You may not want to fly on a 787 after watching this.

[Netpackrat]

, like inadequate wing-root stress risers,

I'm not an engineer, but I generally know enough to have an intelligent conversation with them, and the above doesn't make much sense.

[blackeagle603]

e.g. nick on prop blade = stress riser.

[Rich Jordan]

Another stress riser example: Early M1 Garand operating rods had a right angle where the handle attached to the tube section. And they had a tendency to break there. Early oprods were modified, later ones were made with curved areas to reduce the problem.

Some pics of the variants here (google view of ar15.com)

[blackeagle603]

all over my desk.... some seriously fouled up drilling in this pwb plated thru hole. The drill tearing creates stress risers in the thru hole plating. The copper will work harden an cracks will propogate off those points during thermal cycling (as the differing CTE between laminate and copper work off each other.

Epic nailheading at the post interconnect too. That's also a result of aggressive drilling (either result of idiot drill engineer, or more likely by accountants taking over the factory decision making). The holewall gouging and nailheading both were result of drill costs being cut too much (high feeds/speeds, high chip loads, too many hits/bit, too many resharps per bit, cheap entry material).

[Vonz90]

http://www.amesweb.info/StressConcentra ... DRAOVeQPKE

The traditional way of calculating it (for manual calculations) was to apply a coefficients of stress concentration based on the geometry of the discontinuity. This is not really applicable to the high data rate FEA's and other modern methods, but they do not necessarily capture all of the surface variation either, although some of them take that into account as well.

There is another hybrid method were you subtract material around anything that could increase stress based on the geometry of the discontinuity or surface finish, corrosion potential, etc. and then do the FEA on what is left of your part. You don't see that applied too often though other than very high reliability requirements as that method drives to a very robust (read heavy) part.

Of course, most of the rest of world designs in the stress realm below the fatigue limits anyway (i.e. stresses that will never produce a fatigue failure). Aeronautical types use materials and work in weight limits where that is not possible. In short, every airplane will get to fatigue failure, it is just a question of whether it was designed right so it does not get to that point during it's design lifetime.

[Netpackrat]

Yup... Hence why "inadequate stress risers" would only be an issue if your goal was sabotage. We take great pains to ensure that we do not impart any stress risers, and remove those that are picked up in service (like the mentioned nicks in props, fan blades, etc). I blend a pretty good fan blade, if I do say so myself. Most mechanics don't seem to have the knack for it. My employer has a $20,000 paperweight shaped like a fan blade sitting on the shelf in the inspections department as a training example. It's in nearly perfect condition except for a tiny, difficult to even measure nick that I found in the unblendable area at the root of the blade. People look at it, and shake their head, but they sent it out for overhaul, and the overhaul shop agreed with me. Scrap metal.

[Weetabix]

It's been a while, but when you say "strain" do you mean "stress?"

[Greg]

It's been a while, but when you say "strain" do you mean "stress?"

What I've seen looks OK to me so far.

You do have to deal with both, they're inseparable. Of course I ran away from engineering school.

[Weetabix]

It's caviling, I admit. Stress has to do with the force/area while strain deals with elongation.

[Windy Wilson]

until something fails in flight, the plane is safe.

Until something fails in flight, the airplane didn't crash, either.
The people who ran Santa Monica Airport used to say "no one on the ground has been killed by an airplane from Santa Monica Airport."
And it was true, until that airplane crashed into that apartment building.