Load-Bearing CFRP Wrapped Lined Steel Pipe Frame Lightweight Design Study Note

Literature Overview

This paper by Zhou Qi, Ma Qihua, and Zhou Tianjun from Shanghai University of Engineering Science and Donghua University investigates the application of carbon fiber reinforced polymer (CFRP) wrapped lined steel pipes in load-bearing automotive frames. Published in Modern Manufacturing Engineering (2020, Vol. 4, pp. 57-63), the study addresses a critical challenge in automotive engineering: achieving significant weight reduction while maintaining structural integrity for complex frame geometries that demand high connection reliability. The research is funded by the State Key Laboratory of Fiber Material Modification Open Topic Project (KF1826) and the Central University Basic Research Business Fee Special Fund (2232018A3-02).

Core Technical Approach

The authors propose using CFRP layer-wrapped steel pipes as a hybrid structural element for load-bearing frames, representing an innovative partial material substitution strategy. Rather than replacing the entire steel structure with composite materials—a challenge given the complex connection requirements of frame assemblies—they retain the steel pipe as the primary load-bearing element and add CFRP wrapping layers to optimize the composite action. The optimization methodology employs HyperWorks software's composite optimization module through three sequential steps:

1. Free size optimization — determining the optimal laminate thickness and fiber volume fraction
2. Thickness parameter optimization — refining ply thickness distribution
3. Ply sequence optimization — determining the stacking sequence for maximum performance

After optimization, the results are adjusted to satisfy manufacturability constraints of the CFRP wrapping process.

Key Performance Results

Technical Interpretation and Engineering Practice

Steel Pipe Selection Considerations

From a steel pipe manufacturing perspective, the choice of base steel pipe is critical for this hybrid approach. The steel pipe must serve as both a structural member and a substrate for CFRP adhesion. Key considerations include:

• Surface preparation: The steel pipe outer surface must be prepared to ensure adequate bond strength between the steel and CFRP layers. Surface roughness, cleanliness, and potential primer application are essential for achieving the design laminate performance.
• Pipe geometry: The CFRP wrapping process imposes constraints on pipe diameter, wall thickness, and dimensional tolerances. Seamless pipes or high-quality ERW pipes with tight dimensional tolerances are preferred.
• Residual stress interaction: The welding or forming processes used to create the steel pipe frame may introduce residual stresses that interact with the CFRP laminate stresses, potentially affecting the overall structural response.

CFRP Wrapping Process Challenges

The wrapping process itself presents several technical challenges relevant to pipe manufacturing:

• Tension control: Uniform wrap tension is critical for achieving consistent fiber volume fraction and avoiding voids or wrinkles in the laminate.
• Cure cycle management: The thermoset resin system used for CFRP consolidation requires controlled temperature and pressure profiles, which must be compatible with the steel pipe's thermal expansion characteristics.
• Defect detection: Post-cure inspection of the CFRP layer using ultrasonic testing (UT) or thermography is essential to detect delamination, voids, or dry spots that would compromise the hybrid structure's performance.

Manufacturability Adjustment

The paper acknowledges that the mathematically optimal laminate configuration may not be directly manufacturable. This is a common issue in composite engineering where the optimal ply sequence may require impractical fiber orientations or ply thicknesses. The adjustment process involves:

• Rounding ply thicknesses to commercially available prepreg thicknesses (typically 0.125 mm, 0.25 mm, 0.375 mm, or 0.5 mm)
• Ensuring that fiber orientations align with the wrapping equipment's capabilities (typically 0°, ±45°, and 90° relative to the pipe axis)
• Verifying that the adjusted configuration still meets the minimum safety factors for all design load cases

Reflections and Implications

This research represents a meaningful step toward hybrid metal-composite structures in automotive applications. The 23% weight reduction achieved while maintaining full structural performance under extreme loading conditions is significant for fuel efficiency and emissions reduction. However, several questions remain for practical implementation:

• Crashworthiness: The paper focuses on stiffness and strength under quasi-static conditions but does not extensively address crash energy absorption characteristics, which are critical for automotive safety.
• Joint design: The connection of CFRP-wrapped steel pipes at frame joints requires careful design to prevent premature debonding under cyclic loading.
• Cost analysis: The economic viability depends on CFRP material costs, processing equipment investment, and production volume, which are not addressed in the paper.

The approach has broader implications for the steel pipe industry, suggesting that steel pipes could serve as core elements in hybrid structures where the steel provides formability and connection capability while CFRP provides additional stiffness and strength without proportional weight increase. This concept could extend to structural applications beyond automotive frames, including aerospace, marine, and construction sectors where weight reduction is a primary design driver.