Development of Fracture Toughness Testing echniques for Non-sharp Defects

这一建议的创新之处在于开发了新的检测方法，用于表征缺陷尖端的锐度，并指导非尖锐缺陷样品的断裂韧性测量。这项研究计划的结果将逐步改进基于断裂力学的缺陷评估方法，对包括核能、航空航天、石油和天然气、风力发电和国防部门在内的一系列行业产生真正的整体影响。SI评估程序传统上将缺陷分类为无限尖锐的裂纹或局部变薄区域(LTA)缺陷，未通过LTA几何检查--缺陷尖端半径(R)必须超过其深度(A)，r/a 1-被假定为尖锐裂纹，并根据断裂力学原理进行评估。LTA规则可能适用于一般腐蚀或机械刨削，在这些情况下，壁厚发生变化，并且假定塑性坍塌是失效机制。对于非尖锐缺陷(如焊接/添加剂制造的气孔、点蚀缺陷)，原则上没有明确的失效机制。对于非尖锐缺陷，与裂纹情况相比，期望具有更高的断裂韧性和承载能力。一方面，将钝化缺陷简化为LTA可能是非保守的，另一方面，将其简化为尖锐的裂纹可能过于保守。因此，有必要开发更现实地解释非尖锐缺陷的程序。实现钝化对失效影响的完整表征的关键因素是开发具有代表性和可重复性的缺口试件的断裂试验指南。观察到，与疲劳预裂标准试件相比，钝头的解理断裂和延性撕裂更为复杂。对于具有非尖锐缺陷的试件，起裂和裂纹扩展的测量和定义并不那么简单，即观察到短裂纹在不同的平面上竞争。在试件中引入缺口而不是疲劳预裂，可以被认为是面内约束损失。最近还观察到存在离面约束效应。因此，在典型的断裂韧性测试(例如E-399)中，满足疲劳预裂的最小厚度要求的试件可能导致缺口部件的韧性测量非保守。解理的实验缺口断裂韧性测试将在缺口单边缺口弯曲(SEN(B))试件上设计和执行。改变样品厚度和缺口尖端的锐度，以达到不同程度的缺口尖端约束，并达到平面应变条件。由于与断裂韧性值相关的固有分散性，对断裂韧性结果的验证需要足够多的测试结果。整个初始测试计划将使用S355结构钢。将使用测试试件的解理模型对试件厚度和缺口尖端敏锐度的影响进行参数研究。对修正边界层(MBL)模型和SEN(B)模型进行了研究。3DMBL模型将被用来评估缺口尖端锐度的面内效应和模型厚度对裂纹尖端前方应力分布的面外效应。双参数断裂力学(J-Q)将用来描述不同约束水平和不同缺口尖端半径下的面内应力场，从而使用一个独特的框架来量化这两种类型的影响的应力/应变场与高裂纹尖端约束条件(Q=0)的偏离程度。实验结果将用于验证本任务中使用的SEN(B)模型的结果。从实验工作中得到的韧性值将用于捕捉数值模型中的行为。

The novelty in this proposal is the development of novel inspection methods for flaw tip acuity characterisation and guidance for fracture toughness measurement in samples with non-sharp defects. The outcomes of this research programme will produce a step-improvement in fracture mechanics-based defect assessment methods, of genuine overall impact across a range of industries including nuclear; aerospace; oil and gas; wind power generation; and the defence sector.SI assessment procedures conventionally categorise flaws either as infinitely sharp cracks or local thinning areas (LTAs) Defects failing the LTA geometry check - the flaw tip radius (r) must exceed its depth (a), r/a 1 - are assumed as sharp cracks and assessed by fracture mechanics principles. The LTA rule may be appropriate for general corrosion or mechanical gouges where there is a change in wall thickness and plastic collapse is assumed to be the failure mechanism. For non-sharp defects (e.g. welding/additive manufactured porosity, pitting corrosion defects), the failure mechanism is not, in principle, clearly defined. For non-sharp defects, higher fracture toughness and load bearing capacity are expected compared to the crack case. Simplifying blunt defects into LTAs can be non-conservative, on one hand, and into sharp cracks can be excessively so, on the other. As a result, there is the need to develop procedures that account for the non-sharp defects more realistically.A critical factor for achieving a complete characterisation of bluntness effect on failure is the development of representative and reproducible fracture testing guidance for notched specimens. It has been observed that cleavage fracture and ductile tearing from blunt tips is more complex than from fatigue pre-cracked standard specimens. The measurement and definition of initiation and crack growth is not as straightforward for specimens with non-sharp flaws, i.e. short cracks have been observed to be in competition on different planes. The introduction of a notch into a specimen, instead of a fatigue pre-crack, can be considered an in-plane constraint loss. It has also recently been observed that there is an out-of-plane constraint effect. Thus, specimens that would satisfy minimum thickness requirements for a fatigue pre-crack in typical fracture toughness tests (e.g. E-399) may result in non-conservative toughness measurements in notched-components.Experimental notched fracture toughness tests for cleavage will be designed and performed on notched single edge notch bend (SEN(B)) specimens. Changes in sample thickness and notch tip acuity will be performed to achieve different levels of notch tip constraint and achieve plane strain conditions. The validation of the fracture toughness results requires a sufficiently large number of test results due to the inherent scatter associated with fracture toughness values. An S355 structural steel will be used for the entire initial testing program.A parametric study of the effect of sample thickness and notch tip acuity using cleavage models for specimens tested will be developed. Modified boundary layer (MBL) models and SEN(B) models will be studied. 3D MBL models will be used to assess both the in-plane effects of notch tip acuity and out-of-plane effects of model thickness on stress distributions ahead of the crack tip. Two-parameter fracture mechanics (J-Q) will be used to characterise the in-plane stress fields under different levels of constraint and for various notch tip radius, thus using a unique framework to quantify the level of deviation of the stress/strain fields from high crack tip constraint conditions (Q=0) for both types of effects. Experimental outputs will be used to validate results from the SEN(B) models used in this task. The resulting toughness values from the experimental work will be used to capture the behaviour in the numerical models.