The Experimental Study And Constitutive Description On Cyclic Creep/Relax Behavior Of Magnesium Alloy

Among the best light-weight structural materials with a relatively high strength-to-weight ratio and excellent mechanical properties, Magnesium alloys have attracted special attention of researchers working in the field of mechanics and materials. Its use offers promise for reducing weight and improving fuel economy. However the inferior creep resistance of magnesium alloys at high temperature limits its application to power components such as transmission cases. One means of assessing the structural integrity in these cases is modeling the cyclic creep behavior of magnesium alloys, the topic of my work.First, the stress-strain behaviors were investigated by means of stress controlled cyclic creep experiments of AM50 alloys with different loading patterns on MTS. Experimental results on AM50 have shown that cyclic creep acceleration took place after long dwell times under repeated loadings. The increase of the strain rate during cyclic creep not only affects the creep strain in subsequent cycles but also reduces the rupture life of the specimens. Then a damage constitutive model is developed based on the experimental observations and damage mechanics of continuum materials. Three parts are incorporated: one is the creep stress-strain relation, the second is the damage law derived from the rate of energy release, the third is the evolution rule for internal stress describes the recovery and softening behavior under repeated loading. In order to testify the efficiency of the developed constitutive model, some numerical simulations of cyclic creep processes were carried out. The comparison between computed and measured results shows excellent consistence and indicates that the model developed in this work can describe the cyclic creep and relax behavior.In a word, this joint plastic and creep algorithm analysis incorporating the experimental study and description of constitutive laws of magnesium alloy offers a powerful tool for wide applications of magnesium alloys and predicting the cyclic creep behavior. The ultimate goal is to advance the development of low cost, creep resistant magnesium alloys.At last, in order to investigate the effects of R-ratio and strain amplitude on cyclic mean stress relaxation and life during the cyclic-loaded process, a number of cyclic relaxation experiments strain-controlled of AM60 were also carried out on MTS, the ratio ranged from 0.1 to 0.7 at the different strain amplitude of 0.3% and 0.6%. Results indicated that cyclic mean stress relaxation increases with increasing R-ratio, but life reduces with reducing cyclic strain amplitude. Constitutive relationship was proposed to simulate cyclic mean stress relaxation with strain amplitude equals 0.3%. The calculation results showed that the constitutive relationship has accepted for the cyclic stress relaxation under strain-controlled tests.