Rubber properties : Creep and Set

The deformation of rubber is influenced by the length of time under stress. If the rubber is statically loaded to a give amount, as occurs for example by the support of a machine, then, in accordance with Figure 1, an elastic deformation takes place superseded after a longer period of time by creep. Creep behavior follows an exponential law and is conclude some time afterwards. If the spring is released then its shape return elastically and by creep to a new position at witch recovery is arrested. This represent an inelastic after-effect for a given rubber mix. High elastic grades show a small residual strain and minimal creep. With good elastic qualities, the residual strain (Permanent Set) after prolonged loading and unloading lies between 2 and 5 %, and creep lies between 5 and 10 % of the total elastic strain.

Figure 1. Creep Diagram for soft rubber

Creep and residual strain are proportional to the total elastic strain under a static load. Because of this they are expressed as a percentage of the elastic strain. Thus both effects are independent of the magnitude of load, but they are temperature dependent as show in Figure 2 . The diagram represents a butadiene mix; natural rubber mixes behave more favourably.

Figuer 2 . Temperature dependent of creep , permanent set and modulus for rubber cylinder 20 mm in diameter x 20 mm under stress of 1 MPa

When an oscillating (dynamic) load is superimposed on a static load a further undesirable deformation results from the oscillating load. This is described as Dynamic Set. The dynamic set is dependent on the number of stress reversals and the magnitude of the oscillating load. It also reaches and end value quickly. For vehicle springs the set may take up the same magnitude as creep at full static loading of the vehicle. The spring constants are not influenced by creep and set. The sole effect is a parallel displacement of the spring curve by an amount of the set. In rubber spring design it is useful to allow from 8 to 10 % of the elastic deflection of the spring for permanent deformation through creep and set.

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