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?