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SHEAR BEHAVIOR AND RELATED MECHANISMS IN MATERIALS PLASTICITY

Determination of Critical Strains in Isotactic Polypropylene by Cyclic Loading-Unloading

[+] Author and Article Information
Florian C. Spieckermann

Physics of Nanostructured Materials, Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Wien, Austriaflorian.spieckermann@univie.ac.at

Harald R. Wilhem

Laboratory of Polymer Engineering, LKT-TGM, Wexstrasse 19-23, 1200 Wien, Austria

Erhard Schafler, Michael J. Zehetbauer

Physics of Nanostructured Materials, Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Wien, Austria

Elias C. Alfantis

Laboratory of Mechanics and Materials, Polytechnic School, Aristotele University of Thessaloni, Thessaloniki 54124, Greece

J. Eng. Mater. Technol 131(1), 011109 (Dec 23, 2008) (5 pages) doi:10.1115/1.3030938 History: Received January 28, 2008; Revised June 17, 2008; Published December 23, 2008

By analyzing the deformation of α—isotactic polypropylene through cyclic uniaxial compression at different temperatures—conclusions are drawn on the contribution of the crystalline phase and the amorphous phase to the hardening curve. The deformation of the crystalline phase, which deforms mainly by simple shear of the crystallites, strongly depends on the properties of the amorphous phase. A separation of strain in a relaxing and a quasipermanent part, as introduced by the work of Hiss (1999, “Network Stretching, Slip Processes and Fragmentation of Crystallites During Uniaxial Drawing of Polyethylene and Related Copolymers  ,” Macromolecules, 32, pp. 4390–4403), is undertaken. By this experimental procedure it is possible to characterize the deformation dependence of several physical quantities such as Young’s modulus or the stored energy associated to each loading-unloading cycle. Furthermore specific transition strains, A, B, C, and D, can be determined where the recovery properties change. It is demonstrated that beyond point C the strain hardening can be described by the simple rubber hardening model of Haward (1987, “The Application of a Simplified Model for the Stress-Strain Curve of Polymers  ,” Polymer, 28, pp. 1485–1488).

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Cyclic compression tests of isotactic polypropylene at a constant true strain rate ϵ̇t=4×10−4 s−1. Deformation temperatures were −20°C, 20°C, and 50°C.

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Figure 2

Schematic representation of the division of the strain ϵt in a residual and a recoverable part

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Figure 3

Envelopes of the stress-strain curves of the cyclic compression tests for deformation temperatures −20°C, 20°C, and 50°C

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Figure 4

Residual and recoverable strains for each cycle as a function of the total true strain, for deformation temperatures −20°C, 20°C, and 50°C. The data points were omitted to increase clarity in this representation.

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Figure 5

Recoverable strains for each cycle as a function of the total true stress for the deformation temperatures −20°C, 20°C, and 50°C. The changes in the slope of this curve correspond to the critical strains.

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Figure 6

Recoverable strains related to the critical strains, B and C, as a function of the deformation temperature

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Figure 7

Haward–Thackray plot for deformation temperatures −20°C, 20°C, and 50°C. After the critical point C the hardening can be described by the Haward–Thackray model.

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Figure 8

Crystalline volume fractions for samples deformed up to different strains at T=20°C, as determined by DSC and WAXS. The lines are for guiding the eye.

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Figure 9

Lamella thickness of samples deformed up to different true strains at T=20°C measured by DSC

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Figure 10

Absorbed energy per cycle and unit volume as determined from cyclic loading-unloading experiments for deformation temperatures −20°C, 20°C, and 50°C

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Figure 11

Evolution of the elastic secant modulus for each cycle as determined from cyclic loading-unloading experiments for deformation temperatures −20°C, 20°C, and 50°C

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