[SOUND] Hi, this is module 11 of Mechanics of Materials I. Today's learning outcomes are to continue to define and discuss the material properties associated with stress-strain diagrams, and to define and identify what we're going to call the 0.2% offset yield stress. So here again is our typical stress strain diagram and then I've got the stress strain diagrams for three materials, one of the materials is steel. Again as I've mentioned before, its typically used for machine parts, shafts maybe as in the dry shafts of cars, et cetera. And it's a ductal material so it has a lot of strain before, it allows for a lot of strain before it fractures. The other material is aluminum, one of the other materials is aluminum. It has been typically used in the past for aircraft frames although now we're seeing a lot more composites in aircraft frames. The strength to weight ration in aluminum is better than steel, although steel is stronger. So that strength to weight ratio is a reason they use in aircraft because you can get a lot of strength for less weight, that's what you want to have in an aircraft type application. And you're starting actually to see aluminum used in cars and pickups these days. And then finally the third material I have is stress-strain curve four is magnesium. It's a low density metal about two thirds that of aluminum. It has good corrosion resistance. It has good high temperature properties and it's used in electronic applications and some other specialty applications. So, here's the stress strain diagram for the three typical materials. We've talked a lot about steel where we have a pretty well defined yield point, where we start to get plastic yielding. Some materials, like the aluminium here and the magnesium don't have a well defined yield point. Remember the yield stress is the low stress that starts to produce permanent deformation. So if we can't easily identify that, for those types of materials, we typically use what's called the 0.2% offset yield stress, so here's a graph of a depiction of what I mean by yield stress. As we go up the stress strain curve we go up to a point where if we backed off we would have 0.2% permanent set or permanent offset. And that's then called the offset yield stress, and that's the type of definition we'll use for yield stress, again for materials like aluminum, magnesium, where there's not a well defined yield stress point. So let's again have you do a worksheet on your own. I want you to find the approximate 0.2% Offset Yield Stress for the aluminum curve on this graph, and I've got the solution in the module handouts. [MUSIC]