Load drop at the upper yield point of a polymer

The stress‐system curves have been measured for isotropic and oriented specimens of poly(ethylene terephthalate) in extension, shear, and compression. It was found that for all these types of deformations there was an intrinsic yield drop, i.e., a fall in true stress. It is concluded therefore that the stress for initiation is greater than the stress for propagation of yielding.