|Published on:||December 4, 2017|
|With:||Ted Harris, Joe Woodward, Eric Miller|
|Description:||In this episode your host and Co-Founder of PADT, Eric Miller is joined by PADT’s Senior Mechanical Engineer Joe Woodward, and Simulation Support Manager Ted Harris for a look into recent announcements regarding simulating 3D Printing with ANSYS and 3DSIM as well as a discussion about what users can do when their models are taking too long to solve.|
How can the mechanical behavior of cellular structures (honeycombs, foams and lattices) be modeled?
This is the second in a two-part post on the modeling aspects of 3D printed cellular structures. If you haven’t already, please read the first part here, where I detail the challenges associated with modeling 3D printed cellular structures.
The literature on the 3D printing of cellular structures is vast, and growing. While the majority of the focus in this field is on the design and process aspects, there is a significant body of work on characterizing behavior for the purposes of developing analytical material models. I have found that these approaches fall into 3 different categories depending on the level of discretization at which the property is modeled: at the level of each material point, or at the level of the connecting member or finally, at the level of the cell. At the end of this article I have compiled some of the best references I could find for each of the 3 broad approaches.
The most straightforward approach is to use bulk material properties to represent what is happening to the material at the cellular level [1-4]. This approach does away with the need for any cellular level characterization and in so doing, we do not have to worry about size or contact effects described in the previous post that are artifacts of having to characterize behavior at the cellular level. However, the assumption that the connecting struts/walls in a cellular structure behave the same way the bulk material does can particularly be erroneous for AM processes that can introduce significant size specific behavior and large anisotropy. It is important to keep in mind that factors that may not be significant at a bulk level (such as surface roughness, local microstructure or dimensional tolerances) can be very significant when the connecting member is under 1 mm thick, as is often the case.
The level of error introduced by a continuum assumption is likely to vary by process: processes like Fused Deposition Modeling (FDM) are already strongly anisotropic with highly geometry-specific meso-structures and an assumption like this will generate large errors as shown in Figure 1. On the other hand, it is possible that better results may be had for powder based fusion processes used for metal alloys, especially when the connecting members are large enough and the key property being solved for is mechanical stiffness (as opposed to fracture toughness or fatigue life).
The most common approach in the literature is the use of homogenization – representing the effective property of the cellular structure without regard to the cellular geometry itself. This approach has significantly lower computational expense associated with its implementation. Additionally, it is relatively straightforward to develop a model by fitting a power law to experimental data [5-8] as shown in the equation below, relating the effective modulus E* to the bulk material property Es and their respective densities (ρ and ρs), by solving for the constants C and n.
While a homogenization approach is useful in generating comparative, qualitative data, it has some difficulties in being used as a reliable material model in analysis & simulation. This is first and foremost since the majority of the experiments do not consider size and contact effects. Secondly, even if these were considered, the homogenization of the cells only works for the specific cell in question (e.g. octet truss or hexagonal honeycomb) – so every new cell type needs to be re-characterized. Finally, the homogenization of these cells can lose insight into how structures behave in the transition region between different volume fractions, even if each cell type is calibrated at a range of volume fractions – this is likely to be exacerbated for failure modeling.
The third approach involves describing behavior not at each material point or at the level of the cell, but at a level in-between: the connecting member (also referred to as strut or beam). This approach has been used by researchers [9-11] including us at PADT  by invoking beam theory to first describe what is happening at the level of the member and then use that information to build up to the level of the cells.
This approach, while promising, is beset with some challenges as well: it requires experimental characterization at the cellular level, which brings in the previously mentioned challenges. Additionally, from a computational standpoint, the validation of these models typically requires a modeling of the full cellular geometry, which can be prohibitively expensive. Finally, the theory involved in representing member level detail is more complex, makes assumptions of its own (e.g. modeling the “fixed” ends) and it is not proven adequately at this point if this is justified by a significant improvement in the model’s predictability compared to the above two approaches. This approach does have one significant promise: if we are able to accurately describe behavior at the level of a member, it is a first step towards a truly shape and size independent model that can bridge with ease between say, an octet truss and an auxetic structure, or different sizes of cells, as well as the transitions between them – thus enabling true freedom to the designer and analyst. It is for this reason that we are focusing on this approach.
