Investigation On Fatigue Life And Microstructure Of Rubber Material Under Multiaxial Loading

Rubbers are extensively used in many applications because of their large reversible elastic deformation, excellent damping and energy absorption characteristics. Typical applications include engine mounts and tires for automobiles, vibration isolators, seals, hoses, belts, structural bearings, etc. Since these applications impose large static and time varying strains, durability and mechanical properties are often the primary consideration. It is pivotally important to investigate the fatigue characteristics and mechanism of rubber. In this dissertation, mechanical behaviors properties and microstructure of two formulations filled vulcanized natural rubbers are investigated comparatively.Mechanical behavior properties experiments and theoretical analysis results show that material A has higher elasticity and better adhesion ability between filler and matrix of rubber comparing with material B. A series of fatigue tests under uniaxial and multiaxial loading were conducted. Dynamic fatigue processes of rubber can be divided into three stages of cyclic softening, stable micro-crack growth and crack rapid growth under uniaxial and multiaxial fatigue loading. Two kinds of rubber materials are all cyclic softening, which is known as Mullins effect, and the magnitude of the cyclic softening depends on the maximum strain experienced for strain ratio R=0. The degree of cyclic softening of material B is greater than that of material A. The extent of cyclic softening for nonproportional loading is higher than that for proportional loading at the same equivalent strain amplitudes, which shows additional cyclic softening. The fatigue life of material under non-proportional path is much shorter than that under uniaxial load with the same strain amplitude. The fatigue life of rubber A is much longer than that of rubber B.The effects of axial stress, shear strain range, shear strain rate and their histories on ratchetting behavior were studied, respectively. It is shown that the ratchetting strain depends on the axial stress and cyclic shear strain range. The ratchetting strain increases more rapidly as the constant axial stress and shear strain range become larger. The loading rate has main influence on the initial strain under the same loading amplitude. The prior cycles with higher axial stress and larger strain range greatly restrains ratchetting strain of subsequent cycling with lower ones. Creep behavior plays an important role in the ratchetting strain and its influence on ratchetting behavior can not be neglected. Fracture morphology characteristics of two kinds of materials were investigated by scanning electron microscope (SEM). It shows that failure of material A is induced by cavitation and that of material B is induced by decohesion. For material A, with the decreasing strain amplitude the fracture surface shows more larger diameter globular particles under uniaxial loading and can be seen more obvious and deep crack under multiaxial loading. However for material B, with the decreasing strain amplitude fracture surface shows larger and larger scalelike shape and becomes more smooth under uniaxial loading, but fracture surface under multiaxial loading looks like more smooth than that of uniaxial loading, and shows gradually wave-like morphology.The effect of strain amplitude on crystallinity and crosslink density of rubber were investigated by Fourier Transform Infrared Spectroscopy (FTIR) and swelling balance methods. It shows that with the increase of strain amplitude, absorbance ratio and crosslink density of material B increase under uniaxial and multiaxial loading, this implies crystallinity of rubber also increases with the increase of strain amplitude.However the trend of aggrandizement of crystallinity bands becomes weaker and crosslink density is a larger under multiaxial loading compared with uniaxial loading. Present fatigue life prediction approaches were evaluated for rubber material. It is shown that equivalent strain approach give a good prediction for the fatigue life of two formulation filled vulcanized natural rubbers although it has a lot of shortcomings. Compared with Strain Energy Density (SED) model, the Cracking Energy Density (CED) model represents the portion of strain energy density that is available to be released by virtue of crack growth on a given material plane, so it presents better results in life prediction of two formulations filled vulcanized natural rubbers. Some of approaches based on critical plane which are widely used for metal fatigue are also be testified in this paper, and the results show that Chen-Xu-Huang (CXH) model can give a good result in two formulation filled vulcanized natural rubbers. Wang-Brown model and Smith-Watson-Topper (SWT) model present good prediction of the fatigue life for material B, but give poor predicted results for material A. A new model which considered the effect of non-proportional additional softening on fatigue life has also been introduced, and it shows that the modified model can predict the fatigue life of two formulations filled vulcanized natural rubbers very well.