Welcome back once again to the teaching laboratories here in Kemper Hall on the UC Davis campus. In this segment, we're gonna talk about fracture a concept that indicates how important it is to be aware of small defects in the material, defects on a microscopic and even millimeter scale that can lead to catastrophic fracture. [MUSIC] In introducing fracture toughness to our discussion of important things that we should know about material science, we've really added a fifth very important parameter on top of those big four that we got from the stress strain curve. In this case, Fracture toughness gives us a way to quantitatively determine when a flaw size is critical. And as I said at the opening, these are flaws that can be fairly substantial in size. A few millimeters deep crack on the surface of a material. But we probably can think of no more iconic example of defective material than the Liberty Bell. Which for the lifetime of the United States has represented liberty. But also has a very famous and large crack that is visible on the surface. And in fact that crack is one that was a concern when the Liberty Bell was moved in 1976 on the occasion of the bicentennial of the American Republic. And in order to make the Liberty Bell more accessible to a large number of potential tourists in Philadelphia, it had to be moved to a new location. And in moving, there was substantial concern that that, crack could be a critical one, and by the concept of fracture toughness it could have completely destroyed this great iconic American image. So as a result sophisticated acoustical equipment was used to monitor any potential growth of that crack, and fortunately no such crack growth did occur. So, from something as famous as the Liberty Bell to something as routine as everyday structural components in engineering designs, we have to be very sensitive to the possibility of the motion of such cracks, how they might begin, and how they might grow until they become what we call a critical flaw size. So now, let's look at exactly what these terms mean and exactly what defines a critical flaw that can lead to catastrophic failure.