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A modified neo-Hookean model for semi-crystalline thermoplastics assessed by relaxation and zero-stress creep tests of recycled polypropylene and polyoxymethylene
Linnaeus University, Faculty of Technology, Department of Mechanical Engineering. IKEA, Sweden.ORCID iD: 0009-0009-7242-7613
Lindvall CAE, Sweden.
Linnaeus University, Faculty of Technology, Department of Mechanical Engineering.ORCID iD: 0000-0001-7373-5866
2024 (English)In: Mechanics of time-dependant materials, ISSN 1385-2000, E-ISSN 1573-2738, Vol. 28, p. 43-63Article in journal (Refereed) Published
Abstract [en]

The mechanical behavior of thermoplastics is strongly rate-dependent, and oftentimes it is difficult to find constitutive models that can accurately describe their behavior in the small to moderate strain regime. In this paper, a hyperelastic network model (modified neo-Hookean) and a set of experiments are presented. The testing consists of monotonic tensile loading as well as stress relaxation and zero stress creep. Two materials were tested, polyoxymethylene (POM) and recycled polypropylene (rPP), representing one more rigid and brittle and one softer and more ductile semi-crystalline polymer. The model contains two main novelties. The first novelty is that the stiffness is allowed to vary with the elastic deformation (in contrast to a standard neo-Hookean model). The second novelty is that the exponent governing viscous relaxation is allowed to vary with the viscous deformation. The basic features of the new model are illustrated, and the model was fitted to the experimental data. The model proved to be able to describe the experimental results well.

Place, publisher, year, edition, pages
Springer, 2024. Vol. 28, p. 43-63
Keywords [en]
Recycled polypropylene, Polyoxymethylene, Relaxation, Network model, Viscoelasticity, Viscoplasticity
National Category
Applied Mechanics
Research subject
Technology (byts ev till Engineering), Mechanical Engineering
Identifiers
URN: urn:nbn:se:lnu:diva-124637DOI: 10.1007/s11043-023-09631-xISI: 001058321300002Scopus ID: 2-s2.0-85169843576OAI: oai:DiVA.org:lnu-124637DiVA, id: diva2:1797775
Available from: 2023-09-15 Created: 2023-09-15 Last updated: 2024-11-07Bibliographically approved
In thesis
1. Experimental characterisation and constitutive modelling of thermoplastics under finite strain
Open this publication in new window or tab >>Experimental characterisation and constitutive modelling of thermoplastics under finite strain
2024 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

As part of the global environmental and sustainability agenda, the industry ispreparing for replacing the fossil-based thermoplastics with new, more sustainablealternatives.These novel materials need to be verified and tested for the intended product.Historically, product verification has been done through trial and error methods.As this is both time-consuming and costly, an increased demand for more accuratefinite element analyses (FEA) is seen.Thermoplastic materials are often exposed to large deformations, complexload scenarios, and long-term loading, resulting in substantial creep and/or stressrelaxation during their service life. The mechanical response of these materials iscomplicated and strongly rate-dependent.This thesis consists of two parts. In the first part, a detailed material charac-terization was divided into calibration and verification sets. The calibration setsinclude tensile and compression tests in monotonic loading, stress relaxation, andzero-stress creep. Three-point bending and quasi-static punch tests are used asverification sets. The fossil-based semi-crystalline copolymer Polyoxymethylene(POM) was chosen due to its stable and consistent performance to set the testmethodology for characterizing the mechanical behaviour of thermoplastics.The second part consisted of implementing a novel strain-dependent networkmodel and identifying potential limits and improvements. Three constitutivemodels were evaluated; two were built with the same basic theory, with the maindifference being the use of hypo- vs. hyper-elasticity, where the hyperelasticversion was developed as a part of this thesis. The third model is based onhyperelasticity but with an Eulerian formulation and with a single leg rather thana network model.The material characterization showed that it was able to catch the viscositybehaviour and identify flaws and limitations of the used constitutive models.Some gaps and limitations in the fundamental theory were identified whenpushed to their limits. For the two network models, the risk of unstable modelswhere found based on parameter selection. The two hyperelastic models showproblems in capturing the zero-stress creep behaviour. In the end, all modelspresented in this work performed better than current commercial models.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2024. p. 24
Series
Lnu Licentiate ; 47
Keywords
Thermoplastics, Material characterisation, Viscoelastic, Hyperelastic, Polyoxymethylene
National Category
Applied Mechanics
Research subject
Technology (byts ev till Engineering), Mechanical Engineering; Technology (byts ev till Engineering), Mechanical Engineering
Identifiers
urn:nbn:se:lnu:diva-133244 (URN)10.15626/LnuLic.47.2024 (DOI)9789180822237 (ISBN)9789180822244 (ISBN)
Presentation
2024-11-29, M1051, Hus M, Georg Lückligs väg 1, Växjö, 15:01 (English)
Opponent
Supervisors
Available from: 2024-11-07 Created: 2024-11-06 Last updated: 2024-11-07Bibliographically approved

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Stoltz, BjörnKroon, Martin

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