Fundamental studies of environmentally assisted surface cracking in pipeline steels

在加拿大和世界各地，在处理环境辅助破裂问题上，研究界存在许多两难境地，环境辅助破裂是许多设施和部件失效的主要原因。为了了解破坏机制，已经进行了调查，无论是使用具有侵略性机械载荷的光滑杆试件，如慢应变速率测试，还是使用紧凑的拉伸试件，代表了长裂纹和穿透厚度裂纹的扩展行为，这与现场使用中易受影响的结构中形成的裂纹有很大不同。在后一种情况下，表面裂纹或缩略图状缺陷被启动和扩展。在表面裂纹扩展为穿透厚度裂纹之前，一些结构，例如石油和天然气输送的钢质管道，可能已经失去了它们的适用性。因此，研究表面裂纹的基本行为是非常必要的。*通过对现场使用中发展起来的裂纹的基本分析，已经确定：1)环境辅助的管线钢表面裂纹开始于其表面长度方向的优先扩展，其方式是在现有裂纹附近引发独立于现有裂纹的新的小裂纹，而不是通过在裂纹尖端直接扩展来延长现有裂纹的长度；2)裂纹深度通常保持休眠状态，因为其扩展不利于环境；3)然而，长度方向的扩展增加了深度尖端的机械驱动力，即使深度尖端在几何上保持休眠状态。这项研究旨在为表面裂纹沿其长度方向的扩展提供一个基本的理解，特别是通过在现有裂纹附近引发小裂纹来实现的。分析表明，管线钢的屈服行为和/或其显微组织强烈影响表面裂纹在其长度方向上优先通过不断的小裂纹萌生而扩展，这取决于服役期间遇到的腐蚀环境和可变压力波动的类型。所有这些因素对表面裂纹扩展的影响是相互关联的。这项提案将单独和协同地处理上述因素，以提供对问题的基本了解。预计，这些发现将对开发具有更好的抗环境辅助开裂能力的新材料以及制定更好的管线钢运营策略、实现更高的运营完整性和管道安全具有重要意义。

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.**