IEC TR 62959-2021 pdf free download – Optical fibre cables – Shrinkage effects on cable and cable element end termination – Guidance.
Because different materials are used within cables, when the temperature changes, the cable elements and the sheath expand or contract differentially. If the elements cannot move freely, forces are generated within the cable. If the fibre is stressed by such forces, then optical performance can degrade temporarily. After the temperature reverts to its original value, cable elements return to or close to their original lengths, unless they have undergone shrinkage or are restrained by internal coupling. This reversible thermal material dimension change is seldom independently addressed as a cable characteristic. Annex B describes a suitable test method for determination changes in cable sheath length, and optionally cable’s elements, on short cable samples during a climatic exposure test. Information about the thermal expansion and contraction can be helpful when classifying a cable and to understand the higher attenuation observed during climatic tests.
5.2.3 Irreversible thermal contraction (shrinkage)
Irreversible thermal contraction is specifically relevant for extruded plastic materials in optical fibre cables. During the cooling stage of an extrusion process, the polymer orientation is “frozen”. If the extruded material is exposed to a high temperature, or kept for a long time at room temperature, the frozen-in polymer orientation can relax, and the extruded plastic material can shrink in direction of the extrusion in an irreversible way [6] 1 . The amount and speed of post-extrusion shrinkage can be influenced significantly by the process parameters during extrusion and by the choice of the base material. Zero or negligible shrinkage can be achievable in some cases.
This post-extrusion shrinkage can be reduced by the inclusion of strength members coupled to the plastic. The more rigid the strength members are and the more tightly the plastic materials are extruded onto them or otherwise coupled, the more the force caused by shrinkage is compensated for by the strength members and the more the potential shrinkage is reduced. If the fibre is stressed by such a force within the cable, then optical performance can degrade permanently.
5.2.4 Forces between cable elements caused by thermal changes
Thermal changes cause forces between cable elements that are made of different materials due to different CTEs as well as different shrinkages. Such forces can induce stress on the optical fibres within the cable. In a general sense, the higher the shrinkage or interactive force, the more likely attenuation can be elevated (see 5.5.2). This shrinkage force is a good indicator for the stress applied to the fibre, but still it is not the only influencing factor (see 5.5.3). measurement of the force of a polymer material caused by temperature changes, dynamic- mechanical analysis (DMA) can be used. A defined material sample is fixed at two points and the force between those two points is continuously measured while the temperature is changed. After exposure at high temperature and during lowering of the temperature, a pulling force between the two points is generated resulting from the shrinkage of the material. As shown in Figure 1 , as a qualitative example, the measured forces (f 1 , f 2 ) of two tested polymer materials are significantly different (by approximately a factor of 5 when the temperature is reduced to −40 °C).IEC TR 62959 pdf download.