Constraint effects on creep crack growth

Theoretical models of creep crack growth and test data on specimens of different types and sizes are examined in the context of constraint. The effect of out-of-plane constraint is considered via data obtained from compact tension (CT) specimens of different thickness. The effect of crack depth and loading configuration and hence in-plane constraint on crack growth rate is addressed using data from various specimen types. In particular, test data on Type 316H austenitic steel obtained using circumferentially-cracked bar (CCB) specimens at 550C are compared with an extensive dataset on the same material from CT specimens. It is shown that the CCB data lie below the CT data on a plot of crack growth rate versus crack driving force parameter C* at low C* values but are within the scatter band of the CT data at higher C* values. The value of C* at a reference stress level equal to the yield stress provides a good discriminator of the point at which the data merge, for both the CCB data and other published data. This has implications for use in plant assessments where data collected at higher loads and hence C* values are extrapolated to plant C* levels. An extrapolation method is described. A time-dependent failure assessment diagram approach to incorporation of constraint effects in creep crack growth assessments at high temperature is also outlined.