The Creep Behaviour of Rubbers Reinforced with Particulate or Short Fibres
Elastic stress-strain testing and (primary and secondary) creep testing have been carried out on unfilled polyurethane rubber and two rubber composites filled with 10 vol% and 20 vol% alumina particulates. They have been run at three different temperatures (20°C, 45°C and 65°C) for all three materials. The stress-strain curves of the rubber conformed well to the expectation of the conventional rubber elasticity theory. Furthermore, the true stress-strain plots of those three materials exhibited an approximately linear relationship with stiffness increasing by adding the particulates: the relationships conformed to the composite elastic theory. In order to capture the creep historical strain of these materials with an uncomplicated method, a Miller-Norton empirical formulation, which is conventionally used for predicting the creep behaviour of metal materials, is adapted in this work. The creep behaviour of all three materials could be captured well by using the Miller-Norton constitutive law and the observed dependence on temperature has been used to estimate the activation energy for creep to be ~ 7 kJ mole-1. This showed that the creep process did not involve rupture of covalent bonds (for the range of applied loads used) but was associated with physical processes such as molecular untangling. The fillers enhanced the creep resistance, to a degree that was quantitatively consistent with the expected load transfer between matrix and particulate. After these results, the particulates in polyurethane rubber were replaced by different amounts (5 vol% and 10 vol%) of alumina short fibres. The tensile stress-strain testing and the fixed load creep testing of these fibrous reinforced composite rubbers were done at the same temperatures (20°C, 45°C and 65°C). The samples were made via a blending and extrusion process that produced a degree of fibre alignment along the axial loading direction. Prior milling procedures were used to produce fibres with two different average aspect ratios, which were estimated to be about 10 and 16. True stress-strain relationship from these fibrous reinforced composite rubbers also exhibited approximately linear plots. The dependence of stiffness on the volume fraction and the iii fibre aspect ratio conformed well to the predictions from the Eshelby model. Also, the creep behaviour of all the materials could be captured well by using the Miller-Norton constitutive law with the average matrix stress predicted from the Eshelby model. Thus, it could be seen short fibres were much more effective for reducing creep rate than particulates. To check the agreement of another fibrous reinforced composite rubber with the Eshelby model and the Miller-Norton constitutive law, tensile stress-strain testing and creep testing have also been carried out on silicone rubber, Dow Corning SE 1700, with and without short fibrous alumina reinforcement at two temperatures (20°C and 65°C). It was found that silicone rubber alone is stiffer and shows greater creep resistance than polyurethane rubber alone. Furthermore, when silicone rubber was substituted for polyurethane rubber as the matrix in fibrous composite, its greater stiffness significantly reduced creep strain.
creep, rubber, reinforced particulate or short fibres in rubber
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