Rubber degradation under cyclic loads 1

[steveatt]

Do the properties of "Natural Rubber" degrade quicker under cyclic stress conditions or will it be more environment dependant? The point in case is a car shock ansorber and "perishing" (aging/loss of elasticity) of the rubber.

 

[tomaspin]

Steve,

It depends on cyclic stresses, the geometry of the part, and the environmental conditions.

For example, suspension bushes on a intensively used off highway truck will always fail through fatigue or mechanical damage before ageing becomes an issue, often around the 5,000 hour mark. Mechanical fatigue results in a loss of stiffness- a part is considered to have failed if the stiffness has dropped by 25-30%

Ageing of a rubber part is bought about by ozone attack, UV radiation, and high operating temperatures.
Ozone attack causes a hardening and cracking the rubber surface, but in practice this rarely results in the failure of components. As it is a diffusion process, it is dependent on the exposed area of the rubber, compared to its' volume. For most load bearing parts the exposed area is quite small, and most natural rubber elastomers are compounded to resist ozone attack with the addition of waxes to the polymer. To be frank, degradation due to exposure to fuel and mineral oils is more likely.

Ageing of a rubber depends on temperature to a certain extent, and will result in a gradual stiffening over time (7-10 years is reasonable), and a deterioration in noise isolation performance.For a part loaded in compression it is unlikely that this would cause the component to fail.

To summarise, if you drive your car fast over rough roads, deterioration through fatigue, leading to a loss of stiffness and breakdown of the part is more likely. However, gradual ageing (and stiffening) will predominate in a gently driven vehicle.

For a shock abosorber bush, the shock absorber itself will probably need changing before the bushes fail!

I hope this goes some way to answering your question

Regards

Tom Aspin

 

[TrevorD]

Besides the environmental conditions the natural rubber part is exposed to , including the type and intensity of use it undergoes. Degradation of natural rubber parts will depend very much on the rubber curing formulation and the type of crosslinking formed during vulcanisation. This will all depend on the chemicals and conditions used to cure the natural rubber.

Cyclic loadings can be designed into a rubber part by skilled rubber technologists using thier knowledge of the properities obtained by varying the type and levels of accelerators, thiurams, and sulphur donors.

Newer vulcanisers such as AS100, TBUT and SAA30 can be used to improve the dynamic properities obtained from traditional chemicals such as DPTT and TMTD