The influence of modelling variables in FEM evaluation of fillet welded structures

Sammanfattning: Valmet is a world leading company in the pulp-, paper- and energy industry and is a manufacturer of e.g. paper machines. These paper machines are largely composed of steel frames for which beams and plates are often joined by welding. In order to evaluate the structural integrity of welded components both static and cyclic evaluations of these components are performed. At Valmet the stress in the weld can be determined in different ways, usually an Excel program is used which determines the stresses in the weld by analytical means. For a more complicated geometry a FEA approach is more suitable. The FEA modelling can be done in several ways, the geometry of the structure can be divided into several or one body. Dividing the geometry into separate bodies simplifies the meshing procedure, but the separate bodies have to be joined using contacts. Different boundary conditions can also be considered during modelling e.g. placing a fixed support at the plate face or at the face of the screw holes in the plate. For structures affected by static loads the stress is determined in the smallest nominal area of the weld i.e. the weld throat. For fillet welds, crack propagation mainly occurs at the weld toe or the weld root. The geometry of the weld imposes problems for determining the stress due to the stress concentration present, but methods have been developed to solve this problem. The hot spot method and the effective notch method are two of these methods. By the use of fatigue classes and calculated stresses the fatigue life of the component can be determined. Two different models have been developed in order to perform static and fatigue analysis for which stress and fatigue life has been determined for different modelling variables and evaluation methods. The results from the static FEA show lower stresses compared to the analytical values. Different types of supports did not show a significant influence, but a slight increase in the stress near fixed screw holes was observed. Modelling using separate bodies did not show a large effect on the results except when using the MPC contact formulation for which stresses were slightly larger compared to the reference model. Evaluating using the effective notch method yielded longer fatigue life compared to the hot spot method. When the weld was not explicitly modelled significantly longer fatigue lives were observed, this is not recommended since this most likely overestimates the fatigue life. Similarly in this case the MPC formulation generally showed a some what larger stress and therefor the shortest fatigue life. It was however observed that in this test the compression only support did yield a significantly larger fatigue life.