Speed of sound in material under tension

1. Apr 10, 2010

jimmyct

I have built a loading machine for testing material properties. It applies tension at slow rates (ie 1htz) under load control until the specimen fails. One thing I noticed is that if I tap the material as it is loaded, it seems to give off a different pitch noise when it is under higher tension vs. lower tension of the sinusoidal load.

Ive been trying to figure out the theory behind this today for my own curiosity. The speed of sound (C) within a material is defined as C = (B/p) we B is the bulk modulus and p is density. I dont think either of those is changing as the load is applied. Bulk modulus is more of a constant determined from the slope of a stress strain curve while the density should not change as it is a material property. Any ideas what is being observed?

2. Apr 10, 2010

Studiot

3. Apr 10, 2010

jimmyct

the pitch was my first thought
but I'm not sure its entirely right.

the wiki page that was linked in says that the pitch is related to the primary frequency at which the mechanical vibrations are passing through the specimen in which we can hear.

But Im thinking that the frequency (f) itself is also proportional to speed of sound (SOS) and wavelength(w)
f = SOS/W

So which part is really changing? The wavelength or SOS?

To add more confusion, the link to the subsection of wikipedia page on "pitch" says that as the violin string gets longer, the pitch changes and says this length and pitch are proportional. Then it provides a formula that says the density is also changing. So that leads back to the other side of initial question i asked being that density is one of the parameters for Speed of sound. I just find it hard to believe the density of the steel plates are changing though as there is little strain/displacment on them.