iMechanica - manufacturing - Comments

I am suggesting to treat this problem with FFEMS or ESIS etc

Mike Ciavarella — Sun, 12 Feb 2012 03:48:33 -0500

dear Friends

being in the board of FFEMS, I am suggesting the Editor, Prof. Eann Patterson, to perhaps approach this problem. This was the letter I sent them yesterday.

Mike

Dear FFEMS board members,

I suggest we should take the initiative to look, perhaps in a joint effort as BOARD,
to the A380 worldwide panic about cracks. I have in the mean time
written something of a even too personal perspective on the imechanica
Harvard blog. cracks in A380? Every plane now in the air is an experiment by itself...

You may have heard of Qantas Airways and Korean Air will carry out inspections for wing cracks on their Airbus A380 superjumbo planes earlier than previously scheduled, after European air safety officials ordered global checks, warning of a safety risk if the defects were not fixed.

I would suggest we all give it a look and perhaps join
forces to raise the debate, both as it is instructive to the public, to
the general audience of students in the Harvard blog, and perhaps also
to the journal, as we can introduce ourselves as members of the board. However, I think a common line of action every time we communicate (as we registered also "FFEMS BOARD" in the Imechanica blog is a little too difficult to do. But it is up to Eann to decide even this possibility.

I could try to approach Jaap Scijve yes, as I am doing already with
this email, in case he reads us. I could also contact Schwalbe as
editor of EngFractMechanics, in case the board of FFEMS does not like or
pursue the idea. In fact, so far we are only two to explicitely like
the idea, and we need a lot more to go forward. Perhaps Eann should
take the lead here, suggest how this would work ---

1) a special issue of FFEMS idea? In that case, we need guest editor,
and list of potential authors to invite, including relevant people from
industry (I could try to approach some AIRBUS)

2) a FFEMS "task force" just to write a paper on the subject --- my idea
mainly, as in the http://imechanica.org/node/11898 blog, that every
major innovation brings new problems of fatigue
which in truth we can only expect to very limited extent, as we are
mainly extrapolating. I am particularly worried, for example, of size
effect, when the matter is A380. All the data they use for design,
based on NASA tables of the '60s, are certainly based on materials,
size, and type of vehicles completely different. However, the latest I
heard from them, they wanted to find something LESS conservative for
their A300 who return after 20 years of service with no cracks. I am
sure they will STOP this ideas to find something LESS conservative for
A380 now, and actually maybe they want to know what is going on. Maybe
just manufacturing, or overload with respect to mission loading?

3) I am not familiar with all the procedures. Safe life is not an
approach today. Damage tolerance could be, but I doubt about HCF we
understand enough. Fail safe is a good idea, but if something like a
Japanese disaster
http://en.wikipedia.org/wiki/2011_T%C5%8Dhoku_earthquake_and_tsunami
occurs, say here we had a turbulence which is outside all possible
spectra? Should we say we induced cracks that we did not predict, but
those are within our analysis, IF ONLY WE USED a better spectrum? And
how to reevaluate all this?

Sorry for long email. Obviously I cannot write more, you should
consider this Email as part of my proposal to the Board, which perhaps I
will even add openly on imechanica.org as it contains possibly
interesting elements of technical discussion.

But we need more people to agree to work on it!

Regards
Mike

wallster, both and none, ubt the point is another!

Mike Ciavarella — Sat, 11 Feb 2012 02:18:26 -0500

Dear Wallstet

you are referring here to elementary concepts, which drive us in the design. Obviously, rounding corners is usually beneficial, as it relieves the "elastic" stress concentration. Whether it does so with plastic limit, is a lot less obvious, and whether it does so under fatigue, also less obvious. Just imagine one of the classical counterexample of notches being detrimental not being true. In Stephen Fuch's paper, he shows how compressive part of the cycle can induce in the stress concentration certain residual stresses wich are later beneficial.

However, the point is.

1) how to transfer this elementary knowledge to components different in scale, in details of the assembly, under spectrum loading which includes also extreme events like turbolence (of the type of the accident we are talking about). This procedure can be used at design stage, but nobody even dreams of letting a new plane in the market only with this!

2) we also rely, at design stages, on tables of data obtained by, say, NASA, in the 1960's, and we constantly refer to those. But maybe they are too old now? Or maybe we are starting to extrapolate when we are changing the size of the plane? My friends at Airbus lately were concerned that design of A300's (the good one, not the one under attack, A380), return after 10 or 20 years of service, and have LESS cracks than expected, so their life can be prolonged. The worry with A380 is that cracks are emerging too soon!

3) We have always, and only learned by mistakes. Limiting attention to aircraft only (but remember the example I made on railways fatigue,on which by the way we undestand very little even after 200 years...), see wikipedia

Structural failure of the aircraft

Examples of failure of aircraft structures caused by metal fatigue include the De Havilland Comet accidents (1950s) and Aloha Airlines Flight 243 (1988). Now that the subject is better understood, rigorous inspection and nondestructive testing procedures are in place.

Composite materials consist of layers of fibers embedded in a resin matrix. In some cases, especially when subjected to cyclic stress, the layers of the material separate from each other (delaminate) and lose strength. As the failure develops inside the material, nothing is shown on the surface; instrument methods (often ultrasound-based) have to be used to detect such a material failure. In the 1940s several Yakovlev Yak-9s experienced delamination of plywood in their construction.

4) You know the story of Comet? They were using static design with factor of safety 2, and they thought it was more than enough. Later, after 7 accidents, they decided appropriate criteria for fatigue should be taken into account, for the pressurization of the cabin was enough of a cycle of stress, in case of stress concentrations at the windows. Notice that we had had the experience of railyways, but people do not always transfer technology, so the progress is not linear and "monotonous" function...

5) Companies such as Airbus and Boing, not not rely only on design. They want to rely on experience on testing full scale. Tests are done and under various harsch conditions. Yet, no test can cover the entire spectrum of what can happen in life.

So I repeat my question. Can we be near the discovery of something equivalent to when we found out why Comet were falling down one after the other for no apparent good reason?

Michele Ciavarella, Politecnico di BARI - Italy, Rector's delegate.
http://poliba.academia.edu/micheleciavarella

L-shaped members

wallstedt — Fri, 10 Feb 2012 16:05:06 -0500

This is somewhat related to the crack model question: when I was an undergrad I was taught that the inside concave corners of L-shaped members should be rounded to avoid stress concentrations. Later, I heard experts in the field say that rounded corners are not necessary because ductility in the structures will absorb the stress concentrations. So which is it? Can concave corners be sharp or not?

thak you very much

雪魄梅魂 — Sun, 11 Apr 2010 07:24:18 -0400

My MD thesis gonna about tire analysis. That will be very helpful to me.

Email：fdzhw@yahoo.com.cn