Mechanical response of swollen elastomers under short and long-terms cyclic loading conditions / Loo Mei Sze

Conventional petroleum-derived fuels are no longer considered as futuristic sources of fuel because of their ever-increasing cost and continuous diminution in supply. These energy insecurity issues have motivated the government of Malaysia to explore intensively the prospect of biodiesel as a renewable energy alternative for future substitution of the current depleting energy resources. While the introduction of biodiesel such as palm biodiesel is proven to be environmentally friendly, the compatibility between biodiesel and elastomeric components remains as a major challenge up to this date. Indeed, it is found that the exposure of elastomeric components to biodiesel leads to a material degradation in the form of swelling which reduces their performance in service. Hence, the investigation and modeling of the mechanical responses of elastomers swollen by biodiesel are essential for durability analysis of the components. In this thesis, the mechanical responses of swollen elastomers under short-term and long-term cyclic loading conditions are addressed. The cyclic loading is considered as a short-term loading if the number of loading cycles is less then ten cycles. To this end, the thesis is divided into two parts. The first part of the thesis focuses on the mechanical response under short-term cyclic loading. For this purpose, both dry and swollen elastomeric specimens are subjected to various modes of short-term cyclic loading. The swollen elastomeric specimens are obtained by immersing initially dry specimens in solvents (palm biodiesel and conventional diesel fuel) in the absence of mechanical stress until various desired degrees of swelling are obtained (stress-free swelling). It is found that swollen elastomers show inelastic responses such as stress-softening due to Mullins effect, hysteresis and permanent set which decrease as the degree of swelling increases. Moreover, a first attempt toward the modeling of the Mullins effect taking into consideration swelling is proposed by extending phenomenologically the well-known Continuum Damage Mechanics model and Pseudo-Elastic model. The second part of the thesis deals with the mechanical response of swollen elastomers under long-term cyclic (fatigue) loading. For this purpose, the fatigue tests are conducted at various maximum strains and at constant zero strain ratio. The physical mechanism of fatigue damage is studied by observing the specimen fracture surfaces through FESEM analysis coupled with EDS. Two definitions of end-of-life are adopted: (i) The number of cycles required for a crack to reach 1 mm in length and (ii) The number of cycles required for a complete rupture to occur in the specimens. The Wöhler curves are plotted using the maximum principal stretch as the predictor. It is found that the swollen elastomers have a shorter lifetime compared to the dry ones. More precisely, the elastomers swollen by biodiesel appears to have the shortest lifetime. Moreover, FESEM results revealed that the solvent type and swelling level has no effect on the morphology of crack nucleation and propagation, regardless of the loading condition