How to Perform A Creep Deformation Analysis in Ansys Workbench Mechanical

How to Perform A Creep Deformation Analysis in Ansys Workbench Mechanical

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If you’ve ever hung a heavy item on a plastic clothes hanger and come back later to see it has permanently bent, then you’ve witnessed Creep in action. Creep occurs when materials are subjected to sustained mechanical loads over a long period, slowly deforming and eventually breaking. This process is accelerated at high temperatures, making it a concern for engineering materials.

Creep Deformation Analysis vs. Stress Rupture Analysis

Creep rupture and Creep tests measure the time and rate of Creep deformation in a test specimen under a sustained mechanical load and specific temperature. Creep tests also monitor elongation throughout the test, providing more information than a Creep rupture test.

Before analyzing Creep deformation, it’s essential to determine if it’s necessary for the component. If the component only needs to avoid rupture, a stress rupture analysis suffices. The primary question to ask is whether excessive deformation will affect the component’s function. For example, a turbine blade with excessive Creep deformation would rub against the containing casing, causing wear and vibration issues. In this case, a Creep deformation analysis would provide useful information. Conversely, a reformer tube only needs to avoid Creep rupture before its intended design life.

The Three Phases of Creep

Creep can be divided into three phases: primary, secondary, and tertiary. Primary Creep starts quickly and slows down approaching the secondary phase, which is the longest and occurs at a constant rate. Tertiary Creep has an increased rate of deformation that may lead to rupture, and Ansys typically models primary and secondary Creep.

How to Perform A Creep Deformation Analysis in Ansys Workbench Mechanical

 

The Norton model is commonly used for modeling secondary Creep, using a power-law relationship that expresses the Creep strain rate as a function of stress and temperature.

How to Perform A Creep Deformation Analysis in Ansys Workbench Mechanical2

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.

How to Perform A Creep Deformation Analysis in Ansys Workbench Mechanical3

To turn on Creep deformation effects for a load step, specify the current load step number, and then turn on Creep effects under Creep controls. Set up the initial conditions with Creep effects off, then have a second load step with Creep effects on. Note the Creep limit ratio under Creep controls, which limits the ratio of equivalent Creep strain to elastic strain increment. A ratio of 1 to 10 is recommended for accurate solutions.

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These are the basics of implicit Creep analysis in Ansys Mechanical. For a more thorough example of a Creep analysis, refer to Chapter 35 of the Workbench Technology Showcase.

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