Evaluating Fatigue Life of Structures Under Thermocyclic Loading

Abstract

The issue of evaluating strength and service life is discussed as applied to structures, the performance properties of which are characterized by multi-parametric non-stationary thermal–mechanical effects. The main degradation mechanisms of structural materials (metals and their alloys) are considered. The main requirements to mathematical models describing fatigue damage accumulation are formulated. In the framework of mechanics of damaged media (MDM), a mathematical model is developed that describes thermoplastic deformation and fatigue damage accumulation under combined low-cycle and high-cycle fatigue. The model consists of three interrelated parts: relations that define thermocyclic plastic behavior of the material with account for its dependence on the failure process; evolutionary equations describing the kinetics of damage accumulation; and a strength criterion of the damaged material. A version of the constitutive relations of thermoplasticity is based on the concept of the yield surface and the gradientality principle of the plastic strain rate vector to the yield surface at the loading point. These relations describe the major effects of the cyclic plastic deformation of the material for arbitrary complex loading trajectories. This version of kinetic equations of fatigue damage accumulation is based on a scalar damage parameter and energy principles and considers the main effects of nucleation, growth, and merging of microdefects under arbitrary complex loadings. A generalized form of the evolutionary equation for the fatigue damage accumulation under low-cycle and high-cycle fatigue is proposed. The critical damage value is used as the strength criterion of the damaged material. The effect of the distillate droplet impingement frequency on thermocyclic fatigue life of the heated pipe is numerically analyzed on the basis of the developed version of the constitutive relations of MDM. The numerical results of the fatigue damage accumulation under thermal pulsation are in good agreement with experimental data. It is shown that the proposed MDM model qualitatively, with the accuracy required for practical calculations, describes the experimental results and can effectively evaluate thermocyclic fatigue damage accumulation in structural alloys under combined multiaxial nonproportional thermal–mechanical loading.