Figure 4 shows the result of evaluating the fatigue limit diagram after the mean stress and the stress amplitude
were calculated under the normal service load conditions.
The Basquin law, a stress-life law, was used here to describe the relationship between shear stress amplitude
and fatigue life of ABS.
The relation between deformation amplitude and stress amplitude
is given by equation 3 where [E.sup.*] is the complex modulus.
Pappin  observed this problem and alternatively described the permanent strain with ratio of deviator stress amplitude
to mean normal stress.
Many rubber products, such as tires and belts, are tested by subjecting them to an oscillating load with either a constant peak stress amplitude
or strain amplitude.
The measured maximum stress amplitude
precedes the maximum strain amplitude by the phase angle [delta].
This can be ascribed to the difference of specimen dimensions because the risk volume (specimen volume subjected to a stress amplitude
larger than the 90% of its maximum value [24, 25]) of the NZ specimen in this test is much smaller than that of the full specimen.
It has been shown that the ratcheting behavior is influenced by various factors such as mean stress, stress amplitude
, stress rate, temperature, and so forth.
Only when additional data was obtained, specifically loss and storage modulus, as well as tan (d), combined with an understanding of the effects of stress amplitude
and hysteresis was it possible to understand what happened to the pads molded from NBR-6.
In the cyclic loading tests, the stress amplitude
is 3-16 kN/m and the tensile rate is 20 N/s.
For these tests the choice of stress amplitude
is very important.