Nonlinear Behavior in Aging

Many authors have attributed the yield point and the fragile - ductile transition occurring in many glasses to the decrease in the values of the retardation times resulting from the increase in volume produced by stress in the glassy system. 

The nonlinearity exhibited by the aging process suggests that the characteristic relaxation time Xq of the KWW equation is not a constant but a variable. A nonlinear function can be obtained for the KWW equation by defining a reduced time given by (28,29) 

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