I'm a materials engineer by degree, but I'm learning how to use FEA for my master's project. I drafted a simple tensile specimen and a ran a nonlinear static analysis on it using autodesk simulation mechanical. The material I selected is ASTM A441 with a yield strength of 46ksi. I calculated that my tensile sample needs 2875lbf to acheive this stress, and so I applied this load to one end. To my amazement, when I ran the simulation it perfectly predicted the stress at 46.122 ksi.
Now for my question...I want my model to able to predict the ultimate tensile strength of the sample. If I assume a tensile strength of 66ksi for the material and apply a corresponding load of 4125lbf, the von mises stress shows 77ksi in the material not the expected 66ksi. I'm using the Von Mises Isotropic Hardening plasticity material element, and I'm using a tetrahedryl mesh. Any reason why I cant seem to predict the stress in the material after it has yielded??
What is the stress tensor result (in the direction of the tensile load) on the centerline of the model? (Select some elements across the section near the center of the model, right-click, and hide the unselected element. You may want to consider using a symmetry model or multiple parts so that you have a clean "break" at the center of the model.)
16 years experience with Simulation Mechanical
"Von Mises with Isotropic Hardeing" material model cannot help obtain the ultimate stress. Since the stress-strain curve provided by "Von Mises with Isotropic Hardeing" material model does not follow the real material curve. To better simulate your model, you can use "Von Mises Curve with Isotropic Hardening" and insert the real stress-strain data of the material.
First, define "Von Mises Curve with ..." in "Element Definition"; and then Click "Material" -> "Edit Properties", you can see "Stress vs. strain data".
- Enter tensile stress and strain values. The material is assumed to behave the same in tension and compression.
- The first row corresponds to the yield stress.
- Since the modulus of elasticity, yield stress, and strain at yield are directly related, only the first two items can be entered. The strain at yield will be calculated automatically.
- If the material properties for a part are loaded from a material library, and if the material library does not have the stress-strain curve entered, the stress-strain data will be approximated from the library's modulus of elasticity, yield stress, ultimate stress, and elongation at 2 inches.