The S-N-curve for an elastic material is characterized by the Basquin relationship, = C.N where C is a material constant, is the stress amplitude,

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The S-N-curve for an elastic material is characterized by the Basquin relationship, = C.N where C is a material constant, is the stress amplitude, Nr is the number of fully-reversed- stress cycles to failure, and b is the Basquin exponent approximately equal to -0.08. If a specimen spends 50% of its life subjected to an alternating stress level of oa 250 MPa, 30% of its life at oa 275 MPa and 20% of its life at oa 300 MPa. If oa 250 MPa coincides with the endurance limit of the material below which failure does not occur up to 10 cycles, estimate its fatigue life using the Palmgren-Miner cumulative damage rule.