INFLUENCE OF SIC PARTICLE DISTRIBUTION AND PRESTRAINING ON FATIGUE CRACK-GROWTH RATES IN ALUMINUM AA 6061-SIC COMPOSITE-MATERIAL

The fatigue crack growth properties of an aluminium AA 6061 alloy containing 15 vol.-% particulate SiC and of the corresponding matrix alloy with the same grain size were investigated. The composite was tested in the undeformed condition and after undergoing a plastic prestrain of 0.33% which changes the residual microstress state of the matrix from hydrostatic tension to longitudinal compression. The composite showed superior resistance to crack growth compared with the matrix material irrespective of whether nominal or effective values of stress intensity range DELTA-K were considered. A 50% increase in the nominal threshold value of DELTA-K was observed for the undeformed composites. Prestraining did not affect the intrinsic fatigue crack growth properties but resulted in a twofold increase of crack closure stress intensity. Quantitative fractography showed that SiC particles deflect the propagating crack, thus enhancing roughness induced crack closure. Local relief of DELTA-K by crack deflection and diminished crack tip opening are the main causes of the improved intrinsic fatigue crack growth properties of the composite.