Microplasticity and energy dissipation in very high cycle fatigue – DISFAT

This project aims at studying the mechanisms leading to crack initiation for ductile single-phase metallic materials when they are subjected to stress magnitudes lower than the conventional fatigue limit. In these loading conditions, the number of cycles to failure is higher than 10^9 and belongs to the so-called very high cycle fatigue (VHCF) range. The present project is a fundamental research project but motivated by industrial problems. Some mechanical components, such as pistons, rotating axes, in the transportation and more generally engineering industry, have been designed previously using fatigue resistance data at low cycle rates (<10^7 cycles ; the regime of High Fatigue Cycle, HCF) whereas they endure oscillating loads for a number of cycles higher than >10^9 cycles and finally fail. The main challenge of this project results from the fact that the manifestations of the mechanisms of interest give rise to very low and localized signal owing to the very low stress magnitudes involved. Our strategy is first to analyze the response of metals and alloys having 'simple' microstructure and deformation mechanisms but also controlled initial states, with special attention paid to the surfaces (with reduced roughness and residual stress state, labeled as 'ideal' state surface). From the existing knowledge of cyclic deformation mechanisms (High Cycle Fatigue range), we choose to study face-centered (fcc) and body centered (bcc) cubic metals and the role of two intrinsic properties of solid crystals: the tendency to wavy or planar slip and the lattice friction resistance. Second, the VHCF range will be reached using ultrasonic fatigue machines. The working frequency is 20 kHz. The use of higher loading frequencies than conventional frequencies (30 Hz) brings advantages (1) to provide fatigue limit and near threshold crack propagation data within a reasonable time and (2) to increase the dissipated power (energy per unit of time) and thus to generate temperature variations which can be detect with the current thermal measurement device. Thus, to detect and characterize the irreversible microstructural changes responsible for crack initiation, we propose to get three types of additional data. The first type is the temperature increase at the specimen surface along with the cyclic loading. The dissipation field is deduced using quantitative imaging techniques. The second type is the surface roughening estimated by Atomic Force Microscopy (AFM). The third type is the characterization of the dislocation microstructure underneath the surface where irreversible changes of the surface profile appear using Transmission Electronic Microscope (TEM) for the local scale and Electron Channeling Contrast in Scanning Electron Microscope (ECC/SEM) to get information for larger scales.