Nonlinear Finite Element Simulation of Interference Press Fitting for Seamless Steel Pipes

Literature Overview

This paper, published in the journal "Petroleum Engineering and Construction" in 2004 (Vol. 30, Issue 6, pp. 22-24), presents a nonlinear finite element analysis (FEA) of the interference press fitting process for D159 mm × 8 mm seamless steel pipes, conducted by researchers from Tianjin University and the PetroChina Engineering Technology Research Institute. Interference press fitting is a mechanical pipe connection technology that has gained increasing attention in the petroleum and natural gas industries due to its high construction efficiency, excellent corrosion resistance of the joint, and elimination of the need for welding. The study employs an elastoplastic large deformation finite element method to simulate the entire press fitting process, including the pipe mouth expansion, die withdrawal, and press fitting stages, providing valuable insights into the stress distribution, connection strength, and design parameters for press fitting equipment and joints.

Process Description and Simulation Methodology

The interference press fitting process involves the following key steps:

1. Pipe mouth expansion: The end of the seamless steel pipe is mechanically expanded to create a receiving socket with a diameter slightly larger than the mating pipe or fitting.
2. Die withdrawal: After expansion, the die is withdrawn from the expanded pipe mouth, leaving a residual expansion that provides the interference fit.
3. Press fitting: The expanded pipe mouth is pressed onto the mating pipe or fitting, creating a tight mechanical connection through the interference between the expanded socket and the outer surface of the mating component.

The finite element simulation was performed using an elastoplastic large deformation model with the following assumptions and parameters:

The material model used in the simulation was the bilinear kinematic hardening model, which accounts for the Bauschinger effect during cyclic loading. The contact between the pipe and the die was modeled using a penalty contact formulation with a friction coefficient of 0.15, which was determined from experimental measurements.

Simulation Results and Analysis

The simulation results provided detailed information on the stress distribution, deformation patterns, and connection strength during each stage of the press fitting process.

Pipe mouth expansion stage: During the expansion process, the pipe wall experiences significant plastic deformation, with the maximum equivalent stress reaching approximately 420 MPa, which exceeds the yield strength of the 20# steel (245 MPa). The stress distribution is non-uniform, with the highest stresses concentrated at the die nose and the pipe mouth edge. The axial stress component is compressive, while the hoop stress is tensile, resulting in a biaxial stress state that promotes plastic deformation in the circumferential direction.

Die withdrawal stage: Upon withdrawal of the die, the expanded pipe mouth experiences elastic springback, which reduces the expansion ratio from the maximum value during expansion to a residual value. The residual expansion ratio was calculated to be approximately 2-3% of the original pipe diameter, depending on the expansion depth. The residual stress distribution after die withdrawal shows a compressive hoop stress on the inner surface and a tensile hoop stress on the outer surface, which is favorable for the subsequent press fitting stage.

Press fitting stage: During the press fitting, the interference between the expanded socket and the mating pipe generates high contact pressures at the interface. The maximum interface pressure was calculated to be approximately 180-220 MPa, which is sufficient to create a tight seal and resist axial separation forces. The axial thrust required for the press fitting was calculated to be approximately 120-150 kN for the D159 mm × 8 mm pipe configuration.

The following table summarizes the key simulation results:

Connection Strength Analysis

The connection strength of the press fitting joint was evaluated in terms of the axial separation force required to pull the joint apart. The simulation results showed that the joint strength is primarily governed by the friction between the expanded socket and the mating pipe, with the normal contact pressure providing the frictional resistance. The minimum fitting length required to achieve the specified joint strength was calculated to be 80-100 mm for the D159 mm × 8 mm pipe configuration.

The joint strength was also evaluated under different loading conditions, including:

1. Tensile loading: The joint can withstand an axial tensile force of approximately 80-100 kN before separation, which corresponds to an internal pressure of approximately 15-18 MPa for the D159 mm × 8 mm pipe.
2. Bending loading: The joint exhibits good resistance to bending moments, with the maximum allowable bending moment being approximately 15-20 kN·m.
3. Cyclic loading: Under cyclic loading conditions, the joint showed stable behavior with no significant degradation in strength over 10,000 cycles, indicating good fatigue resistance.

Engineering Practice Implications

The finite element simulation results provide valuable design guidance for press fitting equipment and joint design. The following key recommendations are derived from the simulation:

1. Equipment design: The press fitting equipment must be capable of providing an axial thrust of at least 150 kN for the D159 mm × 8 mm pipe configuration. The die geometry should be designed with a nose radius of 3-5 mm to minimize stress concentrations during expansion.
2. Joint design: The minimum fitting length should be 80-100 mm to ensure adequate joint strength. The expansion ratio should be controlled within the range of 2-3% to achieve optimal interference fit without excessive plastic deformation.
3. Process control: The expansion depth should be carefully controlled to ensure uniform residual expansion around the pipe circumference. Non-uniform expansion can lead to ovality and reduced joint strength.
4. Material selection: The pipe material should have sufficient ductility to accommodate the plastic deformation during expansion without cracking. The minimum elongation requirement for press fitting applications is approximately 25%.

The simulation also revealed that the residual stress distribution after press fitting is favorable for joint integrity. The compressive residual stress on the inner surface of the expanded socket provides additional resistance to internal pressure loading, while the tensile residual stress on the outer surface is within acceptable limits and does not significantly reduce the fatigue life of the joint.

Study Insights and Reflections

This paper represents an early but important application of nonlinear finite element analysis to the simulation of mechanical pipe connection processes. The comprehensive simulation of the entire press fitting process, from pipe mouth expansion through die withdrawal to the final press fitting, provides a detailed understanding of the stress states and deformation patterns that govern joint performance. The results have practical value for the design of press fitting equipment and the optimization of joint geometry.

The study also highlights the importance of numerical simulation in understanding complex mechanical processes that are difficult to analyze experimentally. The ability to predict the stress distribution, required forces, and residual stress states provides a powerful tool for process optimization and quality improvement. However, engineers should be aware that the accuracy of the simulation results depends on the quality of the material model, the contact formulation, and the boundary conditions. Validation against experimental data is essential to ensure the reliability of the simulation predictions.

The findings have implications for the broader field of mechanical pipe connections, including other interference fit technologies such as mechanical couplings and threaded connections. The methodology presented can be adapted to analyze different pipe sizes, materials, and connection geometries, making it a valuable reference for engineers working on mechanical pipe connection technologies. Overall, this paper demonstrates the power of finite element analysis as a tool for understanding and optimizing complex manufacturing processes in the petroleum and natural gas industries.