SN curves come from experiments on test coupons of the material of interest. If you are not provided with the SN curve or cannot find one in the literature, then you can obtain material samples and perform fatigue testing on those samples. Fatigue is most common on ductile metals, however you say the material has a Poisson's ratio of 0.49999 which sounds like it is an elastomer such as rubber. Rubbers have a more complex response to fatigue than ductile metals. Perhaps you can do some research on that.
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