Why Is Water Modeled as Incompressible and Steel as Compressible in Engineering?

[Pooty]

My Finite Element Analysis Professor posed a question to us in lecture and asked that we do a little research on it while at home. We were talking about the Bulk Modulus of Water and Bulk Modulus of Steel. I looked them up and for Water it is 2.2 GPA where as Steel is 160 GPa. He asked, why is it that in Engineering, we model water as an incompressible material whereas we model Steel as compressible when Steel clearly has a much higher resistance to volumetric strain.

I don't deal with water a whole lot in structural engineering but I know that in terms of internal combustion engines, water is always considered to be an incompressible liquid. Can anyone shed some light on this subject? Thanks

 

[Mark44]

This wiki page - http://en.wikipedia.org/wiki/Bulk_modulus - says that the bulk modulus of water increases at higher pressures. See the 2nd table in the Selected Values section.

 

[Pooty]

So I read that, but still don't know how that means we model water as incompressible and steel as compressible. I found this article...

A common statement is that water is an incompressible fluid. This is not strictly true, as indicated by its finite bulk modulus, but the amount of compression is very small. At the bottom of the Pacific Ocean at a depth of about 4000 meters, the pressure is about 4 x 107 N/m2. Even under this enormous pressure, the fractional volume compression is only about 1.8% and that for steel would be only about 0.025%. So it is fair to say that water is nearly incompressible.

The 2nd to last statement makes it sound as if steel still maintains a higher Bulk Modulus than Water even at the bottom depths of the ocean. Am I interpreting this wrong?

 

[nvn]

Hint: Perhaps study hydrostatic pressure on solid ductile materials, and von Mises failure theory, and see if you can develop any ideas.

And please explain why this is an easy question to you.

 

[pongo38]

My suggestion is that with steel, you apply the bulk modulus to problems where its use is worthwhile, and you want and need credible results. Other problems may make its use not worthwhile. In structural analysis generally, the assumption is usually made that the deflections are negligible compared with the member lengths, but detailed problems arise when the deflection is very important. With water, and hydraulics problems in Civil engineering, it just isn't worth being so exact as to take account of water's compressibility, but I'll bet someone will be able to think of a particular issue where it is worthwhile to consider it.

 

[Pooty]

Pongo, Thank you! That is what I was thinking as well. And that is basically what I told my professor today. In civil engineering water is not a material we think of carrying structural loads. I was thinking about Naval Architecture, but any loads imposed on the water in that discipline can disperse in other directions and is not confined to tri-axial loading. An internal combustion engine was the other thing I was thinking of where I here the "incompressible" term thrown around a lot but that makes sense because compared to a petroleum liquid, it is pretty much incompressible. Thanks, I appreciate the insight.

NVN: I said it was easy for two reasons. 1: I wanted people to actually read the thread. 2: My professor stated, this is not that hard of a problem, do not look into it too much. And considering the types of questions I've seen on these threads before, I thought a qualitative type question like this wouldn't be too much of a ball buster for you guys.