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Autodesk Fatigue Wizard

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The Fatigue Wizard software enables CAE engineers of any experience level to utilize results of a finite element analysis to carry out sophisticated and accurate fatigue life calculation. CAE engineers with over 30 years combined experience and knowledge of finite element analysis and durability assessment developed it.

The Fatigue Wizard leads you through the steps necessary to perform complex fatigue life calculation. You can view results as life or safety factors using the Fatigue Wizard post processor, or export and view in Autodesk® Simulation. With minimum effort and time, you can use the Fatigue Wizard to generate customizable reports.

Fully Functioning (SN and EN)

Fatigue Wizard can perform sophisticated fatigue life analyses, from conventional stress-based analyses to low cycle strain-based analyses with plasticity correction. Mean stress corrections can be accounted for using standard methods. Endurance modifiers such as surface finish, temperature, and so on, are all available. For a complete list of the capabilities within the Fatigue Wizard, follow the features link.

Multiple Load Cases (Non-Proportional)

Fatigue Wizard can perform fatigue calculations using stress results from multiple load cases. Results from multiple load cases can be scaled individually using separate load histories of up to 30000 points, and analyzed as a combined load case. Alternatively, stress states from individual load cases can be used in sequence to produce the cyclic stress history.

You can scale each load case and load history individually to determine the sensitivity of fatigue life to load case.

Material Database

Successful fatigue analysis depends entirely on accurate material data. Fatigue Wizard ships with an extensive material database for stress life curves (SN) and strain life curves (EN), and component data for welded joints to BS7608. You can add to the database as data becomes available.

Fatigue Wizard also includes the capability to estimate strain life material data from readily available mechanical properties, using standard empirical relationships.

Post Processing

Fatigue Wizard includes post processing tools. An OpenGL results post processor is included within the wizard interface. You can view life contours or safety factor contours before you step back to perform 'what-if' analyses.

Fatigue Wizard also includes an automated report writer. You can create and export summary reports of the analyses you perform. This feature maximizes the time available for analysis.

Analysis Options

Fatigue Wizard additional 'advanced' forms. With analysis options, you can control the mean stress corrections and FEA stresses to use. You can control the portion of the FE model to analyze, based on a ranked maximum stress



range.

You can perform fatigue calculations using equivalent stresses, such as signed Von Mises, tresca, and maximum absolute principal stress. Alternatively, a critical plane approach is implemented using resolved surface normal or shear stress.

Analysis Speed

Fatigue Wizard offers several choices to aid speed of calculation. You can select the choices individually depending upon the required speed, reliability, and desired output. You can analyze all of the model, or parts of the model. You can analyze the model selectively based on your defined 'groups,' or by a given percentage of nodes based on a ranked nodal maximum stress range.

Topics in this section

● Steps

● Theoretical Overview

● Welding Based on BS7608

● File Formats

● Demonstration Videos

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Step 7 provides a summary of the calculated fatigue results. It is a quick indication of the suitability of your component to withstand the applied loading condition. The traffic light symbol indicates whether the results are acceptable, marginal, or unacceptable when compared to the 'desired' values entered on step 5.

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Traffic Lights

The Traffic Light calculation takes the calculated fatigue result and divides it by the 'desired' result (entered in step 5). The traffic light symbol is then taken as the worst value calculated for the entire model. You see one of the following:

Green indicates that there are no factored results (life or FSF) values less than 1.3.

Yellow indicates that there are no factored results less than 1.0, but there are some f values between the range 1.0 to 1.3.

Red indicates that there are factored results values less than 1.0. THIS REPRESENTS A POTENTIALLY UNACCEPTABLE RESULT. INVESTIGATE IT FURTHER.

Node List

The summary report lists the 200 nodes with the lowest Log10 life or lowest FSF for the different correction methods analyzed.

To plot this data, press the button. The following is a typical graph:

This graph is useful as a simple model quality check. A quick inspection of the graph shows if the lowest life is associated with just a few nodes, which is sometimes an indication of poor loading/restraining.

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The next options in the analysis cycle are as follows:

1. Step backwards through the Fatigue Wizard to change some input data and calculate again.

2. Use the Visualizer to inspect the results graphically. You can also access the Report Writer from the Visualizer.

3. If you are finished with the Fatigue Wizard, click Finish to write the results. Then you can view them in the Results environment of Autodesk Simulation. Use the appropriate menu based on the results calculated under the "Results" pull-down menu.

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Access the Fatigue Wizard visualizer from any step of the analysis in the tree view of a form. The visualizer shows either the basic FEA model, or the FEA model with contoured fatigue results, depending on whether results are available. If a fatigue calculation is performed, the visualizer defaults to a contoured display of results.

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Visualizer Functions:

The Fatigue Wizard visualizer form is divided into several regions:

1. Tree-View

The tree view window displays a list of the constituent parts of a model, and shows the status of the display properties. You can set individual parts to display contoured results (if available), make invisible, or display just the underlying FE mesh of the part.

To set the display properties of a part

1. Click in the tree view window on the part to change. The part highlights in the graphics region, and the tree view icon changes to a solid cyan to indicate that it is selected for change.

2. Right-click anywhere inside the tree view window. On the menu, select any of the following actions:

❍ show (toggle part on if currently not displayed)

❍ hide (toggle part off if currently displayed)

❍ change color (select color for the underlying FE mesh)

❍ toggle results (toggle between contoured results and underlying mesh display)

2. Graphics Region

The graphics region contains the main display of the FE mesh and contoured results. The current display type and results set are displayed at the top left of the window.

Dynamic Viewing

You can pan, zoom, and rotate the FE mesh and contoured results using the following combinations of mouse and keyboard controls.

Fatigue Wizard contains full-time 3D dynamic viewing with your 3-button mouse. You can position the model to see your model from any angle.

Consider the following image that shows the graphics region. Think of an imaginary XYZ axis at the centre of the screen, and an imaginary circle as shown.












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The X axis points along the horizontal, the Y axis is vertical and the Z axis points out of the screen.

Rotation about X and Y

● Move cursor inside imaginary blue circle.

● Press and hold down the Left mouse button.

● Move up and down to rotate about X.

● Move left to right to rotate about Y.

Rotation about Z

● Move cursor outside imaginary blue circle.

● Press and hold down the Left mouse button.

● Move up and down to rotate about Z.

Pan/Move in X and Y

● Move cursor inside imaginary blue circle.

● Press and hold down the Middle mouse button.

● Move up and down to move in Y.



● Move left to right to move in X.

Zoom

● Move cursor outside imaginary blue circle.

● Press and hold down the Middle mouse button.

● Move up and down to zoom.

Using a combination of the above, you can position your model anywhere in space.

Right Mouse Menu

Click in the main graphics region to view a menu containing some standard views and tools.

Results

In a contoured results set display, a contour band is displayed on the right-hand side of the window. The contour bands vary from the minimum to maximum data values of the results set. You can customize as shown here.

You can also view the data as the number of cycles or Log10 number of cycles.