Continuum models are easy to implement and for relatively isotropic processes and materials such as metal fusion, may be a good approximation of stiffness and deformation behavior. We know through our own experience that these models perform very poorly when the process is anisotropic (such as FDM), even when the bulk constitutive model incorporates the anisotropy.
Homogenization at the level of the cell is an intuitive improvement and the experimental insights gained are invaluable – comparison between cell type performances, or dependencies on member thickness & cell size etc. are worthy data points. However, caution needs to be exercised when developing models from them for use in analysis (simulation), though the relative ease of their computational implementation is a very powerful argument for pursuing this line of work.
Finally, the member level approach, while beset with challenges of its own, is a promising direction forward since it attempts to address behavior at a level that incorporates process and geometric detail. The approach we have taken at PADT is in line with this approach, but specifically seeks to bridge the continuum and cell level models by using cellular structure response to extract a point-wise material property. Our preliminary work has shown promise for cells of similar sizes and ongoing work, funded by America Makes, is looking to expand this into a larger, non-empirical model that can span cell types. If this is an area of interest to you, please connect with me on LinkedIn for updates. If you have questions or comments, please email us at email@example.com or drop me a message on LinkedIn.
 C. Neff, N. Hopkinson, N.B. Crane, “Selective Laser Sintering of Diamond Lattice Structures: Experimental Results and FEA Model Comparison,” 2015 Solid Freeform Fabrication Symposium
 M. Jamshidinia, L. Wang, W. Tong, and R. Kovacevic. “The bio-compatible dental implant designed by using non-stochastic porosity produced by Electron Beam Melting®(EBM),” Journal of Materials Processing Technology214, no. 8 (2014): 1728-1739
 S. Park, D.W. Rosen, C.E. Duty, “Comparing Mechanical and Geometrical Properties of Lattice Structure Fabricated using Electron Beam Melting“, 2014 Solid Freeform Fabrication Symposium
 D.M. Correa, T. Klatt, S. Cortes, M. Haberman, D. Kovar, C. Seepersad, “Negative stiffness honeycombs for recoverable shock isolation,” Rapid Prototyping Journal, 2015, 21(2), pp.193-200.
 C. Yan, L. Hao, A. Hussein, P. Young, and D. Raymont. “Advanced lightweight 316L stainless steel cellular lattice structures fabricated via selective laser melting,” Materials & Design 55 (2014): 533-541.
 S. Didam, B. Eidel, A. Ohrndorf, H.‐J. Christ. “Mechanical Analysis of Metallic SLM‐Lattices on Small Scales: Finite Element Simulations versus Experiments,” PAMM 15.1 (2015): 189-190.
 P. Zhang, J. Toman, Y. Yu, E. Biyikli, M. Kirca, M. Chmielus, and A.C. To. “Efficient design-optimization of variable-density hexagonal cellular structure by additive manufacturing: theory and validation,” Journal of Manufacturing Science and Engineering 137, no. 2 (2015): 021004.
 M. Mazur, M. Leary, S. Sun, M. Vcelka, D. Shidid, M. Brandt. “Deformation and failure behaviour of Ti-6Al-4V lattice structures manufactured by selective laser melting (SLM),” The International Journal of Advanced Manufacturing Technology 84.5 (2016): 1391-1411.
 R. Gümrük, R.A.W. Mines, “Compressive behaviour of stainless steel micro-lattice structures,” International Journal of Mechanical Sciences 68 (2013): 125-139
 S. Ahmadi, G. Campoli, S. Amin Yavari, B. Sajadi, R. Wauthle, J. Schrooten, H. Weinans, A. Zadpoor, A. (2014), “Mechanical behavior of regular open-cell porous biomaterials made of diamond lattice unit cells,” Journal of the Mechanical Behavior of Biomedical Materials, 34, 106-115.
 S. Zhang, S. Dilip, L. Yang, H. Miyanji, B. Stucker, “Property Evaluation of Metal Cellular Strut Structures via Powder Bed Fusion AM,” 2015 Solid Freeform Fabrication Symposium
 D. Bhate, J. Van Soest, J. Reeher, D. Patel, D. Gibson, J. Gerbasi, and M. Finfrock, “A Validated Methodology for Predicting the Mechanical Behavior of ULTEM-9085 Honeycomb Structures Manufactured by Fused Deposition Modeling,” Proceedings of the 26th Annual International Solid Freeform Fabrication, 2016, pp. 2095-2106
After playing with that block it seems like it may be time to try a more complex geometry. For business banking, I’ve got this key fob that generates a number every thirty seconds that I use for security when I log in. Might as well sort of model that.
