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Technical Discussions
Massimiliano Pau
Teacher, -
Univ. of Cagliari, Dept. of Mechanical Engineering, Italy, Joined Sep 2002, 25

Massimiliano Pau

Teacher, -
Univ. of Cagliari, Dept. of Mechanical Engineering,
Italy,
Joined Sep 2002
25
07:00 Nov-02-2000
Sound field in water/steel transition

Dear colleagues,

I've recently characterized my Krautkramer focused probe (10 MHz frequency
, 0.25" diameter, 2.5" focus length) by means of the ASTM 1065 test
with the ball target method (1.5 mm steel sphere) in order to obtain the
cross profiles from which to deduce the sound beam diameter.

My question is: since the ASTM test is performed in water, what will
happen when the sound beam passes from water to another material
(such as steel for example as happens in my experimental tests)?

In some book I've found that such a transition would produce
a sort of further focalization, but there is no trace of
quantitative evaluation of this phenomenon.

Any suggestion ?( bibligraphic references are also welcome)

Massimiliano Pau





 
 Reply 
 
Paul A. Meyer
R & D,
GE Inspection Technologies, USA, Joined Nov 1998, 47

Paul A. Meyer

R & D,
GE Inspection Technologies,
USA,
Joined Nov 1998
47
07:19 Nov-02-2000
Re: Sound field in water/steel transition
Hi Massimiliano,
Just as in optics sound is refracted at a material boundary. If you imagine the converging rays of a focused sound field impinging a material boundary, the rays will refracted, either inward or outward, depending on the acoustic velocity of the two materials.
This is discussed in more detail in ULTRASONIC TESTING OF MATERIALS by Krautkramer and Krautkramer. See the chapter on Geometric Ultrasonic Optics.
Regards,
Paul

: Dear colleagues,

: I've recently characterized my Krautkramer focused probe (10 MHz frequency
: , 0.25" diameter, 2.5" focus length) by means of the ASTM 1065 test
: with the ball target method (1.5 mm steel sphere) in order to obtain the
: cross profiles from which to deduce the sound beam diameter.

: My question is: since the ASTM test is performed in water, what will
: happen when the sound beam passes from water to another material
: (such as steel for example as happens in my experimental tests)?

: In some book I've found that such a transition would produce
: a sort of further focalization, but there is no trace of
: quantitative evaluation of this phenomenon.

: Any suggestion ?( bibligraphic references are also welcome)

: Massimiliano Pau





 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1282

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1282
04:34 Nov-06-2000
Re: Sound field in water/steel transition
Massimo:
When I looked at the numbers for the probe you are using I was very confused. A Flat 10MHz 6mm diamter probe radiating into water has a natural focal length (the end of the near zone) of 60mm. The radius of curvature you stated is 2.5" or about 63mm. This means that you are trying to focus the beam at a point further away than its near zone. THIS IS NOT POSSIBLE!

The effect of a large radius on the element will be to smooth some of the outer side lobes but it will not change the focal size or position.

Your measurement of 1.1mm is slightly less than the calculated ideal. For the flat 10MHz 6mm diameter probe the Peak Signal occurs at 59.9mm in water with a velocity of 1500m/s and the 6dB spot diameter is 2.4mm.

If I replace the water with steel (5900m/s) the peak will occur at 15.2mm and the spot size will be 2.4mm.

There seems to be no reduction in spot size base on the velocity of the material. In fact, probe manufacturers have always called this the 40% rule, which means for a flat element the best spot size that could be achieved is 40% of the pelement diameter.

If, however, the radius of curvature of the probe was reduced to well inside the natural near zone you do get a reduction in spot size. Just using a 50mm radius of curvature the spot size in water goes to 1.2mm at 49.4mm waterpath. When I replace the water with steel my calculation breaks down again because the natural focus in steel is 15.2mm and a radius of 50mm is not focusing the beam shorter than the near field in steel.

What you also ask is the effect of ray trace focusing on the beam where part is in water and part in steel. If we assume that the beam is in fact now double focused (once by radius of curvature and once by refraction) then I believe that the spot size will in fact be reduced from the original calculated size in water. I do not know how much, however, the effects of focusing closer and closer for a single medium seem to be that the spot size continues to reduce. Therefore I would expect a similar effect as the water path decreases and the beam is brought closer to the steel.

Finally, you state that you want an "exact" diameter. The calculation i made uses the assumption that I have a monochromatic sound source (one frequency). The reality is we have bandwidth. This will blur the spot to a "fuzzy zone". I think the best you can do is to put a small target in a sample of steel (using Electro-discharge machining -EDM) and repeat the ASTM scanning to see if there is a difference in echo response.

