High resolution multi-axial strain measurement for predicting mechanical response of semi-crystalline polymeric materials

Akhigbe, Eromosele Odigie (2018) High resolution multi-axial strain measurement for predicting mechanical response of semi-crystalline polymeric materials. Masters by Research thesis (MPhil), Manchester Metropolitan University.

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Abstract

Semi-crystalline polymeric materials such as High-Density Polyethylene (HDPE) are generally assumed to have a constant Poisson’s ratio of 0.4. A study that was previously carried out on HDPE polymer by Bhabha in 2015 indicated that the Poisson's ratio may be non-uniform within the elastic region. This research establishes the variation in the Poisson’s ratio of HDPE polymer below 1% strain, however, the Poisson’s ratio stabilised as the strain increases within the elastic region. Poisson’s ratio is a function of strain, both in the transverse and longitudinal direction of the material. Strain is commonly measured using external transducers but can be limited by the difficulty in attaching strain gauge to the HDPE surface and can also alter the measured strain over the attached area if the stiffness of the material is significantly less than the stiffness of the strain gauge. Strain can also be measured using mechanically attached extensometers, which tend to indent soft materials thereby causing local stress within the area of contact. Due to these limitations, two non-contact methods were used in the research. A non-contact optical technique known as Digital Image Correlation (DIC) was employed to investigate the strain and generate strain maps of the surface of HDPE and General-Purpose Polystyrene (GPPS) materials. The strain obtained was then used to calculate the Poisson’s ratio for both materials. The findings of the DIC technique was that the Poisson’s ratio was non-uniform within the nominally elastic range of HDPE and GPPS polymer. The DIC technique is a full-field technique and the results showed non-uniformity in Poisson’s ratio at 1% strain, however, as the strain increases, the Poisson’s ratio stabilises to a constant value. From the DIC results obtained, there is no direct comparison between the behaviour of HDPE and GPPS on the surface. The hypothesis is that the non-uniformity in strain and hence the Poisson’s ratio obtained from the DIC techniques could be related to local relief of stresses occurs when crazes form, which could lead to the modulations in lateral and longitudinal strain in the case of GPPS material. Whilst in HDPE, there are localised movements of the crystal lamellae. Also, a likely explanation could be that at the strain range investigated could only cause significant deformation in the amorphous regions whilst the crystalline regions are unaffected. A further investigation on the internal composition will be required to further understand the results obtained on the surface of the specimens.