3. Result-Set Tabs

For each analysis type (SN, EN, or Haigh) Fatigue Wizard, by default, calculates two or three different result sets. If you are calculatin life, these results sets contain the tree types of mean stress corrections available. If performing a Haigh type calculation, they contain the two methods of calculating a Fatigue Safety Factor (constantR or constantM) .

To display the individual result sets, choose any one of the results sets tabs at the bottom of the main graphics window.

4. Main Toolbar

The main toolbar is visible only if results are available, and is shown at the top of the visualizer form. Use this toolbar for Report tools and Plot Types.

Plot Type

Fatigue Wizard displays two types of contoured results; the traffic ligh' plot, and a standard ten band contoured plot. To switch between the plot types, click the appropriate icon in the main toolbar.

The traffic light plot provides an overview of the regions of the model that show either acceptable fatigue results (green), marginal fatigue results (yellow), and unacceptable fatigue results (red). Fatigue Wizard refers to the desired result you entered on step 5 to determine into which band the results fall.

This display is only a guide to those areas which require your further attention. You determine whether results calculated by the Fatigue Wizard are acceptable. When Fatigue Wizard creates the traffic light plot, any result below the desired value you entered is unacceptable, and it displays red. Any result which is 1.3 times greater than the desired result you entered is acceptable, and displays green. Results calculated by the Fatigue Wizard which fall between the other two are deemed



marginal, and display yellow.

Report

Use Fatigue Wizard to generate an automatic report, summarizing the calculation parameters and results. Access the report using the report button. Edit and save using the buttons displayed in the main toolbar.

For further details on the report, see the page "Report Writer".

ImportantWe recommend that you click OK at the bottom of the visualizer window to close the visualizer. Clicking on the 'close' button of the main window controls exits Fatigue Wizard completely.

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The Report Writer operates like most modern word processors. The report view emulates the paper size you select (Letter, A4, and so on), and is a WYSIWYG view. Almost all of the report is written automatically. You can edit the report.

Icons

PDF Export - Creates an Acrobat PDF file.

RTF Export - Exports a file into various programs.

Copy all to Clipboard - Copies the entire report to the Windows clipboard.

Open previous - Opens a previously saved report.

Print - Prints immediately to the currently defined printer.

Setup - Controls various page/print/pdf options.

Cut/Copy/Paste - Applies to selected text or graphics. Also available on the right-click menu.

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Insert Current Picture - Inserts the current picture in the Step 9 graphics region.

Insert picture from file - Inserts the selected file (BMP, GIF, JPG, and so on).

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Theoretical Overview - WikiHelp

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A German engineer, Albert, carried out the first known fatigue investigations in the early 19th century. It had been understood that a material subjected to repeated loading would eventually fail. However, it was the widespread failure of railway carriage axles after many miles of apparently faultless service which began to draw attention to the phenomenon that later became known as fatigue.

Most engineering components or structures experience some form of cyclic loading. Failure in such components typically means the initiation of small cracks, which eventually link to form large cracks and catastrophic failure. The crack initiation begin at stress levels well below the monotonic yield stress of the material. Over time, (repeated) damage accumulates until eventual failure. It has been widely suggested that over 90% of structural failures occur as a result of fatigue damage.

Two basic approaches are available to estimate the number of repeats of a cyclic load a material or component can withstand.

The early methods utilized material data first collated in the 19th century by Wholer. This technique related the stress in a component to an empirical curve, known as the S-N curve, (or Wholer line) to obtain an estimate of the number of cycles to failure. This technique is still used today and is commonly known as the stress-life approach. This technique uses the component stress and makes the assumption that the stress remains in the elastic region of the material. Therefore, it is best suited for fatigue failure where 100000+ cycles are required for failure.

The second and more modern technique utilizes the material strain to predict failure. Curves relating strain to cycles can be prepared from test or empirical formulae similar to those used for the stress-life approach. The major benefit in using this technique is that the strain-life curve represents the local Strain State of the component, and can account for local plastic deformation. This greatly increases the accuracy of the life prediction at loading levels where local yielding occurs.

Fatigue Wizard uses both techniques to predict fatigue life.

Definition of Loading

Fatigue damage is caused by loading a structure with repeating loads over a period of time. To analyze fatigue failure it is necessary to define the applied load. in the Fatigue Wizard, you can define two basic loading types, the simplest form being the constant amplitude harmonic.

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This loading is fully described by the definition of a stress amplitude sa and a mean stress sm. The repeating cycle is a single harmonic period, that is points 1-2-3-4 in Figure 1.

Figure 1 Constant Amplitude Harmonic Loading

The second type of loading, and a more general case, is the variable amplitude load.

Figure 2 Variable Amplitude Loading

Variable amplitude loading is made up of many individual constant amplitude cycles. These individual cycles can each have different amplitude and mean and can be superimposed or subsequent. Figure 2 shows a simple example of a variable amplitude load. You can see that there are two simple cycles: 1-2-5-6 forms a large amplitude single cycle onto which the smaller cycle 3-4-3 is superimposed. A variable amplitude loading can therefore be broken down into a number of simpler constant amplitude loads. Then they are used in the fatigue analysis to predict damage.

If a variable amplitude load is to be used for fatigue calculation, a method must be applied to extract the ‘sub-cycles’. Fatigue Wizard applies it by a standard rainflow counting algorithm.

Rainflow Counting and Fatigue Damage

Both the stress-life and strain-life techniques of predicting fatigue failure rely on a single cycle damage curve. This damage curve correlates number of repetitions with stress or strain amplitude for a single simple cycle. This curve can be used directly if the structural loading is the simple constant amplitude type. However, if a variable amplitude loading is defined, several steps need to be followed to calculate a life for the applied load.


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First the load history is peak-valley picked. This process scans the time history for any points which are not ‘reversal’ points and discards them, thus reducing unnecessary data. Next the reduced time history is processed with a standard rainflow counting algorithm, which extracts individual single cycles. These single cycles are then used with the single cycle damage curves to calculate a life for each cycle.

After a life is calculated for each individual cycle in the time history, a method of determining the cumulative damage or total life is required. Fatigue Wizard utilizes the well-documented ‘Miners Rule’. This rule states that the total damage accumulated over a load history is the linear sum of the damage of individual cycles.

or

where N is life in repeats and D is the damage.

Stress Life Technique

The stress life technique utilizes a single cycle damage curve known as the SN curve. This curve relates stress amplitude to number of cycles to failure. Since this technique has been used widely for many years, such curves and material data relating to such curves are readily available. The stress value used with the curve depends on the data represented in the curve, and to an extent, the discretion of the engineer. The uniaxial version of Fatigue Wizard uses the ‘worst absolute principal’ stress in its calculations, since the direction of the stresses is assumed constant.

Fatigue Wizard generates the SN curve internally from the material data input by the engineer. Data required are the monotonic tensile strength of the material, and an ‘endurance limit’.



