Characterization Of Creep Constraint For Specimens With A Wide Range Of Geometries And Its Correlation With Pipeline Constraint

The creep damag and fracture are the main failure modes of high temperature components.The creep crack initiation time and crack growth rate of the material are affected by the geometric constraints of specimens and components.Therefore,it is necessary to develop a creep life evaluation method for cracked high-temperature components incorporating constraint effect.Based on the newly developed in-plane and out-of-plane unified crack-tip constraint parameter Ac,a large number of finite element calculations are used to characterize the creep crack-tip constraint of the specimens with a wide range of geometries and sizes,and the relation with the constraints of pipeline cracks with different sizes are analyzed.The effect of material creep properties on the constraint parameters Ac of different geometric specimens and the creep crack growth rate associated with P92 steel constraint were studied.The research work and the main conclusions obtained are listed as follows:(1)Based on a large number of finite element calculations,the influence of the thickness-width ratio B/W,width W and crack depth a/W of different geometric specimens(C(T),SEN(T)and M(T))on the unified crack-tip constraint parameter Ac was studied systematically and the constraint parameter solutions of these specimens were obtained.The results have shown that the thickness-width ratio B/W of the specimen has the greatest influence on the constraint,the second is the effect of width W,and the effect of crack depth a/W on constraint is not obvious.For a certain specimen size W and crack depth a/W,as the specimen thickness-width ratio B/W increases,the specimen constraint increases,the creep crack initiation time becomes shorter,and the crack growth rate increases.When measuring the creep crack initiation time and crack growth rate of materials,the effects of B/W and W on the results should be mainly considered.(2)Systematic comparative analysis and correlation studies on the constraint parameter A?values of the C(T),SEN(T)and M(T)specimens of different geometries calculated above and the constraint parameter Ac values of the axial cracks and the circumferential cracks on the inner surface of the pipelines with different crack sizes(a/t,a/c)were carried Oout.The specimen geometries and dimensions matching with the constraint of pipeline cracks with a wide range of dimensions have been obtained.In the creep life evaluation of pipelines with different crack sizes,these correlation results can be used to select the standard and non-standard specimen sizes that match the constraint of pipeline cracks.For the deep and long high-constrained pipeline cracks,non-conservative assessments that may result from the use of lower constraint standard specimen data can also be avoided.(3)The influence of creep properties of materials on the constraint parameter Ac of the specimens with a wide range of geometries shows that with the increase of the creep exponent n,the constraint parameter Ac decreases and the constraint level increases.The parameter Ac of the specimen with low in-plane constraint and the out-of-plane constraint is sensitive to the creep exponent n,and the sensitivity increases with the decrease of n.For the low-constrained cracked structure with low exponent n,the effect of exponent n needs to be considered in the engineering estimation of the constraint parameter A,.For the high-constrained structure with higher exponent n,the effect of the exponent n can be ignored.The correlations between Ac and n of different geometric constraint specimens have been established.(4)Based on the creep crack growth rate data of P92 steel measured by C(T)specimens with different geometries and dimensions and combined with the finite element calculation of the creep constraint parameter A,of these specimens,the correlation between the creep crack growth rate and the unified constraint parameter A,of P92 steel was studied and established.It can be used for the calculation of constraint-related the creep crack growth life of the cracked structures of P92 steel.