Document Type : Original research

Authors

• Bahereh T. Marouf ¹
• Rasoul Lesan Khosh ²
• Reza Bagheri ¹
• Yiu-Wing Mai ³

¹ Polymeric Materials Research Group, Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran

² Bashpar Pishrafteh Sharif, Tehran, Iran

³ Center for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia

Abstract

Understanding the yielding of semicrystalline polymers such as isotactic polypropylene (iPP) remains challenging due to morphology-dependent deformation mechanisms and the sensitivity of yield stress to temperature, strain rate, and loading mode. Here, yield stress is measured across macro- (tensile, compression) and micro-scales (nanoindentation) over a range of temperatures and strain rates. Nanoindentation-derived yield stresses obtained using the expanding cavity model agree closely with compression measurements, confirming the dominance of compressive fields, while Tabor-derived values correlate with tensile maximum stress. To rationalize the observed temperature and rate dependences, three theoretical frameworks—Eyring’s model, crystal plasticity, and the lamellar cluster model—are comparatively evaluated. The Eyring model captures the logarithmic strain-rate sensitivity and thermal softening but lacks structural specificity; the crystal plasticity model provides a slip-based interpretation with improved agreement at elevated temperatures; and the lamellar cluster model differentiates deformation modes, accounting for the convergence of compression and indentation yields. The combined experimental–modeling analysis demonstrates the utility of nanoindentation for localized yield assessment and highlights the model-dependent nature of structural interpretation in semicrystalline polymers.

Keywords

• Nanoindentation
• Polypropylene
• Yield stress
• Strain rate
• Thermo-mechanical modeling

Main Subjects

• Characterization