Fatigue Analysis Made Easy

Software for durability assessment is now more accessible – and vital – than ever.

Bob Williams
Product Manager
ALGOR, Inc.
Pittsburgh, PA

This article was published in Machine Design, "Will the Design Survive? Ask Fatigue Analysis", May 19, 2005.

Has your company experienced product failure under stresses far below what was thought "safe"? If so, the likely cause of the failure was fatigue. Under long-term cyclic loading, a material can fail at significantly less than its yield strength. While this behavior may seem puzzling and even counter-intuitive, most engineering failures (60 to 80 percent) now occur because of some form of fatigue.

What Is Fatigue?

Typically, fatigue occurs in products such as steel rails, beams, girders and rotating stepped shafts where materials are repeatedly cycled through varying stresses. The stresses either modulate in intensity or actually reverse. Over time (perhaps months or years depending on the frequency), the cyclic stresses cause a weakening of the material. The greater the amplitude of the stress variation is, the greater the fatigue effect – and the shorter the product life.

When a fatigued material fails, the characteristic pattern of crack propagation consists of a splitting that is initially relatively smooth-shaped, but gradually widens over time until catastrophic tearing and fragmenting occurs. In contrast, a material that fails under stresses greater than yield (shortly after application of the load – instantly or within minutes) usually shows jagged tearing and fragmenting throughout.

Computer-Aided Fatigue Analysis

Historically, engineers performed hand calculations using engineering handbooks and Goodman diagrams to verify the fatigue life of product designs; but, hand calculations only consider a few points and may not be accurate for complex geometry including assemblies. Early fatigue analysis software packages automated the process of a stress-based fatigue life calculation and provided more detailed and accurate simulation; but, they were expensive and cumbersome to use – largely, the tool of "guru" analysts.

Now, modern fatigue analysis software packages, such as the one provided by FatigueWizard of Loughborough, United Kingdom, make it more affordable and easier than ever to perform sophisticated and accurate stress- and strain-based fatigue life calculations. Through an innovative wizard interface, FatigueWizard helps complete effective durability assessments of product designs.

How to Use FatigueWizard

You must first use an FEA package to perform a static stress analysis of the model since stress or strain results are needed as input for the fatigue life calculation. For instance, after obtaining stress analysis results in ALGOR, simply choose the "FatigueWizard" menu option (or toolbar icon) to set up a fatigue analysis.

After performing an ALGOR linear static stress analysis of this rotating stepped shaft model, a simple menu option was used to launch FatigueWizard for fatigue life calculation.

The FatigueWizard interface guides you step-by-step through the process. First, choose between two options for the type of fatigue analysis to be performed:

• Strain-based (recommended default) – modern technique that allows for plasticity and is suitable for low- and high-cycle fatigue problems.
• Stress-based – traditional technique used only for high-cycle fatigue problems.

The strain-based option was chosen as the type of fatigue analysis to be performed.

Specify material information using an extensive, editable database or custom values for the elastic modulus and tensile strength. A strain-life curve is then automatically generated, which can be viewed to verify the material data entry. Alternatively, you can enter your own data to create a customized curve.

The built-in material library was used to specify the material properties for the rotating stepped shaft. The strain-life curve was viewed to verify the material data.

Enter data to simulate real-world conditions. A multiplier can be entered to simulate local stress concentrations that were not explicitly modeled, such as welded connections. In addition, a simple pop-up menu is used to specify a surface factor due to the machining method.

For the rotating stepped shaft model, the geometric detail was accurately modeled so that a stress concentration was not needed. A surface factor was specified to account for the effects of the machining method.

Define the load history by specifying values for the load multiplier over time. Options for multiple load cases and transient analysis can also be specified.

For the rotating stepped shaft model, a load history for a single load case was specified using a pre-defined sinusoidal curve included within FatigueWizard.

Specify the desired number of repetitions for the loading cycle that can be withstood without failure. Options are also available for FatigueWizard to calculate either a safety factor or the number of cycles to failure.

The number of repetitions for the loading history was entered to specify that the rotating stepped shaft should withstand ten-million cycles without failure.

Perform the fatigue life calculation. After the analysis has been completed, a summary of the results is displayed, which indicates whether your design is safe based on the parameters that you entered. It also gives the predicted number of cycles to failure.

Although the ALGOR linear static stress analysis had indicated stresses were far below the material yield point, the fatigue analysis indicated the design was unsafe based on the entered parameters. It also predicted the number of cycles to failure at far less than the desired 10-million cycles.

Result contour displays can be viewed in both FatigueWizard and the FEA package. For instance, results such as life or safety-factor contours can be viewed directly within ALGOR's FEMPRO interface. A customizable, HTML-based design report can also be generated in FatigueWizard with minimal time and effort. This report can be conveniently viewed and shared with others.

Result contours of the FatigueWizard analysis were displayed directly within the ALGOR FEMPRO interface. An HTML-based design report was created for convenient viewing and sharing. The report summarized the results for the rotating stepped shaft and suggested possible courses of action to correct the fatigue failure.

Ready for Fatigue Analysis?

As seen with this example, FatigueWizard is a full-featured, easy-to-use tool for performing fatigue life calculations, which allows you to better verify product designs. In particular, the strain-based technique is a valuable feature because it accounts for nonlinear load history and can provide a more thorough, detailed and accurate simulation of fatigue life over the traditional stress-based technique. Plus, the capability to access FatigueWizard from within a familiar FEA interface and view fatigue analysis results within that environment provides convenience and flexibility.

Is your product durable? It's time to find out.