Crack-length determination for the compact tension specimen using a crack-opening-displacement calibration-comparison of COD measured graphically and from the gage output

To implement the increasing trend toward automated testing procedures, crack-length measurement during fatigue crack-growth-rate testing was determined using a COD gage technique. Data were analyzed both graphically and using the gage output measured in millivolts. To compensate for nonlinearity in the load-vs-COD traces, it was found necessary to sample differences in the load millivolt readings to produce smooth da/dN-vs-deltaK curves agreeing with those determined from graphical analysis of crack length.