bending stiffnesses, they are used to nondimensionalize the external torque 

 T and axial force F. From Equation 18, the critical tension is expressed 



as: 



(18a) 



Kinking Tests 



Kinking experiments were conducted on samples cut from two 

 electromechanical cables of different construction. The 1x48 double - 

 armored cable and the 3x19 multistrand cable were selected as test samples. 

 In the experiments combined tension and torsional loadings were applied to 

 the cable, and the measurement taken of the rotational response and the 

 cable tension and torque when a kink formed. The sample length varied 

 from 21 feet to 23 feet 9 inches. Swaged eyes were used as end terminations 

 for the double -armored cables, and cable clamps were used for the multi- 

 strand cables (Figure 12 and 13). The cable samples were suspended under 

 a steel frame 30 feet from the ground; a torquing bar and a known weight 

 were attached to the lower end. The torque was measured by two low- 

 range spring scales attached to the torquing bar. The weight was first 

 attached to the torquing bar, and the cable was allowed to rotate to an 

 equilibrium orientation. Starting from that orientation, two men, each 

 positioned at one end of the torquing bar, walked in a circle to apply 

 torsion and rotation to the lower end of the cable. The cable sample 

 was eventually twisted into a kink and the experiment stopped (Figure 

 14). The torque and rotation were recorded at desired intervals. Finally 

 the rotation and torque at the kink were recorded. The sample was changed 

 and a new experiment started. 



In some of the experiments, the tension and the torque were continuously 

 monitored by a load cell connected to a strip chart recorder. The torque 

 variation during the formation of the kinds was observed and recorded 

 (Figure 15). Photographs were taken during the experiments to facilitate 

 the study of the configuration of various kinds of cable deformation at 

 various load combinations. In addition, the mechanism of kink formation 

 and its variations were observed and recorded. 



The process of cable kinking is more complex than the simple buckling 

 of a short rod. For a rod, both the axial compression force and the torque 

 tend to accelerate the failure of the rods. Once an initial eccentricity 

 is created by the torque, the compression force alone could complete the 

 buckling process by producing higher and higher bending moment through 

 increasing eccentricity. This is not so for cable kinking where the axial 

 force is not compressive. Instead, a tension acts on the cable to reduce 

 any initial eccentricity caused by the torque. Therefore, larger torque 

 is needed to curl the cable into a helical-shape. As the torque increases, 

 the cable deforms locally at a weak point and a half loop is formed. The 

 torsion- induced bending moment is balanced by the tension-induced resisting 



11 



