Numerical Simulation of Mechanical Composite by Roll-Die Drawing of Inner-Outer Composite Tee Pipe

Literature Overview

This paper by Ma Linling and Lei Junxiang from the School of Materials Science and Engineering at Shanghai University of Science and Technology, published in "Nonferrous Metals Materials and Engineering" in 2019, Volume 40, Issue 3, presents a finite element analysis of the mechanical composite process for inner-outer composite tee pipes using roll-die drawing technology. The authors established a finite element model using ABAQUS software to simulate the drawing process of a circular outer tube and a Y-shaped inner core, analyzing the residual contact pressure between the outer tube and inner core to estimate the pull-out resistance. The paper highlights the advantages of roll-die drawing over traditional drawing composite technology, including improved forming quality and reduced processing energy consumption.

Core Technical Analysis

The mechanical composite of tee pipes is a manufacturing challenge that combines the geometric complexity of tee-shaped cross-sections with the requirements for a strong, reliable bond between dissimilar materials. Traditional methods for creating composite pipes include explosion welding, friction welding, and mechanical interference fitting. The roll-die drawing approach described in this paper represents an alternative that leverages plastic deformation to create a mechanically interlocked joint between the outer tube and inner core.

The fundamental principle of the process involves simultaneously drawing both the outer tube and the Y-shaped inner core through a matched die assembly. As the two components are drawn through the die, the outer tube is compressed radially inward while the inner core is expanded radially outward, creating a residual contact pressure at the interface. This residual pressure, maintained after the components exit the die, provides the mechanical interlock that resists pull-out forces.

Finite Element Model Configuration

The finite element model established in this study incorporates several key modeling decisions that directly affect the accuracy of the simulation results:

The Y-shaped inner core geometry presents a particular challenge for the roll-die drawing process. Unlike a simple straight tube, the Y-shaped core has regions of different curvature and wall thickness that deform differently during the drawing operation. The branching region of the Y-shape experiences complex multiaxial stress states that may lead to localized thinning or wrinkling. The finite element model must accurately capture these geometric complexities to provide reliable predictions of the residual contact pressure distribution.

The residual contact pressure analysis is the key output of the simulation. After the drawing process is complete and the components exit the die, the elastic recovery of both the outer tube and the inner core creates a compressive contact pressure at their interface. This pressure is not uniform across the interface; it varies with the local geometry, material properties, and deformation history. The pull-out resistance is estimated from the integral of the residual contact pressure multiplied by the coefficient of friction and the contact area.

Process Analysis and Defect Considerations

The roll-die drawing process for composite tee pipes is susceptible to several defect mechanisms that must be understood and controlled. The following table summarizes the primary defects and their countermeasures:

The connection between residual contact pressure and pull-out resistance is governed by a friction-based model. The pull-out force can be estimated as the product of the average residual contact pressure, the coefficient of friction between the two materials, and the total contact area. However, this simplified model does not account for stress concentration effects at geometric discontinuities, such as the branching region of the Y-shaped core. In practice, the actual pull-out resistance may be lower than the estimated value due to localized stress concentrations that initiate debonding at specific points.

Material Selection and Compatibility

The selection of materials for the outer tube and inner core is critical for the success of the mechanical composite process. The materials must be compatible in terms of elastic modulus, thermal expansion coefficient, and corrosion behavior. For applications in oil and gas pipelines, the outer tube may be a carbon steel such as API 5L X65 or X70, while the inner core may be a corrosion-resistant alloy such as duplex stainless steel or nickel-based alloy. The mismatch in elastic modulus between the two materials affects the residual contact pressure distribution and must be accounted for in the finite element model.

The coefficient of friction between the two materials is another critical parameter that affects the pull-out resistance. In the finite element model, this coefficient is typically assumed to be constant, but in reality, it varies with contact pressure, sliding distance, and surface finish. For the purpose of the simulation, a representative value should be selected based on experimental data for the specific material combination.

Integration with Engineering Practice

In the context of pipeline engineering, composite tee pipes find application in situations where a corrosion-resistant internal lining is required within a structural outer pipe. For example, in sour service environments where hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) are concerns, a carbon steel outer pipe may be combined with a corrosion-resistant alloy inner core to provide both structural integrity and corrosion resistance. The mechanical composite approach eliminates the need for welding between the two materials, which is advantageous when dealing with dissimilar material combinations that are prone to cracking during welding.

The roll-die drawing process is particularly well-suited for producing composite tees because the Y-shaped geometry can be formed in a single operation, avoiding the need for post-welding or post-forming operations. This reduces the number of process steps, improves dimensional accuracy, and minimizes the risk of welding-related defects. The finite element simulation provides a tool for optimizing the process parameters before physical trials, reducing the cost and time associated with process development.

The pull-out resistance prediction is directly relevant to quality control. In pipeline applications, the composite tee must withstand axial loads during installation, operation, and thermal cycling. The pull-out resistance must exceed the maximum expected axial load with an appropriate safety factor. The finite element simulation provides a means to verify this requirement before the component is put into service.

Key Questions and Reflections

One important question that arises from studying this paper is the scalability of the process. The finite element model presented is for a specific geometry and material combination, but the principles should be applicable to other sizes and geometries. However, the accuracy of the simulation decreases as the geometry becomes more complex or as the drawing reduction ratio increases. For very large diameter composite tees, the process forces may exceed the capacity of available drawing equipment, necessitating alternative approaches such as hot forming or welding.

The paper focuses on the mechanical composite aspect of the process but does not address the subsequent quality assurance steps required for pipeline components. In addition to pull-out testing, composite tees used in pipeline applications must undergo non-destructive testing (NDT) to verify the integrity of the composite interface. Techniques such as ultrasonic testing (UT), specifically phased array ultrasonic testing (PAUT), can detect debonding at the interface. Radiographic testing (RT) may also be employed to verify the continuity of the composite bond. The finite element simulation can predict the residual stress distribution, which can be correlated with NDT results to establish acceptance criteria.

Another consideration is the effect of the composite process on the material properties of both components. The cold working introduced during the drawing process increases the hardness and strength of the materials but reduces their ductility. For applications requiring post-weld heat treatment or service at elevated temperatures, the cold-worked condition may be unacceptable. The finite element model should include a material model that accounts for strain hardening and the associated reduction in ductility.

Study Insights and Implications

The research demonstrates the value of finite element simulation in the development and optimization of complex forming processes for composite pipe components. By predicting the residual contact pressure distribution and estimating the pull-out resistance, the simulation provides a quantitative basis for process parameter selection and quality assessment. This approach can reduce the number of physical trials required for process development, saving significant time and material costs.

The methodology is directly transferable to other composite forming applications, including composite elbows, reducers, and straight pipes. The same finite element modeling approach can be applied to different geometries and material combinations, providing a systematic framework for composite component design and manufacturing. The key to successful application lies in accurate material property characterization, appropriate contact modeling, and validation against experimental data.

For the pipeline industry, the mechanical composite approach offers a promising alternative to welded composite pipes, particularly for applications where welding of dissimilar materials is problematic. The ability to produce complex geometries such as tees in a single forming operation, combined with the elimination of welding-related defects, makes this technology attractive for critical pipeline components. Future research should focus on process scaling, quality assurance methodology, and long-term performance validation under realistic service conditions.