Rules for the Assessment of Interacting Cracks Under Creep Conditions

Multiple interacting cracks are generally treated as an enveloping single crack if the prescribed criterion is satisfied in the current fitness-for-service codes. To examine the applicability of the existing criteria under creep regime, in this report, creep crack interaction effects of plates containing collinear cracks or parallel cracks are first analyzed under the framework of reference stress method and finite element computations. An equivalent crack growth rate criterion is then proposed to develop a congruous multiple crack treatment rule for different growth mechanisms. The current combination rule for multiple cracks is examined and modified in terms of the proposed new criterion. The results reveal that the creep crack interaction factor is a combined function of creep exponent $n$ of materials, limit load ratio of cracked structures, and interaction of stress intensity factor. More pronounced interaction is observed for both interacting cracks in the same plane and that in different planes. Although a higher intensity of creep crack interaction is observed, a greater value of $γcreep$ is permissible compared with that of linear-elastic crack interaction denoted by SIF. Accordingly, a modified combination rule for collinear cracks of $S/a=3.0$ is proposed to exclude the nonconservativeness, while the current alignment rule for offset parallel cracks of $H/a=2.0$ is still practicable in terms of the proposed criterion.