Mullins effect in polyethylene and its dependency on crystal content: A network alteration model

Area of Science:

• Polymer Science
• Materials Science
• Mechanical Engineering

Background:

• The Mullins effect, a phenomenon of stress-softening in elastomers, is critical for understanding polymer behavior under cyclic loading.
• Polyethylene's mechanical response, particularly its Mullins effect, is influenced by its crystalline structure, which varies significantly with density.
• Existing models often struggle to capture the complex, strain-dependent, and crystal-sensitive nature of the Mullins effect in polyethylene.

Purpose of the Study:

• To investigate the influence of varying crystal content on the Mullins effect in different types of polyethylene (ultra-low, low, and high-density).
• To develop and validate a unified constitutive model that accurately predicts polyethylene's viscohyperelastic-viscoelastic-viscoplastic response over a large strain range, incorporating crystal dependency.
• To elucidate the underlying deformation mechanisms responsible for the Mullins effect in polyethylene, guided by the proposed model.

Main Methods:

• Experimental cyclic stretching of polyethylene samples with crystal contents ranging from 0.15 to 0.72 over a large strain range.
• Development of a unified viscohyperelastic-viscoelastic-viscoplastic constitutive model decomposing macro-scale response into intermolecular and network contributions.
• Micromechanical treatment using volume fraction concept to consider inelastic deformations in amorphous and crystalline phases, incorporating experimentally-based, crystal-dependent material kinetics.

Main Results:

• Experimental observations confirmed that stress-softening, hysteresis, and residual strain in polyethylene under cyclic stretching are dependent on both crystal content and applied strain.
• The proposed constitutive model successfully captured the Mullins effect in polyethylene across different densities and large strain ranges.
• The model's decomposition into viscoelastic-viscoplastic intermolecular and viscohyperelastic network parts, along with micromechanical considerations, accurately reproduced experimental data.

Conclusions:

• Crystal content significantly dictates the magnitude of the Mullins effect in polyethylene, influencing stress-softening, hysteresis, and residual strain.
• The developed unified constitutive model provides an accurate and comprehensive framework for predicting polyethylene's large-strain mechanical behavior, including the Mullins effect.
• The study offers insights into the inherent deformation mechanisms governing the Mullins effect in polyethylene, validated by the predictive power of the constitutive model.