Immigration reform is a hugely controversial topic these days. You just need to read the comments to online posts to see how strong and different the feelings are. Most of this discussion centers around illegal immigration and the effects it has on the economy and crime. These are complicated issues and I am not qualified to talk about them nor have the intention of posting my view on them. However, there is a whole other side to immigration reform and it has to do with legal immigration. Legal immigration requires a visa and there are many visa categories depending on the purpose of the foreign person. Many of these visas, for example F (students) and J (exchange visitors) visas, do not allow the person to remain in the US once they finish their program. Work visas (H-1B) do allow for permanent immigration but the number of visas per year is capped and the process is expensive, lengthy and not guaranteed for everyone.
Now you might be wondering why legal immigration is important or relevant to science and the answer is nicely presented by Tom Friedman in his Invent, Invent, Invent article. All you need is to visit a science department at your closest university to find out how many foreigners do science and are willing to stay in the US and be productive both economically and scientifically. There are also plenty of scientists and engineers abroad that would like to come to the US and enjoy the technology and research culture any day. Of course, there are many American citizens in the same departments doing science, but point is that the US needs to have policies that not only keeps in, but also invites the best of the best in the world to come to this country. Friedman makes a great point when he says that these scientists will create more jobs than they take and that will benefit every one.
Getting it done requires an immigration reform too and it seems to be neglected/forgotten with the illegal part. Hopefully soon people will get over their fears and politicians over their stupidity and stubbornness and we'll have a decent proposal approved.
Monday, June 29, 2009
Wednesday, June 24, 2009
- Stopping cracks!
This is a post inspired in an article about bone plasticity and fracture toughness published in the most recent issue of Physics Today. No, I will not teach you how to break someone's arm or leg (although if I knew how to it would probably be cool to teach, wouldn't it?). What I want to talk about is the concept of fracture toughness and the mechanisms that increase it in materials.
For a crack to be created you need energy. Everyone is familiar with mechanical energy, you can push, bend or throw a cup and it will break. You could also use thermal energy, heat (or cool) certain objects and they will crack. But it is not at the slightest push that an object will break, there is a minimum of energy that you need to create a crack.
A crack is nothing more than breaking chemical bonds and creating more surfaces (think about it, if you break a plate now you have at least 2!! =P). There is an energy associated with keeping a bond and there is an energy associated with an exposed surface. When it is energetically favorable (that is, breaking the bond has less energy than keeping the bond) the object will crack.
So, that's what a crack is and although learning about how cracks originate is an interesting topic, it is not the most interesting part of fracture theory for me. What I find really cool is how a material deals with a crack once this one is formed. Materials possess a quantity called fracture thoughness, which is a measurement of how hard (or easy) it is for a crack to propagate through. The most critical part of a crack is the tip because here is where the higher stresses (or forces if you prefer) concentrate. Just as you need energy to create a crack, you need energy to grow it. However, just as some materials have mechanisms to prevent cracking (for example, a clothes hanger bends significantly before you can break it) some materials have mechanisms to prevent the cracks from growing (in other words, they increase the fracture toughness). Some of these mechanisms can even be artificially engineered, isn't that cool?
Ok, so what are these mechanisms that increase fracture toughness? One of them is crack deflection. The idea here is to change the direction of crack propagation to eliminate (or at least minimize) the force applied at the crack tip. Crack deflection occurs very often in porous materials and at the interfaces in composite materials. Bone being a porous matrix does exhibit crack deflection.
Another way of increasing fracture toughness is by creating microcracks around the crack tip. In this case the effect is double, first when a force is applied to a material containing both a crack and microcracks, the force is distributed among all of them and therefore can reduce the stress concentrated at the main crack tip and inhibit crack growth. The other way in which microcracks help is by expanding the region around the crack and "closing" its size. Radiographs of damaged bone can show multiple microcracks, although in some cases the microcracks are way too small to be seen by eye.
Lastly, crack bridging can also hinder crack growth. Bridging is, by design, the main fracture toughness mechanism in most fiber-reinforced materials but in monolithic ceramics (i.e. alumina) exhibit grain-bridging. In fiber reinforced materials, the idea is that the matrix cracks easier than the fibers, and thus when force is applied the crack will form but the fiber across the crack will remain intact and support the load. Grain-bridging is a much more subtle idea and it consists of grains in the crack rubbing against each other and carrying the applied force instead of the crack.
Any fracture toughness mechanism will show up in what engineers call an R-curve. If this curve rises with crack extension then you can be certain the material possesses some kind of fracture toughness mechanism. Determining which one, on the other hand, is not always that easy. Now to come back to the Physics Today article, it turns out bone has all three of them:deflection, microcracks and bridging. I am not surprised that bone is really hard to break now.
