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Technical Discussions
Malcolm Maclean
Malcolm Maclean
09:33 Jan-17-1999
Velocity of sound through coatings

Hi again,
Could anyone guide me to information on the velocity of sound through any paint coatings and/or the rate of change of velocity with temperature (dv/dT)in the coatings. I need the information for my thesis study on environmetal effects on the accuracy of UT thickness readings. Some of you may remember my plea for help regarding information on the effects of temperature on probes earlier on this year, all replies were really useful and appreciated. I am aware there are techniques and equipment that can virtually eliminate the coating from the thickness readings and temperature effects, but I would like to assess the effects theoretically but have exhausted my resources for this information.

Thanks for reading.

Malcolm mmaclean@email.menet.net



    
 
 
Rolf Diederichs
Director,
NDT.net, Germany, Joined Nov 1998, 604

Rolf Diederichs

Director,
NDT.net,
Germany,
Joined Nov 1998
604
02:47 Jan-17-1999
Re: Velocity of sound through coatings
: Hi again,
: Could anyone guide me to information on the velocity of sound through any paint coatings and/or the rate of change of velocity with temperature (dv/dT)in the coatings. I need the information for my thesis study on environmetal effects on the accuracy of UT thickness readings. Some of you may remember my plea for help regarding information on the effects of temperature on probes earlier on this year, all replies were really useful and appreciated. I am aware there are techniques and equipment that can virtually eliminate the coating from the thickness readings and temperature effects, but I would like to assess the effects theoretically but have exhausted my resources for this information.

: Thanks for reading.

: Malcolm mmaclean@email.menet.net
------------------------

I assume that you want to measure the thickness of the parent material only.
If you start the TOF measurement on the coating/parent interface you must not be
worry of change of velocity with temperature in the coating.
Precondition that not much lost of echo amplitude occures by increasing of
attenuation in the coating - see for more details my reply to
'Temperature effects on UT Thickness readings research - Malcolm Maclean
18:59:55 1/02/99'

Rolf



    
 
 
Godfrey Hands
Engineering,
PRI Nadcap, United Kingdom, Joined Nov 1998, 291

Godfrey Hands

Engineering,
PRI Nadcap,
United Kingdom,
Joined Nov 1998
291
09:00 Jan-18-1999
Re: Velocity of sound through coatings

: Hi again,
: Could anyone guide me to information on the velocity of sound through any paint coatings and/or the rate of change of velocity with temperature (dv/dT)in the coatings. I need the information for my thesis study on environmetal effects on the accuracy of UT thickness readings. Some of you may remember my plea for help regarding information on the effects of temperature on probes earlier on this year, all replies were really useful and appreciated. I am aware there are techniques and equipment that can virtually eliminate the coating from the thickness readings and temperature effects, but I would like to assess the effects theoretically but have exhausted my resources for this information.

: Thanks for reading.

: Malcolm mmaclean@email.menet.net

The use of A scan displays will allow you to measure the thickness between multiple echoes from the base material, ignoring the thickness of the coatings.

Regards,
Godfrey Hands



    
 
 
Malcolm Maclean
Malcolm Maclean
01:40 Jan-18-1999
In reply to all
: Hi again,
: Could anyone guide me to information on the velocity of sound through any paint coatings and/or the rate of change of velocity with temperature (dv/dT)in the coatings. I need the information for my thesis study on environmetal effects on the accuracy of UT thickness readings. Some of you may remember my plea for help regarding information on the effects of temperature on probes earlier on this year, all replies were really useful and appreciated. I am aware there are techniques and equipment that can virtually eliminate the coating from the thickness readings and temperature effects, but I would like to assess the effects theoretically but have exhausted my resources for this information.

: Thanks for reading.

