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The oncology, so we're gonna start off with the stress strain curve and then talk about some material properties and then later on we'll be joined by mr singh and mr shock akani. So my first question to you all is what is stress because it's a colloquial word that we start to throw around, but actually what's the definition. If you get asked in your exam, Iggy for over unit um area typically um meter square, so so what what's your answer, so it's a force applied over unit area, uh so it's meter square, yeah, so it's of course per unit area, so it's newton's divided over me two squared or whatever unit of area you're using okay. Um There are two different types of materials they can be ice atropic or an ice atropic, So does anyone know what Eissa tropic means uniform stress over various different directions including tension and compare, yeah it's pretty much there, so i psA tropic means that material will behave exactly the same regardless of the direction the load has been applied, whereas an a nice atropic material will behave differently depending on whether it's under compression or under tension. For example, um what would cortical bone be, I tracked. Uh I'm nice atropic, okay okay, so a force can obviously be applied in different directions, so it could be tension when the forces are pulling against each other, pulling away, compression when they're acting towards each other bending forces, as well as sheer forces, So these are important concepts that you'll come across throughout ramachandran, through the different principles were predominantly going to focus on tension and compression this morning and then later on a little bit on sheer, so when a material is loaded, what are the two potential outcomes when that load is then removed refers to elastic or plastic deformation, so whether it can return to the original state or not perfect good, so uh experiences strain and strain is a change in the length in respect to its original length, and then once that force is removed, you will then either have plastic or elastic behavior, we're going to talk about those in the next few slides, strain itself doesn't have any units because it's a change in length and it can be spoken of as a percentage or you can just not use any units at talk, So if we take this bit of material, which is the gray and the dotted line is its original length, and then we apply a force to it or load. We see that it becomes thinner and longer here and that it has changed in length, that change in length is the difference between the purple and the green line and so that would be red, so it's a change in length divided by the green, which is the original length and that will give us a percentage and then that is your strain and that's if the material is loaded. If you then remove the load, that strain will be the permanent defamation in, in that material okay, so as panels has said it can be elastic, so what's elastic panels that means that when the load is removed, it will uh regain its original properties, its original length and size absolutely, so it goes back to its original shape and size, so it's entirely recoverable, different defamation, so plastic panels, so in my case, it does not go back to the original shape absolutely, so the defamation is no longer recoverable, good and then the bit that everyone absolutely hates and the exam is having to draw the stress strain curve so if you practice it and keep practicing it, it becomes a very easy station to ace. The reason the stress strain curve is useful is is that if you just talk about strain as we have just done, it does take into consideration the cross sectional area of the material that you're using, and so this eliminates length and cross section, so you can compare different materials much more easily okay, but remember it is a depending on the material that you're using, you might have different directions that load is applying, and therefore the material might behave differently, and we'll talk about how you deal with that in the exam. At the end of this talk, so the first part of during the stress strain curve in the exam and then understanding in general is that you need to be able to label the access ok So we know that stress is force per unit area, so they're new to um over meter squared on the y axis on the x. We put strain as a, in this case as a percentage okay, so the first part of the graph is linear and what does that represent are you signed in lynette. I think I saw you sorry trying to turn my microphone on uh um so that's the linear part where you've got um the same response to the same amount. I can't explain this very well to the same amount of stress, so um it's a proportional yeah, yeah absolutely so, it's linear and it's proportional, so uh what we would say here is. This is representative of the elastic zone, so if you load the material at any point here and you remove that load, the graph will return back down to zero because there'll be no permanent change in the strain because it's elastic and it's entirely recoverable anywhere in this zone. At the end of that linear portion anything beyond that becomes known as plastic, So any if you uh somewhere over here and you take the force off, you will then become have plastic deformation. You won't return to your original shape, so what is the relationship between stress and strain in this portion, it's got a name, does anyone know what it is hooks. Law is the law books, law, personality awesome. He's that sam hi, sam sorry, I didn't recognize your voice best cool, what's hooks Law, then SAm, where the stress is equal to strain, so you get it's yeah it's a straight, it's always a straight line because it's proportional, yeah, so it's the linear relationship between stress and strain, it's proportional and the gradient of that linear relationship, which is not hooks law, but it's a separate point is known as your young's modular, so you can have a steeper linear portion, or you could have a slightly flatter linear portion, okay so uh this would be how you would work out your gradients, so it's your stress over the strain, so what is um what what's so important about young's model is what does it represent it represents the elasticity of the of the material. It's a little bit more than that so um any other ideas, anyone, so the gradient of elastic portion is your young's modelers and that represents how stiff a material is okay, so it is the gradient of the elastic portion, but the actual gradient itself represents the stiffness of the material. So if you wanted to describe the difference here between ceramic and cortical bone, which one stiffer ceramic the ceramic yeah, absolutely so, a steeper gradient means it has a higher young modulates, which means it's stiffer and that becomes important when we start thinking about what ephemeral stems are made of, for example, so we say here that we have a presumable can see my cursor by the way if we say here that this is a titanium stem with a bit of cancellous bone left and cortical bone, and here we've got a stainless steel stem in a cement and then moving on to our cancellation cortical bone. Why, is it that we're using a titanium stem in an unscented hip. Why are we not using cobalt chrome, as ask them, it's got a similar young's modulates between the titanium and the cortical bone, whereas between the cobalt and chrome and cortical bone, there'd be a mismatch, so what happens when that helps with um the words just on the tip of my tongue. Uh you're going down the right route yeah, uh I know what it is, but I can't think of the word it's okay, we're going to want that anyway, so yeah, so titanium is got a similar young's modular to cortical bone, you can see here there