After you have defined the stress-life or strain-life curves for all materials in your model, you can choose your fatigue options and run the fatigue analysis.
The Fatigue Tool option is available from the Toolbox drop-down menu on the Solution Context Tab or through the right-click context menu.
Worksheet View
The Worksheet displays by default when you insert the Fatigue Tool during a Static Structural or Harmonic Response analysis. It displays default data, in graph form, based on the property settings of the object. The display changes as you make changes to the associated properties. In addition, this graphical Worksheet display is available for the options of the Graph Results menu. See the Fatigue Tool (Group) object reference page for additional information.
Property Definition
Once inserted into the tree, based on your analysis type, define the Details View properties as described below:
Method Selection (Random Vibration analysis only)
Exposure Duration (Random Vibration and Harmonic Response analyses only)
Frequency Selection (Harmonic Response analysis only)
Frequency (Harmonic Response analysis only)
Sweep Rate (Harmonic Response analysis only)
Fatigue Strength Factor (Kf)
This property defines the fatigue strength reduction factor. This setting enables you to
adjust the stress-life or strain-life curve(s) by a specified factor while the fatigue
analysis is running. The default value is 1. You set the property by
making an entry or you can use the slider feature associated with the property. The slider
represents the entry range from minimum (0.01) to maximum
(1). You use this property to account for a "real world" environment
that may be harsher than a rigidly-controlled laboratory environment in which the data was
collected. Common fatigue strength reduction factors to account for such things as surface
finish can be found in design handbooks.
Loading Type
The options of the Type property are described below. Their availability is based upon your analysis type.
Zero-Based (r=0)
Fully Reversed (r=-1)
Ratio
History Data
Non-Proportional (available only for stress-life applications)
Note:
The (r=0), (r=-1), and options are all constant amplitude, proportional loading types and are graphically illustrated in the Worksheet.
The option enables you to import a file containing the data points. This option is a non-constant amplitude proportional loading type. This data is depicted in a graph on the Worksheet. You can specify the number of data points this graph will display using the Maximum Data Points To Plot property in the Options category.
The Non-Proportional option is a non-proportional constant amplitude loading type for models that alternate between two different stress states (for example, between bending and torsional loading). Problems such as an alternating stress imposed on a static stress can be modeled with this feature. Non-proportional loading is only supported for Fatigue Tool objects used with Solution Combination where you select exactly two stress states (only two Base Cases present in the Solution Combination Worksheet).
Scale Factor
This setting scales the load magnitude. For example, if you set this to 3, the amplitude (and mean) of a zero-based loading will be 1.5 times the stress in the body. The graph in the Worksheet window will update to reflect this setting. This option is useful to see the effects of different finite element loading magnitudes without having to run the complete structural analysis repeatedly. Note that this scale factor is applied after the stresses have been collapsed from a tensor into a scalar. Thus any multiaxial stress collapse methods that are sensitive to the sign (Von-Mises, Maximum Shear, Maximum Principal) may not give the same answer had the scale factor been applied to the environment load itself.
Analysis Type
Define the fatigue analysis as either Stress Life or Strain Life.
Mean Stress Theory
This setting specifies how the mean stress effects should be handled.
If Analysis Type is set to Stress Life, choose from None (default), Goodman, Soderberg, Gerber, ASME Elliptical, and Mean Stress Curves. The Goodman, Soderberg, Gerber, and ASME Elliptical options use static material properties along with S-N data to account for any mean stress while Mean Stress Curves use experimental fatigue data to account for mean stress. You can specify the default setting for this property using the Mechanical application Fatigue settings in the Options dialog box.
If Analysis Type is set to Strain Life, choose from None, Morrow, and SWT (Smith-Watson-Topper).
Note: A sample plot of each of these theories is shown at the bottom of the Worksheet view. This plot does not use live data, but is rather a generic representation of each theory. For more information on these theories, see "Metal Fatigue In Engineering" by Ralph I. Stephens, et. al.
Method Selection
This property is only available for Random Vibration analyses. The options include: Narrow Band, Steinberg (default), and Wirsching. This setting specifies the method to calculate the Probability Density Function (PDF) from the available PSD stress response.
