Structural relaxation and evolution of yield stress in epoxy glass aged under shear strain

Hollow cylindrical specimens of annealed epoxy glass were twisted and then aged for various periods of time under shear strain. At the end of the aging process, we twisted the specimens again to determine the stress-strain relations. For specimens aged under a shear strain of 0.005 or 0.01, the stress relaxation behavior was almost independent of the amount of strain imposed, and the value of stress at the upper yield point, regardless of aging time, was almost the same as that of the annealed specimen. On the other hand, for specimens aged under a strain of 0.02 or 0.04, the stress relaxation behavior depended on the value of the strain applied, and the value of stress at the upper yield point first decreased and subsequently increased with increasing aging time. These results led us to the following conclusions: If epoxy glass is strained largely, the originally stable structure becomes unstable. Also, when epoxy glass is aged under strain, the stability of the structure continues to decrease for a short period of time after deformation ceases, and then increases with increasing aging time. (C) 2004 Society of Plastics Engineers.