In order to establish an accurate integrity assessment of structures containing defects at high temperature, it is necessary to clarify the constraint effect on creep crack growth (CCG) property. However, the experimental investigation of constraint effects on creep crack growth has been little studied. In this study, the effects of the out-of-plane constraint and the in-plane constraint were investigated and considered via data obtained from compact tension (CT) specimens with different thicknesses and single edge-notched tensile (SENT) specimen. The results show that the constraint effect induced by specimen thickness (out-of-plane) and specimen types (in-plane) on CCG rate is related to load level C*. In the lower C* region, there are obvious differences in the CCG rate for different specimens, while the CCG rates of the different specimens are within a scatter band in the higher C* region. Meanwhile, the CCG rate da/dt versus C* on log-log scale is composed of two straight lines with different slops instead of a single linear relation. Because the straight line slope in the lower C* region is smaller than that in the higher C* region, it would be non-conservative if the CCG rate in the lower C* was extrapolated from that in the higher C*. Therefore, in order to conduct integrity assessment and life prediction at high temperature accurately, it is necessary to obtain the actual CCG rate in the lower C* (long-term data), and the constraint effect induced by specimen thickness and loading modes should be considered.

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