: Malcolm mmaclean@email.menet.net
Thanks for the replys Rolf & Godfrey and your email Ed.
Rolf, regarding the trigger point on the leading edge of the echo...I noticed the increase in attenuation during my tests when heating up the test piece. To eliminate this problem Iincreased the gain to maintain the 1st BWE at full screen height, thus keeping the trigger at the same level on the echo to eliminate the error. I only wish I had one of the UT sets that you mentioned that does this gain control automatically(can tell me who makes them?).
Continuing on, I am sorry but I don't think I have structured my original question properly (bad habit of mine). I appreciate that reading between multiple echoes on the A scan will eliminate the coating thickness, and I have been guided to manufactures who have designed equipment to do this automatically. I have no problem with that and have included this technique in my tests to great success, also for eliminating the effects of temperature on the probe shoe/delay material.
However, what I am investigating is the effects of the paint when reading from the "1st BWE". I know the paint acts as a delay line but I need to justify the effect ot has on the thickness readings and to do this I need to know the velocity of sound through the paint and any dv/dT values. I would imagine it will likely vary depending on the type of paint used eg. epoxy based, chlorinated rubber etc. I cannot find any velocities for paint in any of the many tables I have, journal articles, text books or other papers. This information will be used to justify why reading between multiple echos is required for thickness checking through coatings.
I hope this has clarified my question a bit better.
Thanks for your patience with my question.
Regards
Malcolm





    
 
 
Norm Woodward
Norm Woodward
09:06 Jan-19-1999
Re: Velocity of sound through coatings(sound measurements in general)
:
: : Hi again,
: : Could anyone guide me to information on the velocity of sound through any paint coatings and/or the rate of change of velocity with temperature (dv/dT)in the coatings. I need the information for my thesis study on environmetal effects on the accuracy of UT thickness readings. Some of you may remember my plea for help regarding information on the effects of temperature on probes earlier on this year, all replies were really useful and appreciated. I am aware there are techniques and equipment that can virtually eliminate the coating from the thickness readings and temperature effects, but I would like to assess the effects theoretically but have exhausted my resources for this information.

: : Thanks for reading.

: : Malcolm mmaclean@email.menet.net

: The use of A scan displays will allow you to measure the thickness between multiple echoes from the base material, ignoring the thickness of the coatings.

: Regards,
: Godfrey Hands

I am often surprized how many of my colleagues will search the literature endlessly for sound velocity data when they have the material, calipers/micrometers, and a calibrated ultrasonic unit, on hand.

On some of the new units (with digital thickness guages), all you have to do is calibrate on a known reference material, then set up on the echos of your unknown, and dial in its measured thickness. The machine will then determine, and display, the velocity. Of course, with the older systems, you will have to do the paperwork yourself. But you must remember that the A-scan is a measure of time, so if you measure your coating thickness, you should be able to determine the velocity data, algebraicly.

As to measuring at higher temps, I think we went down that trail before.


    
 
 
Richard Freemantle
Richard Freemantle
01:33 Jan-20-1999
Re: Velocity of sound through coatings(sound measurements in general)
: I am often surprized how many of my colleagues will search the literature endlessly for sound velocity data when they have the material, calipers/micrometers, and a calibrated ultrasonic unit, on hand.

: On some of the new units (with digital thickness guages), all you have to do is calibrate on a known reference material, then set up on the echos of your unknown, and dial in its measured thickness. The machine will then determine, and display, the velocity. Of course, with the older systems, you will have to do the paperwork yourself. But you must remember that the A-scan is a measure of time, so if you measure your coating thickness, you should be able to determine the velocity data, algebraicly.

: As to measuring at higher temps, I think we went down that trail before.

Futher to above you really need to take into account the thickness of the coatings which I don't think has been stated. Some years ago I did a lot of work on pulse echo reflection coefficient measurments on painted metal substrates. These had coating thicknesses ranging from 7 to 40 microns. The substrates often had a total of four or five different coatings. The effect on thickness measurement was often tolerable but the effect on reflection coefficient measurement (ie from the backwall echoes in the metal substrate) was very significant.
I needed to make velocity measurments on the paint layer to work out the effect of the coatings on the main echoes sequence in the metal substrate. This was not easy, particularly with standard NDT probes since the delay in the coating was often very much less than a wavelength. Seeing this delay on a standard A scan using, for example, a 20 MHz highly damped probe was not easy. In the end I resorted to using very short duration pulses (< 10 ns) excited from a thickness mode PZT element and did the velocity measurements in
the frequency domain. Returning to the orginal posting by Malcolm, I will try and dig out the paint types, and the velocity and density values that I measured, hopefully this will give you a feel for what velocity values you could expect in the paint coatings.

