Deformation bands in oriented polyethylene terephthalate

Polyethylene terephthalate sheet which had been oriented by drawing was subsequently deformed in tension where the primary variable was the angle between the stress and the initial drawing direction. The beginning of yielding was marked by the formation of a narrow deformation band which had the following important characteristics: (1) the shear strain in the band was about 200%, (2) the direction of the band was nearly but definitely not parallel to the initial drawing direction and (3) the direction of optical extinction under crossed polarizers rotated from the initial drawing direction toward the axis of stress. Simple ideas of the structure of oriented polymers are used to explain the main features of the deformation behaviour, including the change in the extinction direction within the deformation band. These ideas are of a sufficiently general nature to be applicable to all oriented linear polymers.