6 Replies Latest reply on Jan 27, 2012 4:44 PM by Jerry Steiger

    Need help on Creep Analysis

    Akash Vandakudri

      Hi Everybody,

       

      I need to conduct Creep Analysis of plastic components 33% glass fiber with nylon 6/66. The material is from the DuPont.

       

      The component needs Creep Analysis check, as these are tested from -40 degree Celsius to 150 Degree Celsius. It has an internal operating pressure of 120 psi.

       

      Kindly let me know how to go with it and what all material data do I require. Also how to find/calculate creep constants for the analysis.

       

      Presently I am having material data which gives me tensile stress from -40C to 150C at different strain levels (0.1% to 3%). Can I calculate C1, C2 and C3 from this data.

       

      Please let me know what more data I require.

       

      Best Regards

      Akash A Vandakudri

        • Re: Need help on Creep Analysis
          Paul Kellner

          I rather doubt that the stress-strain curves you have provide time dependent information. At least you did not so state.

           

          One way to proceed is to perform a DMA test.(1) This test can be used to develop master creep curve data for your material for any reference temperature.

           

          I have the older Cosmos/M which provides two possible creep laws, Bailey-Norton and Exponential. Simulation has been incorporating Cosmos/M functionality over the years, but you will need to verify with SW that they are using this law.

           

          Bailey-Norton has four parameters, C0, C1, C2 and CT. The exponential law has six paramaters. So, subject to verification I would say you are provided with Bailey-Norton. Bailey-Norton is of the form

           

                  εc = C0 σ(C1) t(C2) e(-CT/T)
                C1>1 and 0<C2≤1

           

          You would have to find the Ci that match the master curve at the reference temperature for which you are doing your creep study. Note that the master curve in DMA is the log of the apparent modulus versus time.

           

          (1) Dynamic Mechanical Analysis, Sepe, Michael P., Plastics Design Library, Norwitch NY, 1998, Chapter 4

               ISBN 1-884207-64-2

            • Re: Need help on Creep Analysis
              Akash Vandakudri

              Dear Paul,

               

              Thanks for your reply and I am so sorry reply this late to your message, as I was completely involved in different analysis.

               

              Now I have asked the material supplier to give the material constant for creep and they are working on it.

               

              Regarding the setup for the analysis, I would like to ask some more doubts to you:

               

              1) I am doing nonlinear elasto-plastic analysis with creep effects. The material is glass filled nylon. So what are the material properties I need to give for this. Should they be given for specific temperature or one at room temperature.

                   1 Yield Stress

                   2 Youngs Modulus

                   3 Tangent Modulus

                   4 Poisson's Ratio

                   5 Creep constants

                   6 Hardening factor

               

              2) Regarding the analysis has to be conducted on filter with internal pressure of 120 psi and operating temperature at 40 degree celsius.

                  1 Should I give temperature to entire body alon with internal pressure for time period

                  2 Should I need to do nonlinear dynamic analysis

                  3 Boundary conditions will be structural contraints only.

               

              3) Should I need to do any additional setup for the analysis in study properties and setup the temperature properties.

               

              I am confused about whether the analysis run for temperature range or specific temperature only and check for its temperature effects.

               

              Kindly help in solving this analysis, as very new to this type of analysis.

               

              Best Regards

              Akash A V

                • Re: Need help on Creep Analysis
                  Paul Kellner

                  Akash Vandakudri wrote:

                   

                       1 Yield Stress

                       2 Youngs Modulus

                       3 Tangent Modulus

                       4 Poisson's Ratio

                       5 Creep constants

                       6 Hardening factor

                   

                  DMA will give 2 and 5.

                  A tensile test at the temperature of interest will give 1

                  The supplier should be able to give 4

                  Don't know that that material strain hardens so 6 may not be necessary.

                  You calculate 3 from the information in the tensile test.

                   

                  Since you say this is a filter it is likely immersed in some fluid. That fluid will likely significantly change the material properties of the plastic. That makes it all the more important to do specific material testing. The vendor is not likely to have that data for the plastic saturated with a fluid. If the fluid is water there is published data on it's effects.

                   

                   

                  Akash Vandakudri wrote:

                   

                  2) Regarding the analysis has to be conducted on filter with internal pressure of 120 psi and operating temperature at 40 degree celsius.

                  Are you saying that there is a 120psi pressure drop across the filter?

                  What failure modes are you expecting?

                    • Re: Need help on Creep Analysis
                      Akash Vandakudri

                      Hi,

                       

                      Thanks for you quick reply. Regarding the material I do now understand that creep analysis is done for a particular temperature at one time (say -40degree celsius) and not for entire range(-40degree celsius to 120 degree celsius).

                       

                      I am interested in checking at -40 degree celsius, so I will extract all the above material properties for only -40 degree celcius and not for any other temperature.

                       

                      Last reply I said its filter, but its not filter, but filter assembly (outer body) and it is not immersed in any liquid. It has fluid within it opearting at 120 psi(burst pressure).

                       

                      We need to check filter assembly at -40 degree celsius opearting at 120 psi. The material is 33% glass filled nylon 6/6. We are seeing brittle failure of the plastic when operated at -40C.

                       

                      Do I need to run nonlinear dynamic analysis with temeprature and internal pressure with repect to time varying loads.

                       

                      Kindly let me know on this, so that I can start doing deck setup for hhe analysis and run the analysis.

                       

                      Best Regards

                      Akash A V

                        • Re: Need help on Creep Analysis
                          Paul Kellner

                          Quite a difference between 40C and -40C. What is the glass transition temperature for your material? What happens to its notched impact strength? What is it's elongation at -40C?

                           

                          Whether it is immersed or the fluid is on one side, it still has an effect. What is the fluid?

                           

                          As soon as you start looking at non-linear analysis time history becomes important.

                           

                          You don't need to do dynamic analysis unless your load (pressure) changes so rapidly that inertia comes into play. Generally an analysis in which inertia comes into play and one in which creep comes into play are at two extremes of time history loading, the former rapid loading and the later long term static loads.

                           

                          Beside all that, when injection molding glass filled nylon the fibers tend to become oriented certain ways depending on the way the mold fills. You can get knit lines which greatly affect the material properties if the gating isn't right.

                            • Re: Need help on Creep Analysis
                              Jerry Steiger

                              Akash,

                               

                              As Paul says, the knit lines are very important in an injection molded part, especially when it is glass reinforced. Are you working with the molder of the parts? Can you gets some short shots that show you where the knit lines are? Can you run a mold filling analysis like Moldflow to check on the knit lines and the material alignment? Either of these could give you a lot of understanding about your failures.

                               

                              The strength at the knit line for a glass reinforced material is usually much lower than in the bulk of the material, as the knit line has almost no glass reinforcing the part. It can be weaker still depending on how the parts are molded. If the pressure and temperature are low, then the knit line will be very weak.

                               

                              If the flow fronts meet head on, the knit line will be weak. If the flows run along almost parallel, the knit line may be nearly as strong as the bulk material.

                               

                              Glass filled materials are much stronger and stiffer in the flow direction than in the directions across the flow. I suspect that the creep behavior is different depending on the orientation as well.

                               

                              Jerry Steiger