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학습트랜스크립트
00:00Remember that we have some cracks when some stress is bigger than yield strength.
00:12We are also talking about some brittleness or ductileness.
00:21If that material is ductile, it can store more energy inside.
00:27If it cannot absorb that much energy, that material becomes quite brittle.
00:34See, cracks having sharp tips.
00:40The edge can be this sharp, or it can be some blunt tips like this.
00:48Blunt tips or sharp tips.
00:52When we have these kinds of two types, then the plastic material deforms at a crick tip, which blunts the crap.
01:03Like this deformed region.
01:06So if we have this sharp edge that looks brittle, that's what we have been talking about in the previous slide all the way.
01:18And then also the energy balance on the crack.
01:22So it might be a little bit difficult to understand.
01:26How can that save energy when a crack occurs?
01:32So here, like they are saying like elastic strain energy.
01:37Before the crack occurs, they can absorb energy inside.
01:42We will see that in a few slides later.
01:47That there is some test way how that material can absorb the energy.
01:53So energy is stored in material.
01:55See, like energy can be stored when this crack occurs.
02:00So it is elastically deformed.
02:04So when it moves like this way, they need some sort of energy to make that way.
02:12This energy is released when the crack propagates.
02:16You know what that propagates means?
02:18It's moving and then not simply like just moving or generating another sort of crack.
02:32So creation of new surfaces requires energy.
02:35This is also quite important thing.
02:38Whenever we need the surface, we learn about surface energy.
02:44All the surface has surface energy.
02:48To see that surface, we have to add energy to see that surface.
02:57So creation of a new surface, it doesn't need to be that large.
03:04But in the case of water, that's much higher than any plastic surface.
03:10You know, most plastic surface, the energy is the scale of like 20 or something, 20, 30 energy per square meter or whatever.
03:23But when we have this much scale, then water is around like 7, 30 or 78 or something.
03:35No, I think 72.8 something.
03:38So that's much higher.
03:40So like two, not just two, like three times higher.
03:46So that means it's easier to make surface with the plastic, but it's not easy to make surface with water.
03:56That might be why the water gathers.
04:02That's high surface energy.
04:05When we have a high surface energy, that means that it doesn't want to go that way.
04:10It wants to lower the energy.
04:14Surface energy can be lowered.
04:16But that's a little bit different story.
04:18So I'm sorry.
04:19Let's focus on what we have here.
04:22Forget about it.
04:24But anyway, you know, when this crack occurs, obviously the face, new face will be generated by this, the energy stress.
04:38No, I'm sorry.
04:40Stress is not energy.
04:41Stress is a force.
04:43If the force is applied on a tip or on a certain space, then the moving distance generates energy.
05:00That energy can be stored to this surface or material reset.
05:09And criterion for crack propagation.
05:12So, you know, like a crack occurs.
05:15I mentioned that like a critical stress and then the crack tip stress, when we have that crack tip.
05:25If that crack tip is greater than just regular critical stress, then a crack occurs.
05:34And also, that is also calculated this way.
05:38You know, remember this A?
05:40What's A?
05:41Do you want to go back to the slide here?
05:45See, this is the A.
05:47If we have the void, then half of this distance is A.
05:54If it looks like the circle, that could be radius.
06:00But it's not a circle.
06:06So it's like a half long distance and half of the long distance.
06:12That's A.
06:15And then critical stress is something like a table, tableized the value and young modulus.
06:24And then this is a surface energy, right?
06:27Surface tension.
06:28Sorry, it's not surface energy.
06:30Surface tension is a little bit different from surface energy.
06:33You just figure out what's the main difference between them.
06:37It's very, it's not like a force, you know, a certain, what is that, like a standard area, that could be an energy.
06:51But it doesn't have that, the energy scale for the value.
06:56But here, like a specific surface energy on unit surface area.
07:08That's more correct one.
07:10And one half length of internal crack, that's what we have learned.
07:16In chapter 8, you always think about, remember this A is like a void, that one half length.
