Update 4/18

It's crunch time, and I'm not talking about lunch. The bulge test has to give me what I need in terms of a thesis. I will need to propose a thesis real soon.

What do I know already?

CP Ti grade 2 is the material I am working with. That is, commercially pure titanium grade 2. This material is used a lot in industry. It has superior corrosion resistance and has a high strength to weight ratio. This means it can supply good performance with a lesser amount of material. Because of its corrosion resistance, it is applicable in chemical, medical, and aerospace industries. It is quite expensive though, so that limits its reach into other industries that can easily use other cheaper material.

The forming limit diagram is helpful in determining how much stretching can take place before the material fractures. It plots the major true strain vs minor true strain against each other. What does this mean? Well, here's a picture:

From this picture, you can see some cool things. The darker red line is what people want to achieve with testing. Uniaxial tension is achieved with a tensile test. It's where the material is stretched in one direction and shrinks in the other direction. Poisson's ratio is involved in there too. The left side of the FLD for CP Ti gr2 has been tested for already at NIU. Micro limited dome height was used to get that data. However, the right side is more difficult to get with that method. There is friction involved with the limited dome height test. So here is where my thesis comes in. 

The bulge test uses nothing but air to deform the material. That means that there is essentially no friction involved, which is good for data. For a perfect circular bulge, there is equibiaxial tension. Both the major and minor axes are strained by the same amount. Perfect! This can find the tip of the right hand side FLD. My thesis is essentially just trying to find that right hand side.

Is this enough of a thesis? I don't think so. For this reason, I will try to use different diameters of bulge to find out if there is an effect on the strain path. Perhaps the smaller diameter bulge tests behave slightly differently. Who knows? Furthermore, I can take pictures at different points of the bulging process. This can give me a strain history at various pressures. Maybe this is important in some way?

This has been studied on the macro scale before, but we are working with 38 micron thick material here. This means that the macro scale experiments may no longer be an accurate model for our purposes. It's time to do some more literature review and ask some questions.

Some questions:

1. What macro scale experiments have been conducted already on this material?

2. What experiments of similar material have been conducted already?

3. If other material has been tested, what can be expected to be the same or different?

4. What constitutes a macro vs micro size? 

5. Have other materials shown variations in macro vs micro characteristics?

Update 4/6

Oh boy, do I have a bunch to talk about! I kind of forgot about this blog, honestly. I've been able to gain a lot of traction with the bulge test, and I think that's the way that I'll end up going with my thesis. Yes, the biaxial tensile test machine is still in the works, but I have actually performed bulge test experiments already.

So where shall I go with this? I ended up getting a new MultiMoto shield for the arduino. It worked flawlessly, so I am now able to control the speeds of the four linear actuators independently. All I would need to take care of with that is to make speed vs PWM input curves for each actuator. That way the speeds could be perfectly controlled as desired. The other MultiMoto shield that only half worked (before spring break) was returned to the seller. They tested it and determined that it was indeed a dud! Yay, it wasn't my fault that the older shield didn't work! Hallelujah.

For the biaxial machine, though, we still need some grippers to hold the 38 micron titanium specimen. Yes, that may be difficult. The grippers themselves aren't too difficult to make, but aligning them is a challenge in and of itself. The current clamping method is quite rickety at the moment.

But Hallelujah, the bulge test is where the fun has been happening! Imagine bolting some metal together with some o-rings and just pumping in nitrogen to 400 psi. Yeah, it's quite nerve-racking. But after a few times you find out that it's actually not that bad of a pop when a failure occurs.

Anywayyyyy, I got the machined parts back from the machine shop and was super pumped when my o-ring grooves fit the o-rings perfectly. I love how quickly the machine shop gets stuff done, too. It's definitely a change of pace from some of my past experiences.

Above you can see the beautifully shiny 303 stainless steel parts. Aren't they pretty? <3 Those o-ring grooves are meant to hold the nitrogen in the system, and they sure have been doing a good job. Hurray for the random internet source I used! 

Since we were really scared and didn't know quite what to expect, I clamped the bulge test parts underneath a table and put a blast shield in the way. This reminds me a lot about my senior design project where we just spun a bunch of magnets at a couple thousand RPM. I should consider safety a bit more in the future maybe. In the picture above, though, you can clearly see some bulging occurring in the titanium sheet.

And above here you can see some bent metal. Except instead of titanium it was aluminum. This aluminum burst at pressures of maybe 100 psi or less. Very weak! Because the pressure was still so low, the metal didn't fly off anywhere. That's what I need to happen with the titanium samples. However, that will be quite unlikely. Therefore, a different plan of attack is necessary.

Here you can see a picture of a bulge test of the titanium at 430 psi. The 440 psi test burst, so there aren't any good pictures of that. Notice the crease around the edge of the 20 mm diameter bulge? Yeah, that's the sign of a stress concentration (more on that later).

Here is a picture of all of the samples we tested. Titanium bursts at around 440 psi while Aluminum burst at quite low pressures. The 2 mm bulge test on the bottom left of the picture did not show much promise at all for us. We simply cannot achieve enough pressure to bulge that small diameter.

We tried using strips of Ti over whole blank sizes of other material. 100 micron thick Al did not work well. Two 50 micron blanks of brass was too strong. A single 50 micron brass blank underneath a strip of Ti seemed to work well. By using different strip geometries, different strain paths can be achieved throughout the testing. Also notice that the Ti on the bottommost sample still failed at the edge of the bulge. This indicates a stress concentration. I have had the inside radius of the tooling cut down to a 1.5 mm radius. This has worked well thus far.

More later.