What is stress cracking?

Stress cracking describes the occurrence of fine to distinct cracks in a material, which are caused by the interaction of mechanical (tensile) stress and chemical and/or physical effects. This form of damage often occurs without significant plastic deformation and can lead to sudden failure of the component.

In elastomer and plastics technology, the tendency to stress cracking is an important aspect of material selection and product development. Stress cracking is particularly important for safety and function-critical components such as Seals, Valves or Rubber-metal composite parts This is important because it often begins unnoticed and can lead to sudden failure (even after months or years).

 

Factors influencing stress cracking

The development usually depends on two factors:

  • Mechanical stress: caused by external loads, internal stresses from the manufacturing process or assembly forces
  • Chemical influences: aggressive polar media, ozone and UV radiation in particular generate radicals that have a chain-splitting effect, whereby the following are particularly important Unsaturated polymers (R polymers) are susceptible.
  • Physical influences: non-polar media such as oils and fats do not act directly on the polymer chains, but have a swelling or washing effect Plasticiser from.

 

If chemical or physical influences are added, cracking can start and progress much faster than would be the case with purely mechanical fatigue.

 

Prevention of stress cracking

To prevent stress cracking, the mechanical load and media impact must be considered. Possible measures are

  • Suitable choice of material: Select an elastomer that is resistant to the media used. M and Q polymers are generally more suitable than R polymers. For critical media, media-resistant materials such as FKM, EPDM or VMQ used.
  • Low-stress construction: round off edges, avoid notches, provide uniform wall thicknesses and reduce excessive pretension.
  • Gentle installation: Avoid overstretching, tilting or constriction.
  • Suitable operating conditions: Keep temperature, vibrations and pressure peaks within the permissible range, as they can accelerate the formation of cracks.
  • Ageing protectionUse of stable compounds with suitable stabilisers to minimise embrittlement and loss of performance over time.