Figure 3 SN Curve

The SN curve takes the form shown in Figure 3. SN curves are always plotted on log-log scales, as shown. The curve starts from the monotonic tensile strength suts at log(N)=1. It is then common to approximate the range up to a value of 1000 cycles by a straight line to a value of 90% of suts. The main slope of the SN curve extends from the 1000 cycle point to a life value corresponding to the ‘endurance limit’ and a stress amplitude equivalent to send. The behavior beyond the endurance limit then depends on whether the material is a ferrous alloy or non-ferrous alloy. It is typical for steels to show a real endurance limit and a flattening of the SN curve. Conversely, aluminium alloys and other non-ferrous alloys show a less distinct flattening of the SN curve at high cycles. Fatigue Wizard allows the user to distinguish between these two cases.

Mean Stress Effects

You usually present SN curves for a fully reversed test. This means that the stresses applied cycle between equal tensile and compressive states. In realistic structural loading, it is more common for the cyclic loads to oscillate around a non-zero mean state. This non-zero mean state has a significant effect on the life to failure. A method is required to account for the presence of a mean stress in the cycle when using the standard SN data.

Fatigue Wizard allows you to calculate the fatigue life using ‘no-mean’, ‘Gerber’ and ‘Goodman’ corrections.

Stress Concentration

It is usual for FE analyses to explicitly model and calculate stresses in areas of local concentration, so any scaling of nominal stress is unlikely to be required. However, should any linear scaling of the calculated FE stresses be required Fatigue Wizard provides a general scaling factor Kt.

se = Kt.snominal

Where se = scaled elastic stress

Endurance Limit Modifiers

SN curve data is usually presented from tests on standard sized polished specimens. Applying this ‘laboratory’ test data to real engineering components sometimes requires some correction. The corrections usually take the form of ‘modifiers’ applied to the specimen endurance limit.

Modifiers can be applied to take account of size, surface finish, stress gradients, surface treatments, ‘loading type, and so on. These modifiers are well documented in many engineering texts and readily available to the engineer if required.

send-modified = send . nsize . nsurface …

where n is the appropriate modifier

Fatigue Wizard supplies standard modifiers for surface finish effects and statistical effects and a general modifier to input a calculated combined value. Fatigue Wizard modifies the SN curve by scaling the specimen endurance limit as above, and rotating the SN curve about the 1000 cycle point.



Figure 4

Strain Life Technique

The strain-life technique was developed in the 1960’s as an improvement to the stress-life technique for predicting fatigue failure in material, which undergoes plastic deformation. The principle is the same, in that strain amplitude (rather than stress) is measured or calculated, for a cyclic load, and compared to an empirical graph of strain amplitude against life. The advantage over the stress-life approach is that there is no inherent assumption in the test data regarding totally elastic deformation.

Strain-Life Curves

Strain-life curves describe the fatigue behavior of laboratory tested specimens. Specimens are usually tested under fully reversed strain controlled load and correlated against life to crack initiation. An empirical curve of strain amplitude against cycles to failure is plotted.

For low levels of strain where deformation is completely elastic, the fatigue behavior correlates to the stress amplitude and

Where sf’ is the fatigue strength coefficient and b is the fatigue strength exponent, both extracted from the log-log fit of stress amplitude against life.

For high levels of strain where deformation is predominantly plastic the fatigue behavior correlates best with strain amplitude such that



Where ef’ is the fatigue ductility coefficient and c is the fatigue ductility exponent, both extracted from the log-log fit of plastic strain amplitude against life.

The general strain life curve or Coffin-Manson curve is then described as a combination of these two individual straight line curves, such that

Figure 5 Constant Amplitude Strain Life Curve. Coffin-Manson relationship.

Approximate various Strain-Life coefficients that are not available

There are various methods to approximate strain based material data from the more readily available monotonic data, such as tensile strength, modulus, and so on. The method used by the Fatigue Wizard is the method used by Seeger/Beste. It is available in many texts on strain based fatigue calculation. This technique approximates the strain life curve (strain based coefficients) using the following formulae:

Steels:

● K' = 1.65 * UTS



● n' = 0.15

● Sf = 1.5 * UTS

● b= -0.087

● c= -0.58

● Ef = 0.59 (if UTS/E<0.003)

● Ef = 0.59*(1.375-125*UTS/E)

Others:

● K' = 1.61 * UTS

● n' = 0.11

● Sf = 1.67 * UTS

● b= -0.095

● c= -0.69

● Ef = 0.535

Mean Stress Correction

The constant amplitude life curve described previously is generated through material testing under fully reversed loading, that is, with zero mean shift. In realistic engineering loading cycles, it l inevitably exists with a non-zero mean strain.

To correct for the existence of non-zero means in the strain cycle, Fatigue Wizard uses two corrections to the simple strain life curve, namely the Morrow correction and the Smith-Watson-Topper correction.

Strain-Life curve with Morrow correction

Strain-Life curve with SWT correction

Elastic-Plastic Material Data

Since strain-life data is extracted from materials, which are subjected to cyclic loading beyond the elastic limits, it is necessary to define a cyclic stress-strain material curve.

Fatigue Wizard utilizes two well-documented empirical formulae for the definition of cyclically stable stress-strain curve.

The cyclic stress-strain curve is constructed by using the Ramberg-Osgood relation to determine material constants K’ and n’. That is, the cyclic hardening coefficient and the cyclic hardening exponent.

Ramberg-Osgood relationship



To generate a complete cyclic stress-strain curve, Masing’s Theory is used such that:

By combination of these two relationships a full hysteresis loop of the cyclically stable stress-strain curve can be generated from the two material constants.

Figure 6 Cyclically stable Stress-Strain Curve

Neuber Correction

Many FE based fatigue codes use the results of elastic FE analyses. This enables you to carry out relatively simple FE calculations without incurring expense and time of running non-linear solutions. If a strain based life calculation is to be performed later using these elastic stresses, put a method in place for converting the FE based elastic stress to an equivalent elastic-plastic stress (and strain).

Of the many methods proposed for this conversion, Fatigue Wizard utilizes the most common, the Neuber correction.

Stress-Life Approach.

● Read elastic stress data from FE Results.

● Extract Worst Absolute Principal Stress.

● Scale Elastic Stresses by Kt.

● Extract Cycles from Time History of Load by Rainflow Counting.

● Modify SN Curve According To Endurance Limit Modifiers.



● Calculate Damage for Cycles With Mean Stress Correction.

● Sum Damage According To Linear Miners Rule.

● Output Fatigue Life or Safety Factor for all Nodes.

Strain-Life Approach

● Read elastic stress data from FE Results.

● Extract Worst Absolute Principal Stress.

● Scale Elastic Stresses by Kt.

● Carry out Neuber Correction To local stress.

● Construct complete local stress/strain time history using cyclically stable stress/strain curves.

● Extract all closed hystereses loops in time history.

● Construct Single Cycle Strain-Life Curve from Material Data.

● Modify Strain-Life curves according to endurance limit Modifiers.

● Calculate Damage for Cycles With Mean Stress Correction.

● Sum Damage According To Linear Miners Rule.

● Output Fatigue Life or Safety Factor for all Nodes.


● Stress Based Fatigue Analysis

● Strain Based Fatigue Analysis

● Fatigue Safety Factor

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Stress Based Fatigue Analysis

The nominal stress, SN, approach to calculating life requires minimal input to describe the material fatigue characteristics. The SN curve is generated from the material tensile strength and your input for material endurance limit.

