Approach to consider welding residual stresses in fatigue design using direct numerical simulations

该项目的总体目标是开发一种方法，包括焊接残余应力的影响，通过直接模拟疲劳评估。首先，将测定和比较小尺寸试样和大型结构中的焊接残余应力。由于缺乏结构约束，用于实验研究的小试样中的残余应力通常相对较低。调查将显示在什么范围内的残余应力存在于用于疲劳试验的小规模样本，以及它们将如何在较大的组件不同。使用数值焊接模拟，广泛的板厚度和试样尺寸将进行评估，以确定几何参数对残余应力的影响。不同尺寸的模型的模拟结果，将允许在小尺度试样和大结构的残余应力之间的比较。由于残余应力可以通过叠加的外部载荷重新分布或松弛，对于疲劳评估，加载后稳定的残余应力是相关的。使用数值模拟，针对压缩和拉伸之间的不同载荷水平（即，不同载荷比）来计算该重新分布。因此，将针对残余应力水平和载荷情况的不同组合确定影响疲劳的有效残余应力，并针对小型和大型部件进行比较。在疲劳试验中，将研究残余应力如何影响疲劳行为。由于残余应力在整个板厚上一般是不均匀的，因此对裂纹萌生和扩展的影响将是不同的。此外，两种不同的焊缝几何形状与不同的残余应力分布和应力集中系数进行比较：十字形接头，横向加载方向，和加劲肋纵向加载方向。随后，将残余应力的影响与平均应力的影响进行比较。总的来说，测试结果将提供与平均应力相比，残余应力对不同焊缝几何形状影响的信息。最后，模拟和实验的结果将进行评估，以开发一种方法来考虑使用直接数值模拟疲劳评估中的残余应力。为此，必须确定模拟中需要的精度水平以及如何读取残余应力以获得保守的结果。通过模拟加载过程中残余应力的重分布，确定了有效应力比。然后用它来计算应力比修正。因此，将验证基于板表面上的残余应力值的这种程序是否适用，或者是否需要进一步校正。该项目的结果将进一步提高未来设计程序的效率和安全性。

The overall objective of this project is to develop an approach to include the influence of welding residual stresses in fatigue assessments through direct simulations. First, welding residual stresses in small-scale specimens and in large structures will be determined and compared. Residual stresses in small specimens used for experimental investigations are often relatively low due to the lack of structural constraints. The investigations will show in what range residual stresses are present in small-scale specimens used for fatigue tests and how they will differ in larger components. Using numerical welding simulations, a wide range of plate thicknesses and specimen dimensions will be assessed in order to identify the influence of geometrical parameters on residual stresses. The simulation results for the models of different dimensions, will allow a comparison between residual stresses in small-scale specimens and large structures. Since residual stresses may be redistributed or relaxed by superimposed external loads, for fatigue assessments the stabilized residual stresses after loading are relevant. Using numerical simulations, this redistribution is calculated for different load levels between compression and tension, i.e. different load ratios. As a result, the effective residual stresses affecting fatigue will be determined for different combinations of residual stress levels and load cases and compared for small and large-scale components. In fatigue tests it will then be investigated how residual stresses affect fatigue behaviour. Since residual stresses are generally not uniform over the plate thickness, the influence on crack initiation and propagation will be distinguished. Furthermore, two different weld geometries with different residual stress distributions and stress concentration factors will be compared: cruciform joints, transversal to the loading direction, and stiffeners longitudinal to the loading direction. Subsequently the residual stress influence will be compared to that of mean stresses. Overall, the test results will give information on the influence residual stresses have on different weld geometries compared to mean stresses. Finally, the results of the simulations and experiments will be evaluated to develop an approach to consider residual stresses in fatigue assessments using direct numerical simulations. For this purpose, it has to be determined which level of accuracy is needed in the simulations and how to read out residual stresses in order to obtain conservative outcomes. The effective stress ratio is determined by simulating the residual stress redistribution under loading. It is then used to calculate a stress ratio correction. Thus, it will be verified if such procedure based on a residual stress value on the plate surface is applicable or if further corrections are needed. The results of this project will further improve efficiency and safety in future design procedures.