07:24That's good words, like one half length of internal crack.
07:29For ductile material, replace this, the surface energy with surface energy plus this value, plastic deformation energy.
07:39So when crack occurs, they do not, they do not deform themselves.
07:46So they didn't use any other energy to change their shape.
07:54So no extra energy is required for that shape.
07:59But in the case of this, like a plastic deformed ductile material,
08:04we apply some energy that can be stored to deform the shape.
08:13So that's also one of the energy we should use for the deformation.
08:21Okay, here like a fracture toughness range.
08:26Metal, so they have quite high value.
08:34And then polymer is quite low value.
08:37And the composite has the in between them.
08:41So we can control that.
08:44Typical like plane strain fracture, the values.
08:49Remember this value depends on temperature and strain rate.
08:58That will be learned in the later slides.
09:01And also microstructure.
09:03Higher strength VR, like a hardening can reduce the value often by doing some.
09:12That's why it has like this variable.
09:18By doing some sort of extra work, we can change the values a little bit.
09:25Then let's see design against the crack growth here.
09:38To prevent the crack growth, we also have to learn about something.
09:47So control stress, we can reduce some values by lowering loads.
09:54That's obvious, right?
09:56So here, let's see design against the crack growth condition here.
10:06And that's like a fracture toughness.
10:10Regularly, if the energy or applied stress is greater than this fracture toughness, then obviously that crack will occur.
10:23So we have to keep that material lower than this value.
10:30That's normally what we have to do.
10:32But in this case, we would like to know about the crack growth.
10:37So we gave the intentionally higher force rather than this, the acceptable required fracture toughness.
10:52That is equal to Y is like a sample size.
10:56And then this is a stress.
11:00And this is Pi's number.
11:03And the A is, do you remember that?
11:06What was it?
11:08Like half of the void size?
11:12So based on this one, if we have a large size and then if it has a large void, that most highly stressed cracks grow first.
11:28So what can we control here against the crack growth?
11:37So we cannot control this sample size for specific purpose.
11:44Let's say I'm trying to make a computer a keyboard or the airplane wings.
11:55Then we already have certain size for that purpose.
12:00So obviously we have to make a smaller value.
12:06This one should be smaller than regular value for this.
12:12So this design must be smaller than this.
12:18This is sample size and this is the value we can get from the table in the previous slides.
12:27And then that's the possible max void size before the actual crack occurs.
12:37So let's say if this can be one centimeter for the airplane wings or 0.7 millimeter for any kind of a keyboard, whatever.
12:55I've never seen keyboard is broken by just my finger typing or something.
13:03But anyway, this is sample size and then this one.
13:09So the stress, how much we are going to apply to that material.
13:16For example, for my keyboard, I do not beat the keyboard with my whole energy.
13:31Sometimes that occurs.
13:32I saw many keyboards, not just the keyboard itself, but you know, what is it?
13:47Some other, when it has like a high force, they are angry or something.
13:54But also we can control this like the void, but I don't think we can control this void size.
14:03But if we can control that, so we can control this one or we can control the less load.
14:12Isn't it obvious to say that?
14:15I guess.
14:16But anyway, that's the way we can go to.
14:21So here, actual aircraft, the wings.
14:26That's quite, I think that's quite tough one.
14:31Let's say we are, we have to design a aircraft wings.
14:37Can you make it?
14:39Would you like to?
14:41Okay.
14:42Consider a simplified wing skin panel under tensile stress.
14:48Due to aerodynamic loads, right?
14:52When the airplane is flying, the, what is that?
14:56Some Bernoulli equation, something, which we don't care much about.
15:01So the, the, the, the, the values are given here.
15:08So this is the, we already, the new, like a critical stresses based on this equation.
15:21And then it's already given here.
15:24Then the sample size, we don't know yet.
15:27And then also the largest flow is nine millimeter.
15:30That's here.
15:31Largest.
15:32The flow is nine millimeter.
15:37So we can put here half of that, right?
15:42Or, um, whatever that can be changed.
