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Why fracture and failure mechanics is so important? From Southwest Boeing 737 cracks to future Boeing 787 safety

L. Roy Xu's picture

Even Jay Leno tried to understand fatigue cracks at his Yesterday’s Tonight Show, our research on fracture and failure mechanics is received great attention from the general public. Read more after this photo--

  Southwest jet

 

Recent news about crack propagation inside metal Boeing 737 fuselage gave us a clear warning: well-established elastic-plastics fracture mechanics still could not prevent unexpected fatigue crack initiation, and then a dynamic fracture process (read this article “Boeing didn't expect 737 cracks so soon (link)”)  These cracks inside metals are often called mode-I opening cracks.  From research viewpoint, how to detect the potential fatigue crack location is critical. Modern structural health monitoring technique is still not sensitive enough to do that, even sensors are embedded inside new Boeing 787 Dreamliner. We need fracture mechanics knowledge to answer general public’s questions: why a dynamic crack stopped (arrested) and left a 5-foot hole, and  previous lab or full scale fatigue experiments are reasonable because unexpected fatigue cracks started from the middle of the whole fatigue life (15 v. 30 years)?

A great advantage of the new Boeing 787 aircraft is that its half structural materials are composite materials. As a result, composite materials significantly reduce the total aircraft weight and increase the fuel efficiency.  A potential disadvantage is that B787 structures will have more complicated failure modes. In addition to fatigue cracks in its half metal structures, complicated composite failure mode will occur. Especially the mode-II in-plane shear crack will become a major failure mode, mainly along the interfacial joints of composites and metals. Failure of composite materials is usually progressive so a sudden big hole inside the fuselage would be rare, and the pilots will have enough warning time to land safely. Therefore, fracture mechanics becomes more important since B 737 aircraft represented the past 30 years, while B787 aircraft will dominate most Airlines in the next 30 years.  Why should we study failure mechanics, especially for composite failure? Simply because we should “protect ourselves, family, friends and more….”   For more technical details, I include some web links of top scholars' fracture mechanics research, and discussion inside iMechanca:

Theoretical fracture mechanics: http://esag.harvard.edu/rice/   http://www.seas.harvard.edu/suo/

Experimental fracture mechanics, http://rosakis.caltech.edu/

iMechanca posts: 1)  Mixed-Mode Fracture. Curved Crack Path:  node/8036 ,
2) cohesive zone modeling of interface fracture: node/7396 

 3) Fatigue node/7705

Please help--My work is too focused on breaking all kinds of materials and understanding crack propagation phenomena, I don’t know how to use Facebook or Twitter yet. If you have these accounts, send my article to your family and friends—Dr. L. Roy Xu on April 6, 2011.

Comments

I disagree with the statement that the fatigue crack was unexpected. The crack originated at the expected location and propagated the way it was predicted. The crack arrest mechanisms (tear straps) worked as designed. What is under scrutiny now is that the number of GAG cycles it took for a crack to generate and propagate this way was shorter than expected. I don't think this has to do with the analysis or predictions being innacurate. It has to do with low-cost airlines flying a spectra that was not expectted and hence the maintenance scheme would have to be updated.

 On the other hand, the 787 is not half metal, it is 50% composite in weight, but in volume that percentage is a lot higher. While I agree that composites have other types of failure that are more critical than fatigue, I disagree that they are a potential issue, as these failure modes are known and the airplane is designed for it's structural elements to be able to carry 150% of the highest load it would see in service. If one of those failure modes presented itself during normal operation it would be of great concern given that it would point to a fundamental design error, rather than to something more subjective as the detection of a crack or the planning of a maintenance scheme.

L. Roy Xu's picture

My “unexpected fatigue crack” expression refers to its early initiation (only half design life), NOT its location, please read these articles again. As a previous aerospace engineer, I feel surprised that you still use the safety factor conception to design a modern airplane. The current approach is called “damage tolerance design”, which is based on fracture and failure mechanics.

Roy: Thanks for the blog. I just finished the chapter of fatigue in my fracture mechanics class. I will post this link to my class. At least, get the students to think about it a little.

My two cents:

Totally agreed: When designing with composite materials, it has to be "damage tolerance design". The reason is simple: their mechanical properties are too sensitive to too many things, expected and unexpected. They have little tolerance to flaws, compared to metals. In this case, risk accessment is important. However, how to apply it to a civilian aircraft seems to be a great challenge. Remind that it is hundreds of people riding that craft called B 787, vs. a few on Discovery who were aware of and accepted the high risk. For the same reason, as Rui pointed out in another thread, the maintainance schedule could become too frequent, necessarily, and take toll on the operation cost...

Elastoplastic fracture mechanics works fine, for monotonlic loading. For cyclic loading, I am not so sure.

Do we understand the damage mechanisms in composite materials, especially, under fatigue loading? If someone answers "yes", I am anxious to hear about it.

Respectfully,

Bo

L. Roy Xu's picture

Bo,

Very important points. US Air Force’s F-111 swing-wing aircraft was a typical design failure due to fatigue crack propagation using the safety factor design in 1960s. From 1970s, US Navy’s same swing-wing aircraft F-14 has a long safety record due to its damage tolerance design (see photo). I know the damage tolerance design is required for all US military aircraft. However, we’re waiting for B 787 engineers’ explanation: whether it’s required for this new aircraft to be used in next 30 years (directly related to your and my safety).

external image f14_3.jpg

 Damage tolerance design limits the critical crack length (for metals) or damage size (for composites) during the whole life of an aircraft. So we must examine any damage of the aircraft quite often. As a simple example for the safety factor design, if the average material strength is 150MPa and the applied maximum stress 100MPa, the safety factor is 1.5. However, there is one sample with a low measured strength 100MPa (I did a lot of experiments and it’s normal), then material will fail and lead to the accident of Southwest B737-300 aircraft. I notice that the rare model-III out-of-plane shear crack was involved in the last stage of the crack propagation in that aircraft. Roy

 Thanks for clarifying your statement about the unexpected fatigue crack. From your sentence "From research viewpoint, how to detect the potential fatigue crack location is critical" it sounded like the location was what you were talking about.

 As a current aerospace engineer that works on the aircraft in mention I can't comment on how damage tolerance is covered, but it is.

 However, I can say the fatigue problem in composites is understood enough to know that it is not what is critical for the design, unlike a metal airplane.

I think it is premature, even irresponsible, to say that getting on a 787 is a high risk. Where's the basis for that? So far the flight test program has only shown that the plane behaves as designed, how does that translate into a high risk airplane?

 

With metals, we can predict with confidence, certainly, to a certain degree. Since we can't predict 100 % sure, we apply a safety factor--for scheduling maintainnance as well as in the design stage. From my limited experience with composites, my guts feeling tells me to wait and see. However, if you are the designer of B 787, you can't afford doing that. You have to think ahead, and do your best to assess the risk. If you can't tell the risk is low or high, it is high risk--we take a conservative approach, don't we? I guess this is probably the reason for that F22 is so costly and why Boeing is in its current awkward position.

Cheers.

Bo

 

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