Fatigue Failure
Monday's seaplane crash in Miami is beginning to look like a case of fatigue failure. There have been several reports of a wing separating from the plane before it crashed into the sea. Fatigue is an insidious thing, everything seems to be going along find until one day, apparently without cause, BOOM!, something breaks.
It may have happened to you. You aren't doing anything different from usual, suddenly your key snaps, or the beater on your mixer falls off, or (one I actually saw once) your tire is suddenly driving along ahead of your car.
Why should this matter to you? If you pass over or under bridges, walk into large buildings, drive or ride in cars, busses, or airplanes; a fatigue related failure might kill you. One spectacular failure related to fatigue was the collapse of the Mianus River Bridge on Interstate 95 in Connecticut back in 1983. Yes, the chances are small, but an understanding of why it happens can be nice to understand.
Did you ever wonder why most holes in structural members are round (or have rounded corners)? There are two reasons for this, one is that drills are round, the other is that a sharp corner focuses stresses. Because of this focusing of stresses, tiny cracks will often start to form. Once they've formed, they tend to grow. Very slowly at first, then a bit quicker, and eventually, BOOM!, you get that catastrophic failure. (Of course, very often that BOOM! is something small, so we hear "click!")
The cause of the initial crack can be a sharp corner (engineering or manufacturing failure), or it can be corrosion (maintenance failure), or it can be something like a bolt that wasn't tightened enough (assembly failure). Regardless, the result can range from inconvenience to death.
Have you ever looked at one of those things that broke after many years? What you probably saw was a very smooth break much of the way across followed by a section that looked like it was torn apart. That's a classic fatigue failure.
In order to understand these failures, we can consider a bridge beam. These are usually large I-beams (though they may have many other shapes). If we imagine one of them installed on a bridge, it may have to support some percentage of the weight of a tractor-trailer (or even several of them!) So, let's assume that every time a tractor trailer drives across the bridge, this beam is loaded with 50,000 lbs (25 tons) and it flexes a little bit. (I often look out airplane windows and watch the wings of the plane flex - the tips move up and down.) Some metals have a fatigue limit, below which they will not fail due to fatigue (perfect situation), other metals are subject to fatigue regardless of the load applied. (When I searched to confirm terms, I found this link that gives a more in depth look at the topic.) Anyway, over time, a tiny crack is formed, then it begins to grow. During this phase, though the crack may not be visible, it can often be detected. This is the important part of this post.
There are processes that can detect cracks in material too small to see. Without going through the processes, suffice to say, it can be critically important to do at least occasional testing to determine if crack develpment is occurring. This type of testing is also often done during production of large parts to ensure their long and safe function. One of the big companies involved in making these products is Magnaflux. There's little value to testing most small items, but when lives are at stake, testing must be performed regularly.
In the case of airplanes, the Federal Aviation Administration (FAA) generally requires inspections related to the age of the plane. (Note: planes ages are usually considered in flying hours, rather than years.) One of the difficulties facing those trying to evaluate fatigue problems is that your testing must not destroy the item you are testing. (It would make no sense to "break" an airplane in order to say, "Yep, that one used to be good enough to fly.") So, all testing must be non-destructive testing.
The plane involved in the Miami crash dates from the 1940's and likely had a great many flight hours. If well maintained, a high flight hour plane can be safe, but I think we can reasonably expect to see an advisory from the FAA mandating additional inspection and, perhaps, replacement of some parts on this type of aircraft.
It may have happened to you. You aren't doing anything different from usual, suddenly your key snaps, or the beater on your mixer falls off, or (one I actually saw once) your tire is suddenly driving along ahead of your car.
Why should this matter to you? If you pass over or under bridges, walk into large buildings, drive or ride in cars, busses, or airplanes; a fatigue related failure might kill you. One spectacular failure related to fatigue was the collapse of the Mianus River Bridge on Interstate 95 in Connecticut back in 1983. Yes, the chances are small, but an understanding of why it happens can be nice to understand.
Did you ever wonder why most holes in structural members are round (or have rounded corners)? There are two reasons for this, one is that drills are round, the other is that a sharp corner focuses stresses. Because of this focusing of stresses, tiny cracks will often start to form. Once they've formed, they tend to grow. Very slowly at first, then a bit quicker, and eventually, BOOM!, you get that catastrophic failure. (Of course, very often that BOOM! is something small, so we hear "click!")
The cause of the initial crack can be a sharp corner (engineering or manufacturing failure), or it can be corrosion (maintenance failure), or it can be something like a bolt that wasn't tightened enough (assembly failure). Regardless, the result can range from inconvenience to death.
Have you ever looked at one of those things that broke after many years? What you probably saw was a very smooth break much of the way across followed by a section that looked like it was torn apart. That's a classic fatigue failure.
In order to understand these failures, we can consider a bridge beam. These are usually large I-beams (though they may have many other shapes). If we imagine one of them installed on a bridge, it may have to support some percentage of the weight of a tractor-trailer (or even several of them!) So, let's assume that every time a tractor trailer drives across the bridge, this beam is loaded with 50,000 lbs (25 tons) and it flexes a little bit. (I often look out airplane windows and watch the wings of the plane flex - the tips move up and down.) Some metals have a fatigue limit, below which they will not fail due to fatigue (perfect situation), other metals are subject to fatigue regardless of the load applied. (When I searched to confirm terms, I found this link that gives a more in depth look at the topic.) Anyway, over time, a tiny crack is formed, then it begins to grow. During this phase, though the crack may not be visible, it can often be detected. This is the important part of this post.
There are processes that can detect cracks in material too small to see. Without going through the processes, suffice to say, it can be critically important to do at least occasional testing to determine if crack develpment is occurring. This type of testing is also often done during production of large parts to ensure their long and safe function. One of the big companies involved in making these products is Magnaflux. There's little value to testing most small items, but when lives are at stake, testing must be performed regularly.
In the case of airplanes, the Federal Aviation Administration (FAA) generally requires inspections related to the age of the plane. (Note: planes ages are usually considered in flying hours, rather than years.) One of the difficulties facing those trying to evaluate fatigue problems is that your testing must not destroy the item you are testing. (It would make no sense to "break" an airplane in order to say, "Yep, that one used to be good enough to fly.") So, all testing must be non-destructive testing.
The plane involved in the Miami crash dates from the 1940's and likely had a great many flight hours. If well maintained, a high flight hour plane can be safe, but I think we can reasonably expect to see an advisory from the FAA mandating additional inspection and, perhaps, replacement of some parts on this type of aircraft.
posted by Scooter at 1:05 PM
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