Effect of Geometrical Discontinuities on Mode Shapes Stress Maps Using Infrared Thermography

Thermoelastic stress analysis (TSA) is an experimental technique that provides maps of stress variation in loaded solids related to the variation of temperature, under linear and adiabatic conditions. Traditionally, TSA has been employed in mechanical components under monotonic harmonic excitation measuring with infrared thermography. Hence, the amplitude of the temperature signal in a pixel can be related to the stress amplitude, after proper calibration of the thermoelastic properties. The main purpose so far has been the analysis of stress maps for fatigue and fracture mechanics by applying a load at a low frequency, about 10 Hz, but high enough to avoid heat conduction. However, recent studies have exploited harmonic excitation for a new use, which is the evaluation of stress maps associated with mode shapes. These studies have shown the capability to provide the shape information at much higher frequencies, of hundreds and even thousands of Hertz. In this work, the authors evaluate the capabilities of this methodology, based on harmonic excitation of mechanical components, to identify the effect of discontinuities on the stress maps associated with mode shapes. It is based on the fact that each mode shape is unique, changing the stress distribution from one to another. For this purpose, different kinds of geometrical discontinuities on flat components were evaluated. First, their natural frequencies were identified, and, afterwards, a harmonic excitation was applied at those frequencies. The response of the specimen was recorded with an infrared thermocamera. The analysis of the thermal signals, related to stress, reveals that how much the maps are altered depends on the location of the discontinuity regarding the modal shape of the sound specimen. In this way, with a set of some mode shapes, it is possible to identify, at least, one that highlights the effect of discontinuities.