General Mechanical

General Mechanical

Why the Maximum Equivalent (von-Mises) stress is bigger than yield and Ultimate Tensile Stress?

    • tiger17361
      Subscriber

      hi everyone,

      i am doing stress analysis of turbine blade using the aerodynamic loads (pressure), Temperature profile of the blade at 15500 RPM. the model outline is attached. the material properties (800 C) is used are also attached. rotational velocity is 15500 rpm. but i am getting max stress much much more than UTS. what is the problem point which i am not considering. please help if anyone knows how to fix this.

      i used all the material properties at 800 C.

      yield strength of material is 870 MPa at 800 C and the stress i got from analysis is 2889 MPa.

    • David Weed
      Ansys Employee
      Regarding your question, the properties that you are referring to in Engineering Data will not have any bearing on the actual behavior of the material model during the simulation; they are not incorporated into the material model. Those are primarily for calculating Stress Tools results. You can read more about this feature in the Mechanical help guide here: https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v221/en/wb_sim/ds_Stress_Tools.html. Based on your input in Engineering Data, your material will behave in a purely linear elastic fashion.
      If you want material behavior which incorporates yield stress, you will need to choose an appropriate plasticity model, e.g., bilinear isotropic hardening, etc., which can be chosen from the side panel within Engineering Data:

      You will need to supply data for your material model based on experimental results or publications. You can read more about the material models which are available in the Engineering Data User's Guide here: https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v221/en/wb_eda/eda_structural_properties.html
      In addition to this, you'll need to take into account two factors when incorporating a plasticity model:
      Use the ERESX,NO command under the Analysis branch so that any plastic strain results, which are nonlinear quantities, will be copied from the integration points to the nodes rather than linearly extrapolated. Linear extrapolation or a nonlinear quantity will cause errant values to show on the plot (typically, stresses that are higher than yield), so it is best to simply copy results from gauss points to nodes. Also, if you have multiple load steps in your analysis, you want to set the command object so that it will be applied for all load steps. This can be accomplished by going to the details window of the command object and setting the Step Selection Mode value to 'All'.
      When reviewing stress contour plots, look at the unaveraged result rather than the averaged result. Again, this will result in a more accurate representation of the stresses/strains.
      I hope that this is information is helpful to you.
    • tiger17361
      Subscriber

      thanks for your precious time and guidance. you are right i have changed my engineering data based on my model.
      as i am solving a linear static referring this...
      i am mainly concerned about von mises stress for my this problem. and also the deformation only.
      i first solve my part in CFX to get temperature and pressure profiles. i have solved this part. now what i want is that these pressure and temperature profiles to import into ansys mechanical so that i can solve for stresses. attached are the pressure and temperature profiles generated using CFX. and then same have been imported as shown in picture.
      as i imported these pressure and temperature loads in ansys mechanical i can see that these have been imported. sharing with you the images
      100% mapped loads from CFX to Ansys mechanical. uptill this point m good. next
      i put rotational velocity and fixed support as shown.

      now my setup is complete....
      but when i go for structural analysis it doesnot show me the temperature loads
      even then i solved my problem and get some value of stress....(which is much much greater than yield strength of the material is used)
      but then i suppressed the temperature load and got same value of stresss, it means structural analysis is not incorporating my temperature loads. but i want to use this temperature profile.....
      please guide me on two point :
      i m solving for only von mises stress , which comes out to be higher than yield strength of my material.. what does it mean.. as per my understanding.. material will yield so i am not safe ....... but in practical application that component is in operation... my stress value may be too too high... what is the problem with my part.
      why my temperature profile is not being incorporated even it is imported. i need stress results with the temperature and pressure loads.
      i think i have explained my problem in detail to you. will be highly obliged if you look into my problem. thanks
      waiting for your helpful comments.

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