Stress Relaxation Behavior Of The Spring Steel

The stress-relaxation behavior, a common phenomenon for metallic materials undergoing long service, is one of the most important failure types of the various elastic elements. There are more researches on compression, tension and bending stress-relaxation behavior of the coil spring, but the stress-relaxation in rectangular cross-section torsion spring is rarely reported. Therefore, a specific stress-relaxation test device for such torsion springs is designed. The stress-relaxation mechanism of silicon-manganese alloy spring steel is discovered through the detailed experiments and the existing dislocation theory, which has important theoretical significance and engineering value for improving the quality of such spring parts, extending their service life and increasing machine reliability.By using the stress-relaxation test device self-developed for the torsion spring, the stress-relaxation tests for different rectangular cross-section torsion springs at various temperatures are completed. The torque rules varying with time for silicon-manganese spring steel under the conditions of fixed twist angles are obtained. The results show that anti-stress relaxation behavior of silicon-manganese alloy spring steel under room temperature is good, but there is a significant stress-relaxation phenomenon under high temperature. In the stress-relaxation process of the rectangular cross-section torsion spring under a constant temperature, the initial load is determined by the specimen size, torsion angle and modulus of elasticity. At the initial stage of stress-relaxation, the equivalent creep increases with time rapidly, and then the growth rate slows down with the continuation of time. At the second the growth rate of equivalent creep is gradually stable. On the side of stress-relaxation mechanism, the stress-relaxation is the process that the micro plastic deformation gradually accumulated, which must be exacerbated further by high temperature. On the premise that the total strain remains the same, the elastic strain is reduced gradually with time and the stress decreased accordingly, which is consistent with the stress-relaxation law of the creep model.The stress-relaxation behaviors of silicon-manganese alloy spring can be described by using the dislocation theory, where the movable dislocation density is decayed in a power function of time. On the base of the new movable dislocation density function, elasticity constrained torsion formula of rectangular cross-section bar and the existing stress-relaxation dislocation theory, a new stress-relaxation kinetic equation is proposed in this thesis for silicon-manganese alloy spring. By using CREEP subroutine, ABAQUS secondary development interface, the new stress-relaxation kinetic equation is converted to a modified time hardening creep model based on implicit finite element method. Meanwhile, the torsion stress-relaxation behavior of silicon-manganese alloy spring steel under400℃is numerically simulated by using the new model. At some key points of the rectangular cross-section torsion spring, the simulated results show that the law of the Von Mises equivalent stress varying with time is consistent with that of the torque in the experiment.