next to each other on this graph okay, so actually the load is shared between the stem and the bone, but actually if we were to put in a cobalt chrome stem, there's a big jump between the stiffness or the young's modular of cobalt chrome and of the cortical bone, and in that situation, the load is predominantly uh felt or loaded through the stiffer material and so you end up with stress shielding. If the load is going through a cobalt chrome stem in the femur, so you'll end up with some bone loss on the, on this side where we've got a cemented stem were using stainless still in this situation, which has still got a higher young's modulates than cortical bone, and so you should expect some mismatch, but because you've put cement in, there is a gradient of mismatch and so actually it distributes more equally across all of the bones, so you don't see stress. Your d. In that situation, you don't have something stiff against something that's less stiff, okay, so then we'll move on in our stress strains, we've got to the point where we've gone from elastic into the next phase of the graph and if at this point, we decided to the green point, we decided to remove that load, it will fall back down perpendicular to the linear region and that will be the level of defamation that you have here in the green dotted line okay and this is because we're in the plastic zone now so just bear that one in mind they could very easily ask that in the exam, and before my exam, I'll be honest, I didn't realize that that was the case of how you would have drawn it out. If they ask you to explain how much defamation there was um there is a transition point between elastic and plastic and that's known as the yield point and you can divide that up and I'll just talk you guys through that into three sections, So if you zoom in on that section, now, so this is going from the linear portion into what we think is the plastic portion, but actually when you zoom into it, there's three sections of it, the first point is the limit of proportionality or the proportional limit, which stops here, and that's where the line is still linear, and it's still obeying hooks law up until that point. If you release the load, it's elastic, and it returns back to its original shape. When you move into the next phase, you can see it's no longer linear okay, it would be carrying on like this otherwise, and it's flattened off, but in this phase, it's still elastic even though the relationship is not linear, and so in this phase if you were to move the load, it would still go back to zero because it's elastic, and this point, then where it transitions into the third section gets known as your limit of elasticity. Beyond that, it's all plastic. If you release the load in this point, you will then fall per particular back down to the X axis, and there'll be a degree of defamation that is permanent. Everyone happy with that transition point hopefully good, so then we're in this plastic zone and at some point you'll see a little bump that forms I've kind of exaggerated it a bit here and this is the process of strain hardening, does anyone know what that is anybody. No so strain hardened. Incurs, when you need quite a lot of stress to be able to produce any further strain okay so that's why there's a sudden increase, and that leads us up into our ultimate tensile strength from that point, uh the ultimate tensile strengths, the maximum force that can be um put through that material before it fails. After that point, you have this rapid fall in the stress, and that's called necking, does anyone know what necking is failure of the material yes, it's beginning of the failure of materials a little bit more to the definition, and that so at this point it fails, Necking is actually the rapid reduction and cross sectional area where as it starts failing, okay. It's a bit like a bottle neck, so it rapidly reduces in its cross sectional airing and then you get fracture or failure okay and that is the stress strain curve that you need to draw in your exam. If you're asked to talk about it and the key points that you would want to be bringing out in the conversation. Any questions about the stress strain curves over can ask question is mike you mentioned stress shielding. I've heard that used locks and might have used it once, but I don't really know what that means, so stress holding occurs in a couple of different ways so stress yielding can happen in this situation when you've got a really stiff material against something that's not as stiff, and so you end up putting the force gets transmitted through the stiffer I object, so then the bone is not being loaded. The bone doesn't have the same room modeling, and so you will end up with stress yielding in the bone, so you end up with, for example um approximal bone loss. When you see that there's almost like a ghost appearance in the greater trochanter, you can also see it in fully coated stems, un cemented stems, where a similar process all of the loading is going through the distally fixed component and there's no load going through the proximal bone does, that make sense, make yes so what does cement due to stress shielding, It's meant dissipates those forces so because that takes something that's stiff to something that's less stiff back up the stress strain. There's a gradient of young's module iss the whole bone and the, the bone and the implant are loaded okay, so it softens the jump from one material to the other. Yeah going through the cement okay, you don't have something stiff next to something not so stiff okay, produces a great plus when you've got a taper slip stem. The other components that are working. There is the fact that it's relying on the hoop stress is the whole bone is being loaded by the mechanism of that stem. It's not purely just about your own schedule, yeah no worries okay, I'm gonna move on um So this is obviously we're talking about cortical bone at the moment. Why if that's an issue tropic how do we know what direction This has been loaded in. If it behaves very differently in each direction, so what you would do if they said to you that they wanted you to draw a graph in compression is, is that you turn this round okay, so this is actually in, uh If you, this is this would be an under tension if we turn it around okay, so this becomes the bottom left corner here and you would then redraw your axes and label them as compressive stress and strain and then redraw exactly the same graph and that's how you then describe that this is now an under compression because they behave in different directions, different ways in different in the way the force has been loaded in different directions okay, so you need to be able to just draw and label that stress strain curve and be able to understand the principles along each phase of the stress strain curve and then remember that if they say this is an an ice atropic uh material, then you need to clarify with them, do they want you to draw under tension or compression and if you wanted to, you could just draw this out from the beginning, but I would just draw a basic one to begin with and see where they take you okay, can, I just double check the previous slide you had you just mentioned that we normally draw the stress strain curve under tension, but here you know when you're yeah under tension, you turn it around okay, so this is in the bottom left and under confession, it's just worth clarifying that. If anyone then says you just have to if you want to make a point of it saying that this is under compression and you can just flip it around and say if it was yeah because you could do it the way, but you have to have drawn both out to show the difference, but that is only if you're drawing something for an an ice