Engineers hate doing original work. If you do original work you have to work shit out from first principles, you have to experiment and build scale models and calculate forces and bending moments and see if they exceed your material strengths. This is lame and boring and takes up valuable time you could be using to drink whiskey or oppress the native Indian population or whatever engineers used to do back in the 1800's.
Fortunately, engineers somehow developed exactly one ounce of professional solidarity and poured it into the writing of handbooks. Handbooks are lovely: if your boss hands you a requirement document, you can haul out your copy of the CRC Handbook "Selection of Engineering Materials and Adhesives” and look up the appropriate material for the forces and torques involved in your design, which unfortunately still requires you to avoid day drinking and do some maths. Here, let’s look at the CRC handbook’s documentation on some materials. I’m gonna avoid the equation stuff for now and stick to flowcharts and tables.
Let’s say we’re looking for a metal that we want to use for something on a boat. Maybe this is some steam piping or something. Boats insist on getting wet, often in seawater. That’s a huge pain in the ass but you work for a naval contractor so you knew what you were getting into, you’d best get started.
(disclaimer: if you’re a real engineer you need to be a bit more careful than this I am skipping the boring steps)
At first you think “fuck I’m gonna have to look at reactivity series.” but fortunately to your rescue comes this handbook that’s hopefully on a bookshelf at your office. You open to the section on metals:
Alright cool let’s say we need a medium strength material and we can’t post-process it because we don’t have passivation equipment in the factory and they’re not buying it just for this. Looks like stainless steel then. The machine shop guys aren’t gonna be happy about this but they haven’t updated your phone number on the corporate directory so you can probably get this one in under the wire. Cool, what do I know about Stainless Steel. That’s right, nothing, I’m an electrical engineer, we don’t even have a material science class to fall asleep in, I’ll be right back.
Ugh, that’s like 30 pages. Fine, I’ll read it. At least I don’t have to get out the strength testing hydraulic stretch thing. Last time I used that a chunk of steel almost got me in the neck.
Looks like we want Austenitic steel. Duplex sounds expensive and this part isn’t that important, it’s probably fine. I don’t know what Austenitic steel is, what does the book say about that.
Man I hate it when I pick a chart and it just sends me to a bigger chart. The more specific this gets the more expensive it’s probably gonna be so let’s just pick something that sounds good. We’re gonna weld this so we don’t have to worry about machining, let’s try 316L stainless, and if the maths doesn’t work out then I guess we just cry about it. 316L comes in pipe according to the book, so we’re fine there. If it’s bad for pipe then it’s the manufacturer’s fault for making it pipe shaped.
Alright hopefully at this point you’ve already designed your item and calculated what the forces and minimum tolerances could be. If you haven’t done that, I guess go look if what you want to make is on McMaster Carr and just copy it. Now take those minimums you calculated and see if it exceeds these:
(aw damn I did end up with a couple equations in here I lied earlier sorry)
If something doesn’t meet your requirements, go back up the list and pick a new material, you messed up. If it’s fine, I mean, probably check the data sheet for actual 316L steel, you’re an adult with a job and you’re signing off on this, do it right. But if that’s all good, congratulations, you managed to choose the appropriate material for a job and you didn’t have to look up what “grain structure” or “galvanic series” means once.
Now it’s just a matter of making sure you replace all your “TODO FIND MATERIAL” notes in the drawing with 316L Stainless Steel, hand off to the fabrication department and hope to god they don't come to your office you over frivolous matters like “unheard of screw thread dimensions” or “impossible weld geometries.”
Handbooks are a way to offload a lot of the mathematics, design and testing you’d otherwise have had to do yourself to a Boeing machine shop with a team of women in a room somewhere performing dozens of strain calculations for eight hours a day in the 1940′s.
