Deterioration of the parameters measuring the fatigue of rubber compounds as a result of thermal aging was investigated. The energy to break, tear strength, fatigue life, and fatigue crack propagation rate of EPDM rubber compounds reinforced with three different types of carbon black before and after different periods of thermal aging were measured and compared through a series of static and cyclic loading tests. The experimental results indicated that the fatigue resistance of EPDM rubber compounds with different types of carbon black was consistently reduced because of changes in the molecular structure and crosslink density as a result of thermal aging. Meanwhile, the intrinsic flaws of carbon black-reinforced EPDM rubber compounds after 6 months of thermal aging were smaller, regardless of the type of carbon black. Therefore, the effects of thermal aging on the fatigue of rubber compounds are significant and should be taken into account in designing rubber components.

The dynamic properties of rubber bearings frequently used as isolators in structures could be significantly deteriorated because of the change of microstructure in rubber caused by cyclic compression and thermal aging. As a result, a catastrophic failure of bridges and buildings unexpectedly occurs when they are subjected to earthquake attack. Here, the dynamic properties of neoprene rubber bearings before and after different cycles of compressive loading or various periods of thermal aging were first measured and compared to each other. On the basis of the experimental results, the effects of cyclic compression and thermal aging on the stiffness, energy absorption, and equivalent viscous damping coefficient of neoprene rubber bearings are investigated. It is found that the deterioration of dynamic properties of neoprene rubber bearings caused by either cyclic compression or by thermal aging is significant and should be taken into account in designing rubber bearings.

Rubbers deteriorate when they are exposed to ultraviolet irradiation for long periods of time. By conducting a series of hardness measurements and simple tension tests, the static properties of neoprene rubbers before and after exposure to various durations of ultraviolet irradiation were first measured. It is found that the Shore A hardness and tensile modulus of neoprene rubbers after exposure to ultraviolet irradiation are increased but their elongation at break, tensile strength, and energy to break are significantly decreased. On the basis of a complex spring model of a vibration system, the dynamic shear properties of neoprene rubbers before and after exposure to different durations of ultraviolet irradiation were then determined from the experimental results of dynamic transmissibility tests. It is also found that the storage modulus, loss modulus, and loss factor of neoprene rubbers are drastically affected by the duration of ultraviolet irradiation they experienced.

ABSTRACT:The fatigue failure mechanism of rubber bearings under cyclic compression is important in evaluating their fatigue lives and thus is analyzed theoretically and numerically here. At first, the stress distributions in a bonded rubber cylinder derived from three different existing models were utilized to calculate the cracking energy densities within it. Next, the location of fatigue crack initiation and the direction of subsequent crack propagation in circular rubber bearings were consecutively determined. Furthermore, finite element numerical results were compared to those obtained theoretically from the three models to check their validity in predicting the fatigue crack initiation and propagation in circular rubber bearings. Based on the quasi-statically theoretical and numerical results, it is found that the fatigue cracks initiate first at the outermost boundary between rubber and steel plates and propagate later inwards to the center of circular rubber bearings. The corresponding fatigue failure mechanism obtained theoretically and numerically is consistent with experimental findings reported previously.

The fatigue life of circular rubber bearings under cyclic compression is theoretically and numerically analyzed based on a previously proposed fatigue failure mechanism. The energy release rate at any point in circular rubber bearings under cyclic compression, which depends on the cracking energy density and crack length along the predicted crack propagation path, is derived first theoretically. Then, the corresponding fatigue crack growth rate and fatigue life are determined numerically by introducing the fatigue parameters of three different rubber compounds before and after suffering from thermal aging. Meanwhile, the effects of intrinsic flaw size and maximum compressive stress on the fatigue life of circular rubber bearings are also investigated. It is found that the enlargement in the Regime 1 range of the crack growth rate of rubber increases the fatigue resistance of circular rubber bearings. Therefore, the effects of the mechanical properties, intrinsic flaw size, threshold value, and maximum cyclic compressive stress on fatigue life are significant and should be taken into account in designing rubber bearings.