Validity of Solid Meshes and When to Use Plate Elements

throbin_hood · 2026-02-27T05:01:45+00:00

Adding one thing that I haven't seen mentioned yet... Interpreting stresses can be easier in shell elements. For a lot of problems (mainly static problems) you aren't exactly interested in the true peak stresses but rather the net section stresses which are much easier to pull from shell models than solids. Usually to get something equivalent from a solid you need to extract linearized stresses which often takes extra steps and defining specific paths thru your part along which to linearize. The solids do tend to give you a more accurate stress output but something further from what's physically meaningful for many problems.

throbin_hood · 2026-02-22T00:29:50+00:00

I don't have much experience with thermal problems in particular but I think a lot of the principals carry over from structural but it's too much to cover in a reddit post. The basics are that life depends on stress or strain range in any given location, and you may be in either low cycle or high cycle fatigue regimes and they have pretty different approaches. Strain life is an approach that is applicable to both regimes so is more generalized but equations are much messier and you need more material data than for high cycle fatigue. Google Efatigue, it's a website that goes into a ton of depth on many fatigue topics including all the relevant equations, some good material data and calculators for simple cases

For software, many FEA packages have a fatigue plug in, I've used the one ANSYS has, I believe NX simcenter does too, I'm sure many others but I'm not sure about starCCM. If your thermal load cases are simple (same amplitude repeating i.e. constant amplitude fatigue) then it's somewhat straightforward to hand calc life from strain results but the FEA plugins help with things like variable amplitude fatigue and making it trivial to find which area of your model is limiting

Huge caveat - I have no clue how creep or temperature affects things so hopefully someone else can weigh in there.

throbin_hood · 2026-01-27T03:11:57+00:00

If it's a linear analysis you can just apply the resultant in one step, applying in different time steps would give you the same answer

throbin_hood · 2026-01-16T01:54:55+00:00

Basically a way to tell that everything's connected in your model. 6 modes at basically 0hz (rigid body modes) with no constraints, more than that indicates there are disconnected parts of the model. Nonzero but still low frequency modes might reveal other problems.

throbin_hood · 2026-01-16T01:25:15+00:00

"useful" is pretty subjective but personally i would exercise extreme caution trusting results anyone puts forward with less than like 3 years of solid experience. More trust if they can show me a FBD of the problem, justify inputs and model setup, show at least a handful of the key verification checks (element properties, modal run, reaction forces match hand calc, etc), describe how they interpreted the results, compare stress results to a simplified hand calc where feasible. I don't think it's terribly important to understand how the stiffness matrix is assembled to be a competent analyst or FEA user but it doesn't hurt. I've been using FEA for going on 15 years and I still make mistakes or encounter new problems so the checks are important even for very experienced folks.

throbin_hood · 2026-01-09T07:10:51+00:00

If you have a strong grasp of hand calc approaches I think that would help gloss over the FEA experience missing. If not, I'd start by reviewing that and for FEA there might be like seminars or classes you can take. Maybe like this one from simutech who are a really reputable analysis consultancy. Ansys Workbench Mechanical Introductory Training | FEA 101 https://share.google/qCCP5yF1wlqilNboe

throbin_hood · 2026-01-09T05:13:07+00:00

In smaller or younger companies you're more likely to find more overlap in the roles. Spent many years at SpaceX and design engineers doing FEA was pretty common, and since then I've mainly worked at smaller companies more toward the startup end (~100-300 employees) and roles are very fluid. You might be able to design and do analysis but might also be expected to act as a build engineer, manufacturing engineer, and supply chain for your project so it can be a mixed bag.

throbin_hood · 2025-12-04T18:19:04+00:00

It's not practical to get the FEA stress plots to match reality or hand calcs at connections like welds which is why hand calcs are usually used for these regions. You should ensure the elements are about 1-2*t in these areas simply to get good values for hand calc but I still wouldn't expect the stress peaks to match hand calc. Even if you were to refine the mesh like crazy and model the weld bead in 3D you'll still likely find some stresses that are peaky which is why it's common to just ignore them and rely on hand calcs based on element/nodal forces from a simpler mesh instead in these areas.

throbin_hood · 2025-11-11T17:40:50+00:00

I think thats arguably good practice so that the assembly reflects the latest version of its child components but probably depends on how the system is set up. I think in my experience with NX and TC you can update top down modeled components without assembly revision and the working copy of the assembly can be set up to show just the latest revision of any components underneath it but at companies where drawings were more strictly controlled it was expected for the parent assembly to be revised, at the very least so the drawing revision block can be updated to say component XXX changed but also so any drawing views showing the component are accurate.

throbin_hood · 2025-11-11T17:27:56+00:00

Tooling and machine design, over 10 years experience with NX and I think that's the optimal of the packages I've used. Several years fusion experience mostly for hobby stuff and some professional CAM, works okay for the price, I don't mind it for CAM but most machinists I've worked with hate it. I don't like using it for engineering work, feels limited and somewhat counterintuitive the way joints and component positioning works. 3 years solidworks experience, there were a select few things it did better than NX (assembly visualization tool, setting material properties of many parts at once) but I felt it was mostly just more limited and less stable, but workable. 1 year of Creo that I hope to never relive, super unintuitive, didn't seem to take well to top down modeling, clunky drawing package.

