The FAA is pretty important, they are the regulatory body that says we have to make planes that don’t break. Thanks to them, we have jobs.
I typically work with parts made of metal. Metal can break in essentially two ways.
- Static Failure: A big force is applied all at once, and it’s too big for the metal to handle. So the part get’s bent out of shape, or even breaks apart. Think of bending a paper clip.
- Fatigue Failure: A smaller force is applied over and over again. Applied only once, the force is not big enough to break the part. Applied 20, 30, or 40 thousand times, the part will get tired (fatigued) and might break on the 40,001st time. Think of an old trampoline finally breaking after thousands of jumps.
Obviously parts are designed so that they don’t break in static failure. A jet engine would be useless if it broke the first time you turn it on. What the FAA is interested in is how long these parts will last, because as seen above, fatigue failures are often sudden and catastrophic. So, how many flights can the plane make before you need to repair or replace parts? That is the problem that we structures engineers need to solve.
Major Rotating Parts
These are what I currently work on. The people who design planes want them to go somewhere. To make the plane go, the people who design jet engines want to take big titanium rings and spin them really fast. We make sure they won’t break when you do that over and over and over again.
Compressor/Turbine Blade Vibration
There are blades all around that big titanium ring, so now it looks like a big titanium pinwheel. The blades want to vibrate, we make sure they don’t break if/when they do.
Sometimes parts get dented and scratched. Not a huge deal on a phone case or a car door. On a big titanium pinwheel though? Spinning in the sky? With people strapped to it? Important. We make sure damaged parts can be repaired and put back on the plane.
How do we do this?
Lots of math, computers to do said math, and testing to validate our math. When you put a force on a metal part, it gets stressed. Unlike in humans, in metal we can calculate and quantify that stress into a tangible number. Then we can compare it to the strength of that metal to see if/when it will break. I’ve heard this analogy: If stress is an amount of water, then a material’s strength is the amount of water the glass can hold. Too much water and the glass overflows, i.e. part failure.
Calculating stress by hand is easy for simple shapes. It’s been tried, tested, and documented thoroughly. At the simplest level, if you know the force acting on a part, and you know its cross-sectional area, you can calculate the stress. Piece of cake.
σ = F/A
Where σ is your stress, F is your known force, and A is your known cross-sectional area.
If your part looks like a big titanium pinwheel, and has a bunch of different forces acting in a bunch of different directions, it get’s trickier. There are steady forces like a hoop load from spinning. Think Gravitron ride, the metal ring wants to pull itself apart. There can also be fluctuating forces that cause vibration. These forces are all happening at once, so the math gets really intense really fast.
That’s where Finite Element Analysis (FEA) comes in. Using this software, we structures engineers can break these complex parts down into thousands of little cubes like the one in the picture above.
We then ask a computer to do a calculation like the one above for each of those cubes, all at once. If time weren’t an object, we could technically do this by hand. It would take years. Time is money though, and we want to get these planes in the sky. So that’s part 1, we use FEA software to calculate the amount of water that’s gonna go into our glass.
That doesn’t do us any good if we don’t know how big the glass is, though. The second part of the puzzle. The idea of material testing in the aerospace industry is quite simple for steady forces. Take a strip of metal, stretch it until it breaks, and write down how hard you had to pull to get it to break. It’s similar for vibration forces too. Take a strip of metal, shake it until it breaks, and write down how hard you had to shake it to get it to break. Once you have that number, and the amount of water (stress) from part 1, you can use math to estimate how many flights the part can withstand before breaking.
Sounds simple when put like that, but there are many many factors that come into play when comparing material test conditions to the actual part conditions. Some big adjustments that need to be made are things like:
- Temperature – the inside of a jet engine is hot, much hotter than a comfy test lab.
- Surface finish – are there imperfections in your part, or is it polished to a mirror finish?
- Nicks, dents, and scratches – this is where we enter the realm of repair analysis. If a part is scratched, the material can be significantly weakened. Testing needs to be done for this.
Those are just a few, there are loads more. We use all of them to figure out how much water our glass can hold. Even after all that testing and precise FEA analysis though, there is still some error that needs to be put to bed.
The final puzzle piece, part 3. This is where we find out if we forgot about any extra water that might get poured into the glass. If it does, we need to figure out where the heck it came from.
The parts we designed and analyzed are made, instrumented, then put into a test engine. We attach strain gauges to measure the actual amount of stress the part is subjected to. We run the engine and compare the measured stress to the stress we calculated in part 1. If they match, hooray! The multi-million dollar test proves that we were right, and we can start selling jet engines. If not, company execs are pissed and its back to the drawing board for us measly engineers. We need to figure out how to either make the glass bigger, or reduce the amount of water we pour in.
So, that’s what I do.
It’s fun, frustrating at times, rewarding at others, and to me it’s just plain cool. Hopefully after reading this you can appreciate the amount of effort put into making flying safe for the masses.