StrainLife fatigue is applicable to low cycle fatigue, but you must have sufficient test data in the low cycle range. If the design life is only 20 cycles, why don't you just run a cyclic non-linear analysis for 20 load steps and compare the results to a design criteria (e.g., total deformation, amount of accumulated plastic strain, etc.). You need to know the non-linear material properties and select an appropriate non-linear material model (e.g., bilinear kinematic hardening).
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