Competence & customer service

Flex Life

Flex Life testdevice

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


Low Flex Life


  • normally not exposed to significant mechanical stress

Products / materials:

  • mainly CuNi alloys


  • primarily used in resistance heating cables

Medium Flex Life


  • often subjected to strong vibration and constant movement

Products / materials:

alloys with

  • increased tensile strength
  • good electrical conductivity


signal cables in:

  • automotive
  • military
  • aircraft
  • aerospace
  • traffic
the finished product is exposed to a high degree of tension and drag, as the cable is pulled through small diameter holes
  • 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
  • medical equipment
these conductors provide the precision and reliability that are vital for patient safety

High Flex Life


  • highest demands on flex life

Products / materials:

alloys with

  • very high flex life
  • somewhat reduced electrical conductivity


  • industrial applications
  • sensor cables in high wear automotive situations (for example ABS leads)
  • signal cables in robotics

Test method

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.

Test method Flex Life
Test method Flex Life

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.

Graph Flex Life test
Graph Flex Life test

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:

  1. low flex life
  2. medium flex life
  3. high flex life

Related topics on our website




GTS Test Solutions, Berlin



Please contact us for further information.