16 Replies Latest reply on Jul 27, 2009 6:55 AM by laurent klopfenstein

    Polymer failure modes

    Derek Bishop

      How do you define the allowable limits of a polymer? One suggestion I heard recently was to take the yield stress at a 10 % proof stess point and then make the allowable limit a quarter of that. I want to stay in the elastic region and avoid problems with creep. I'm doing some FEA on a plastic platform that has to support in excess of 13 tonne (metric) for an extended period of time. The polymers are EPS and HDPE. How about limits on strain. I understand this is important with composites. How about with polymers? I had a long search on the net and could not find a scrap. Any assistance here would be helpful. Oh and if you know of any good plastics design books that would be helpful also. Thanks in advance.

        • Re: Polymer failure modes

          First of all, you have to understand for polymer, the yield stress might in a wide range which depends on manufacturer and batch, I will recommend you to get the relative information from www.matweb.com , and you have to choose from one manufacturer and the lower value of yield stress should be used.

          After that, you have to give a large allowance of FOS due to the property of Polymers:


          • Plastics will deform permanently  under load  even the load is relatively low.
          • When subjected to static low  stress / strain a ductile / brittle transition will occur at some point in  time resulting in brittle failure
          • Cyclic stressing will result in  a ductile / brittle transition resulting in brittle failure at low stress  level
          • Premature initiation of  cracking and embitterment of a plastic can occur due to the simultaneous  action of stress and strain and contact with specific chemical  environments (liquid or vapor)
          • Re: Polymer failure modes

            Polymer failure modes

             

            Mechanical Modes 

            • Deformation and distortion due to creep & stress relaxation,
            • Yielding,
            • Crazing
            • Brittle Fracture  due to Creep rupture (static fatigue),
            • Notched creep rupture, Fatigue (slow crack growth from cyclic loading), High energy impact
            • Wear & abrasion,

             

            Thermal Modes   

            • Thermal fatigue
            • Degradation – thermo-oxidation
            • Dimensional instability
            • Shrinkage
            • Combustion
            • Additive extraction

             

            Chemical Modes   

            • Solvation, Swelling, dimensional instability and additive extraction
            • Oxidation
            • Acid induced stress corrosion cracking (SCC)
            • Hydrolysis (water, acid or alkali)
            • Halogenation
            • Environmental stress cracking (ESC)
            • Biodegradation
              • Re: Polymer failure modes
                laurent klopfenstein

                Hi

                 

                We are making many simulations for plastic par ( PA6, PP, PBT or PA66 for example ) I have already posted a picture showing the limit to considere depending on the art of stress that you have. A plastic is up to 0,2 % strain elastic and then start the plastic area. The problem ist how long at which temperature !. The picture I have posted give you some indication for a part design to withstand 10 to 15 years.

                 

                https://forum.solidworks.com/thread/25201?tstart=0

                 

                We are working for an automotiv supplier and already test and confirm those limit many time

                 

                best regards

                  • Re: Polymer failure modes
                    Derek Bishop

                    Laurent,

                     

                    Can you explain in general terms how you determine the allowable  stresses for the plastic. The graphs don't help me much.

                     

                    To find a yield stress I understand you use a 0.2% strain offset. Knowing the yield stress, the allowable stress could be 2/3 x Yield Stress ie. FOS = 3/2 or 1.5.

                     

                    What kind of safety factors do you use and how do you determine these?

                      • Re: Polymer failure modes
                        laurent klopfenstein

                        Hi,

                         

                        so you do not have any offset, the 0,2% is just more or less the theorie.

                         

                        to explain the graph let s take an example.

                         

                        1) You want to design a plastic housing ( PA66GF30 for example ). The housing will be under pressur coming from a spring and the temperatur is from -40°c up to 120°c.

                        In that case we are taking the strain/stress curve from the PA66GF30 @ 120°c and looking for a maximal stress around 0,5 % ( like the graph say ).

                         

                        2) If there are exeptional force comming from an other source, we will do the simulation with a maximum allowed stress at 1% Strain

                         

                        3) if the Housing have some cliping function, we will simulate the bending of the clip with a maximal stress of 0,5 x Max Strain @ room temperature.

                         

                        The safety coeficient or FOS is not really possible to calculate because it will not only depending on the fiber orientation but from the" cold joint" as well !

