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.
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.
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 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.
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.