Today I looked at some information about holography. It seems to me that most student projects about the subject are making holograms, and while that may be worthwhile and interesting, it is not suitable for a study, question-and-answer science competition type of project. However, holography's application in nondestructive testing is very interesting, and that could lend itself to a science fair project. Also, diffraction gratings made by holography fascinates me, and I think the question of the feasibility of the construction of diffraction gratings for spectroscopy by way of holography is an interesting idea. Doug, a student working with holography here, said that some of his diffraction gratings from holography are better than actual diffraction gratings.July 12, 2001
Holographic interferometry - holometry
- detect movement
- detect structural flaws
- measure growth (ie instantaneous growth of plants)
I have decided to see if I can compare photoelasticity (with cross polarizers) with holography in terms of measuring stress (force) and strain (displacement). I would like to see if I can measure just one value - namely stress or strain - on both, but I don't know whether that is possible - I am pretty sure that one can measure only one of the two values for each method. In the case of being able to measure the same values in both methods, I will compare their sensitivities. In the case of being able to measure only one value for each method, I will see if the stress and strain have a relationship. I believe they should; for example, they do in a spring (F = kx). The correlation will probably not be linear in the case of stress and strain on an object, but if some correlation is found, that would be interesting, and possibly worthy of pursuing for the rest of Simon's. The problem may be that it has already been done many times, which it probably has.July 16, 2001
To look up:
- Photoelastic coefficients
Note: "The stress field at any point in a photoelastic specimen can be related to its index of refraction through Maxwell's stress optical laws."
Holographic interferometry - double exposure - both images are seen simulataneously. Bright fringes occur where the object didn't move, and dark fringes occur where it did (??). "If the image turns black except at the bottom, the weight is too great."
Today, I wanted to see if I could figure out the stress and strain in terms of different moduli (0% compressibility, 100 % compressibility), using calc, algebra, and formulas. I would mathematically figure out what happens to objects if they are 100% or 0% compressible. I also wanted to start holographic interferometry with a stressed object and measure the strain, and I wanted to start photoelasticity with polarizers and objects that can be stressed and measure the stress. However, I knew that I wouldn't start; I at least wanted to get started on figuring out how to get these experiments together. Also, Doug showed me how to make holograms and develop them, and I wrote down the basic steps in my log book. I also took some basic notes in my book about what happens to objects in the cases of no compressibility and compressibility. All materials exist in between - density changes (compressibility), but also mass gets displaced and deformity results (no compressibility). The ratio of the two processes is different for each substance and shape. Also, changes in displacement can occur in different directions. Finally, things could get even more complex - for example, let's say someone puts a force in the x direction, a force F, creating displacement in directions x, y, and z. Now, if the object is not a homogeneous cube, it is possible for the same force F, if applied in the y direction, to create different displacements in the directions x, y, and z. If there are formulas similar to the formula for a cube, F = kx, these formulas would require 9 different constants, and that is assuming that the correlation between force and strain is linear. The strain tensor would be a 3 x 3 tensor, and it would be different for each substance and probably would be adjustable according to the shape. In conclusion, it is complicated, and I would rather not get involved with these ideas. I still want to work with stress and strain and relate them to holography and photoelasticity.July 18, 2001
Today, Dr. Metcalf explained birefringence in terms of formulas. I was interested in it; I saw how birefringence was basically a phase difference caused by different directions' of an object having different indices of refractions, which can be caused by stress or the way the object is made. The fringes follow the stress field (the path of stress), and that is why they are curved and not straight. I like birefringence and stress fields.July 20, 2001
Also, Miriam Rafailovich at Material Sciences explained how no birefringence studies have taken place on materials with clay nanocomposites. The clay is supposed to increase the mechanical strength of the substance. I would like to do a birefringence study on materials with clay. This would probably be doable. I could use a device called a Stress Opticon, which is a photoelastic device - it has two crossed polarizers enclosing a space where the object is placed, and screws are placed such that they can provide stress on the object. The birefringence on the samples the company (Vishay Measurements Group, Inc.) provides is great and extremely noticeable. I could use some substance without clay, and blend the same substance with clay, create the same shape of both materials, and use them in the stress opticon to observe the difference in birefringence. This is directly related to stress fields, so I could show how the stress fields change as one adds clay to a substance, if these fields do in fact change. Anyway, this is interesting, and I will try to do this experiment.
