An investigation of preload relaxation behauvoir of three zinc-aluminium alloys

Abstract

Preload relaxation tests have been carried out on three sand-cast Zinc-Aluminium alloys, No3, No5 and No2. The equipment used for load relaxation tests consists of a load-monitoring device, an oil-bath and a data-acquisition system. A load cell monitoring device was used to monitor the load loss in an ISO-metric M6 x 1 steel screw set into sand castings of alloys No3, No5 and No2 and tightened to produce an initial preload of 6 kN. The castings were held at constant temperature in the range 80-120oC in an oil-bath.

The oil-bath maintains the desired test temperature throughout the experiment. All tests were conducted in duplicate for periods of up to 160 hours. A parametric equation of the form: ln L = a [Q/RT – lnB – lnt], was used for a quantitative analysis of the preload relaxation behaviour of alloys under different testing conditions, where L is the load after time t, Q an activation energy for load relaxation, R the universal gas constant and a and B are constants. For all alloys, the initial load loss was high, decreasing gradually with time but not ceasing.
The results showed that the load loss increased rapidly with temperature and almost all of the relaxation curves approximated to a logarithmic decay of load with time. Alloy No2 had the best resistance to load loss, with No5 next and No3 worst at all temperatures. The lower resistance to relaxation of alloy No3 was mainly due to the lower relaxation strength of copper-free primary dendrites whereas in alloys No2 and No5, the higher copper contents contribute greatly to their relaxation strength in the form of second-phase particles.