A Study On Fatigue Life Of Rubber Material Under Nonproportional Loading

In this paper, to investigate the multiaxial fatigue properties and uniaxial ratcheting behavior of vulcanized natural rubber, a series of tests including both proportional and nonproportional loading paths based on small specimens were performed. The experiments show that the rubber material is cyclic softening, which is known as MULLINS effect, and the magnitude of the softening depends on the maximum strain experienced for strain ratio R=0 loading style. After a short period of significant softening, the stress-strain response under constant-amplitude cyclic loading exhibits a logarithmic trend of softening both in uniaxial and multiaxial loading paths. However, the multiaxial loading will lead to a shorter fatigue life.Test ratcheting experiment results show that the ratcheting strain depends on loading condition,such as load amplitude, mean load and loading rate. In addition, loading history is found to have significant influence on ratcheting behavior. The result of strain recovery test indicates that permanent damage and viscoelastic properties of NR are responsible for the accumulation of ratcheting strain under cyclic loading.Present fatigue life prediction approaches were evaluated for rubber material. It is shown that equivalent strain approach gave a good prediction for the fatigue life although it has a lot of shortcomings. Compared with SED model, the 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. But the equivalent stress model gives the worst correlation.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 SWT model and CXH model can give a good result. A new model which modified Fatemi-Socie's model has also been introduced, and the prediction result shows that the modified model can predict the fatigue life of rubber material very well.