Residual stresses and ductile fracture

EDF有兴趣开始一个博士学位，研究残余应力对延性断裂的影响，特别是对延性撕裂阻力的影响，达到约2mm的增长。这个问题在过去已经使用了许多不同的方法进行了研究，所以在定义一个新项目之前，有必要简要地回顾一下技术的现状。实验证据这里我们有极限载荷的问题。如果裂纹起裂发生在接近Lr=1时，则很难确定残余应力的任何影响，因为预测的影响既小又被实验散射掩盖。脆性断裂发生在裂纹驱动力较低的情况下，在残余应力场中几乎没有塑性诱发松弛的低lr情况下，设计和执行小规模试验相对简单（例如见[1]）。也可以在Lr范围内进行测试，以检查塑性的影响。大多数与核部门直接相关的延性材料具有较高的断裂韧性和相对适中的屈服强度，并且随着撕裂的进行，裂纹扩展阻力上升。在工程用钢（例如，见[2]）中，很难设计出在极限载荷下不发生起裂和撕裂的小规模试验。因此，残余应力的任何影响都被通常的实验散射所掩盖。STYLE项目在eshete 1250管道上进行了一次大型断裂测试，其中包含一个修复焊缝，残余应力足够高，对裂纹萌生和撕裂的早期阶段产生了可测量的影响，因为启动发生在Lr<0.5[3]。然而，这是一个单一的、复杂的、昂贵的测试，它可能应该被视为一个验证基准，而不是科学研究的基础。Sherry和同事们通过使用一种模型材料来解决这个问题：低韧性铝合金仍然会因微空洞接合而失效。这类材料允许使用小样品和标准试验机。引入残余应力的方法是多种多样的。准确估计它们的大小和分布通常不是直截了当的，必须注意引入方法不会改变被测试材料的性质（或者任何性质的变化都是很容易理解的）。

EDF are interested in starting a PhD examining the impact of residual stresses on ductilefracture, specifically on ductile tearing resistance up to about 2mm of growth.This problem has been examined using a number of different approaches in the past, so it isworth reviewing the state of the art briefly before defining a new project.Experimental evidenceHere we have the problem of the limit load. If crack initiation occurs close to Lr=1then it is difficult to identify any impact of residual stress since the predicted effectsare both small and masked by experimental scatter.Brittle fracture occurs at low values of the crack driving force, and it is relativelystraightforward to design and perform small-scale tests where fracture occurs at lowLr where there is little plasticity-induced relaxation in the residual stress field (forexample see [1]). It is also possible to test over a range of Lr to examine the effectsof plasticity.Most ductile materials of direct relevance to the nuclear sector have high fracturetoughness and relatively modest yield strength, and the crack growth resistance risesas tearing proceeds. It is difficult to design small-scale tests that do not undergoinitiation and tearing at the limit load in engineering steels (for example, see [2]).Thus, any effects of residual stress are masked by the usual experimental scatter.The STYLE project performed a single large-scale fracture test on an Esshete 1250 pipecontaining a repair weld, where the residual stresses were high enough to have ameasurable effect on crack initiation and the early stages of tearing, since initiationoccurred at Lr<0.5 [3]. This however was a single, complex, expensive test, whichperhaps should be regarded as a validation benchmark rather than a basis for ascientific investigation.Sherry and co-workers have approached the problem by using a model material: alow toughness aluminium alloy that still fails by micro-void coalescence. This class ofmaterial allows the use of small specimens and standard testing machines.The methods used to introduce residual stresses are varied. Making accurateestimates of their magnitude and distribution is often not straightforward, and caremust be taken that the introduction method does not alter the properties of thematerial being tested (or that any property changes are well understood).