Mechanical Strength

In order to ensure the long-term durability of a sensing installation it must be granted that the sensing fibres are not damaged by the mechanical stress that is applied during all the stages of handling, deployment and service life.

The issue is secondary for temperature sensing cables, where the cable structure is designed to avoid any stress transfer to the fibre, but it is particularly relevant for strain sensing applications where the optical fibres are intentionally deformed in order to sense the substrate strain.

For strain sensing applications, in order to ensure the durability, the maximum strain level that can be measured in practice (maximum measurable strain emeas,max) must be set reasonably below the failure elongation of the optical fibre that is used as Brillouin sensing element.

The maximum measurable strain should not to be confused with the maximum elongation that can be tolerated by a strain sensing cable until the embedded optical fibre fails (cable failure elongation efc): in fact most cables include stress transfer sheath(s) characterized by a relatively low yield point that, above a certain stress limit exhibit a plastic behaviour, limiting the maximum strain that is transferred to the optical fibre and preventing the fibre failure up to a larger cable deformation.

For further information you may refer to: 

Optical fiber cables for Brillouin distributed sensing applications