Observations on Photo-Emission and the Process Zone of a Fatigue Crack

Cyclic loading of a solid material induces a cyclic temperature fluctuation. The thermal cycle is asynchronous with the loading cycle when the strains are elastic. This is commonly known as the thermoelastic effect. In differential thermography, a sensitive infrared camera is used to measure this temperature variation; typically of the order of a few tens of milliKelvin. Since the magnitude of the temperature variation is proportional to the dilational strain, values of the surface elastic stress can be derived. Recent advances in infrared camera technology and data processing algorithms have enabled values of effective stress intensity factor ranges and the location of fatigue cracks to be determined from temperature fluctuations around the crack tip on the specimen surface. Careful observation in the region near a crack tip reveals deviations from the asynchronous behavior of the photon signal relative to the load cycle. Phase shifts up to π/8 are commonly observed. The spatial distribution of these phase shifts exhibits characteristic features which are described and discussed. The features in this distribution appear to be associated with high elastic strain gradients, regions of plasticity and contact between the crack faces. Interpretation of the phase shift and its spatial distribution may lead to a better understanding of the mechanics of fatigue crack growth.