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...
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.
Another Roland "story"...
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.
Our government believes that if it isn't broken (yet) don't fix it. Actually, well, a lot of businesses think that too.
Do you Gentoo?
The people who will probably pay the most attention are those who are developing pressure vessels (especially nuclear pressure vessels). Crack propagation and arrestation are very important to understand especially in brittle fracture or stress corrosion cracking contexts. When you are talking about releasing high pressure steam or radioactive material if a system breaks you tend to care a lot about understanding where it is safe to operate.
An easy way to break a pressure vessel is to add just a little chlorine at high temperatures or too much stress at lower temperatures. This doesn't only apply to pressure vessels. There are a surprising number of roofs of swimming pools that collapse and kill people. Chlorine at reasonable temperatures causes SCC (stress corrosion cracking) on the steel supports and then a lower temperature later reduces the ability of crack arrestation. Result: lots of children dead. Chlorine is the bane of steel.
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.
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?)