An experimental/analytical comparison of three-dimensional deformations at the tip of a crack in a plastically deforming plate I: Optical interferometry and experimental preliminaries

A critical examination of measured and computed three-dimensional deformations in the vicinity of a crack in an elastic/plastic plate subjected to three-point-bending are presented in a series of three papers. The aim is to apply interferometric measurement methods to the large plastic deformation at the tip of a crack in a ductile steel, and to assess the current ability to model that deformation numerically. Comparison between the precise interferometric measurements and the finite element model, which has been closely matched to the physical specimen, including internal tunneling of the crack, indicates good agreement up to 75 percent of the ultimate failure load; above that level, discrepancies increase to levels on the order of 25 percent. It also develops that out-of-plane measurements are less than optimal for deducing in-plane stress and deformation states. This finding implies consequences for working with caustics and similar methods when applied to cracks in plastically deforming solids. This first paper presents a description and assessment of the experimental methods and outlines the fracture experiment itself. The second paper describes the material characterization to closely match the finite element model to the physical fracture specimen, as well as a description of the finite element model itself. The third paper presents the results of the comparison between experiment and model, as well as some additional results from the numerical model alone.