If you have children of a certain age, then right now your life is all about Katniss Everdeen and The Hunger Games. If you don’t know the story, essentially it’s about a sixteen year old girl trying to survive a kill-or-be-killed entertainment show in a post-apocalyptic world. Now Katniss’s weapon of choice is a recurve bow, which got me thinking–after my efforts to design a pumpkin chunkin catapult, could I use SolidWorks Simulation to analyze a bow? My first decision was to choose what kind of bow. Now, being English, there was really only one choice–a wooden English longbow. A quick trawl of the Web resulted in many plans, and in pretty short order I had an unstrung bow built in SolidWorks.
The strength of the bow comes from both its shape and its materials. The belly of the bow (the part facing the archer) has to resist compression, while the back of the bow (facing the target) has to resist tension. The yew longbow stave achieves this by using the heartwood for the belly and the sapwood for the back, forming a natural laminate. To create this laminate inside SolidWorks Simulation, you need to split the bow stave into two volumes and apply two different custom materials to it, but the material definition itself also requires a bit of subtlety. Wood is an orthotropic material; its strength with the grain is much greater than its strength across the grain, so when defining the two custom materials, reference geometry has to be selected to set the grain direction .
With a stiff spring in the place of the bow chord and the model quartered to speed up the solution, I was ready to draw the bow by setting a maximum draw. I must say that I was very happy with the results; both the maximum principle stress (good to see regions of tension and compression) and the displacements give reasonable results.
But is my bow usable? Could I draw this bow? By plotting the string/spring draw reaction over time, we can see that the maximum draw load is about 110N, for ¼ of a bow. Giving a total bow draw of 440N, or 99 lbf, this would indeed be a powerful bow, but one that I would not be able to draw to its full capacity.
I guess its back to the drawing board, but now I can use the design scenarios and optimization tools within SolidWorks to design a longbow for any given draw pull. Something that the trial and error approach of a craftsman would take years to obtain.
Now I know you are asking yourselves what longbow design has to do with the modern world. Just substitute the longbow for any flexible component of your own design–do you know how it behaves under load, or are you just guessing? SolidWorks Simulation takes the guess work out of design. It's something that, with a bit of training and engineering common sense, every designer can use.
Want to learn more about SolidWorks Simulation? Check out our First Look at Simulation video to see how simulation could improve your own designs and reduce prototypes.