FATIGUE-CRACK INITIATION AND GROWTH IN NOTCHED 2024-T3 SPECIMENS MONITORED BY A VIDEO TAPE SYSTEM

摘要

Fatigue-crack initiation and early stages of growth in notched 2024-T3 aluminum alloy sheet specimens were monitored during constant-amplitude fatigue tests conducted for a range of stress ratios. A video tape system was developed to detect fatigue cracks 0.1 mm in length on the sheet surface and to monitor their growth up to a length of 1 notch radius (2.5 mm).nFatigue cracks initiated either at the notch corner or on the notch surface. Most of the first fatigue cracks started as notch-corner cracks, and the crack-initiation period was defined as the number of cycles for which the crack was 0.1 mm in length on the sheet sur¬face. For each of the stress ratios used, the results showed that at the low stress levels, the crack-initiation period constituted at least 90 percent of the fatigue lives. However, for the negative stress ratios at the high stress levels, cracks started as early as at about 40 percent of the fatigue lives.nFor early stages of the growth of corner cracks, a linear relationship existed between the logarithm of crack length and the number of cycles. For cases of elastic cyclic stresses at the notch, experimental stress intensity factors were derived from the measured corner-crack growth rates. The factors agreed well with those calculated by several approximate analytical methods from the literature on corner-crack stress intensity factors.