The Heat Energy Dissipated in a Control Volume to Correlate the Fatigue Strength of Bluntly and Severely Notched Stainless Steel Specimens

In previously published papers by the authors, the specific heat energy loss per cycle (the Q parameter) was used to rationalize about 120 experimental results generated from constant amplitude, push-pull, stress- or strain-controlled fatigue tests on plain and notched hot rolled AISI 304 L stainless steel specimens as well as from cold drawn un-notched bars of the same steel, tested under fully-reversed axial or torsional fatigue loadings. It has been shown that Q can be estimated starting from the cooling gradient measured at the critical point immediately after the fatigue test has been stopped. Concerning notched specimens, it was noted that a 3 mm notch tip radius was close to the limitation of applicability of the adopted temperature sensor, consisting in 0.127-mm-diameter thermocouples, because of the 1.5-to-2 mm diameter spot of the glue which prevented to measure the maximum temperature level. In this paper, the fatigue-damage-index effectiveness of Q parameter was investigated, carrying out fully reversed axial fatigue tests on 4-mm-thick AISI 304L specimens, having 3, 1 and 0.5 mm notch tip radii. The cooling gradients were measured by using an infrared camera, characterized by a 20 m/pixels spatial resolution. As a result, all new fatigue data could be rationalized using the same scatter band published previously by the authors.