Flex Life – what is it?
In Germany the term “Biegewechselfestigkeit“ (bend strength) is used to describe the test result when a single wire or bunched or stranded conductor is subjected to a constant reverse bending process until the conductor breaks. Internationally we call this “Flex Life“. Measurement of this process is normally carried out according to the standard method as described in ASTM B470. The results demonstrate the failure behaviour of conductors that will be used in constantly changing environments and applications. Identical constructions made of different materials can be tested and the results compared.
Following a series of experiments carried out by LEONI we can confirm the following correlation:
- The higher the tensile strength of the tested material, the greater the flex life before wires break
- The finer the individual wires in a conductor, the greater the flex life is before wires break
For the flex test (also called the reciprocal bend test) a sample is secured to the test apparatus according to defined conditions (length, weight, bend radius).
Then it is bent first in one direction (1), then back to the central vertical position (2), then in the other direction (3) and finally back to the centre again (4).
This is one complete cycle.
The following results can be recorded:
- Number of cycles completed before the whole sample breaks
- Number of cycles completed before a predetermined resistance value is reached (as individual wires break the resistance of the conductor increases)
- Completion of the preset number of cycles without either of the above failures
The set up is as follows:
- Sample length 70 cm
- Weight load dependant on sample cross section
- Bend angle 60°
- 30 cycles per minute
- Sampling rate 5 HZ
- Test current 1 A
Comparison of test results
A number of factors including the number and diameter of the individual wires, lay length and lay direction can all significantly affect the flex life of a stranded conductor. Therefore it is imperative that when comparing results for different strands that all parameters must be identical.
The experiments were carried out by LEONI on a series of AWG28/36 stranded conductors.
A representative cross section of LEONI Histral® products were tested in both hard and soft condition up to a maximum tensile strength of 1200 N/mm².
The chart clearly demonstrates that strands in hard condition have a significantly better flex life compared to the same strands when annealed. Soft wires are very quickly stretched along the bend point leading to metal fatigue and then wire breaks.
The hard strands fall into 3 distinct groups, those with:
- low flex life
- medium flex life
- high flex life
Advantages in application
- 1. low Flex Life
- The materials tested were mainly CuNi alloys primarily used in resistance heating cables and not normally exposed to significant mechanical stress.
- 2. medium Flex Life
- These materials combine increased tensile strength with good electrical conductivity. This combination is typically required for signal cables in automotive, military, aircraft, aerospace and traffic applications, where the finished product is exposed to a high degree of tension and drag, as the cable is pulled through small diameter holes. In automation technology applications cables are often subjected to strong vibration and constant movement causing conductors with poor flex life to fracture thereby leading to machine failures. In medical equipment these conductors provide the precision and reliability that are vital for patient safety.
- 3. high Flex Life
- Various industrial applications provide ideal environments for conductors with very high flex life and somewhat reduced electrical conductivity. These alloys function exceptionally well when deployed as sensor cables in high wear automotive situations (for example ABS leads) and as signal cables in robotics which places the highest demands on flex life.
GTS Test Solutions, Berlin