atropic material and I think in the exam if they didn't bring it up, I wouldn't create myself a whole. I think they would let you draw it in the first instance and then they might ask you if you're doing well to progress on to talking about the difference of an ice atropic materials. I think if you're drawing this out, you're probably doing very well, wouldn't you say yeah the this is in here because this has come up and somebody got asked this um in this format, so I think unless it's one of those things if you haven't given it any thought before the exam and you just get thrown well how are you going to deal with this analyze atropic material. I personally would feel quite on the spot and not necessarily have uh would think on my feet to do this so just in case um everyone happy. So far, Hopefully anyone know what toughness is the cross sectional area under the curve, It's just the the area under the curve yes. There's the area under the curve of your stress strain curve and it's the force that's been absorbed before that material fails. Yeah, so anyone know what brittle is or why don't we, I did warn you guys about pen and paper, so why don't we draw a stress strain curve of a brittle material as practice nice to see you working hard day. G, it's just about to say luke, do you want to talk us through your stress during curve of brittle material, you want to show us your drawing luke hello, so that's gonna get that's gonna get my paper don't worry, but essentially it would be a steep curve, so with a brittle material you wouldn't have much change in strain and then it would have an early failure, so I'm going to have a steep curve that's going to go to failure that's what I'm thinking okay, so what is it that that material, what is brittle that you're showing us in that graph them um undergo well. I think I think the properties are that it's um yeah it doesn't undergo much change in a significant change in length, so not a lot of strain before it fails, whereas a uh non brittle materials so pliable material was yeah, undergoes a lot of strain, and it has a large area under that's it, yeah sorry, so, following on from area under the curve, so it has a smaller area under the curve as well, is that right, no I'm just digging a hole you're doing fine. You're due your graph for us, who else has got a graph to show us I saw dillinger frantically drawing away there. I don't know where's the camera do you okay good, so you've drawn it right luke, so what is it that that material hasn't got much of uh it's plastic where's it's plastic portion uh non existent, so your uh brittle material has got minimal plastic defamation before it fails okay, so you'll go up your elastic limit have very little plastic defamation and then it will it will suddenly fail quite quickly. You know in terms of uh I would try to not dig yourself a whole and mix different um uh what's the word, I'm looking for a different elements of it, so you started talking about having a stiffer uh because I because I haven't yeah that's fine yeah. I know why you're doing that because you're thinking about ceramic and ceramic is stiff, yeah, so it has a steep gradient, but I guess there's nothing stopping you having a less stiff material that could still be brittle, yeah so don't dig yourself a but yes and keeping it pure brittle is that a material has very little plastic deformation before it fails okay uh So ready with your pens again, what is a doctor material and you can draw your stress during curse for that, I'll pick on someone else luke, Don't worry, christie, are you there all right, I'm just having to drive into work to see some patient's I'm listening while I'm right uh who asked Perry exam david yeah hi, hello so uh I've drawn sort of a flatter curve, um but I suppose it doesn't it's just that the uh the oh we've lost you david can anyone else hear david where fractures were in excellent fractures. Think you cut out briefly, then david okay, can we hear you do be a signal. I think he's got to be we can't hear you david okay, so david computer anything to get out of teaching. Huh No, I'm sure he's not okay, so your doctor material has got a very long plastic period before it fails, so you can draw whatever linear portion you want, but you need to really overemphasize and draw a really long plastic area before it fails, so that would be how you would draw a duct are material, so what is hardness well. I've given you the answers already, so it doesn't help, does it, hardness is a surface property and I think we'll probably end up hearing about some other surface properties later on during the day, but it's the ability or resistance to scratch that the material has okay, and I think probably first heard about this in most scale in g. C. S. E. S, So this is why we sometimes use diamond tip bears because they're hard in case the diamond is the strongest material on most scale. Um Why don't you um each with your pen and paper draw a stress drinker for an imaginary material, so you can make decide what properties it has and then we'll talk through some of the graphs. I don't see drawing. Iggy immediately switch is his comer off, yeah ms spaces, A terrible taskmaster isn't she, it's a very dry topic. Otherwise, if someone so do you know what I was going to give the same talk, I've given before, probably it's the reason I volunteered for the topic. Cause I've already got presentation on it. Uh It's again he's trying to make it as interesting as you can with pictures, although I learned from the last talk that I gave that all my references are about 20 years out of date, so they're relevant to me, but clearing no one else you they'll be relevant to you as well, hi, rash right then who would like to talk through their imaginary material Warren, warren's teaching later, I'll let him off the hook, wish you want to talk through yours, yeah hi, fish, can anyone hear fish. I can't, I think he's just changing out his pajamas right someone must have drawn a material come on guys, sudden sam on it come on sam, then just let me turn my camera. Thank you. So my material is uh fairly ductal, but it's got a it's got a fairly steep young's modulant as well, um so it's a stiff material, but it's also ductal yeah. There, we are, you got quite reasonable area under the curve, so it, so it's also tough. It's also tough what kind of metal of drama and sam, what kind of uh johN tendon uh stainless steel yeah, he could have come up with an imaginary name. It's an imaginary uh lovely nicky, have usual one hi, nicky morning, okay, what properties has yours got oh, you're muted sorry, I've gone for something. I think that would be very very plastic and soft, yeah, very long, plastic area, hasn't it yeah that makes it very doctor, Yeah, so I don't know a soft metal would be something like nickel or lead. I don't know it could be imaginary. It's just about talking about the principles, Don't worry, I'm not going to be as hard as mr singh mike come on, you look like you're desperate to show me yours me uh story behind it oh oh well, then what properties has it got so something that's got a very long elastic portion, maybe not a metal. Ok, doesn't have to be a metal, so yes, it's got a very long elastic portion. It had quite a short plastic, didn't it yeah, so it would be quite brittle wouldn't it yeah, so something that really stretches, but then once it reaches its uh ultimate tensile strength, it drops off and then suddenly snaps so useless well, it depends what you're using it for, doesn't it if you want the last is to be quite good okay, cool any questions, guys can, I just ask a mask question about the area under the curve. Uh In mind the audience mike yeah so no if you have two materials like that uh They both have the same area under