Residual stress from welding is important for material performance, as it combines with applied loading and can lead to premature failure of the welded component. Measuring residual stress is useful as it provides critical data that can be used for structural analysis.
In the context of welded pipes, measuring residual stress typically requires sectioning the pipe to obtain a reasonable pipe length for transport to a laboratory and for the execution of the measurements. However, sectioning the pipe to reduce the length may affect the residual stress on the weld, as sectioning creates a new free surface and results in redistribution of the residual stress within the part.
This case study documents results from a numerical experiment performed to assess the effects of trimming the length of a girth welded pipe on the residual stress within the welded region. The results provide insight into the amount of redistribution of residual stress for several different lengths, which is useful for measurement planning.
The study considers a three-dimensional (3D) finite element (FE) model of a steel pipe (Young’s Modulus E = 28,000 ksi, Poisson’s ratio ν = 0.33) with a circumferential (girth) weld. The pipe outer diameter is D = 36 inch, with wall thickness T = 1 inch. The initial pipe length is assumed to be 1.5D, or 54 inch. The weld is 1 inch wide and is located at the mid-length of the pipe. Figure 1 provides an illustration of the overall pipe geometry.
A quarter-symmetric model was used, with symmetry boundary conditions. Residual stress was introduced in the weld region via a constant and isotropic thermal strain. Note, since the intent of this study is to evaluate the change in residual stress due to reducing the length of the pipe, the exact residual stress distribution is less important – the residual stress used here is a simplified representation. Hoop and axial residual stress components are shown in Figure 2. The color scale ranges from -40 to 80 ksi. The top shows the hoop residual stress component on the entire quarter-symmetry model, while the bottom images show the axial and hoop residual stress components zoomed in to focus on the distribution through the thickness. The discontinuity observed in the hoop stress is expected and occurs at the transition of the thermal strain input area (thermal strain is non-zero within the weld region, and zero outside the weld region).
To assess changes in residual stress in the weld, subsequent steps were performed to simulate trimming the length of the pipe. Material is removed symmetrically, maintaining the weld at the mid-length of the pipe. Figure 3 shows the initial pipe configuration (54 inch long) and each subsequent pipe length analyzed. The parts outlined in red illustrate the material removal areas, and the remaining pipe length is indicated for each configuration. Six different pipe lengths were considered, with shortest length of approximately 1.83 inch, or 0.05D (in this case, the majority of the pipe length consists of the weld itself (1.00 inch), with little remaining material on each side of the weld).
Figure 4 shows the hoop residual stress (left) and axial residual stress (right) as a function of depth through the thickness from the outer diameter of the pipe, as illustrated by the red arrow. The results were taken at the center of the weld. The result for the initial pipe length (54 inch) is shown using a black line with circle symbols. The hoop residual stress is tensile through the thickness of the pipe. Reducing the pipe length to 27.5 inch does not result in any practical changes in hoop and axial residual stress. Further reduction to 14.25 and 7.63 inch shows a small change in hoop stress, but the axial stress is nearly unchanged. A pipe length of 4.31 inch (0.12D) is when larger differences appear, with a noticeable reduction in magnitude of tensile hoop residual stress. The axial residual stress is also visibly different at the pipe outer diameter (x = 0 inch) and inner diameter (x = 1 inch). Sectioning the pipe further to 2.66 and 1.83 inch causes large changes in residual stress (particularly the hoop residual stress component).
To quantify the change in residual stress, the % stress loss relative to the initial pipe length (54 inch) is shown in Figure 5. The loss is shown at two selected points: at the outer diameter (OD), and at a point near the mid-thickness of the pipe. The loss at the OD is shown with a black line (and black dot on pipe sketch), while the loss near the mid-thickness is shown with a blue line (and blue dot on pipe sketch). The hoop stress (left) at both the OD and mid-thickness exhibits small changes (under 2.5%) up to the 7.63 inch pipe length. At 4.31 inch pipe length, the loss at the OD is near 10% in hoop stress and near 18% in axial stress. Reducing the pipe length further increases the % stress loss to nearly 48% (in hoop stress) for the extreme condition (1.83 inch pipe length). The results are similar for the axial stress, with noticeable changes for pipe lengths under the 7.63” condition analyzed.
The results for this specific pipe geometry and residual stress configuration suggest that when the shortest pipe length possible is desired when sectioning, an approximately 0.21D pipe length is a reasonable choice to consider while producing minimal changes in the weld residual stress field.