General Mechanical

General Mechanical

Updatting Normal Stiffness

    • RoyalFlowers
      Subscriber

      Hello Everyone,


      I have a question about updating normal stiffness. In my model I have bone-screw contact. Screws are modeled as cylinders and have bonded contact with bone holes. Because in the reality there are some penetrations between spongy bone and screws, I assumed the Pure Penalty Bonded contact with normal stiffness factor 1. Then for these contacts updating stiffness option was selected as never, and for  the rest frictional or bonded contacts updating stiffness was set to in each iterartion. Now the question is that why did total displacement (displacement of loading center) reduce? Because by default normal stiffness factor of bonded contact is 10 and selecting lower values and never updating should reduce global stiffness of our system and displacement should increase. Hence, I am confused a little bit about the differences between updating normal stiffness in each iteration or never. Because on the one hand, we reduce contact normal stiffness factor for convergency, but on the other hand it is recommended to use update stiffness in each iteration to allow the software to change normal stiffness according to stress and penetration values in contact area. Hence, what is the role of small normal stiffness factor when it will be later changed by the software? When we have different contact types with different normal stiffness values, how does work updating stiffness option? Does it take a global normal stiffness value for all contacts  in each iteration?


      Area with Bonded Contact of K=1


      I am grateful if someone can advise me in this regard.


      Best regards, Shabnam Samsami


       


       

    • RoyalFlowers
      Subscriber

      Hello Everyone,


      I would be grateful if someone could answer this post.


      Best regards, Shabnam Samsami

    • Wenlong
      Ansys Employee

      Hi Shabnam,


      The normal and tangential contact stiffness is calculated based on the current mean stress of the underlying elements and the allowable penetration (imaging springs), and that is why it is recommended to update it each iteration. If you use a scale factor of 0.1, it means it will multiply 0.1 to the contact stiffness calculated at each iteration.


      Hope that helps. For more information, you can refer to the MAPDL Contact Technology Guide:


      https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v194/ans_ctec/Hlp_ctec_realkey.html#strdetsif3jla072099


      Bests,


      Wenlong

    • RoyalFlowers
      Subscriber

      Dear Wenzhang,


      Thanks a million for your informative reply.


      The point that made me confuse was normal stiffness factor and updating, because it is not clarified like this in the ANSYS help. It is exactly mentioned in the help:


      KEYOPT(10) = 0: the normal contact stiffness will be updated at each iteration based on the current mean stress of the underlying elements and the allowable penetration, FTOLN, except in the very first iterationon. So before I thought that with updating and selecting normal stiffness factor, imported k factor is taken only for the first step and later k factor will be changed by the software. Now your explanation made it clear that this thought was wrong. If we select updating stiffness and import a desirable K factor, software calculates normal stiffness in each step and multiply that with our defined factor for normal stiffness. Hence it is recommended because normal stiffness value should be defined based on the stress and penetration of underlying elements.


      I really would be thankful if you confirm that my understanding is correct. 


      Best regards, Shabnam Samsami

    • Wenlong
      Ansys Employee

      Hi Shabnam,


      Right. I agree with what you said.


      Best Regards,


      Wenlong

    • RoyalFlowers
      Subscriber

      Hi Wenyzhang,


      Thanks a million for your informative answers.


      Best regards, Shabnam Samsami

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