Dynamic Fracture Initiation and Propagation in Metals: Experimental Results and Techniques

Citation

Zehnder, Alan T. (1987) Dynamic Fracture Initiation and Propagation in Metals: Experimental Results and Techniques. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/f3zy-j846. https://resolver.caltech.edu/CaltechETD:etd-03052008-085910

Abstract

Dynamic fracture initiation and propagation in ductile and brittle materials was studied experimentally using the optical method of caustics in conjunction with high speed photography. The drop weight impact test, previously used only for studies of fracture initiation, was adapted to study both dynamic fracture initiation and dynamic fracture propagation.

The results show that for a relatively brittle, quenched and tempered, high strength 4340 steel the dynamic fracture propagation toughness depends on crack tip velocity through a relation that is a material property. In addition, the effect of stress waves on the dynamic response of different specimen geometries is discussed and the micromechanisms of failure for this heat treatment of 4340 steel are investigated.

Extension of the optical method of caustics to applications in elastic-plastic fracture was studied with the goal of learning how to measure dynamic fracture initiation toughness in tough, ductile materials. Static experiments were performed on different specimen geometries of a ductile 4340 steel and 1018 cold rolled steel, and were compared to small scale yielding, plane stress, finite element results. Issues studied that are related to the applicability of caustics are the extent of the dominance of the plane stress HRR field, the effect of plasticity on the accuracy of caustics from the elastic region outside the plastic zone, and the extent of the crack tip region of three dimensionality.

The above approach to caustics in ductile materials was based on the assumption of validity of the HRR field. A novel approach to the use of caustics with ductile materials was taken that eliminates the concerns over the region of dominance of the HRR field, etc. In this approach a calibration experiment was performed relating the caustic diameter to the J integral for a particular specimen geometry under conditions of large scale yielding. This approach was successfully applied to optically measure for the first time the J integral under dynamic loading. Measurement of the J integral by means of strain gages was developed and applied to obtain J simultaneously with the caustics measurement.

At the same time (and on the same specimens) additional measurements were made including, load, load-point displacement, strains near the crack tip and out of plane displacements (measured with interferometry). These results are compared with excellent agreement to a three dimensional finite element simulation of the specimen.

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