For a crack to be created you need energy. Everyone is familiar with mechanical energy, you can push, bend or throw a cup and it will break. You could also use thermal energy, heat (or cool) certain objects and they will crack. But it is not at the slightest push that an object will break, there is a minimum of energy that you need to create a crack.
A crack is nothing more than breaking chemical bonds and creating more surfaces (think about it, if you break a plate now you have at least 2!! =P). There is an energy associated with keeping a bond and there is an energy associated with an exposed surface. When it is energetically favorable (that is, breaking the bond has less energy than keeping the bond) the object will crack.
So, that's what a crack is and although learning about how cracks originate is an interesting topic, it is not the most interesting part of fracture theory for me. What I find really cool is how a material deals with a crack once this one is formed. Materials possess a quantity called fracture thoughness, which is a measurement of how hard (or easy) it is for a crack to propagate through. The most critical part of a crack is the tip because here is where the higher stresses (or forces if you prefer) concentrate. Just as you need energy to create a crack, you need energy to grow it. However, just as some materials have mechanisms to prevent cracking (for example, a clothes hanger bends significantly before you can break it) some materials have mechanisms to prevent the cracks from growing (in other words, they increase the fracture toughness). Some of these mechanisms can even be artificially engineered, isn't that cool?
Ok, so what are these mechanisms that increase fracture toughness? One of them is crack deflection. The idea here is to change the direction of crack propagation to eliminate (or at least minimize) the force applied at the crack tip. Crack deflection occurs very often in porous materials and at the interfaces in composite materials. Bone being a porous matrix does exhibit crack deflection.
Another way of increasing fracture toughness is by creating microcracks around the crack tip. In this case the effect is double, first when a force is applied to a material containing both a crack and microcracks, the force is distributed among all of them and therefore can reduce the stress concentrated at the main crack tip and inhibit crack growth. The other way in which microcracks help is by expanding the region around the crack and "closing" its size. Radiographs of damaged bone can show multiple microcracks, although in some cases the microcracks are way too small to be seen by eye.
Lastly, crack bridging can also hinder crack growth. Bridging is, by design, the main fracture toughness mechanism in most fiber-reinforced materials but in monolithic ceramics (i.e. alumina) exhibit grain-bridging. In fiber reinforced materials, the idea is that the matrix cracks easier than the fibers, and thus when force is applied the crack will form but the fiber across the crack will remain intact and support the load. Grain-bridging is a much more subtle idea and it consists of grains in the crack rubbing against each other and carrying the applied force instead of the crack.
Any fracture toughness mechanism will show up in what engineers call an R-curve. If this curve rises with crack extension then you can be certain the material possesses some kind of fracture toughness mechanism. Determining which one, on the other hand, is not always that easy. Now to come back to the Physics Today article, it turns out bone has all three of them:deflection, microcracks and bridging. I am not surprised that bone is really hard to break now.
Tuesday, June 23, 2009
- Welcome post
Hello all,
Science under the influence is an experiment where Squashed, The Savage and I will try to regularly write about science-related stuff we know or would like to learn more about and in some cases about general topics that we find interesting too.
Although the three of us do very similar research on a daily basis, we come from very different backgrounds and somehow we hope we can present and discuss topics in a way that is accessible to the average non-science person (at least most of the time) but also with each one's personal touch. Of the three of us, two are males and one is a female.
We'll do our best to tell the science without mistakes, but don't say we didn't tell you that you should read at your own risk. =P
A bit of background on how we decided to start SUI:
The Savage and el Charro were talking about how good of an exercise it would be to write about science and also to have the freedom to write about whatever you want and reach an audience. Somehow the idea of having a blog sounded good and here we are. Squashed, who was in the room at the time, decided to join us in this journey.
We hope you enjoy reading our blog and feel free to comment!!!!
Science under the influence is an experiment where Squashed, The Savage and I will try to regularly write about science-related stuff we know or would like to learn more about and in some cases about general topics that we find interesting too.
Although the three of us do very similar research on a daily basis, we come from very different backgrounds and somehow we hope we can present and discuss topics in a way that is accessible to the average non-science person (at least most of the time) but also with each one's personal touch. Of the three of us, two are males and one is a female.
We'll do our best to tell the science without mistakes, but don't say we didn't tell you that you should read at your own risk. =P
A bit of background on how we decided to start SUI:
The Savage and el Charro were talking about how good of an exercise it would be to write about science and also to have the freedom to write about whatever you want and reach an audience. Somehow the idea of having a blog sounded good and here we are. Squashed, who was in the room at the time, decided to join us in this journey.
We hope you enjoy reading our blog and feel free to comment!!!!
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