What exactly makes a bolt "aerospace grade"?

Question

I'm reading various articles on Tesla S electric car and many of them (one example) say that the car body is very strong because of using aerospace grade bolts. What are those I wonder? I found this review on aerospace fasteners that says

The key difference is quality. Aerospace products need to be more durable in order to withstand a lot of high pressure and temperature environments, such as leaving the earth’s atmosphere or exposure to burning rocket fuel. Additionally, aerospace products must be lightweight.

So what are those aerospace grade bolts? Are they just made of stronger (and perhaps lighter) alloys or do they have any additional properties?

Answer 1

Disclaimer: I worked as an aerospace engineer for 15 years for the USAF. Our organization managed the 53 Federal Stock Group ⁽¹⁾ (among others), which includes Bolts, Screws, etc. By this I mean to suggest I have some (dusty) knowledge of this subject.

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While the quality control is very much a part of the process, as suggested, that alone is not the only difference. How the bolt is produced (the manufacturing process used) is also key.

So, for example, it is common to have bolts with "rolled threads".

Quoting from some industry literature ⁽²⁾:

Often, rolled threads are required by design because of their superior tensile, shear, and fatigue strength. Other processes remove material to produce the thread form, but thread rolling displaces the material with hardened steel dies...

The result of moving the material grains (molecules) into the shape of the thread rather than weakening it by removing material, is that the grains become denser at the critical parts of the thread, especially in the root and on the flank below the pitch diameter...

                       

                                                          ^{Figure 1: non-rolled thread}

So, looking at the "not rolled" thread (fig. 1), one can see that threads which are cut (for example on a lathe) have exposed grain boundaries. Such a condition is undesirable for certain applications, for example under high impact loads. These areas would be prone to fracture/failure.

Compare the above illustration to the "rolled thread" illustration:

                       

                                                              ^{Figure 2: rolled thread}

Here (Fig.2), as explained above, one can see that the grain boundaries flow around the contours of the thread, alleviating the exposed grain boundary condition and thereby producing a superior thread.

So, in conclusion, it is both the quality control requirements and the manufacturing process used to produce the bolt which make it aircraft quality.

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Sources:

1. FEDERAL SUPPLY CLASSIFICATION Groups and Classes (PDF)

2. Horst Engineering: Thread Rolling

Answer 2

Parts for use in aircraft have to be certified. This means they need to be produced from standardized materials, using a standardized and traceable process. The part must be checked thoroughly during production, and the entire history of the part must be recorded. The tests and paperwork often account for most of the cost of the part. \$100 bolts are not uncommon.
"Aerospace grade" means that the materials and process used to create the part are suitable for use in aircraft parts. I suspect aerospace suppliers will sell you two versions of the same part: one with the entire papertrail for use in aircraft, and the other without the papertrail for use elsewhere.

Answer 3

While not a matter of materials science, my A&P gave me his explanation of the subject when my plane was in for annual:

Mechanic: These two screws are identical. This one comes from Home Depot and costs 25 cents. This one is comes from an aviation supplier and is certified for use in your plane; it costs \$2.00 per screw. If I use the Home Depot one and it fails in flight, I'm liable. If I use the \$2.00 one, the manufacturer is.

Me: Well, what's different between the two?

Mechanic: About $1.75 of liability insurance.

It's an exaggeration for sure, but the principle is important: If a screw breaks on your patio deck, there's no presumption of liability or fitness. It's a screw; it broke. It happens.

But if the screw fails on a plane and you lose an aileron, that's a different story. The lawyers are coming for somebody's head. So while a huge portion of the cost of materials covers the liability of the manufacturer, the insurance company isn't going to grant a liability policy unless the manufacturer can demonstrate that the bolt won't fail in flight, that literally every reasonable precaution, testing procedure, manufacturing technique, and quality control mechanism known to the industry is being employed to ensure that these bolts are safe. Because if the manufacturer misses even one reasonable precaution, then the insurance company will have to pay out.

Answer 4

I have little knowledge of the aerospace industry other than that gleaned from growing up in Derby (UK, East Midlands) which is the home of Rolls Royce.

In terms of materials used there are a number of specialised alloys used in aero engines and in particular Nimonic which is used for making bolts for aero engines. Nimonic was developed by Henry Wiggin and Company, Birmingham UK.

My brother worked for a company in Derby and operated a CNC lathe cutting the bolts (I assume threads, milling etc). The alloy is very expensive so running up a bill on a bad day is too easy.

Links

Rolls Royce Civil Aerospace

Article from Derby Evening Telegraph

1951 Advert for Nimonic

Hope this is of some interest.

Answer 5

"Aerospace" fasteners of all sorts are rigorously tested an monitored in order to be able to be qualified to meet strict NAS requirements. Batch after batch, year after year, you get a dependable part.

National Aerospace Standards page of Aerospace Industries Association

Also used are large number MS drawings (Military Standards) and various US military specs

Quite a bit of effort and certification goes into assuring very high grade hardware meets its requirements fully and is not counterfeit.

High grade fasteners must meet very strict metallurgical composition standards. Heat treating and other processes must be properly controlled. Fabrication processes must be consistent and closely monitored. Production lots are sampled and tested on a schedule to assure standards are being met. There is a very large collection of strength, class of fit, temperature range, fatigue tolerance requirements that 'hardware store' fasteners do not even know about. Aircraft parts come with material certs, process certs, strength testing certs. (not each individual screw of course, but lots provided to distributors)