Relaxation phenomenon in epoxy glass aged under post-yield strain

Relaxation phenomenon in epoxy glass aged under shear strain larger than an upper yield point was studied. After aged under post‐yield strain for various aging periods, cylindrical specimens of epoxy glass were twisted clockwise (in the same direction as the prestrain) or counterclockwise (the opposite direction to the direction of the prestrain). The evolution of yield points was significantly different from that of the specimens aged under preyield strain. There exist two knee‐like yield points on stress–strain curves of specimens twisted counterclockwise: one evolved toward to an upper yield point and merged the other knee‐like yield point whose stress value was almost independent of aging time. On the basis of the experimental results, we proposed a combined relaxation model of two relaxation mechanisms: one is relaxation results in an isotropic structure whose center in stress space is the stress value in the terminal zone and the other is kinetic relaxation of the isotropic center. The combined relaxation indicated the possibility of phase transition caused by postyield strain, and therefore the higher yield stress than that of an annealed specimen was not resulted from strain‐accelerated aging, but presumably resulted from a structural change under postyield strain. POLYM. ENG. SCI. 46:630–634, 2006. © 2006 Society of Plastics Engineers