A Thermal Imaging System for Crack Growth Quantification in Thermo-Mechanical Fatigue Specimens

High temperature materials intended for turbine engine or advanced airframe applications must undergo extensive thermo-mechanical fatigue (TMF) testing to predict their behavior in extreme operating environments. Figure 1 provides an illustration of a typical TMF test machine where hydraulic load cylinders impart cyclic tensile loads while quartz heat lamps and nitrogen cooling jets are used to thermally cycle the test specimens over temperatures ranging from -20°C to 1000°C. The purpose of these tests is to determine when crack initiation occurs and then to measure the rate at which fatigue cracks grow. The combination of high specimen temperature, access limitations imposed by the test chamber, and test specimen surface oxidation make the desired crack length measurements difficult. Although traditional measurement techniques such as the electric potential difference (EPD) method are effective for single, relatively straight cracks, they are unable to identify the presence of multiple cracks or accurately measure the length of abnormal crack morphologies such as bifurcated or sawtooth [1]