The SN curve in the Fatigue Wizard is constructed as a straight line, on a log-log plot, from a point equating to 90% of the tensile strength at 1000 cycles, passing through the defined endurance limit at (typically) 1E7 cycles.

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Life calculation is then performed as a simple linear interpolation between these points.


Most SN test data is generated under conditions of fully reversed loading, that is, with a mean stress of zero. The mean stress in a cycle can affect the component life. Therefore, several methods of accounting for the effect of mean stress are proposed for the SN-based fatigue-life relationship. The Fatigue Wizard utilizes two methods to calculate an equivalent (zero mean) stress; the Gerber correction and the Goodman correction. Compressive mean stress is ignored.

Gerber Correction

Goodman Correction

Where

Graphical example



From FEA, for a given node, it is possible to calculate a mean stress (Sm) and an amplitude stress (SA) .

If the FEA stress falls within the . zone (Goodman) or .+ . zone (Gerber) the nodal stress is acceptable.

The software performs this calculation for all nodes in the model.

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Strain Based Fatigue Analysis

Cyclic Stress and Strain

A material subjected to a repeating constant amplitude strain eventually creates a repeating stable stress/strain hysteresis loop. You can plot several of these hysteresis loops, created at various strain amplitudes, on the same axes. Then, if a curve is plotted through the tips of all of the hysteresis loops, a 'cyclic stress-strain' curve is generated. The intent of this artificial curve is to represent the stable behavior of the material (after initial cyclic softening or hardening).

The cyclic stress strain curve is defined as:

where

K' = cyclic strain hardening coefficient

n' = cyclic strain hardening exponent

Local Strain Based Fatigue-Life Relationship

You can cycle a smooth test specimen between fixed strain limits until it develops a crack. If you cycle several such specimens between various elastic strain limits, you can plot the resulting number of cycles to crack against elastic strain amplitude. If you plot them on a log-log axes, then the points approximate to a straight line. Basquin first proposed this relationship.

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Manson and Coffin later proposed that a similar relationship exists between the plastic strain amplitude and number of cycles to failure.

The total strain-life relationship can therefore be defined as:

Where

Two methods of defining these strain-based material constants are provided within the Fatigue Wizard.

Advanced

The parameters are entered directly to the Fatigue Wizard to define the strain-based fatigue life relationship.

Approximate

The Fatigue Wizard approximates the parameters based on your input for the material tensile strength and the type of material (steel or other). Seeger's method is used to calculate appropriate material constants, and details are available in standard texts. This technique is an approximation. Use it only in the absence of actual test data.

The Seeger technique approximates the strain life curve (strain based coefficients) using the following formulae:

Steels:

● K' = 1.65 * UTS

● n' = 0.15

● Sf = 1.5 * UTS

● b= -0.087

● c= -0.58

● Ef = 0.59 (if UTS/E<0.003)

● Ef = 0.59*(1.375-125*UTS/E)

Others:

● K' = 1.61 * UTS

● n' = 0.11



● Sf = 1.67 * UTS

● b= -0.095

● c= -0.69

● Ef = 0.535


Several methods of accounting for the effect of mean stress in the cycle are proposed for the local strain-based fatigue-life relationship. The Fatigue Wizard utilizes two methods; the Morrow correction and the Smith-Watson-Topper correction.

Morrow Correction

The elastic part of the strain-life relationship is corrected by subtracting the cycle mean stress. This correction is based on the observation that the effect of mean stress is greater at long lives.

Smith-Watson-Topper Correction

Smith Watson and Topper suggest that fatigue life is a function of the product of the strain amplitude and maximum stress in the cycle. This factor leads to a strain-life relationship of the following form:

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Fatigue Safety Factor

The basis for a Fatigue Safety Factor (FSF) calculation is a perceived ‘infinite life’ of the component. To achieve the perceived ‘infinite life’ beyond N number of cycles, construct a diagram that shows values of mean stress and alternating stress, which achieve N cycles (or infinite life). You can generate these diagrams experimentally. However, for fatigue analysis, it you can approximate these diagrams from monotonic material properties, such as ultimate strength, yield strength and the fatigue strength.

Fatigue Wizard allows the definition of two different FSF type diagrams known as the:

● Goodman diagram.

● Haigh diagram (more generic).

By default the FSF is calculated using the standard ‘Goodman’ diagram as shown here.

Goodman Diagram

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Generic ‘Haigh’ Diagram



The generic ‘Haigh’ diagram is constructed with six points. The points are calculated from input of the material UTS (tensile and compressive), material yield (tensile and compressive), and the material fatigue endurance limit. The key points in the ‘Haigh’ diagram are then defined as shown in the previous diagram.

NoteValues of ultimate strength and yield strength do not have to be the same in compression as they are in tension. This note can apply to materials such as cast irons, which show greater values of compressive strength.

Calculation of FSF

The basis of the Fatigue Safety Factor (FSF) calculation is the ratio of a point representing the stress cycle in question (mean and alternating), and an intersection point with the ‘infinite life’ line.

Fatigue Wizard calculates two standard methods of forming the intersect with the infinite life line; the method of constant stress ratio (constR), and the method of constant mean (constM).

Method of Constant Stress Ratio (constR)

The basis of the method of constant stress ratio calculation is the assumption that when scaling stresses in the cycle to the ‘infinite life’ line, both the mean and alternating stresses are scaled with the same ratio.

Method of Constant Mean (constM)



The basis for the method of constant mean calculation of the FSF is the assumption that when scaling stresses in the cycle to the ‘infinite life’ line, only the alternating stresses are scaled.

The FSF calculation gives a safety factor with respect to ‘infinite life’. In calculating this safety factor, the assumption is that there is no damage summation of stress history constituent cycles (as is the case with a ‘life’ calculation). An FSF is calculated for all cycles individually, and the worst safety factor is reported. This report assumes that if the material/component can give infinite life when subjected to the worst cycle, then any subsequent cycles within the stress history do not have any further effect. This assumption is reasonable for ferrous materials that show a distinct endurance limit. However, exercise care if you apply the same assumption to a material that does not show this phenomenon.

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S-N Curve Derivation

The S-N data found in BS7608 is based on Stress Range and not Stress Amplitude as used by Fatigue Wizard for material curve data. Fatigue Wizard uses two data points for the derivation of weld S-N curves (see below).

Point (A) corresponds to 0.9xUTS at 1000 cycles and point (B) corresponds to the number of cycles for a stress of 1.0 units.

BS7608 defines a Stress Range curve, typically based on points at 1e5 and 1e7 respectively.

Stress Amplitude is equal to half the Stress Range. Therefore, you can create a modified S-N curve with an identical slope to the BS7608 curve, but offset to account for the Stress Amplitude. You can use the modified curve in subsequent calculations.

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Material Databases - WikiHelp

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Material Databases

The Fatigue Wizard ships with material databases in ASCII format stored at the location selected upon installation. The parent material libraries have the extension of .mat, and the weld material libraries have the extension of .wld.

