Engineering Data includes example materials, some of which contain fatigue curves derived from engineering handbooks. You can also define custom fatigue curves to suit specific needs. During a fatigue analysis, the Fatigue Tool uses the curve date to calculate life, damage, safety factors, and other fatigue-related results for each material in the model. For example, if Young's Modulus is temperature dependent, then the fatigue calculations are carried out using the Young's Modulus computed at the reference temperature of the body.
Important: The Fatigue Tool may produce incorrect results if your analysis includes a Material Assignment object scoped to elements. To ensure accurate results, only scope materials to bodies.
- Strain-Life Parameters
For the strain-life approach, the materials must have Strain-Life Parameters defined. The following four strain-life parameter properties and the two cyclic stress-strain parameters must have data defined:
Strength Coefficient
Strength Exponent
Ductility Coefficient
Ductility Exponent
Cyclic Strength Coefficient
Cyclic Strain Hardening Exponent
- S-N Curve
For the stress-life approach, the materials must have S-N Curve defined. The alternating stress, or stress-life (SN), mean curve data can be defined with respect to mean stress, r-ratio, or temperature. The Interpolation method (Log-Log, Semi-Log, or Linear) can be defined. The curve data must be defined to be greater than zero.
Mean Stress
Use this definition if experimental SN data was collected at constant mean stress for individual SN curves.
R-Ratio
Use this definition if multiple SN curves were collected at a constant r-ratio. The r-ratio is defined as the ratio of the second loading to the first: r = L2 / L1. Typical experimental r-ratios are -1 (fully reversed), 0 (zero-based), and .1 (to ensure that a tensile stress always exists in the part).
Temperature
Use this definition if multiple SN curves were collected at different temperatures.
If you choose the Field Variable while defining an S-N Curve material property, and you enter multiple S-N Curves (Cycles/Alternating Stress) for different Temperatures, the Fatigue Tool automatically uses the appropriate S-N curve for the interpolation based on the temperature at each node of the body.
Note:The Fatigue Tool supports temperature dependent fatigue for Static Structural and Transient Structural analyses only, and not for frequency-based fatigue.
When you specify stress-life (SN) mean curve data with respect to and the Type property of the Fatigue Tool is set to , the application uses an average temperature for the calculations.
It is possible to define multiple SN curves to account for different mean stress or r-ratio values. The values of mean stress/r-ratio values are only important if multiple curves are defined in the Engineering Data workspace and the option is specified for the Mean Stress Theory property in the Fatigue Tool .
- Linear/Bilinear S-N Curve
For a Frequency-Based Fatigue analysis, the S-N Curve material properties enable you to examine frequency-based fatigue formulations using either a linear or bi-linear curve for Stress (S) versus the number of life cycles (N). Material property options include:
Linear
Use this definition for single slope SN curve data.
Bi-Linear
Use this definition for when you have two sloped of SN curve data.
Note:
See the Add Material Properties section of the Engineering Data User's Guide for the steps to add material properties.
Refer to the next section, Fatigue Stress Life Versus Strain Life, for additional information about strain-life parameters.
Note that in Engineering Data, in the Display Curve Type drop-down menu, you can plot either a Strain-Life or Cyclic Stress-Strain curve.