Slashdot Mirror
Tracking the Cracks
Roland Piquepaille writes "Israeli physicists from the Weizmann Institute have used a new approach to study how materials break. In a short news release, brilliantly titled "Breaking news", they explain their new method for analyzing the progression of a forming crack. The news release even says that it could have help engineers predict 'exactly how much pressure the levees protecting New Orleans could withstand before giving way.' This method could be used by engineers and material scientists in a vast variety of applications."
never design something so it will hold exactly what it needs to stand up against. Unless you're building for suicidal adventurers, people will appreciate headroom. Especially people behind levees...
Plumbers should especially pay attention.
No existe.
In the article they say taht they have applied their method to a variety of materials, namely plastic, glass and metal. There is a common thread there though, in that all three are higly regular materials. In an earthen levee, or even a contcrete one, the materials used to make are way more irregular than what they have tested their methods with. It sounds like the connection to New Orleans levees is really premature to me.
I'm currently involved in the making of 3D nano devices. One of the steps involved in the making of these things is the breaking of a silicon wafer. This is currently the least reliable step in our process, and we sure are very interested in ways to improve this.
-- Cheers!
The news release even says that it could have help engineers predict 'exactly how much pressure the levees protecting New Orleans could withstand before giving way.'
No, it doesn't. That's a rhetorical question in the first paragraph.
There was a program about the Orleans aftermath over here in the UK (it was an edition of 'Horizon' on the BBC), which showed not only that the levvies had only been built for a smaller scale hurricane (not surprisingly...), but also that the designers/builders hadn't taken into account the clay-like consistency of the soil they were being laid into and so they literally just got ripped straight out of the ground.
Talk about missing out engineering 101. Idiots.
The question was not *IF* a hurricane would flood New Orleans, just *WHEN*.
Experts had been warning for this for years but somehow the levees were not reinforced.
"Scientists all over the world have experimented with crack(ing)"
God spoke to me.
I hope the title was meant to be punny, because a model on cracking is nothing new. There are currently many models that work for crack propagation in composites (of metal, polymers and amorphous materials). Every research groups CLAIMS that their model allows them to gain the best insight, because saying anything else:
forfeits further research dollars.
Until the model is explained in further detail and some source code is released, rather than the typical hand-waving, hype and money generating BS, this "breaking news" is nothing but hype.
-PhD student. Metallurgical Sciences.
... for they let in the light." - Spike Milligan
"All you have to do is be fragile and grateful. So stay the underdog." Chuck Palahniuk, Choke
Why is this article posted on /.? At best this is a report of a minor advance in a well established field. Hundreds of such advances are made in every field every week. Yes, PR department at Weitzmann Inst called it a breakthrough but that doesn't make it into one.
Is it possible to limit the science postings to real science news? Maybe have editors who know the field evaluate the postings before hand.
The engineers had taken into account the soft soil and the levees were designed to go through this and into more stable soil. However they were not set as deep as the design called for. This is not a design issue, it's an issue with the contractor and inspectors. It is possible that there were also design issues that compounded the problem but I have not heard of any major ones. Incidently you say "clay-like soil consistency" like it's a bad thing. Stable clay soils are exactly the type of earth you want to construct earthen levees, damns, subgrade, etc. out of since it compacts so well and won't move once it is compacted. This as opposed to the material the levees were built in that had a lot of biological material in it and was very unstable.
I thought this was going to be about tracking people who crack software.
This is only true if your engineers are completely incompetent and mix the concrete incorrectly in places. Otherwise, concrete is much more regular at microscopic level than glass or plastic, which are both amorphous solids. And if your engineers are that incompetent, a good model for cracking is not going to save you. As to larger-scale irregularities, they're generally irrelevant if you know the structures of the component grains or regions.
:-P
We already know the strengths of the materials we use to build things, because we test them beforehand. The reason new models are important is that they give us a better understanding of _why_ the strengths turn out the way they do.
P.S. A +5 insightful attached to an assertion that a mostly crystalline solid is less ordered than an amorphous solid makes me cry inside. Everyone makes mistakes, but moderators aren't supposed to encourage it
...it's really a sad day for America when we require a goddamn ACT OF CONGRESS to make our DVD players work properly. ~
Yep, that's because the raise in stress at the crack tip is proportional to it's radius. By drilling it out you increse the radius and decrease the stress.
