General Mechanical

General Mechanical

Why the stresses from shell elements are higher or lower than the stresses from solid elements

    • Alejalo88
      Subscriber

      Why the stresses obtained from the FEMs with shell elements are higher than the stresses obtained from the FEMs with solid elements at some locations and lower at other locations.

      The question is related to the finite element analysis of welded tubular joints and the stresses located at the Chord Crown, Chord Saddle, Brace Crown and Brace Saddle locations.

      Please see the documentation attached including the Ansys code for two of my models.

    • 1shan
      Ansys Employee
      ANSYS staff are not allowed to download attachments, please insert inline images of your model to help support your query. Also, have you performed a mesh convergence study and made sure that the mesh is fine enough? If yes, how much is the difference between both cases? There are bound to be some differences because of the assumptions made in 2D elements, but if the geometry is thin enough the mesh accurately captures the geometry, the difference shouldn't be very high. Also, if there are transverse or out of plane loads make sure to use at least 3 elements across the thickness of the model.
      Regards Ishan.
    • Alejalo88
      Subscriber
      Hi Ishan Thanks for your reply!!!!!
      I will try to explain my question with pictures then:
      The subject is about the stress analysis of a welded tubular T joint in Ansys Mechanical APDL as seen on the picture below (the thickness of the tubular members is 8mm). A unit axial load is applied at the top of the brace (vertical member). A symmetry boundary condition is applied at the XY and YZ planes (only a quarter of the T joint is modelled). A fixed support is applied at the end of the chord member (horizontal member).
      I have 2 finite element models:
      The first model meshed with SHELL181 elements, the mesh density is adjusted until I get a mesh convergence of less than 3% between the nodal and the element solutions at the location of highest stress.
      3 shell sections are specified:
      A first shell section type is applied to the brace member (vertical member) specifying a thickness of 8mm, 5 integration points and a midplane section offset.
      A second shell section type is applied to the chord member (horizontal member) specifying a thickness of 8mm, 5 integration points and a top plane section offset.
      A third shell section type is applied to the geometry representing the weld specifying a thickness of 7.82mm (average thickness of the weld geometry along the tubular joint intersection), 5 integration points and a midplane section offset.
      The axial load is applied using TARGE170 and CONTA175 elements at the top of the brace (vertival member) as shown on the picture below (force-distributed constraint):
      The fixed support is applied using TARGE170 and CONTA175 elements at the end of the chord member (horizontal member) as shown on the picture below (coupling constraint):
      The second model meshed with SOLID186 elements, I have 5 elements through the thickness of the tubular members and I get a full convergence between the nodal and the element solutions at the location of highest stress.
      The axial load is applied using TARGE170 and CONTA174 elements at the top of the brace (vertival member) as shown on the picture below (force-distributed constraint):
      The fixed support is applied using TARGE170 and CONTA174 elements at the end of the chord member (horizontal member) as shown on the picture below (coupling constraint):
      I am reading the 1st principal stresses at the locations as shown on the picture below for both finite element models.
      The results show me that at the chord saddle and at the chord crown the stresses for the solid elements are lower than those for the shell elements.
      But at the brace saddle and at the brace crown the stresses for the solid elements are higher than those for the shell elements.
      The deformed shape of the structure looks like on the picture below. It is apparent that on the chord member I am having more bending than membrane behaviour while on the brace member I am having more membrane than bending behaviour (Am i correct?).
      And so my question is which element should give higher stresses in which behaviour.
      Is it correct to assume that the shell elements give higher stresses in bending and the solid elements give higher stresses in membrane behaviour?????
      Thank you so much in advance Alejandro Santacruz
    • Alejalo88
      Subscriber
      Sorry I forgot to mention the actual values obtained from the models:
      Shells chord saddle: Solids chord saddle:
      178.73(10^-4) MPa 174.3(10^-4) MPa
      Shells chord crown: Solids chord crown:
      66.933(10^-4) MPa 65.705(10^-4) MPa
      Shells brace saddle: Solids brace saddle:
      90.343(10^-4) MPa 102.12(10^-4) MPa
      Shells brace crown: Solids brace crown:
      13.725(10^-4) MPa 13.987(10^-4) MPa
      As you can see the values pretty much agree for both elements but this is a thesis and I have to explain the variations.
    • Alejalo88
      Subscriber
      Shells chord saddle: - Solids chord saddle:
      178.73(10^-4) MPa - 174.3(10^-4) MPa
      Shells chord crown: - Solids chord crown:
      66.933(10^-4) MPa - 65.705(10^-4) MPa
      Shells brace saddle: - Solids brace saddle:
      90.343(10^-4) MPa - 102.12(10^-4) MPa
      Shells brace crown: - Solids brace crown:
      13.725(10^-4) MPa - 13.987(10^-4) MPa
Viewing 4 reply threads
  • You must be logged in to reply to this topic.