# Torsion results don't match hand-calcs **URL:** \ **Category:** General Questions\ **Tags:** results\ **Created:** [December 5, 2023, 1:30pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317 "2023-12-05T13:30:28Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![JKnight](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JKnight](https://prepomax.discourse.group/u/JKnight)\ **Post date:** [December 5, 2023, 1:30pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/1 "2023-12-05T13:30:28Z")
I trying to start small and build up to a slightly more complex study. For now, I’m analyzing a simple tube in PreProMax, and I got widely different results from hand-calcs. PreProMax says: 0.006999728 Rad (~0.4 deg) Hand Calcs says (13deg)[[Torsion of a Thin-walled Open Tube | Online Calculator](https://calcdevice.com/torsion-of-thin-walled-open-tube-id209.html)] EDIT: this is the wrong formula, leaving it here to keep the thread coherent) 8mm OD 6mm ID 300mm length 316 Stainless Steel I followed this tutorial: [https://www.youtube.com/watch?v=2upPxL45OZg](https://www.youtube.com/watch?v=2upPxL45OZg) The mesh is 1mm to 0.1mm, with the other settings remaining default. I applied 500 N⋅mm to a rigid body from a reference point at one end, and made a fixed boundary condition on end-surface of the other end. Then I look at UR3 for the rotational displacement in the Z-axis. 1. What other information should I include here to ask for help? 2. If I upload the .pmx file to share, do I also need the .STEP file separately? 3. Where should I start looking for my problem? I’d like to apply more force, but I expect I’ll need a non-linear study for more deflection. I need to learn more before I can do that. ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [December 5, 2023, 1:37pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/2 "2023-12-05T13:37:04Z")
Please share the .pmx file. No need for a separate .step file. You can upload it to some hosting website like WeTransfer, Google Drive or Dropbox and paste the link here. The formulas are for open tube, I assume that your model accounts for that. It would be easier to start from a closed pipe. Formulas from Roark’s: ![roark](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/a/a4facec2fc0007d9a65509da7ec71018053b65e0.png) K is used in θ=(TL)/(KG) ---
**Author:** ![ANYS](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/anys/32/7737_2.png) [@ANYS](https://prepomax.discourse.group/u/ANYS)\ **Post date:** [December 5, 2023, 1:41pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/3 "2023-12-05T13:41:24Z")
> [@JKnight](#): > > settings remaining default Rotations are nonlinear effects. Default iteration scheme may lead to error. Try to switch to automatic with a minimum of 10 increments for the whole period. Let us know if that fix your discrepancy. ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [December 5, 2023, 1:43pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/5 "2023-12-05T13:43:02Z")
You have to set access to “Anyone with the link” and then copy the link and share it here. ---
**Author:** ![JKnight](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JKnight](https://prepomax.discourse.group/u/JKnight)\ **Post date:** [December 5, 2023, 1:53pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/6 "2023-12-05T13:53:07Z")
2nd attempt at a link > **[1601\_Nail1.pmx](https://drive.google.com/file/d/1reqYg0KzKUjkIhsi-4E-0HAeYGStGQcw/view?usp=sharing)** > > Google Drive file. ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [December 5, 2023, 1:55pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/7 "2023-12-05T13:55:38Z")
As I suspected, your part doesn’t have a cutout so you should use the formulas for a closed pipe. I shared the ones from Roark’s above. I would also refine the mesh - it’s very coarse in your model. ---
**Author:** ![JKnight](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JKnight](https://prepomax.discourse.group/u/JKnight)\ **Post date:** [December 5, 2023, 1:58pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/8 "2023-12-05T13:58:35Z")
(I assume you also replied on reddit?) You’re right - I’ve checked my math and discovered that I was using the wrong formulas. Thank you for straightening me out. It’s always nice to discover the FEA was right and the hand-math was wrong. I’m starting with this shape/loading because I’m designing a part that will be a derivative of this. It will get various slots/holes/changes in wall thickness. As deflections get larger, I’ll need to move to non-linear analysis, no? ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [December 5, 2023, 1:59pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/9 "2023-12-05T13:59:56Z")
> [@JKnight](#): > > (I assume you also replied on reddit?) No, that wasn’t me. > [@JKnight](#): > > As deflections get larger, I’ll need to move to non-linear analysis, no? Right but let’s correct the linear analysis and get a sufficient agreement with the analytical approach first. I got 0.00693 rad from the formulas above. ---
**Author:** ![JKnight](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JKnight](https://prepomax.discourse.group/u/JKnight)\ **Post date:** [December 5, 2023, 3:57pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/10 "2023-12-05T15:57:30Z")
I’ve been interupted a few times, but I got there: > **[1601\_Torsion.sm](https://drive.google.com/file/d/1Lx9qfShQO9wsvuzQv8QDjSqF25GrfZvQ/view?usp=sharing)** > > Google Drive file. this SMath sheet shows that PreProMax and hand-calcs agree. ---