                         

                        we have a mold simulation software integrated in Solidworks ( Simpoeworks ) and doing a mold simulation before stress simulation to see a little bit what happen.

                         

                        regards

                          • Re: Polymer failure modes

                            So the main concern is the Stain level but not the stress level?

                            And the Stain level is the main contribution factor of creep?

                              • Re: Polymer failure modes
                                laurent klopfenstein

                                Hi

                                 

                                The strain give me the allowed stress.

                                By the way, I have to add something, just looking the Von mises is not enough ! you have to look the principal stress as well, at least the first !.

                                 

                                Let me give you an exemple, if a volume of the part you are simulating is growing in the way that the shap stay exactly the same but is bigger ( like a zomm effect , a ball stay a ball but bigger ) the Von mises will be zero !! ( yes look the formel to understand when all stress a equal ).

                                but the first Pricipal stress will gibe you the value.

                                 

                                to understand the what is the principal stress :

                                For a 3D element (a cube) there will be 3 normal stresses and 3 shear stresses. They are often given in a global coordinat system.

                                 

                                Now imagine that you create a local coordinat system for each element. And imagine that you rotate the coordinate system so that there is no shear stresses on the cubes surfaces. The only stresses present for the rotated cube are "pure" normal stresses. Those are the principal stresses.

                                 

                                So my advice :

                                in Solidworks Simulation, always compute the Von mises and first Principale stress

                                 

                                regards

                                  • Re: Polymer failure modes
                                    Derek Bishop
                                    Okay thanks, that helps to explain what the graphs are about. Seems to me that you are setting your limiting criteria as strain like Shaodin says. But I can see that you can get an allowable stress based on the strain. You talk about fibre orientation in a previous post. I understand that strain criteria is usually used when working with fibre reinforced plastics. I'm not sure if that is a normal apprach when talking about a polymer.
                                    • Re: Polymer failure modes
                                      Derek Bishop
                                      Not sure about you comments regarding checking principal stresses. Von Mises stresses are all about the stresses due to the distortion of an element. If the body doesn't undergo distortion then the stresses are not a problem. In the example you gave, the cube with the same stress on each face undergoes  no distortion. Von Mises theory suggests that in this scenario the principal stresses which are all the same will not be a problem. The theory suggests that "a body that is stressed in this way can withstand enormous hydrstatic pressures without damage". [refer R.C. Juvinall "Fundamentals of Machine Component Design" 4th edition pg 246]
                                        • Re: Polymer failure modes
                                          laurent klopfenstein

                                          Hi,

                                           

                                          It s really interessting what you said about the hydrostatic pressure, I will try to see if I can read it.

                                          but from the experience that I have und regarding some discussion I had with Proffesor in some seminar about no linear simulation the Problem of the Von mises formel is really there.

                                          If you take a cube and pull all the 6 surface with the same force the Von mises result will be 0 but the material will be under stress, the question if the material will withstand or not are still open, but you agree on the fact that the stress is there, so the question is which stress ? and the answer is the principal stresses.

                                           

                                          the other problem is when the stresse is not the same but very close to another, in that case you think that everythink is acceptable and in the reality the part will break. The principal stress as to be checked every time that you are close to the break limit.

                                           

                                          thanks for your coment

                                           

                                          regards

                            • Re: Polymer failure modes
                              Derek Bishop

                              Thanks for your responses Shaodun. My topic heading may be misleading. You have provided a lot of infromation regarding the modes of failure. I'm trying to focus on criteria to set when doing FEA on a plastic model. The main criteria would limits on stress and strain. I've provided an example of a way to specify a limit on stress ie a qurter of the yield stress. Assume for the moment that the  various properties of a plastic are known ie stress strain curves, Young's Modulus, Shear Modulus and Poisons ratio. What do you set the stress limit at to avoid creep? Are you aware of any strain limiting criteria for polymers. I've listed the polymers that are being used in the original post.

                                • Re: Polymer failure modes

                                  Hi Derek:

                                   

                                  Frankly speaking , I do not have much exprience in Polymer FEA, but my major in college is Polymer processing, so base on my experience, I will prefer to have 2~4 times of FOS for stress and strain. If your part is made by extrusion and machining, you can use lower FOS, if your part is made by injection molding, the FOS should be higher.

                                   

                                  Polymer is complicated material, I will not prefer use FEA to verify the design for long term usage.