Swelling of elastomers in biodiesel and the resulting mechanical response under cyclic loading / Chai Ai Bao

Chai, Ai Bao (2013) Swelling of elastomers in biodiesel and the resulting mechanical response under cyclic loading / Chai Ai Bao. PhD thesis, University of Malaya.

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Abstract

The environmental and economic concerns have raised the popularity of biodiesel as the replacement for the conventional fuel. However, the incompatibility of engineering rubber components with biodiesel affects significantly the performance of the components. Majority of the compatibility studies focus on evaluating the degradation of mechanical properties in the rubbers due to contamination of different types of biodiesel. Nevertheless, the resulting mechanical responses of swollen rubbers, in particular under cyclic and fatigue loading conditions, are rarely investigated. In engineering applications where elastomeric components are concurrently subjected to fluctuating mechanical loading and exposure to aggressive liquids such as biodiesel, it is crucial to investigate and to model the mechanical responses of these components for durability analysis. The first part of this research involves experimental works to investigate the effect of swelling, due to biodiesel diffusion in the elastomers, on the macroscopic mechanical response of elastomers under cyclic compressive loading. First of all, simple immersion tests on stress-free elastomeric specimens were conducted and the resulting mechanical responses were evaluated. The focus of this work is on the effect of swelling on the inelastic responses classically observed in elastomers under cyclic loading conditions, i.e. stress-softening due to Mullins effect, hysteresis and stress relaxation. The results show that inelastic responses decrease significantly when the degree of swelling increases. Secondly, swelling tests on uniaxially-stressed specimens were conducted. For this purpose, an original compression device was developed to investigate the effect of uniaxial mechanical loading on the swelling of rubber in solvent. The apparatus comprises of four stainless steel plates and spacer bars in between which are specifically designed such that compression can be introduced on the rubber specimens while they are simultaneously immersed into biodiesel. Thereby allowing coupled liquid diffusion and large strain to take place. Different pre-compressive strains and biodiesel blends were considered. At the end of each immersion period, the resulting swelling and mechanical responses of rubber specimens under cyclic loading conditions were investigated. Special attention is given to the stress-softening phenomenon. The second part of this work can be regarded as a first step towards modeling of stress-softening in swollen rubber. For this purpose, the pseudo-elastic model and the two-phase model were considered and extended in order to account for the degree of swelling. Results show that the proposed models were qualitatively in good agreement with experimental observations.

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