19 Replies Latest reply on May 20, 2017 11:15 AM by Ruben Dyck

    Simulation

    Ruben Dyck

      I have been working with SW for about a year now and at this point I am barely going into simulation. That part is entirely new for me. I'm wondering, could someone explain the basics of interpreting results? I know how to set up and run basic simulations but I feel very insecure about interpreting the results and would really appreciate an explenation in layman's terms. Can't find a good summary.

       

      • What does stress measure?
      • What does displacement measure?
      • What does disfiguration measure?
      • What about the factor of safety?

       

      What do the colors represent? How far can I go and still know that the product will be safe?

        • Re: Simulation
          Ryan Dark

          Hi Ruben,

          Hopefully I can help clear up some of these questions for you.

          1. Stress is a normalized value for how much "load" an object is under.  For a specific geometry with a specific simulation load setup on it the stress field should be (approximately) the same for all materials (for loads that cause small displacements; displacements so small you could not see them with the naked eye).  You would then compare that loaded stress against the failure stress (called "yield stress") of the specific material you have applied for the analysis.  When the loaded stress exceeds the yield stress the model is considered to have failed.  Often a factor of safety is applied so that a load stress ~25-50% of the yield stress is considered a failure (so we don't kill people, eh?).
          2. Displacement measures the actual displaced form of the model under load.  It should relate directly to a measurable movement on the design under the loading condition you have put it under.
          3. I don't know what that is referring to within Simulation.  Can you expound?
          4. The calculation for factor of safety is (Yield Stress)/(Load Stress).  If a model has a maximum load stress of 10,000 psi and the material has a yield stress of 30,000 psi you have a factor of safety of 3.

          The colors on the plot refer to the legend that is also on the screen.  The colors are trying to show a visual representation of the stress or displacement distribution on your model under load.  You will need to decide what Factor of Safety you want for your design which will dictate that maximum load stress which will in turn tell you how much load your design can take.

            • Re: Simulation
              Ruben Dyck

              Thank you Ryan. This helps a lot.

               

              Just to clarify, yield stress would be the load that the material and design could take and still be safe, right? And then load stress would be the load that the material or design would have under the current simulation, right? And the factor of safety would show this at a glance, correct?

               

              I'm a bit confused with colors. One post I found seemed to suggest that in the FOS, blue is safe (good) and red is unsafe (bad). But I wonder if mine is inverted. Mine starts with red at the bottom and the higher it goes, it turns into blue. The others (stress, displacement, etc.) are inverted though - blue at the bottom, red on top.

               

              I'm attaching a picture of my tension results and FOS results. My assumption is that the structure is on the safe side. Am I correct?simulation 1.JPGSimulation 2.JPG

                • Re: Simulation
                  J. Mather

                  Are you familiar with Scientific Notation?

                   

                  Look at the upper range on your FoS.  2.607E+07 Who cares?

                  FoS Chart Options.png

                  Change the Chart Options to range between 0 and 15 (over say 15 or so is irrelevant, don't know why in the world SolidWorks defaults to max calculation).

                   

                  Also - be aware that Factor of Safety is a misnomer and does not mean "safe", it is not a measure of "safety".

                  The part in this thread (see link) would have to be formed well below FOS 1 simply to manufacture.

                  I need help modeling a part

                   

                  FoS is  = Sy/Smax within the linear elastic range, below 1 the part has gone into plastic deformation (see  I need help modeling a part ).

                • Re: Simulation
                  J. Mather

                  Ryan Dark wrote:

                  The calculation for factor of safety is (Load Stress)/(Yield Stress). If a model has a maximum load stress of 10,000 psi and the material has a yield stress of 30,000 psi you have a factor of safety of 3.

                  I think you have your formula  reversed.  10,000/30,000 is not equal to 3

                    • Re: Simulation
                      Ruben Dyck

                      Thank you J. Mather.

                       

                      Sorry for my questions. I need a simulation for dummy's book. I am not an engineer and just self-taught at solidworks. So to confirm, do I understand this correctly?

                       

                      FOS = stress yield / stress maximum. The basic concept is the following. Stress yield is how much stress the part can handle, right? Stress maximum would be the maximum stress exerted according to the simulation. An FOS of 1 would mean that it was right at it's limit, so that would be BAD. The higher the number of FOS, the BETTER and further away from it's limit and therefore safer? Is that correct?

                       

                      I'm curious, how do you figure out the stress yield of your part? Or the actual FOS? Sorry my SW is in Spanish. I made the changes you suggested and I'm attaching the results as a picture.

                      simulation 3.JPG

                      Would this parts' minimum FOS be 1.4? Would I interpret this correctly if I concluded that all the red parts are ok, but the green/blue areas are more at risk? I still don't understand how I should interpret this. At what number should I be warned and take a hint that I need to change the design or use thicker material, etc? What reference can I use?

