Significance of the load shift in single-edge notch tension tests

Abstract

When single-edge-notch (SEN) specimens are tested in tension, the load is usually applied at the extremities of the specimen through single pins aligned along its center line. Upon loading the specimen, an initial positive moment which tends to assist in opening the crack is created. This moment will continue as long as there is no appreciable plastic deformation ahead of the crack and no significant change in the initial specimen geometry. As the load increases, however, the load center line considerably shifts relative to the specimen center line giving rise to a negative bending moment which tends to close the crack. Furthermore, if the specimens are to be made of highly ductile materials (e.g., line-pipe steels), the shift will be much more pronounced; thus leading to substantial discrepancies between theoretically calculated and experimentally obtained crack-opening-displacement (COD) values. The ductile fracture analysis of the edge-cracked strip problem, in general, is based on the assumption of small displacements and rotations in which case reference to the underformed geometry for equilibrium is fully justified. In this paper, it is shown that if one uses small displacement theory to solve such a problem then extreme caution should be exercised in prescribing the nature of the external loads to account for the severe geometrical changes that actually occur during the tests.