Procedure for structural design for long-term cyclic strength

Abstract

A new procedure is proposed for estimating the strength of high-temperature structures taking account of creep and fatigue. This procedure is a further development of the theory of separating the strain cycle into specific components of creep strain and instantaneous ductility (SRP-model). Intergranular and intragranular failure are considered. It is shown that if intergranular failure may only develop during creep, then intragranular failure is connected with both types of strain. A nonlinear equation is obtained for damage summation whose results agree satisfactorily with experimental data with small spreads of total strain.Many elements of power generation equipment operate under conditions of the interaction of creep and fatigue. Considerable attention has been devoted to evaluating the strength of this equipment both in our country and abroad. It is particularly important to resolve this problem for atomic power engineering objects [1][2][3]. Approaches developed in this branch for structural design may be used successfully in planning installations for oil and gas processing, and also chemical production equipment operating under elevated temperature conditions.In [4] on the basis of an SRP-model [5] a procedure was proposed for evaluating material endurance with high-temperature fatigue. In accordance with this procedure, the number of cycles to failure N ƒ is calculated by the equation (1) where D′ p is failure strain amplitude for a prescribed temperature with N ƒ =0.5; c ≈ 0.5; ∆ε equ is the equivalent spread of irreversible strain in a tension half-cycle, determined by the equation ∆ε equ = ∆ε in + λ cp ∆ε cp + λ cc ∆ε cc + λ pc ∆ε pc ;(2) ∆ε in = ∆ε pp + ∆ε cp + ∆ε cc + ∆ε pc is the spread of irreversible strain in a tension half-cycle; ∆ε pp is the spread of instantaneous plastic tensile strain after instantaneous plastic compression; ∆ε pc is the spread of instantaneous plastic tensile strain after instantaneous plastic compression under creep conditions; ∆ε cc is the spread of tensile strain due to creep after compression under creep conditions; ∆ε cp is the spread of tensile strain due to creep after instantaneous plastic compression; λ cp , λ cc , λ pc are strain cycle characteristics. Experiments have shown that the greatest material damage is caused by creep strain in a tension half-cycle when there is no creep in the compression half-cycle (cp-cycle). If in a cycle there are only strains during compression (pc-cycle), ∆ ′ = − ε equ p f c D N 2 2 ( ) ,