12 Replies Latest reply on Oct 10, 2017 4:04 PM by Jim Steinmeyer

    How to evaluate the resulting stresses????

    Qmb Eid

      i did a model time dynamic analysis.... but i had a results that shows that there is a pick stress on the body !!!
      how can i evaluate the results if it is bigger than the yield strength and the strength limit of the aluminum???!!!!
      is it acceptable to have this small deformation area!!!??????
      please how  knows please answer me >>>

       

       

       

       

      Снимок1.JPGСнимок2.JPG   

        • Re: How to evaluate the resulting stresses????
          Ryan Dark

          Hi Qmb,

          According to your images the yield stress of the model is 190 MPa while the selection on the model is experiencing between 196 MPa and 299 MPa.  The blotchiness of the results plot would indicate that the mesh is too coarse for this to be a converged stress plot but given the range of values you are seeing across the selected entity you almost certainly have exceeded yield on that portion of the model.  You would need to reduce the mesh element size in this region and rerun the study to confirm though.

           

          Assuming that the additional testing you do with smaller mesh element sizes still exceeds 190 MPa then you can only conclude that the model has failed to withstand the loading applied to it.  The results for displacement and stresses above yield would be considered invalid since this is a Linear type study (only valid in the linear region of a material).

            • Re: How to evaluate the resulting stresses????
              Qmb Eid

              thanks i first thought this i a yield stress for a static studies and not working in the liner dynamic studies.....
              please one more question ... is it acceptable to have this small deformation area ..???  where the deformation even not through the body but only on the surface>> and the stress accepted on the rest of the body

                • Re: How to evaluate the resulting stresses????
                  Jim Steinmeyer

                  That is where your engineering judgement comes in. Is it a critical component? What is the use of the part? What would be the results of a failure? How critical is the undeformed shape of the component?

                  Lots of questions that we can not answer for you. I have seen places where we would accept this slight deformation and there have been instances where this definitely would not be acceptable.

                    • Re: How to evaluate the resulting stresses????
                      Qmb Eid

                      it is a camera ... study shock test
                      01.JPG02.JPG
                      do you think that i have to ignore this small deformations in this place and other places where there is also like this small deformations ?

                        • Re: How to evaluate the resulting stresses????
                          Jim Steinmeyer

                          First I am going to say that you get what you pay for, and since i am not being paid or really seeing all the constraints and forces my opinion is probably not worth much. But, in looking at where most of the the stresses are they appear to be on sharp edges or corners other than the ones around the mounting holes. If possible I would look to placing a radius on those edges. With the majority of the part showing a low stress I suspect that a radius will spread out the stress sufficiently in a hurry. The same would hold for around the bolt holes, If you place a small boss to simulate a washer I suspect the stresses will rapidly decrease. How ductile is your material? Not being able to read the language your property manager is in I don't know what you are using. If you are using mild steel I might suspect that the very small areas will deform plasticly only a very small amount and then the areas around it with low stresses will come into play and stop deformation. Again, I do not know what the loads or the material are and only you can make that determination. My first choice would be to improve the design to reduce the stresses. That is why we use simulation after all.

                            • Re: How to evaluate the resulting stresses????
                              Qmb Eid

                              thanks for your helps
                              the material is aluminum  АЛ2 it is like AL-2024
                              the loads is a shock impulse 300g - 1.2 msec.

                                • Re: How to evaluate the resulting stresses????
                                  Qmb Eid

                                  i changed the edges to a radius and got these results
                                  in your opinion can i say that this deformation is acceptable???

                                   

                                   

                                  01.JPG02.JPG

                                    • Re: How to evaluate the resulting stresses????
                                      Jim Steinmeyer

                                      Again, not being familiar with the material I don't know how it will react to localized deformation. he material might yield and stretch slightly, or it might crack which would lead to the crack propagating further. What is the cost / loss if the bracket fails in testing? If you have a $30 part that fails and you can go back and build it up, no big deal I would give it a try. If you have a $30,000 mold that will need to be reworked or scrapped, that is a big deal.

                                      We don't know your circumstances, only you can make the determination if the risk outweighs the reward for not eliminating all of the hot spots before making a test part. I don't know the reason for the lower rib being recessed where the hot spot is. Maybe something fits in there and you need clearance there. Maybe it is there only for looks and you could either place a chamfer or a much larger radius there to eliminate it. Maybe it can be a quick and easy fix so you should do it, maybe there are lots of challenges to work around and it is important to keep the design as close to current as possible. Maybe this is for a class and the instructor will be grading you on how well you are able to use FEA to make a product that will work the first time.

                                      Part of being an engineer is being able to look at the circumstances of the product you are designing and being able to make your own decision on if changes are required. Here we get accused of throwing steel at problems. Our parts go on a $250,000 product that is needed every day. Not having a feeder to feed your cattle because a $40 part is broken is not an option, so adding a few extra dollars to the part is how we deal with it. If someone can't video record his bike ride to show a couple of friends because a $15 part broke, probably not a real problem. A little Duct Tape and a Popsicle stick can get them through. Maybe.