Condition Assessment by Thermal Emission (CATE) for In Situ Monitoring of Fatigue Crack Growth

The cost and size of instrumentation for thermoelastic stress analysis (TSA) have often been an inhibiting factor for its use in industrial applications. This has been alleviated to some extent by the development of packaged infrared (IR) bolometers which have become popular for non-destructive evaluation of structures. Recent work has demonstrated that an original equipment manufacturer (OEM) microbolometer, combined with a single circuit board with dimensions equivalent to a credit card, can be used to detect cracks of the order of 1 mm long and to monitor their propagation. The monitoring system costs about one tenth the price of a packaged bolometer and can provide results in quasi real time without the need for calibration. The system uses the principles of TSA to acquire thermal images and evaluate the amplitude of the thermal signal over the field of view, i.e., a map of thermal emission amplitude. Feature vectors are extracted from the time-varying maps of thermal emission amplitude and used to identify changes in them that occur when a crack initiates or propagates in the field of view. Since the technique does not generate TSA data but uses uncalibrated thermal emission data, it has been named Condition Assessment by Thermal Emission (CATE). The CATE system's crack detection capability has been evaluated in laboratory conditions and compared against a state-of-the-art IR photovoltaic effect detector. It was demonstrated that the CATE system is capable of detecting cracks as small as 1 mm at loading frequencies as low as 0.3 Hz. Evaluations in industrial conditions on large-scale structures are being concluded and imply that there will be little loss of capability in the more demanding applications.