Stress Concentrations and Finite Element Analysis (FEA) | K Factors & Charts | SolidWorks Simulation

all right so if you look at a problem like this you might go way back in your memory banks to the earliest parts of this course and you might think that this looks similar to some of the stuff that we did back then okay this is saying find the maximum normal stress carried in this part when it is subjected to this 5,000 pound tensile load acting in the part right so we know exactly how to do this what do we do okay the maximum stress is going to occur wherever you have the smallest cross-sectional area some one says which I agree with that so we would say we go all the way back to our first very first things that we learned in here we would say that the normal stress here is going to be equal to what okay force over area the force is 5,000 pounds and then you would take that and divide by what okay we need to figure out where is the area the minimum what's one possibility for where the area is the minimum okay the little part where there's a reduction in the width of it where it goes from three inches down to two inches on the right end of it okay so how much cross-sectional area do I have right there two inches times okay you see up here we've got the thickness of this part so 0.5 inches okay and this makes the math work out really nice so we can mostly talk about the idea right so what does this end up predicting as far as the stress that's going to be felt over there in that front tip of this part okay 5,000 pounds per square inch easy stuff right well I don't know if you guys knew this or not but SolidWorks which is a you know the piece of software that you guys have become familiar with here SolidWorks also has the ability to take parts like this and tell you under a particular amount of load how much stress is going to occur in the part so what I'd like to start with today is to pull up SolidWorks and let's see what SolidWorks says is the worst-case stress okay real quick before I do that is there another candidate for the worst-case stress could be at the whole right well if we look at the whole so this is kind of on the the right end what if we look at the whole okay they're 5,000 pounds over what okay same thing basically but you would get to it by staking three inches minus one inch because basically you take the total width and subtract off the material that's missing because of the hole and you take that and you multiply it by half an inch okay so I also did that on purpose to where they wind up being about the same okay so but like I was saying SolidWorks has the ability to tell us how much stress is felt in a part and so let's take a look at SolidWorks and see what it says the stress might be in a part like this okay so I'm gonna go into SolidWorks here and I'm gonna go into file new and we're going to start a new part some of you might be using a more recent version of SolidWorks than me I apologize for that if it throws anybody off but this is still what I've got on my computer and it hasn't changed that much all right so the first thing you want to do once you start it apart here is we want to actually draw the part right we haven't drawn it yet so we didn't go in here and all the part that we're talking about so to do that let me start a sketch I'm gonna put this sketch on the front plane and let me you know kind of just start by roughly drawing something that is about what I want something like this okay now when you do this you there's a little bit of care you need to take because it's going to apply a lot of relations as I'm clicking through it's going to apply a bunch of relations I was as I put this little shape together I was doing it rather quickly but I was trying to keep it from adding too many relations right now all I really want is horizontal or vertical relations for all of these things so now what I'm gonna do is take this point right here and this point right here I'm gonna say merge those points and it makes this you know funky looking shape here what else you think I might be able to do to sort of make this the proper shape okay I'll tell you what let me put the hole in the middle of it too okay tell you what I'm gonna actually put the hole such that it the hole is centered on the origin this doesn't matter that much but we'll just go ahead and assume that that's what I've got there and the hole was how big okay one inch so I'll go ahead and and dimension that okay and then it was between this edge and this edge down here how far was it three inches okay between this edge and this edge was two inches okay one of things I can do to make sure this is centered I can apply a relation here and I can say let me get out of my dimensioning tool here I want to make this little segment right here the same length as this segment right here okay so I'm gonna make these two things equal to each other that'll sort of center up this thing now let me do this I want to make this piece right here collinear with that piece as well right so that'll kind of make the shape do more like what I want it to let me show you another trick you can do you can add Center lines all right so if I draw a center line from corner to corner like this one of the things that allows me to do is that center line is not going to be part of whatever part is created it's just there to help me draw things so I can take that and choose the midpoint okay choose the midpoint select midpoint right and if I want I can make that if I select those two that things at the same time I can make those two things coincident with each other which is going to kind of Center that hole in this part okay now what else do I need to do to sort of you know fix the rest of my dimensions that might matter did I have this dimension down here okay we can flip back over