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  • Writer's pictureNed Patton

It has to be a System

I want to talk about design of composites and why it takes a system engineering or system design mindset and process to come up with a good composites design. And by system design what I mean is that you have to take into account and understand the thing you want to make completely, and you have to define it completely including how you’re going to make it, right at the outset of the design process. Last week to illustrate this, I used the example of making a bow for archery. You have to know everything about the bow that you want to make before you can even select the string (fiber) and glue (resin) to use to make the main pieces of your bow, or to select a fabrication process – meaning how you are going to make it. While this sounds difficult, it really is not all that hard to do, you just have to have the right approach – what I call a holistic approach – to designing and making things out of composites.

The reason behind all of this is that when you make the material you also make the part. So, everything about your part has to be well defined before you start making anything. You can certainly buy pre-made things like fiberglass structural parts – I-beams, channels, etc. – that will work in your design, but that isn’t really a composites design – it’s just taking a metal design and applying it to composites. To make a true minimum weight composite structure – which is the most common motivation for using composites to start with – you have to design the entire structure from the ground up. This includes how you connect it to the rest of whatever you’re making as well as how you’re going to make it. It also includes what it has to weigh, what it has to do, how stiff or flexible it needs to be, what sort of environment it is going to live in, how it has to look (aesthetics?), and any number of other things that are important to you and to the thing that you plan to make. This is called “System Engineering” or your “System Architecture” because it creates a set of requirements that you can design your composite part or assembly from. This is the most important thing to get right at the outset of your composites design.

As we can see from the figure posted at the top of this week’s blog, it starts with the geometry. To an engineer, this is basically what the thing looks like, how big it is, and how it connects to whatever it is connected to. And this geometry at the outset of your design doesn’t have to be that well defined – you just need a sense of how big it is and what the general shape needs to be. This is the beginning of the creative part of designing something using composites, and this is where engineering and art coincide. The big difference between a composites design and a metal design or a design using conventional materials is that with composites, the shape of the part can be anything that the designer/architect/artist can dream up, as long as that shape fits the intended function of what you’re trying to make. In an earlier blog post I talked about the evolution of composites design in the road bike world. The all composite frame of the new Look bike has a very different shape than does the initial one that Greg La Monde rode to victory in the Tour de France.

While on the surface this may seem to be hard to accomplish or to get right, in reality it is really fairly simple. You just need to understand the basic concepts, have an idea how the strings and glues go together, and what combination of string, glue, and fabrication process produces what properties, and the entire world of composites opens up to you. What I’m talking about here of course is that composites design/fabrication is an iterative process where everything needs to be taken into account in each iteration of the design.

And that’s what I mean by a “systems engineering” approach or mindset that is needed to create a good design using composites. The systems engineering side of the design process gives you what this thing has to accomplish, what it has to look like, what it has to weigh, how strong it is, how flexible it is and in what areas it needs to be flexible or stiff, and how it interacts with the rest of whatever it interacts with.

I’m going to go back to the bow and arrow example here to illustrate this. A bow needs to have a region right around where you grab it – the handle – that is very stiff. Then the two arms of the bow have to have the same stiffness and flexibility in the same areas on each side of the handle. And it is common to have a varying stiffness as you go out from the handle – as in stiffer close in and more flexible farther out, or vice versa. Let’s say you want your bow to get more flexible farther away from the handle than it is closer to the handle. This is your first design variable – you can vary this stiffness in any way that makes sense for how you shoot an arrow. The bow could bend more toward the handle and less farther away from the handle to give you a longer draw. Or it could get more flexible nearer the tips of the bow to give you more immediate power and a shorter draw. Or you could do something fancy and vary the stiffness in a way that gave you both longer draw and more power. And you probably want the bow to be as light as you can make it so that it is easy to carry around all day. In total, all of this information becomes the systems design or systems architecture for your bow. The rest of the bow has a lot more details, like where the string attaches on each end, how you wrap the handle, how it fits your hand, and where you place the arrow on the bow when you shoot it, etc. In total, this is the systems architecture and systems design of your bow.

This is just one example. I’m sure you can come up with others. And I’m sure that once you do enough digging into the properties of each of the strings and glues as well as what they do in combination to make a composite, you will gain a good understanding of what you need to do to make anything you want to make. Just think of what you want to make, how it interacts with its surroundings, review the process flow above, and figure out the basics. Then iterate on your design until you like what you have. It’s really that simple. And it really is not that hard to do, you just need to broaden your view and see the entire picture of what you want to make. Then make the magic happen.

In other posts I talk about the string in composites – in particular carbon fiber. Where did it come from, when did the first carbon fibers become available, and why is it so strong and stiff. It all starts with the Periodic Table of the Elements, and the properties of the element carbon. Look for it under "Strings"

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