Stress Relaxation Dynamic Properties And Service Life Evaluation Of Helical Compress Spring

The stress relaxation in metallic materials is a common phenomenon, and one of the most important failure forms of the elastic elements. However, there are no standards of stress relaxation testing for springs. Likewise, it should be further research on the relaxation mechanism and the conditions when stress relaxation occurs. Therefore, investigations on stress relaxation property and preventing technology are the most important for improving the quality, lengthening the service life of accessories, ensuring the reliability of the equipment, which has a huge significance of technology and economy.In order to study the rule of the stress relaxation of helical compress spring and realize the evaluation of its service life, the paper designed a new type of device for the stress relaxation dynamic testing, This device achieved a continuous measurement and automatic data recording as well as the relaxation curve displayed for spring stress relaxation at the temperature from ambience to 300℃. The results show that the precisions of experiments are improved remarkably by adopting the new testing device. The dynamical equations of the stress relaxation for the helical compress springs made by 60Si2Mn and T9A have been deduced at the temperature of 80～200℃. The calculation and analysis of the stress-relaxation energy over the temperature involved are performed. Further more, the characteristics of stress relaxation for the spring at different applied loads have been studied. According to the kinetics equation, the degree of the stress relaxation at a certain temperature during a period can be calculated. As a result, a reliable evaluation for the service life of the helical compress spring can be provided. The microstructural changes accompanied with stress relaxation have been investigated by using optical microscope (OM) and transmission electron microscope (TEM) before and after the stress relaxation tests. Combined with relaxation thermodynamics and dynamics, the mechanism of stress relaxation of spring has been studied.The experimental results show in the semi-logarithmic coordinate of△P/P0-lnt, the curves can be divided into two stages and both are in linear relation. On the first stage, the rate of stress relaxation is higher, and lasts a short time; on the second stage, the rate of stress relaxation is comparatively lower. The relaxation curves at the temperature of 80～200℃are in accordance with the Arrhenius equation. Therefore, the evaluation of service life of the springs can be calculated in a short time at a valid temperature above the room temperature. By analyzing of the microstructural changes accompanied with stress relaxation, it can be concluded that the stress relaxation of spring resulted from the dislocation movement and atomic diffusion, is a thermally activated process with elastic strain-energy as the driving force, and it makes dislocation structure, stress distribution and substructure homogeneous, impels material interior structure and property tend to stabilization.