here and look at it but what was that dimension okay four inches okay and then this dimension right here probably needs to also be specified two inches all right so now I've got my shape that's fully defined here and I'm going to turn that into a feature by doing an extruded boss/base right that's what SolidWorks uses to create this actual shape and some of you might be wondering on the original that I had I had Phillips I had like little curvy things on these interior corners all right I left those off at this stage on purpose because one of the things I'm going to play with after a little bit is changing the values of those and it's easier to do that if you use an actual Filat feature at those locations instead of drawing it into the into the figure so anyway just to explain why I'm doing what I'm doing there okay we're gonna make this half an inch thick right and then I'm gonna just do this as a mid plane not this doesn't really matter very much but that basically makes it to where the origin is going to be centered in the middle of that hole okay so there I've made the basic shape of the piece and what I want to do now is add the fill it's so I want to fill it on this little edge right here okay but I'm gonna also select another edge right and it's kind of cool sometimes if you just hover in the right spot it'll let you pick an edge that's not necessarily the thing you see when you look at the figure and the orientation it's in right here in this you know at this moment so anyway I'm gonna pick that as well so those two edges I'm going to fill it and then I believe we had those set at half an inch right here right so go ahead and set these at half an inch okay when I hit check it has now made the part that I'm going to try to model using the simulation tools of SolidWorks okay so that part should all be review I'm just making that part in the first place should be old hat for everybody at this point let's start the new stuff okay I'm gonna show you that on my instance of SolidWorks here I have this tab up here that says simulation okay many of you if you're looking at SolidWorks right now you might not have that tab in your version of SolidWorks yet if you don't it's not that big a deal you can go up into tools and under tools there's a thing down here that says add-ins and under add-ins you can enable SolidWorks Simulation alright we have a license for that here at our site you know so you can you can do this these simulation things under here so and if you don't have the option to do that it could be that whenever it was installed on your machine maybe someone didn't take the effort to actually make sure it wasn't that part of it was installed on your machine you might need to go back and and have someone at our yeah even in one of our offices here look at how to get that installed on your computer if that's something you're interested in doing okay so this is the simulation tab and the first thing you want to do under the simulation tab is go up here to study advisor okay and under study advisor what we're gonna do is go to new study okay over on the left side it gives you a few options for what kind of study you're about to do right and we're gonna do a static study so that's the very first option up there at the top of the list now it has populated the upper bar up there with a number of things that we're going to need to do to do this study so the first one that they have there is apply material alright and it gives you a big whole library of certain kinds of material I didn't tell you what kind of material this was on the very first page so what do you think we should pick here all right you might want to pick a structural steel or something let me say this it doesn't really matter as long as you don't have to answer any questions about how much it deforms right that's why it needs to know what kind of material it is is that it would need to know the elastic modulus of the material to know how much it would start deforming under this load and the thing is internally the system that it's using to solve for this needs to simultaneously evaluate how much the thing to forms along with how much force stresses I should say it has in the various pieces of it so anyway all that to say it doesn't really matter but if you want to pick something that's roughly a structural steel this ASTM a36 that's basically the structural steel it's a very similar kind of steel to what our structural steel is in the data tables that we've been using in the class so we'll pick that one okay hit apply and then close I like how it changes the color just slightly it says hey I've made that that steel material alright next thing you need to do is define a portion of the body that's not going to be allowed to move okay in SolidWorks they call that a fixture right and the way I recommend that we look at this is why don't we fix the left end and then we can apply loads to the right end and that'll do the same thing as our original picture even though up here I wrote it as a five thousand to the left on the left side and a five thousand to the right on the right side it's not going to be much different for us to go ahead and put a fixture on the left end and apply the load on the right end okay so I'm going to do that here under fixtures advisor right I'm going to insert a fixed geometry okay and I want to put apply that knot to this face right here but I want to and I don't want it on the edge either I want it on the face back there that right now I can't see so a lot of you are familiar with being able to rotate using the arrow keys on a keyboard you could certainly rotate that around and pick that face on the backside but let me show you another thing in case you haven't seen it before if you right click on here almost always it will give you select other