An automated reversing d-c potential system capable of measuring crack growth rates in compact-type, edge-notched, and surface-defected specimens under a wide variety of environmental conditions is described. Measurements can be made with ease during cyclic or static loading conditions. The unique characteristics of the system software, which give the operator a wide choice of test routines, including constant load range, constant stress-intensity range, and programmed load range or programmed stress-intensity range, are discussed. Detailed procedures are then given for conducting a “controlled K” cyclic test on a surface-defected specimen.
The system has outstanding stability and sensitivity. The high frequency of measurement permits averaging of many readings within a given cycle to obtain a single data point or to detail a given cycle. Techniques for averaging data to enhance the resolution of growth rate measurements are discussed.
Finally, results from several tests are described to demonstrate the versatility of the system in making measurements on different specimen geometries over a wide range of testing conditions.