Now, we'll discuss in more detail the elastic modulus and the two strength parameters, the yield strength and the tensile strength. So let's look at these one by one. First of all, the modulus of elasticity, again, we've already said, is in that area leading up to the yield strength, is that slope, very straightforward. Might emphasize that, that is given by the Hooke's law, which we'll define in a moment. Again, we've defined the yield strength as that 0.2% offset, so again, a reference point. And effectively, the formal definition, even though that break from the perfect tangency there is the onset of plastic deformation, then sort of the practical engineering definition of the boundary between elastic, and plastic deformation is that intersection point that defines the yield strength. So 2 is the Y.S. yield strength. So now the tensile strength. That requires a little additional commentary. We've already talked about the elastic modulus and yield strength, what's the situation with this maximum in the overall stress strain curve? The overall stress strain curve clearly is implying that somehow [COUGH] the material's becoming weaker beyond this maximum point here. But that's not actually the case. We have to think back again to our overall sample, which we simplified the grip ins on it and just for the sake of some quick sketches. But the point is that as we pull on this material, that is, can't discuss here, but as you see in some of the discussion and the associated textbook, there's this issue called the Poisson's ratio, which so simply acknowledges that there's some very slight, almost imperceptible to the unaided eye, some narrowing of the cross sectional area of the material, the cross sectional diameter, slightly less. As you pull on the material you're making it longer, you're also making it slightly narrower. But beyond that, as you continue to plastically deform the material, permanently deform it beyond this maximum point, what actually begins to happen is, let me bring in our little eraser again here. I am going to erase the middle section. What has really occurred is that we end up that a little bit of a narrowing of the cross sectional diameter. What appears like, kind of a little coke bottle look to it. I call this necking down, this is a this is called a necking. Looking down of the cross sectional area says stress concentration reaches a certain point. And some of you in civil and mechanical engineering may go in to this in more detail and courses on continued mechanics in your more advanced studies, but for our purposes here we'll just acknowledge that as the stress concentration reaches a certain point, then the material begins to draw down in the center.