The purpose of tensile strength test is to verify the ability of optical fiber cables to withstand the pulling tension applied during installation and residual tension after installation. International standard bodies such as IEC and EIA/TIA etc has created test procedures and acceptance criteria based on these factors. Optical loss test (Attenuation ) and fiber strain test are conducted during and after applying the tensile load on the cable.
Tensile strength test is also a confirmation on the design compliance of the manufactured cable. Fiber optic cable design engineers calculate the tensile strength of the cable based on theoretical values of Young’s modulus and Area of cross section of the materials used to make cable. The manufacturing process will affect the final characteristics of these materials after processing.
As per the major international test procedures we need approximately 150 meters of optical fiber cable to conduct tensile strength test. You can add a few more meters to this to do cable end preparation and splicing. Either the optical fibers can be tested individually or as a group by splicing. Attenuation and fiber strain shall be monitored. Additionally, we can measure cable elongation during tensile strength test.
We need a specially customized tensile strength set up as shown in the below diagram to conduct the test. An OTDR, Power meter, Light source, Fiber strain monitoring equipment are also required to monitor optical loss and fiber strain. Tensile machine shall be able to control the load increase generally stated as jaw speed expressed in mm/minute. A 40 mm/minute speed is considered to apply the load uniformly and consistently on the cable without giving sudden jerks on the cable during tensile loading.
Essential parts of tensile loading machine are the load cell and tension display unit. Output of the load cell shall be given to the fiber strain monitoring equipment so that the equipment’s computing unit can plot a graph showing Load versus fiber strain.
Wind the cable on to the tensile machine set up and clamp firmly the ends to the fixing device. Cable ends are prepared for splicing optical fibers. Randomly select fibers from each tube to at least two groups of concatenated fibers, one for attenuation and other for fiber strain. We recommend to make three group of concatenated fibers. First group of concatenated fibers can be used for monitoring attenuation using OTDR. Second group of concatenated fibers can be used to monitor attenuation change in dB using power meter and light source. Third group of concatenated fibers can be used to monitor the fiber strain. At zero tension, mark the cable sheaths at specified locations say 5 meters exactly for measurement of cable elongation.
Apply tension slowly on the cable until it reaches the specified tensile load, say 2700N. This load will vary according to the cable type and design. Basically you need to apply the designed load. Once we reach the specified load, keep the load for 10 minutes and monitor the attenuaion in dB/km, change in attenuation in dB and fiber strain in % as a function of load in Newtons. During this 10 minutes, we can do the measurement for cable elongation. Measure between the markings we made at zero tension. The length must have increased by a few millimeters. From the initial and final lengths between the marks, we can calculate the cable elongation in percentage.
At the end of 10 minutes, release the load gradually. The attenuation in dB/km and dB shall be checked. Monitoring of fiber strain shall be continued to the end of the test. The fiber strain shall come to the zero value after release of the load. This ensures that there are no residual strain on the fibers after release of the load. Under the load the change in attenuation shall not be more than 0.1dB. If the cable construction is good, change in attenuation will show typically less than 0.05dB. Fiber strain shall be below 0.6% under the maximum tensile load. Cable elongation shall be less than equal to the designed value for cable elongation, typically less than 1%.
Cable shall be inspected for any cracks on the sheath after testing. Optical fiber cables if designed properly will elongate within the elastic limits, meaning they will return back to the initial length after release of the load.