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mvass last won the day on October 14 2019

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About mvass

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  1. Sure: Optistruct 2018 (student version). I've placed the input file in your dropbox. Thank you.
  2. Hello all. I know there is a very good tutorial on the subject (https://www.youtube.com/watch?v=236ofmwz1X4) however I would like to share my latest experience when comparing stress results obtained from two different solvers i.e. MSC NASTRAN and Optistruct. By default NASTRAN calculates stress using the command: STRESS(PLOT,SORT1,REAL,VONMISES,BILIN)=ALL, meaning that (if you neglect the other terms) a bilinear extrapolation is used to get the stresses from the element's centroid to the nodes. Since optistruct/RADIOSS calculates stress at the element's centroid by default, I supposed that the equivalent command in the .fem input file for optistruct would be: STRESS(SORT1,H3D,REAL,ALL,BILIN) = YES (I have replaced "VONMISES" with "ALL" to have all stresses in my output) and performed two examples of simple static analyses: one with shell elements: the well known hole in plate and another with a bracket meshed with solid elements (4 noded tetras). In both cases the results from MSC NASTRAN and Optistruct were identical, as long as in Hyperview I activated the option "use corner data" (participation of element to the node). Without "corner data" activation the difference in VM stress between NASTRAN and Optistruct for the hole in plate problem was 175 MPa (NASTRAN) to 157 MPa (Optistruct). Now: I performed an analysis of a 3d bolted connection (two plates connected with a 3d bolt) with bolt pretension as a first step and a tensile force acting on the top plate of the connection as the second step. Obviously, this problem involved contact between the plates and the bolt. The analysis converged and the results were correct since I knew from published results (and NASTRAN) that max. stress should be approx. 80 MPa. I received the same results with optistruct, but without the "use corner data" activated, although I have used the command described before for stress. If I activate the corner data in Hyperview, then my stresses almost double in magnitude (156 MPa). So, this contradicts with what I have found with my previous tests (hole in plate and 3d bracket). Does it have to do with the "contact"? Thank you in advance for your reply.
  3. Dear PrasannaK, I had a quick look on your approach. It seems that the contact properties defined need some serious adjustments, because if you run the model as per the updated file, the lower plate performs a large rotating motion. I'll have a further look and let you know, although as mentioned before I think I can solve the problem by: a) creating a "freeze" contact between the two plates and simulating the bolt with 1D beam element connected with RBEs to the bushing and plates.
  4. Dear PrasannaK, thank you for kindly replying to my question. I'll have a look on your file later this afternoon, however I must say that I have made some progress myself yesterday: I had to define a "freeze" contact between the two plates, because the axial movement (slide contact, with friction), was causing the "hot spot" on the bushing that I mentioned in my previous post. I am not however sure if that approach is physically correct. As to the contacts, I count the following: bushing-plate 1, bushing-plate 2, plate 1 - plate 2, bolt - bushing. If you also include the bolt's head, then it is bolt head-bushing, bolt head-plate 1, so yes that makes 6 contact surfaces in total. Let me check your file and I'll contact you again. Thank you for taking the time to check my model!
  5. Dear all, Lately I am having some problems solving a problem that might seem "trivial", however I am still having problems getting some meaningful results. As seen from the files attached, the problem consists of two aluminium plates connected with a steel bolt and a bushing. There is a press fit of 0,2mm (if I remember well) between the bushing and the two plates. The upper plate has one of its sides fixed and a compressive force of 90 kN is acted on the lower plate. Although I have simulated the contact* between all components (plates, bolt, bushing) and ran a quassi static NL analysis, I keep getting extremely high stresses at the area seen on the attached picture. It is probably some mistake with the contact there but I cannot find it. Please have a look on the attached .hm file. There are no contacts defined to allow you to use your own approach to the problem. You can use any of the latest HM versions to setup the problem. Thank you in advance for your kind assistance. *: a number of approaches used: friction, no friction, slide, freeze etc. bolt_coupon.hm
  6. Thank you for your reply. As mentioned in my question, I am aware that Simsolid can show the contact forces between two bodies. Word to developers: Add this, and above all the ability to assign composite layups to parts (including post processing tools) and Simsolid will definitely become the software of choice for many-many companies... Respectfully, mvass
  7. Is it possible to see the stress results due to contact? I've seen that forces (resultant and components) between contacting surfaces can be evaluated, but I haven't seen any options for contact stresses. Thank you.
  8. Answer: The use of "R-trias" elements of the right size does the trick...
  9. Hello all, I would like your expert opinion about the surface mesh I am trying to perform on the curved areas of a simple wing (leading edge). Since I would like to export the mesh for a CFD analysis, I am trying to mesh the surfaces with "trias" elements. However I am not happy with the mesh on the leading edges which exhibit a high curvature. I know the result will be better with "quad" elements, however as I mentioned before I would like "tria" elements on my surfaces. The "surface deviation" option meshed the surface (as per attached example) but with too many elements. Can you please have a look on the attached model (note: HM version 10)? Thank you in advance for your assistance. surface_deviation.bmp elevator.hm
  10. Thank you for your reply. In my study, I am not looking for post buckling and my solution scheme does not include a velocity BC like the example shown. Besides the obvious constraints, the load is exerted on the model via an enforced displacement. In addition, I am not looking on a transient NL analysis. But I guess it wouldn't hurt if I'll try to solve the problem with NLPARM and TSTEP card, provided that it is possible. Should I get something worth to write about, I'll post it here.
  11. Hi, any views on my updates about the problem? Thank you in advance
  12. Second update: I managed to solve the afore-mentioned problem, by specifying a value of 1.0 (instead of the default 1.1) to the DTSCI entry of the NLPARMX card (see the attached picture). However the question still remains: Why the solution is different when the default values are used? How come there is no warning/error in the previous situation? nlparmx.bmp
  13. OK, here is my update to the problem. Unfortunately I cannot share the model with you, however, please find attached the .out file and the solution scheme from the .fem (input file). As mentioned before, the solution converges normally but the results are not correct. It is a NL buckling problem (compression of a panel) and I am supposed to have a max displacement (magnitude) on the panel of 20mm. This doesn't happen and the max. displ. magnitude I get is 10mm. My solution was a NL geometric (impl) that I set up in Optistruct and automatically translated to Radioss (version 2017.2). I tried to run the problem as quasi-static NL with large displacement activated in optistruct but it didn't converge. Thank you in advance for your assistance. CCCC_v2_rad_s1_0001.out problem_fem_extraction.txt
  14. I agree with you. That's why I was surprised to see different results when NINC was 50. Maybe I couldn't see where the mistake was, however there was no error shown in both runs (NINC=50, NINC=25). Thanks for the advice about the param option.
  15. Thanks for your reply and sorry for my late feedback. I cannot remember what I did with the latest version, however I'll give it a go with NLSTAT and large displacement option and contact you again. However I still cannot understand why when you specify that you want a NL analysis with fixed number of increments (constant DT), the problem converges with less increments completed (and the result doesn't capture the phenomenon as it should).
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