How is the S-N curved defined for spectral fatigue using the Mechanical Fatigue Tool?
The correct method to define the S-N data in Engineering Data for spectral fatigue is a bit counter-intuitive for two reasons:
- 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).
- 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.