Mechanical properties of the polypropylene copolymer measured by the impression method

Abstract

An impression test using a flat indenter with three different diameters 0.5, 1, and 2 mm is proposed to study the plastic flow of a polypropylene copolymer. By comparing the flow characteristics of the impression test with the uniaxial tension and compression tests, it is found that a yield drop occurs in the compression, and geometrical necking appears in tension, but no load drop and only strain hardening is found in the impression tests. Furthermore, the plastic behaviors of the impression before the yield are also similar to those of the compression test because there is a correlation of the stress‐strain curve between impression and compression by only adjusting the scaling factors. The plastic zone size underneath the indenter is only slightly larger than the indenter diameter and, as the load is applied, the mean pressure across the face of the indenter is smaller than that of the metals due to the large intermolecular distance in the polypropylene copolymer. The impression test is also used to measure the strength, elastic modules, strain‐hardening exponent, and strain‐rate sensitivity in a manner of shallow penetration and miniature deformation, which indicates that the impression test can be employed to study the mechanical properties of the thin‐film polypropylene copolymer. © 1996 John Wiley & Sons, Inc.