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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."
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.
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.
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.
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. ~
You may not want to fly anymore. Airplanes are typically designed with a factor of safety of less than 1.5. An FOS of 10 is usually overkill, do I really need to design a bridge so that it will hold ten times the highest load it will ever carry in it's lifetime? In a word, no.
Henry Petroski's classic To Engineer Is Human : The Role of Failure in Successful Design shows its age a bit, but it's a great read on structural engineering, factors of safety, and failure to learn from the mistakes of the past. http://www.amazon.com/gp/product/0679734163/sr=1-1 /qid=1139177043/ref=pd_bbs_1/002-3727742-0917603?_ encoding=UTF8
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