Influence of Microstructure and Surface Roughness on Fatigue Initiation in Extruded Aluminum

• Comprehensive experimental data on the fatigue properties of extruded Al6082-T6 is used together with crystal plasticity simulations to investigate the microstructure and surface roughness influence on fatigue initiation. Thesurface roughness of the specimens used in the experiments was mapped with White Light Interferometry (WLI) and the location of the fatigue initiation sites was identified and characterized in each sample. With surface geometries based on the WLI scans, densely meshed non-linear FE models were used to identify the locations of the highest macroscopic stress concentrations. A somewhat surprising outcome of the experimental study is that the majority of fatigue failures were initiated at surface irregularities that did not correspond to the most significant macroscopic stress concentration. Since the depths of the surface irregularities are comparable to microstructure features - such as the grain size - the present study explores to what extent microstructure variations can be acause for the observed material behavior. The numerical investigation is performed by using crystal plasticity simulations and simulation models which are based on the actual surface geometries and material microstructuresencountered in the experiments. Grain structure and texture are taken into account and the simulations reveal that variations in the microstructure can indeed make initiation of fatigue more likely to occur near surface notches thathave macroscopic stress concentrations lower than maximum. Different frequently employed fatigue initiation parameters (FIP) are investigated and it is shown that a FIP based on a modified Fatemi-Socie criterion adds valuable information on local slip activity. The predictions obtained by using this criterion are to some extentconflicting with the results based on other FIPs, for example based on accumulated plastic strain or stored energy.A key observation is that macroscopic stress concentrations alone are insufficient and crystal plasticity simulations provide a competent additional tool in analyzing fatigue initiation mechanisms in polycrystalline samples. Not only the magnitude of stress concentrations, but also the stress gradients near surface irregularities appear as important aspects to consider when analyzing fatigue initiation. In addition, highly misoriented grain interfaces, which constitute significant barriers to plastic slip, provide internal domains in the material where initiation of fatiguedamage appears more likely to occur than at the stress concentrations that are due to the surface roughness. The present investigation clearly highlights the importance of considering the influence of microstructure heterogeneities on fatigue properties in polycrystals.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Teknisk mekanik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Applied Mechanics (hsv//eng)

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