So the first thing I do is start up a new model and orient myself on to the sketch plane:
Then I use the line and arc tools to create the basic shape. Play around a bit. I found that a lot of things I had to constrain in other packages are just assumed when you define the geometry. A nice thing is that as you create geometry, it locks to the grid and to other geometry.
I dragged around and typed in values for dimensions to get the shape I wanted. As I was doing it I realized I was in metric. I’m old, I don’t do metric. So I went in to File and selected SpaceClaim options from the bottom of the window. I used the Units screen to set things to Imperial.
This is the shape I ended up with:
I took this and pulled it up and added a couple of radii:
But if I look at the real object, the flat end needs to be round. In another tool, I’d go back to the sketch, modify that line to be an arc, and regen. Well in SpaceClaim you don’t have the sketch, it is gone. Ahhh. Panic. I’ve been doing it that way for 25 some years. OK. Deep breath, just sketch the geometry I need. Click on the three point arc tool, drag over the surface, then click on the first corner, the second, and a third point to define the arc:
Then us pull to drag it down, using the Up to icon to lock it to the bottom of the object.
Then I clicked on the edges and pulled some rounds on there:
OK, so the next step in SolidEdge would be to do a thin wall. I don’t see a thin wall right off the top, but shell looks like what I want, under the Create group on the Design tab. So I spinned my model around, clicked on the bottom surface I want to have open and I have a shell. A thickness of 0.035″ looks good:
My next feature will be the cutout for the view window. What I have not figured out yet is how to lock an object to be symmetrical. Here is why. I sketch my cutout as such, not really paying attention to where it is located. Now I want to move it so that it is centred on the circle.
Instead of specifying constraints, you move the rectangle to be centered. To do that I drag to select the rectangle then click Move. By default it puts the nice Move tool in the middle of the geometry. If I drag on the X direction (Red) you can see it shows the distance from my start.
So I have a couple of options, to center it. The easiest is to use Up To and click the X axis for the model and it will snap right there. The key thing I learned was I had to select the red move arrow or it would also center horizontally where I clicked.
If I want to specify how far away the edge is from the center of the circle, the way I did it is kind of cool. I selected my rectangle, then clicked move. Then I clicked on the yellow move ball followed by a click on the left line, this snapped the move tool to that line. Next I clicked the little dimension Icon to get a ruller, and a small yellow ball showed up. I clicked on this and dragged it to the center of my circle, now I had a dimension from the circle specified that I could type in.
After playing around a bit, if found a second, maybe more general way to do this. I clicked on the line I want to position. One of the icons over on the left of my screen is the Move Dimension Base Point icon. If you click on that you get another one of those small yellow balls you can move. I dragged it over to the center of the circle and clicked. then I can specify the distance as 0.75″
I’ve got the shape I want, so I pull, using the minus icon to subtract, and I get my cutout:
If you look closely,you will notice I put rounds on the corners of the cutout as well, I used Pull again.
The last thing I want to do is create the cutout for where the bank logo goes. It is a concentric circle with an arc on the right side. Saddly, this is the most complex thing I’ve ever sketched in SpaceClaim so I was a bit afraid. It was actually easy. I made a circle, clicking on the center of the outside arc to make them concentric. The diameter was 1″. Then I made another circle of 2″ centered on the right. To get the shape I wanted, I used the Trim Away command and clicked on the curves I don’t want. The final image is my cutout.
Now I can do the same thing, subtract it out, put in some rounds, and whalla:
Oh, and I used the built in rendering tool to quickly make this image. I’ll have to dedicate a whole posting to that.
But now that I have my part, it is time to play with move in 3D.
Tyler, who is one of our in-house SpaceClaim experts (and younger) pointed out that I need to start thinking about editing the 3D geometry instead of being obsessed with controlling my sketches. So here goes.
If I wanted to change the size of the rectangular cutout in a traditional CAD tool, I’d go edit the sketch. There is no sketch to edit! Fear. Unknown. Change.
So the first thing I’ll do is just move it around. Grab one of the faces and see happens.
