Numerical Prediction of Creep Rupture Life of Ex-Service and As-Received Grade 91 Steel at 873 K

Abstract

Infallible creep rupture life prediction of high  temperature steel needs long hours of robust  testing over a domain of stress and temperature. A substantial amount of effort has been made to  develop alternative methods to reduce the time  and cost of testing. This study presents a finite  element analysis coupled with a ductility based  damage model to predict creep rupture time  under the influence of multiaxial stress state of  ex-service and as-received Grade 91 steel at 873 K. Three notched bar samples with different  acuity ratios of 2.28, 3.0 and 4.56 are modelled in commercial Finite Element (FE) software,  ABAQUS v6.14 in order to induce different stress  state levels at notch throat area and investigate  its effect on rupture time. The strain-based  ductility exhaustion damage approach is  employed to quantify the damage state. The  multiaxial ductility of the material that is  required to determine the damage state is  estimated using triaxiality-ductility Cock and  Ashby relation. Further reduction of the ductility  due to the different creep mechanisms over a  short and long time is also accounted for in the  prediction. To simulate the different material conditions: ex-service and as-received material,  different creep coefficients (A) have been  assigned in the numerical modelling. In the case  of ex-service material, using mean best fit data  of minimum creep strain rate gives a good life  prediction, while for new material, the lower  bound creep coefficient should be employed to  yield a comparable result with experimental  data. It is also notable that ex-service material  deforms faster than as-received material at the  same stress level. Moreover, higher acuity  provokes damage to concentrate on the small  area around the notch, which initiates higher  rupture life expectancy. It also found out that,  the stress triaxiality and the equivalent creep  strain influence the location of damage initiation  around the notch area.