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

Literature Overview

The paper by Wang Yanqun, Li Chunrun, and Zhang Tianli, published in Petroleum Engineering Construction in 2004 (Vol. 30, No. 6, pp. 22-24), presents a nonlinear finite element analysis of the interference press-fitting process for D159 mm × 8 mm seamless steel pipes. The authors are from Tianjin University School of Mechanical Engineering and CNPC Engineering Technology Research Institute. Interference press-fitting is a pipe connection technology that has gained increasing adoption in oil and gas pipeline construction due to its advantages in construction efficiency and joint corrosion resistance.

Technical Background

Interference press-fitting (also known as press-fit connection) is a mechanical pipe joining method where a connector is forcibly expanded or compressed onto the pipe end to create a permanent, leak-tight joint. Compared to traditional welding, this method offers:

• Elimination of hot work hazards in hazardous environments
• Faster connection speed in the field
• Reduced need for welder qualification and welding procedure qualification
• No heat-affected zone, preserving pipe material properties
• Good corrosion resistance when combined with external coating

The D159 mm × 8 mm specification represents a common medium-diameter pipeline size used in gathering systems and distribution networks.

Finite Element Model Development

The authors developed a three-dimensional axisymmetric finite element model using elastic-plastic large deformation theory. The model accounts for:

Material Model

• Elastic-plastic constitutive behavior with strain hardening
• Von Mises yield criterion
• Isotropic hardening law based on experimental stress-strain data
• Material properties of the specific seamless steel pipe grade

Contact Model

• Frictional contact between pipe and connector
• Coulomb friction model with appropriate friction coefficient
• Large sliding contact algorithm for handling significant relative displacement

Boundary Conditions

• Symmetry conditions applied to reduce model size
• Appropriate constraints to simulate the actual press-fitting fixture

Simulation Process Stages

The press-fitting process was simulated in three sequential stages:

Stage 1: Pipe End Expansion (Flaring)

The pipe end is pre-expanded to facilitate insertion of the connector. The simulation reveals:

• Axial compressive stresses develop in the pipe wall during flaring
• Hoop tensile stresses increase at the flare region
• Plastic deformation is concentrated in the flared zone
• The required axial pushing force is calculated for equipment design

Stage 2: Connector Withdrawal (Demolding)

After the connector is formed around the pipe end, the die or forming tool is withdrawn:

• Elastic springback occurs in the connector and pipe
• Residual stresses redistribute
• Contact pressure between pipe and connector is established
• The magnitude of residual interference fit determines joint integrity

Stage 3: Press-Fitting (Final Assembly)

The final interference fit is achieved through controlled compression:

• Radial compressive stresses in the pipe wall
• Radial tensile stresses in the connector
• Contact pressure distribution along the interface
• Overall joint stiffness and load-bearing capacity

Key Results

Stress Analysis

Required Axial Forces

• Flaring force: Calculated value provides equipment design basis
• Press-fit force: Determined for hydraulic press capacity selection
• The forces are significantly higher than elastic estimates due to plastic deformation

Residual Stress Distribution

The residual stress state after press-fitting is critical for joint performance:

• Compressive residual hoop stresses in the pipe wall enhance resistance to internal pressure
• The interference fit creates a self-locking mechanism that resists pull-out
• Stress concentrations at the transition zones must be evaluated for fatigue life

Required Fit Length

The simulation determined the minimum engagement length required to ensure adequate joint strength under design loads. This parameter is critical for connector design and manufacturing.

Engineering Application Value

The finite element simulation provides quantitative data for:

1. Equipment design: Press-fitting machine capacity, die geometry, and stroke requirements
2. Connector design: Optimal geometry, wall thickness, and material selection for the connector
3. Quality assurance: Acceptance criteria for residual interference and contact pressure
4. Joint performance prediction: Pull-out resistance, pressure containment capability, and fatigue life estimation

Comparison with Conventional Methods

Study Insights

This paper represents an early but significant contribution to the numerical simulation of mechanical pipe joining technologies. The nonlinear finite element approach, accounting for large deformations, material plasticity, and frictional contact, provides engineering insights that are difficult to obtain through physical testing alone. The simulation results enable rational design of press-fitting equipment and connectors rather than relying on trial-and-error approaches.

For engineers involved in pipeline construction technology selection, this work demonstrates that press-fitting can be a viable alternative to welding for specific applications, particularly where hot work restrictions apply, construction speed is critical, or continuous corrosion protection is paramount. The quantitative stress analysis provides the foundation for establishing acceptance criteria and quality assurance protocols for press-fitted joints.

The methodology presented—using nonlinear FEA to analyze a complete forming process with multiple stages—is broadly applicable to other mechanical forming operations in the pipe and tube industry. The key insight is that numerical simulation, when validated against experimental data, can significantly reduce development time and cost while providing detailed internal stress information that is inaccessible through measurement alone. Engineers should consider similar simulation approaches for any novel joining or forming process where internal stress states are critical to performance.