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The creep crack initiation (CCI) location and time of axial surface cracks with different sizes in cracked pipes have been comparatively predicted by finite element calculations based on creep ductility exhaustion model and creep fracture mechanics considering constraint effects by using two new creep constraint parameters ( R ∗ and A c ). The effects of calculation methods of the parameters C ∗ , R ∗ and A c along the crack fronts on the prediction accuracy of CCI time have been analyzed. It has been found that with decreasing crack length, the CCI location changes from the deepest part to near surface part along the crack front, and the composite parameters composed of C ∗ and constraint parameters can predict the CCI location. The CCI time of pipe cracks increases with decreasing crack depth and length due to the decrease of crack-tip constraint level. The excessive conservatism in conventional CCI life assessments using single parameter C ∗ can be significantly reduced by using the two-parameter concept ( C ∗ - R ∗ and C ∗ - A c ) considering constraint effects. When the average values of C ∗ , R ∗ and A c along the crack fronts (instead of the values at the deepest locations of crack fronts) are calculated and used in CCI life predictions of cracked pipes, the prediction accuracy with respect to the finite element calculations based on creep ductility exhaustion model can be significantly improved. The C ∗ values calculated by reference stress method can lead to extra conservatism in CCI life predictions.

The creep crack initiation (CCI) time of six specimen geometries in ASTM E1457 standard has been predicted by using numerical simulations, and the constraint effect on CCI time was analyzed. The results show the specimens with different geometries, size ranges and side-grooves recommended by ASTM-E1457 produce different CCI time data due to different constraint levels. With increasing constraint levels in specimens, the CCI time is reduced. The quantitative correlation formulas of CCI time with creep constraint parameters have been established, and they may be applied in CCI life assessment of high-temperature components for accounting for constraint effect and improving accuracy.

Three-dimensional creep constraint of six specimen geometries in ASTM E1457 standard was characterized by using two constraint parameters, and creep crack growth (CCG) rates in these specimens were predicted by using numerical simulations. The results show that the specimens with different geometries and size ranges recommended by ASTM-E1457 have different constraints, thus produce different CCG rate data. With increasing specimen thickness and side-groove depth, the constraint and CCG rates of all specimens increase. It needs to consider constraint effects on CCG rate when predicting component life using laboratory data. The quantitative correlation formulas of CCG rate with creep constraint have been established.

By using stress dependent creep ductility and strain rate model in a ductility exhaustion based damage model, the creep crack initiation (CCI) behaviour in Cr–Mo–V steel specimens with different geometries and dimensions (different constraints) over a wide range of C* has been predicted by finite element simulations. The predicted creep crack initiation time agree well with the existing experimental data. In low and transition C* regions, the constraint induced by specimen geometries and dimensions has obvious influence on CCI time. With increasing constraint level of specimens, the CCI time decreases due to the increase of stress triaxiality ahead of crack tip. Different CCI trends and constraint effects on CCI behaviour in a wide range of C* result from the interaction of crack-tip stress state and stress dependent creep ductility of the steel. It is suggested that in CCI life assessments of high temperature components, the long-term CCI time data at low C* region should be obtained and used, an...