Variation in Cyclic Deformation and Strain-Controlled Fatigue Properties Using Different Curve Fitting and Measurement Techniques

The strain-life approach is now commonly used for fatigue life analysis and predictions in the ground vehicle industry. This approach requires the use of material properties obtained from strain-controlled uniaxial fatigue tests. These properties include fatigue strength coefficient (σ_f′), fatigue strength exponent (b), fatigue ductility coefficient (ε_f′), fatigue ductility exponent (c), cyclic strength coefficient (K′), and cyclic strain hardening exponent (n′). To obtain the aforementioned properties for the material, raw data from stable cyclic stress-strain loops are fitted in log-log scale. These data include total, elastic and plastic strain amplitudes, stress amplitude, and fatigue life. Values of the low cycle fatigue properties (σ_f′, b, ε_f′, c) determined from the raw data depend on the method of measurement and fitting. This paper examines the merits and influence of using different measurement and fitting methods on the obtained properties. To evaluate the effect of variations in properties on fatigue life, life predictions were performed on the SAE keyhole specimen subjected to the transmission load history.