Stress Component
Because stresses are multiaxial but experimental fatigue data is usually uniaxial, the stress must be converted from a multiaxial stress state to a uniaxial one. A value of 2 times the maximum shear stress is used. You can choose from several types, including component stresses, von Mises, and a signed von Mises, which takes the sign of the absolute maximum principal stress. The signed von Mises is useful for accounting for any compressive mean stresses.
Note: For a fully reversed case (no mean stress), the Max Principal and Abs Max Principal options will give the same result because alternating stress is always calculated using Abs Max Principal stress.
Exposure Duration
This property is only available for Random Vibration and Harmonic Response analyses. It specifies the duration (in time) the loading is applied. The resulting damage is for this entire duration. The default setting is one second (1). This means that the calculated damage is based on damage per second.
In a Harmonic Response analysis, the number of cycles is internally calculated as the Exposure Duration multiplied by the chosen frequency.
Frequency Selection
This property is only available for Harmonic Response analyses. You use this property to select a analysis. Options include:
(default): This option specifies that only the stress at the defined Frequency is considered for the fatigue analysis.
: This option enables you to manually specify frequency values and their corresponding Exposure Durations in the Tabular Data window. Stresses at each of these distinct frequencies are considered for the fatigue analysis and fatigue damage is accumulated.
: This option enables you to manually specify frequency values and their corresponding Scale Factors in the Tabular Data window. A analysis is performed from the lowest to highest frequency value in steps of 0.5 Hz using the Sweep Rate and the appropriate amplitude scaling at each step. The fatigue damage is accumulated.
Frequency
This property is only available for Harmonic Response analysis. It specifies the frequency (in Hz) for which the stress response is calculated and used for the fatigue analysis. The phase angle for which the maximum stress occurs is automatically chosen for a chosen frequency.
Sweep Rate
The Sweep Rate property is only available for Harmonic Response analysis. It is only available when the Sine Sweep option is selected for the Frequency Selection property. This property specifies the rate of frequency sweep in Hz/s units.
Units Name
This property enables you to specify the name for the Life Units. The unit options include:
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Selecting the option displays the field. Enter the name for your customized unit name in this field. The specified unit is reflected in the Details view for all applicable fatigue settings.
1 "Unit" is Equal To
Where "unit" is either cycle or block based on the Units Name selection. Modify the field’s value based on the desired number of cycles or blocks for the units.
Bin Size
This option appears only if Type is set to History Data (non-constant amplitude loading). This setting defines how many divisions the cycle counting history should be organized into for the history data loading type. Strictly speaking, this is number specifies the dimensions of the rainflow matrix. A larger bin size has greater precision but will take longer to solve and use more memory.
Use Quick Rainflow Counting
This option appears only if Type is set to History Data (non-constant amplitude loading). Since rainflow counting is used, using a "quick counting" technique substantially reduces runtime and memory, especially for long time histories. In quick counting, alternating and mean stresses are sorted into bins before partial damage is calculated. This means that with quick counting active, calculations will be performed for maximum of binsize. Thus the accuracy will be dictated by the number of bins. Without quick counting, the data is not sorted into bins until after partial damages are found and thus the number of bins will not affect the results. The accuracy of quick counting is usually very good if a proper number of bins are used when counting. To see the effects of using quick counting, compare the results of constant amplitude loading to simulated constant amplitude loading from a load history file. With quick counting off, the result should match exactly but with quick counting on, there will be some error depending on the bin size and alternating stress value in relation to the midpoint of the bin the count is sorted into.
Infinite Life
Stress Life Analysis
This option appears only if Type is set to History Data (non-constant amplitude loading) and defines what life will be used if the stress amplitude is lower than the lowest stress on the SN curve. It may be important in how damaging small stress amplitudes from the rainflow matrix are.
Strain Life Analysis
Since the strain-life method is equation based it has no built-in limit, unlike stress-life for which the Fatigue Tool uses a maximum life equal to the last point on the SN curve. Thus to avoid skewed contour plots showing very high lives, you can specify Infinite Life in a strain-life analysis. For example, if you set a value of 1e9 cycles as the Infinite Life, the maximum life reported is 1e9.
Maximum Data Points To Plot
This option is only applicable for History Data loading and allows you to specify the number of data points to display in the corresponding graph that appears in the Worksheet. The default value is 5000 points. The graph displays the full range of points and all points are used in the analysis. However, depending on the value you set, every second or third point may not be displayed in the interest of avoiding clutter and making the graph more readable.