Analysis of plastic wrinkling behavior in tube hydroforming process

Wrinkling behavior of tube in hydroforming mostly occurs in plastic state. The corresponding critical load for wrinkling is the sum of the initial yield load and the plastic wrinkling load. For plastic wrinkling, constitutive equation is in linear form when it is computed from the initial yield point using linear-hardening model. Formulation to calculate the critical stress for wrinkling during tube hydroforming process is given first. And the effects of main mechanical properties, tube dimensions and axial-circumferential stress ratio on wrinkling behavior are analyzed. The results show that: the modulus of elasticity and yield strength are the main mechanical parameters affecting the resistance to axial wrinkling, and the variables λ of the both determine the changing of resistance to axial wrinkling. When λ is greater than zero, the absolute value of critical stress for wrinkling will increase. However, when λ is less than zero, the absolute value of critical stress for wrinkling will decrease. The effect of stress ratio on wrinkling depends on the time of the smallest critical stress's arising for wrinkling. When the wrinkling corresponding to the smallest critical stress arises before necking, the absolute value of critical stress for wrinkling will decrease first and then increase as the absolute value of stress ratio increases. When it arises at necking, the absolute value of critical stress for wrinkling will increase as the absolute value of stress ratio increases.