Deformation index

The deformation index is a parameter that specifies the mode of control under which time-varying deformation or loading processes occur in a solid. It is useful for evaluating the interaction of elastic stiffness with <a href="/facts/Viscoelasticity/pDhHgx2T">viscoelastic</a> or <a href="/facts/Material_fatigue/zmJY8hMt">fatigue</a> behavior. 
If deformation is maintained constant while load is varied, the process is said to be deformation controlled. Similarly, if load is held constant while deformation is varied, the process is said to be load controlled. Between the extremes of deformation and load control, there is a spectrum of intermediate modes of control including energy control. 
For example, between two rubber compounds with the same viscoelastic behavior but different stiffnesses, which compound is preferred for a given application? In a strain controlled application, the lower stiffness rubber would operate at smaller stress and therefore produce less viscous heating. But in a stress controlled application, the higher stiffness rubber would operate at small strains thereby producing less viscous heating. In an energy controlled application, the two compounds might give the same amount of viscous heating. The right selection for minimizing viscous heating therefore depends on the mode of control.

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Deformation index