ME 310: da/dN Testing

Hey guys, another research update for you all! So last time, I talked about fatigue pre-cracking my rail specimens and how the fatigue process worked. Basically, I needed to fatigue pre-crack my specimens in order to get a crack that would mark a path for the rest of the crack to follow. This process is to help with da/dN testing, which is a test that measures the change in crack length per change in number of cycles. Since I had fatigue pre-cracked a dozen specimens, I then began conducting the da/dN tests.

So da/dN…. What does that mean and why is it so significant? Well, as I mentioned before, it’s basically the rate at which a crack grows for a given cyclic loading on a specimen. What we are essentially trying to find from this type of test are 2 things:

1. The cyclic loading at which there is no longer a crack growth rate- this is important because it allows us to find the Kmin Threshold value. This is the minimum stress intensity factor (proportional to load) that a crack starts to grow at. In other words, a what load do you stop creating a crack on the specimen.

2. The slope of the da/dN vs Delta K graph. Basically, as long as the increase or decrease in crack growth rate maintains itself steady and moderately linear, then we can characterize the behavior of the crack for a given load and predict what the crack growth rate will be at a given time.

So the past month has consisted of these tests, but just like the fatigue pre-cracking, it has been a little tough figuring out the best way to go about these experiments. To give you an example, I ran a crack growth rate test a several weeks back and obtained the following results:

da_dN Vs Delta K

Now compare this experiment to one that was performed by a certain railroad company.

A36 steel da-dN test

So as you can see, there is definitely something wrong with what I was doing and it took time to figure out what was wrong. Apparently, the reason why my data was so distorted was because I had stored the data points was based on the number of cycles and not on the crack length. So in some instances, I would receive negative crack growth rates because the strain gauge would measure a smaller crack length at certain instances (remember it’s cycling, so there is sinusoidal amplitude that can give you positive and negative numbers), which would cause a negative difference. So although you may think, “Duh, isn’t that obvious??”, small little issues tend to occur very often and one always has to be on the lookout for small problems like these in order to obtain accurate results. Nonetheless, I think I’m starting to get the hang of all this da/dN testing and can now start running these tests with ease. Here’s a graph of one of the good tests that I ran:

L112 dadN test

So as the change in stress intensity factor Delta K increases, the crack growth rate obviously increases because the specimen’s area gets smaller  and because your putting larger load on it as well.

So that’s where I’ve been essentially with these tests. I’ll be sure to get a fracture toughness test in soon enough and hope that the tests are valid. More updates to come!

Last Weeks of Class

As the semester winds to a close, I’m starting to find it increasingly difficult to get out of bed to go to class each morning. In fact, in the past three days combined, I’ve only gone to three classes. That’s not something to be proud of.

However, it’s not the first I’ve felt this way. Throughout the semester, particularly after a round of exams, the stress catches up to me and I feel slightly burnt out, so I take a week off. And then the next time I go to class, everything my professors write on the board may as well be hieroglyphs cause I don’t know what’s going on anymore. Then I have to catch up, stress out, and repeat. It’s quite the vicious cycle.

So stay in school (or class, in this case) kids. Especially since the amount of money you pay per credit is pretty obscene. Missing a class is like throwing hundreds of dollars down the drain.

Registration Woes

The Co-Op program comes with it’s pros and cons. The pros: a secure internship position for two rotations, easy application process, and experience. The cons: a very inflexible schedule.

Now I still believe that the pros far outweigh the cons, however, the fact that I have to spend fall semester of my Junior year at the internship means that I’ll have to take quite a few classes over the summer. I’m fine with that. What I’m a bit upset about is that ME 21, a lab that most people take during the fall semester of junior year, isn’t offered over the summer. In addition, ME 21 is a pre-requisite course for two more lab classes.

Now if I were smart and new that I would be participating in the Co-Op position in the fall semester of sophomore year, I could have scheduled to take ME 21 this semester. But back then I wasn’t sure whether or not I would secure a position.

Either way, I’m going to have to load up on classes spring semester to catch up.

A Combined Rankine and Brayton Cycle

One of the biggest challenge mechanical engineers face is how we can make a system more efficient. Many conventional methods involve making the system larger and bulkier, which is why the most efficient cycle we have, the Rankine Cycle, requires so much material that we have to create entire power plants. Power plants are expensive, immobile, large, but they are the most efficient.

