Kinetics of preload-relaxation behaviour of threaded fasteners in a zinc-rich alloy sand castings

A zinc-rich sand-cast alloy, ILZRO16, has been tested for preload relaxation. The equipment used for load-relaxation tests consists of a load-monitoring device, an oil-bath and a data-acquisition system. A load cell was used to monitor the load loss in an ISO-metric M6 × 1 steel screw set into sand castings of the alloy and tightened to produce an initial preload of 6 kN. The castings were held at a constant temperature in the range 80-120 °C 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 155 h. A parametric equation of the form: ln L = α[Q/RT − ln B − ln t], was used for a quantitative analysis of the preload-relaxation behaviour of the alloy 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 α and B are constants. 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. The results of preload relaxation for ILZRO16 were also compared with those of two other zinc alloys, ACuZinc5 and ACuZinc10 under the same testing conditions. The comparison showed that ILZRO16 had much higher resistance to load loss than ACuZinc5 and ACuZinc10 under all testing conditions.