One form of kinking occurs as a two-step mechanism. A cable can 

 rotate and accumulate torque, which is released in the form of a loop 

 when the cable becomes slack. This loop is pulled into a kink, causing 

 permanent deformation when the cable becomes taut again. Even if the 

 kink itself did not leave the cable inoperable by breaking the conduc- 

 tors, a kink severely reduces the cable life by exposing wires to 

 corrosion and producing a weak point under cyclic loading. 



Kinking can be formed by improper torque balancing. As discussed 

 earlier, even though manufacturers claim that they can make "torque- 

 free" or "non-rotating" electro-mechanical cable, there is no contra- 

 helically-wound cable manufactured that can demonstrate this capability. 

 Parallel filament synthetics do not have inherent residual torsional 

 stresses and should be torque-free. 



Fatigue usually is the result of cyclic loading, such as running 

 over sheaves during deployment and recovery operations and action over 

 a pulley or sheave due to ship motion. This repetitive bending, lead- 

 ing to fatigue failure, is difficult to detect because the internal con- 

 ductors are being damaged. The copper wires are cold-worked due to the 

 bending, and the result is usually an electrical, not a mechanical, 

 failure. To prevent fatigue failure: (1) the cable should not be sub- 

 jected to any reverse bending during sheave passage; (2) both the recom- 

 mended sheave groove size and minimum bending radius for that particular 

 electro-mechanical cable should be used to design the proper sheaves; 

 and (3) a sample of the cable should be fatigue tested to determine its 

 expected working life. 



Improper sheave groove size and rubbing of overlaying armoring con- 

 tribute to abrasion of the external wires of the cable. These wires be- 

 come notched, and fatigue failure occurs under continued bending and ten- 

 sile loading. Protection against abrasion can be provided by polyethylene 

 or urethane jacketing over individual strands and/or over the entire 

 cable. 



Terminations are subject to fatigue due to bending and twisting of 

 the cable just above the connection, which must by nature be rigid. This 

 action can deteriorate the wires in the termination and birdcage the 

 external armor above the connection. Birdcaging can permanently deform 

 the armor and increase the possibility for corrosion. Fixed terminations 

 must be relieved by swivels and ball or pinned joints to eliminate bending 

 and torsional moments. 



Since the use of synthetic materials as armoring is relatively 

 recent, the failure experience of E-M cables using these materials is 

 not known at this time. 



Design and Testing of E-M Cable 



In order to guard against these probable failure mechanisms, the 

 user should realize the limitations of his particular E-M cable. This 

 requires knowledge of the cable mechanical properties, including parameters 

 such as strength, bending, and torsion. 



To design for strength the user must decide what maximum working 

 load he will encounter and the safety factor he desires for the specific 

 application. Once the working load and safety factor are determined, the 



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