In order to understand the sensitivity of the ratchetting
behaviour to the stress loading rate history for the cerium
oxide filled vulcanized NR, some multi-step ratchetting
tests were carried out. Fig. 9(b) shows the effects of previous
stress loading rate on the ratchetting behaviour of the
cerium oxide filled vulcanized NR. In this case, uniaxial
ratchetting tests with four-step loading were carried out
under a mean stress of 2.5 MPa and stress amplitude of
2.5 MPa. The stress loading rates were 0.02 MPa/s, 0.2 MPa/
s, 2.0 MPa/s and 0.2 MPa/s, respectively. From Fig. 9(b), it
can be seen that the ratchetting strain increases little when
the stress loading rate is increased from 0.02 MPa/s (step 1)
to 0.2 MPa/s (step 2). We can obtain the same result when
the stress loading rate is increased from 0.2 MPa/s (step 2)
to 2.0 MPa/s (step 3). When the stress loading rate is
decreased from 2.0 MPa/s (step 3) to 0.2 MPa/s (step 4), the
ratchetting strain increases but the amount of increase is
small. It is concluded that the previous loadings with low
stress loading rate can restrain the accumulation of ratchetting
strain of the subsequent loadings with high stress
loading rate. But, the effects of the previous loadings with
high stress loading rate on the ratchetting strain of the
subsequent loadings with low stress loading rate is
negligible.