Static and Fatique Analysis of a Fork

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Fatigue Analysis

It is observed that repeated loading and unloading weakens objects over time even when the induced stresses are considerably less than the allowable stress limits. This phenomenon is known as fatigue. Each cycle of stress fluctuation weakens the object to some extent. After a number of cycles, the object becomes so weak that it fails. Fatigue is the prime cause of the failure of many objects, especially those made of metals. Examples of failure due to fatigue include, rotating machinery, bolts, airplane wings, consumer products, offshore platforms, ships, vehicle axles, bridges, and bones.

Linear and nonlinear structural studies do not predict failure due to fatigue. They calculate the response of a design subjected to a specified environment of restraints and loads. If the analysis assumptions are observed and the calculated stresses are within the allowable limits, they conclude that the design is safe in this environment regardless of how many times the load is applied.

Results of static, nonlinear, or time history linear dynamic studies can be used as the basis for defining a fatigue study. The number of cycles required for fatigue failure to occur at a location depends on the material and the stress fluctuations. This information, for a certain material, is provided by a curve called the SN curve.

Stages of Failure Due to Fatigue

Failure due to fatigue occurs in three stages:

Stage 1 One or more cracks develop in the material. Cracks can develop anywhere in the material but usually occur on the boundary faces due to higher stress fluctuations. Cracks can occur due to many reasons. Imperfections in the microscopic structure of the materials and surface scratches caused by tooling or handling are some of them.

Stage 2 Some or all the cracks grow as a result of continued loading.

Stage 3 The ability of the design to withstand the applied loads continue to deteriorate until failure occurs.
Fatigue cracks start on the surface of a material. Strengthening the surfaces of the model increases the life of the model under fatigue events.

Performing Fatigue Analysis in Solidworks :

Create one or more static, nonlinear or modal time history dynamic studies.

Create a fatigue study (constant amplitude or variable amplitude) and define its properties. Make sure to set the proper options before running the study.

Notice the events type shown with the name of the Loading folder in the Simulation study tree. Right-click the Loading folder and select Add Event to define a fatigue event based on a reference static study, or on a particular solution step from a nonlinear or modal time history dynamic study. To define a fatigue event based on multiple reference studies, set Loading Type to Find Cycle Peaks.

( Variable amplitude events usually contain large amount of data. The Function Curve dialog lets you import text files containing the data as load history curves. The data become available when defining a variable amplitude fatigue event )

You must define at least one SN curve for each material. Click the Fatigue SN Curves tab in the Material dialog box to define SN curves.

Right-click Result options and select Define/Edit to request results at all nodes or at boundary nodes only. If the study is based on variable amplitude events, you can request the Rainflow matrix chart at the desired locations.

For variable-amplitude fatigue studies, right-click a fatigue event icon and select Plot 3D Rainflow matrix or Plot 2D rainflow matrix to plot a chart for the bins of the input load history

Run the fatigue study.
If the results of a reference study, used in defining an event, are not available, the program runs the reference study automatically before running the fatigue study.

View results:

Double-click an icon in a results folder to display the associated plot.
To define a plot, right-click the Results folder and select the Define Fatigue Plot.
To list results, right-click the Results folder and select List Fatigue Results.
For studies with variable amplitude events, right-click the Results folder and select Define Matrix Charts to view Matrix Charts.
Click here for more result viewing options.

If you run a study before meshing it, the program meshes the study automatically before running it. You can also request to run the study by checking Run analysis after meshing in the Mesh PropertyManager.

When you run a study that does not have any result folders, the software creates the folders and plots specified in the Result Options for the study type. If result folders are present, the software updates the existing plots.

For more : http://help.solidworks.com
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