Study on the mechanism of cumulative fatigue damage of advanced engineering materials under service loadings

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In this project, fatigue crack initiation and propagation behavior undre constant and variable amplitude loadings were investigated on advanced engineering materials such as titanium alloys, powder metallurgy (P/M) aluminum alloys, metal matrix composites (MMC) and ceramics.Crack opening behavior of titanium alloys under variable amplitude loadings was found to depend on the microstructure of materials, which resulted in the different crack growth rates against the identical stress intensity factor depending on the materials. Acceleration of crack growth by load reduction was observed for the small fatigue cracks, while retardation was found for conventional long fatigue cracks on the titanium alloys as well as high strength structural steels.As to the SiC reinforced aluminum matrix composites, i.e.SiC particle reinforced P/M composite and SiC whisker reinforced casting composite, fatigue crack initiation and growth mechanisms were found to be defferent depending on the geometry of SiC and production processes. A fatigue crack initiated in the vicinity of SiC particle in the P/M composite and a single or a few cracks propagated until fracture. On the other hands, in the casting composite, fatigue cracks easily initiated at the whisker/matrix interfaces or at the sites where SiC whisker was pulled out and some microcracks were found to coalesce to form a main crack. Furthermore, SiC reinforcement irrespective of particle or whisker was found to be benefit for fatigue characteristics of MMC due to the increase of fatigue crack growth resistance.Through the direct observation of fatigue crack growth behavior under a single overload on a 3% silicon iron, it was found that the retardation behavior was closely correlated with deflected or branched cracks.Finally, crack growth acceleration under repeated two step loadings was found on a sintered silicon nitride, to the contrary of the retardation behavior usually observed on the metallic materials.