[MUSIC] Hi, and welcome back. In today's module we're going to continue looking at fatigue. Last time we wrapped up fully reversible stresses, this time we're going to look at a case study, and the stresses here are not quite fully reversible, but they're fluctuating in fatigue. So the learning objective for today's module is to understand how critical it is to incorporate all of the factors that are affecting your design in fatigue analysis from the environmental factors, to the geometry, to stress concentration factors. So remember how we talked about fatigue causes catastrophic failure with not a lot of warning and the failure occurs below the ultimate strength. This case study is going to be a prime example of that. Some of the areas that are going to be really interesting in the case study is you'll see a big issue with stress concentration factors and environmental factors in reducing the endurance strength along with a number of high cycles. So keep those in mind as we go through this failure. All right, so this is the failure, this is a fatigue failure. It's Aloha Airlines flight 243. It's a Boeing 737 and you can see that this is a failure that occurred in flight. So this was while the aircraft was flying and the top quarter half of the fuselage has come off. So let's look at some details. So it was an 89 passenger flight in the Hawaiian Islands where they were jumping from one island to another. And it underwent explosive decompression in flight. Where a quarter of the fuselage, the top quarter of the fuselage was ripped off of the aircraft during the flight. The plane managed to land an emergency landing in Maui and there was one fatality and 65 passengers were injured. So if we dig a little deeper. If we start to look at an investigation. This is a Boeing 737. When you get into manufacturing you'll see a number of components will often have different numbers associated with it. So even though it's a Boeing 737-297 the production number is 152 which is going to come to be really important in a little bit. It was built in 1969. This aircraft production number 152 had 90,000 flight cycles which means it flown 90,000 flights and it was the second highest in the industry. So the second highest aircraft to see all these flight cycles. Essentially, it got to a really high number of flight cycles before a lot of its counterparts did because it was hopping from island to island. So would be going from like Maui to Kauai several times a day. When an aircraft takes off, it goes from a relatively high pressure state to a relatively low pressure area. So from sea level and then it goes up to high atmosphere with lower pressure. So you get these cyclic hoop stresses that occur in the aircraft during flight so you go up to low pressure and then back down to high pressure, and that's one hoop stress cycle. So you're constantly getting these fatigue cycles in aircrafts during flight cycling. Investigation indicated there was no other issue and there was no pilot error that resulted in this failure. So if we dig a little bit deeper to see why this occurred, we can take a look at the aircraft. To the right you can see the aircraft that's failed, that's the 737. And then on the left you can see a side view of a 737, this is the part of the aircraft that came off during flight, it's roughly 18 feet of the top of the fuselage came off of the aircraft. If we look at the FAA flight failure reports. So the report into what caused this. The aircraft accident report. You can see that it was part of section 43, the top part of section 43 in red here is what came off, right here. And so all of these sections, 41 through 46 are under relatively high pressure during these hoop stresses. So it's not entirely surprising this was the area that failed. In order to understand the failure a little deeper, you need to understand the anatomy of the fuselage. So the fuselage is that big cylinder in the plane that everybody is sitting in. And essentially aircrafts had these aircraft skins that are covering the fuselage. And they're connected through rivets and bonded lap joints. So what happens is the skins come in. And you'll have, so here you can see the aircraft's skins going through here. There are these big metal sheets that cover the aircraft. And so you'll have a metal sheet come up and another one will overlap. And what they'll do is will stick some rivets through which will mechanically fasten one sheet to the other. And on the Boeing 737 they also had what they called cold-bonded lap joints which is essentially they glue epoxy, they put an epoxy sheet between the two aircraft skins and they glue them together. So they have two fastening methods. The rivet and the epoxy. Now, on production planes 1 through 291, we're looking at plane around 150. So we're in this region, this is the plane that failed. They had cold-bonded lap joints, which just means that they stuck an epoxy sheet in between these two aircraft skins, and that epoxy sheet cured at room temperature. So there was a couple of tricks with this, you had to make sure both of the surfaces of the aircraft skins were very clean. They did that by etching the aircraft's skin a bit down. And then the epoxy sheet had to be completely dry. In addition, this particular aircraft operating in the Hawaiian Islands, is exposed to a lot of salt water, so there's a corrosion issue. Now past production plane 291, Boeing changed their process and they used hot-bonded lap joints. So an epoxy bond that cured at temperature and they also had a thicker geometry. So let me show you what I mean by this. Here's a figure directly out of the FAA accident report. This on the left, this is the design that the plane that failed had, on the right, this is the redesign. So you can see the epoxy would come in here. Here are the rivets, and if you look at the redesign the epoxy is coming in still here it's a slightly easier more repeatable process and then they have more material across to take the load. Now if you look here they're calling this a knife edge, and what they're talking about is that this particular skin cuts down to a corner right at the rivet. So if we looked at the hole where the rivet went through, we would see this really kind of sharp angle in the aircraft skin. And that's a great place for a stress concentration