Regards,

Richard Freemantle


    
 
 
Norm Woodward
Norm Woodward
05:51 Jan-20-1999
Re: Velocity of sound through coatings(sound measurements in general)

: Futher to above you really need to take into account the thickness of the coatings which I don't think has been stated. Some years ago I did a lot of work on pulse echo reflection coefficient measurments on painted metal substrates. These had coating thicknesses ranging from 7 to 40 microns. The substrates often had a total of four or five different coatings. The effect on thickness measurement was often tolerable but the effect on reflection coefficient measurement (ie from the backwall echoes in the metal substrate) was very significant.
: I needed to make velocity measurments on the paint layer to work out the effect of the coatings on the main echoes sequence in the metal substrate. This was not easy, particularly with standard NDT probes since the delay in the coating was often very much less than a wavelength. Seeing this delay on a standard A scan using, for example, a 20 MHz highly damped probe was not easy. In the end I resorted to using very short duration pulses (< 10 ns) excited from a thickness mode PZT element and did the velocity measurements in
: the frequency domain. Returning to the orginal posting by Malcolm, I will try and dig out the paint types, and the velocity and density values that I measured, hopefully this will give you a feel for what velocity values you could expect in the paint coatings.

: Regards,

: Richard Freemantle

I can understand the limitations of trying to determine the velocity of sound of a coating on an actual part; the thicknesses would (or should)likely be less than 75 microns (three mils.) However, if one had access to the original raw materials, as I alluded to in my original post, it would seem that one could carefully make up a useful specimen by keep adding and drying/curing coats until the build-up produced a measurable sound path.

BTW, my original remarks were a general observation, not an attack on Malcolm's approach. It seems that often we hope that NIST or Krautkramer, et al, will hand us ultrasonic data on some fairly exotic material, when most of us have the materials and equipment to do it ourselves, at least to a sufficient degree of accuracy.

Norm Woodward


    
 
 
Malcolm Maclean
Malcolm Maclean
02:04 Jan-20-1999
Re: Velocity of sound through coatings(sound measurements in general)

: : Futher to above you really need to take into account the thickness of the coatings which I don't think has been stated. Some years ago I did a lot of work on pulse echo reflection coefficient measurments on painted metal substrates. These had coating thicknesses ranging from 7 to 40 microns. The substrates often had a total of four or five different coatings. The effect on thickness measurement was often tolerable but the effect on reflection coefficient measurement (ie from the backwall echoes in the metal substrate) was very significant.
: : I needed to make velocity measurments on the paint layer to work out the effect of the coatings on the main echoes sequence in the metal substrate. This was not easy, particularly with standard NDT probes since the delay in the coating was often very much less than a wavelength. Seeing this delay on a standard A scan using, for example, a 20 MHz highly damped probe was not easy. In the end I resorted to using very short duration pulses (< 10 ns) excited from a thickness mode PZT element and did the velocity measurements in
: : the frequency domain. Returning to the orginal posting by Malcolm, I will try and dig out the paint types, and the velocity and density values that I measured, hopefully this will give you a feel for what velocity values you could expect in the paint coatings.

: : Regards,

: : Richard Freemantle

: I can understand the limitations of trying to determine the velocity of sound of a coating on an actual part; the thicknesses would (or should)likely be less than 75 microns (three mils.) However, if one had access to the original raw materials, as I alluded to in my original post, it would seem that one could carefully make up a useful specimen by keep adding and drying/curing coats until the build-up produced a measurable sound path.

: BTW, my original remarks were a general observation, not an attack on Malcolm's approach. It seems that often we hope that NIST or Krautkramer, et al, will hand us ultrasonic data on some fairly exotic material, when most of us have the materials and equipment to do it ourselves, at least to a sufficient degree of accuracy.

: Norm Woodward

Norm,

No offence taken, I am still learning. You have to understand that my field experience limited me to carrying out thickness checks and nothing more elaborate, so I hope you can be patient with me.

I have four 20mm thick steel specimens each with 1000 microns coating (measured mechanically) of different types of paint. Rather than making up more specimens by building up measurable coats, can I run this one by you for your comments.

ASTM E 494 (X2.7.6.3 - Pulse echo twin probe method) Calculates unknown velocity (V?) as
V? = Vsteel x (actual thickness / indicated thickness).

