In this lecture I will talk about Structural Design of Wind Turbine Blades. Structural design is one of the most important areas of engineering. We are surrounded by structures, and it's important that they are safe, that they are reliable, and that they are efficient and do not fail, so people are hurt. The learning objectives of this lecture is that you should be able to formulate the basic principles of structural design. You should be able to explain the difference between loads, boundary conditions and strength. And you should be able to explain what the load carrying capacity of a wind turbine blade is. When we do a structural design it's important to know what other loads that are applied to this structure. We should also understand how is the structure interacting with the surroundings, how it is connected to the surroundings? We called that the boundary conditions. Then all structures have a purpose and this purpose gives the structural requirements to the structure. So by knowing the loads, the boundary condition and the structural requirements, we can design the structures, and we should keep in mind that it must not fail. So here's a few examples of simple structures. A tennis racket. What are the loads? The load comes from the ball hitting the strings in the racket. And the boundary condition comes from the players grip on the tennis racket. But it can fail. And in this case it failed because a frustrated player hit the racket to the ground. And that's important lesson because often structure failure is caused by loads that the structure is not designed for. What about a bicycle? What are the loads? The load comes from the rider on the bicycle. His weight on the saddle and the handle bars and the forces from his feet onto the pedals. And what are the boundary conditions? Well it's where the tires are connected to the road. Then let's look at wind turbine blades. So when wind turbines placed, we have basically two types of loads. We have loads from the wind working on the blades in the flap-wise direction. And we have loads from gravity working on the blades in the edge wise direction. And the boundaries is where the blade is connected to the hub. Now how is typical blade designed? Well it's a compromise between the aerodynamic and the structural requirements. So from an aerodynamic point of view we want thin air foils. But from a structural point of view, we want thick air foils that can carry the loads. So the flap-wise loads coming from the wind is often taken by a load-carrying gurtel inside the blade while the edge-wise loading, coming from gravity, is carried by strengthening the leading edge, and the trailing edge of the blade. And as you can see in the picture. These blades are very flexible. Now in order to design them, we must understand the behavior of the blade at full scale, the whole blade. But then, when we zoom in, we should also be able to understand and design the different components of the blades. And as we zoom in further, we get to the composite materials, the layers, the laminate. And further on, we come to the material, the fibers, individual fibers, the matrix around the fibers, and the interface between the fiber and matrix. And in order to design, we should understand and know the failuar mechanisms at all these length scales. Now here's a case example where we have made a final element analysis of a wind turbine blade. So we have loaded the blade in the flap-wise direction, and we set that load to 1. So this is the flap-wise direction. Then we rotate the load in different angles around the wind turbine blade and we scale the load until the blade fails. So then this red curve, it's a load carrying envelope. And as you can see from this, the blade is strongest in the flap-wise direction. And it's weakest in the edge-wise direction towards the trailing edge. Now in order to see if that is critical, we must compare the loads with the load carrying capacity. So here the black curve is the loads. And luckily enough, the loads are highest in the flap-wise direction, and lowest in the edgewise direction. So it's not critical. But if we now scale the loads until it reaches load carrying envelope, then we can see, what is the most critical part of the blade. And here, we have scaled it until it touched the load envelope curve. And we see in this direction, it's most properly, most critical. So, we have actually tested this blade in that direction, as you can see in the video. So what you see here is a full scale test of a 34 m wind turbine blade. First you will see the loading of the blade in high speed. The blade is loaded in the direction I showed on the previous slide. And the loading comes from wires attached to the blade at 4 points along the blade. Winches at the floor, then are used to load the blade towards the floor. note how buckles occurs along the trailing edge. Now you will see, the failure of the blade in real time. You can hear the cracking of the compositive fields. And now in slow-motion. What happens is that the buckles at the trailing edge become so large, that the adhesive bonded trailing adge fails and you see how the bond is opening up several meters along the blade. You can also see that adhesive glue is falling out of the adhesive bond. Now the blade is offloaded again, shown in high speed. From this test we have learned what critical part of the blade is in this load direction. And we have learned how it fails. And all this fits very well with our final element calculations. Now to sum up, in this lecture you should have learned the basic principles in structural design. You should have learned the difference between loads, boundary conditions, and strength, and you should be able to explain what the load carrying capacity of a wind turbine blade is.