A study of the rapid deformation behaviour of a range of polymers

Abstract

Polymers are increasingly being used in applications where they are rapidly deformed. However, compared with metals, relatively few studies of their mechanical properties at high rates of strain have been published. This paper describes an investigation of the rapid deformation behaviour in compression of a number of widely used polymeric materials. The necessity of properly characterizing polymers is discussed, as the variation of commercial grades bearing the same name is considerable, and furthermore these materials are much more susceptible to change during storage than say metals. The importance of thermal properties to rapid, and hence adiabatic, deformation is pointed out, and tables of such properties are presented. Extensive use was made of high-speed photography (interframe time 7 (is) to study qualitatively the behaviour of solid discs of polymers at strain rates of 2.5 x 10 3 s -1 . The framing speed was sufficiently fast to capture fracture initiation and subsequent failure of all the polymers studied, including polycarbonate (PC), which fails in an almost explosive manner. The darkening of heat-sensitive film in contact with deforming discs was also investigated. Quantitatively, this technique was used to check the applicability of Avitzur’s analysis (Avitzur ( Israel J. Technol . 2, 295-304 (1964)) of a deforming annulus to polymers. Agreement was found to be good and hence friction could be measured during deformation at high rates of strain for the first time. Studies were also carried out to determine the best lubricant for rapid compressive testing. Petroleum jelly was found to reduce the friction closest to zero. An optically identical system was set up in an Instron mechanical testing machine both to perform friction studies and to explore deviation from incompressible behaviour. Agreement with Avitzur’s analysis was found to be poorer, and no lubricant was found to reduce friction below about 3-4 %. PC, with a very high value of frictional stress, showed evidence of a change in volume. Allowances were made for the elastic indentation of the anvils. Higher strain rates were achieved by using an instrumented drop-weight machine and a direct impact Kolsky bar, both developed in this laboratory. Care was taken to eliminate sources of error, including friction and calibration errors. The strain rate sensitivity of the polymers ranged from 5—15 MPa per decade of strain rate. However, most showed some softening as the strain rate was raised from 10 3 to 10 4 s -1 , the exceptions being polybutylene teraphthalate (PBT) and polyvinylidene difluoride (PVDF).