Bob Crantz wrote:

How about sending an acoustic pulse along the rod rigging the same as a time
domain reflectometer works. The pulse speed is a property of material and
tension and the reflectance a function of acoustic impedance. Cracks would
raise the impedance. The cracks could be located along the length of the rod
by simply applying the pulse signal at one point.

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

Cheers
Marty

Sounds good to me, I don't belive tension (or compression) has anything

to with the speed of transmission in a solid.

But to get the expected measurements in a solid structure that is being
read for resonent feedback variances, wouldn't said feedback be
effected if the the solid had a wide range of motion? Seems like it
would require isolation for relaible readings. Out of my depth on this
one, folks....just guessing based on what I've read from Bob C.

RB
35s5
NY

"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

http://www.faqs.org/docs/sp/sp-172.html

Bob Cranz is correct, this would work for cracks in the rod, even when
it was attached at both ends. When rod fails, where does it fail, I'd
bet at the ends where TDR would be harder.
Given identical types of ends and repeatedly stressed, any engineer
would expect that solid rod would fail before cable type. A crack in a
single strand of wire cannot propogate anywhere except in that strand.
A crack in solid can go all the way through and probably will because
the crack itself becomes a stress concentrator.

Given identical types of ends and repeatedly stressed, any engineer
would expect that solid rod would fail before cable type.


This guy is a funny troll. Even the makers of standing rigging admit
that rod is more durable. It's also a supperior system due to lower
weight. Just take care of it. Surveyors who have no market interest in
selling either also say rod is longer lived.
This guy is some engineer.

RB
35s5
NY

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

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

Bob Cranz:

Do you do NDT?
(Non-Destructive Testing)

Not primarily, electromagnetics R&D. DC - Millimeter Wave, IR-visible. Lots
of sensor work.

Amen!




wrote in message
oups.com...
Bob Cranz:

Do you do NDT?
(Non-Destructive Testing)

Cool, I used to do some IR sensor work on Star Wars, now I do x-ray
optics.