as one of the options in this menu what that does is it allows you to select things that may not be on the you know the side that you want to actually work on right the side that's closest to you so notice there you know I can kind of pick that that face that's back there and choose that as my fixed face see that anyway maybe you've seen that before but some people haven't and they like to like to see those nice tricks like that okay so there's a fixed face back there the next one in the lineup here is external loads so let me go under there and I'm going to apply a force okay over here in the the little bar it says you know this right here would be faces on which to apply this force so I'm gonna apply this force on this face and you might notice there that the arrows look like they're applying a compressive force is that what we have it's not so what we're gonna do is couple things it's ready to give me these SI units I don't want to use SI units because I define my original problem in u.s. units so let me change that to us and then under the force I'm gonna go ahead and put in the amount of force that I'm applying on that face right 5,000 pounds and I can reverse the direction right so there's that force I'm putting on that face I'll hit check okay we're gonna skip connections advisor even though that's a really cool kind of it's got a lot of cool tools in that connections advisor because it allows you to figure out how multiple parts will interface with other parts you can do things like no penetration like you can have one part in another part and you can sell them not to penetrate one another and it'll calculate those contacts between different parts but anyway we don't need to do that for this for this problem so I'm gonna skip that and the next one on the list is run and there's actually a few options underneath run and I'm not going to choose any of them just yet because I'm gonna make a strong point here if you're doing one of these simulations like this make sure at this stage you save alright because it might actually start trying to calculate this for you and it very well might crash your computer and when it does you will have lost everything that you were just working on right so we're gonna save right now okay save and let me go in I'll do a new file folder here okay and I don't really care what I call it but let's say just demo or something like that so now I've got it saved and I'm gonna even though you don't have to do this I'm gonna show you multiple steps under this run because I think it's gonna be instructive to help us know kind of what SolidWorks is doing under the hood just a little bit okay under here you might notice there's an option that says create mesh so I'm gonna hit that what meshing does is it takes the body as has been defined and it splits it up into little bitty pieces right they're called elements so it splits it up into these itty-bitty little elements and it will basically define that whole piece of geometry that I've set up up there as a chain of all these little elements so let me actually show you this it gives you a little slider bar to make it either coarser or dense or finer okay so what do you think it means to make that mesh finer smaller elements and more of them okay what do you think that does to the solution it'll take longer to do but it it will probably be more accurate and it will certainly be better at figuring out gradients of stress and stuff right so if you if there are fast changing values along the geometry you need to have a larger number of these elements to be able to capture that right so anyway let me slide this kind of more to the fine end a little bit here and and go ahead and hit check now I'll tell you that it's not uncommon for SolidWorks to experience some some difficulties during a mesh alright this time it actually made it just fine yeah how coarse does it go that's a good question let's play with it right so create mesh can we go all the way I don't know why it's hopping over to this other thing but you can see there it can go pretty coarse you know there's those members or those little pieces now are these they're actually kind of little tetrahedrons right that this thing is all made up with all these little tetrahedrons because that's a relatively simple shape to be able to model how stress propagates through a little tetrahedron and by stacking all of them up you can figure out how to stress propagates through the whole thing but anyway that's the coarsest end of the scale and we could run it like that but let me let me go back to something a little bit more fine here all right so there's the mesh and what do you think you do after that you could save again it's not that hard to make a mesh you know so I don't think I'm gonna hit save again but let's go ahead and hit run okay know me sometimes this runs that ran relatively quickly right and what it has now done is figured out what this thing is going to do is showing you an exaggeration of the deformation that that part would experience under that load that I just put on there this isn't the actual amount of deformation it does but it takes all of the little deformations and sort of you know magnifies them so that you can see basically what shape the thing takes on okay but that's not necessarily what I want to see over here on the left it has automatically come up with a few results for me to look at right so if I want to see stress I can right click on here and hit show right and what it's showing me there is with this scale this color scale that oh that I have over there on the right it's showing me how much stress exists in various parts of the piece like in various zones of the piece okay so I don't particularly like the units that it just gave me there so tell you what I'm gonna go in here and right