Okereke, Michael ORCID: https://orcid.org/0000-0002-2104-012X and Akpoyomare, Ambrose I. (2017) A constitutive model for semi-crystalline polymers: A multiple viscoelastic relaxation processes implementation. In: COMPLAS2017: XIV International Conference on Computational Plasticity: Fundamentals and Principles, 5-7 Sept, 2017, Barcelona, Spain. (Unpublished)

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Abstract

The constitutive modelling of semi-crystalline polymers seeks to obtain reliable predictive tools for a wide range of their mechanical responses. Such efforts have continued to occupy computational material scientists. Although significant advances have been made regarding amorphous polymers, thanks to works by Buckley [1], Boyce [2], and Govaert [3], there is significant research scope for developing similar predictive modelling fidelity for semi-crystalline polymers. The presence of crystalline and amorphous phases in semi-crystalline polymers presents interesting constitutive modelling challenges.

In this study, a physically based, three-dimensional constitutive model has been developed for simulating a wide range of features observed in deformation and processing of semi-crystalline polymers. The constitutive mathematics is based on a one-process Grass-Rubber model for amorphous polymers proposed by Buckley and colleagues [1]. The model philosophy exploits the presence of multiple relaxation processes associated with the different mechanics of the crystalline, amorphous and pseudo-amorphous parts of the polymer. The model development reasoning is inspired by a well-known physical framework of rate-dependent deformation that establishes a correlation between the observed transition in flow stress of a material and the secondary β-transition of the viscoelastic behaviour. Here, two dominant relaxation processes were identified - the α- and the β-processes. Each process was modelled using the bond-stretching and conformational stresses constitutive mathematics of the Glass-Rubber model.

The model has been implemented numerically into a commercial finite element code through a user-defined material subroutine (UMAT) and validated against compression test results carried out on an isotactic polypropylene across an unusually wide range of strain rates [4]. In this study, the model predicts quite well the experimentally observed nonlinear mechanical responses like: temperature and rate-dependence, adiabatic heating effects, structural rejuvenation and post-yield de-ageing of polypropylene. It provides a viable modelling tool that can be utilized for design involving semicrystalline polymers at room temperature as well as exploring the processing response at elevated temperatures.

Item Type:	Conference or Conference Paper (Paper)
Uncontrolled Keywords:	Polypropylene; Semicrystalline polymers; Constitutive modelling; Mechanical relaxations; Two-process model
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Faculty / School / Research Centre / Research Group:	Faculty of Engineering & Science Faculty of Engineering & Science > School of Engineering (ENG)
Last Modified:	05 Oct 2017 11:38
URI:	http://gala.gre.ac.uk/id/eprint/17705

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