ACP - Element coordinate system, draping

Hi, I am working with curved shells made of orthotropic material.


Would the element coordinate systems of the shell orientate themselves according to the curvature without ACP? Or is ACP required for that to happen?


Also, what's the difference between turning the "Draping" option on and off when creating Fabrics, Stackups, Oriented Selection Sets and Modelling Groups in ACP? In Modelling Groups the options are "No Draping" and "Internal Draping".


Hope someone can shed some light on this, thanks in advance!

Answers

  • sharveysharvey San Diego, CAMember

    Hello @jinglun,

    For the first answer, the shell element will have it's normal pointed outward automatically, so the x-y directions will be inplane to the shell and follow the curvature. But for orthotropic materials, the in -plane orientation is typically critical since the moduli can vary (x vs y). This is where ACP will allow you to setup the rosettes, draping, etc. to get the orientations correct. For a simple flat plate, one may not need ACP, but for general composite structures, it becomes key.

    Regarding the draping

    When we specify a rosette and the reference direction follows the x direction of the rosette, this works for simple surfaces. Like a plate, box, etc. But for general complex surfaces, if you actually lay down a fabric, the orientations will shear based on the shape. Notice when a fabric is put on a sphere, the mesh shears. So in the image below, the blue shows very little shearing at the top of the sphere (I set this as the seed point). But notice as I get further away, to make the fabric conform to the sphere, the fabric will shear. This means that the fiber orientation will change.


    So, when you turn on internal draping by setting to internal draping, it computes this effect. It takes computational resources to calculate this so keep that in mind when doing for many plies. If you leave as no draping, then the rosette is projected onto the surface, and you can look at the reference direction arrows in ACP to show what that looks like.

    Draping for a specific material can also be assigned to an Oriented Selection Set (OSS). In that case, the Reference Direction of the OSS is adjusted. All associated modeling plies use this draped reference direction. This avoids running multiple draping simulations on the same surface. If Internal Draping is applied to a modeling ply associated with a draped OSS then the OSS’s draped Reference Direction is ignored and an independent draping simulation is started for the Modeling Ply.

    Does that answer your questions? Thank you.


    Regards,

    Sean

  • Hi @sharvey , thanks for the detailed explanation! Yes you answered my questions, but I have new questions now:


    "the shell element will have it's normal pointed outward automatically..."

    Does this apply to solid elements too?


    Are the computed element coordinate systems preserved when a solid model is generated from ACP?


    Referring to your last paragraph, do you mean that specifying draping for both OSS and Modelling Ply is redundant?


    Do the locations of Orientation "point" under OSS, and Draping "seed point" under Modelling Ply definitions matter for curved (symmetrical & assymetrical) geometries?


    Thanks again!

  • sharveysharvey San Diego, CAMember

    Hello @jinglun,

    Correct for shells. Now for solids The element coordinate system follows the shell convention where the z axis is normal to the surface of the shell. The nodal ordering must follow the convention that I-J-K-L and M-N-O-P element faces represent the bottom and top shell surfaces, respectively. So to ensure that you get this ordering, ACP extrudes the shell to the solid element to ensure this proper orientations. So the answer is yes, ACP preserves this.

    Regarding the draping, it is about efficiency and accuracy tradeoff. Doing at the modelling ply lets you have differences for each ply (if needed) the material draping has a woven vs UD coefficient setting, but more importantly the seed point and direction. If each ply was to be "tacked down" at different locations, you would need different seed points, and potentially direction. That being said, it can be computationally intense to do every ply especially if you have dozens to hundreds of plies. So, by doing at the OSS level, we are doing once (per OSS), and using that draping computation for all the modeling plies that use it. Of course if you specify at modelling ply, it will override the draping from the OSS.

    The orientation point under OSS, is important to define, but where you pick (which element face) is not as critical, as long as you pick a face that is on the desired geometry surface (element mesh) so it then uses that seed point and orientation to make sure all elements faces connected will point consistently inward or outward.

    The Draping seed point is where the draping mesh will start to build from. The Seed Point is the starting point where the ply is laid into the mold. At this location the fiber direction is unchanged and the draped fiber direction is equal to the theoretical one. The Seed Point can have a big influence on the final result of the draped fiber angles. It is important. Think of it like you are placing the ply onto the surface and where you start to tack it down and it won't move or shear at that seed point, but then it can start to shear going away from the seed point. If you see high draping angles, you can change the seed point and direction.

    Hope this helps.

    Regards,

    Sean

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