Multi-axial fatigue in magnetorheological elastomers using bubble inflation

For materials in modern machines, fatigue strength is probably the most critical physical property that needs to be understood. In particular, the high dynamic loading experienced by machine parts necessitates understanding fatigue properties in life limiting components. However, rubber fatigue is imperfectly understood and even less is known about fatigue resistance in adaptive or smart elastomers. Preliminary research into the equi-biaxial fatigue behaviour of magnetorheological elastomers (MREs) is described here. Test samples were fabricated by incorporating carbonyl iron particles, typically of 6–7 μm in diameter, in room temperature vulcanised (RTV) silicone rubber. Physical testing was conducted using a bubble inflation testing system and test samples were fatigued at stress amplitudes between 0.75 MPa and 1.4 MPa under engineering stress control. S–N (Wöhler) curves of stress amplitude (σa = S) versus cycles to failure (N) are presented. Stress–strain behaviour throughout the fatigue process is also described. For a stress amplitude of 0.75 MPa and zero minimum stress, stress softening was observed for the entire test, though it was particularly pronounced in the first 100 cycles of testing. A limiting value of complex modulus (E*), observed previously in dynamic testing of conventional elastomers, was determined in these tests.

► The equi-biaxial bubble inflation method is appropriate for determining fatigue life in MREs. ► Stress softening continued throughout cyclic tests, but was pronounced in the first few cycles. ► Stress softening occurred more rapidly for materials tested at higher stress amplitudes. ► There is a limiting value of E* at which fatigue failure will occur in silicone based MREs. ► The limiting E* for all tests on the MREs was within limits of ±2.63% of the mean value of E* at failure.