Mechanical Anisotropy of Oriented Polymers Part 1 A Yield Criterion for Uniaxially-Drawn Poly(ethylene terephthalate)

Measurements of tensile and shear yield stress have been made on strips cut at various angles to the draw direction from films of drawn amorphous poly( ethylene terephthalate). The data were well fitted by a criterion of the von Mises type but with modifications to allow for the anisotropy of the samples and also for a built-in compressive stress in the draw direction. In the tensile experiments a sharp neck is usually formed at an angle to the tensile direction and this angle is predicted with good accuracy by an application of the theory of plastic potential. The built-in stress is closely related to the retraction stress which develops when the films are heated but not allowed to contract. In general the tensile yield stress and the tensile modulus are also closely correlated.