Influence of Ratcheting Strain on Tensile Properties of A356 Alloy

Ratcheting, a special kind of fatigue damage, is known to occur under the influence of asymmetric stress-controlled cyclic loading. Engineering structures undergo premature failure once ratcheting damage happens to these. The present study aims to describe the effect of the imposition of previous ratcheting damage on the tensile properties of aluminium A356 alloy. A356 aluminium alloy is being potentially used in automobiles, aerospace components, etc. In such applications, chances of ratcheting deformation cannot be ruled out. Thus, in this investigation, a few ratcheting tests were carried out at ambient temperature on as received A356 alloy with varying mean stress (am) and stress amplitude (ga) up to 2000 cycles. The magnitudes of cm and am were chosen in such a way that the maximum stress (umax) during cyclic loading must lie above the yield strength of the alloy and below the ultimate tensile strength. All the ratcheting tests were done at a cyclic load frequency of 0.05 Hz using a servo hydraulic universal testing machine (BISS). The stress rates were varied accordingly in order to avoid any buckling of the specimens. The strain accumulated during the cyclic loading was captured using a 12.5 mm dynamic extensometer fitted at the gauge portion of the specimen. It was aimed to achieve at least 250 stress-strain data points per cycles during each test. Postratcheting tensile tests were carried out on the ratcheted specimens at a nominal strain rate of 6.66 x 10-4/s using a screw-driven universal testing machine (Instron). Study of fracture surfaces generated after tensile tests were done using scanning electron microscope (SEM). The results of ratcheting tests indicated that the material was cyclically hardenable in nature obtained through the reduction in stress-strain hysteresis loop areas, with increasing number of cycles. Ratcheting strain increased with increased in the magnitude of cm or am. Post-ratcheting tensile tests showed that in general, the yield strength (YS) and ultimate tensile strength (UTS) increased with increasing amount of ratcheting strain. Interestingly, post-ratcheting tensile elongation also increased in all the ratcheted specimens as compared to that of the as-received specimen. The fracture surfaces showed presence several shrinkage pores which caused favorable sites for rise of stress concentration resulting initiation and growth of cracks. The elongation depends on the nature of the cracks generated during tensile loading and the extent of quasi-cleavage facets. (C) 2017 Elsevier Ltd. All rights reserved.