Finite Element Analysis of Hollow Drawn Steel Tubes Using ANSYS LS-DYNA

Literature Overview

The paper by Xue Longquan, He Yafeng, Liu Rongchang, Dai Chunying, and Chen Juntao, published in the Journal of Plasticity Engineering in 2005, presents a three-dimensional nonlinear finite element analysis of the hollow drawing process for steel tubes using the ANSYS LS-DYNA module. The research was conducted at Xi'an University of Technology and Hebei Normal University of Science and Technology. The study dynamically simulated the entire hollow drawing process to obtain field variable distributions, analyze the mechanisms of transverse and longitudinal cracking defects, and establish relationships between process parameters and drawing force.

Process Modeling and Simulation Setup

The hollow drawing process involves pulling a steel tube through a die to reduce its diameter and wall thickness while maintaining or adjusting the hollow geometry. The finite element model was developed using LS-DYNA's explicit dynamic analysis capability, which is well-suited for capturing the high-strain-rate deformation and contact interactions inherent in tube drawing processes.

The model incorporated:

• Geometric modeling: Accurate representation of the initial tube geometry, die profile, and plug configuration
• Material properties: Elastic-plastic constitutive model with appropriate strain hardening behavior for the steel grade used
• Contact conditions: Frictional contact between the tube and die, and between the tube and plug
• Boundary conditions: Displacement-controlled drawing with appropriate loading rates

The simulation was validated against known process characteristics and typical drawing force measurements from industrial practice.

Three-Stage Drawing Process Characterization

One of the most valuable contributions of this study is the identification of three distinct stages in the drawing process based on the variation pattern of drawing force:

This three-stage characterization provides a practical framework for process monitoring and control. In industrial practice, monitoring the drawing force curve in real-time can help operators identify abnormal conditions and prevent defect formation.

Defect Mechanism Analysis

Transverse Cracking

Transverse cracks (circumferential cracks) typically occur near the die exit and are associated with excessive tensile stresses in the circumferential direction. The finite element analysis revealed that these cracks initiate when the local strain exceeds the material's ductility limit, particularly in regions of high strain concentration. Factors contributing to transverse cracking include:

• Excessive die angle leading to high contact stresses
• Insufficient lubrication increasing friction and localized heating
• Material with inadequate ductility or unfavorable grain structure
• Drawing speed beyond the material's strain rate capacity

Longitudinal Cracking

Longitudinal cracks (axial cracks) develop along the tube length and are primarily associated with the interaction between the plug and the tube inner surface. The analysis showed that longitudinal cracks initiate when the compressive stress from the plug exceeds the material's capacity, combined with tensile stresses from the drawing force. Contributing factors include:

• Plug diameter mismatch leading to excessive interference
• Poor surface finish on the plug creating stress concentrations
• Material with internal defects or inclusions
• Inadequate lubrication at the plug-tube interface

Process Parameter Optimization

The study established quantitative relationships between key process parameters and drawing force:

Die Cone Angle

The die cone angle directly affects the drawing force and deformation uniformity. A smaller cone angle reduces drawing force but increases the contact length, potentially leading to excessive friction heating. A larger cone angle increases drawing force and may cause localized strain concentrations. The optimal angle balances force efficiency with defect prevention.

Friction Coefficient

Friction at both the die-tube and plug-tube interfaces significantly influences the drawing force and deformation pattern. Higher friction increases drawing force and may promote defect formation. Lubrication strategy is therefore critical for achieving consistent quality and extending tool life.

Wall Thickness

The initial and final wall thickness affect the drawing ratio and strain distribution. Thicker walls require higher drawing forces and may be more susceptible to cracking if the drawing ratio is too aggressive. The relationship between wall thickness and drawing force is nonlinear, with diminishing returns at higher thicknesses.

Engineering Practice Applications

Mold Design Optimization

The finite element analysis results provide a rational basis for die and plug design. Engineers can use the simulation to:

1. Optimize die cone angle for minimum drawing force and maximum quality
2. Design plug geometry to prevent longitudinal cracking
3. Determine appropriate clearance and contact zone dimensions
4. Evaluate alternative die materials and surface treatments

Process Design Guidelines

The three-stage characterization enables systematic process design:

• Initial stage: Use controlled drawing speed and adequate lubrication to prevent surface defects
• Flow stage: Monitor drawing force for consistency and adjust parameters as needed
• Steady stage: Maintain optimal parameters for consistent quality output

Quality Control Integration

The defect mechanism analysis supports the development of targeted quality control measures:

• Incoming material inspection: Verify ductility, grain structure, and absence of internal defects
• In-process monitoring: Track drawing force curves for early detection of abnormalities
• Post-process inspection: Apply NDT methods (UT, MT, PT) to detect residual cracks

Reflections and Insights

This research exemplifies the power of finite element analysis in understanding and optimizing metal forming processes. The ability to visualize field variable distributions provides insights that are difficult or impossible to obtain from physical experiments alone. For engineers involved in steel tube manufacturing, this type of analysis is invaluable for process development and problem-solving.

The three-stage characterization of the drawing process is particularly practical. In industrial settings, operators can use drawing force monitoring as a real-time quality indicator. If the force curve deviates from the expected pattern, it signals a potential problem that requires immediate attention, such as die wear, lubrication failure, or material variation.

The defect mechanism analysis has direct implications for quality control strategy. Rather than relying solely on post-process inspection, engineers can implement preventive measures based on the identified root causes. For example, if transverse cracking is a recurring problem, the solution may lie in adjusting the die angle or improving lubrication rather than simply rejecting defective products.

From a career perspective, this research highlights the importance of integrating numerical simulation with practical engineering knowledge. The finite element model is only as good as the assumptions and boundary conditions applied, and these require deep understanding of the physical process. Engineers who can combine simulation capability with hands-on experience are best positioned to develop robust manufacturing processes.

The study also underscores the importance of process parameter optimization. Small changes in die angle, friction coefficient, or wall thickness can have significant effects on drawing force and product quality. Systematic parametric studies, as conducted in this research, provide the data foundation for rational process design rather than trial-and-error approaches.