Copper has high fatigue strength

Copper conductors can withstand larger vibration amplitudes and for much longer than aluminium conductors without cracking or breaking

Fatigue occurs when a material is subjected to repeated loading and unloading stresses. If the stresses are above a certain threshold and the number of repetitions is large enough, microscopic cracks begin to form. Progressively, a crack can reach a critical size and then propagate suddenly, leading to a fracture.

Fatigue strength is defined as the value of stress at which failure occurs after a given number of cycles. These are the comparative values of fatigue strength for high conductivity copper and low alloyed aluminium respectively:

Material

 

Fatigue strength (N/mm²)

No. of cycles x 106

HC Al

annealed

20

50

half-hard (H8)

45

50

HC Copper

annealed

62

300

half-hard

115

300

It can be seen that copper can withstand higher stress levels for longer durations than aluminium.

In conductor applications such as in equipment installations and machinery, aluminium conductors are subject to nicks, scratches or “ringing”. These flaws can lead to fatigue failure when subjected to movements due to repeated expansion and contraction or vibration. The significantly higher rate of thermal expansion in aluminium when exposed to thermal cycling due to load changes can also create sufficient movement such that minor flaws may deteriorate into areas of high resistance, causing hot spots or even breakage of the conductor.  These small flaws can deteriorate into large breaks in the wiring, causing long-term interruptions in service. Copper, being harder and more flexible, avoids nicks and breaks, but even if these should occur, its higher fatigue strength ensures a higher degree of reliability.

Another application area in which fatigue strength plays a role is overhead transmission lines. Due to wind excitation, the electrical conductors experience so called aeolian vibrations in the 5 to 50 Hz range. These conductor oscillations can lead to fretting fatigue, mainly in the suspension clamp regions, spacers or other fittings and thus an interruption in service. By comparison high tension low sag (HTLS) lines with copper conductors perform much better in this respect due to copper’s superior fatigue strength.

References

  1. High Conductivity Copper for Electrical Engineering, ECI publication no. Cu0232, Feb 2016 
  2. White Paper – A Comparative Evaluation of Copper and Aluminium Wires and Cables in Building Installations, ECI publication no. Cu0172, Dec 2015
  3. Fretting Fatigue Phenomena on an all Aluminium Alloy Conductor, Marco Boniardi, Silvia Cincera,Fabrizio D’Errico, Chiara Tagliabue, Dipartimento di Meccanica, Politecnico di Milano, Via La Masa 34, 20156 Milano, ITALY
Powered by Zendesk