The following two phenomena observed in torsional shear experiments are str
udied in this paper: (1) micro-defects are periodically initiated under uni
form loading; and (2) the alternative evolution of periodic micro-defects o
ccurs when they uniformly grow to some extent. Micro-defects are modeled as
concentrated strain and suitable boundary value problems are posed for the
se initiation and evolution processes. With the aid of the equivalent inclu
sion method, the existence of bifurcated solutions is presented in a transp
arent manner, and the stability of these solutions are examined. It is show
n that such unstable bifurcated solutions correspond to the phenomena obser
ved in the experiments. Variational problems corresponding to the boundary
value problems are formulated and the bifurcation and stability analyses us
ed for the boundary value problems are presented in a more general manner.
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