Fracture Toughness

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Introduction to High-Throughput Materials Development

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Georgia Institute of Technology

Introduction to High-Throughput Materials Development

71 оценки

This course is an introduction to high-throughput experimental methods that accelerate the discovery and development of new materials. It is well recognized that the discovery of new materials is the key to solving many technological problems faced by industry and society. These problems include energy production and utilization, carbon capture, tissue engineering, and sustainable materials production, among many others. This course will introduce the learner to a remarkable new approach to materials discovery and characterization: high-throughput materials development (HTMD). Engineers and scientists working in industry, academic or government will benefit from this course by developing an understanding of how to apply one element of HTMD, high-throughput experimental methods, to real-world materials discovery and characterization problems. Internationally leading faculty experts will provide a historical perspective on HTMD, describe preparation of ‘library’ samples that cover hundreds or thousands of compositions, explain techniques for characterizing the library to determine the structure and various properties including optical, electronic, mechanical, chemical, thermal, and others. Case studies in energy, transportation, and biotechnology are provided to illustrate methodologies for metals, ceramics, polymers and composites. The Georgia Tech Institute for Materials (IMat) developed this course in order to introduce a broad audience to the essential elements of the Materials Genome Initiative. Other courses will be offered by Georgia Tech through Coursera to concentrate on integrating (i) high-throughput experimentation with (ii) modeling and simulation and (iii) materials data sciences and informatics. After completing this course, learners will be able to • Identify key events in the development of High-Throughput Materials Development (HTMD) • Communicate the benefits of HTMDwithin your organization. • Explain what is meant by high throughput methods (both computational and experimental), and their merits for materials discovery/development. • Summarize the principles and methods of high throughput creation/processing of material libraries (samples that contain 100s to 1000s of smaller samples). • State the principles and methods for high-throughput characterization of structure. • State the principles and methods for high throughput property measurements. • Identify when high-throughput screening (HTS) will be valuable to a materials discovery effort. • Select an appropriate HTS method for a property measurement of interest. • Identify companies and organizations working in this field and use this knowledge to select appropriate partners for design and implementation of HTS efforts. • Apply principles of experimental design, library synthesis and screening to solve a materials design challenge. • Conceive complete high-throughput strategies to obtain processing-structure-property (PSP) relationships for materials design and discovery.

Из урока

High-Throughput Property Measurements

This module covers techniques to experimentally conduct property measurements suitable for high-throughput screening; optical, electronic, mechanical, chemical, and thermal properties are considered.

Introduction9:05

Instrumented Indentation Test13:37

Measurements using Indentation Methods9:21

Fracture Toughness9:51

Indentation Testing - Polymers and Coatings8:22

Abrasion, Scratch, and Buckling6:22

Познакомьтесь с преподавателями

Dr. Richard W. Neu
Professor
The George W. Woodruff School of Mechanical Engineering
Dr. J. Carson Meredith
Professor, Associate Chair for Graduate Studies, and J. Carl Pirkle Sr. Faculty Fellow
School of Chemical & Biomolecular Engineering

[MUSIC]

Hi, this is Rick Neu.

In this lesson we're going to look at high-throughput methods

to measure fracture toughness.

The learning outcome is to explain the principles and methods for

measuring fracture toughness.

So first of all let's review what a conventional

fracture toughness test might look like.

This is a compact tensile specimen.

It's not really compact in the sense that we need to be compact.

The wheat varies anywhere from a quart of an inch to an inch to

could be much larger.

In fact, sometimes it has to be really large if

your material has a fairly good amount of ductility.

And basically what you do is you apply a load after you've pre-cracked

the specimen usually in fatigue.

You apply a load.

And you measure the load at which fracture occurs, and

that load is correlated to a measure of fracture toughness.

One measure of fracture toughness is the critical stress and

tensity factor, this Kc.

And so we'll just use that as an example.

And this is a standard test.

There's actually two ASTM standards that deal with this particular test, E399 and

the more comprehensive E1820.

To measure the crack growth and you typically put a clip gauge on here.

Say you get load displacement information from this sort of test.

That isn't going to work for high-throughput.

An so we pointed out the best way to do

a high-throughput test on small samples is through indentation but in this

case instead of using spherical indenters like we did for strength and modulus,

we're going to purposely use a sharp indenter to create a stress concentration.

If you press a sharp indenter in something that's brittle, what happens is cracks

will form in the body where tensile stresses naturally will form.

