In order to sense the deformation of the element under test, the sensing cable must be glued or fixed to the element and it is strictly needed a tight mechanical coupling between the external sheath of the cable and the sensing fibre that is housed inside the same.
Such tight coupling represents a key difference from conventional telecom cables in which the design is aimed to protect the fibres by mechanically insulating them from the sheath, and for this reason strain sensing applications always require specific products.
Moreover, due to such tight coupling, strain sensing cables may require specific handling cares to avid fibre damage during manufacturing, stocking and installation.
The axial stiffness of the sensing cable may perturb the strain measurement by locally increasing the apparent stiffness of the substrate. This is a common issue of strain sensing i.e. also for electrical resistive strain gauges and it is addressed by making sure that the equivalent stiffness of the strain sensor and glue layer is negligible with respect to the stiffness of the member under test.
Strain sensing requires transferring the deformation of the element under test (substrate) to the sensing element, and this is normally obtained by an interface layer capable of providing mechanical coupling by chemical adhesion and/or interlocking. The stiffness and thickness of the interface controls the strain transfer and is a key element for the accuracy performance of strain sensing.
Creep, that is also known as “cold flow” in elastomer technology, is a plastic permanent deformation that slowly increases in time even at very low strain levels. Creep is typical of rubbers and very soft polymers that are used for strain transfer in some sensing cables.
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