15:53But here, that the, the same, the same aircraft.
16:00So here, use the same material.
16:03That means, like a, the K values are the same.
16:07And the largest flow is four millimeter.
16:10So this must be two.
16:12In this case, 4.5.
16:14Or this case, two.
16:16So, um, both are the same materials.
16:21So, so the, the stress might be the same for the both, uh, critical stress should be the same.
16:34So, uh, the Y should be the same.
16:37And then A is half.
16:40So, um, uh, if we have that, uh, the equation, then here.
16:49Um, maybe, I don't think they have to use, like, nine millimeter for here.
16:57It should be 4.5.
16:59And this point should be, like, a two.
17:02But the ratio is the same, right?
17:06So the, the result might be the same way.
17:09So if we do that, uh, we can calculate the.
17:18critical stress.
17:20For, for this airplane.
17:24It's a little bit higher, obviously.
17:27It has, like, uh, the higher floors inside with the same, um, material.
17:34So it can, uh, it can survive at lower the failure stress.
17:41And then, uh, this is the, um, impact test.
17:49Before the linear elastic fracture mechanism, uh, became standard.
17:55Impact tests were developed to gauge notch, notch, um, toughness at high loading rates.
18:03So, uh, impact loading.
18:09This is how we can calculate that, or what's that some, um, um, the energy absorption.
18:17See, the hammer is located quite high, uh, high position.
18:22And then, uh, there's a choppy, uh, uh, sample here.
18:28The specimen is struck by, uh, some pendulum.
18:32So the height difference.
18:35It started here, and then, it, it hit the specimen.
18:39And then, it stops around, like, a final height.
18:42It, it, it will swing between this, this height, and this height.
18:47A little bit.
18:48A little bit.
18:49And then, it will stop here.
18:53The first, final height.
18:55The height change must be related to some, um, what is that, uh, the potential energy.
19:04Where did it go?
19:07This energy must go to that, that material.
19:14See?
19:15So, it absorbs the energy.
19:18The height difference, right?
19:20Before and after.
19:21Fracture gives the absorbed energy.
19:24High absorbed energy implies better notch toughness under those, uh, test conditions.
19:31Of course, these results vary with temperature, strain rate, and material condition.
19:39Though impact energy isn't the same as fracture toughness, it's very useful for quality control,
19:46material comparisons, and assessing ductile to brittle transition behavior.
19:56So, uh, this one is quite important, like, influence of, uh, temperature on impact energy.
20:03We just, uh, learned that energy can be stored to the, the material.
20:09If it doesn't, uh, if the material does not accept this kind of impact energy, that's more brittle.
20:18Then, if we can absorb quite large amount of energy, that's ductile.
20:24See?
20:25So, uh, this impact, impact testing may give us some, uh, hint or the, the information about
20:34the ductile, if that material is ductile or brittle.
20:39Let's see what it tells.
20:43Because, like, many BCC material, BCC material, metals, and then the ferric stairs, and everything, a lot,
20:52show ductile to brittle transition.
20:55That means, like, here, the face-centered cubing metals, or those materials, do not show this kind of, like,
21:04ductile to brittle transition temperature.
21:08Of course, uh, the, obviously, at low temperature, something is, uh, the, uh, toned down with many pieces.
21:17So, um, uh, I cannot give you exact example, but if you have glass, the, the falling down to the, the floor, at low temperature,
21:31it may have the, uh, higher number of pieces, um, when they are broken down.
21:40So, here, um, the impact energy is, uh, the high, at high temperature.
21:51But as temperature drops, impact energy goes down, and that's brittle behavior.
21:56That's what we have learned in the previous, the, the slides.
22:01The transition region is characterized by mixed behavior here around.
22:06Um, FCC metals, like, uh, many aluminum alloys, do not exhibit a sharp transition.
22:19They tend to remain relatively ductile down to low temperature.
22:25That's good for, uh, application, right?
22:29So, uh, plotting impact energy, impact energy,
22:34vs. temperature, reveals a transition temperature range.