Now a disclaimer: I didn’t learn any of this in class. I was trying to come up with stuff for my Thermodynamics research project (this was ultimately scrapped because it wasn’t undergrad level), so I don’t understand it fully. If you combine a Brayton cycle and a Rankine cycle, you can improve the efficiency drastically. You see, a lot of heat is wasted through a standard Rankine cycle, which is why we add reheat and regeneration, which takes up space. What if instead of adding reheat and regeneration processes, we added a Brayton cycle?

A Brayton cycle begins by drawing in air from the surroundings through a compressor. The air is then put through a combustor to heat it up, and the heated air is used to rotate a turbine, and some of that work from the turbine goes back to power the compressor.

What if we took the heat generated by a Rankine cycle and used that to power a Brayton cycle? Then we’d have to add less fuel to the combustor to heat up the air and we wouldn’t waste as much.

Here’s an extremely simple version of what it would look like.

brayton-rankine

Steps 1-6 are the processes of a simple Rankine Cycle, and steps 7-10 are the processes of a Brayton cycle. If there’s a heat exchanger connecting the two, then you can see how the heat generated as waste by a Rankine Cycle can instead be used to power a Brayton cycle.

I don’t know what it would look like in real life, but something worthy to note is that the Brayton cycle is what is used for modern gas turbines and air breathing jet engines.

Distortion/Deformation of Fluid Particles

A new concept we’re learning in Fluids is the deformation of fluid particles as they move along a streamline. As a refresher, a streamline is a path traced out by lines tangent to the particles velocity at any given time. Therefore, if the velocity is constant, we can say that the streamline also traces out the particles path.

The four types of changes can be divided into two categories: linear and angular.

4.png

The specific equations are pretty complicated to derive, so I won’t go into the details. But think about a fluid particle moving through a river – it’s easy to see how it’s shape wouldn’t remain constant.

But why learn this stuff? This subject leads into another important concept – measuring the the amount of fluid that passes through a certain area, AKA flux. This can be used to measure how much waste is being dumped into a river, etc.

 

Uncle Robert’s Power Plant

Our initial plan for the thermodynamics research project was scrapped. The initial project consisted of a “Jigsaw,” a concept which divided students into groups, and each individual within a group would be responsible for researching and learning about a certain topic. For the purposes of the class, the different subjects would have been the Otto, Diesel, and Rankine cycles.

Now, however, we’ve decided to do something more akin to Aunt Ada’s Treehouse, and those of you who’ve taken Professor Best’s class know what I’m talking about. For Thermodynamics, we’ll ask students to measure how much power would need to be generated by a power plant to supply energy to a neighborhood, with different needs and power consumption. The goal is to have the students develop an idea of not only how to calculate the work generated by a Rankine Cycle, but to also see what the purpose of learning the Rankine Cycle is for.

As for the Otto and Diesel cycles, which are rather simple, a simple video of a cut out would work. As for application, all the students need to know is that the cycles are used for car engines and the differences/tradeoffs between the Otto and Diesel cycles.

Thank Heavens for Short Physics Labs

I’ve already talked about how much I liked Physics II labs in comparison to the Physics I labs I had to take freshman year. Good grief, I hated Physics I labs. However, the first few Physics II labs weren’t a stroll in the park either. More often than not, the labs spanned the entire three hours, especially when we were working with circuits.

However, now that we’ve moved past circuits and moved on to optics, things have gotten a lot smoother. Smoother than a…anyway, they’ve gone really well. Not only have they gone by faster (1-2 hours at most), but I feel like I actually understand what’s going on.

One of the most recent labs involved converging lenses, and how we could build a miniature, ghetto telescope in the lab. The diagram of what we constructed looked something like this.

converging lens

The object, D, is illuminated with a light source. Then, we determine a focal length for one lens, place it at the determined distance, and then look at the image through that lens with another lens. Kind of like two lenses on either end of a telescope. The first lens flips the image, and the second one magnifies it. By flipping the image, I mean it’s flipped right-side up, as images travelling through a converging lens are upside down. Our eyeballs are an example. Our brain just flips those images again so that we see the world right side up. pretty cool stuff.