FATIGUE CRACK-PROPAGATION CHARACTERISTICS OF 6061 ALUMINUM-ALLOY REINFORCED WITH SIC WHISKERS

The fatigue crack growth characteristics of SiC whisker reinforced cast 6061 aluminum alloy matrix composites were investigated. The samples of MMCs were tempered under three aging conditions. The role of crack closure was also appreciated from a compliance measurement. Furthermore in the higher DELTAK region, the fatigue crack growth characteristics were evaluated employing the experimentally obtained and converted J-integral value. Fatigue crack growth resistance including fatigue thresholds for MMCs exceeds that for the unreinforced alloy. However, fatigue crack growth rates of MMCs are remarkably accerelated in the stage II b, and the da/dN-DELTAK curves of MMCs and unreinforced alloy intersect during the stage II b. Moreover, MMCs fracture unstably and rapidly at much lower growth rates than the unreinforced alloy in the stage II c. In all the matrix aging conditions, the underaged MMC exhibits superior fatigue crack growth resistance. This is considered primarily due to crack-closure from the da/dN-DELTAK(eff) plots which subtract out the influence of crack-closure, and also due to crack tip shielding such as damaged zone, crack deflection and uncracked ligament. In the stage II a of MMC, the plastic zone is so small that the stresses ahead of major crack are not sufficient to fracture SiC whiskers. Therefore the crack propagates smoothly in the matrix and when it comes at a whisker, the crack tends to avoid a whisker, thereby promoting whisker pull-out. With increasing DELTAK level, the fracture process zone develops within the plastic zone, so the main crack propagates toward subcracks ahead of crack tip and crack deflection becomes remarkable. Therefore in the stages II b and II c the fracture surface becomes rough with increasing DELTAK.