Micro-mechanical Modelling of Polymeric Closed-cell Foams: Effect of Microstructural Variability on the Mechanical Properties

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dc.contributor.advisor Das, R en
dc.contributor.advisor Battley, M en
dc.contributor.author Chen, Youming en
dc.date.accessioned 2017-04-27T21:47:03Z en
dc.date.issued 2016 en
dc.identifier.uri http://hdl.handle.net/2292/32692 en
dc.description.abstract Polymeric closed-cell foams are increasingly used as the cores of sandwich structures in marine, railway and wind power industries due to their lightweight, good mechanical properties, high impact-absorption capacity, moist resistant, excellent acoustic isolation, and low thermal conductivity. The mechanical behaviour of foam cores is critical for the integrity of sandwich structures. Current polymeric closed-cell foams exhibit widespread microstructural variability, which has a considerable effect on the mechanical properties of the foams. This research aims to characterise the microstructural variability in polymeric closed-cell foams and investigate how the microstructural variability affects the macroscopic mechanical behaviour of the closed-cell foams. The research involves micro-mechanical modelling using the finite element method, imaging techniques and statistical analysis for realistic foam microstructure characterisation, and numerical and experimental testings for prediction and validation of foam mechanical properties. Optical microscopy and X-ray computed tomography were utilized to image the microstructures of several foams (M80, M130, C70.75 and C70.130). From the captured images, a number of major foam parameters were characterised using image processing and analysis techniques. Finite element micro-models based on the CT images, Laguerre tessellation models integrating various levels of variations in cell size and wall thickness, the Kelvin and Weaire-Phelan models were developed to study the effects of these foam parameters on the global compressive and shear responses of closed-cell foams. Along with with these numerical foam models, irregular honeycomb specimens were manufactured using 3D printing and experimentally tested under compression in order to explore the deformation and failure mechanisms in foams. Cells in the polymeric foams studied are fairly isotropic, and cell walls are mostly straight. In M130 foam, the relative density of the top section is around 14.14% lower than those of the middle and bottom sections, which leads to lower stiffness and strength of the top section. In M80 foams, relative density, cell size and cell wall thickness varies along the thickness of foam panel direction. In C70.75 and C70.130 foams relative density is reasonably homogenous. However, these two foams have secondary pores imbedded in cell walls. Measured cell size approximately follows a normal distribution and cell wall thickness nearly follows a lognormal distribution. Cell wall thickness shows more scatter than cell size. The stiffness and strength predicted by the image-based models are in good agreement with experimental values. The shear stiffness, and compressive and shear strengths predicted by the Laguerre tessellation models integrating the relative density, cell size and cell wall thickness distributions measured from M130 foam are around 20% lower than those from datasheet. Based on Laguerre tessellation models, the compressive and shear stiffnesses and strengths of closed-cell foams are found to decrease with increasing cell size and cell wall thickness variations. For a given level of variation, the effect of cell size variation on the compressive and shear stiffnesses and strengths of closed-cell foams is comparable to that of cell wall thickness variation on them. Strength is more sensitive to cell size and cell wall thickness variations compared to stiffness, and compressive strength is more sensitive to cell size and cell wall thickness variations than shear strength. The combined effect of cell size and cell wall thickness variations on stiffness is approximately equal to the sum of the individual effect, but the combined effect on strength is less than the sum of the individual effect. In M130 foam cell wall thickness variation is the main factor that causes the strengths of the foam to deviate from its potential ideal values. For an isotropic closed-cell foam, the Weaire-Phelan structure is the one that provides the highest compressive stiffness and strength of all the structures studied to date. However, the shear strength of closed-cell foams would not vary much with cell shape. The highest shear strength is achieved when the foams have uniform cell size and cell wall thickness. During compressive tests on closed-cell foams, large and thin cell walls that form a small angle with the compressive loading direction buckle elastically during the elastic regime of compression, leading to a reduction in the global stiffness of the foams. Plasticity sets in the buckled cell walls soon, further diminishing their load-carrying capacity and causing more cell walls to buckle and yield. When a plastic zone expands nearly across foam specimens, the global stress reaches its maximum value, followed by cell collapse in the plastic zone. During shear tests, vertical cell walls that are nearly parallel to the shear loading direction are mainly subjected to in-plane shear and buckle during the initial elastic regime of shear. Shear buckling is less detrimental to the load-bearing capacity of cell walls than compressive buckling. Therefore, the global stiffness degrades less rapidly in shear tests compared to compressive tests. Plastic deformation initiates from the buckled cell walls and further reduces the global stiffness of the foam. The stiffness of closed-cell foams nearly linearly depends on the Young’s modulus of foam base materials, and the strength of closed-cell foams nearly linearly depends on the yield strength of foam base materials. The Young’s modulus of foam base materials has only a minor effect on foam strengths. The yield elongation of foams is closely related to that of foam base materials, which implies that foam ductility is mainly inherited from foam base materials. Compressive tests with the irregular honeycomb specimens show that large and thin cell walls are weak spots where failure initiates. However, the susceptibility of a cell wall to failure is also determined by its neighbouring cell walls. It is more reasonable to consider weak cells as a region consisting of a number of cells, rather than an individual cell or cell wall. The relationships of foam mechanical properties with the properties of foam base materials and foam microstructures established in this research provides useful information for manufacturers to improve the mechanical properties of structural foams. The deformation and failure mechanisms obtained in this work could assist in developing constitutive relations and failure envelops for closed-cell foams. The Laguerre tessellation models proposed are capable of predicting the response of closed-cell foams with reasonable accuracy and computational efficiency, which could be used as a predictive tool and micro-scale model for multi-scaling modelling of foams in the future. en
dc.description.sponsorship moved, 6/11/17, fz en
dc.publisher ResearchSpace@Auckland en
dc.relation.ispartof PhD Thesis - University of Auckland en
dc.relation.isreferencedby UoA99264938014102091 en
dc.rights Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher. en
dc.rights.uri https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm en
dc.rights.uri http://creativecommons.org/licenses/by-nc-sa/3.0/nz/ en
dc.title Micro-mechanical Modelling of Polymeric Closed-cell Foams: Effect of Microstructural Variability on the Mechanical Properties en
dc.type Thesis en
thesis.degree.discipline Mechanical Engineering en
thesis.degree.grantor The University of Auckland en
thesis.degree.level Doctoral en
thesis.degree.name PhD en
dc.rights.holder Copyright: The author en
pubs.elements-id 623647 en
pubs.record-created-at-source-date 2017-04-28 en

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