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[FAQ]

The correct method to define the S-N data in Engineering Data for spectral fatigue is a bit counter-intuitive for two reasons: 1.The S-N curve can only be defined as a linear or bi-linear curve (in log-log space). To directly define the S-N curve, you must define the Fatigue Strength Coefficient(s) and the Fatigue Strength Exponent(s). If you enter the S-N data using in the table, the Fatigue Tool fits a straight line between the first data point (fewest cycles) and the last data point (most cycles). 2.The Fatigue Strength Exponent (m) that Engineering Data uses to define the linear or bi-linear S-N curve for spectral fatigue is the negative inverse of what is traditionally called the “fatigue strength exponent” in Basquin’s equation (elastic portion of the E-N curve). In traditional usage, the fatigue exponent is a negative value. For spectral fatigue using the Mechanical Fatigue Tool, it will be a positive value.