You can edit these files with a simple text editor to add new materials, and so on. You can also create new files and copy existing files that display in the Fatigue Wizard.

The Fatigue Wizard does not read the first line of the file, which contains a description of the comma delimited data items to follow. They are, respectively;

Symbol Description Data type Name or Title 1 Name, description, or first title of the materia Text Both Title 2 Reference, class, or second title of the material Text Both E Modulus of Elasticity Numeric Both Poisson Poisson's ratio Numeric Both UTS Ultimate Tensile Strength Numeric Both Yield Yield Stress Numeric Both SF Fatigue strength coefficient Numeric Strain B Fatigue strength exponent Numeric Strain C Fatigue ductility exponent Numeric Strain Ef Fatigue ductility coefficient Numeric Strain n Cyclic hardening exponent Numeric Strain K Cyclic hardening coefficient Numeric Strain Endurance Endurance limit Numeric Stress Cycles Cycles at Endurance limit Numeric Stress

Important notes

● All data values MUST be entered in units of Force = N, Distance = mm (I.E stress in N/

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mm2.)

● Enter data in all fields.

● Failure to enter the correct data type (text or number) results in an error when you run the Fatigue Wizard.

● Save the material library in the "Addins￦Fatigue Wizard" folder where the Autodesk Simulation software is installed.

The weld database follows the same format as in the previous section, with the following exceptions;

● The descriptor SAE denotes the standard used (BS7608, and so on).

● The descriptor BS970 denotes the weld class (F, F2, and so on).

You can use only the stress-based method with welds. Enter the strain data into the database, but you can enter 'dummy' values.

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Spectrum File

With Fatigue Wizard, you can perform a fatigue damage calculation using a ‘load spectrum' or ‘duty cycle’. This type of analysis is a computationally efficient method to assess a structure or component in terms of its durability over various loading conditions.

You control the spectrum analysis with a comma separated (.csv) text file containing the description of the applied loads and repetitions of each load. To edit these files, use a text editor, such as Notepad, and import into the Fatigue Wizard in Step 4.

Below is an example of the spectrum file format, and a description of each of the required values

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Using the example above, the damage summation is performed as follows:

● Row 1 contains the total number of discrete loads that make up the spectrum file. In this case there are 11 discrete loads, so there are 11 columns in the spectrum file.

● Fatigue Wizard then reads each column of data from the spectrum file and creates individual ‘load histories from each column.

● Rows 3 to row 5 contain multipliers for load cases 1 to 3 of the FEA . These multipliers are applied in addition to any multipliers input directly on step 4 of the Fatigue Wizard. The description of this single load history is identical to a transient analysis based on load cases from the FEA.

● The load history defined by row 3 to row n is then repeated a defined number of times. The number of repetitions is defined by the number in row 2.

● A damage is calculated for the cycle and number of repetitions described above.

● Fatigue Wizard then repeats the previous steps for each column in the spectrum file.

● A total damage is summed over all of the load histories and reported.

The example spectrum file in the comma separated format is as follows:

11

10000000,3500000,3100000,2500000,2100000,1600000,600000,130000,50000,10000,5000,$

0.5,0.125,0.25,0.375,0.5,1,1,1,1,1,1,$

0.1,0.25,0.5,0.75,1,2,3,5,10,15,20,$

0.5,0.125,0.25,0.375,0.5,1,1,1,1,1,1,$

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To start the Fatigue Wizard, in Autodesk Simulation, open a model. Before you launch the Fatigue Wizard, verify that the model has results. Then, click Analysis Analysis Fatigue Analysis or Tools Utilities Add-ins Fatigue Wizard Execute to launch the Fatigue Wizard. The wizard guides you through the following steps.


● Step 1

● Step 2

● Advanced Strain-Life

● Step 3

● Step 4

● Step 5

● Step 6

● Step 7

● Visualizer

● Report Writer

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Step 1

In Step 1 of Fatigue Wizard, select a form of fatigue analysis to perform. You can perform damage based life calculations, or a fatigue safety factor calculation based on a traditional 'Haigh Diagram'.

For a description of the different analysis methods, click the hyperlinks that follow, or see the





























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Theoretical Overview section of the Fatigue Wizard Help.

Fatigue Life Calculation

● Strain Based (EN)

● Stress Based (SN)

Fatigue Safety Factor Calculation

● Haigh Diagram

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In Step 2 of Fatigue Wizard, enter the material properties to use in the fatigue analysis. The form presented depends on the type of fatigue analysis you chose in step 1.

For example, different material data is required for the definition of a strain-based analysis than a





























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stress-based analysis. The Fatigue Wizard tailors the form presented at Step 2 to highlight only the data that is required for the chosen analysis type.

However, some input to this step is common to all analysis types.

Stress Units

Verify that the material data used within the Fatigue Wizard analysis shares the same units that you used in the preceding Finite Element Analysis. If you overlook this requirement, large errors occur in the fatigue analysis results.

If the stresses are read in from a proprietary FEA software package, the units system can be picked up automatically from the stress files. In this case, the stress units are preselected, and all other units are not available for selection. If your Finite element analysis software does not support the preselected stress units, check the unit system used in the stress analysis. Then seclect the appropriate unit system on this form of the Fatigue Wizard software.

Material Graph

As a visual check of the material data being entered into the Fatigue Wizard, show the data

graphically by clicking on the chart button .

Material Databases

You can enter the required data directly into the relevant boxes on the form. Or, if available, select a predefined material from the editable material database. Use the buttons in Data Entry methods to select the material databases:

Select from parent metal data, weld material data, or user-defined S-N data, respectively.

To import data, select the appropriate icon. In the dialogbox, select a material from the list of available options. Following is a typical view of the parent metal database.














Step 2 - WikiHelp

To edit or create a material database

1. Click the Lock icon.

2. Make the appropriate changes to the table.

3. Click the Save icon.

4. Enter a new file name to create a database, or accept the existing file name.

NoteSave all material database files to the [install folder￦Addins￦Fatigue Wizard] folder. If you save material database files to other locations, they are not read into the Fatigue Wizard.

5. Click Save.

You can also select weld material data (Stress based method only).


Step 2 - WikiHelp

The weld SN curves are, mainly, taken from BS 7608 : 1993 Code of Practice for Fatigue design and assessment of steel structures.

If unit information can be derived from your FE model, then Fatigue Wizard automatically selects them. Alternatively, material data values can be automatically converted to four common unit systems by selecting the appropriate option.

The material data selected is then placed automatically into the relevant input boxes. You can still enter your own data values.

The material data entered varies, depending upon your choice of analysis method in Step 1.

SN (Stress Based Life) Data Entry

If you elect to perform a stress-based life calculation in step 1, you are required to enter two further inputs for the SN material data in addition to the Elastic modulus and the Ultimate tensile strength:

● Polished Endurance Limit

● Cycles at Endurance

For more information on these material data, see "Theoretical Overview: Stress Based Fatigue Analysis".

