Heat dissipation measurements in low stress cyclic loading of metallic materials: From internal friction to micro-plasticity

The present paper deals with a procedure to measure the very small quantities of heat generated during the very first cycles of mechanical loading on metallic specimens. A dedicated procedure is presented to reach the required sensitivity in terms of heat sources with some original features with respect to the present state of the art, assuming that the heat sources are uniformly distributed within the specimen as such low strain levels. Then steel (cold rolled and annealed) and aluminium specimens were tested at two different aspect ratios (Rσ=0.1Rσ=0.1 and Rσ=-1Rσ=-1). It was shown that for small stress levels, the heat generated was more or less constant with cycle number. This energy was associated to visco-elastic behaviour (internal friction) and it was shown that the same energy could be measured with the two stress ratios when plotted against strain rate (stress amplitude at constant frequency, here 15 Hz). Then, after a certain stress level, an initial outburst of energy was evidenced over about 10–15 cycles before a constant level was reached again. This was associated to micro-plastic adaptation. This procedure provides an advanced tool to tackle the problem of very early fatigue damage detection and is aimed at providing some physical justification to the procedures of rapid fatigue limit detection by self-heating tests.