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Primarily, rubber is used in place of metallic, ceramic, and other rigid materials because it will provide a greater deflection for a given force than these other materials. Most uses of rubber are based upon this characteristic. Rubber is used as an engineering material in resilient stiffness characteristics become critical to the function of the product, appropriate test specification must be established.

Dynamic Methods
Applications where rubber is used as vibration isolators are dependent upon the behavior of the rubber under dynamic operating conditions. Rubber is stiffer dynamically than in a static mode; and, since the static to dynamic stiffness ratio varies with individual compounds, it may be advisable to specify the dynamic characteristics of the rubber for such applications. When dynamic stiffness or spring rate is specified, and is critical to the rubber product performance, the complete conditions and methods of measurement must be established between customer and rubber manufacturer.

Factors Affecting Static and Dynamic Load Deflection Characteristics
Age – The aging of rubber compounds over a period of time is a complex process. The normal net effect of aging is an increase in modulus or stiffness. The magnitude of this change is dependent upon the specific material involved and the environmental conditions

Dynamic History
The load-deflection characteristics of a rubber product are affected by the work history of that specific product. The initial loading cycle on a new part, or a part that has been in a static state for a period of time, indicates a stiffer load-deflection characteristic than do subsequent cycles. In static testing this effect becomes stabilized and the load deflection characteristics normally become repeatable after two or four conditioning cycle.

Temperature has an effect on spring rate- the higher the temperature the lower the spring rate, and the lower the temperature the higher the spring rate of a rubber product not under continuous tension.