[MUSIC] Hi, this is Rick Neu. In this lesson we're going to look at ways to make Mechanical Property Measurements using Indentation Methods. The Learning Outcome is to explain the indentation methods for measuring elastic modulus and strength in particular. So first look at Elastic Modulus. Now few things about Elastic Modulus we need to know. One is relatively insensitive to microstructure, so what that really means is it's possible to even use 1st principle calculations to determine some of the elastic constants. In other words we can possibly use high throughput computational approaches to learn a little bit about about the elastic modulus too. So for example the VASP code could be used to determine this. Using, it's a code to do Quantum Mechanical Molecular Dynamics analysis. But there are exceptions and there reasons why we need to be able to measure elastic modules. For one we need to validate the results that we get from these computation of codes. But the other is the fact that modules can be sensitive to microstructure in some cases. If your microstructure's full of porosity, it's not going to follow what a quantum mechanic code gives you. If there's multiple phases that form, they could interact in a way that's not predictable by first principles. And anisotropy due to crystallographic texture and other features from deformation or thermal processing can play a role. So these things make it difficult to predict the elastic modulus totally from first principles, so what do we do? Well most viable high-throughput method for measuring elastic modulus are based on indentation test, measuring the force-displacement response. Because we can get fine spatial resolution, the rapid measurement as we pointed out and you can automate the mapping of the surface. So how might we do this? Well we can use instrumented nanoindentation. That's what we covered in a previous lesson. That's pretty much a static measurement. But because if you're just interested in determining elastic modulus, you're not trying to apply a load to permanently deform the material you're probing. You can use some methods that use lighter loads. So it allows you to use atomic force microscopy methods. And there's a couple ways you can do that. There's this Dynamic Force Modulation method and there's an Atomic Force Acoustic/Ultrasonic Microscopy method. Just to explain the difference between these method you have to look at is basics schematic of Atomic Force Microscopy basically consist of an indenter? Tip, it's on a cantilever, end of the cantilever there is some piece of electric oscillator that moves that cantilever up and down. And there is someway to detect displacement at the tip. And so that the cantilever could be in touch with the body below or may be its not in touch but usually when we are doing these sorts of experiments. We want the tip to be in touch with the body because we're looking at the compliance of the tip with the body. And so now we can look at, we can oscillate it into that body and look at the compliance. R, it turns out if the flexural resonance of that cantilever is affected by the compliance of the body or the stiffness of the body which of course relates to the modulus. So you can measure the flexural resonance of a cantilever and correlate that to the modulus. And so there's a couple of ways you can do this and clearly, this can be made high-throughput, so now let's turn to strength. Now what do we mean by indentation strength? And so here is some indentation stress strength curves that were generated on a material library that had a thermal gradient. And this particular material where it was cold and a higher strength where it was hotter the strength went down. And so there's three curve shown here. The blue, the red, and the one near the bottom. I think it's black and if you did a macroscopic tensile test, a conventional ASTM ISO type test. And compared the strength you get from that test to the indentation yield strength measured using the methodology we explained in the previous lesson. They aren't the same so here's a comparison that shows those three different microstructures, the as-received. Some intermediate temperature and one that was exposed to temperature 413 degrees C. And what you see is the indentation yield strength is higher but twice as high as the yield strength you measure in a macroscopic test. And this is due to the fact that it's a fairly complex data stress under the indentation test. In fact, there's a scaling relationship that you can find if you understand the mechanics of the material fairly well. And you can see it's roughly a factor of two. But the most important thing to note is the fact that they do scale, materials that tend to be have higher strength and lower strength in a tensile test will also scale the same way in the indentation test. So you get the highest indentation yield strength in the condition where it would have the highest yield strength. And so we can use this still high through put measure to tell us something about the relative strengths of the material. Now the other aspect that you have to deal with when you're dealing with metals in particular, is the microstructure and so the sampling size. The thing with spherical indentation is you could actually adjust the size of your indentor. So this is a little schematic that illustrates this and illustrates a microstructure. So, at the bottom there's a metal, a polycrystal metal. It could be some other material that has some distribution of grains or other sorts of features, something that has a scale to it. And so, on the left we have a microindentor, it's bigger indentor, and on the right a nanoindentor. Nanos hunt a thousand times smaller than a micro. And so just to illustrate that that would be our large identer. The large identor might be sampling many many many grains here. But as you make the identor size smaller you do this by making the radius smaller, you're sampling less grains. You're smaller yet, almost sampling just one grain. And you get down to the nanoscopic scale, you may be just sampling essentially a single crystal. Because your indentations all encompassed within that single grain. And clearly you're going to have different responses, depending on this scale. Or if you do nano indentation, you have to relate that crystal response back to the macroscopic response. And we have relationships and we know how these things do scale, so we can make these connections. So in this lesson, we've explained the indentation methods for measuring elastic modulus and strength. In particular, we focused on a few key things. For elastic modules, we explored different methods that can be used based on indentation. Was either nano indentation or atomic force microscopy. With strength we look at two issues that we have to deal with, the issue of scale and the fact that you can use different instruments that can indent at different scales. And we learned a little bit about the meaning of this indentation strength and how it compares to a traditional measure of yield strength. So what's next? We'll take a look at one other mechanical property and that is Fracture Toughness, thank you. [MUSIC]