Select the "My material is a Steel" check box to alter the way in which the SN curve for the material is defined beyond the endurance limit. When selected, the SN curve follows a horizontal line beyond the defined endurance limit. That is, below the endurance limit of the material, there is no damage. Therefore, the material exhibits infinite life.

If you do not select "My material is a Steel,"' the SN curve is modified at stresses below the endurance limit to give a different slope. The slope used, below the endurance limit, is proportional to (2k-1) where k is the slope of the main SN curve.




Step 2 - WikiHelp

EN (Strain Based Life) Data Entry:

If you elected to perform a strain-based life calculation in step 1, you are required to enter two further EN material data in addition to the Elastic modulus and the Ultimate tensile strength.

To input the required data to describe the strain-life curve, you have two options: the Approximate method and the Advanced method.

Approximate Method

Because the methods used to determine strain-life data are expensive, and the time that this method has been widely used is relatively short, this material data is often difficult to find for non-standard materials. To overcome this problem, several methods have been adopted which approximate the strain-life data from the readily available monotonic data (modulus and tensile strength).

The Fatigue Wizard uses one of these empirical methods. If the material that you are using is not included in the materials database, select the 'Approximate' button on the form. This forces Fatigue Wizard to calculate approximate data for the strain-life curve. With this empirical method, in some cases, inaccuracies occur in the final fatigue results. As with all fatigue analysis, accurate material data is crucial.

The "My material is a Steel" box is important if you use the Approximate method. The empirical method uses different data for steels and non-ferrous materials. In a similar way to the SN curve exhibiting different behavior at high cycles, the EN curve shows different trends for ferrous and non-ferrous materials. Therefore, set the 'My material is a Steel' check box appropriately.

Advanced Method

If you have access to strain-life material data that is not included in the database, then you can enter it directly in Fatigue Wizard.

In this case, select the 'Advanced' option on the form. A separate form opens for entry of the six required parameters to describe of the necessary cyclic stress/strain curve and strain-life curve.

See a more complete description of these advanced parameters in "Theoretical Overview: Strain Based Fatigue Analysis".

FSF (Haigh Diagram) Data Entry

If you elected to perform a fatigue safety factor calculation in step 1, the material data input for the fatigue safety factor calculation requires the same data as the SN life calculation method (modulus, tensile strength , endurance limit, and number of cycles to failure), with the added requirements necessary to describe the 'Haigh Diagram'.

There are two methods to calculate a fatigue safety factor. You can select a basic Goodman diagram, or a more advanced method in which the Haigh diagram is defined with more flexibility.

Goodman method

If you select Goodman, the Fatigue Wizard sets up with a traditional Goodman diagram for its calculation of fatigue safety factor (fsf). The following is an example of the Goodman diagram.




Step 2 - WikiHelp

The Goodman diagram uses the same value for strength in both tension and compression. This factor is a severe limitation of the Goodman line for materials such as cast iron.

Advanced method

If you require more control over the shape of the 'infinite life' curve or Haigh Diagram, select the Advanced option. Although this method does not provide complete control over the definition of the shape of the diagram, it is more versatile than the Goodman approach. This versatility is important for materials such as cast irons, which show different monotonic properties in tension and compression. Using the Advanced method, it is possible to define separate values for ultimate strength and yield strength in both tension and compression.

If you click the Advanced button, a separate form loads where you can input the additional data.

on the additional form, the boxes for Tensile Strength and Fatigue Endurance limit are automatically filled with the data from the main form of Step 2. Therefore, we recommend that you complete all data required on the main Step 2 form before you enter the Advanced properties entry form.

The entry of compressive data, both Ultimate Compressive Strength and Compressive Yield Strength, are entered as magnitudes with no account of sign. Fatigue Wizard automatically assigns a negative sign to these compressive values.

If you select the Advanced option on the main form, you are required to enter additional properties before you proceed to the next steps of an analysis.

The following is an example of an 'Advanced' Haigh Diagram.


Step 2 - WikiHelp

For more details about creating advanced Haigh diagrams and the theory of the fatigue safety factor calculation, see "Theoretical Overview: Fatigue Safety Factor".

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In this step, you define your own Strain-Life curve. This graph shows a typical Strain-Life curve.

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To create the graph, we entered the material tensile strength and modulus of elasticity.

There are various methods for approximating strain-based material data from the more readily available monotonic data, such as tensile strength, modulus, and so on. The method we use in Fatigue Wizard is the same one used by Seeger/Beste. It is present in many texts about strain based fatigue calculation. Basically this technique approximates the strain life curve (strain based coefficients) using the following formulae:

Steels:

● K' = 1.65 * UTS

● n' = 0.15

● Sf = 1.5 * UTS

● b= -0.087

● c= -0.58

● Ef = 0.59 (if UTS/E<0.003)

● Ef = 0.59*(1.375-125*UTS/E)

Others:

● K' = 1.61 * UTS

● n' = 0.11

● Sf = 1.67 * UTS

● b= -0.095

● c= -0.69

● Ef = 0.535

For more control over the creation of this curve, select the Advanced option, or click the View/Edit button. When the Advanced Strain Life Data dialog box displays, enter data.

The following image illustrates these parameters.












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The addition of two curves defined by the parameters entered in the Fatigue Wizard dialog box creates the Strain-Cycle curve (red).

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In step 3, if you perform fatigue analysis on stress results from a finite element program, consider any geometric effects that are ignored in the stress results.

For example, it is possible that the finite element model does not perfectly represent the geometry





























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of the part being analyzed. It can miss geometric features that lead to stress concentrations. Examples are features such as:

● Fillets

● Holes

● Notches

● Threads, and so on

If these features are not present in the finite element model, modify the stress results to take the effects of these features into account.

You can use accepted values of stress concentration associated with the features type and dimensions. These values are readily available for many different geometric features in texts such as Roark or Peterson.

A value of Kt is used to convert the nominal stresses.

You can use the same approach in the fatigue analysis. However, some materials exhibit different sensitivity to stress concentration with respect to fatigue life. That is, the same geometric stress concentration changes the fatigue life differently, according to the material type. This effect is known as a notch sensitivity.

To take account of the notch sensitivity, it is common to apply a factor Kf to the finite element results: Kf being equivalent to Kt, but factored to take account of the notch sensitivity of the material. If a value of Kf is not known, you can be conservative and apply the Kt value.

The Kf value is entered into Fatigue Wizard in Step 3 using the box shown:

Kf>=1.0

If you are certain that the finite element model is accurately predicted stresses, then use a value of 1.0 (default) for Kf.

In addition to the stress concentrations normally associated with small geometric features, there are a number of additional factors to account for in a fatigue analysis. All of these additional factors have a potential to modify the nominal material properties obtained from specimen testing.

Normally, SN and EN material data are previously measured using specimens of a standard size, standard temperature, and standard polished surface finish. In a ‘real’ engineering component, if the conditions are not the same as these ‘ideals,’ use some means of factoring the materials properties (from test).

These factors are sometimes referred to as the endurance limit modifiers. You can add an endurance limit modifier for surface finish. Use the drop-down menu, or apply any other modifier using the ‘miscellaneous’ option.

