Crack Growth Monitoring in Ceramic Matrix Composites by Combined Infrared Thermography and Acoustic Emission

The current study proposes a novel methodology for measuring crack growth in composite materials using combined infrared thermography (IRT) and acoustic emission (AE). The technique is tested across a SiC-fiber-reinforced ceramic matrix composite while no apparent factor is limiting its usage on composite materials of different nature as well. Compact tension specimens were loaded in tension with unloading/reloading loops and the thermally dissipated energy due to crack growth and other damage mechanisms was captured by IRT with a 100 Hz sampling rate. Crack growth was established by identifying the time instances where the maximum temperature, hence also damage, occurred and then quantifying, by means of control lines, the damage span within the thermograph corresponding to the specific instance. The high accuracy of the proposed technique was validated against optical measurements of crack length. The theoretical crack length predicted by the elastic compliance technique was found to overestimate the experimental findings by at least 25%. Knowledge of the critical level of damage accumulation for material structural health was made possible from AE descriptors such as activity during the unloading part of the cycles. In this study, AE was particularly successful in closely following the actual crack growth measured by IRT, an observation that brings out the potential of the technique for quantitative measurements.