General Mechanical

Top/Bottom vs Top and Bottom Stress in a Solid184 layered element

• pcrot
Subscriber

I obtained a model of a composite structure that was modeled in ACP.  The mesh that was passed to the Static Structural component system is a mesh of SOLID184 layered elements with each ply consisting of 2 or 3 elements per ply thickness.

Question 1: What is purpose of using layered elements if the mesh already has 2 or three elements per ply?  The layer element itself implies it contains information regarding plies within its element thickness (Z-direction), even though the element is 1/2 or 1/3 the actual ply thickness.   Was this potentially an oversight by the original structural analyst?

Question 2: For the above case, what is best way to analyze the composite component stresses?  Top, Bottom, or Top/Bottom?  In fact what does Top/Bottom actually mean?

• Sheldon Imaoka
Ansys Employee

For question #1, ACP always writes out layered elements, even if you have 2-3 elements per ply. This is intentional, to provide consistency with the element definition. As you may know, homogenous SOLID185 uses material (MAT) definition whereas layered SOLID185 uses section definition (SECN) for material properties. By always using layered elements, even if you have a mix of SOLID185 elements, you can rely on all of them having section definition rather than a mix of MAT and SECN, which can be confusing. (We may have 2-3 elements per ply in parts of interest but we may also have 2+ plies per element in parts not of interest in the same assembly, which can cause this situation of a 'mix' to occur.)
For question #2, top/bottom is intended for layered elements with multiple layers, which is different from the case you describe. If you had an element with 3 layers, for example, using top/bottom allows you to focus on the appropriate ply. If you have 2-3 elements per ply, you essentially have homogenous elements, so you can ignore this option/setting.
Regards Sheldon

• pcrot
Subscriber
Sheldon Even though the model has 2 or 3 elements per ply as defined by ACP, the elements and therefore,the stress result is of a layered element andrequires selecting the option of top, bottom, or top/bottom... and there is adefinite difference between the three... so which option(s) can be ignored? I do not thinkone can ignore if there is no option that provides a "homogenized" result for a layered element.
My question 2 wasan attempt to get an explanation ofthedifference between Top and Bottomversus Top/Bottom, and especially ifthere aremultiple layered elements per ply.Will selectingTop or Bottomresult in stresses of thetop or bottom of the element and then somehow averaged with adjacentply elements through thickness? And what does Top/Bottom really mean... what can one expect from this option.
I have concluded, rightly or wrongly,and without any description of these options from ANSYS documentation, the best andmost conservative option to report out is Top/Bottom since this appears to provide both the maximum and minimum stresses within the ply structure.
Regards
Paul
• Sheldon Imaoka
Ansys Employee
Hi Paul Sorry if I misunderstood your question, but if you are reviewing results per ply (Sub Scope By: Ply) or by layer (Sub Scope By: Layer), leave results to show top/bottom (Position: Top/Bottom). In this way, for a given element, the actual results for top and bottom per element will be shown, which is what you want. ("Position" is referencing position of element, not position of ply/layer for the situation you have.)
The reason why we have this setting is because, if you had an element with a definition of more than 1 layer, you may wish to see what is occurring at the topmost or bottommost part of a given layer (ply), so a user may wish to change it to the non-default option of "Position: Top/Bottom". However, in your case, you just want to use the default setting of "Position: Top/Bottom".
Think of the "Position" as being an option per element for your case - do you want only the top, only the bottom, or both top & bottom results shown on a given element? To see this more clearly, change "Display Option: Averaged" to "Unaveraged". If you review results with "Position: Top/Bottom", you will see gradient through the thickness direction. On the other hand if you use "Position: Top", the result should be uniform through an element's thickness since you are only showing results for top of the element(s).
BTW, it may be easier for you to just use "Sub Scope By: Layer" and review all results for each element at once. You can then use ply-based results if you want to focus on results of a given ply.
Regards Sheldon

• pcrot
Subscriber
Thanks! That is helpful. Sometimes it takes a couple tries to the question right.