Surface Finish














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Fatigue Wizard is coded with a variety of surface finish modifiers, which you can quickly choose. Use the drop-down menu, and choose the appropriate finish for the component (or surface) you are analyzing.

Miscellaneous

To take account of any further endurance limit modifiers, input a further single value under the ‘miscellaneous’ input. Enter a single value between the range 0 and 1.0.

● Size effects

● Temperature effects

● Loading effects

● Reliability of data

A more complete description of these effects, and the values associated with each, is available in most engineering texts on fatigue analysis.

Combination of modifiers in Fatigue Wizard

When you apply modifiers within Fatigue Wizard, understand how they affect the analysis, especially if you apply them in combination.

For more information on endurance limit modifiers, see "Theoretical Overview".

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In step 4, define the cyclic loading, which is used later in the fatigue analysis.

The FE model is previously solved, and has linear static stress results for either a single or multiple load cases.





























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To define a cyclic event and a fatigue calculation, the analyst must assign a ‘load history’ to each of these load cases. The load history describes how the stresses calculated in the static FE model vary with time. In Fatigue Wizard, you can scale and combine any one or more of the load cases by a ‘load history’ to create the final cyclic stresses.

Fatigue Wizard automatically reads in the results for all load cases analyzed in the FE model. Use the methods described in the following sections to turn load cases on and off, define individual load curves to load cases, and define scalar multipliers for each load case.

This step takes the results of your analysis and multiplies them by Load History curves. These curves define the Fatigue Event.

For example:

You performed a Linear Static Stress analysis on your FE model.

This step takes the results of your analysis and multiplies them by a Load History curve. This curve defines the Fatigue Event.

For example:

Single Load Analysis

You performed a Linear Static Analysis using the loading shown in the following image, and reviewed your stresses.

The load P is a static constant load.

But, in reality this load can be applied and then removed many times, as in the following image. If necessary, evaluate the fatigue performance of your model under this cyclic event.

For example:

Or














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It is easy to create these conditions with Fatigue Wizard.

The following steps describe how to apply a load history to the static event. Click the "Loadcurve" field for the required load case within your Linear Static Analysis.

This form displays, where you can input load curve data by various means.

Enter Load curve data into the form directly, or import it using the common CSV file format (from Microsoft Excel, for example).

If a CSV file is not available for importing, enter the data directly into the form. Then you can save the load curve data to a CSV file and use it in subsequent fatigue analyses.

This example illustrates an imported CSV file (from Excel):


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When the load curve data is entered or imported into the form and accepted, you are returned to the main 'Step 4' form. The form is updated to indicate that load curve data is entered and assigned to this load case. The load curve for loadcase1 is "Defined" and "Active."


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If necessary, after the load curve data is defined for a load case, you can remove it from the fatigue calculation by 'de-activating' the row. Click the "Active" cell to toggle its status. You can scale the load curve data. Enter a multiplier (set as 1.00 by default) in the "Scalar" cells.

TipYou can press the "Active" title bar to toggle all your load cases if you have a large number of load cases to "turn on."

Important notes on Load curves

The load curves defined previously are MULTIPLIERS to your Linear Stress results for each load case considered. Consider the following examples:

Fatigue Stress used in calculations = Linear Static Stress x Load curve Multiplier.

For example, if the loading applied to your Linear Stress model represents a maximum load, for example, 100N, then your load curve defined for fatigue analysis has a maximum value = 1.0.

Fatigue Stress used in calculations = 100N x 1.0 = 100N.

You can apply a load of 1N to your Linear Static model and then define your fatigue load curve to have a maximum value = 100. This technique is a convenient way to work, and is the most common way to apply fatigue loads.

Fatigue Stress used in calculations = 1N x 100 = 100N.

Multiple Load cases

If more than one load case is solved in the analysis, you can use more than one of these load cases in the fatigue analysis. Two methods exist within the Fatigue Wizard for utilizing more than one load case; the method of load case superposition, and a transient method. For more details on the theory of each method, see "Theoretical Overview".



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Load case Superposition

If more than one load is applied to the model simultaneously, you can assign separate load histories to each load case, and perform the necessary stress scaling and superposition.

Follow the steps described previously for each of the load cases required in the fatigue analysis. Load histories applied to each load case do not have to have the same number of points, but must all start and finish at the same value of ‘time’. If not, Fatigue Wizard warns you.

After you assign load histories to each load case, you can asign a multiplier to each load case. You can turn on or off the individual load cases using the ‘active’ button.

To view all load cases on a single graph, press the "view all loadcurves" button. . In the view that displays, ensure that all curves start and end at the same times.

Transient Analysis

A transient analysis differs from the method of superposition in that no load histories are applied to the individual load cases. Instead, Fatigue Wizard takes the results from various load cases within a Linear Static Analysis and uses them to define the sequence of stress.

The stress history that constitutes the fatigue cycle is made up of the individual stress states sequenced one after another. No load case superposition is necessary.

To enter transient information, select the load cases to define your cycle, and enter a stress scale factor, if necessary.


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The order of the stress states in the cycle is determined by the order they appear in the FE model. Therefore, if using the transient method, set up the FE model correctly for subsequent fatigue analysis.

Spectrum Analysis

A spectrum analysis is a simple extension of the previously described transient analysis. In the same way, the spectrum analysis does not perform any load case superposition. Stress histories are created within the Fatigue Wizard from sequences defined from individual load cases. Fatigue Wizard takes the results from various load cases within a Linear Static Analysis and uses them to define the sequence of stress.

The spectrum analysis differs from the simple transient analysis in that an additional text file is used to define multiple sequences of stresses. The text file is used to define both the sequence of stress (constituting a single stress history), and a number of repetitions of this history.

Fatigue Wizard then performs a damage summation over all of the repetitions of each of the individual stress cycles defined by the spectrum file.

To set up a spectrum analysis, select the "Spectrum Analysis" box. Then you can use the standard file selection icon that appears to associate a spectrum file. The spectrum analysis then takes the scale factors from the form of Step 4, and those read from the spectrum file, to create multiple stress histories. Each of these discrete stress histories is used to calculate a damage. Then this damage is all summed over the number of repetitions of the stress history, also defined within the spectrum file. A Total damage/Life is then reported over the entire spectrum.

The following example describes the process to perform a spectrum analysis:

Example spectrum file

5 10000 5000 20000 100 10 $


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0 -1 1 0 0 $ 0.05 1 0 2 -5 $ 0 -1 1 0 0 $

● The first row of the spectrum file defines the number of load histories defined in the file (in the example there are 5)

● The file is then considered to be separated into the same number of columns as there are load histories.

● The top row of each of these columns defines the number of repetitions of this load history. In the example above, the load history defined by column 1 is repeated 10000 times, the load history of column 2 is repeated 5000 times, and the load history of column 3 is repeated 20000 times, and so on.

● The remaining rows of each of the columns are multipliers of the stresses for each of the load cases in the FEA. (Note: these multipliers are applied in addition to multipliers specified on the main Step 4 form!)

● These multipliers, for each column, are used to define the multiple load histories, which constitute the overall load spectrum. In the previous example, there are three discrete load histories to make up the complete spectrum.

