Evaluation of the limit state and cyclic life of structural materials under thermal fatigue

Abstract

P ecu liarities o f th e M a te ria l D am ag e u n d e r Iso th e rm a l a n d N on Is o th e rm a l C o n d itio n s. The life o f m aterials under therm al fatigue is generally determ ined from the calculation relationships that correspond to a fatigue or quasi-static fracture m ode under low -cycle m echanical loading under isotherm al conditions. A ttem pts to use the sam e calculation relationships for therm al fatigue conditions yield an incorrect result despite the sim ilarity o f m any processes under low -cycle m echanical fatigue in isotherm al conditions and under therm al fatigue, such as the developm ent o f the local plastic strains and form ation o f cracks in the m ost heavily stressed zones (with the accum ulation o f the num ber o f cycles); an increase in the stress and strain am plitudes leads to a decrease in the num ber o f cycles to fracture; a sim ilar effect o f the stress and strain concentration, stress state m ode, and m any other factors is observed.A t the sam e time, there is a fundam ental difference betw een the low-cycle m echanical fatigue and therm al fatigue. U nder conditions o f low-cycle mechanical loading at constant tem perature, the sam e zones o f the m aterial are in turn subject to plastic tensile and com pressive strains. In this case, the alternating cyclic elastic-plastic deform ation is caused by a change in the resistance to the elastic and inelastic strains (the B auschinger effect). Therm al stresses occur due to the therm al strain constraint, and therefore, they generally decrease rapidly w ith increasing tem perature due to an increase in the plastic strain that has a tendency © A. P. GOPKALO, 2009 ISSN 0556-171X. Проблемы прочности, 2009