Creep

Creep is a time dependent increase in deformation under constant load . If an elastomer component is subjected to a static preload, then this load cause a progressive increase in deformation as a function of time.It can be important in a wide variety of application, such as suspension engine rubber mounts and building mounts etc. Creep is expressed as a percentage of the initial deflection . Thus

Creep at time t (%) = 100*(l – l0)/l0

where l0 is initial deflection and l is the deflection at time t .

The initial deflection must be measured at a define initial time to , which should be about 10 times longer than time taken to apply the deformation. Creep rate is express as creep divided by function of time. This function may be the logarithm of time if the relaxation mechanism is physical or time if it is chemical . Creep in rubber material consist of both physical (due to molecular chain slippage) and chemical(due to molecular chain breaking) . Physical creep rates (A) decrease in time and are usually expressed as a percentage of the original deflection per decade of time. Chemical creep rate (B) at a constant temperature are approximately linear with time, and thus the total creep is given by

creep(%) = A log10(t/t0) + B(t – t0)

**Engineering with Rubber (2001)

Pic1 Creep test