The room temperature physical and mechanical properties of the high conductivity coppers in the annealed condition do not differ significantly from one another.
In the as-cast condition, high conductivity copper has a tensile strength of 150-170 N/mm2. The changes in structure brought about by hot working raise the tensile strength to the order of 200-220 N/mm2. Upon further working the resulting mechanical properties of a particular form are influenced by the amount of cold work given to the material which has the effect of raising its tensile strength, proof stress and hardness but reducing its elongation. The effect on the mechanical properties of cold work (reduction in area) by rolling is shown in Figure 2.
The maximum tensile strength obtainable in practice depends on the shape and cross-sectional area of the conductor. The larger the cross-sectional area of a conductor the lower its tensile strength, since the amount of cold work that can be applied is limited by the reduction in area which can be achieved.
For the usual sizes of busbar conductors in the hard condition, tensile strengths from 250 N/mm2 up to 340 N/mm2 can be obtained depending on the cross-sectional area.
The 'proof stress' required to produce a definite amount of permanent deformation in the metal is a valuable guide to its physical properties. Proof stress is defined as the stress at which a non-proportional elongation equal to a specified percentage (usually 0.2) of the original gauge length occurs.
As with the tensile strength, the proof stress varies with the amount of cold work put into the material (see Figure 2).
Figure 2 - Effect of cold rolling on mechanical properties and hardness of high conductivity
British Standards applicable to busbar conductors do not specify hardness measurement as part of the testing requirements. It can however be more quickly and easily carried out than a tensile test and is convenient therefore as a guide to the strength of a conductor. The results have to be used with discretion for two reasons:
a) Unlike ferrous materials the relationship between hardness and tensile strength is not constant (see Figure 2).
b) A hardness test is usually only a measurement of the outer skin of the material tested. If the conductor is of large cross-sectional area and has received a minimum amount of cold work the skin will be harder than the underlying metal. Consequently, variations in hardness may be obtained dependent on where the measurement is made in relation to its cross-section.
As a guide, typical hardness figures of the temper range of conductors supplied are:
Annealed (O) 60 HV max
Half-hard (½H) 70-95 HV
Hard (H) 90 HV min.
It is well known that the exposure of cold worked copper to elevated temperatures results in softening and mechanical properties typical of those of annealed material. Softening is time and temperature dependent and it is difficult to estimate precisely the time at which it starts and finishes. It is usual therefore to consider the time to 'half-softening', i.e., the time taken for the hardness to fall by 50% of the original increase in hardness caused by cold reduction.
In the case of HC copper this softening occurs at temperatures above 150°C. It has been established experimentally that such copper would operate successfully at a temperature of 105°C for periods of 20-25 years, and that it could withstand short circuit conditions as high as 250°C for a few seconds without any adverse effect.
If hard drawn conductors are required to retain strength under operating conditions higher than normal, the addition of small amounts of silver at the melting and casting stage produces alloys with improved resistance to softening. The addition of 0.06% silver raises the softening temperature by approximately 100°C without any significant effect on its conductivity, at the same time appreciably increasing its creep resistance.
Figure 3 - Typical creep properties of commercially pure copper and aluminium
Creep, another time and temperature dependent property, is the non-recoverable plastic deformation of a metal under a prolonged stress. The ability of a metal to resist creep is of prime importance to design engineers.
The creep resistance of oxygen-free HC copper is better than that of tough pitch HC copper. This is due to the very small amounts of impurities which remain in solid solution in oxygen-free copper, but which are absorbed in the oxide particles in tough pitch copper. Some typical observations are shown in Figure 3. Tough pitch copper creeps relatively rapidly under low stress at 220°C. The addition of silver to both oxygen-free and tough pitch coppers results in a significant increase in creep resistance.
Fatigue is the mechanism leading to fracture under repeated or fluctuating stresses. Fatigue fractures are progressive, beginning as minute cracks which grow under the action of the stress. As the crack propagates the load bearing area is reduced and failure occurs by ductile fracture before the crack develops across the full area.
Conditions for such failures can be set up in a busbar system rigidly clamped for support and then subjected to vibrating conditions. Support systems are discussed in detail in Section 8.
The high conductivity coppers are ductile and in the correct temper will withstand severe bending and forming operations. As a general guide to bending, copper in the half-hard or hard temper will bend satisfactorily round formers of the following radii:
Table 6 HC copper minimum bend radius
| Thickness | Minimum bend radius |
| Up to 10 mm | 1t |
| 11-25 mm | 1.5t |
| 26-50 mm | 2t |
where t = bar thickness
Material of thicknesses greater than 50 mm is not normally bent; however, it is possible to do so by localised annealing prior to bending.
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