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occurs for example by the support of a machine, then, in accordance with Figure 1, an elastic deformation takes place superseded after a longer period of time by creep.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.

Relate Content

1. What is the standard test for oil seal.

Tire design : Tire stiffness

The vertical tire force (Fz) can be calculated as a linear function of the normal tire deflection (z) measured at the tire center.

Fz = kz*z

the coefficient kz is call tire stiffness in the z direction. Similarly, the reaction of a tire to a lateral and longitudinal force can be approximated by

Fx = kx*x

Fy = ky*y

When kx , ky are called tire stiffness in the x and y directions.

The stiffness curve can be influenced by many parameter. The most effective one is the tire inflation pressure. Lateral and longitudinal force/deflection behavior is also determined experimentally by applying a force in the apporiate direction. The lateral and longitudinal force are limited by the sliding force when the tire is vertically loaded. The coefficient kx and ky are called tire stiffness . They are measured by the slope of the experimental stiffness curve. When the longitudinal and lateral force increases, part of the tire footprint creep and slide on the ground until the whole tire footprint starts sliding. At this point. the applied force saturates and reaches its maximum supportable value. Generally, a tire is most stiff in the longitudinal direction and least stiff in the lateral direction.

kx > kz > ky

Tire with a larger of plies have higher damping, because the plies’ internal friction generate the damping. Tire damping decreases by increasing speeds.

Polyurethane applied to Flexible Gaiters

Polyurethane is applied to automotive industry . The ball joints for steering linkages and front suspension mechanisms of cars have to be protected from wear otherwise play rapidly occurs, which is reflected back to the steering wheel with a resultant loss in steering control. The conventional method of protection is by packing the joint with grease and containing the grease with a flexible gaiter. These gaiters are subjected to the effect of the grease inside and the effect of moisture on the outside and have to operate over serveral years without deterioration. To operate satisfactority they also have to resist the damage caused by small stones thrown up by the tires. The high tear strength of polyurethane is an obvious advantage here, but another equally important advantage is that in the environment of grease, water, ozone and general exposure to weathering the polyurethane elastomers or nitriles. Initially these gaiters were made from millable types of polyurethanes and were two to three times the cost of similar gaiters made in neoprene. Even so the practical advantage gained dictated fairly widespread use, although it is considered that further material improvements are still required and injection-moulded gaiters are already replacing some millable types.