Third year PhD student at IRDL, Brest, in collaboration with Safran Landing Systems, Vélizy-Villacoublay, on the influence of surface integrity and coating on the high cycle fatigue properties of 300m steel by thermographic measurements under cyclic loadings. This thesis is a part of an industrial chair called "Self-Heating", involving two laboratory (IRDL & P' Institut) and four companies (Naval Group Research, Safran Aircraft Engines, Safran Landing Systems, SafranTech) in eight theses and four post-docs on the determination of high cycle fatigue and very high cycle fatigue properties from thermographic measurements under cyclic loadings.
300M, an ultra-high strength steel used in aeronautics for landing gears, is generally shot-peened and coated with High Velocity Oxygen Fuel sprayed WC-10Co-4Cr to improve corrosion resistance and tribological behavior. In the literature, few process parameters have been tested to characterize the effect of surface integrity on fatigue properties. The self-heating method under cyclic loading is used to quickly determine fatigue properties by coupling the fatigue mechanisms with dissipation mechanisms. This paper focusses on the results obtained on the bare and shot peened materials.
For bare 300M steel, analyses show a good correlation between the steel self-heating model and the conventional fatigue data, as well for mean properties as for its scattering. Several parameters have been tested, such as load ratio, coupon volume, loading history and material orientation.
For shot peened 300M steel, correlation is not direct, as tests only determine the average dissipation whereas the configuration is highly heterogeneous. A comprehension and predictive model has been built by splitting the global dissipation into several dissipation areas. To investigate the dissipation of those areas, the fatigue mechanisms and the influence of shot peening, the bare steel is tested under different mean stresses, load ratios and plastic pre-deformations. Those results help to parametrize the model and so to interpret the temperature signal as a characterization of fatigue properties. This is the first step to build a lifespan forecast model based on temperature measurements to study shot-peening and coating influence on fatigue properties.