Learning Forum

[javat33489]

Hi all. Strange things are happening at ANSIS 2022 R2. My model is a cylindrical part - a rod, with different faces, looks like in the picture, this is an example (the real part has threads at the ends and different grooves). One end is fixed, the other end is set to move. While moving, I look at the force reaction and compare with the test results of real samples. I use bilinear isotropic hardening and large displacements. So the model stretched by 45 mm and reached the value of the force at which the value of the tensile strength of steel was exceeded, this is 29 tons. The tests of the real sample are the same, the product breaks with a force of 29-30 tons, BUT the stretching of the real sample is only 7 mm. Why such a big difference with the calculation? 7 mm and 45 mm.

 

[peteroznewman]

Do you have an experimental stress-strain curve for the steel that the rod is made from?  Maybe the bilinear isotropic hardening material model is not a good match to the actual material data.

What flaws were in the actual sample that were not in the model?

What were the measured dimensions of the actual sample and how much do those numbers differ from the model?

Where did the actual sample fail?  Where does the model predict the failure occurs?  If the actual sample failed at the threads at the end, then you can adjust the model to include them.

[javat33489]

The model is fully consistent with the sample. The hardening model was taken from the library data - this is stock steel without processing, previously it always coincided in different calculations of the samples (even if there is a slight difference).

In the calculation, the model is torn in a different place, because the real sample broke on the thread. I think this is the case, but why then did the loads coincide?

If the actual sample failed at the threads at the end, then you can adjust the model to include them.

- I don't understand this, can you explain in detail?

[peteroznewman]

The threads represent a stress concentration and so the failure occurred at that location.

Edit the geometry to represent the cross-section of the pipe including the profile of the threads and any reduction in wall thickness from steps in the thickness as well as the thread profile. Use small elements to capture the high stress gradients around these stress concentrations.

You could even do a small breakout model of just this local region.

[javat33489]

small breakout model - what does it mean? Crop the model to the right place?

[peteroznewman]

You don't need the whole length of the tube when you know where it failed. On a cropped model, you can put a lot of elements to capture the details of the geometry around the thread where the failure occurred.