This work deals with very high cycle fatigue tests on high strength steel subject to the corrosive action of salt solution. Tests were carried out at constant load ratio R = -1 on corrosion treated and non-treated specimens and sea water flow specimens of R5 steel, which according to an International Classification Society of offshore systems is steel graded with the highest ultimate tensile strength to manufacture mooring chains and accessories intended to position mooring applications such as mooring of mobile offshore units, mooring of floating production units and mooring of offshore loading systems. Fatigue endurance on pre-corroded specimens was quite dispersed but with a mean value of 360 MPa, whereas for the non corroded specimens the fatigue endurance was located between 420 MPa at 105 - 106 cycles, and 380 MPa at 109 cycles. Concerning the sea water flow specimens, important increase in fatigue life is observed in reducing the applied load. Dispersion on fatigue endurance for the pre-corroded and sea water flow specimens was related to the complex process of corrosion on the specimen surface; nevertheless, the resulting pitting holes were associated to fracture origin in these specimens. Furthermore, fatigue failure origin was systematically localized at a circular, elliptical or irregular surface pitting hole formed during the corrosion process; special attention was focused on the pitting holes related to fatigue failure. Finite element results were obtained for the hemispherical voids placed on the specimen surface in order to determine the corresponding stress concentration; these results were associated with the real fracture origin pitting holes observed on specimens with pre-corrosion and sea water flow corrosion. Finally, some conclusions were obtained concerning pitting holes, stress concentration and fatigue life for the pre-corroded and sea water flow corroded specimens.
This work deals with rotating bending fatigue tests at high speed (150 Hz), carried out on aluminum alloy AISI-SAE 6061-T6 for which the highest experimental stress inside the specimen is close to the elastic limit of material. Simulation results are obtained by Visual Nastran software in order to determine the numerical stress and strain distributions inside the specimen; then, this information is used for the experimental set up. A general description of experimental test machine and experimental conditions are developed in first sections, following section present the experimental results and discussion about the observed failure origin related to discontinuities and the associated high stress zones. A principal contribution of this work is related to the analysis of fracture surfaces and its correlation with experimental fatigue endurance: a simple model is proposed for the prediction of fatigue life of this aluminum alloy under high speed rotating bending fatigue tests, based on the surface density of associated fatigue macro-plastic deformation zones close the crack initiation.
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