the holographic equipment. Incremental loading and measuring allowed observation of the development of the plastic zone in a realtime analysis. With knowledge from normal tensile tests that strains smaller than 0.5% are in the elastic region, the size of the plastic zone at the crack tip could be accurately determined. An important application of this measuring method is in studying the crack behaviour in small seams, such as EB-welded joints. This has led to the development of a correction equation for fracture toughness testing of such specimens. (IFAM)