AN EXPERIMENTAL-STUDY OF THE EFFECT OF A FLAW AT A NOTCH ROOT ON THE FATIGUE LIFE OF CAST AL-319

Smooth and notched specimens of a 319 cast aluminum alloy containing c asting defects were fatigue tested in the as cast and hipped condition s. Hipping is a process in which the material is subjected to a high p ressure at high temperature to eliminate flaws and then slowly cooled. The materials were tested under fully reversed constant amplitude loa ding and under a variable amplitude load history consisting of underlo ads followed by constant amplitude small cycles. The stress ratio and the number of constant amplitude small cycles following an underload w ere adjusted so that the crack did not close and remained fully open f or all the small cycles. Three notch sizes of 1.0 mm, 3.0 mm and 6.0 m m in diameter. with a natural flaw at the center of the notch root, we re 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 n otch root. The fatigue lives observed showed the following: 1. A notch size effect is observed for notched as cast and hipped Al 319 specime ns having a 0.6 mm diameter flaw at notch root under constant and vari able 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 frac ture mechanics approach was used to model the fatigue behavior of the cast aluminum material under variable amplitude loading. In calculatin g the fatigue life of the cast aluminum, the flaw was modeled by a sph erical cavity having the same diameter as the natural flaw. The size a nd location of the flaw at the notch root were varied to examine their effect on fatigue life. Fatigue life predictions were in good agreeme nt with the experimental results. (C) 1998 Elsevier Science Ltd. All r ights reserved.