the curve. Yeah They're two very different materials yeah and that's all right, but so that what the area under the curve is toughness, but what is it do you remember how we define toughness, so it's uh the energy that it can take before it breaks yes. So different materials may take. It may have different characteristics, but ultimately you can still absorb the same amount of energy before they snap is the the easiest way to think about it okay and it doesn't mean because the material is if the material is stiff and brittle versus as you have drawn, not particularly stiff, but do they are still brittle. They can still absorb you know energy before they'll give out okay and it's just the question of how much they can absorb okay, yeah they've got different properties, but overall they have the same toughness, yeah you're happy, yeah it doesn't matter what it doesn't matter if they're brittle or elastic, that's not what the toughness is about, so their properties that that material has in the same way that if we were to say each of us could you, and I I'm very sure you're very tall, that's a characteristic, if we could both only maximally lift 100 kg, we could, that would be our maximum that we could take. We would still have very different properties. I'm sure in your tour, yeah okay, yeah is that help, I don't know, yeah, but we could still lift the same, but we can still lift the same way. Yeah no worries um great, I'm gonna stop screen sharing then and then it's over to mr singh. Uh This is a are we going to stick with this platform. Interestingly yes, okay uh Should I just share the screen rather than share slides, It wasn't message, yeah you, I find it easier to share screen than to share slides okay, but you're not going to be able to see us once you go, yeah how interesting I know I can't see you once I go, you're right just make it slide show, and rest it. Otherwise, we can see all of you uh oh God, I'm gonna rattle through some of this because uh hang on Amrix is not working. Iggy, what was the thing that he has to do to make I'm sharing a window. It just says yeah I know but we can't see your slide show. We're seeing your home like the first screen of power point wiggy mr singh, You know how to change this. I know how I remember doing it with jim. Actually, I'll just share my entire street last week, Yeah, so you should see your screen on the right, yeah, look at that, so can you see that now, Yeah, yeah, yeah, perfect, so oh well I need to just rattle through the side. It's a slideshow um which bit is this one is this about else we're all not can we talk about strange hardening. Actually you're doing viscoelastic properties and then you're doing um fatigue failure by the way sure do you mind if I do them out of water just cause I do a lot of water, so this is um strain hardening, basically the definition as I put there that the plastic defamation increases resistance to further deformity, which is why you get a dip. Uh once you go past the yield point, but then the material gets harder again because, because literally those those factors that I mentioned there and then sometimes you rely upon that, So the other picture that's now come up shows that particularly with metals such as steel, if you cold work metal alloys, then um you change its properties so that you can basically again rely on its strain hardening to again affect its ultimate toughness and its behavioral characteristics okay. Um so uh the necking was that but I'm gonna say I'm gonna hammer through uh these sorry it will come out um t, and then so I just go onto fatigue failure. I know it's slightly article to ultimately hopefully won't matter kate. Will you just remind me to stop and I know they will be due a coffee break at some point, although in fairness, I don't think this will take that particularly long. Um. The reason I've put a picture of a paper clip up here is basically to help try and understand what fatigue is so perhaps for those of you who are exam going or about coming up to the vibe, ear's would anyone have a definition of fatigue failure in their minds that's my coffee machine turning off sorry uh um is it failure after a psychical load below the ultimate tensile strength, yeah so that's right, so that's basically right, so there's just one other word, I'd throw in there luke, apart from that was a really good definition, which is basically the material fails at a repeated number of cycles so that's another factor and then at a level of stress which is underneath the ultimate tensile strength. The only word I think you didn't put in there was stress, but um yeah that's basically the definition of it, so fails repeated loading a stress level below the ultimate tensile strength. Guys Do you, so do you know about graphs that we would normally draw for these things as well, so what I'm basically getting at is the s. N. Curve. Um I think I'd have to come out of it to be able to see uh to see you guys wouldn't I um so here we go, I guess where let's just go back into this um so. Um For example, I know that you know when we talk about fatigue failure, everything that we put in every material you put inside a patient depending on how you've intended that material to be used is ultimately a race and particularly in fracture fixation as you can see here. It is always a race between bone healing and metals or materials failing because ultimately, otherwise we would never have to rely on the bone to heal. We can always alter our constructs too to such a stage where it wouldn't fail, and I think the reason why you have to be this is relevant particularly setting of trauma is you have to ponder how you're going to fix your fracture and how then you want it to heal because that may affect your choice of implant and how strong your implant is, so for example, you've seen in this particular image that you can see here. The titanium nails broken. The sub dropped fracture hasn't healed one of the bolts distantly has broken, so all that's happened here is that fracture has gone on to a non union. The patient has been weight bearing through their titanium rod. The rod is a load um bearing device rather than a load share ing device, and ultimately through repeated cycling's of mobilizing the metal has given out and this has been the ultimate result. Um this is very different, for example, if you want to fix something rigidly where you don't want callous to form and then you end up basically well like screws, compression, absolute anatomical stability, absolute rigid, absolute stability, then you're going to rely on those cutting cones to heal that fracture that inevitably takes a lot longer than callous formation, and so you may have to think about that the material you put in there has to therefore be strong enough to withstand the extra period of of time that you might want to fix so one of the common issues for example particularly with femoral fractures is um I tend to bridge the vast majority of my family fractures in period prosthetics, or if there are implants where I'm forced to perhaps use a plate and screws rather than a nail, So if there's an implant at the top of the bottom, you're often forced to use a plate and screws and then the questions with that are, do you fix it anatomically with a lag screw or do you try and bridget uh and there are pros and cons to, to both. I've favored bridging, um simply because I find that the fracture will tend to heal before the plate will snap, so the reason I'm going through all of this with you. Uh In relevance to fatigue failure is a lot of the basic sciences you learn is relevant to your clinical practice. It's not just theory you learn out of a book in the few months before an exam and it has no bearing on your practice. I mean it is all extremely