Elastic guided waves in helical multi-wire armors

Area of Science:

• Mechanical Engineering
• Materials Science
• Applied Physics

Background:

• Cylindrical structures like cables and pipes are protected by armors composed of helical wires and polymeric sheaths.
• Understanding elastic guided wave propagation in these complex armors is crucial for structural health monitoring (SHM) and non-destructive evaluation (NDE).
• Existing methods may not efficiently capture the intricate contact mechanics and symmetries present in armor structures.

Purpose of the Study:

• To propose and validate a numerical method for investigating elastic guided wave propagation in armors.
• To assess the feasibility of using these waves for NDE and SHM of armored cylindrical structures.
• To analyze the influence of interwire and sheath contact on wave propagation characteristics.

Main Methods:

• A semi-analytical finite element method (FEM) is employed, utilizing twisting coordinates to exploit the screw symmetry along the structure's axis.
• Rotational Bloch conditions are applied to the cross-section to handle the high-order discrete circular symmetry.
• The 3D problem is reduced to a 2D unit cell model for efficient computation of contact problems and dispersion curves.

Main Results:

• Numerical results for a power cable armor show longitudinal waves predominantly propagate within the wires.
• Above a model-identified limit frequency, modal velocity of longitudinal waves approaches that of a single free wire.
• Modal attenuation is consistently higher in the armor compared to a free wire due to contact with viscoelastic sheaths.

Conclusions:

• The developed numerical method accurately models elastic guided wave propagation in armors.
• Two specific wave modes show potential for effective NDE of armors.
• Mechanical contacts, including interwire friction and sheath interaction, significantly influence wave propagation and attenuation.