It moves back and forth, pretty simple. The same tools for specifying the start and stop points are available. Now, if I ctrl-click on all four surfaces the whole thing moves. That is pretty cool.
Note: I’m using the undo all the time to go back to my un-moved geometry.
Another Note: As you select faces, you have to spin the model around a lot. I use the middle mouse button to do this rather than clicking on the spin Icon and then having to unclick it.
That is enough for this post. More soon.
As I explored ANSYS SpaceClaim in my first try, it became obvious that a lot of capabilities that are in multiple operations in most CAD systems, are all combined in Pull for SpaceClaim. In this posting I feel like it would be a really good idea for me to really understand all the things Pull can do.
Not very exciting or adventurous. But there is so much in this operation that I feel like I will miss something critical if I don’t read up first. It states:
“Use the Pull tool to offset, extrude, revolve, sweep, and draft faces; use it to round, chamfer, extrude, copy, or pivot edges. You can also drag a point with the Pull tool to draw a line on a sketch plane.”
Let’s think about that for a second. What it is basically saying is if I pull on an object of a given dimension, it creates an object that is one higher dimension. Point pulls to a curve, a curve pulls to a face, and a face pulls to a solid. Kind of cool. The big surprise for me is that there is no round or fillet command. To make a round you pull on an edge. This is change.
I started by reading my block with a hole back in.
This fillet pull thing scares me so I thought I’d confront it first. So selecte Pull, and selected an edge:
Then I dragged it away from the block. Nothing. You can’t create a surface that way. Then I dragged in towards the center. A round was created.
If anything, too simple. Back in my day, adding a round to an edge took skill and experience!
So next I think I want to try and change the size of something. Maybe the diameter of the hole. So I select the cylinder’s face. Is shows the current radius. I could just change that value:
Instead I drag, and while I do that I noticed that there are two numbers, the current radius and the change to the radius! Kind of cool. No, really useful.
You use tab to go between them. So I hit tab once, typed 3 then tab again (or return) and I get a 8 mm diameter. I like the visual feedback as well as the ability to enter a specific change number.
Next thing that I felt like doing was rounding a corner. Put a 5mm round on the corner facing out:
So I grabbed the point and dragged, and got a line.
Remember, it only goes up one entity type – point to curve. Not point to surface. So I ctrl-clicked (that is how you select multiple entities) on the three curves that intersect at the corner:
Then I dragged and got my round.
This are all sort of dragging straight. After looking at the manual text it seems I can revolve and sweep as well with the Pull operation. Cool. But what do I revolve or sweep around and along? Looking at the manual (and it turns out the prompt on the screen) I use Alt-Clicking to define these control curves. Let’s try it out by revolving something about that line I mistakenly made.
I click on one of the curves on the round. then Alt-Click the line – It turns blue. So there is a nice visual clue that it is different than the source curve. Now I’ve also got spinny icons around the curve rather than pull icons.
So I drag and… funky revolved surface shows up. I had to spin the model to see it clearly:
Let me stop and share something special about this. In most other CAD tools, this would have involved multiple clicks, maybe even multiple windows. In SpaceClaim, it was Click, Alt-Click, Drag. Nice.
Using the Pop=up Icons
As you play with the model you may start seeing some popup icons near the mouse when you select geometry while using pull. The compound round on the block is complicated, so I spun it around and grabbed just one edge and pulled it in to be a round. Then I clicked on it and got this:
Not only can I put a value in there, I can drop ones I use a lot. I can also change my round to a chamfer, or I can change it to a variable radius. This is worth noting. In most other CAD tools you pick what type of thing you want to do to the edge. Here we start by dragging a round, then specify if it is a chamfer or a variable.
The variable radius is worth digging more in to. I clicked on it and it was not intuitive as to what I should do. Let’s try help. Search on Variable Radius… duh. Click on the arrow that shows up and drag that. There are three arrows. The one in the middle scales both ends the same, the one on either end, well it sets the radius for either end.
Clicking on a control point and hitting delete, gets rid of them.
That’s just one icon that pops up. Playing some more it seems the other icons control how it handles corners and multiple fillets merging… something to look at as I do more complex parts.
