Quantification of stress redistribution due to mismatch in creep properties in welded branch pipes

Abstract

A B S T R A C T This paper reports steady-state stress distributions within the weld metal in a welded branch component, via detailed three-dimensional elastic-creep finite element analyses. The creep exponent and constants for the base and weld metal are systematically varied to simulate under-matching, even-matching and over-matching conditions in creep. Various loading conditions are also considered to see the effect of the loading mode. It is found that the mismatch effect in creep on steady-state stresses within the weld metal can be uniquely quantified by the mismatch factor, defined as a function of creep exponent and constant. Ratios of section-averaged (effective and maximum principal) stresses for the mismatched case to those for the even-matched case are linearly dependent on the mismatch factor. A = creep constant, see Eq. (2) E = Young's modulus MF = mismatch factor, see Eq. (1) or Eq. (3) n = creep exponent P = generalized load P L = limit load t = time t red = redistribution time, see Eq. (4) ε = (creep) strain σ = stress σ 0.2 = strength corresponding to 0.2% inelastic strain; 0.2% proof strength σ 1 = maximum principal stress σ e = von Mises effective stress σ h = hydrostatic stress σ ref = reference stress ξ = coordinate along the centre line in the weld metal, see Fig. 2Subscripts b = referring to the base material w = referring to the weld metal m = referring to the mismatch condition avg = referring to section-averaged stress values (averaged over the ligament within weldment)

Mismatch effect in creep properties on creep crack growth behavior in welded joints

“…Thus, three cases of MF > 1, MF = 1 and MF < 1 refer to creep soft material, creep match and creep hard material, respectively. This type of design of configurations of mismatch in creep properties was widely used in the literature [5][6][7][19][20][21].…”

“…However, only a few mechanical analyses for the material constraint effect on CCG behavior due to the mismatch in creep properties between different materials in welded joints could be found in the literature. Lee et al [19] and Han et al [20,21] defined a mismatch factor about creep constant and exponent of Norton's law between different materials in weldment to quantify the mismatch effect in creep properties on the distribution of steady-state stress. Tu et al [22,23] and Xuan et al [24] studied the influence of material mismatch on the evaluation of the time-dependent fracture mechanics parameters C(t) and C ⁄ .…”

Quantification of creep stresses within HAZ in welded branch junctions

“…For overmatched cases, magnitudes of (σ e ) m /(σ e ) b are slightly different, but variations are similar to those for undermatched cases. As we have already quantified stresses in the weld in our previous works, 3,4 the results in Fig. 4 suggest that quantification of the HAZ stresses can be also possible.…”

“…). In our previous work, other loading modes (out‐of‐plane bending to the branch pipe and in‐plane bending to the run pipe) were also considered, and the results were found to be essentially similar. For internal pressure, pressure was applied as a distributed load to the inner surface of the FE model, together with axial tensions equivalent to the internal pressure applied at the end of the branch and main pipe to simulate closed ends.…”

“…A = creep constant, see Eq. (1) E = Young's modulus FE = finite element HAZ = heat-affected zone M, P = in-plane bending moment and internal pressure MF = mismatch factor, see Eqs (2)(3)(4) or Eq. (6) n = creep exponent r, R = branch and main pipe mean radius, respectively t, T = branch and main pipe thickness, respectively ξ = coordinate along the centre line in the weld metal, see Fig.…”