Mechanism for fatigue striations as formed under variable negative R-ratio in Al-based structural alloys

Effect of varying negative R-ratio on the fatigue striation patterns was examined for Al-based V95-T4 (analogue of 7057T6) and D16T (analogue of 2024T3) alloys. Three different sequences of cyclic loads and of the varying maximum and minimum stress levels were applied in air atmosphere to the test alloy panels 10 × 160 × 460 mm in size. The tests were conducted at room temperature with the loading frequencies in the range of 0.5–1.5 Hz. Fractographically, the magnitudes of striation spacing were found increasing for increasingly negative R-ratios. To estimate the effects of negative R-ratio on the crack growth rate correction-function surfaces were constructed for the stress-intensity factor KI. The fractographic analysis suggests that the unloading portions of cycling trajectory are only responsible for the fatigue striation effects. Such a mechanism appears dominating in the positive as in the negative ranges of R-ratio.