**Author:** ![JKnight](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JKnight](https://prepomax.discourse.group/u/JKnight)\ **Post date:** [December 5, 2023, 6:28pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/11 "2023-12-05T18:28:54Z")
I added PEEQ, and still got good results (0.0698 instead of 0.0699, slightly closer to hand-calcs) When I increased the torque 10x and re-ran the study, I noticed this in the output: ||b|| getting too small with scaling, boost it statically what does b mean in this context? ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [December 5, 2023, 6:47pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/12 "2023-12-05T18:47:39Z")
> [@JKnight](#): > > I added PEEQ So you added plasticity to the material definition ? Because PEEQ is just an output variable for plasticity. > [@JKnight](#): > > ||b|| getting too small with scaling, boost it statically It’s an array used by PaStiX, you would have to examine the ccx source code to find its meaning. ---
**Author:** ![JKnight](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JKnight](https://prepomax.discourse.group/u/JKnight)\ **Post date:** [December 5, 2023, 8:00pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/13 "2023-12-05T20:00:40Z")
yes, I added 1 point of plasticity for the material. My study didn’t converge though. I killed it after 27 min. ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [December 5, 2023, 8:10pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/14 "2023-12-05T20:10:16Z")
It’s better to cover a larger range with plasticity and add some hardening. But of course, the values and load magnitude have to be reasonable. ---
**Author:** ![JKnight](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JKnight](https://prepomax.discourse.group/u/JKnight)\ **Post date:** [December 6, 2023, 9:16pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/15 "2023-12-06T21:16:54Z")
I’m struggling with the details of how to do this, and what sources to trust. My understanding is that we’re trying to put in more data points in the relevant region of the stress/strain curve. I assume the software does linear interpolation between them. Stress up to yield is assumed to be linear, so we’d enter Yield-stress and 0? or 0.2%? Then we need some additional points along the curve in the plastic region. Should I be grabbing these from a stress/strain curve? I’m currently working with 316 Stainless Steel (1.4401 in some parts of the world). I found [this curve](https://ars.els-cdn.com/content/image/1-s2.0-S1738573320304587-gr9_lrg.jpg) from as figure 9 in [this paper](https://www.sciencedirect.com/science/article/pii/S1738573320304587). I tried to grab some values from first part of the graph 205 MPa, 0 mm/mm 310 MPa, 0.01 mm/mm \<–QUESTIONable measurement! 675 MPa, 0.2 mm/mm 900 MPa, 0.44 mm/mm Is there a more accepted method for acquiring / entering this data? ---
**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\ **Post date:** [December 6, 2023, 9:40pm UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/16 "2023-12-06T21:40:45Z")
Check this thread: [Help Non Linear Aluminium FEM Beginner - #6 by FEAnalyst](https://prepomax.discourse.group/t/help-non-linear-aluminium-fem-beginner/1311/6) Basically, you need true stress vs true **plastic** strain data points. Here you can find some helpful information, including formulas to calculate true stress/strain from engineering stress/strain and plastic strain from total strain: [https://classes.engineering.wustl.edu/2009/spring/mase5513/abaqus/docs/v6.5/books/gsx/default.htm?startat=ch05s02.html](https://classes.engineering.wustl.edu/2009/spring/mase5513/abaqus/docs/v6.5/books/gsx/default.htm?startat=ch05s02.html) You can use a bilinear model (hardening recommended) like this, for instance: Re ——- 0 Rm —— A where Re is yield strength, Rm is tensile strength (UTS) and A is elongation at failure. Or you can specify more data points to define a while nonlinear stress-strain curve (keeping in mind what I said at the beginning about the required stress and strain measures). > [@JKnight](#): > > I assume the software does linear interpolation between them. Between them - yes. But outside of the specified data, it assumes plateau (horizontal line). You should avoid this due to potential convergence issues - do the linear extrapolation manually by specifying higher stress-strain values as the last point. ---
**Author:** ![lucas\_bueno](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/lucas_bueno/32/1168_2.png) [@lucas\_bueno](https://prepomax.discourse.group/u/lucas_bueno)\ **Post date:** [December 7, 2023, 1:34am UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/17 "2023-12-07T01:34:53Z")
> [@FEAnalyst](#): > > you need true stress vs true **plastic** strain data points. Special attention to this @JKnight ! Some people may be familiar with Nastran in which some non-linear solutions use engineering stress and strain… In CCX we need to use the true ones. The results can be very different. ---
**Author:** ![JKnight](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@JKnight](https://prepomax.discourse.group/u/JKnight)\ **Post date:** [December 7, 2023, 2:21am UTC](https://prepomax.discourse.group/t/torsion-results-dont-match-hand-calcs/1317/18 "2023-12-07T02:21:53Z")
Thank you for clarifying. I believe I can use the curve in the top-half of the image I posted. I wish I had more of the data they used to create the graph.