If you’ve ever hung a heavy item on a plastic clothes hanger and come back a while later to see that the hanger has permanently deformed, then you’ve seen a phenomenon known as “Creep” occurring firsthand. When materials are subjected to sustained mechanical loads over long periods of time, they tend to slowly deform, and eventually rupture. While this failure mode is usually not a concern at room temperature for most engineering materials, the process is accelerated at high temperatures.
Creep Deformation Analysis vs. Stress Rupture Analysis
A Creep rupture test is performed by applying a sustained mechanical load to a test specimen which is maintained at a specific temperature and the time to rupture is measured often over the course of months or even years. A Creep test is similar to a Creep rupture test, but in a Creep test, the elongation of the test specimen is monitored throughout the test. This gives information on the rate of Creep deformation that a Creep rupture test does not provide.
Before beginning an analysis, it is important to determine if a Creep deformation analysis is necessary. If the only concern is ensuring that a component does not rupture after some time, a stress rupture analysis is all that is required. The key question to ask is if the excessive deformation will interfere with the function of a component. For instance, a turbine blade that experiences excessive Creep deformation would begin to rub against the casing that contains it, causing wear and vibration issues. In this case, a Creep deformation analysis would provide useful information. On the other hand, a reformer tube used in the production of hydrogen does not have tight clearances, and the only concern is making sure that a Creep rupture does not occur before the intended design life. In this case, a Creep rupture analysis using a Larson miller curve may be all that is required.
The Three Phases of Creep
Creep can be broken down into primary Creep, secondary Creep, and tertiary Creep. Primary Creep begins at a fast rate and then slows down as it approaches the secondary phase. Secondary Creep is the longest portion of the Creep curve and occurs at a constant rate. Finally, during tertiary Creep, there is an increase in the rate of deformation and may culminate in rupture. Ansys has the capability to model primary and secondary Creep, but Tertiary Creep is not typically analyzed because rupture is impending.
The Norton model is commonly used for modeling secondary Creep. It is a power-law relation that expresses the Creep strain rate as a function of the stress and temperature.
Setting up a Creep Analysis in Mechanical
The actual process of setting up a Creep deformation analysis is relatively straightforward. First, add a Creep material model to the material in the Engineering Data section of Ansys Workbench. In the below, example, the Norton model was chosen to model secondary Creep.
To turn on Creep deformation effects for a particular load step, click on Analysis Settings, specify the current load step number, and then turn Creep effects on under Creep controls. You may want to use the first load steps to set up the initial conditions for the Creep analysis with Creep effects turned off, and then have a second load step where Creep effects are turned on. Finally, note the Creep limit ratio under Creep controls. To ensure an accurate solution, this setting limits the ratio of the equivalent Creep strain increment to the equivalent elastic strain. Typically recommended values are between one and ten.