Theses and Dissertations Collection

This document describes the experimental approach used to identify crack-tip opening loads due to the plasticity-induced crack closure under fatigue loading conditions using a data acquisition camera (DAC) and DaVis, an advanced data analysis software. The primary pieces of equipment used in the experimental work are as follows: (1) a loading frame, which uses hydraulics to supply and control a cycling load applied to a compact tension (CT) specimen, (2) the DAC itself, set up on a stable tripod to collect the images required for specimen analysis by this method, and (3) a computer equipped with the DaVis software, used to analyze and process the images collected by the DAC. The DAC was used to collect images of the crack closing and opening during fatigue loading. The goal was to determine the exact instant of crack-tip opening during loading and unloading. Images were collected in series for each applied cycle, with separate data sets collected for the loading and unloading portion of each cycle. Data was collected at three ΔK levels, i.e., 10, 15, and 20 MPa∙√m, to identify changes in opening load as the crack increased in length. Using the DaVis software, the images collected were used to compute crack opening displacements, from which it could be determined whether or not the crack was open or closed at a given point in the cycle. As expected, the data collected indicated that opening load changes with increasing ΔK value. The computed opening load values were consistent with those computed using other methods, as will be laid out in this paper.