Module 10: Temperature Effects and Creep

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En provenance du cours de Georgia Institute of Technology

Machine Design Part I

745 notes

Georgia Institute of Technology

Machine Design Part I

745 notes

“Machine Design Part I” is the first course in an in-depth three course series of “Machine Design.” The “Machine Design” Coursera series covers fundamental mechanical design topics, such as static and fatigue failure theories, the analysis of shafts, fasteners, and gears, and the design of mechanical systems such as gearboxes. Throughout this series of courses we will examine a number of exciting design case studies, including the material selection of a total hip implant, the design and testing of the wing on the 777 aircraft, and the impact of dynamic loads on the design of an bolted pressure vessel. In this first course, you will learn robust analysis techniques to predict and validate design performance and life. We will start by reviewing critical material properties in design, such as stress, strength, and the coefficient of thermal expansion. We then transition into static failure theories such as von Mises theory, which can be utilized to prevent failure in static loading applications such as the beams in bridges. Finally, we will learn fatigue failure criteria for designs with dynamic loads, such as the input shaft in the transmission of a car.

À partir de la leçon

Material Properties in Design

In this week, we will first provide an overview on the course's content, targeted audiences, the instructor's professional background, and tips to succeed in this course. Then we will cover critical material properties in design, such as strength, modulus of elasticity, and the coefficient of thermal expansion. A case study examining material selection in a Zimmer orthopedic hip implant will demonstrate the real life design applications of these material properties. At the end of the week you will have the opportunity to check your own knowledge of these fundamental material properties by taking Quiz 1 "Material Properties in Design."

Module 10: Temperature Effects and Creep9:22

Module 11: CTE mismatch12:52

Rencontrer les enseignants

Dr. Kathryn Wingate
Academic Professional
Woodruff School of Mechanical Engineering

[SOUND] Hi and welcome back.

In today's lecture,

we're going to cover the impact of temperature on various materials.

This is going to conclude Unit 1, Material Properties in Design.

So the learning outcomes for today's module is one,

to understand the impact of elevated temperatures on steels and aluminums.

And two, to understand when in design you should be concerned about creep.

Before we do that, let's take a step back and think of all of the places in

the product life cycle that we need to think about temperature in design.

And to do that,

I'm going to use the example of a cube satellite which is also called a cubeSat.

It's a smaller satellite, about the size of a shoebox and

it can be sent up on orbit.

It's very common for various university themes to design these and

they do rather smaller satellite tests in space.

So if we look at this cube satellite,

we might start out with a block of 6061 aluminum that we're going to

turn into the main structure, which we call the bus of the satellite.

So here we have 6061 aluminum block, and

it's already gone through some material processing.

It might of been tempered, it might go through other types of heat treatment and

so as a designer, you need to be aware of that type of temperature.

The next step is you're going to go through some fabrications,

so maybe you take a block of 6061, and you see and you see it down.

The aluminum will see some temperatures during that process.

Other components are going to come in and get attached to this structure and

they'll need to go through various types of thermal testing.

Such as thermal cycling, maybe thermal shock, thermal vac,

where they pull a vacuum, and then run a bunch of different cycling tests on it.

Thermal cycling tests, where you can go anywhere from

negative 20C all the way up to 250 or 300C.

And so there is all sorts of testing that will be done.

The next step is you're going to through some type of shipping.

So you're product at this point, the satellite is completely assembled.

And now it's going to get shipped, and

you could end up shipping down in Arizona or where it could be quite hot,

like 110, 120 degrees Farenheit in the back of a truck.

Or you could be going through Saint Paul, Minneapolis where it gets quite cold.

So negative 15 degrees Farenheit and then into a storage facility.

And you'll go through some types of cleaning processes.

And then you're going to go through pretty much one of

the roughest environments for a satellite which is launch,

these satellites are launched in the tip of a rocket.

And it can be quite the environment from both a mechanical and

a thermal environment.

And then you'll go through deployment where if you

have any structures on the satalite that need to open out that will occur.

So the solar panels might come out or

where antenna might be deployed at that point.

And then you're going to hit your operational orbit and

you'll hit various temperatures on orbit in your operating condition.

So for example,

one of the cubes that Reconzo hits negative 10C up to 50C on its orbit.

Other satellites can hit much more extreme temperatures depending on

the orbit they're in.

So this is the lifecycle for a satellite.

Now with most products you have to worry about repair.

Typically, satellites can be repaired, but for a car or for your laptop, or

for a cellphone you'd have to think of if there's going to be any types of repair

processing that's could also cause higher low temperatures.

So when you look at the design you have to look very far out of its operating

conditions, you should never just be looking at the temperatures and

operating conditions.

You need to look at the entire life cycle of the product when you're considering

temperature.

So let's look at some modes that impact material properties due to

elevated temperature.

So today,

we're going to take a look at the impact of elevated temperature on steel.

And what happen is that causes a decrease and

strength with an increase in ductility.

So here you can see a chart that shows the temperature

that the steel is experiencing on the x axis along with

the percent of room temperature strength it maintains on the y axis.

This chart is from MIL HNBD 5J and this is a low-alloy steel.

And you can see right around room temperature the steel is at 100% of its

room temperature strength.

And then as you get up closer to 1,000 degrees Fahrenheit you have

an extreme decrease in temperature.

So you're only at around 40% of its room temperature strength.

So this is something that you really have to consider.

Especially when you're looking at heat exchanger design or engine design.

So what's happening here the mechanism is that your getting these

metallurgical changes in the structure and

you're also getting a degradation of the heat treatment.

So you can see the same phenomenon also happens at aluminum here.

We have another chart this is 6061 aluminum and again from MIL HNBD 5J.

And again you can see that there's temperature

on the x axis in percentage strength on the y axis.

We see this decrease in temperature were if we get

up to 500 degrees Fahrenheit you're at almost 50% of the room strength.

So aluminum does not handle temperature as well as steel does.

There's another type of failure mode due to temperature and that's called creep.

So creep is a very interesting a mode of failure, but

it's also kind of terrifying, because what happens is,

is your yield, the material will yield under the yield strength.

And so creep occurs, typically in ferrous metals at

elevated temperatures for long periods of time.

And what happens is, you get this time dependent deformation under load.

Where your material is actually going to continue to deform over time and

will carry less and less load, or will have less and less strength over time.

So if we look at this chart on the right to your screen, you can see this is for

a generic metal alloy at a temperature above room temperature.

So we could say, maybe it's at 500 degrees Fahrenheit,

so you can see if you were to load it instantaneously.

So quite rapidly within a few minutes, you get a certain strength.

If you were to apply the same load over 100 days you would

find that the strength of the material has decreased significantly.

And if you were to apply that same load over 10 years,

you would see your strength is even lower.

So this is a very dangerous mechanism and it's important to know

if the metal that you're dealing with has a history of creep.

Especially when you're looking at a design that again, has elevated temperature for

long periods of time with a load applied under these long periods of time.

So it has to be under load for a long period of time.

So those are the two mechanisms that are very important to keep track of at

high temperature.

There's a couple of other temperature impacts.

There's a temperature impact called CTE mismatch,

and that stands for coefficient of thermal xepansion mismatch.

That occurs when you have two different materials that are bonded

together or acting closely together and we'll talk about that in the next lecture.

The other impact that you have to think about is cold temperature.

There's a mechanism called a ductile to brittle transition temperature.

And we'll talk about that in a later lecture in

the static failure unit of our course.

So that's it for today, and I'll see you guys in the next module.

[SOUND]

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