If for a 20mm thk steel sample with 1mm paint, my indicated ultrasonic thickness for example was 23mm, would I be right by saying the indicated thickness of the paint was 3mm and therefore the velocity for the coating will be:-

Vpaint = Vsteel x ( 1mm / 3mm)?

Would this be a correct approximation? I am a bit limited with probes at the moment, the highest frequency twin I have is 5Mhz and the highest single is 10Mhz (without delay). So this may restrict me for other methods of measurement.

Mal


    
 
 
Norm Woodward
Norm Woodward
03:10 Jan-20-1999
Re: Velocity of sound through coatings(sound measurements in general)
:
: : : Futher to above you really need to take into account the thickness of the coatings which I don't think has been stated. Some years ago I did a lot of work on pulse echo reflection coefficient measurments on painted metal substrates. These had coating thicknesses ranging from 7 to 40 microns. The substrates often had a total of four or five different coatings. The effect on thickness measurement was often tolerable but the effect on reflection coefficient measurement (ie from the backwall echoes in the metal substrate) was very significant.
: : : I needed to make velocity measurments on the paint layer to work out the effect of the coatings on the main echoes sequence in the metal substrate. This was not easy, particularly with standard NDT probes since the delay in the coating was often very much less than a wavelength. Seeing this delay on a standard A scan using, for example, a 20 MHz highly damped probe was not easy. In the end I resorted to using very short duration pulses (< 10 ns)excited from a thickness mode PZT element and did the velocity measurements in
: : : the frequency domain. Returning to the orginal posting by Malcolm, I will try and dig out the paint types, and the velocity and density values that I measured, hopefully this will give you a feel for what velocity values you could expect in the paint coatings.

: : : Regards,

: : : Richard Freemantle

: : I can understand the limitations of trying to determine the velocity of sound of a coating on an actual part; the thicknesses would (or should)likely be less than 75 microns (three mils.) However, if one had access to the original raw materials, as I alluded to in my original post, it would seem that one could carefully make up a useful specimen by keep adding and drying/curing coats until the build-up produced a measurable sound path.

: : BTW, my original remarks were a general observation, not an attack on Malcolm's approach. It seems that often we hope that NIST or Krautkramer, et al, will hand us ultrasonic data on some fairly exotic material, when most of us have the materials and equipment to do it ourselves, at least to a sufficient degree of accuracy.

: : Norm Woodward

: Norm,

: No offence taken, I am still learning. You have to understand that my field experience limited me to carrying out thickness checks and nothing more elaborate, so I hope you can be patient with me.

: I have four 20mm thick steel specimens each with 1000 microns coating (measured mechanically) of different types of paint. Rather than making up more specimens by building up measurable coats, can I run this one by you for your comments.

: ASTM E 494 (X2.7.6.3 - Pulse echo twin probe method) Calculates unknown velocity (V?) as
: V? = Vsteel x (actual thickness / indicated thickness).

: If for a 20mm thk steel sample with 1mm paint, my indicated ultrasonic thickness for example was 23mm, would I be right by saying the indicated thickness of the paint was 3mm and therefore the velocity for the coating will be:-

: Vpaint = Vsteel x ( 1mm / 3mm)?

: Would this be a correct approximation? I am a bit limited with probes at the moment, the highest frequency twin I have is 5Mhz and the highest single is 10Mhz (without delay). So this may restrict me for other methods of measurement.


: Mal

My experience with dual or twin transducers is somewhat limited, but the method, and calculations, seem correct, and the answer seems plausible. The velocity may seem smaller than the literature values for various polymers, but there may be entrapped solvents and partially-cured vehicle in the coatings which would produce the lesser, but correct, value.

I hope the data will be accurate enough for your subsequent calculations and analyses.

Norm


    
 
 
Tomaz Godicelj
Tomaz Godicelj
02:54 Apr-12-2000
Uncontact measurment of alumunium narrow strip
We are producing Al narrow strip with dimensions as follows:
- width: 223 mm
- thickness: from 2,00 to 10,00 mm (tolerance +- 0,03mm)
We want to measure the thickness permanently during production (after cold rolling).
During cold rolling are present vibratoins (+- 2mm).
If you have any solution, please let me know.
Tomaz Godicelj


    
 
 

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