click on stress one here and go to edit definition and under here it says Newton's per square meter I'm gonna change that to psi okay a lot of you might be wondering about what is this von Mises stress okay we won't be able to get entirely into that but I will go ahead and give you at least a little bit of it you might remember when we talked about pressure vessels that when you have multi axial states of stress there's another step you have to do to apply a some sort of a failure criteria and figure out what's going to actually fail the material take into account multiple directions of stress at the same time okay one of the ways of doing that involves finding this von Mises stress which you can kind of think of that as an equivalent stress that you can compare to a tensile test value right and it's an one number that you can compare to a tensile test value and it's valid to sort of know whether or not you're close to breaking the material okay it's based on in case you want a little bit of terminology it's based on a failure criteria called the distortion energy criteria alright but we'll save that for a future time together all right so this is what the what the stress is let me change it to psi here right so what's my maximum stress according to SolidWorks I know this is kind of small text up here okay 1.2 to 1 times 10 to the fourth psi and it says where does it occur Yeah right inside of this circle right here right okay so this is kind of interesting this isn't what we would have predicted using our formulas that we've already used force over area what's going on okay so that materials that have discontinuities in them discontinuities can be things like a sudden change in cross-section right a hole is a sudden change in cross section right over there you might notice where the thing has actually reduced in in size right these little areas right over here it's reduced in size what do you know why does it have this little yellow spot right here okay that that's another spot where it has actually started to concentrate stress at that location because there was a change in geometry alright there's a change in what the cross-section looked like and this happens for a lot of different kinds of changes in geometry of a part if you suddenly go from one size to another then a lot of times we'll concentrate stress at that location where you've changed from one thing to another and it will actually have more stress at those locations than you would have predicted even after having accounted for the missing material because of the hole which we already did that right but there's even more than that just because there's a discontinuity in sort of the fabric of the part okay so that's kind of interesting what do you think might affect that this stress value okay he says the radius radius of what the radius of the Filat right let's play with that a little bit let's go back into the part right this fill it right here let's change that definition okay let me let me change this to where is it there it is right up there I'm gonna change it to instead of 0.5 let me make it point 1 okay once I've done that and it's going to need me to remesh the part right because I've changed what the geometry was that I that the part is made up so it needs to actually redo that mesh step so let me go up in here and hit mesh again okay I'll leave it there the same you know relative level of density but you'll notice here that now we've got this much tighter radius right here okay so it meshes and then we'll go ahead and solve it all right so go back into stress here show that piece what's going on in there now we've got some pretty strong you know red spots let me ask you this do you trust the kind of the shading of this these red spots right here is there a reason in your mind why we should have a really strong red zone right here that doesn't even necessarily look like it's symmetric with the other side does that seem like that makes sense okay so he says that's due to the mesh right this means that I probably don't have a fine enough mesh to actually capture the changes in stress that happen right there at that zone so what should I do about it you might want to make a finer mesh and this is always you know I always feel like I'm playing chicken here a little bit because I usually find that if I take this thing all the way to the fine end I'm much more likely that something's gonna crash when I try to mesh it you guys feel lucky what you got okay his question is what exactly is the limiting factor in terms of the computational power of your computer in order to be able to do these calculations let me kind of describe this for you a little bit so each one of these little elements that it makes let me go ahead and hit mash and we'll see how how this goes all right looks like we made it so each one of these little elements that it creates right here each one of them has to be in equilibrium each one of them has relationships that are applied to it that predict how much it deforms right and each one of them interacts with all the other ones around it right so there are all these relationships that are set up per element right how much is it deform whether or not it's an equilibrium how does it interact with all the pieces that are around it and the result is a really large system of equations like look at the number of elements and there's multiple equations per element right and it has to solve that giant system of equations in order to figure out all those forces that involves a lot of things but one of the big things it involves is you need a good bit of RAM like it needs to be able to store a lot of the data that it's actually operating on as it does that process you know obviously having a good processor to be able to do the floating-point calculations and all