Connection Methods for Carbon Fiber Composite Pipe Fittings in Stratospheric Airship Rigid Skeleton Structures

Literature Overview

This paper, published in Composites Science and Engineering (Vol. 2020, No. 6, pp. 89-94) by Xiao Chang et al. from Shanghai Jiao Tong University, addresses a specialized aerospace engineering challenge: the connection of carbon fiber reinforced polymer (CFRP) pipe fittings used in the rigid skeleton structure of stratospheric airships. The research was driven by specific engineering requirements of a stratospheric airship project, making it a practical, application-oriented study.

The authors investigate three existing simple connection methods—flange joint connection, slot joint connection, and mortise-and-tenon joint connection—through axial tensile testing and applicability analysis. Based on these findings, they propose a novel threaded sleeve connection method and validate it through both experimental testing and finite element simulation.

Connection Method Comparison

The three existing connection methods were evaluated based on tensile strength, assembly convenience, and structural suitability for stratospheric airship applications.

The proposed threaded sleeve connection method addresses the key limitations of existing approaches. In a stratospheric airship rigid skeleton, the pipe fittings must be lightweight, strong, and easily assembled and disassembled for maintenance. The threaded sleeve method achieves all three objectives: the sleeve provides a high-strength mechanical connection, the threaded interface allows for easy assembly and disassembly, and the method is compatible with the slender, lightweight pipe geometry typical of airship skeleton structures.

Material and Manufacturing Considerations

Carbon fiber reinforced polymers present unique challenges for connection design. Unlike metallic pipe fittings, CFRP is an anisotropic material with direction-dependent mechanical properties. The tensile strength of CFRP along the fiber direction can exceed 1000 MPa, but transverse tensile strength is typically only 30-50 MPa, and interlaminar shear strength is even lower. This anisotropy has profound implications for connection design:

The stratospheric environment presents extreme conditions: temperatures ranging from -60°C to +50°C, very low atmospheric pressure, and intense UV radiation. The connection method must maintain structural integrity under these conditions. CFRP has excellent thermal stability, but the connection interface—where two CFRP members meet—is typically the weakest point in the structure.

The threaded sleeve connection addresses this challenge by distributing the load over a large contact area and avoiding stress concentrations. The threaded interface converts tensile loading into a combination of bearing stress and shear stress at the thread flanks, which are more favorable loading modes for CFRP than pure tension or compression.

Finite Element Validation

The authors validated the proposed connection method through finite element analysis, which provides detailed stress distribution information that is difficult to obtain experimentally. The FEA model accounts for the anisotropic material properties of CFRP, the contact mechanics at the threaded interface, and the geometric nonlinearities associated with large deformations under high loads.

The key FEA findings include:

• Stress distribution at the threaded interface is relatively uniform, with no severe stress concentrations
• The maximum stress in the CFRP material remains below the failure threshold under design loads
• The connection achieves the required tensile capacity with an adequate safety factor
• Assembly and disassembly forces are within practical limits for field operations

The combination of experimental validation and FEA provides a robust technical basis for the proposed connection method. This dual-validation approach is consistent with aerospace engineering practice, where analytical models are always confirmed through physical testing before being accepted for design use.

Study Insights and Implications

This paper demonstrates a systematic approach to engineering problem-solving that is highly relevant to pipe fitting design more broadly. The authors did not simply propose a new connection method; they first surveyed existing methods, identified their limitations through testing, and then developed a solution that addresses the specific requirements of the application. This methodology—survey, test, analyze, propose, validate—is a template that should be followed in any fitting connection design project.

The threaded sleeve connection method has potential applications beyond stratospheric airships. In any application where CFRP pipe structures require modular assembly and disassembly—such as wind turbine blades, aerospace structural components, or high-performance sporting equipment—the threaded sleeve method could be adapted. The key design parameters would be the sleeve length (which determines load-bearing capacity), thread pitch (which affects assembly ease and load distribution), and the CFRP layup schedule (which must be optimized for the specific loading conditions at the connection).

One area for future development is the integration of the threaded sleeve connection with traditional metallic pipe fitting standards. In many engineering applications, CFRP structures must interface with metallic components, and the connection method must accommodate thermal expansion mismatches and galvanic corrosion concerns. The threaded sleeve method, being a purely mechanical connection without adhesive or welding, is inherently compatible with dissimilar material joints, which is a significant advantage.