to start so essentially at this joint right here, you can see, right here, this is a rivet coming through the two skins and joining them together. And one of these aircraft skins is coming down to a pretty sharp corner, and that's a stress concentration area. And that's exactly where the cracks started. So they started at these rivet holes at the knife edge around the rivet hole and they start to propagate out. So let's take a step back and look. The idea was that these lap joints would carry the load. And that's not what happened. So let's take a look at the sequence of events that occurred to lead to this failure. So typically in a failure this catastrophic, Boeing and the FAA, they're going to release tiger teams and they're going to go to the failure site and they're going to try to figure out what exactly happened and they're going to start very broad. And when you have a failure this catastrophic, it's almost always that multiple things went wrong to cause this failure. So this might be their initial high level diagram. It's very common to draw a fish bone diagram when you're trying to do a failure analysis and they try to think of all the causes that could have been related to this failure. So they might look at the weather, if there's pilot error, anything wrong with the design, anything wrong with the analysis, the manufacturing process, was the inspection and maintenance done correctly, anything weird in the flight pattern. And when they did this, they immediately were able to rule out there was no pilot error, there's no weather anomaly that caused this failure. So if we start to look at what caused the failure, let's start up on Boeing's side. In the manufacturing the cold bonded lap joints, it turned out that process was difficult to do with high repeatability. So some of the lap joints would fail and that would mean that all of the load would be carried by the rivets. This lap joint process was tied with the fact that over in the flight pattern you're flying over the ocean in this really corrosive environment. So salt and air and salt water could get into these lap joints and cause them to fail more rapidly. Over on the design and analysis side the lap joints were designed to take the load. When they failed, the load transferred to the rivets and it turned out the holes next to the rivets had these really sharp edges where there was a stress concentration factor. And this is where the cracks started to form and propagate. Boeing didn't do a full plane fatigue testing on a 737, it did do some smaller level fatigue testing on a 737. Now here's the deal, so Boeing figures out that this design can possibly have a failure, and they figured it out well before This failure occurred. So they released an alert, an official FAA alert and it said, hey, if you have one of these aircraft, you need to inspect the rivets and the lap joints to make sure the lap joints haven't been disconnected and that there's no holes forming at the rivets, right? So Boeing does absolutely the right thing here, they changed their design. They released an alert and they alert all of the airlines that have this aircraft to start inspecting for this failure mode. And so now we jump over here and we see the reason why this aircraft is the first one across everybody to fail is because it had a high number of cycles because it was constantly going up to altitude and coming back down and going up and coming back down. because it's flying several flights from relatively small islands everyday. So its life, its N number was a lot higher than the rest of the aircrafts released out on the field. And so the main problem comes in here in inspection and maintenance. So despite the alert for whatever reason, Aloha Airlines' staff wasn't fully trained and they were not able to catch the fact that several of their aircrafts did have these delaminated lap joints and the cracks starting to form at the rivet hole. So if you go through the FAA report, it will walk you through the entire failure analysis. But all of these factors combined for this to occur. So if we look at the sequence of events. So there's these cold bonded lap joints. And they're going to get thrown. They have a relatively thin geometry, it's difficult to do this process repeatably so there could be issues in the bond due to the surface. They get thrown into this highly corrosive salt water environment and they start to cycle a lot faster than any of the other aircraft released in the field. And so what happens is the cracks start to form between the sheets in the lap joint. So if this is the aircraft's skin, remember we had like a rivet going through here and then we had these epoxy joints. And they're going to delaminate, which basically means that they're going to detach. And now all of the load is coming through these rivets. So load transfers to the rivets and now cracks start to form, this is actually what the rivet looks like. Cracks start to form right here, at the edge of the rivet, because that hole had a high KF, it has a high stress concentration factor, so the cracks form, and there's a lot of these rivets throughout the sheets. So you would see something like this and what would happen is throughout the entire aircraft the cracks would be forming along the rivets, growing and then connecting along the fuselage. So it starts out relatively small at the rivet hole and then they connect along the fuselage. Eventually you get rapid crack propagation and the remaining material can no longer take the hoop stress and then you get this catastrophic failure which looks like this. So this is a really great example of a fatigue failure. It shows you the importance of the stress concentration factor in geometry. It shows you the importance of how the endurance limit can get reduced in certain saltwater corrosive environments, and it shows you the impact of a high number of cycles that would cause this aircraft to fail first. So a lot of really interesting phenomenon came together and it shows the fatigue mechanisms quite well. So next time, so far in this class we've looked at relatively simple loading fully reversible fluctuate or fully reversible loads and fatigue. And next time, we're going to get into more complex loading which are fluctuating loads and fatigue. I'll see you next module. [SOUND]
