[MUSIC] Hi and welcome back. Today, we're going to start unit 1, Material Properties in Design. This unit is really critical for mechanical engineers in design. These are a lot of material properties that you need to take into account when designing various components. So today, we're going to begin module 3, where we examine the strength of materials. The learning outcomes for this module are to understand the quantitative and qualitative meanings of strength. To understand how stress and strength are different, and understanding units of strength and then the relative values of common engineering materials. So let's start off with coming out with our notation for strength. In this course, we'll be using capital S to denote strength, and subscripts to indicate what type of strength that is. So for example, for a yield strength, we would see capital S subscript y. And for ultimate strength, we will see capital S subscript ut. What is strength? Strength is the point at which something of interest occurs. So for example, at yield strength we start to see a really large increase and strain over small increases and stress and past that point you get permanent plastic deformation. Before the yield strength you get pure elastic deformation. Strength is an inherent material property of the object or the component. It's based off of materials atomic and molecular structure and the processing and treatment that material has seen before it became the component it is. And so I think in general a great way to think about strength is capacity. Strength is the capacity of material to take stress. So when students first get out off the initial mechanics of materials class. We often see that are confused at what the differences between strength and stress. So I'm going to asked you at home to pause and write down some of the key differences you can think of between stress and strength. So what is the some critical differences of strength and stress. First of strength is going to be denoted by capital S and stress in this course will using nomenclature sigma. Strength is a property it's due to the structure, the processing and the treatment of the material. So you can see the structure of a certain metal here where stress is a state that the material is in. It can be due to mechanical or thermal loading, but it's a state in the material that comes from a load being applied to it. So when we look at strength and stress, for strength, we get to the value of a material strength from testing and from test data either from the National Institute of Standards in testing or from various military standards. Where for stress we actually can calculate the stress on an object through analysis and possibly through FEA modeling. Overall you think of strength for stress. Strength is the capacity of the object and stress is the state the object is in. And kind of an analogy that you can use here is if we look at a glass of water, strength would be how much water can the glass hold? It's the capacity of the glass and the stress would be how much water is in the glass? So the units of strength on in the English units, it's in pounds of force per inches squared or PSI typically an engineering especially in metals most of the values of strings were in the ksi regions. So typically an English, you'll hear people referred to things to 50 ksi and ksi means kilopounds per square inch. In metric units, it's Newton per meter squared which is a Pascal and for most engineering materials metric strengths would be in megapascals and then some really strong models will be up in the gigapascal range. So down at the bottom of the slide here you can see the various strengths of different commonly used engineering materials. So first we have steel this is, a weaker steel. It's 1020, a hot rolled steel and you can see the yield Strength is about 30 ksi. A much stronger grade steel, 4140 quenched and tempered steel is around 208 ksi. So the range of strengths of steel is quite large. Aluminum, really common aluminum, 7075 T6 is right around 78 ksi and titanium is a little bit stronger not quite as strong as the stronger steels at 120 ksi. Those are all metals, when we go ahead and we look at a plastic like nylon which you see it's very commonly used in various toys and a lot of mechanical gear boxes you use in like kitchen equipment. It's yield strength is much lower, down in the 6.5 ksi region. So key take aways for today's lesson. Strength, the notation in this class is a capital S. It's an inherent material property of the object based off of the atomic and molecular structure of the material. And the processing and treatment the material has gone through as it becomes a specific component in your designing, like a gear or a bearing. Strength can be thought of as a capacity where stress is something that's applied to the object either through a thermal load or a mechanical load or residual stresses due to processing. One of the things I'm going to ask is that prior to watching the next module, which is module 4, that you go ahead and you complete worksheet one based off of your prior knowledge. So in taking this class you should of already completed a mechanics of materials class or a deformable bodies class. And this is just a review to make sure that you're understanding the material well enough to move on through the course. All right, we'll see you next time. [SOUND]