Overview of Durability Simulations

When you have the appropriate role, you can define a durability analysis case or an FKM analysis case to simulate the useful life of metallic products.

Note: This section discusses non-FKM durability analysis cases only. For FKM analysis cases, see FKM Assessments.

Durability analysis requires additional configurations of your model and scenario. You can create a durability analysis case and configure it using frames from one or more structural analysis cases. If multiple structural analysis cases contribute to the fatigue loading, they must all share the same FEM representation. During the fatigue calculation, the fatigue solver automatically reads the appropriate frames from the structural analysis cases and generates the appropriate output. (For more information, see Frames from Structural Analysis Cases Referenced in the Fatigue Loading.) Durability analysis cases can use frames from structural cases containing steps such as static steps, static perturbation steps, or implicit dynamic steps.

This page discusses:

Basic Process of Fatigue Analysis

In most cases, you define the fatigue analysis in a simulation where you have already run the structural analysis case, so your simulation already has stress results. The following figure depicts the basics of the process, with structural analysis cases followed by durability analysis cases. The process is organized by phases of the definition.



Detailed Discussion of the Steps for Fatigue Analysis

The full process of creating a fatigue analysis includes two sets of steps: one set you perform in the structural analysis case and another you perform in the durability analysis case.

Configure the Structural Analysis Case

Perform the following steps to configure an existing structural analysis case for subsequent use in a durability calculation:

  1. Apply an appropriate material to the model. For more information, see Material Palette.

    Choose a material with fatigue options defined in its simulation domain to share a model between structural and durability analysis cases. For more information, see fe-safe Durability Properties.

    The fatigue options also include the configuration of a fatigue algorithm, which can describe finite life or infinite life behavior. The app requires a fatigue algorithm and the associated fatigue options in the simulation domain to run the durability analysis case. For more information, see About Fatigue Algorithms.

    You can apply a new material after the initial solve of the structural case, but making such a change invalidates the stress results and requires you to run the simulation again. For more information, see About Analysis Cases.

    Tips: If a suitable material is not available in the database, you can do either of the following:
    • Create a new material from scratch with appropriate fatigue behavior.
    • Import the Elastic-Fatigue material briefcase, which provides a set of over 300 materials suited to elastic-fatigue analysis.
  2. Optional:

    Revisit the mesh to determine whether it is suitable for a fatigue analysis. You might need to switch to the Structural Model Creation app to review the mesh. For example, you might need to run a mesh convergence study in the area of interest to make sure the mesh density captures stresses accurately. As with material assignment, changes to a mesh invalidate previously solved structural analysis cases.

  3. Configure the output requests for the solid geometry in your model.

    Most fatigue failures occur on the exterior surface of a solid body. As a result, you should use stresses at element-nodes or stresses averaged at nodes rather than those at interior (Gauss) integration points to ensure the most accurate fatigue life prediction.

    For structural analyses that involve elastic-plastic material behavior, you should use elements with nodal integration elements.

    For structural analyses that involve elastic behavior only, you can use elements with nodal integration points or those with interior (Gauss) integration points. In the latter case, the app extrapolates the stresses to the element nodes (without nodal averaging) before it performs the fatigue calculations, and the fatigue analysis results are output at the element nodes.

  4. Configure the output requests for the shell geometry in your model.

    As with output requests for the solid geometry, confirm that the stresses are output on the top and the bottom.

Define the Durability Analysis Case

Perform the following steps to define the durability analysis case for the fatigue analysis.

  1. Create a durability analysis case.

    For more information, see About Durability Analysis Cases.

  2. Select the finite element model that you want to use for the durability analysis case. If more than one FEM representation is present in the simulation, choose the FEM representation that is shared with the structural analysis case to be used in the fatigue loading.

    For more information, see Applying a Finite Element Model to a Simulation.

  3. Define the fatigue loading history. Fatigue loading defines one or more events that describe the fatigue load history. These events refer to frames from steps defined in the structural cases. You do not need to solve the structural cases before you define fatigue loading.

    Define the Defining Fatigue Loading.

  4. Optional:

    Define the surface finish. You can specify a base surface finish for the entire model or a local surface finish for a specific region of your model. A local surface definition supersedes the base surface finish where they overlap.

    For more information, see Defining Surface Finishes.

  5. The app generates durability output automatically.

    The app calculates field output for fatigue at the same locations requested in the structural analysis case.

    The variables that are output depend on the fatigue algorithm you chose in the fatigue-related material options for the materials applied to the model. For more information, see About Fatigue Algorithms.

Run the Simulation Job

When you run the simulation, you can run the durability analysis case separately from the structural analysis cases, or you can run them both in sequence. For more information, see Running a Simulation from an App.

The Run dialog box lists the individual analysis cases in order. Select the check boxes next to the analysis cases that you want to run, and specify the number of CPUs you want to allot to the solver for each analysis case. From the Units options, specify that the simulation runs using MKS units.

Postprocessing

Physics Results Explorer enables you to display output from the structural analysis case and durability analysis case. For more information about the results available for durability analyses, see About Durability Output. For information about the results output for structural analyses, see the Abaqus Output Guide.

Frames from Structural Analysis Cases Referenced in the Fatigue Loading

A simulation object can contain several structural analysis cases, and each structural analysis case can also include multiple steps.

For a durability case, you must refer to at least one frame from one step to create a Fatigue Loading definition. But fatigue load histories can be much more complex. Fatigue loading events can contain references to frames from more than one structural case in the same simulation object. Below is a diagram of a simulation object that contains two separate structural cases, each containing one or more static steps. The fatigue loading in the durability analysis case references frames from all three steps.



If the simulation object includes multiple structural cases, the fatigue loading does not have to reference all of them.