When the analysis type is set to Initiation (crack initiation or strain-life or local strain analysis), the form as shown appears.
Specific parameters that can be set on this form are the Analysis Method, the Plasticity Correction, the Stress Strain Combination parameter (to be used during the fatigue analysis), the Certainty of Survival criterion, and whether to calculate Biaxial parameters. The following table describes these parameters in detail.
This part of the form allows for a factor of safety analysis, if desired. The
parameters and their meanings are also described in detail over the next few pages. The options under the Run Factor of Safety Analysis toggle are disabled until this toggle is activated.
For ae=0, Signed Tresca, Signed von Mises and Max Abs Principal should be the same. If aeis negative, Max Abs Principal is the best choice. If it is positive, Signed Tresca is the best choice. These comments apply to the crack initiation approach. If using stress life, generally it is best to stick with Max Abs Principal.
Main Index
The following table describes each parameter in detail:
Parameter Description
Analysis Method Acceptable values of the Analysis Method (sometimes referred to as Mean Stress Correction) are S-T-W (Smith-Topper-Watson), Morrow or Strain-Life. These methods are described in detail in
Fatigue Theory
(Ch. 14). Although one of the above must be selected, sensitivity study allows the comparison of results using all of these correction or analysis methods. Of the two methods, the Smith-Topper-Watson is most commonly used, especially with variable amplitude loadings. However, it cannot deal with whollycompressive cycles (for which it will predict zero damage) and when the loading is predominately compressive, the Morrow correction will be more conservative. The Morrow method may also be useful where mean tensile stresses are very high and the Smith-Topper-Watson method may give answers that are over-conservative.
Plasticity Correction
This option selects the method used to carry out the conversion from elastic to elastic-plastic stresses and strains. Acceptable values for this widget are: Neuber, Mertens-Dittmann, Seeger-Beste, or None. The default is Neuber. The other options, the Mertens-Dittmann and Seeger-Beste methods, are based on the Neuber method but include modifications which cause them to give better answers (and more conservative) results when geometries are unnotched, or plasticity is not highly localized. The latter two methods are very similar, with the Seeger-Beste method being more conservative. Both require an elastic strain concentration or shape factor which is a function of the shape of the cross section of the component and the type of loading.
For instance, the shape factor for a rectangular beam in bending is 1.5. The shape factor for a notched beam could be estimated from the product of the shape factor for the unnotched geometry and the stress concentration factor of the notch. When the shape factor ap tends toward infinity, both methods reduce to the Neuber method.
These shape factors are input via the Materials Information form. See Elastic-Plastic Corrections(p. 1264) for an explanation of these various correction methods.
Biaxiality Analysis It is possible to request a Biaxiality analysis to be performed. This requires, first, that the stresses be aligned to the surface of the component. If the stresses from the FE analysis are not aligned to the surface, it is possible to accomplish this by selecting theCalculate Normals(p. 69) from the Job Control form before submitting the analysis. This option only makes sense when using nodal stress values or elemental stress values for shell elements. For the latter, the stresses must be aligned by the FE code as opposed to allowing MSC.Fatigue to do it by requesting it to calculate normals. The biaxial parameters which are calculated inform the user as to the amount of multiaxiality present in the component due to the loading applied and allows for the user to determine the validity of the fatigue analysis. Please refer to
Multiaxial Fatigue
(Ch. 6) for further discussion of this feature.Biaxiality Correction
This option selects the method used to correct the treatment of material properties in the application of the Neuber method in order to take account of the biaxiality of the loading. Acceptable values include: None, Material Parameter, or Hoffman-Seeger. If None is selected, the software will carry out the Neuber method using the chosen Strain Combination and the uniaxial cyclic stress-strain curve. The Hoffmann-Seeger method uses the biaxiality ratio to convert the combined strain parameter to an equivalent strain (based on the von Mises Strain) before carrying out the Neuber correction and then recalculating the elastic-plastic stresses and strains. It is applicable to the Maximum Absolute Principal and the Signed Tresca strain combinations. In the case of the Signed von Mises, it reduces to the unmodified Neuber method. The Material Parameter
modification (Ratio) method works by calculating a new cyclic stress-strain curve for each node or element on the basis of the mean biaxiality ratio. The new curve relates Maximum Absolute Principal stress and strain amplitudes. This method is therefore only
applicable when using this strain combination. The latter two corrections require the mean biaxiality ratio. Therefore, the Biaxiality Analysis toggle must be set on to select these methods. They should only be used where surface resolved stresses are available. See Biaxiality Correction Options(p. 432) for more explanation of the different correction methods. These methods require that the loading be approximately proportional. The Neuber method in conjunction with the Maximum Absolute Principal strain is the method of choice if the mean biaxiality ratio is close to zero. Otherwise the biaxiality is better taken into account either by using the Neuber correction in conjunction with a yield criterion-based parameter such as Signed von Mises or Signed Tresca, or by using the Hoffmann-Seeger or the Material Parameter modification (Ratio) method.
Parameter Description
Main Index
Stress/Strain Combination
This option menu selects the stress or strain parameter used in the fatigue analysis. The six multiaxial stress/strain components defined by the stress or strain tensor are resolved into one uniaxial or
combined value for fatigue calculations for each node for each time step. This is necessary since the fatigue damage models used in MSC.Fatigue are based on theories which deal with uniaxial stress or strain. These stress or strain scalar combinations can be either one of these components, X Normal, Y Normal, Z normal, X-Y Shear, Y-Z Shear, Z-X Shear, Max. Abs. Principal, Max. Principal, Min. Principal, Signed von Mises, von Mises, Signed Max. Shear, Signed Tresca or Critical Plane. Note that the shear strain components are engineering shear strains (two times the tensor shear strains). When the stress state is uniaxial or the maximum shear stress is more than half the absolute maximum principal, the most appropriate selection is Max. Abs.
Principal. In other circumstances, the Signed von Mises will be more conservative. The sign on the Signed von Mises and Signed Tresca is taken from the sign of the absolute maximum principal value. It is necessary to sign these stress/strain parameters otherwise non-conservative fatigue life estimates will result.
Certainty of Survival
This defines the Certainty of Survival based on the scatter of the
e
-N curve. For example, to be 96% certain that the life will be achieved, set the slider bar at 96. This value is used to modify thee
-N curve according to the standard error scatter parameter (SE), so the design criterion parameter will be meaningless if the value of SE is 0.A Design Criterion value of 50 leaves the
e
-N curve unmodified.Factor of Safety Analysis
This option will cause a type of Factor-of-Safety or over design analysis to be performed. It informs the user how much stress may be modified for optimization purposes based on fatigue life. This analysis is in addition to the normal fatigue calculations and must be requested. This analysis method can be very useful for those
components which predict an infinite life providing a measure of the risk of fatigue failure. The results of this analysis are stress factors for each location (node or element) for which the analysis has been performed. A value of one suggests that the specified life will be exactly attained whereas a factor less than one means the desired life will not be attained. Factors greater than one are, therefore, most desirable. Certain parameters must be supplied in order to proceed with this analysis. They comprise the remainder of the parameter descriptions in this table.
Design Life For a Life based Factor of Safety analysis enter the target design life.
Maximum Factor for Calculation
Enter a maximum factor (default is 100) to be used in the analysis.
This number can be lowered to speed up the calculation if it is known that the maximum stress factor of interest will be less that the default.
Material Cutoff This toggles the usage of the material cutoff value in the analysis.
Parameter Description