Gotta understand a little bit about why cracks would form,

when you're indenting a brittle material with a sharp indenter.

So first of all, you have to realize tensile stresses are induced as

the plastic zone size around the indenter increases in a certain regions.

Now during loading the tensile stresses are below the indenter and so

that's shown and you create what's known as median cracks.

During unloading you get spring back and

you get tensile stresses that tend to form lateral to the surface,

and these are called lateral cracks.

And then there is a hoop stress around the indenter

at the tips of those hoop stress is where they are magnified the most and

you get cracks formed there, those are radial cracks.

And often what happens is the median cracks and

the radial cracks link up to form this median radial cracks.

And so, when you indent something there's going to be subsurface cracks,

which you often can't see, unless you're use some sort of an x-ray type source,

but there'll be cracks along the surface that you do see.

And the length of the cracks relates to the fracture toughness of the material.

In fact, if the materials not brittle the cracks won't form.

So in some sense there's a threshold

where your fracture toughness is high enough where cracks don't form at all

There's really two types of indenters that are often used in this.

One is a vickers type indenter.

So this has four corners and the fracture toughness

is based on the length of the radial surface cracks that form.

The crack length, turns out, varies as a function of load.

So as you increase the load, make the indent larger,

your crack length also grows in size.

And in fact, for a range of loads,

generally, you get the same measure of fracture toughness.

Work done by Lawn, Evans and Marshall, back in the early 1980's,

showed that fracture toughness can be correlated to

the properties of the material, the Young's modulus and

the hardness, the load applied and the size of the cracks.

Later an extensive study on many materials was done and basically a similar form,

a little bit more comprehensive form shown at the bottom was developed.

And this is fairly realistic in relating some measure of fracture toughness

to the crack lengths that form with some scaling factor and

I show this scaling factors that are typically used.

So, that's a Vickers Indenter, now if you go to

Berkovich Indenter this Indenters are commonly use in nanoindentation.

You lose symmetry and you only have three corners.

So this modifies that equation, but just slightly.

It actually depends on the number of radial cracks that form.

And in fact, it basically just affects the scaling factor in front.

In fact the ratio changes from four

radial cracks to two radial cracks, that ratio is 1.073.

And so that basically, you come up with a similar relationship,

that relates those properties to the cracks that measure.

Now you have to measure all these different things.

But this can be done.

I mean, and this is just an application, example application.

Let's say you want to measure the fracture toughness of silicon carbide,

a brittle material.

And so you start out with a diamond indenter, we use a particular instrument.

There's a few out there.

Hysitron Triboscope is one of the common instruments you use to do

these measurements.

You apply the load and you perform in-situ imagine.

No cracks are observed if you don't load it high enough.

So there's some threshold where there's no cracks and

once you go above that threshold the cracks appear.

And then once you've continuously increased the load your cracks will grow.

Now what you typically do to get the E and H.

The Young's modulus and the hardness is you use the load displacement information

below that threshold where there is no cracks,

because once there is a crack that changes your modulus hardness measurements.

So you get that information from the same instrument.

And then for loads both the threshold, you measure the crack and

then you plug that into one of these factor toughness relationships.

In this example We just used the simpler relationship and

found that the fractured toughness was around 3MPa square root of meter and

it was found you can measure this at different load levels and

it was relatively independent on load up to 40 millinewtons.

And on the left it shows an image of the indent and

there's cracks coming from the corners that are measured.

So in this lesson, we've explained the principles and methods for

measuring fracture toughness.

And the way to do it is using indentation methods but using sharpened indenters.

They provide fine space resolution, rapid speed and

measurement, and possibility of automation.

But we should point out there's still some limitations.

You can only really obtained quantifiable information

when the materials is relatively brittle.

If your material has fairly good fracture toughness capability already Cracks aren't

going to form in the indent and you're not going to get any additional information.

But the way this could be used is, for example, on the right here

we have one of our samples, material libraries, with a gradient.

And let's say we put an array of indents and

go to this fracture mechanics protocol.

And if you look closely, you'll see that there's a region on here

where cracks form at the indents and there's other regions where they don't.

And so we can quickly assess that well, that region or those processing

conditions, those microstructures are more brittle than other parts on that sample.

And so we can quickly identify what conditions might lead to

lower fracture toughness.

So what's next?

We'll turn our attention to chemical properties.

Thank you.

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