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) over bar, 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) over bar 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) over bar inside a volume surrounding the tip of a fatigue crack. The resulting averaged heat energy per cycle (Q) over bar* is an appropriate candidate to correlate crack growth data generated from fracture mechanics tests, especially at relatively high applied Delta K values, when the Linear Elastic Fracture Mechanics hypotheses are violated. The present paper is focussed on the use of (Q) over bar* as an elastic-plastic fracture mechanics parameter. First of all, the size of the structural volume, where (Q) over bar is to be averaged to calculate (Q) over bar*, was defined with reference to an AISI 304L stainless steel. After that, the averaged heat energy (Q) over bar* was determined during crack propagation tests starting from the temperature maps measured in situ by means of an infrared camera. The use of (Q) over bar* 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) over bar*. and Rice's J-integral has been discussed and formalised.