"Physics is to math as sex is to masturbation." -R. Feynman
Well, I heard the rumor that the Israelis had taken over the dealing in XTC, but hey, now that I heard it confirmed on slashdot that they're now taking over the crack trade as well, my, gosh.
I'm still trying to figure out what people mean by 'social skills' here.
Janine Gordon Associates specializes in placing favorable PR pieces, rather than direct advertising. See their case histories page, where they boast about how they plant stories. (Note: annoying all-Flash site.)
This is a Roland the Plogger story, of course. But, for once, none of the links benefit his search engine ranking. So one wonders if Janine Gordon Associates uses Roland the Plogger.
I believe the factor of safety for stell structures is in the order of 1.5. As for earthworks the factor of safety can be up to 3. A factor of safety of 10 is not needed. But you DO have to maintain the structures so they are still at their designed capacity.
I don't have the details about the New Orleans levees, but I honestly doubt cracking had anything to do with it. Such huge works are rarely made of expensive materials such as concrete. It's just too huge. Usually only the sections of levees protecting the most "critical" areas would be expected to be made of concrete. Most of it would be made of earth. This seems to be validated by some info that can be found on the levee failures
http://www.csmonitor.com/2005/1103/p02s02-ussc.htm l
One thing that you need to do when building any kind of water retaining structure is to have your impermeable cutoff wall deep enough to prevent percolation of water when the water is at its maximum. Either the water level was maintained higher than the levee height (which would have been difficult since the water would just have overtopped the levee... unless the whole area beyond the levee was already flooded, but then the water on the other side would have prevented that kind of percolation and resulting erosion) or the original design assumed the water would never reach the top of the levee. Either that or the flow properties of the underlying soil were compeltely erroneously estimated
In any event, cracking had nothing to do with any of the described failures. The comment in the article was pure "buzzword". I think it's the kind of research that will be of more use to mechanical and structural engineers than to geotechnical engineers.
In summary, the factor of safety was probably good enough to resist the forces applied to the levees. At least in terms of strength it probably was. Where the design seems to have been lacking is in terms of erosion protection. Erosion will weaken an earth structure.
I like my dinosaurs feathery, and my pterosaurs hairy (or is it pycnofibery?)
Where the heck are you getting this idea that concrete is more regular than glass? You seem to be talking of a laboratory prepared CEMENT mixture. Yes, certainly if you prepare glass and cement in a laboratory condition, your hardened cement is likely to be more regular. There are, many considerations that makes this untrue for real life construction.
The first thing to understand is that concrete is NOT cement. Concrete is a MIXTURE of cement and agregate. You can use all kinds of things for agregate, gravel and sand being the most common. Sometimes some fly ash from blast furnaces is added. Engineers normally use lower factors of safety for concrete than steel because the uncertainties are greater. When you test concrete to failure, sometimes the fractuers cut across the agregate grains where the cement bond was stronger than the agregate, other times it will follow a path around strong agregate particles.
The other thing to know about concrete is that is it NOT made in a factory, under controlled industrial conditions and unit testing. Sure, you may get your concrete mixed at a concrete plant and the trucks, but eventually it has to get to the field. Then it must be placed... and the experience and professionalism of the workers is very variable. Furthermore, concrete needs to cure in place. The water content of the concrete during this stage is important since it needs water for the chemical reactions to harden the concrete. But then again there is an optimum value. The chemical reaction is also helped by high temperatures. So weather conditions and placement conditions will affect the final product.
And of course, portland cement is a strong alkali. It can actually react with the agregates themselves which can build up stresses and cause cracks inside the concrete independently of external stresses. You may have witnessed this alkali-aggregate reactivity in concrete if you see cracks in concrete that seem to be humid, even what it hasn't been raining, and somtimes oozing a bit of white foam.
In final analysis concrete is a highly nonuniform construction material.
It can also added that most of your levees, and most likely the sections that failed, are probably earthworks. Therefore whatever the uniformity or lack thereof of the concrete, it would have done nothing for the leveees. Cracks are only meaningful in materials that need resist tensile or bending stresses. Needless to say, that is NOT how earthworks are desined.
I like my dinosaurs feathery, and my pterosaurs hairy (or is it pycnofibery?)