Home > Case Studies > Conductor Fatigue Testing

Conductor Fatigue Testing

Comprehensive In-Lab Services to Meet International Testing Standards
Project Objective

The objective of the Fatigue Test was to subject the conductor to various vibration tests, and to observe the extent of any damage. The results could then be plotted on a Wöhler curve (stress versus number of cycles to failure) in order to estimate the endurance limit of the Aluminum Conductor, Steel Reinforced (ACSR) conductor.

Fatigue in conductors is caused mainly by the presence of alternating bending stresses.  In stranded conductors, the alternating stresses result from the bending back and forth of the conductor at the supporting clamp during vibration. However, that bending also causes the strands of the conductor to slip relative to each other in the vicinity of the supporting clamp.    
Data from the fatigue tests on conductors are normally presented as bending stress (σ) plotted against number of bending cycles (N) to failure of a conductor strand. 

Scope of Work

A total of three (3) samples of ACSR conductor were used, each approximately forty (40) meters long.  Strands were tightly secured with hose clamps to prevent the outer wires strands from loosening relative to the inner wires. The ends of the conductor were terminated with epoxy-resin dead ends.     
The conductor was tensioned to approximately 20% of the conductor’s Rated Tensile Strength (RTS).  An electromagnetic shaker was mounted near the centre of the span and the shaker was rigidly mounted to the conductor through a force sensor.  
Two (2) strain gauges were placed on the top strands of the conductor, one (1) at the north end and one (1) at the south end, to measure the actual bending stress. The test was carried out in a temperature-controlled laboratory. 
Each conductor sample was subject to one of five (5) specific number of vibration cycles: 10, 20, 30, 40 and 50 million cycles.  Two (2) tests were performed at 30 million cycles and one (1) test for 10, 20, 40 and 50 million cycles.
For each of the specific number of vibration cycles performed on the ACSR conductor, the samples were vibrated at a bending amplitude where damage should occur based on EPRI fatigue test results (i.e. stress greater than 8.5 MPa). 
After the conductor had completed the number of cycles, it was dissected to assess the amount of damage.  Ideally one (1) broken strand would result.  If more broken strands were found this would indicate that the conductor failed before the number of cycles had been completed, or that a lower bending amplitude would have been sufficient to break only one (1) strand.  If no strands were found to be broken this would indicate that more cycles, or a higher bending amplitude are required for failure.
Test Results
The bending stress level that the conductor can withstand indefinitely without damage is known as the Endurance Limit. The fatigue data collected for the ACSR conductor sample in this test series, up to 50 million fatigue cycles, suggests an estimated Endurance Limit of approximately 20 MPa.  If the conductor vibrates at a given bending stress above this limit for a number of cycles, there is a risk of fatigue failure.  If the conductor vibrates at a given bending stress below this limit, for a number of cycles, it is considered to be within the safe limit.
Client Benefits 
By understanding the endurance limit of a given conductor, a client can design a system to operate within the safe boundary limit.  If the conductor operates in a system outside its safe boundary limit, additional mitigation strategies can be implemented to reduce the risk of fatigue failure.