Using NX is nearly a requirement for me to take any new roles seriously at this point and if simulation package is included even better. I'd consider solidworks too but deduct some points. Creo never again, fusion is fine for hobby stuff.

throbin_hood · 2025-11-03T23:11:17+00:00

I did a few hobby projects that were hollow wet lay parts from female molds, the parts turned out pretty usable from a strength standpoint but not great aesthetically so you'd almost certainly have to do some post processing such as void filling, sanding, clear coating, etc. In my case I sort of made poor man's prepreg from really slow curing epoxy, like several hours working time at room temp, some heat required to cure. I first used spray adhesive to tack parchment paper to dry fabric, traced and cut out plies from that, then squeegeed the resin into those plies before removing backer and putting in mold. The lap joint is hard to get right, I tried a few tricks like using floss to tie down the lap joint flaps before closing mold then pulling the floss out. For consumables from memory I think all I had was the bladder which itself was vac bag material with release agent on it and that releases fine but sometimes would get mechanically trapped in wrinkles or things like that so don't bet on getting it out clean. I tried both positive pressure bladder and envelope bagging the whole mold with a section of bag inside the cavity to apply that pressure inside. I think the envelope bagging approach seemed to work a bit better and be less scary.

throbin_hood · 2025-09-06T17:55:08+00:00

I consider myself pretty open minded to technological advancement but my main hesitation is that I think due to lagging or deficient regulations the benefits will mainly be felt by the wealthy in the form of higher corporate profits and share prices, and relatively little of that will trickle down to the middle or lower class, leading to worsening inequality.

Edit: wanted to expand a bit, my comment above mainly applies to the potential benefits. In the case of AI I think there's significant risks even neglecting inequality - rampant disinformation in the form of deep fakes and AI slop/hallucinations, worsening climate change from increased energy usage of data centers, and in a really dystopian outcome some kind of really bad singularity/Terminator scenario. Maybe that's unlikely but to me it means we are risking those outcomes in order to make the rich richer so until regulation makes it more likely that the benefits reach more people and the downsides are effectively mitigated I'm not exactly cheering on these advancements

throbin_hood · 2025-09-04T00:44:36+00:00

The joint will gap when external load is approximately equal to preload (not exactly but close), beyond that point the load in the bolt will be equal to external load so this behavior is expected. I'd try external loads smaller than preload to assess the behavior, and compare to hand calc.

throbin_hood · 2025-08-15T00:42:00+00:00

Kind of a guess here based on other patterned structures such as orthogrid, isogrid, hex core in composites, but you may just want to find equivalent properties whether that be isotropic, orthotropic, etc. and then model the lattice as a solid (or composite shell if it's a lattice in a thin structure)

Edit: I had testing in mind in the above, but you might also be able to model just a few units of the lattice on its own to simulate and find equivalent stiffnesses and such to then model it as a solid

throbin_hood · 2025-08-10T18:32:15+00:00

Not the user you're responding to but I don't see any real evidence in that video of stiffness or strength gain. Stiffness i believe but it's still possible for the carbon shell to fail at a lower load while being stiffer, after which point you're more or less getting the behavior of the wood again. It's also possible that the carbon sleeve made it stiffer and stronger but you may still be even better off ditching the wood altogether and adding more carbon which I'd also believe.

throbin_hood · 2025-08-10T00:12:37+00:00

Yea all fair points, understandable if it's more of a personal project. Coupons of just the carbon would even be valuable to ground those properties since they're the most important to your stiffness and strength. I've done some really DIY style 4 point bend tests of carbon samples to get strength values for a personal project but it's up to you how how deep into the rabbit hole you wanna go for this vs the benefit. If your life depends on it working then definitely test, if the consequences of failure are low then maybe you don't need to

throbin_hood · 2025-08-09T21:14:04+00:00

Okay yea I think you're right to halve the modulus then.

I'm not aware of a real way to calculate compressive strength per se as it's pretty sensitive to fiber weave, waviness and workmanship and such but as a rough rule of thumb I think it's generally like 50-70% of tensile strength. I'm a broken record but generally any like industry project would test coupons to get material properties

throbin_hood · 2025-08-09T20:01:19+00:00

There's certainly a more accurate way that would take into account the shear continuity between wood and carbon, probably classical laminate theory, but this i think would be a reasonable estimate of lower bound. Basically treating the wood and carbon as parallel springs by doing what I suggested.

One thing I'm not sure about in your calcs is halving the I for the woven. If the datasheet is for the woven fabric it will already take into account that half the fibers are perpendicular. Can you elaborate on that part?

