13 Replies Latest reply on Aug 11, 2013 11:55 PM by Jared Conway

    Factor of safety is not displayed for SLS material

    Samio Roi

      Hello,

       

      I have tried analysing(stress simulation) a component based on a material which I have defined and configured. Following is the data sheet.

      http://www.3dsystems.com/sites/www.3dsystems.com/files/DS_DuraForm_PA_US.pdf

      But with the above data being fed in, I am not able to view the factor of safety value which seems to be 0 all throughout.

       

      Much appreciate your help.

       

      Samio

        • Re: Factor of safety is not displayed for SLS material
          Justin Strempke

          What sort of analysis, and what do you have your failure criteria set to under the material properties?  I think if it's set to 'Unknown' as most plastics are your FOS won't compute.

            • Re: Factor of safety is not displayed for SLS material
              Samio Roi

              Thanks for the quick reply Justin.

               

              All materials under Solidworks have been defined under "linear elastic isotrophic". Is their any other data which is to be fed desired(factor of safety) results. Basically I am performing a force applied analysis on the component with a few faces acting as rigid fixtures.

               

              I desire to know when would the component break or optimize it.

               

              Also  I would like to know if i can use the 'elongation at break' data in the fed input of the material?

                • Re: Factor of safety is not displayed for SLS material
                  Justin Strempke

                  So which material property are you trying to compare to for FOS?  I was thinking you had to set a failure criteria (drop-down list beneath the analysis type), but I think this just pre-populates your FOS plot options.  Your material data doesn't list a yield (plastics are tricky), but you should be able to set your stress limit to an input value or use the Ultimate value option.

                   

                  I'm not entirely sure a linear model is what you need, either - depends on your setup.

                  • Re: Factor of safety is not displayed for SLS material
                    Jerry Steiger

                    Samio,

                     

                    I would be leary of trusting the elongation at break data for use in determining whether your part will break. If the material behaves in a very brittle manner (stress increases linearly to the breaking point), then it would probably be OK, but the sintered Nylon definitely does not behave that way and there must be a lot of non-linearity. (14% elongation with a Tensile Modulus of 1586 MPa would mean an Ultimate Tensile Stress of 222 MPa, not the 43 MPa that they list.)

                     

                    Even if you approximate the stress-strain curve and make your own non-linear material model, I would still be hesitant to trust the FEA results for predicting breaking.

                     

                    Jerry S.

                • Re: Factor of safety is not displayed for SLS material
                  Jared Conway

                  Justin is on the right path. Post a screenshot of your material definition.

                   

                  The likely issue is that you're missing a property required for the failure criteria or you've chosen one and not plotting that criteria.

                   

                  I might suggest trying a simple block and then posting it and your sldmat file.

                    • Re: Factor of safety is not displayed for SLS material
                      Samio Roi

                      material defination SLS.png

                       

                      Well justin mention, I have used the factor of safety plot based on the ultimate strength of the material and it has worked.

                       

                      Jerry has mention the material strength to be around 222Mpa. This has led me into confusion whether I should be accepting the results provided by solidworks. At the same time I feel the data sheet cannot mislead into such large deviations.

                       

                      Really appreciate your help.

                       

                      Samio

                        • Re: Factor of safety is not displayed for SLS material
                          Jared Conway

                          hi samoi, can't tell if you have a question here or not. and if so, what it is.

                            • Re: Factor of safety is not displayed for SLS material
                              Samio Roi

                              Hi Jared,

                               

                              Above in the replies, Jerry mentions that the material should have a strength of 222 mpa . I would like to know if the present value of 43 Mpa which I have used is correct or not. If not how do you reach a value of 222Mpa and define a non-linear material.

                               

                              samio

                                • Re: Factor of safety is not displayed for SLS material
                                  Jared Conway

                                  as per jerry's response, the only way to answer that question is to understand how your component will fail.

                                    • Re: Factor of safety is not displayed for SLS material
                                      Samio Roi

                                      Jared,

                                       

                                      It is of utmost interest to me to know how the component fails (as you pointed out). Even though this holds my best interest but I cannot determine or gather data for the exact curve. Even solidwork's predefined plastics are defined under the category of linear models with most having breaking criteria as "known".

                                      The only factor I feel remains constant and does not vary with the material property is the stress plot on the model. I have been using this plot and comparing against the tensile strength (1586 Mpa) of the material, looking out in the stress plot if anything is beyond the FOS value (1586/2=793Mpa) I desire.

                                       

                                      I would like to know if I am on the right track, if not then how else do I approach this. Also the FOS plot shows that minimum FOS value is 4 and the displacement plot shows that the maximum displacement is 80 mm. This is not reliable.

                                       

                                      Regards,

                                       

                                      Samio

                                        • Re: Factor of safety is not displayed for SLS material
                                          Jared Conway

                                          hi samoi, you've got 2 streams of thought in your last response

                                           

                                          the first is what the software is doing

                                           

                                          the second is the physical interpretation

                                           

                                          i might suggest that you concentrate on one or the other. i think you're confusing yourself by looking at both at the same time.

                                           

                                          from what i understand, you don't have a material curve. all you have is E. no problem, you just have to deal with the fact that the software will model it as a linear material eventhough it may act nonlinearily.

                                           

                                          it sounds like your goal is to have a FOS of 2 based on tensile strength. no problem. for the FOS plot, you need to choose tensile as the "comparison" for that to be used to calculate the FOS.

                                           

                                          regarding displacements not being reliable. you'll have to elaborate.

                                           

                                          also, you should re-read jerry's comments about that material type.

                                  • Re: Factor of safety is not displayed for SLS material
                                    Jerry Steiger

                                    Samio,

                                     

                                    I was just pointing out that if you assume a linear response and use the reported values of the Tensile Modulus of 1586 Mpa and 14% elongation at break, then the stress at break would be 0.14*1586=222 Mpa. In real life the Ultimate Tensile Stress appears to be 43 MPa, so the real stress/strain curve is strongly non-linear. If you assume a linear material response, it will only be valid for small strains.

                                     

                                    Jerry S.