**4. Discussion and future work**

A comprehensive procedure for the determination of the material constants for the Liu and Murakami creep damage model, based on experimental data, has been described. Particular attention has been given to ensuring a constant of multiaxiality value (α) which is highly appropriate to crack tip conditions. These constants have been applied, for a 316 stainless steel at 600˚C and a P91 steel at 650˚C, to a user subroutine for the Liu and Murakami model which has been used in conjunction with Finite Element package ABAQUS, in order to provide numerical predictions for creep crack growth in both compact tension specimen and thumbnail specimen geometries. Comparisons of the model predictions to corresponding experimental data for multiple specimen geometries, in terms of both crack growth and final crack length/ profile, show extremely close correlation.

Also shown is the effect that side-grooves have on the crack profile in a CT specimen and the ability of the Liu and Murakami creep damage model to predict this more uniform crack profile observed in side-grooved CT specimens.

### **Nomenclature**

**Figure 13.** thumbnail crack specimen FE mesh.

186 Computational and Numerical Simulations

102.3kN.

**Figure 14.** Tested specimen photo to FE damage contour comparisons (a) 78.7kN (b) 90.8kN (c) 90.7kN (d) 91.7kN (e)

*Roman symbols A* Liu and Murakami Creep Law Coefficient *B* Liu and Murakami Creep Law Coefficient *n* Liu and Murakami Creep Law Constant *P* Load *q*2 Liu and Murakami Creep Law Constant *Sij* Deviatoric Stress *ta* Time to Crack Length, *a t <sup>f</sup>* Failure Time *T* Temperature *Greek symbols α* Multiaxiality Constant *ε*˙*eq <sup>c</sup>* Equivalent Creep Strain Rate

*σeq* Equivalent Stress *σr* Rupture Stress *σ*1 Maximum Principal Stress *χ* Liu and Murakami Creep Law Constant *ω* Damage *Abbreviations CT* Compact Tension *FE* Finite Element
