Realistic loading time histories are often difficult to obtain. A measurement of these is required during typical and extreme operating conditions. This is normally carried out by a dedicated measurement team using transducers and data acquisition equipment. MSC Fatigue time history files are automatically generated using the nCode data acquisition products (see More Information about nCode International, 1464). This type of integration contributes to the overall efficiency of product life assessment.

Cyclic materials data are required for modeling fatigue. These are normally obtained by testing smooth specimens in a laboratory using constant amplitude loading. The raw materials test data must be processed to obtain the cyclic properties used in MSC Fatigue. The nCode materials test analysis products provide the additional tools required to capture and process the raw data (see More Information about nCode International, 1464).

The quality of the FEA results and hence, fatigue life estimates is critically linked to the FEA mesh quality. It is important to represent the geometry accurately in the critical areas (not necessarily over the whole model). Good representation of the geometry will be aided by the use of quadratic elements which can better represent curved shapes. Quadratic elements are also more accurate when analyzing complex stress fields in the regions around stress concentrations. Joints between elements of widely differing sizes, especially where stress gradients are high, will give erroneous results. The refinement of the mesh until no significant change in local stress occurs (h-convergence) is strongly recommended since correct global stiffness is a necessary but not sufficient condition for good fatigue analysis.

Any fatigue analysis should ultimately be checked out by testing. Fatigue testing often takes a long time and is expensive to set up and run. However, these drawbacks are not a reason to avoid testing, but it is important that the usefulness of the test is maximized. In practical terms, this means monitoring the test and correlating it with the MSC Fatigue analysis. MSC Fatigue allows the user to output a local elastic strain-time history which could be compared with local strain measurements under test conditions.

It is often possible to shorten variable amplitude tests, using fatigue editors which remove the non-damaging portions of the time history. The benefit in reducing test times and hence costs are very attractive, especially if the product can be put on the market earlier. Such editors are part of the associated software systems from nCode (see More Information about nCode International, 1464).