Designing Cantilever Beams for Controlled Fracturing in MEMs Accelerometers

[TylerJames]

Here's the issue. I'm doing some research involving MEMs where we will be making micro-accelerometers using cantilever beams with a proof mass on the end. I want to design these beams to break at specific accelerations (2 G's, 5 G's, 10 G's etc.) and I'm not sure how to go about this. My thought was use these forces to find the moment on the end of the beam and then use Max Stress=Mc/I to find the other properties of the beam. Is this the correct equation for this situation? I'm also not sure what the stress that a beam breaks at is called (ultimate strength, yield stress?). Just some general info on how to work through this process would be great, I haven't had much luck on google.

 

[kjohnson]

When you say break, what do you mean...i hardly doubt that it will be a straight up failure. Also a lot of this depends on the material used and i know mems sensors will most likely not be using a nice linear-elastic material such as steel. There is a lot of different stuff to consider as well such as the amount of deflections you are expecting as most basic engineering formulas are valid for vary small strains only. MEMS devices are a whole different ball game.

 

[collpitt]

Firstly, I think what you are talking about here is the fracturing of the device and not breaking. The parameters which can be used to control this phenomenon would be the, tensile strength of the substrate material, and the device dimensions.
For example does your cantilever beam have a proof mass? Then in order to reduce the fracturing point you should reduce the thickness of the beam and make your proof mass more bulky(which is logical). You can also control this by changing the material, for eg by using steel you get a strength of ~ 20 Mpa whereas Silicon carbide would give you a strength of ~3-5 Mpa.