3-DIMENSIONAL EFFECTS NEAR A CRACK TIP IN A DUCTILE 3-POINT BEND SPECIMEN .2. AN EXPERIMENTAL INVESTIGATION USING INTERFEROMETRY AND CAUSTICS

An experimental investigation is undertaken to assess three-dimensional effects near a crack front in a ductile three-point bend specimen. The possibility of using the optical method of caustics for the measurement of the J-integral in the presence of large-scale yielding and three-dimensional fields is also investigated. Experiments using the optical methods of caustics by reflection and Twyman-Green interferometry are performed simultaneously on either side of the test specimen. The load and load-point displacement are also measured. The experimental results are compared with very good agreement to the results of a finite element simulation of the experiment. The caustics experiments are used to obtain a calibration relation between the value of the J-integral and the caustic diameter for load levels up to fracture initiation. It is proposed that such a calibration be used in dynamic fracture initiation experiments for the measurement of the time history of the dynamic J-integral. © 1990 by ASME.