You are right Marty, it is the case of S and P waves in geology. They travel
at two different velocities, both dependent on tension.

The TDR wants the P wave. Inducing an S wave will cause a P wave. The S wave
can be damped by putting clay on the rod to reduce transverse vibration.

The two waves can be separated by their velocities and the design of the
receiver transducer. If the receiver transducer is sensitive to only axial
forces, then it will see only P type waves. Likewise for the transmit
transducer. If it can displace only axially wrt to the rod, everything is
ok.

Good job on catching the distinction. A fine point that only a learned,
diligent man such as yourself would catch.

Try this link:

http://quake.wr.usgs.gov/research/ph...shearwave.html

Note they can measure cracks too.


Amen!

"Martin Baxter" wrote in message
...
Bob Crantz wrote:

"Martin Baxter" wrote in message
...

Sounds good to me, I don't belive tension (or compression) has anything
to with the speed of transmission in a solid.

Assume a completely rigid guitar neck. Adjusting the tension of the
string
adjusts its resonant frequency. If the length does not change, but the
resonant frequency does, then the velocity in the string must change.

C = tension/linear density

I don't think so, you are thinking of resonance in a transverse plane,
not propagation of a sound wave through the medium. Your example is for
a transverse wave, the TDR mode you propose to employ is in fact a
compresion/rarefaction, not unlike the P wave in siesmology. The problem
is going to be "injecting" this pulse, if you just tap the side of the
rod you will generate transverse waves rather than the longitudinal wave
you are seeking. I do not think that that transverse waves will be
reflected by a crack in the rod or the connectors between rod sections.

See http://prola.aps.org/abstract/PRD/v7/i6/p1590_1 for the general
equation.


Cheers
Marty