Abstract
The effect of pressure on the yield behavior of polymeric solids was investigated at room temperature. Tests were carried out on samples of six thermoplastics; polycarbonate, polyvinyl chloride, polypropylene, polyethylene, nylon 6.6, and ABS plastics.
An apparatus has been designed and constructed which allows the determination of the yield stress of a polymer specimen in thin-walled cylinder under internal pressure, while the sample is simultaneously subject to a hydrostatic external pressure. As the external pressure increases, the normal stress applied to the plane of the maximum shear stress in the thin-walled cylinder subjected to internal pressure changes from tension to compression.
In the state of hydrostatic compression under high external pressure, the pressure dependence of yield stress fitted in well with Coulomb yield criterion. In the state of hydrostatic tension under low external pressure, the yield surface (the Mohr envelope for yielding) could not be explained by the Coulomb yield criterion.
Furthermore, the deformation bands produced under hydrostatic pressure in polycarbonate have been examined by measuring the angles of bands after the load was removed. These bands were found to be inclined at 50° to the direction of the maximum principal stress. The angle of 50° was independent of hydrostatic pressure.