relevant okay, so um we're going to move on to the next uh the next graph which is called the s. N. Curve. Again, perhaps for those of you who are um exam going, are there any of you guys who were able to to draw a curb or to explain or to at least talk through it uh. From a peri exam point of view okay, sorry, bit of for he back uh we've got oh looks at his camera, okay, luke, might be ready or cat. I might join in on my phone as well, okay cause I can't as you say I can't see any green chairing and rest, you can flick in between Can you yeah and then you can just go into the medal once and then you can go back to your slides okay, let's just see if I can I think if I perhaps um but okay in that case, um who okay cool, so quite a bit of feedback and rush yes uh is that better, yeah good, so um I think the only person I can see that this is kate and mike um so okay does anyone again. I guess does anyone able to define it even if you're able to talk through it with me uh. So exam wise, uh Penelope is on Vicky's on lynette that's my I've got stress on the y axis. A number of cycles on the x axis. Essentially if you can load a material at different stresses uh and this is then how many cycles they go through, so if you load the material under the endurance limit um uh It will that it will essentially be infinite cycles. And then when you go above the endurance limit, it's then uh how many cycles that particular material can go through before failure. Um so the number of cycles and more often it's a logarithmic scale, um which just as you've drawn there and then the only question is that you know how they do you know how they actually calculate it. It's not always a nice discreet curve When they calculate this in the lab, what they do is, they literally will cycle through a material until it fails and then they'll well how many cycles and what the stress level was. And you put a cross on the curve. And then they'll repeat, they'll lower the stress and they cycle it again, and they do this a lot, and what it generates is a number of crosses that then basically you join up to make the line that you've just drawn there so uh Perhaps as mr Hopwood has shown there, it is there's the sM curve, so a log arrhythmic scale, the stress It's interesting depending on what graphs you look at some. Also have stress is a log arrhythmic scale as well, some don't the line the dash line that you can see there is um supposedly the endurance limit, so sam, um Perhaps the next question then is do all materials have an endurance limit uh no, because the ones that don't have an infinite and I think his aluminium one that uh will just go on well. They don't know if it'll go on forever, but certainly when they tried to, so, I guess the question, then is that if with materials like aluminium that have a, they don't have an obvious endurance limit. Do you know you do you appreciate that they calculate an arbitrary endurance limit. Do you know how many cycles, I don't know no so, it's it's it's 10 to the seven is it, I think that's what Ramachandran says, but I'm not mistaken, but you basically choose 10 million cycles as an arbitrary number because that is a lot of cycles for any material uh. And then you calculate uh the stress at that point and then you say that that is arbitrary arbitrarily. It's a it's endurance limit basically okay. So if you are making an airplane wind, do you want to be above your endurance limit or below your endurance limit above oh okay, I guess on the curve, then sorry on the curve, yeah yeah where do you want it to be do you want to sit, so you want it below on the curve line or below the dotted line below the dotted line on the curve, Yeah right because you just don't ever want that airplane wing to fall off yeah, so again the endurance is just as you've said SAM is the stress at which materials can theoretically stand infinite cycles. Um So this is uh um and just as the example you suggested as well, you can see that it never levels out, it just keeps going right the way all down to the bottom, and uh the endurance limit uh is set at 10 million cycles just because otherwise you know you can't give it a number as it were. Um So again the airplane wing is the example that I gave you you'd want to be well below it because ultimately you don't want your airplane wing to ever fall off. Um So the next question is where do hips operate. Do they operate above or below the endurance limit, look okay, and so again the idea behind that is that again you've got uh depends upon uh so it is below and the idea is that hopefully you've got a lot of contact area yeah because you've got a lot of contact area. Your stresses are often lower just simply because if we remember, stresses fought over unit area. So my next question is where do we think knees operate, leading questions invariably and can anyone rationalize um why they operate where they do would they operate, so it is leading oh sorry I was going to, I don't know, I'm just using the principles you have given us, but the I would presume they are the opposite to hips because the contact area between the implants is low in comparison, that's right, so they operate above the endurance limit because obviously the amount of contact at any point is smaller, so they undergo just by their nature and by their very design a lot more stress, because you know you're in essence putting your poly uh through different uh type, well different levels of stress and so it affects its behavior, so the relevance as you can see on the graph as I've outlined because again as with anything um if you are end up if you end up talking about this in your exam, it's always that question about how where is it relevant to your clinical practice um And so talking through the stress strain service okay, but invariably hips and knees are good to throw in simply because it makes it relevant to clinical practice. So the idea is that hip polyethylene um will does undergo somewhere, but they you know they are crosslink polly's they have better wear properties and they're less likely to undergo fatigue resistance or fatigue failure, but they wear out. Like you know just as it was, as you no doubt get delayed unless you've already had um the talk on, where, but you know, invariably that polly particularly wears out knee. Polly's tend to fatigue fail. They don't wear out in the same way that hips do where you see e, centric loading and they are they, they are often made of ultra high molecular weight polyethylene rather than X linked, and the reason is is that the higher ultimate tensile strength is well sorry, the ultimate tensile strength is higher and as a result it's fatigue failure properties or it's resistance to fatigue failure is higher, okay, so it's not like it's less likely to fatigue fail, but as a polyethylene, it has poorer wear properties, but again again, given that it's not likely to wear out in that you're not likely to rub through the poly, you can rely on that a bit less whereas as we say with hips because they're in such high contact, you need their wear properties to be better, but then because they're not likely to fatigue fail, you can use a slightly different type of poly uh I was just clarifying missing sorry um so when we say endurance limit for hip replacements, um do we say hip replacements are above the endurance limit so or rather below, so it's just a wording thing the word thing isn't taking so if I take you back to that so there you go there's the hips, the bearing surface, the poly okay, I guess to be more specific, then um this would refer to the endurance limit, No this would refer to fatigue failure of poly okay when we talk about hips because that's what we're talking about, we're talking about the polyethylene here and hips