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A unique fatigue lifetime equation offers the possibility of predicting residual fatigue lifetime of a material. Derivation of this lifetime equation is based on a correlation between the total and plastic strain amplitude during cyclic loading. A ratio determined as the reciprocal value of the elasticity modulus degradation is evaluated from the continuous measurements of the specimen resonance frequency in a resonance testing machine. For successful application of this procedure, coefficients and exponents for a fatigue curve, Coffin/Manson curve and cyclic stress-strain curve, or coefficient and exponent for the new fatigue lifetime equation are needed. This new equation and the well-known equations of cyclic plastic response are fully compatible. Measurements of dynamic elastic (secant) modulus of a virgin specimen and of a specimen after previous cyclic loading then provide an opportunity to evaluate the residual fatigue life.
The verification of the proposed procedure was performed using a carbon steel with 0.485 mass % carbon at a test frequency of 20.2 kHz, using three different rates of increase for the total strain amplitude before starting ordinary fatigue testing (loading ramps).
elastic modulus degradation, carbon steel, ultrasonic frequency, cyclic stressstrain curve, Coffin/Manson curve, fatigue life equation, loading ramp, lifetime prediction
Professor, University of Transport and Communications, Žilina,