Solid Phase Sheet Forming of Thermoplastics—Part II: Large Deformation Post-Yield Behavior of Plastics

This paper examines the phenomenology of the large strain, post-yield behavior of the three thermoplastics—polycarbonate, polybutylene terephthalate and polyetherimide—from the point of view of the sheet forming process. A photographic technique is utilized to measure the large nonhomogeneous strains observed in polymers. This technique makes possible the mapping of the strain field for flat specimens in the form of strain contours. Amorphous polymers polycarbonate and polyetherimide are shown to behave differently from semicrystalline polybutylene terephthalate. All three polymers are shown to neck in a stable manner, with the necked regions undergoing very large strains. The mechanical properties of the necked/drawn polymer are shown to be substantially different from the properties of the virgin polymer.