Effect of tensile prestrian on fatigue strength of a solution treated precipitation hardened Aluminium alloy and design and development of a plane and rotating bending fatigue testing machine

Abstract

A versatile plane and rotating bending fatigue testing machine with continuously variable strain control has been designed and developed. Attachments for measurement of cyclic stress and cyclic strain have been developed for this machine. This machine has been used to study the effect of tensile prestrain on the low cycle fatigue properties of B51S aluminium alloy in solution treated and precipitation hardened condition. A few experiments have also been conducted to show the capability of this machine for plane and rotating bending fatigue tests. The effect of tensile prestrain on the fatigue strength of B51S aluminium alloy in the solution treated and precipitation hardened condition has been studied at fatigue lives in the range of 10³ to 10 cycles. At 4.5% nominal prestrain, the fatigue strength decreases by 3 to 7%. At higher prestrain, a slight improvement in fatigue strength is noted. Under fatigue loading, this material neither strain hardens nor strain softens. Tensile prestraining of this material does not produce any significant change in this characteristic. The cyclic strain at half life vs. fatigue life curves of virgin and prestrained materials have been studied at fatigue lives in the range of 10³ to 10 cycles. At lives less than 2 × 10 cycles, and at all cyclic strains, the fatigue life of the prestrained material is slightly lower than that of the virgin material. The cyclic stress-strain curve obtained from multiple step tests is very close to that from multiple specimen tests. The cyclic stress-strain curves of virgin and prestrained materials lie slightly below their respective static stress-strain curves. The relative disposition of the static stress-strain curves in the inelastic regions for virgin and prestrained material shows qualitatively the relative disposition of the corresponding S-N curves.