CREEP-FATIGUE INTERACTION IN NON-LINEAR VISCOUS MATERIALS.

摘要

In this dissertation, the effect of creep-fatigue interaction on the mechanical behavior of nonlinear viscous materials is studied in detail. The effect of strain rate, frequency, wave shape and stress ratio on the mode of failure under creep-fatigue interaction is carefully examined. To evaluate the state of creep-fatigue damage under multiaxial stresses, a totally strain-dependent uniaxial creep damage law is generalized by employing the maximum principal strain criterion. In addition, a stress-dependent uniaxial fatigue damage law is generalized by utilizing both maximum principal stress and maximum shear stress criteria. Based on the hypothesis that creep damage and fatigue damage interact in an additive manner, a continuous creep-fatigue interaction damage law which accounts for the important effect of strain rate on the rate of damage accumulation is proposed.;The proposed damage law is employed for a continuum mechanical analysis of a plane-strain thick-walled tube subjected to a pulsating internal pressure. The associated boundary value problem is divided into two successive time stages of damage. In the first stage, where the local value of damage is everywhere less than the critical value of damage, the time and location of the initial local rupture is calculated. In the second stage, the local value of damage equals the critical value of damage along a damage front which moves through the material. This stage of the problem falls under the category of moving boundary problems. Consequently, a nonlinear integro-differential interface equation is developed to calculate the radius of the damage front and the remaining life of the tube. It is found that intergranular creep damage becomes dominant at lower frequencies and transgranular fatigue damage becomes dominant at higher frequencies. In addition, it is found that the propagation of the critical-damage front leads to a radical redistribution of stresses.