Characterisation and modelling of transversely isotropic flexible viscoelastic foam

TA Engineering (General). Civil engineering (General);TJ Mechanical engineering and machinery

Jebur, Qusai Hatem ; Harrison, Philip

University:University of Glasgow

关键词: flexible foam, transversely isotropic, viscoelastic, microstructure offoam, closed-cell foam;   

Others  :  http://theses.gla.ac.uk/4497/1/2013JeburPhD.pdf

来源: University of Glasgow

【 摘 要 】

Polymer foams have unique properties covering a wide range of mechanical and physical behaviours. This makes them important engineering materials that can be used in numerous applications including packaging, impact energy absorption, cushioning and spacers in sandwich construction. In general, the mechanical response of foam materials is not isotropic, though for most design purposes they are treated as such. The validity of this assumption depends on the degree of anisotropy of the foam and in some cases may be questionable. Most polymeric foams tend to display at least some degree of anisotropy that can usually be related back to their manufacture process. The aim of this investigation is to look at the extent of mechanical anisotropy in commonly used polymer foams and explore options currently available in modelling such materials, using established analytical theories and constitutive models implemented in a commercially available finite element code as a starting point. Attempts are made to relate the transversely anisotropic macro-scale response to the microstructure within the material.A melt-extruded closed-cell Low-Density Polyethylene (LDPE) foam has been chosen as a representative foam material with which to explore the topic.Uniaxial compression, simple shear and relaxation tests have been used to characterise the material’s response. Two techniques have been used to determine the Poisson’s ratio of foam under uniaxial compression. As expected, results reveal the LDPE foam to be a strongly transversely isotropic material that is both viscoelastic and highly compressible. The stiffness and strength of the foam are almost three times higher in the extrusion (or principle) direction used to manufacture the foam, when compared with the properties in the transverse directions. It is also noted that the foam’s mechanical behaviour depends on the specimen size.For larger specimens measuring 80x80x80mm3, the modulus at small strain and the yield stress, are approximately twice that of smaller specimens measuring 10x10x10mm3. The compressive behaviour of the LDPE foam is also rate dependent. The yield stress of foam increases approximately linearly with the natural logarithm of the compression strain rate. The Poisson’s ratio values decreases with increasing compression strain rates. While the energy absorption efficiency of foam increases with strain rate. Micro-CT and optical microscopy has been performed to determine the average microstructural cell shape and dimensions within the LDPE foam. Results indicate an average cell geometry that is elongated in the foaming direction by about 20% compared to the transverse direction. A combined analytical and numerical modelling strategy has been employed to provide a better understanding of the relationship between microstructure and macroscopic behaviour. Combined use of analytical and numerical modelling shows that it is possible to give a good prediction of the foam’s macroscopic response based on an understanding of the inherent anisotropy within the foam’s microstructure.