In recent decades, some unexpected fatigue failure occurred in welded joint of metal structures under cyclic loading. In many cases, the cause for the failure could not be detected. A study at NC State University revealed that welding procedure could be one of the factors that was not appropriately considered in current design methodologies. The welding procedure can influence the strain response near weld toe in two ways: one is by generating residual stress, and the other is changing material properties in the heat affected zone (HAZ). It was the primary objective in this investigation to determine the influence of these two factors on strain response by conducting experiments and performing numerical simulations of welded piping joints.;In the experimental study a series of residual stress data were measured, using x-ray and neutron diffraction techniques, for welded piping joints. The measured results revealed that the initial maximum compressive residual stress of stainless steel piping joints is higher than the yield stress of base metal. Moreover, the axial residual stresses of stainless steel piping joints are mostly relaxed after 5 cycles. The change of mechanical material properties due to high temperature exposure was studied by conducting experiments on tubular specimens. It was obtained that some mechanical material properties changed after subjecting to high temperature cycles, and the changed material was correlated to the peak temperatures. A modified thermo-mechanical material heterogeneity model was then developed to improve the initial residual stress simulation at the weld toe. The heterogeneous material properties coupled with the Chaboche model were used for subsequent fatigue response simulation. Quarter-point elements were applied at the stress concentration locations. The analysis results showed that fatigue response and residual stress relaxation can be simulated well. Final two simulations in this research showed that the presence of initial residual stress influences strain amplitude and strain mean, both of which could influence the fatigue life of welded joints.
展开▼
机译：近几十年来，在周期性载荷作用下，金属结构的焊接接头发生了一些意外的疲劳破坏。在许多情况下，无法检测到故障原因。 NC State University的一项研究表明，焊接工艺可能是当前设计方法中未适当考虑的因素之一。焊接程序可以两种方式影响焊趾附近的应变响应：一种是通过产生残余应力，另一种是改变热影响区（HAZ）中的材料特性。本研究的主要目的是通过进行焊接管道接头的实验和数值模拟来确定这两个因素对应变响应的影响。在实验研究中，使用X射线和X射线测量了一系列残余应力数据中子衍射技术，用于焊接管道接头。测量结果表明，不锈钢管道接头的初始最大压缩残余应力高于母材的屈服应力。而且，不锈钢管道接头的轴向残余应力在5个循环后大部分被释放。通过在管状样品上进行实验研究了由于高温暴露引起的机械材料性能的变化。结果表明，某些机械材料性能在经受高温循环后发生了变化，并且变化后的材料与峰值温度相关。然后，开发了改进的热机械材料异质性模型，以改善焊趾处的初始残余应力模拟。结合Chaboche模型的异质材料特性用于后续疲劳响应仿真。在应力集中位置应用了四分之一点元素。分析结果表明，可以很好地模拟疲劳响应和残余应力松弛。该研究的最后两个模拟结果表明，初始残余应力的存在会影响应变幅度和平均应变，这两者都可能影响焊接接头的疲劳寿命。
展开▼
