Hi and welcome back. In this module, we're going to cover the ductile to brittle transition temperature, and this is part of Unit 2: Static Failure. The learning objectives for today's module are to understand why it is critical in design to know if a material is going to behave in a ductile or brittle manner and to also understand the importance of the ductile to brittle transition temperature. So to get started, there's something materials have called notch impact energy. And what notch-impact energy is, is a measurement of the energy required to fracture a notched sample of a material. So, what happens is the testing engineers will take small samples of the material with a notch in the middle and they'll impact the material with a weight on a pendulum, and it will come down and it will hit the material. And utilizing that test, they can measure the amount of notch-impact energy that material has. That test is called a Charpy V-notch test. And if a material has low impact energy, it's behaving in a brittle manner, and if it has high impact energy, it's behaving in a ductile manner. There's a couple of other ways to determine brittle versus ductile. Specifically, you can look at the fracture surface and see what type of fracture has occurred, but notch energy testing is a very common mechanism. If you're looking at notch energy test data, it's important that you're comparing apples to apples. So your notch energy test setup has to be the same across all different types of material samples in order to compare the materials to each other. So let's think about why this is important. Why does a designer care if a material is ductile or brittle in design? Take a few minutes and think about that. So, a designer cares if a material is ductile or brittle because brittle failures are catastrophic, and they happen with little to no warning. So, below we have a US World War II tanker and you can see that the hull is almost cracked directly in half, right? And so this was a catastrophic failure and it occurred with no warning, and it's a brittle failure, where ductile failures you can typically inspect for. You'll start to see some sort of yield or deformation. But, a brittle failure, it's a rapid fracture. And so, designers have to be very aware if the material that they're going to use in their design is going to behave in a brittle or a ductile manner. And so that's why we have this impact energy test is to determine if your impact energy is over on the high side, you're probably going to behave in the ductile manner. Where if your energy, impact energy, is on the low side you're going to behave in a brittle manner. Now, let's take this impact energy on our y-axis, so our impact energy is increasing on our y-axis, and add in a temperature component. And so if our x-axis is the temperature, and it's going from very cold to very hot, and this is the behavior of a material. So I run an impact energy test at a low temperature for this material and then at a slightly higher one and then at a slightly higher temperature and I keep the same material and keep increasing the temperature and run an impact energy test, for some materials, what you'll see is something called a ductile to brittle transition temperature, where at a certain temperature, the material starts to behave in a more ductile manner and you'll see the impact energy increase. Or, you can think of it vice versa, as you lower the temperature of the material, it will behave in more and more of a brittle manner. So there's this range, a temperature range, where if cold is bad and you get a brittle failure there's a range in temperatures where your material is really behaving in between a ductile and a brittle manner. And typically you want to operate above the ductile to brittle transition temperatures so your material is behaving in a ductile manner. So here you can see some test data from NIST, which is the US National Institute of Standards and Testing, and what they've done is they've run impact energy tests. So you can see the impact energy increasing on the y-axis, at various temperatures you can see increasing at the x-axis for a couple of different steels. So there is two 4340 steels. Low energy steel has a different heat treatment than the high energy steel. And then T200 is another type of steel. And you can see that the ductile to brittle transition temperature is very different depending on your type of steel, the heat treatment it's been through, the carbon content, the alloying that it's gone through. And so it's important to understand what the temperature is for the specific material that you are using. So this is kind of a fun case study, the classic example of brittle fracture failure is these World War II tankers. And so this tanker split in half. It had been actually in the water for almost a year. It had just finished its sea testing and it was sitting in the harbor and all of a sudden this crack just propagated through almost through its entire hull. You could hear the fracture occurring over a mile away. And initially, what they did is they thought that there were bad weld joints in this tanker but this started to happen across not only this tanker type but a number of the Liberty Ship tankers being built on the Atlantic side. This tanker was built on the Pacific side. And what they figured out later is that the steel that they were using in these tankers had a ductile to brittle transition temperature that was above the temperature of the water. So, when they placed the ship in the water, it cooled to below its ductile to brittle transition temperature. And then if there was any fracture propagation sites or any starts of fractures it would propagate very quickly. The other thing they determined is that these ships were the first ships that were all welded. Before, they had been using rivets where you would have a plate and you'd rivet it to another plate, so if you had one plate crack, it wouldn't continue on into the next plate. Because the plates were directly welded together, the cracks continued through the whole ship, and so you'd get this quite dramatic failure mode. So what they ended up doing is developing new steels with better impact energy properties so they can handle the cold temperatures. And this whole case study is to remind you to keep in mind where your operating temperatures are and what material you're using and if it will behave in a brittle or a ductile manner based on your operating temperature and its impact energy. So, that's it for today's module. The next module we're going to talk about stress concentration factors, which will tie in really well to ductile and brittle materials. I'll see you next time