Fundamental studies of environmentally assisted surface cracking in pipeline steels

点击翻译按钮获取中文摘要

There exist many dilemmas in the research community when it comes to dealing with environmentally assisted cracking, a major cause of failures of many facilities and components, in Canada and worldwide. Investigations to understand failure mechanisms have been conducted, either using smooth bar specimens with aggressive mechanical loading, such as slow strain rate testing, or using compact tension specimens, representing the growth behavior of long and through-thickness cracks, which are very different from the cracks formed in susceptible structures during field service. In the latter case, surface cracks or thumbnail-shaped flaws are initiated and grown. Before the surface cracks are growing into a through-thickness crack, some structures, for example, steel pipelines for oil and gas transmission, may have already lost their serviceability. Therefore, it is very desirable to study the fundamental behavior of surface cracks. From a fundamental analysis of cracks developed from field service, it has been identified that 1) the environmentally assisted surface cracks in pipeline steels started with preferential growth along their surface length direction by initiating new small cracks in the vicinity of but independent from an existing crack, rather than by extending the length of the existing crack through a direct growth at their crack tips; 2) the crack depth often remains dormant because its growth is not environmentally favorable, 3) however, the growth in length direction increases the mechanical driving forces at the depth tip even though the depth tip remains dormant geometrically. This research is targeted to provide a fundamental understanding of the growth of surface cracks along their length direction, especially by initiating small cracks in the vicinity of but independent from existing cracks. It has been analyzed that the growth of surface cracks in their length direction preferentially through constant small crack initiations is affected strongly by the yielding behavior of pipeline steels and/or their microstructures, depending on the type of corrosion environments and variable pressure fluctuations encountered during service. All these factors are mutually related to each other in terms of their effect on surface crack growth. This proposal will address the above factors individually and synergistically to provide a fundamental understanding of the problem. It is expected that the findings will be critical in the development of new materials with improved resistance to environmentally assisted cracking and in the formulation of better operating strategies for pipeline steels, achieving higher operation integrity and pipeline safety.