                      • Re: Simulation
                        Ryan Dark

                        Yes, that was reversed.

                        • Re: Simulation
                          Ruben Dyck

                          I did some more digging on the internet and I think I'm starting to get the hang of it =) please let me know what you think.

                           

                          I believe that the stress measure gives me the stress yield which in this case would be 351,571,000. The maximum stress in this case would be 252,032,192. So my FOS would be 351,571,000 / 252,032,192 = 1.4.

                          simulation 1.JPG

                           

                          I changed some colors, so here's the final FOS result:

                          Simulation 2.JPG

                           

                          Obviously most of the part is well off and more than strong enough. However the green and yellow / orrange areas could be concerning. However, since the most stressful point is still 1.39, it means that it would be fine as long as it's not overloaded. It could actually take an additional 40% of the intended load before breaking. If we wanted to be super sure however, we should make some changes and make sure that max point reached at least 3.0. Am I interpreting this correctly?

                            • Re: Simulation
                              J. Mather

                              Ruben Dyck wrote:

                              .... It could actually take an additional 40% of the intended load before breaking.

                              FoS is not a measure of "breaking" (fracture).

                              Are you familiar with the stress/strain curve for the material?

                                • Re: Simulation
                                  Ruben Dyck

                                  No, I was not familiar with the stress / strain curve for the material. I did some digging and found this graph: Strain.JPG

                                  So the FOS would be a measure of breaking, fracture, but it weasure how much the material could take and still be safe, before starting to be damaged, correct? Were my assumptions on the simulation I ran correct?

                                    • Re: Simulation
                                      J. Mather

                                      Ruben Dyck wrote:

                                       

                                      1. So the FOS would be a measure of breaking, fracture, but it weasure how much the material could take and still be safe, before starting to be damaged, correct?

                                      2. Were my assumptions on the simulation I ran correct?

                                      1. Everything you have written is incorrect.

                                      I am not sure whether this is because of difference in first language (is English your native language) or lack of education in the topic, or did you simply miss a word here and there (go back and read exactly what you wrote).  (Are you a student?  If so, can you ask your instructor to join this discussion?)

                                       

                                      2. How can anyone say without seeing your file(s)?

                                        • Re: Simulation
                                          Ruben Dyck

                                          Your assumption is correct. English is not my first language. I'm from Mexico. I am not educated in this topic as my background is in administration and only self-taught in SW.

                                            • Re: Simulation
                                              J. Mather

                                              If I start to go through step-by-step, do you have the time (about 15 weeks) and stamina to stay with me?

                                                • Re: Simulation
                                                  Ruben Dyck

                                                  Thank you for the offer! That's very generious. That would depend.

                                                  • How much time would you expect to go into it every week?
                                                  • What format would we be talking about? Disuccions, skype or other video conference, etc.?
                                                  • How much would you charge?

                                                   

                                                  We are currently very busy and I have very little free time. But let me know how you would envision it, and I'll get back to you. Again, thanks for the offer.

                                              • Re: Simulation
                                                Ruben Dyck

                                                I gave it some thought and realize changing the other chart ranges from 0-15 would be a bad idea and really only applicable for the FOS.

                                                 

                                                As stated before, the only background I have is high school physics which I don't remember much. Also, I am not a student and have no instructor. We have assistance from a Mexican company, but while their assistance for SW in general has been ok, they seem to know little more than me about Simulation. What I'm trying to figure out is the basic assumptions behind simulation results that can better help me interpret them. I'm still having trouble understanding FOS and how that applies to the design. What I meant to write before was the following:

                                                 

                                                "So the FOS would NOT be a measure of breaking / fracture, but rather a measure of how much stress the material could take before beginning to get damaged. The breaking / fracture point would come later with more weight as indicated by the stress / strain curve for material."

                                                 

                                                In other words, let's say I design a structure that is supposed to hold a distributed mass of 1,000 kg and I prepare the simulation as such. If the minimum FOS is 1.4 it means that the structure could hold 1,400 kg and be ok. If I'd begin loading 1,500 kg or more, it could be under too much stress and as the load increases eventually it might get to a breaking or permanent damage point. If the FOS were 2 it would mean that it could support 2,000 kg, no problem. Is this correct or am I missing the boat entirely?

                                    • Re: Simulation
                                      John Stoltzfus

                                      Must be Friday or whatever - when I seen this post in simulation I just couldn't help thinking it and now I can't get the tune outta my head "Oh Rubie Don't Take Your Love To Town" by Kenny Rogers