Plastic deformation mechanisms of Polybutylene terephthalate (PBT) are investigated at 120°C, 150°C and 180°C using tensile experiments, 2D digital image correlation, wide and small angle X-ray scattering measurements.

Plots of nominal stress vs true strain (ε), true stress vs ε and Haward-Thackray representation reveal the same I, II, III regimes in strain ranges virtually independent of temperature. Between ε = 0 and the yield strain εY ≈ 0.3 (I) the layer morphology is preserved, the interlamellar distance increases in the drawing direction and the strain is localized in amorphous regions. Between ε ≈ 0.3 and ε ≈ 1.0 (II), amorphous chains remain describable as gaussian coils while transition from spherulitic to fibrillar morphologies occurs. Beyond ε ≈ 1.0 (III) the chains are taut, the fibrillar morphology is established, additional stress causes the transitions between polymorphs, respectively from isotropic amorphous to smectic and from α to β crystalline phases.