and their polyethylene operate below the endurance limit because we just discussed the highest surface area, lower stresses, knees, and they're poly operate above the insurance limit because low contact surface area, higher stress, and as I said the reason, it's then relevant because of the different types of plastics that we use in each bearing surface, and why thank you, I can so um not sensitivity, does anyone have a definition for that. It's a material where a surface surface material property, where if there's in um incongruity such as cracks or scratches, for example, a brittle material such as ceramic have a high notch sensitivity, yeah what does that lead to. Then again, potentially it leads to a material failure yeah so that's right, so it's the increased risk of fracture If the surface is in in homogeneous, which is basically just as you've outlined, digging okay, so I mean the theory is that obviously ceramics you know, I I guess through normal scratch profiles of ceramics versus a metal. As you appreciate they have a different scratch profile, but that in itself is a different subject uh slightly, but you know scratches, ceramics tolerate scratches much better but if they had a little deeper scratch and it's such that it might lead to a crack, you get much because it's a much more ductile material, sorry, much more brittle material you the notch may cause it to crack further. So very relevant example, I've got in my car at the moment is a few weeks ago, I got a big chip on my windscreen and now I've got a 40 centimeter crack on it after not having actually done anything particularly different. The idea is that that chip caused a notch sensitivity in the glass, which is also very ductal, sorry, brittle, a big button, and as a result, it has allowed the glass that you know the rest of the glass and the windscreen to fail through no particular trauma versus a metal, which because it is um duck till you know even if you get cracks in it, you don't likely get the same propagation of these uh uh of fractures um at a, at a lower tensile strength, so that's where notch sensitivity comes in, so it just means that when you're dealing with your ceramic heads, um you have to be nice to them and that's why we you know when you put them on your trunnion and you hit it just make sure you hit it with something with a good amount of coverage over, It's something that's not too hard or not likely to damage your your ceramic head. Because you may find that one day the patient comes back and it's all splintered simply because you know you created a little crack in it either as you reduced it or scratched it as you reduced it or indeed, as you tap the head off um and that's so well before I go into time defended behaviors which I believe was after the coffee break um Does anyone have anything they want to ask about fatigue failure or any other principals. I can try and explain any easier could you one of the things I'm not sure I understand that well is the, is, then the, is the fatigue strength which I think is it something different to the endurance limit. When you're talking about the s. N. Curve, oh that's a good question some I don't know if I know the answer to that to be perfectly honest um the term fatigue strength I've always just referred to as fatigue failure because in essence um what do you understand of it um what do you think is different about it. I might learn something I just remember reading it in the, when the one of the va nasca pitchbook, so I'm trying to find it again, but it's not something I really understood as I read it and I'm just wondering if there was any easier way to yeah. I mean if you talk me through, I might be able to rationalize it for you, it might just as it's gone over, perhaps your head, it'll go over mine. Um I'm trying to find it so I can I've never used fatigue strength. I've always talked about fatigue failure, so I'm just very quickly googled, it, sorry, guys uh and it seems to be that the two term knowledge is that just used is inter, changed okay. I think you have to talk about fatigue failure in the exam. It would be again keeping it quite pure and I think less examiners would be confused by it, is it. The vival book uh because I'm on 1516 of the s. N. Curve, page 1516 of the bone ask a bitch digital vision okay how to find fatigue strength just can. I can just find endurance limit. If if anyone's got the bone ask a which viable book, third edition the latest yeah, because they question it what is the difference between fatigue strength and fatigue limit as one of the questions and the ultrasound for some materials that I think becomes horizontal at higher end values or there is a limiting stress level called fatigue limit, sometimes called the enduring limit below which fatigue failure will not occur for other materials, aluminum and copper, magnesium. They do not have a fatigue limit. The sm co, continues its downward trend at increasingly greater end values. As such, fatigue will ultimately occur regardless of the magnitude of stress for these materials. And fatigue restaurant is specified as fatigue strength, which is defined as the stress level which failure will occur for some specific number of cycles. I tend to the 7 10 38 okay that's fine. Then that does make sense. It's just it's another way of you know how I said to you that um for materials like aluminium um they never fail yeah and so I've said that the endurance limit uh is set arbitrarily at 10 to the million cycles. What I between that is basically that is your fatigue strength um It's kind of truly calling because it's not a true endurance limit is it yeah. It is actually uh you know you set it at an endurance limit because you want to be able to, you know work with that material and have some idea of where it might yeah, um but I think you would just call that where you have to perhaps set an endurance limit rather than the material having a true endurance limit as it's fatigue strength. Yeah Yeah so just for context, um when I went to visit one of the labs in cindy's, they always put it to 10 million cycles for 10 years for any new implant, which, which, which is um industry yeah, and I think that's just it that is just an industry standard big number again, so I think that's why um ramachandran and that's why the 10 the number 10 to 7 exists. I guess because it's just a large number of cycles basically assuming assuming one million is one year um I think that came from, I think that came from charlie when he had his um gen arthroplasty yeah Any other questions so far in fatigue failure. Otherwise, uh mr singh will if you're happy to carry on with fiscal elasticity and then have a coffee break at 10 30 ish and then free body diagrams afterwards okay. If that's okay hold on. If you're happy, thank you very much so um biscoe elasticity otherwise known as time dependent behaviors okay. Um This is I'm afraid, I'll try and make this as interesting as possible. It's extremely hard to make this particular topic interesting. It is ultimately something you just have to know. Um I'll try and make it relevant and it is relevant to your clinical practice certainly to mine as a hip surgeon um uh So that it hopefully doesn't become too dry. A topic basically so viscoelastic city um is stress and strain behavior that is time and rate dependent, So I think hopefully as as I've tried to perhaps highlight in previous talks that I've given a lot of stuff in the basic vie vers, starts with a definition and the definitions your starter for 10. So you kind of know where the rest of the vibe is going to go, but it is also therefore, quite important to have certain definitions in your mind because if you deliver them without hesitation, I think it then sets usually the president's