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Fiber optic cable relaxation coefficient

Specialty cable knowledge brief Published 07 Oct 2024

Prepared for preliminary research by the ZITHA SPECIALTY knowledge desk. Confirm fire performance, mechanical protection, loss budgets and installation requirements against current standards and product documentation.

The relaxation coefficient quantifies the viscous or time-dependent stress relaxation behavior of fiber optic cables under mechanical load, reflecting how strain is transferred from the cable jacket to the optical fiber.Definition and Importance

The relaxation coefficient in fiber optic cables describes the rate at which stress or force in the cable decreases over time when held at a constant displacement. It is a key parameter for understanding viscoelastic behavior, which affects long-term performance, strain sensing accuracy, and mechanical reliability of fiber optic systems embedded in structures or subjected to cyclic loading . In distributed fiber optic sensing (DFOS), the relaxation coefficient is critical because the strain measured in the fiber core is influenced by the cable's coating and jacket layers. A higher relaxation coefficient indicates more smoothing of strain over time, which can reduce the apparent strain in the fiber compared to the actual structural strain .

Factors Affecting the Relaxation Coefficient
  1. Cable Construction:
    • Tight buffer cables have the fiber directly surrounded by a protective buffer, providing strong mechanical protection and faster strain transfer.
    • Loose tube cables allow the fiber to move within a gel-filled tube, which increases the relaxation effect and reduces direct strain transfer .
  2. Material Properties:
    • The viscoelastic properties of the coating layer (e.g., acrylate, polyimide) and the jacket material influence the relaxation coefficient. Softer or more compliant materials generally increase the relaxation effect .
  3. Embedding Environment:
    • When cables are embedded in concrete or other structures, the interface between the cable and the host material affects strain transfer. Displacement discontinuities or cyclic loading can amplify the relaxation effect .
  4. Loading Conditions:
    • The relaxation coefficient depends on the magnitude of applied displacement or force and the loading rate. Slow loading or holding at a constant displacement allows more stress relaxation to occur .
Measurement and Application

Stress relaxation tests are commonly used to determine the relaxation coefficient. In these tests, a fiber optic cable is loaded to a specified displacement or force and held constant while the force decay over time is recorded. The resulting data can be used to:

  • Calibrate fiber optic sensors for accurate strain measurement.
  • Select appropriate cable types for structural health monitoring.
  • Predict long-term mechanical behavior under sustained loads . Understanding the relaxation coefficient is essential for designing reliable fiber optic sensing systems and ensuring that the measured strain accurately reflects the structural behavior rather than the cable's internal viscoelastic response.

Fiber optic cable relaxation coefficient - ZITHA TECH SPECIALTY CABLES (Pty) Ltd [PDF]

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This reference is intended for preliminary specialty-cable research. Compatibility, link budgets, fire performance, installation methods, test limits and applicable standards must be verified for the specific project.

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