Corrosion fatigue fracture behaviour of a SiC whisker-aluminium matrix composite under combined tension-torsion loading

We describe an investigation into the fatigue fracture behaviour under comb ined tension-torsion loading of a SiC whisker-reinforced A6061 aluminium al loy fabricated by a. squeeze casting process. Special attention was paid to the environmental effects on fatigue fracture behaviour. Tests were conduc ted on both the composite and its unreinforced matrix material, A6061-T6, u nder load-controlled conditions with a constant value of the combined stres s ratio, alpha = tau(max)/sigma(max) in laboratory air or in a 3.5% NaCl so lution at the free corrosion potential. The corrosion fatigue strength of b oth the matrix and composite was less in the solution than in air. The domi nating mechanical factor that determined the fatigue strength in air was ei ther the maximum principal stress or the von Mises-type equivalent stress, depending on the combined stress ratio. However, in the 3.5% NaCl solution, the corrosion fatigue strength of both materials was determined by the max imum principal stress, irrespective of the combined stress ratio. In the ca se of the matrix material, crack initiation occurred by a brittle facet nor mal to the principal stress due to hydrogen embrittlement. However, in the composite material, the crack was initiated not at the brittle facet, but a t a corrosion pit formed on the specimen surface. At the bottom of the pit, a crack normal to the principal stress was nucleated and propagated, resul ting in final failure. Fitting corrosion was nucleated at an early stage of fatigue life, i.e. about 1% of total fatigue life. However, crack initiati on at the bottom of a pit was close to the terminal stage, i.e. about 70% o r more of total fatigue life. The dominating factor which determined crack initiation at a pit was the Mode I stress intensity factor obtained by assu ming the pit to be a sharp crack. Initiation and propagation due to pitting corrosion and crack growth were closely examined, and the fatigue fracture mechanisms and influence of the 3.5% NaCl solution on fatigue strength of the composite and matrix under combined tension-torsion loading were examin ed in detail.