that stuff that's good too so it's not necessarily my exact area of expertise but there you know it's almost not much of anything that it doesn't rely on in order to do a calculation like that this computer has 16 gigs of ram and it's about I think it's a three gigahertz processor okay all right so shall we run this and we'll see what happens okay while it's doing this this is one of the longest ones probably that it's going to take to run you can actually force it to show you I ran out of time there there's a thing that you can actually see how the solutions converging if you're doing a problem that's that's like really heavy alright alright so we're still looking at the same little spot and let's show what the results are it looks like we might still have a little bit of stuff going on because of the mesh but it's a lot better than it was right did it change the value a little bit right and a little bit more than nothing right so anyway we're at one point five one eight times ten to the fourth psi at that location is it worse here than at the whole yeah what changed yeah that fill it radius made a big difference it's a strong function of just how much of a radius you have right there yeah yep why should white kay so his question is the first iteration of this that I ran they both had the same radius and yet one of them that the hole looked like it was a worse case than the reduction all right I'll say it's that way because it is that way right but you know we will because they aren't the same right these they're it's different situations so we shouldn't necessarily have an expectation that it would end up being the same but I'll show you a little bit of maybe a secondary confirmation that maybe we're not too far off okay so let's go ahead and and grab that value one point five one eight times ten to the fourth okay psi and will say this is the results from our finite element analysis okay and actually this really wasn't our result for this exact problem was it okay this was the result when fill it radius was was what okay point one okay shall we go back I should have written it down when we were there but shall we go back and change this again so that we figure it out for our original case okay go back in here change this back to 0.5 okay Ramesh and then run the study okay he goes pretty quick if you if you're working on a harder problem you can kind of see how the various variables converge all right so we'll show this and now our worst case scenario happens at the hole right so we'll look at these two cases this one for the hole what's that maximum value 1.2 to 1 times 10 to the fourth that's the same as it was before wasn't it so our changing of the mesh size because we changed the mesh size a little bit from the first time we ran it to what we did just now didn't have much of an effect on what that value was that's actually a pretty good indicator that our mesh was probably fine enough for that question before I changed it again so anyway so let's write that down 1.2 2 1 times 10 to the fourth ok so this is for the for the reduction and then we had another one 1.2 to 1 times 10 to the fourth psi ok and this is for the hole so what I was about to show you is this people needed to be able to do these calculations before finite element software was readily available right people have been designing mechanical parts for a lot longer than a tool like SolidWorks or something like it we're available to be able to find whatever these stress concentrations were through this finite element method so how did people do it before ok there's a couple ways these things are based on some interesting equations that you can actually come up with manual solutions for right they're not easy to come up with and sometimes they're not even possible to come up with other times people have done you know studies that might be called photoelastic studies so there's a way that you can coat a piece of material with a particular coating and then pull on it and look at it with a polarizer light and you can actually see the part you can see the various you know stresses that act in the part by looking at these cut these changes in color bands that go across the part there's there's some pretty neat things people have figured out how to do in order to figure out what these stress concentrations are before you could you know really reliably use computers like this and based on all the results of that the results have been put together into certain charts and I know this one's on its side here but this is the reference material that you have available for you to use that's one of them right there okay this is a case where you've got a hole in the middle of a plate this is a case where you've got a reduction in the cross-sectional area those are basically are two cases that we're dealing with right here what do you think K might be all right let me show you for all of these you know if you wanted to do these by hand they're all based on this idea that our actual maximum stress once you've considered stress concentrations is going to be whatever we said the stress was before which now we're going to call that a nominal stress right that's what we would have said the stress was before we knew anything about stress concentrations multiplied by this K factor and that K factor is called a stress concentration factor it's just you just multiplied by it and it tells you what the actual max is as opposed to what you would have predicted before you knew anything about the stress concentration effect okay so if we can figure out those K values then we should be good to be able to calculate what these maximum stresses are and the K values can be found off of these charts so for our problem what do we need to do let's look at the hole first okay let's orient ourselves with what