The other popup I want to look at is the Up To one. It looks like an arrow on a surface. In other tools I extrude, cut, revolve all the time to some other piece of geometry. This is the way to do it in Space Claim. Let’s say I want to pull a feature to the middle of my hole. First I sketch the outline on a face:
That is enough for pulling and for today. In the next session it may be time to explore the Move command.
This post is a table of contents to a series about ANSYS SpaceClaim. After over 31 years of CAD use, it has become difficult for me to learn new tools. In this series I will share my experience as I explore and learn how to use this fantastic tool.
Thirty-one. That is the number of years that I have been using CAD software. CADAM was the tool, 1985 was the year. As some of our engineers like to point out, they were not even born then.
Twenty-one. that is the number of years that I have been using SolidEdge. This classifies me as an old dog, a very old dog. As PADT has grown the amount of CAD I do has gone way down, but every once in a while I need to get in there and make some geometry happen. I’m usually in a hurry so I just pop in to SolidEdge and without really thinking, I get things done.
Then ANSYS, Inc. had to go and buy SpaceClaim. It rocks. It is not just another solid modeler, it is a better way to create, repair, and modify CAD. I watch our engineers and customers do some amazing things with it. I’m still faster in SolidEdge because I have more years of practice than they have been adults. But this voice in my head has been whispering “think how fast you would be in SpaceClaim if you took the time to learn it.” Then that other voice (I have several) would say “you’re too old to learn something new, stick with what you know. You might break your hip”
I had used SpaceClaim a bit when they created a version that worked with ANSYS Mechanical four or five years ago, but nothing serious. Last month I attended some webinars on R17 and saw how great the tool is, and had to accept that it was time. That other voice be damned – this old dog needs to get comfortable and learn this tool. And while I’m at it, it seemed like a good idea to bring some others along with me.
These posts will be a tutorial for others who want to learn SpaceClaim. Unlike those older tools, it does not require five days of structured training with workshops. The program comes with teaching material and tutorials. The goal is to guide the reader through the process, pointing out things I learned along the way, as I learn them.
A link to the table of contents is here.
The product I’m learning is ANSYS SpaceClaim Direct Modeler, a version of SpaceClaim that is built into the ANSYS simulation product suite. There is a stand alone SpaceClaim product but since most of our readers are ANSYS users, I’m going to stick with this version of the tool.
This is what you see when you start it up:
I’ve been using the same basic layout for 20 years, so this is a bit daunting for me. I like to start on a new program by getting to know what different areas of the user interface do. The “Welcome to ANSYS SCDM” kind of anticipates that and gives me some options.
Under “Getting Started” you will see a Quick Reference Card, Introduction, and Tutorials. Open up the Quick Reference and print it out. Don’t bother with it right now, but it will come in handy, especially if you are not going to use SpaceClaim every day.
The Introduction button is a video that gets you oriented with the GUI. Just what we need. It is a lot of information presented fast, so you are not going to learn everything the first viewing, but it will get you familiar with things.
Here I am watching the video. Notice how attentive I am.
Once that is done you should sort of know the basic lay of the land. Kind of like walking into a room and looking around. You know where the couch is, the window, and the shelf on one wall. Now it is time to explore the room.
It is kind of old school, but I like user guides. You can open the SpaceClaim User Guide from the Help line in the “Welcome” window. I leave it open and use it as a reference.
The best place to learn where things are in the interface is to look at the interface section in the manual. It has this great graphic:
The top bit is pretty standard, MS office like. You have your application menu, quick access toolbar, and Ribbon Bar. The Ribbon Bar is where all the operations sit. We used to call these commands but in an object oriented world, they are more properly referred to as operations – do something to objects, operate on them. I’ll come back and explore those later. Over on the left there are panels, the thing we need to explore first because they are a view into our model just like the graphics window.
The Structure Panel is key. This is where your model is shown in tree form, just like in most ANSYS products. In SpaceClaim your model is collection of objects, and they are shown in the tree in the order you added them. You can turn visibility on and off, select objects, and act on objects (using the right mouse button) using the tree. At this point I just had one solid, so pretty boring. I’m sure it will do more later.
Take a look at the bottom of the Structure Panel and you will find some tabs. These give access to Layers, Selection, Groups, and Views. All handy ways to organize and interact with your model. I felt like I needed to come back to these later when I had something to interact with.
