Mechanics of Time-Dependent Materials

, Volume 16, Issue 4, pp 361–379 | Cite as

Compression of polypropylene across a wide range of strain rates

  • M. I. OkerekeEmail author
  • C. P. Buckley
  • C. R. Siviour


Three grades of polypropylene were tested in uniaxial compression at room temperature, across a wide range of strain rate: 10−4 s−1 to 104 s−1. One grade is a conventional polypropylene homopolymer. The two other grades are the polypropylene forming the matrix phase of a continuous glass fibre-reinforced thermoplastic composite prepreg, with and without blending with a carbon-black master batch. Tests at the highest strain rates were performed using a compression split Hopkinson pressure bar. The test specimens, for all the three rates, were imaged using appropriate digital cameras in order to observe the deformation process. In addition, the images obtained were analysed digitally to obtain true strain measurements for the medium rates category. All three grades of polypropylene showed pronounced strain-rate dependence of compressive yield stress, increasing by factors of up to 4 across the range of rates. At the lowest rates, there was close agreement between the yield stresses for all three materials, and also close agreement with the Eyring theory. Considering the highest strain rates, however, yield stresses increased more rapidly with log(strain-rate) than would be expected from a linear Eyring prediction and values for the three materials diverged. This was attributed to the contributions made in each material by both alpha and beta relaxation processes. Also prominent in the medium- and high-rate experimental results was pronounced post-yield strain softening, greatest at the highest strain-rates. This resulted from a combination of thermal softening from adiabatic heating, and structural rejuvenation as often seen in glassy polymers in quasi-static tests.


High-rate compression Polypropylene Strain-rate dependence 



The authors are grateful to Cristophe Bourban of Gurit Suprem, Switzerland, for providing the PLW and PLB grades of polypropylene used in this study. The authors also appreciate the help of colleagues—Jens Wiegand, Nik Petrinic, Robert Gerlach—in analysing SHPB results as well as the help of Neil Warland, Richard Froud and Richard Duffin in preparing test specimens and setting up the camera. The Cordin 550 high-speed camera was provided by the EPSRC Engineering Instrument Pool, we are particularly grateful to A. Walker for his advice and support while using this camera.


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Copyright information

© Springer Science+Business Media, B. V. 2012

Authors and Affiliations

  • M. I. Okereke
    • 1
    • 2
    Email author
  • C. P. Buckley
    • 1
  • C. R. Siviour
    • 1
  1. 1.Department of Engineering ScienceUniversity of OxfordOxfordUK
  2. 2.Department of Engineering SystemsUniversity of GreenwichChatham MaritimeUK

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