it's giving us on this picture along the bottom down here at the bottom what is this race you have radius versus diameter how do you know what it means okay I know this didn't actually pop very very well on the the colors here so let me put this on here this is saying that the radius of this hole is R okay what's D okay you might be able to see these a little bit better it says you've got 1/2 D available on each side of the hole okay so D is the total amount of material that's left over there all right so for our problem that we have up here R is half an inch and D is what 2 inches because got three inches minus one inch okay so we're dealing with R over D of 1/2 over to right which is 0.25 what do I do with that find it on here that'd be this location right here right maybe I'll do that in a little different color so we can see it a little bit better find this point on the curve and read across to here do your best shot at you know reading what that value actually is keep in mind this is like 2.4 2.6 that in the middle would be 2.5 so what's our best guess as to what that K value might be just reading it off of this chart ok 2 point 4 3 sounds good to me right the 2.4 you're very confident of that the next digit you're gonna make your best guess right just by looking at the chart okay so two point four three and that so that's K and so for our problem up here what is now our maximum stress we would predict at the hole okay maybe I'll put that right here okay all we're going to do is take that K factor of two point four three and multiply it by what we just calculated a second ago right five thousand pounds over three inches minus one inch times 0.5 inches right all of that stuff all the stuff in the red brackets right here is the nominal stress and I'm multiplying it by my stress concentration factor in order to come up with what this actual maximum is right so two point four three times five thousand we already calculated that twelve thousand one hundred and fifty does that look relatively close to what we came up with using SolidWorks we came up with this is this this here is the same thing as twelve thousand two hundred and ten psi those are pretty close right what SolidWorks said versus what this chart said that stress is going to be at that hole okay shall we do the other one the other ones just a little bit there's a little bit more tricky nough stew it because there's another there's instead of just one curve there's a set of curves right so I can go down here and now instead of finding R over D only which by the way let's do this for the once I reduced that fill it down to point one so in other words I'm now working on this where this sort of this being point five this is now point one okay so R over D is gonna be what point one inch over what's D two inches it's the you know width of the narrower part okay this will be what point zero five well what else do I need to know D over D right these are all all these different curves are different capital D over lowercase D what's capital D okay it's that larger width of the two parts so for us capital D is what three inches lowercase D is two inches all right so three over two is equal to one point five right so that means we're not on that curve we're actually on this curve right pointing at that curve right there so how do we use this well we grab our R over D value of point zero five it's gonna be right in here somewhere we read up okay to the point where we hit that curve one point five once I hit that curve I read across to here keep in mind halfway up right here was two point five okay so you like the idea of two point seven looks reasonable enough to me excuse me to point you're right to point seven was that middle spot excuse me I see what you're saying now this point that I drew right here was to point seven and so this point that I'm picking off now is what do you think okay two point seven two or something like that maybe okay so two point seven two we'll be that K two point seven two times 5,000 psi so what does that give us 13600 okay we wound up a little bit further off on that one right not necessarily a huge problem but it's you know two different ways of coming at it and you know it could be could be a couple of errors in there right it could be that maybe SolidWorks maybe we still needed a more fine mesh in SolidWorks maybe we didn't have that mesh fine enough where we really figure out what their actual max is maybe we didn't do a real good job of reading exactly what that stress value was off of this chart right like maybe I should have not had 2.72 maybe I should have two point seven three or two point seven four all right it's it's not necessarily easy to read off of this chart exactly what that should be so my point with this is this the using these charts gets you to some level of precision it's not perfect right you're not gonna be able to go out to sixteen digits and get exactly the same result of someone else right but it gives you a really good idea as to how much stress you know is actually being experienced in the part once you've accounted for these stress concentration factors alright questions yet okay I yeah go ahead okay he wants to know for the exam just how wide are these margins I'll show you a couple of different things that we could do what if I had solved this problem and instead of telling you that the radius of that curve right there was point one two instead of 0.1 everything else is the same I make the radius of curvature for the Filat 0.12 instead of 0.1 okay what that would do is it would shift over to where R over D would calculate to now 0.06 instead of 0.05 so then I could read up this line up to where I hit this curve and holy moly that gives you a really solid value of 2.6 that's one of the things that we commonly do on exams to