● A damage calculation is then performed for each individual load history and multiplied by the number of repetitions defined. This process is repeated for each load history, and the damage summed over all load histories to give a total life.

Note

● The order of the stress states in the cycle is determined by the order they appear in the FE model. Therefore, if using the spectrum method, set up the FE model correctly for subsequent fatigue analysis.

● A more detailed description of the spectrum file is contained in "File Formats: Spectrum File".

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In step 5, specify any number of cycles (for a life calculation) or any fatigue safety factor (for a Haigh diagram analysis). These ‘desired’ values are used only in post-processing of the results. They take no part in the fatigue calculation (except when you select the ‘safety factor’ calculation on step 5, which is described later in ‘safety factor calculation’).


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The form layout of Step 5 depends upon whether you elected to perform a ‘life’ calculation or a ‘Haigh’ diagram calculation on Step 1. Fatigue Wizard tailors the form presented at Step 5 accordingly, and different input is required. In all cases , you are required to input data into the "Desired Life" box. Fatigue Wizard uses this data to determine whether the calculated life or fatigue safety factor is acceptable, marginal, or unacceptable when compared to your ‘desired’ outcome.

If on Step 1 you elected to perform a ‘Haigh’ diagram fatigue safety factor calculation, the options "Cycles to failure" and "Safety Factor" calculation are unavailable, since the choices are no longer applicable.

Cycles to failure or Safety Factor

If you select a life calculation on Step 1, then Step 5 of the Fatigue Wizard gives you a choice of calculating either "Cycles to failure" or "Safety Factor". Do not confuse the safety factor selected on step 5 with the ‘fatigue safety factor’ calculated from a ‘Haigh’ diagram or a ‘Goodman’ diagram (and selected on Step 1). The safety factor selected on Step 5 is based on a cycles or damage calculation. The Fatigue Wizard takes the ‘desired’ number of cycles from Step 5 and performs an iterative process of scaling the stresses and recalculating the damage until it achieves a minimum life equal to the entered ‘desired’ life. The resulting scale factor to be apply to the stresses to achieve the ‘desired’ life is output by Fatigue Wizard.

Currently, you cannot control which load cases are scaled within the safety factor calculation. Fatigue Wizard applies the same stress scaling to all nodes and all load cases that are active in the fatigue calculation.

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In step 6, select the parts of the model to use in the fatigue calculation, and set some of the advanced calculation options.

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In Fatigue Wizard, you can perform the fatigue calculation on selected parts (groups) of the model.

If you wish to perform a fatigue calculation on only some groups of the model, select the groups in step 6. You can turn all groups on or off using the toggle button.

If you turn groups off for the fatigue calculation, the results for the nodes in these groups is automatically set to ‘infinite’ life. It is critical that you verify that there are no potentially critical areas within these groups.

Analysis Options

The design of Fatigue Wizard is meant to help you perform fatigue analyses. If you have little , experience, use the defaults for certain analysis options, which are suitable for a general purpose, time efficient analysis.

If you are experienced, change these defaults to better suit the fatigue analysis and improve the overall accuracy of the predicted fatigue results. The analysis options provide control of:

● Analysis filter

● Mean Stress Effects ( life calculations )

● Fatigue Safety Factor calculations (Haigh diagram Method)

● Stresses used in the fatigue calculation (Von Mises, Principal, Shear or Critical Plane, and so on)

Access all analysis options from the single form:














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Analysis Filter

Runtimes for performing fatigue calculations on large models or smaller models with complex load histories can be lengthy. If run times are excessively long, you can reduce run times. Define a filter, which automatically selects only a defined percentage of the nodes for fatigue processing. The selection of this top percentage of nodes is based on the maximum stress range that the node sees in the stress history.

In order to set the analysis filter, move the slide bar to whatever percentage of the model (nodes) are required for analysis.

ImportantUse caution if you use the technique of filtering the ‘worst’ nodes. In certain situations, it is possible for the filter to miss nodes that are critical locations. Use this option only with a good understanding of the possible effects. We recommend that you perform a final analysis on all nodes to ensure that all critical locations are included.

If you use the analysis filter, all nodes that fall outside of the filter are assigned infinite life.

Mean Stress Corrections ( Life Calculations)

By default Fatigue Wizard calculates result sets using three approaches to account for mean stresses. If you do not require calculation of any of these methods, switch them off in the analysis options form to improve the speed of analysis.

An SN (stress-based) calculation uses:

● No Mean Stress Effect

● Gerber Correction

● Goodman Correction

An EN (strain based) calculation uses:

● No Mean Stress Effect

● Morrow Correction

● Smith-Watson-Topper Correction

Further details of these mean stress correction methods are available in the theory sections of the Help.

Fatigue Safety Factor Calculation Methods

If you elect to perform a fatigue safety factor (FSF) calculation, by default, Fatigue Wizard uses two different methods to calculate the FSF from the Haigh diagram.

● Constant R (stress ratio)

● Constant Mean

If you do not require either one of these calculation methods, switch them off in the analysis options.

Further details of these calculation methods are available in the theory sections of the Help.

Stress Options

The Fatigue Wizard software takes linear stress results from an FEA analysis to calculate the fatigue results. Within the analysis options, you can specify how the Fatigue Wizard utilizes these stress results within the fatigue processing. By default (general case) the Fatigue Wizard utilizes the signed Von Mises approach. In some situations, this method introduces errors to the calculation because no account is taken of the change in direction of principal stresses. If you suspect this situation, select


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one of the ‘critical plane’ methods.

The following are the stress values you can select and use in the fatigue calculation.

● Signed Von Mises

● Max Abs Principal Stress

● Max Shear (Tresca)

● Critical Plane (Normal Stress)

● Critical Plane (Shear Stress)

You determine which stress value is most appropriate for the calculation. However, some stress types are not available for strain-based analyses. These stress types are unavailable if they are not applicable to the current analysis.

If you select either of the ‘critical plane’ approaches, you increase solution times significantly. The cause of the increase is the resolution of stress tensors into many planes (18 by default). A separate fatigue calculation runs on each of these planes. Before you use critical plane approaches, perform an analysis. Use one of the stress invariant methods to identify any fatigue critical areas of the mode. Then perform only a critical plane analysis on these isolated areas.

For details of the stress types that you can select, in the previous step, refer to the theory pages of the Fatigue Wizard Help.

Start The Analysis:

To begin the fatigue calculations and initiate the fatigue analysis, click ‘GO’ (stop light). You can monitor progress through the stages of the fatigue analysisr. Details of the current analysis phase and estimated analysis time is reported.

To stop the calculations that are in progres, click the 'STOP' (stop light).

When the calculations complete, you see a summary of the worst fatigue results for any of the mean stress correction methods chosen. The visualizer icon in the treeview window changes to indicate that fatigue results are generated and ready for viewing.

ImportantAfter you complete the fatigue calculations, if you move backwards in the Fatigue Wizard steps (using the back button, or the treeview window), the results are lost. The Visualizer icon in the treeview window changes to indicate that results are not available.

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