for the rest of it. It also hopefully means that you understand what it perhaps uh is I'm gonna stop the full slide shakers again. In terms of the order of things, but I guess just for with the elasticity. The key things are that it is you know as we've already been discussing stress and strain behavior, but it is just that it is time and rate dependent. So the first one uh is creep, not just horror movie. Um Does anyone have any ideas of what creep is and there'll be four of them as as you might recall, so this is the first of the four, so does anyone have a definition for creep defamation, honest defamation under constant load, for example, um p. M. M. A, or bone cement, So instead of loading under constant, we're talking about the stress strain yeah under stress, yeah, yeah, so the idea is again it's just if you keep these terms in your mind, then they don't become too interchangeable, so let's see so this is the graph for it and again as with the others, you will end up needing to keep different graphs in your mind, so as Ziggy said a constant load, I'm going to reframe it as a constant stress, so I apply a constant stress to a material and over time that material stretches or it's strain increases, so creep is increased strain under constant stress, okay, so now perhaps the other side of this, what is stress relaxation reduction in stress under a constant stress. Oh sorry, yeah sorry under well, I always say that to cut some loads, a reduction in it's a reduction strain under a constant road. Um So in this okay, some in this one, the strain changes under a constant stress. If you think of stress relaxation is the opposite of not the opposite of creep, it's the other side of the coin of creep, can you redefine it again, So in this one in this graph, strain changes, stress stays the same, so in stress relaxation. What happens so time dependent reduction in strain under a constant stress. Yes, yeah basic that's it, it's they're just literally the opposite side. Thick stress over time, so stress relaxation is uh much of what I do go to meditation, but this is this is what it is there okay, So do you notice just sorry, if I take you back time is the bottom is the x axis but the y axis in creep is strain Why access in stress relaxation is stress, so the idea is you've you've got your constant defamation okay, but the amount of stress needed to maintain that defamation goes down, so your stress relaxes this. Trying to remember this was quite hard for me. Um Trying to remember which way around it was the only way I could ever think of it was to think of it of the term stress relaxation, so if you say stress and relax stress in my mind goes down, so in stress relaxation, stress goes down over time, and because stress is changing the strain is the same and then creep, I just used to remember it's the other way around, so stress stays the same and strain increases, so they're literally as you say the opposite, but it's just how easily you can try and remember that I mean I'd say the only way i can remember it was to think of stress, relaxation, relax, means something going down potentially, and that's how I used to think of it okay, so stress relaxation, constant strain, the amount of stress needed to maintain that strain goes down, so how is it relevant okay, so you've all put in hem, ease or total hicks, you've all used broaches. Um There won't be anyone here who hasn't and the idea behind this is that if you're in a rush and you keep hammering broaches into a femur, you're all at risk if you are to brisk about it, of splitting the femur because a brooch is nothing more than a wedge. You are putting down a log much like the diagram on the right. So the idea behind this is that when you hammer broaches in and if it's a bit tight, but you want it to go a bit further, just wait, even waiting 10 20 seconds, allows an element of both creep and stress relaxation to occur so that you can knock the broach out, knock it back in, again wait, knock the broach out, knock it back in. Again, for those of you who have operated with me, you have heard me use the phrase I'm just waiting a moment to you know let the femur get used to the idea that I'm violating it and that's all you're basically doing you're just letting the femur get used to the forces that you're putting upon it um so that it it has time to relax, so that you don't basically cause it to fracture so and again, the other side of it is basically if you think of stress relaxation is it's like putty or played oh. Uh For those of you've got children, I'm absolutely certain you've played with play doh, and you can see that it's behavior changes as it changes, So for example, if you got a lump of played oh, that's nice and flexible, you can pull it apart and if you do it slowly, you'll be able to get it all stretched out like the picture on the left, and that's because you're applying your stress. At a lower rate, your strain is able to increase versus the other side of it is if you just get a lump of play, Doh and yank your part, so you apply a much bigger stress, then it would just breaks up into clumps um and that's the other way to potentially think about that. Do you guys understand that in the relevance of either play, dough and or bone um or do you want me to discuss it in a different way, no that's clear okay, so um Again, this is just graphical to show that I can't what material this is for, um but it basically just shows that your rate of application affects the behavior as well, so it's not just about the well it's the time and the rate, sorry, so in this one, this is a bit of the well as you can see the rate of change the strain rate affect its behavior, so the higher the strain rate, you basically apply higher stress over less time so there's your played oh you're ripping it apart or if you apply a low strain rate, you apply a low stress but over more time and then that's where you can get your play doh to stretch out and again this is where um I guess it's that question of not only when you're hammering in your broach like, I said whether you're in a rush to do it and you're doing it quickly or whether you knock it until it gets tight, just wait for a little bit knock and then knock it in again because you need that time to allow these other behaviors to occur, but by giving it time, you're also affecting the rate at which you apply your forces, does that make sense. This goes back to the play dough, and things again, yeah, yeah okay um hysteresis uh is the final one which I must admit I think this is just something I learned uh unless miss spaces got another way of explaining it, I never really um understood it perhaps as well in terms of applying it to my clinical field, so history says is stress and strain you load the material and as it unloads it has a different pattern of relationship between the stress and the strain and the idea is that the gap between the two lines is released as heat, and the theoretical reason why the behavior is different is that the energy is lost due to its internal frictions of the materials, as the molecules shift around, they apply friction at each other, that friction generates heat, and as a result, the stress strain behavior of it changes so um this is where it starts to become a bit bit uh This is truly just the definition, I'm sorry to say you have to learn so the four things to keep in mind our creek stress relaxation time, time rate, pendant behaviors, and then history sis okay. So again I know as I said it's a little bit dry but are there any other facets of this topic that you want me to go back to or to discuss in any different ways could you just go back to the strain rate dependency. Um stuff, please because it's the it's the it's the graph and which which line