make sure I mean we don't always do this but this is something that is commonly done so that people have a little bit more of a good feeling about it when they've done it right they can be pretty sure that's probably the number we were shooting for 4k okay as long as you did it right now another thing that we can do there is will a lot of times give you results that are only a couple of precision digits of precision right will say is it twenty-five hundred psi or is it twenty-seven hundred psi or is it twenty-nine hundred psi right well we'll make those more broad so that you can feel like yeah think of maybe got that right right although we were - we were that much off he says for this one we were almost two thousand psi off that seems like a lot right and I would say it does seem kind of like a lot but it's not 2000 psi difference between one person reading the chart and another person reading the same chart right it's 2,000 psi difference between a totally other method finite element method where we're not even sure what sorts of you know we're not super sure for that one what sorts of error we might be introducing because of the nature of that method being an approximation anyway right so anyway all that to say you're going to be closer than that being that it's two people trying to read the same chart and answer the question on an exam yeah yeah okay he says what if you wanted to apply on a SolidWorks Simulation you wanted to apply a load to the inside of a hole okay if you want to know what probably the most realistic way to do it is it's to go ahead and also model the pin okay and then put the pin in the hole right and then specify no penetration right what that does is SolidWorks will say alright I've you know solid original say I've been instructed to not allow the pin to move through the material that it is now inserted into so once it once it detects that there's going to be contact between the pin and the hole it will say I'm not going to let it go through the one I'm going to calculate how much force exists on that surface where the two parts contact each other so that's probably the most realistic way to do it short of that the other thing that you can do is you can apply a fixture to that hole right like instead of applying the fixture over here on the left end a quick and dirty way to do this would be to fly the fixture just on that hole and then apply a load to that now that's not exactly right because in real life only half of the hole is going to be interfacing with the pin because the pin is either going to be pushing to one side of the hole or to the other side of the hole but it's a way that you can at least get somewhat close maybe yeah all right so you want to know could you put the whole thing together and then load it and see what it would do if you're building trying to model a truss in this thing okay there's a few reasons why that becomes difficult okay so one of them is you end up needing a fairly fine level of detail at the holes in order to actually be able to resolve the kinds of forces and stresses that occur at the hole and if you use that level of detail across the whole truss you end up with a lot of elements like a insane amount of elements so there are some tricky things you can do to make it have a tighter mesh in the locations where you need a tighter mesh and to go to a coarser mesh in locations where you don't need the tighter mesh it's a little bit outside the scope of what I was planning on teaching you today okay so someday maybe you will be able to use this to do it a you know an evaluation of a larger structure that's not necessarily what my goal is for you this is mostly I wanted to at least introduce the idea that you can calculate some stresses in SolidWorks it'll show you about insights that have to do with the stresses that might occur in members that might not have occurred to you unless you had this to show you right we might not have known about stress concentrations unless we had done some sort of an experiment with this and it starts showing us hey it concentrated that stress a lot in these particular locations maybe we should do something about that you know I'm saying so there's a couple of goals that I had with today's but it's mostly an intro right just there's this tool available the nice thing about SolidWorks is it's very easy to learn believe it or not it's one of the easiest ones of these packages to learn in my opinion and so if you want to play with it I highly encourage you to start messing around and see what you can make it do the other factor that would happen with one of these wooden trusses that we're building that this is this method is not even going to be able to do it without their you know we'd have to actually introduce a whole other sort of set of tools that SolidWorks had if we wanted to be able to do this but this is not going to evaluate buckling right that's a whole other kind of analysis that it's not doing in this set up okay so it'll find stresses but it won't evaluate whether or not the thing it's buckling using our technique that we just used right here buckling is another module that's in here but again it's it's more effort to learn how to use not saying you can't learn it just saying it's a little more effort to go in there and figure out how to use it alright one last comment and then I'll let you go okay what if this is a structural steel and a 36 structural steel which means it has a yielding strength of about 36 ksi and what is the ultimate strength approximately of a structural Steel's in - and remember okay 66 ksi we can look that up if you want but in there in our reference material here up here at the top okay we have 36 ksi for structural steel and 66 for your ultimate strength what that means is you have a lot of deformation that this