Object
This test method is intended to establish the ability of a fiber optic cable to withstand mechanical twisting. The primary purpose of this procedure is to measure any variation in the optical power transmittance of a fiber when the cable is subjected to torsional forces external to the cable sheath. A secondary purpose is to evaluate the possibility of physical damage that may occur as a result of such stresses.
Sample
The specimen shall be a sample of fiber optic cable having a total length sufficient to permit the appropriate clamping and twisting, and long enough to permit optical transmittance measurements as required by the detail specification.
Apparatus
The twisting apparatus consists essentially of two cable gripping devices or clamps, one fixed and one that can rotate, supported as appropriate, the distance between them being adjustable. The rotating clamp is connected to suitable turning equipment (e.g. a torqueing lever). Any clamp supports, gripping devices or torqueing equipment used shall all be such as to permit access to both ends of the cable specimen for optical testing as may be required.
The cable gripping devices shall be such that
they may be tightened around the cable sufficiently to prevent movement within the grip,
the clamps hold the cable firmly in a straight line,
the clamps induce neither localized twisting damage on the cable caused by the inside
edge of the clamp nor undue localized concentration of pressure on the cable,
the process of clamping does not induce any significant or accurately measurable attenuation increase (or no more than a negligible increase) in the specimen. If required by the detail specification and/or to minimize specimen bending from a straight configuration, use weights or an appropriate loading mechanism to apply a tensile load to the cable gripping fixture. In any case, one end of the gripping fixtures shall be free to move longitudinally to respond to foreshortening of the cable.
The apparatus shall include optical transmittance equipment to measure the change in optical throughput as required in the detail specification, and specified in Method A (transmitted power) of IEC 60793-1-46:2001.
Requirements
The acceptance criteria for the sample under test shall be as stated in the detail specification.
Typical failure modes include loss of optical continuity, increase in fibre loss and damage to the cable sheath or core components.