Introduction
When calibrating a material model it is important to select the correct type of strain control in MCalibration. In a traditional compression test it is not possible to get tensile stresses, and you can achieve this also in MCalibration by selecting “strain control compression” as the control variable. Similarly, in a tension test of a thin specimen you cannot get a compressive stresses, and you can achieve this also in MCalibration by selecting “strain control tension”. This article explains the difference between these control types and the traditional “strain control”.
1. Compression Testing
Consider a uniaxial compression experiment in which the strain is ramped to -30%, and then unloaded back to 0% again. If the material is undergoing any amount of viscoplastic deformation then it likely that the top loading platens is not in contact with the test specimen at the end of the test. In other words, the test specimen is always exposed to a compressive stress state. It it not possible to have a tensile stress in a traditional compression test.
1.1. MCalibration (Default Strain Control)
If you set up a compression test in MCalibration, then the default setting is to follow the experimental strain history. In this case the stress can become positive during unloading (see image below).
1.2. MCalibration (Strain Control Compression)
MCalibration also supports a different option called “Strain Control Compression”). If this control type is selected then the predicted stress cannot be positive. In this case MCalibration will internally switch to load control at the time the top loading platens stops being in contact with the test specimen.
The stress predictions in this case are shown with a blue dashed line in the figure below.
2. Tension Testing
In most cases a tension test specimen can be loaded to both tensile and compressive stresses. The exception to this is if the tension specimen is very thin. In this case the specimen will buckle very easily and the stress will always be positive.
This scenario can be captured in MCalibration by setting the control type to “Strain Control Tension”, see image below.
The predicted stress-strain response in this case will be like the blue dashed line shown in the figure below.
Summary
- For some cyclic compression tests if is better to use “Strain Control (compression)” in MCalibration. This will prevent MCalibration from calculating tensile stresses.
- For thin tensile specimens it is better to use “Strain Control (tension)” in MCalibration. This will prevent MCalibration from calculating compressive stresses.