material can do before it reaches the amount of stress that it takes to actually break it this is a what's called a ductile material structural steel is a very ductile material means you can bend it and stretch it quite a bit before it actually will start to you know want to crack okay so if you have that in mind and you go back to our problem that we're trying to solve here and you actually let me go to the SolidWorks thing where is the only material that's reaching these high stresses yeah like not very much of this material is reaching those super high stresses and what if you go out here like it's hard to see it here but it says out here you're at more like 4 ksi out here right or maybe sorry like - ksi or so out here ok so what do you think I'm getting at with this so where I'm going with this is this it's not always the end of the world if your material yields a little bit locally at one little spot right we're not saying it broke we're saying it it deformed permanently once it reaches those high levels of stress so think about what would happen on this if you reached high levels of stress enough to deform it permanently right here in this little zone so then it deforms permanently what happens then okay so what it'll do it'll it'll move stress away from that zone and force the other material that's still here in this cross-section right it'll force that material to carry more stress as it yields a little bit and that transfers the stress outward into the material that's not currently holding a lot of stress right and so here's here's the principle we can draw from this there's a lot of times where when people design with ductile materials they ignore stress concentrations and this is exactly why right if your design designing with ductile materials you sometimes as a matter of fact you know very often you don't care what this maximum stress is as long as it's super localized because your presumption is once I put it in service it's going to yield a little tiny bit right there and that will equalize the stresses across all the material that's they're available to hold the load and I don't really care if it yields a little bit locally right there in one spot okay I know that's a little bit hard to think about but there's you know I'm trying to one of the reasons I tell you this is I'm trying to actually give you a little more confidence again in our earlier calculations we did in the quarter cuz we did things like you know hey you know how much force can it hold before it breaks right and you know now that you're applying this kind of thinking to it thinking I wonder if we considered stress concentrations on all those problems that we did earlier in the quarter what I'm gonna tell you is if they're made out of ductile materials is a pretty good chance that it wouldn't matter anyway because the presumption would be that the material would yield a little bit and then that would just transfer load that had been concentrated all at one little spot in to more of the material and you're fine okay and that was about all I wanted to say any other questions yes sir okay so you might be saying what about these other ones that are here in the reference material so let's kind of scroll through here so we these are the ones we looked at just now okay are there any other ones here's another one what's this one okay it's the same geometry as what we just calculated but instead of carrying an axial force what's this one carry a bending moment a flexural influence that's happening on it okay and it has its own set of curves for dealing with that flexural version okay there's another one down here somewhere right here this one is for torsion so if you have a shaft that reduces in diameter and you're twisting it right how do you know that this one is torsion versus the other one that's a bending moment okay this one's pretty obvious it's showing you that you got these two bending moments that are being applied to it this one's a little bit less obvious cuz they didn't do a real good job on this one of labeling the torque so for this one let me just go ahead and make that clear say this one's for torque and if you want a little clue other than you know me telling you that they did show you that it's a roundish shape by kind of how they drew it right it does give you the idea that this shape is circular but anyway I know that's that may not be you know enough or totally desirable but let me just go ahead and say this entry that we've got here is for torsion and this will allow you to calculate what these stress concentration factors are for that okay here's another one what's this one oh come on what's that one there okay it's another flexural one but what's the difference on this one it's basically got two little notches cut in it right so it's a little bit different than the last one the last flexural one that we looked at this is also let me say this these charts that we have in your in your reference material that you'll be able to use on an exam or whatever these are by no means exhaustive right there's actually books and books that people have put together of charts like this for various kinds of cases we just picked a few of them that might look interesting make it easier for us to write a particular problem and they throw that we threw those in the reference material but once you know how to generally how to read one of the charts the process is the same regardless of what kind of geometry you're looking at okay we good all right let's call it a day

Channel: TheBom_PE

Views: 46,896

Keywords: Swanbom, TheBom_PE, SolidWorks, tutorial, education, lecture, engineering, stress, concentration, factor, finite element, FEA, simulation, mesh, chart, peterson, K factor

Id: AXL4ll3aRB8

Length: 63min 22sec (3802 seconds)

Published: Thu Nov 07 2019