An Overview of Time Dependent Crack Growth Models Used for ERW Seam Weld Analyses

In response to the National Transportation Safety Board (NTSB) Recommendation P-09-1, the Department of Transportation (DOT) Pipeline and Hazardous Material Safety Administration (PHMSA) initiated a comprehensive study to identify actions that could be implemented by pipeline operators to significantly reduce longitudinal seam failures in electric resistance weld (ERW) pipe. As part of the project, Task 4 in Phase II was designed to validate existing failure prediction models and, where gaps exist, refine or develop the models needed to assess and quantify defect severity for cold welds, hook cracks, and selective seam weld corrosion (SSWC) (the primary ERW/Flash Weld seam threats) for failure subject to loadings that develop both during hydrotests and in service. These models would then be used to develop new software to support integrity management of seam welds with enough flexibility to benefit from the experience gained during this project. The purpose of this paper is to review the time-dependent crack growth model used in the development of the PipeAssess PI™ pipeline integrity management software. The model will be discussed in the context of its underlying theory, validation, and application to a set of test cases. Both the stress-activated creep model and consequential tie to fatigue crack growth models are presented, which describe crack growth under hydrostatic holds and subsequent pressure cycles. Full-scale experiments are used to validate the models. The reports generated during the course of the project are publically available and are located at the PHMSA website: HTTP://PRIMIS.PHMSA.DOT.GOV/MATRIX/PRJHOME.RDM?PRJ=390.

Crack Growth of a Polycrystalline Nickel Alloy under TMF Loading

Thermo-mechanical fatigue (TMF) is an important factor for consideration when designing aero engine components due to recent gas turbine development, thus understanding failure mechanisms through crack growth testing is imperative. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180°OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles.