An experimental study of the effect of a flaw at a notch root on the fatigue life of cast Al 319

Abstract

Smooth and notched specimens of a 319 cast aluminum alloy containing casting defects were fatigue tested in the as cast and hipped conditions. Hipping is a process in which the material is subjected to a high pressure at high temperature to eliminate flaws and then slowly cooled. The materials were tested under fully reversed constant amplitude loading and under a variable amplitude load history consisting of underloads followed by constant amplitude small cycles. The stress ratio and the number of constant amplitude small cycles following an underload were adjusted so that the crack did not close and remained fully open for all the small cycles. Three notch sizes of 1.0 mm, 3.0 mm and 6.0 mm in diameter, with a natural flaw at the center of the notch root, were examined under constant and variable amplitude loading. In another series of tests the natural flaws in the as cast material were modeled in the hipped material by a notch made with a circular drill at the notch root. The fatigue lives observed showed the following: 1. A notch size effect is observed for notched as cast and hipped Al 319 specimens having a 0.6 mm diameter flaw at notch root under constant and variable amplitude loading. The fatigue limit stress ranges for hipped Al 319 under constant amplitude loading were 117 MPa, 90 MPa, and 80 MPa for 1.0 mm, 3.0 mm, and 6.0 mm diameter edge notches respectively. The corresponding values under variable amplitude loading were 25 MPa, 18 MPa, and 8 MPa. 2. The effect of natural flaws or casting defects, on fatigue strength, in the 319 cast aluminum alloy can be modeled by a drilled hole of the same size. A crack growth analysis based on a fracture mechanics approach was used to model the fatigue behavior of the cast aluminum material under variable amplitude loading. In calculating the fatigue life of the cast aluminum, the flaw was modeled by a spherical cavity having the same diameter as the natural flaw. The size and location of the flaw at the notch root were varied to examine their effect on fatigue life. Fatigue life predictions were in good agreement with the experimental results.