Influence of Compressive Pre-strain on Tensile Fatigue in Two Types of Aluminum Alloys

Abstract

The effect of pre-strain on fatigue behavior in various kinds of materials has been reported on several times thus far (Ref 1-11). That being said, however, the number of actual systematic research works on this topic is quite few, and it is still unclear whether pre-straining is of any benefit in fatigue properties, presumably depending on the material and its microstructure. Furthermore, in particular, the effect of compressive pre-strain on pulsative (tensile) fatigue behavior has not been reported on in correlation with cyclic plastic strain response for the materials strengthened by different mechanisms, which is very important in engineering application. However, it has so far been considered that the measurement of cyclic plastic strain response under the condition of tensile fatigue is extremely difficult because of its smallness. In the present study, the influence of compressive pre-strain on fatigue properties in two types of aluminum alloys (5056-H34 Al alloy and 6061-T6 Al alloy) has been investigated. 5056-H34 Al is the alloy strengthened mainly by solid solution, while 6061-T6 is the precipitation-strengthened alloy (H34: work-hardened and stabilized, T6: solid-solution treated and artificially aged). The mechanical properties, which were obtained by the tensile test at the strain rate of about 1.5 9 10 2 /s, are shown in Table 1. The UTS and the 0.2% proof stress of 6061-T6 with precipitation strengthening are higher than those of 5056-H34. The specimen is cylindrical in nature with the gauge length of 13 and 7 mm diameter. Compressive pre-strains of 0.4 to 3.2% were applied to these alloys prior to fatigue testing and the fatigue lives have been measured and compared to those in the materials without pre-straining. The stress-controlled fatigue test was conducted using Dynamic Servo FT-5, SAGInoMIYA, Japan at a frequency of 10 Hz under the pulsative (zerotension) condition of sine wave, i.e., the stress ratio R = 0 in air at room temperature. Furthermore, in order to study the cyclic stress-strain response (CSSR), similar fatigue tests were carried out during the repetitions until 1,000 cycles at the frequency of 1 Hz for both specimens with pre-straining and without