22:40This informs the safe of operating limits for the structures like ships,
22:46and bridges, and storage tanks in cold climates.
22:51So, uh, some material may show, like, this temperature might be 4 degrees Celsius, or 25 degrees Celsius,
23:01or negative 10, something.
23:05So, um, if we use some material showing, like, 4 degrees Celsius for, uh, this DBTT, uh, ductile brittle
23:17Then, when the temperature goes down, lower than 4 degrees Celsius, it will show a brittle, uh, fracture.
23:29But above this temperature, it'll show ductile, uh, fracture.
23:35So, uh, what's the main difference? Do you remember that?
23:40In the case of brittle, there is no warning.
23:44There is no sign to see that kind of a final, uh, structure.
23:50But in the case of the ductile samples, there's a warning.
23:56It shows some bulges, signs, or, like, um, bending.
24:01See?
24:02That's, uh, that's quite important to, uh, design the, some material for, some, the applications.
24:12Right?
24:14See here, like, uh, the Titanic, they had, uh, they had a rupture.
24:20And then, also, liberty shapes, also.
24:23It's 1957 and 1996.
24:28It's not that old story.
24:31Okay?
24:34So, steels were used having DBTTs just below room temperature.
24:39That's the problem.
24:41See?
24:43Uh, in, uh, in components are successful to, uh, ductile to brittle,
24:49transition.
24:50A key strategy is to ensure service temperature stays above DBTT.
24:58Yeah.
24:59Then how can we do that?
25:04Like, uh, so let's say, it says we have to keep that material for that application.
25:13Let's say it's like the Titanic should stay above the DBTT, for example, 10 degrees Celsius.
25:21What does that mean?
25:22We have to run this ship only in the summer or spring, fall, when the temperature is lower
25:31and higher than 10 degrees Celsius.
25:34But that's not happening, right?
25:37So we didn't know that much about this kind of characterization before.
25:42So they used, they used the Titanic at low temperature and then the crack occurred.
25:50So what should we do?
25:53We should use some other material rather than iron, right?
26:01So regular, the iron, that's interesting, right?
26:07So the textbook says like, uh, the, the ferrous material, um, that's happening.
26:20Okay.
26:21Here another, uh, the, the word term, fatigue.
26:26You know, what fatigue is in Korean?
26:31Fatigue is 피로.
26:35You know, um, when we are doing our work a lot, like diligently, sometimes we get exhausted.
26:45Then that accrued, that's kind of a fatigue, right?
26:50Let's move to this fatigue, uh, word.
26:54Failure on the cyclic stress or fluctuating stress.
27:01It doesn't have to be like exactly the same wave cyclic.
27:06And sometimes it's, it's something like this.
27:12So, um, open below this yield strength.
27:16And then, um, let's say some, the yield strength is here.
27:29Then, um, you know, like, uh, the sigma max is greater than yield strength.
27:35That'll obviously break it down.
27:38And then if that yield strength is higher than that, what's gonna happen?
27:43So we don't expect any breakage or something.
27:47No, that's not true.
27:49So in the previous slides, we didn't have that time, um, the variable.
27:56From, from now, we have like a time.
28:00So although that, uh, the stress is lower than, uh, the yield strength.
28:07When the time, the variable is added, what's gonna happen?
28:13That's what we want to talk about.
28:16So, uh, the, the often below the, uh, the yield strength.
28:22And then the well below the static tensile strength, fatigue is, uh, is also occurring.
28:29We have to consider.
28:31Then also like fatigue is the responsible for the majority of service failures.
28:37That's quite, uh, interesting, right?
28:42So the, here, uh, I, I read something.
28:50The, it says like a 90% or some 95% is responsible for, particular for most of the failure of this, the material itself.
29:05So stress varies with time.
29:09So time, this is the important thing for, uh, this fatigue one.
29:17Cycling frequency is given here in the, the black line.
29:22And then, um, the key parameter are like, uh, the, the stress M.
