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Micro mechanical study of the stress corrosion cracking of the 316L austenitic stainless steel in chloride medium

机译：氯化物介质中316L奥氏体不锈钢应力腐蚀开裂的微观力学研究

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Because of the synergetic actions of a corrosive medium and mechanical loading which occur in stress corrosion cracking, modelling this kind of damage on actual industrial structures can be very challenging. However, when studying one single couple material / medium, it is possible to only consider the mechanical aspect of damaging. Establishing this sort of purely mechanical simulation implies to have extensive information on the behaviour of the propagating cracks. Several types of tests are commonly used to gather experimental data on crack propagation, among which are tests conducted on fracture mechanics sample (CT, WOL…) and slow strain rate tests (SSRT) on smooth tensile specimens. The first category of tests is mainly used to obtain the propagation rate of long cracks and the so-called Kiscc, the mode I stress intensity factor below which no propagation can be detected for constant displacement tests. The SSRT are preferred to study initiation, mechanisms and short cracks propagation (below Kiscc)- However, as the analyses of the cracking of these tensile tests are essentially statistical, it is somewhat difficult to separate the behaviour of a single short crack from mechanisms resulting from cracks interaction. For instance, for alloy 600 in primary water [1][2], the transition of crack growth rate from a slow to a fast regime for cracks attaining the Kiscc can be interpreted as a mechanism existing for the single crack [3] or as a result of a 'natural selection' within the cracks population.In order to obtain experimental data on the propagation of one single short crack, we have developed SSRT on micro-notched tensile specimens. This new kind of sample allows the initiation and the propagation of one single crack at notch's tip. The experiments were conducted for an austenitic stainless steel (316L) in a model chloride medium (30% MgCb at 117℃). The use of interrupted tests permits us to obtain crack propagation rate in function of the global loading. Modelling these tests, as well as WOL tests, with the finite elements method gives us access to the local mechanical parameters, as strain or strain rate, needed in global stress corrosion cracking simulations. Besides, the predominance of crack branching in SSRT compared to WOL tests has been investigated. The use of EBSD (electron back scattering diffraction) showed the roles of twin and grain boundaries in branching, as well as the confined plasticity on the crack path, revealed by the creation of subgrains boundaries.
机译：由于在应力腐蚀裂纹中产生的腐蚀性介质和机械载荷的协同作用，因此在实际的工业结构上对这种损伤进行建模可能非常具有挑战性。但是，在研究一种单一的偶合材料/介质时，可能仅考虑损坏的机械方面。建立这种纯机械模拟意味着要掌握有关扩展裂纹行为的大量信息。通常使用几种类型的测试来收集裂纹扩展的实验数据，其中包括在断裂力学样本（CT，WOL…）上进行的测试以及在平滑拉伸试样上进行的慢应变速率测试（SSRT）。第一类测试主要用于获得长裂纹的扩展速率和所谓的Kiscc，即模式I应力强度因子，对于恒定位移测试，该模式下的应力扩展因子无法检测到扩展。 SSRT是研究起爆，机理和短裂纹扩展（在Kiscc之下）的首选方法-但是，由于对这些拉伸试验的裂纹进行的分析本质上是统计的，因此很难将单个短裂纹的行为与机理分开来。从裂缝的相互作用。例如，对于一次水中的合金600 [1] [2]，裂纹扩展速率从慢速过渡到快速的过程，可以使裂纹达到Kiscc可以解释为单个裂纹[3]的存在机制或为了获得单个短裂纹扩展的实验数据，我们在微缺口拉伸试样上开发了SSRT。这种新型样品允许在槽口尖端处引发和扩展单个裂纹。实验是在模型氯化物介质（117％的30％MgCb）中对奥氏体不锈钢（316L）进行的。使用间断测试可以使我们获得随整体载荷变化的裂纹扩展速率。使用有限元方法对这些测试以及WOL测试进行建模，使我们能够访问局部应力腐蚀裂纹模拟中所需的局部力学参数（如应变率或应变率）。此外，与WOL测试相比，SSRT中裂纹分支的优势也得到了研究。 EBSD（电子背散射衍射）的使用显示了孪晶和晶界在支化中的作用，以及在裂纹路径上的局限可塑性，这通过创建亚晶界得以揭示。

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