The Mechanics of Equibiaxial Hydrostatic Deformation in Solid State: Isotactic Polypropylene

Ravtf Saraf, Roger S. Porter

Research output: Contribution to journalArticlepeer-review

24 Scopus citations


Equibiaxial, hydrostatic deformation of isotactic polypropylene (i-PP) of an intermediate weight average molecular weight (2.9 x 105) has been achieved by uniaxial compression between two axially aligned circular cylinders at the desired draw temperature {TDF£), Independent measurements of load (i.e., stress) and displacement (i.e., strain) are made during the course of deformation. A systematic variation in TDR(from 30—130°C) at a ram speed of 0.25 in./min. has been studied. The stress—strain data corrected for machine compliance has been compared to a theoretical model. This model assumes a rigid-plastic behavior with a hydrostatic pressure effect. The shear stress at the wall, caused by metal—polymer contact, is coulombic friction. As the compressive force (and, hence the hydrostatic pressure) is increased, the frictional shear stress (in the central portion of the compressed sample) exceeds the maximum shear stress (also changing with hydrostatic pressure), causing the i-PP to yield on the surface (referred to as “stiction”). The friction to “stiction” transition is continuous on the stress—strain curve, but is explicitly observed as a “skin” under an optical microscope with crossed polars. The model quantifies the onset of “stiction” and gives a threshold friction coefficient (depending on material properties) below which the polymer will not shear at the wall! The static friction coefficient between metal and polymer is the only empirical parameter to fit the stress—strain data (within 5%) up to compression ratios >700%. A quantitative measurement of the Bauschinger effect is obtained by simultaneous measurement of yield stress under compression and tension in a single experiment.

Original languageEnglish (US)
Pages (from-to)59-94
Number of pages36
JournalJournal of Rheology
Issue number1
StatePublished - Jan 1987
Externally publishedYes

ASJC Scopus subject areas

  • General Materials Science
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering


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