29:31So let's see what's happening here.
29:33Oh, here.
29:34I'm sorry.
29:35I did 90%.
29:37So, uh, the, although this M means like a mean, averaged one, uh, the max is quite here.
29:49And then minimum is, uh, uh, I don't think that can go to some, this is, I'm sorry.
29:56This is not a zero, like a maximum stress and the minimum stress.
30:01Yeah, obviously minimum stress can, uh, exist, right?
30:05The mean, the average value is around here.
30:08Then the, the, the difference, the mean to the max is, uh, is considered as a, a key parameter S.
30:20So, um, the, if that, that fluctuating value is large, the mean value is showing quite large amount of S.
30:40But if that is not, if it, the, the, is very small, then this S is also quite small, and then mean value is also close to this max value.
30:59So what's going to happen?
31:01That's, uh, that's what it, what it want to explain here.
31:11So there are two types of fatigue behavior.
31:14Let's take a look.
31:15Um, just like this figure explains everything in detail.
31:21So here, stress amplitude, that's S, what we just learned about.
31:26And then this is N, cycles, the number of cycles to failure.
31:31So, uh, remember that S, when we have this one, the mean value, and this is S.
31:40If S is the certain amount, stress amplitude is large, large over this S fatigue, this is unsafe area.
31:58Let's say we, we, we, we run, we ran these cycles up to 10 to the fifth, and, or 10 to the seventh, and then seventh, and then seventh.
32:11And then this, this value is quite large.
32:15That's unsafe.
32:17Right?
32:20Right?
32:21But when, uh, the number of cycles is quite low, the number is low, then this, like, this amount will, the amplitude is still the same.
32:36And that's safe area.
32:40So do you, do you see how we can read this figure?
32:45So, um, if we have this amount of the amplitude, that's this one, how long can we use this part?
32:57See, here, we can use that part up to the cycle of 10 to the fifth, or I don't think we can use 10 to the sixth, right?
33:10So the, safely, we can use up to 10 to the fifth.
33:20And, uh, here, there's one more thing, which is quite important.
33:24If that amplitude is small, so let's say this point.
33:30So we use it forever.
33:33It's still keeps, uh, under this, like a safe area.
33:38So if I ask you to, um, ask you, if the, how, how long can I use this part with this amplitude?
33:50Forever.
33:51Right?
33:53But that's one, uh, one type.
33:58Let's take a look at the other type.
34:00The other type looks a little bit different.
34:03For some material, there is no fatigue limit.
34:07It said, here, the fatigue limit is, uh, showed this line.
34:13But here, it's going down, down, down, down, down, down, down, down, down, down, down.
34:23So if we have a certain amplitude here, we can use up to this number.
34:30But safely, we may need to replace that part after using it up to 10 to the seventh.
34:40So, uh, here, case for steel.
34:45Case for aluminum.
34:47Um, so, you know, have you tried that?
34:51Like, uh, the aluminum foil, you just, uh, crack it a little bit.
34:57And then, finally, the crack occurs.
35:02But this one may occur that way too.
35:04But, um, if it's not like a bending, if it has such energy stored, that steel may stay forever in that way.
35:21That's, that's, that's this explanation.
35:26I hope you understand this one clearly.
35:29So, how can we improve this fatigue life?
35:33Um, like a sample amplitude, if it goes this way, uh, increasing the, the mean, uh, stress.
35:48Near zero, or compressive, the, but this one.
36:02And then moderate tensile, this one.
36:05And then largest tensile, on this one.
36:09So, uh, if we increase the, uh, the bean value.
36:15That means the
36:17if we increase the this value then
36:22amplitude might be smaller.
36:29That might be one way.
36:33Also, series 0 or
36:36compressive event.
36:37We will see how that
36:38compressive one view helps this way.
36:42It's one of the ways we can do some compressive one, you know, what is that?
36:51Let's say we have a material here, compressive means this one, then if some avoid occurs,
37:01this compressive force may reduce this void, but moderate tensile, tensile is the other way, right?
