Crack growth threshold testing using flat-bottom hole specimens

Abstract

This study benchmarks a relatively new axisymmetric specimen concept for use in threshold testing, where the crack initiates and grows from the corner of a flat-bottomed circular hole in a cylindrical specimen while the bottom of the hole is cyclically loaded with a flat-ended plunger. The Flat-Bottom Hole (FBH) specimen is configured so the stress intensity naturally decreases as the crack grows radially outward at constant cyclic load, favoring self-centering, highly circular crack growth, and thus enabling tightly repeatable results especially for threshold testing. Because the specimen achieves K-shedding without load shedding, the opening at every point along the crack is constantly increasing during constant load amplitude growth, minimizing the threat of remote closure. Contrary to most conventional specimens, the circular crack front makes no intersection with a free surface, resulting in a crack front of uniform constraint and, where applicable, a uniform crack closure state. In practice, the specimen can be made quite small in size compared to other specimens of similar function.Two configurations are described, one for ambient testing, and one for elevated temperature testing. The K-solution and displacement solution obtained using FRANC2D are presented. Test results include data for Ti 6A-4V forging and Inconel 718 forging.