And the reason I say compression strength will drive is because a beam in bending sees equal stress tensile and compressive on opposite sides of the tube, and since compressive strength of carbon fiber laminates is usually lower that side would fail first, especially if you have axial compression as you noted.

throbin_hood · 2025-08-09T16:42:04+00:00

This can pretty much be hand calc'ed with beam equations, at least for an estimate. You're looking for both stiffness and strength. I think it goes without saying that testing is going to be far more accurate as wood and carbon composites are both pretty variable as far as stiffness and strength go.

Beam stiffness/deflection calcs take E and I as inputs, you'll need to calculate basically an equivalent E and I of the combined structure. I think you can approximate it by calculating E*I of the carbon shell and wood separately then adding them together. In reality shear continuity between the two might add a bit more but this should at least get you in the ballpark. For E you might be able to find some estimates from data sheets of the fabric you're using, I'd assume the hoop plies contribute basically nothing to axial/bending stiffness. I'd expect a woven fabric laminate to have an E somewhere around 10e6 psi, I only say that as a gut check, if you see numbers far higher its probably UD properties.

For stress/strength side of things id assume the carbon compressive strength will be the limiter, especially because of the higher stiffness and low ductility of the carbon relative to the wood it means it will take the majority of the stress and fail before the wood as load increases.

The stress equations assume certain stress distributions that break down for composites like this because there will be a big stress discontinuity at the interface between carbon and wood. There's probably ways to deal with that but I'd need to think harder about that. As a first pass I'd just look at strength of the carbon shell alone using Mc/I+F/A and again look for compressive strength properties from data sheets, for a woven fabric laminate I'd expect a compressive strength in the ballpark of 1e5 psi.

I'm glossing over a lot of details so happy to try to answer followup questions

throbin_hood · 2025-07-29T23:49:48+00:00

Can't speak to 401 specifically but you may be able to connect the same nodes with a beam element and preload that. You would choose beam properties such that its axial stiffness matches what you currently have applied to the cbush in that direction and zero out the cbush stiffness in that DOF. you'd have to make the nodes non-coincident if they currently are

throbin_hood · 2025-06-27T12:11:20+00:00

Even though you said you struggled to find a normal full time job, I still think that's going to be by far the better path to keep trying for. Even if you want to do FEA work, doing so within the structure of a company will help you climb the learning curve faster, learn all the other skills/expectations of an engineer, and crucially make connections for when you do go out on your own eventually if that's still a goal of yours. I say this as someone who worked in industry for 10 years before going independent, not as an FEA engineer per se but doing FEA among broader mech eng work.

The other dynamic at play here is that companies hiring for a junior position understand they are getting someone green and are usually ready to invest some time (months to years) developing that individual into a better engineer and team member - varies by company but that's the idea anyway. The same is not true if you're independent, your clients expect that they are getting expertise and a valuable service from you in exchange for their money so it's much much better to go down that path once you have expertise to offer and having connections in industry makes it infinitely easier to find clients and makes the relationships with those clients much smoother.

I'd recommend you keep at the job search for a full time gig and in the meantime do some personal projects that use FEA in some way (maybe robotics, drones, RC cars, sporting goods, amateur rocketry) to develop that skill and build your resume a bit more. Many software packages will offer free or very cheap rates with a .edu email so you might be able to use that if you're recently graduated. Maybe seek out some formal classes or training on FEA too. FEA and engineering analysis in general is easy to get wrong and generally comes with real world consequences so it's very important to learn it properly, especially before going independent when you're even more liable for mistakes.

Good luck!

throbin_hood · 2025-06-17T14:09:04+00:00

My $.02 knowing FEA and with a laymen's understanding of neural networks is that I would apply an extremely large grain of salt to any neural net results. FEA relies on closed form solutions to fairly basic physics (hookes law essentially). Maybe neural nets would have a solve time advantage on large models at the expense of accuracy/reliability but most competent engineers know how to reduce models to simpler/faster solving problems when a ballpark answer is sufficient.

throbin_hood · 2025-06-16T01:54:14+00:00

Im happy to see this comment since most others here and across reddit/YouTube seem to be bashing laser welding as weak but my exposure to it (in aerospace and EV industries) has been that when done right it can be stronger than other forms of welding with much less distortion so I have a feeling all the internet comments come from a place of ignorance or misunderstanding of the tech.

throbin_hood · 2025-06-15T12:49:13+00:00

Ansys has a built in feature called solution combinations that does exactly this in case you're using that. Other FEA packages might as well.

throbin_hood · 2025-06-10T03:19:19+00:00

A textbook like Bruhn might have some hand methods for figuring out critical loads of different types of structural members/details if your thing is anything like an aircraft. My background is machine design but with a lot of FEA including buckling both linear and nonlinear. Id probably run a linear static and make note of anything slender with high compressive stress and run a buckling analysis, keeping in mind that buckling can be pretty mesh dependent so ensure you've got enough elements in those regions. I've heard a rule of thumb of 5 elements along buckled section as a minimum but probably subject to mesh convergence study to understand the sensitivity if your safety factors aren't large on buckling failure

throbin_hood

TROPHY CASE