Thin-walled pipes need to consider structural strength and stiffness issues. Strength is mainly dominated by membrane stress, bending stress, axial tensile/compressive stress, and hoop compressive stress. It is necessary to verify whether the pipe stress is within the allowable stress range of the material. The radial flattening, axial deflection, and indentation deformation at pipe support regions dominate the stiffness issue of the structure. It is necessary to ensure that key data such as radial deformation and deflection are below the allowable limits specified by codes.
For thin-walled pipes, in addition to strength and stiffness issues, buckling stability must also be considered. This is especially important for pipes under external pressure conditions, such as submarine pipelines, buried pipes, and vacuum pipes, which may be at risk of circumferential local buckling and radial instability collapse.
02
Case Study
A nonlinear buckling instability calculation is performed on a thin-walled pipe using the Static Structural module in the ANSYS Workbench platform.
The nonlinear calculation requires the use of nonlinear structural steel material from the material library.
Enter Model, the thin-walled pipe geometric model is shown below.
Complete the meshing.
The pipe support region is simplified as fixed and axially movable boundary conditions.
A pressure value of 0.2MPa is applied on the outer surface of the pipe. This pressure will cause buckling instability of the pipe at a certain point in time.
Then control the relevant parameters of the analysis.
- Auto Time Stepping=On, enable automatic substeps, automatically refine substeps based on iterative convergence, smoothly pass through the instability point of sudden stiffness change.
- Define By=Substeps, control increments by number of substeps.
- Initial Substeps=100, initial 100 substeps, initial load increment is only 1/100 of the total load, smooth loading, will not directly skip the buckling critical point.
- Minimum Substeps=50, minimum 50 substeps, minimum single load increment is 1/50 of the total load.
- Maximum Substeps=1.e+006, maximum 1 million substeps.
- Solver Type=Direct, use the direct solver, the stiffness matrix is nearly singular when local pipe instability occurs.
- Large Deflection=On, enable large deflection, thin-walled pipe instability deformation belongs to geometric large deformation, element coordinates and stiffness matrix are updated at each substep.
In the nonlinear controls, enable Stabilization
- Stabilization=Constant, constant stabilization damping mode, suitable for nonlinear buckling calculation of thin-walled pipes under external pressure.
- Method=Energy, stabilization energy method, adds adaptive artificial damping to elements, suppresses numerical oscillation at the moment of buckling through dissipating stabilization energy.
- Energy Dissipation Ratio=1.e-004, energy dissipation ratio, the maximum ratio of stabilization dissipation energy to structural strain internal energy.
Solve:
Post-processing: view the stabilization energy curve:
From 0 to 0.54721s, the stabilization energy value approaches infinitely close to 0. The pipe is in the linear elastic stage, with small deformation and stable stiffness. The structure has no instability trend and does not require additional artificial dissipation to stabilize the iteration. 0.54721s is the mutation point, where the dissipation energy instantly jumps to a maximum value, which is an instability signal. When the external pressure load reaches 54.72% of the total load, the thin-walled pipe loses local stiffness and buckling indentation occurs.
The pipe experiences local indentation instability at 54.72% of the design external pressure of 0.2MPa, indicating insufficient structural buckling safety margin.
Thank you to every reader for your
likes, shares, and recommendations
!
Your support is the driving force behind our continuous creation!
No comments yet