37:10tensile is this way, that will increase the size of the void.
37:18So if we have compressive force or tensile strength, that will give different direction
37:28for the void. So if we do some sharp pinning, just do some compression, that will also
37:40improve the fatigue life, and also carburizing.
37:49And remove stress concentrator, that's what we have learned in the slide maybe 12 or something.
37:56Remember this, like this side, this one.
38:02Okay, so there's another word, the creep.
38:12You know what this creep means? Creep, what's the creep?
38:20This is weird word, and just look it up.
38:23There's a song, which title is creep. That's nice.
38:31And a song, I guess.
38:34Okay.
38:39Some sample deformation at a constant stress, previous time again.
38:43So in the previous, the many terms like stress, or yield, or something, they do not have any terms like
38:57the time. But we are now having some term with the time.
39:06Now let's see what this time can do.
39:15So obviously, it's time dependent.
39:19The permanent deformation on the constant load, typically at elevated temperature.
39:25So it's at low temperature, we just learned about some, or is that DBTT or something.
39:32But now high temperature, we see some creep because, you know, like tensile strength is applied to the material.
39:42When we applied the high energy or temperature, their kinetic energy will be increasing.
39:50And then they do some lower interaction between them.
39:56And then they will be released.
39:57So often this, the LB temperature means like a,
40:06higher than zero point, the half of the melting temperature in Kelvin.
40:12So that's normally one that will be shown in the next slide here.
40:18We will see that very soon.
40:20So metals like some nickel-based superhaloids or ostent type steels and polymers at moderate temperature can exhibit this creep.
40:37This creep shows the three different stages.
40:42The primary is like this area and secondary and tertiary.
40:49And decreasing creep rate as work hardening domination occurs.
40:57And then this, the steady state occurs.
41:01The roughly constant creep rate.
41:04Designers often use this way.
41:06And then the, finally, rupture occurs like accelerating creep due to damage and necking.
41:17Finally, rupture occurs.
41:18The creep is a very important sign and matter for turbine or boiler, reactor, engine which has some relation to temperature.
41:36And then the creep rate is highly temperature sensitive and stress sensitive too.
41:45See, when the low temperature, it shows only this one.
41:52And then if we increase the temperature, then creep strain is very sensitive to showing this rupture at the lower time.
42:05So increasing temperature is not a good way for the creep material.
42:18I think the creep exists for the most of the material.
42:26But the temperature might be different.
42:29Okay.
42:29So prediction of a creep rupture lifetime is quite important because we want to know when that rupture may occur.
42:38So the data is all given here.
42:42And then the equation is also given here.
42:45You know, this is the empirical equation.
42:48So there is no logic between them.
42:52So the L value is here and then temperature and then the time to failure.
43:03That's what we are going to learn.
43:06So like this value was the, it says like a 140 MPa.
43:14So 140 MPa is given here.
43:17That value is written here and then temperature.
43:22And this is time to failure rupture when the rupture may occur.
43:28So if we do the calculation, that's 233 hour.
43:33So we need this kind of like the figure to understand this whole the way.
43:43Okay, that's the end of chapter 8.
43:54Engineering material is not as strong as predicted by theory due to that some void or flaw.
44:05Flows act as stress concentrators that cause failure at stress lower than theoretical values.
44:23Sharp corners produce a large stress.
44:25So when we have a chance, then we should have this way.
44:35And then also tri-axial position is not a good way.
44:39X, Y, Z position.
44:41And this is X, Y is tri-axial concentrant.
44:47So that's the water related problem.
44:51Failure type depending on temperature and stress, obviously.
44:56There are many different like fatigue time dependence and creep is also time dependence, right?
45:03And there are many different variables we have to consider to understand the failure.
45:14So remember that we just learned about the failure, some rupture or the failure by after the fatigue, okay?
45:30Okay, we will see in the next slide, the next chapter.
45:36Thank you.
45:37Robin?
45:49You
45:51of course
45:56You
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