TIP: If you are like me, you probably tried to drag these panels around and hosed up your interface. Go to File > SpaceClaim Options (button at the bottom) > Appearance and click the “Reset Docking Layout” button in the upper right of the window. Back to normal.
The options panel changes dynamically as you choose things from the ribbon. If you click on the Design > Line you get this:
And if you click on Pull you get this:
Keeps the clutter down and makes the commands much more capable.
Below that is the Properties Panel. If the Options panel is how you control an operation, then the Properties panel is how you view and control an object in your model. No point in exploring that till we have objects to play with. It does have an appearance tab as well, and this controls your graphics window.
At the bottom is the Status Bar. Now I’m a big believer in status bars, and SpaceClaim uses theirs well. It tells you what is going on and/or what to do next. It also has info on what you have selected and short cut icons for selection and graphics tools. Force yourself to read and use the status bar, big time saver.
The last area of the interface is the graphics window. It of course shows you your geometry, your model. In addition there are floating tools that show up in the graphics window based upon what you are doing. Grrr. #olddogproblem_1. I’m not a fan of these, cluttering up my graphics. But almost all modern interfaces work this way now and I will have to overcome my anger and learn to deal.
For most of the 30+ years that I’ve been doing this CAD thing, I’ve always started with the same object: A block with a hole in it. So that is what we will do next. I have to admit I’m a little nervous.
I’m nervous because I’m a history based guy. If you have used most CAD tools like SolidWorks or ANSYS DesignModeler you know what history based modeling is like. You make a sketch then you add or subtract material and it keeps track of your operations. SpaceClaim is not history based. You operate on objects and it doesn’t track the steps, it just modifies your objects. SolidEdge has done this for over ten years, but I never got up the nerve to learn how to use it. So here goes, new territory.
Things start the same way. But instead of a sketch you make some curves. The screen looks like this when you start:
The default plane is good enough, so I’ll make my curves on that. Under Design>Sketch click on the Rectangle icon then move your mouse on to the grid. You will notice it snaps to the grid. Click in the Upper Left and the Lower Right to make a rectangle then enter 25mm in to each text box, making a 25 x 25 square:
Next we want to make our block. In most tools you would find an extrude operation. But in SpaceClaim they have combined the huge multitude of operations into a few operation types, and then use context or options to give you the functionality you want. That is why the next thing we want to do is click on Pull on the Edit group.
But first, notice something important. If you look at the model tree you will notice that you have only one object in your design, Curves. When you click Pull it gets out of sketch mode and into 3D mode. It also automatically turns your curves into a surface. Look at the tree again.
This is typical of SpaceClaim and why it can be so efficient. It knows what you need to do and does it for you.
Move you mouse over your newly created surface and notice that it will show arrows. Move around and put it over a line, it shows what object will be selected if you click. Go to the inside of your surface and click. It selects the surface and shows you some options right there.
Drag your mouse over the popup menu and you can see that you can set options like add material, subtract material, turn off merging (it will make a separate solid instead of combining with any existing ones), pull both directions, get a ruler, or specify that you are going to pull up to something. For now, we are just going to take the default and pull up.
As you do this the program tells you how far you are pulling. You can type in a value if you want. I decided to be boring and I put in 25 mm. Geometry has been created, no one has been hurt, and I have not lost feeling in any limbs. Yay.
On the status bar, click on the little menu next to the magnifying glass and choose Zoom Extents. That centers the block. Whew. That makes me feel better.
Now for the hole. It is the same process except simpler than in most tools. Click on the circle tool in Sketch. The grid comes back and you can use that to sketch, or you can just click on the top of the block. Let’s do that. The grid snaps up there. To make the circle click in the middle of the grid and drag it out. Put 10 in for the diameter. A circle is born.
Now choose Pull from the Edit section. There is only a Solid now?
SpaceClaim went ahead and split that top surface into two surfaces. Saving a step again.
Click on the circle surface and drag it up and down. If you go up, it adds a cylinder, if you go down, it automatically subtracts. Go ahead and pull it down and through the block and let go. Done. Standard first part created. Use the File>Save command to save your awesome geometry.
That is it for the getting started part. In the next post we will use this geometry to explore SpaceClaim more, now that we have an object to work on. As you were building this you probably saw lots of options and input and maybe even played with some of it. This is just a first look at the power inside SpaceClaim.
Click here for Post 2 where the Pull command is explored.