COHERENT MOLECULAR SLIP UNDER SHEAR IN A PLASTIC CRYSTAL

We present experimental results on visco-elasticity and plasticity of a plastic crystal submitted to a shear deformation. In a temperature range 10°C below the melting point, an oscillating stress-strain curve is observed with a period double of the lattice parameter. This periodic nonlinear effect could be explained by the simultaneous slip of molecules between two adjacent crystallographic planes, near the sample-glass interface. Compared to this Frenkel-like surface mechanism, a dislocation-based mechanism cannot be excluded. © 1990 IOP Publishing Ltd.