Effect of strain history on craze microstructure

Crazes formed under a constant tensile strain in polystyrene (PS) have a dense network of fibrils with an extension ratio $\text{λ ? 4}$, but a midrib of higher λ forms by drawing fibrils from the craze-matrix interface in the high stress region just behind the craze tip. Stepwise increases in tensile strain during craze growth should thus produce layers of fibrils of different λ, which can be revealed by transmission electron microscopy (TEM) of crazes in stepwise strained PS films. When the time interval between strain increments of 0.5–1% is one minute, TEM images show ‘ridges’ of lower λ fibrils, corresponding to the position of the craze-matrix interface at the time of the strain increments. The ridges appear to be the analogue of the bulge remaining on a macroscopic fibre which has been allowed to stress age by stress relaxation before resuming drawing and imply that rapid stress ageing must occur near rthe craze-matrix interface so that more material is drawn into the craze in preference to increasing the λ of the existing fibrils.