represents which I think I got this in a mock fiver and I couldn't really describe which was which and which was a high strain or low strain, and over sort of time here, I guess I mean it's a subtle graph in so much that the lines I'm afraid are particularly divergent. It is because I guess it is from taking from an experiment of an actual material, but you can see the three curves as they get darker in color. If you look at the rate of application, so that's point you know of the strain, so it's 30.0.5 millimeters a minute, so you're pulling this material apart by 0.5 or half a millimeter minute versus the top line which is five centimeters or 50 millimeters minute that's obviously quite a difference as you can see, and the idea is that in order to do that you have to give it a higher well you have to give it a higher stress that the yeah you have to give it a higher stress to be able to pull it apart okay, so you have much higher strain rate. Because as you pull it apart, you basically have to give it more stress, but it needs you have less time versus the graph on the bottom, which has a lower strain rate. Because perhaps argue it's a fractionally less steeper because you're applying a low stress, but over more time okay, that's um I'm still trying to digest it, Yeah, I guess it's it's the question about if you have you have you played with play, doh sam, yes, yeah, So if you think of it as if you want to make a long piece of play, doh, you've got to do it carefully and slowly you're using low stress and more time you're giving it a low strain rate because you're pulling it apart slowly, but because you're using a lower strain rate, the plato becomes a nice long snake. Yeah, this is if you now just try and pull the play doh like by five centimeters within a minute, if you try and pull it apart. Really quickly, you apply high stress over a lower time and as a result, the play doh just breaks uh so, depending on how much stress you use over how much time affects the strain rate, the rate at which the material lengthens, and as a result how it will then behave. Yeah. I understand that it's just then translating it onto drawing a graph. So again tam, I think if you are ever asked to draw this, I would like to think you are again doing quite well, um put it that way. I think you'll spend your you know the common ones that you end up talking about and certainly being able to draw a graph for would be um stress, relaxation, and creep. I think you have to be able to draw those. Ramachandran has a slightly different picture, I guess of time dependent strain behavior, but all you basically have to stay is that you know. All you basically have to explain is that the behavior or the strain characteristics of the material is dependent upon how quickly you apply your stress to it uh and then you can draw an exaggerated version of that. I'm pretty certain, I don't I can't remember if Ramachandran has a graph of that or not. I think yeah fast, fast, it's just one straight line and then slow is a curve with the ultimate yeah, perhaps that would be a better example because it's more exaggerated way of drawing it. Yeah you certainly have to be able to draw history since, but that's quite an easy one yeah as it were, and again time dependence, trained behavior that's a hard one. I'm not sure when we really um think about that in our clinical work, um to be honest. Play. Doh was the best example that I could come up with that. It seemed to be relevant. I guess some of it may apply to bone cement as well, perhaps whilst it's going off, but again because I think it's not necessarily something I can easily explain In terms of clinical relevance, it becomes hard to uh perhaps understand it. Beyond just learning it can, I ask a question, so say, for example, at five minutes, the stem is not going down you're only about out with the first hole, would you have to to longer okay, say we say assuming that I didn't leave it too long and this situation where we push it in at five minutes, we then push it slowly or push it quickly or hit it hard or hit it quick or hit it slower. I've got to be honest. I g, I think at that point I would just get the stem in, but that's not based upon understanding of time dependent strain behavior, that's just cause I want to hit the factor in to be honest, I want to get, I don't want my stem sitting proud. Um So this is the question Again, I guess before that it's uh yeah that's a hard one mg. I don't know, I would just want to get my stem in the right place, and I must admit I would just hit it down and hit it quite hard because I wanted to go down in the right way. I guess probably the right answer is actually to just constantly maintain load and push it in slowly and allow the cement to displace around it uh. And I suspect if you really wanted to do it in the textbook, you would want a low strain rate because you want your cement to be displaced as your stem goes in, So I would suspect the right answer is to just constantly push it, but do it slowly, but the difficulty there is the properties of the material that you are pushing it into is also changing yeah, so uh I guess the examples of our yeah, yeah. I guess if you are putting it into play doh, in the femur and the property of that is not changing. Um you want I suspect um to be able to use a lower stress and more time to displace the play, doh to get your stem down because the characteristics of the play doh aren't gonna change versus trying to push it hard, where everything I suspect would get jammed and it won't it won't then go in. Thanks. Um Perhaps the only other place where this might be relevant uh. In that case, I'm just trying to think of other examples are impaction grafting uh In theory, and the idea with impaction grafting is that you do you when you put in the chips of bone into a defect and you start hitting it. You don't want just want to hit it hard, two or three times high stress, less time because the bone that you've impacted in, isn't going to coalesce, isn't you know isn't going to change its length. What you wanted to do is to hit it in lots of times. Um The other example of that is when you put in an unscented stem, for example, doing a peri prosthetic um the tape is blind stems like the restoration, the arcos, where you put them into a whole, um and basically it is a taker you're supposed to hit them 100 times, not very hard, okay, and the idea behind that as you constantly hit it, you're using a low strain rate to allow the bone to to accommodate the stem. Whereas if you just get your stem hit it really hard, two or three times apart from the risk of breaking the femur, it will jam in its first instants, but then what happens is that over months, the stem then tends to subside because the idea is that it got jammed in the first instance, but over time as the bone relaxes, you get substance as well, which is a combination of well all of these time and read dependent behaviors. I think that's how I could perhaps rationalize it have you all got brain ache okay. I think that that was the last one on on this fabulous thank you, mr seeing any questions, guys, or yeah nothing. I've must have seen this one before yeah, so, I'm gonna have a coffee break um mr shock akani, oh no mr singh is going to restart, isn't he on three bodies next, so uh should we come back at 10 30 kate. Given this is the hip term am I just doing the three body hits, presumably, we don't want to do the um if we can focus on the hips, we've we've got a bit of time left, so if anyone has a specific one that they would like you to go through, Yeah then we could I know there's been some slightly odd curveballs, haven't there of recent years. Yeah okay, so come back at 10 30 then, and uh we'll take from that. Thanks am rush.