Today, I experimented with birefringence and learned about the processes that are performed at the Material Sciences buildings. I will need a lot of help with the making of shapes and blending, and I will have to learn how to do these processes; I don't want to bother the people over there too much. Also, I need to call Vishay Measurements Group (the manufacturer of the photoelastic device, the Stress Opticon) because they did not provide an instruction manual with the device, a manual that may help in terms of how to determine how much stress is being applied and how to connect the device to measuring devices, etc. It may be helpful. I will call them Monday.July 23, 2001
I need a substance that is not too rigid. If the substance is very rigid, the stress will have very little effect on the substance, and no birefringence may be noticeable. Plus, the material may break if I try to put too much stress on it, in an effort to see the damn birefringence. So, I will try to get a relatively flexible substance. Also, the idea of clay nanocomposites has the complication that the clay makes the substance cloudy, and I can't see the birefringence if I can't see through the material. I will try to reduce the percentage of clay in the substance, but this solution has the corollary disadvantage of having less of an effect on the birefringence, since there is less clay.July 24, 2001
Also, I need bigger samples to work with. The molds they use in material sciences are tiny in comparison to what would be ideal for the stress opticon. Plus, even if I don't use the stress opticon (which is possible because I would rather work in the Laser Teaching Center, and I may not be able to move the stress opticon there), the fringes will be smaller. I would like a bigger shape.
In the case of my not being able to use the stress opticon in the Laser Teaching Center, I will have to design an apparatus that does the same thing. This is not so bad. I can use an overhead projector as a source of light, put crossed polarizers on top, and put the device in the middle. I will have to clamp the object in the middle and figure out how to put measured stress on the object, but it is very probable that I can. Also, this solution has the corollary advantage of the projector's being able to project the image of the birefringence on a screen or wall, and I could photograph this image.
I called the Vishay Measurements Group company. Some complications arose; I will call again tomorrow.
I am well on my way to getting my project started. Mike, a fellow Simons fellow showed me how to blend, and we blended polystyrene (PS) with 5% clay. Hopefully, if the final substance is thin enough, I will be able to see birefringence, in which case my project would work. I also drew a shape on a circular piece of steel that will be the mold for my samples, and it is a rectangle, about 12.5 cm x 1.7 cm, and this size is ideal for the stress opticon. I know that the PS alone will be clear and its birefringence would probably look nice in the stress opticon; I had tiny molds of PS that showed a little birefringence, but it was impossible to put the molds in the device so that I create stress on it effectively. Also, I didn't want to put so much stress on the tiny piece of PS that it would bust. I will try to get someone to cut the mold out of the steel in the physics shop, since I can't do it myself. However, this will take a long time (about an hour) since it is so hard to cut through this thickness of metal in a precise way. So, tomorrow, I hope to have the mold and learn more about blending and molding, and then I can start making my samples for observation.July 26, 2001
I have my cutout in the stainless steel. I made a sample mold of PS just to see what would come out. It wasn't that bad. It was a little annoying to get it in the stress opticon, and the sample made had some bubbles, but I think it will work. I will blend enough PS and clay (5%) to try out molds of that soon.July 27, 2001
Things to do:August 27, 2001
- Get envelopes and stamps
- Get mail from Vishay Measurements Group (the manual). If it doesn't come in the mail by Monday, call again and ask for fax or remail to more specific address.
- Make sure everything in Stress Opticon is ok; pieces must not be missing, or Dr. White will be understandably angry. Set aside box for stray pieces.
- Blend clay and PS.
- Make molds of PS and of PS w/clay. If the PS w/clay turns out to be too cloudy to see through, try lower concentration of clay. I don't think I can try thinner molds of PS w/clay, as it is already so thin as to be annoying.
It has been a while since I have done much work because of college visiting. My new cutout has been made; it is about half an inch thick, so I can use it in the Stress-Opticon, and the plastic supporters work because the samples are thicker. I made a couple samples of PS and the birefringence is very good; however, there is much birefringence because of the imperfections in the sample in addition to any stress I put on it. Also, I need to blend clay and PS so I can start making samples out of that blended material; however, I need to find someone to show me how to use the Brabender machine; there a few things I do not know about the machine (the Brabender blends materials). So, I will come back Wednesday, by which time Dr. Rafailovich should be back from Chicago. If I can get good samples made out of PS alone and PS with clay, I will be able to do a substantial amount of work; whether or not the work will be worth the time taken will be found out. This is a picture of my cutout:August 29, 2001
A big problem that I knew would be important now seems to create a dead end to my project. When I create samples of PS, there are many imperfections which lend themselves to apparent birefringence even without stress. What makes this a bigger problem is that the added stress is very interrelated with the already present birefringence and imperfections; since these imperfections will never be identical in samples, they create a problem. The observable birefringence after stress is added is irrelevant, since the imperfections play a role. Therefore, I need perfect samples; this is pretty much an impossibility, since the machines in Materials Science are not equipped to make perfect, homogeneous samples. I am not sure what I will do.