The heat energy dissipated in the material structural volume to correlate the fatigue crack growth rate in stainless steel specimens

The heat energy exchanged in a unit volume of material per cycle, Q , has been previously proposed and adopted by the authors as a fatigue damage indicator, because it accounts for the response of the material to the external applied loads as a result of energy conversion from the intrinsic mechanical dissipation. Q can be determined by means of temperature measurements performed in situ during fatigue tests, at least for certain classes of metallic and composites materials. Originally conceived as a point-related physical quantity, recently a theoretical frame and an experimental procedure have been defined to average Q inside a volume surrounding the tip of a fatigue crack. The resulting averaged heat energy per cycle Q* is an appropriate candidate to correlate crack growth data generated from fracture mechanics tests, especially at relatively high applied K values, when the Linear Elastic Fracture Mechanics hypotheses are violated. The present paper is focussed on the use of Q* as an elastic-plastic fracture mechanics parameter. First of all, the size of the structural volume, where Q is to be averaged to calculate Q* , was defined with reference to an AISI 304L stainless steel. After that, the averaged heat energy Q* was determined during crack propagation tests starting from the temperature maps measured in situ by means of an infrared camera. The use of Q* resulted promising because it could take into account crack acceleration due to excessive plasticity. Finally, in the context of energy-based approaches to fatigue crack propagation problems, a link between the averaged heat energy Q* and Rice’s Jintegral has been discussed and formalised.