Ed


: Hi Massimiliano,
: Just as in optics sound is refracted at a material boundary. If you imagine the converging rays of a focused sound field impinging a material boundary, the rays will refracted, either inward or outward, depending on the acoustic velocity of the two materials.
: This is discussed in more detail in ULTRASONIC TESTING OF MATERIALS by Krautkramer and Krautkramer. See the chapter on Geometric Ultrasonic Optics.
: Regards,
: Paul

: : Dear colleagues,

: : I've recently characterized my Krautkramer focused probe (10 MHz frequency
: : , 0.25" diameter, 2.5" focus length) by means of the ASTM 1065 test
: : with the ball target method (1.5 mm steel sphere) in order to obtain the
: : cross profiles from which to deduce the sound beam diameter.

: : My question is: since the ASTM test is performed in water, what will
: : happen when the sound beam passes from water to another material
: : (such as steel for example as happens in my experimental tests)?

: : In some book I've found that such a transition would produce
: : a sort of further focalization, but there is no trace of
: : quantitative evaluation of this phenomenon.

: : Any suggestion ?( bibligraphic references are also welcome)

: : Massimiliano Pau





 
 Reply 
 
Bhanu
Bhanu
09:29 Mar-01-2001
Re: Sound field in water/steel transition
If I understand clearly, you mean to inquire about the characterstic of a wave entering a steel test piece immersed in water.v=0.23 in/msec
I such a case the sound wace is focussed nearer to the transducer in the test piece than would be focussed in water.
There is a formula to calculate the depth of focalpoint within the test part and the water path distance.
: Massimo:
: When I looked at the numbers for the probe you are using I was very confused. A Flat 10MHz 6mm diamter probe radiating into water has a natural focal length (the end of the near zone) of 60mm. The radius of curvature you stated is 2.5" or about 63mm. This means that you are trying to focus the beam at a point further away than its near zone. THIS IS NOT POSSIBLE!

: The effect of a large radius on the element will be to smooth some of the outer side lobes but it will not change the focal size or position.

: Your measurement of 1.1mm is slightly less than the calculated ideal. For the flat 10MHz 6mm diameter probe thePeak Signal occurs at 59.9mm in water with a velocity of 1500m/s and the 6dB spot diameter is 2.4mm.

: If I replace the water with steel (5900m/s) the peak will occur at 15.2mm and the spot size will be 2.4mm.

: There seems to be no reduction in spot size base on the velocity of the material. In fact, probe manufacturers have always called this the 40% rule, which means for a flat element the best spot size that could be achieved is 40% of the pelement diameter.

: If, however, the radius of curvature of the probe was reduced to well inside the natural near zone you do get a reduction in spot size. Just using a 50mm radius of curvature the spot size in water goes to 1.2mm at 49.4mm waterpath. When I replace the water with steel my calculation breaks down again because the natural focus in steel is 15.2mm and a radius of 50mm is not focusing the beam shorter than the near field in steel.

: What you also ask is the effect of ray trace focusing on the beam where part is in water and part in steel. If we assume that the beam is in fact now double focused (once by radius of curvature and once by refraction) then I believe that the spot size will in fact be reduced from the original calculated size in water. I do not know how much, however, the effects of focusing closer and closer for a single medium seem to be that the spot size continues to reduce. Therefore I would expect a similar effect as the water path decreases and the beam is brought closer to the steel.

: Finally, you state that you want an "exact" diameter. The calculation i made uses the assumption that I have a monochromatic sound source (one frequency). The reality is we have bandwidth. This will blur the spot to a "fuzzy zone". I think the best you can do is to put a small target in a sample of steel (using Electro-discharge machining -EDM) and repeat the ASTM scanning to see if there is a difference in echo response.

: Ed


:
: : Hi Massimiliano,
: : Just as in optics sound is refracted at a material boundary. If you imagine the converging rays of a focused sound field impinging a material boundary, the rays will refracted, either inward or outward, depending on the acoustic velocity of the two materials.
: : This is discussed in more detail in ULTRASONIC TESTING OF MATERIALS by Krautkramer and Krautkramer. See the chapter on Geometric Ultrasonic Optics.
: : Regards,
: : Paul

: : : Dear colleagues,

: : : I've recently characterized my Krautkramer focused probe (10 MHz frequency
: : : , 0.25" diameter, 2.5" focus length) by means of the ASTM 1065 test
: : : with the ball target method (1.5 mm steel sphere) in order to obtain the
: : : cross profiles from which to deduce the sound beam diameter.

: : : My question is: since the ASTM test is performed in water, what will
: : : happen when the sound beam passes from water to another material
: : : (such as steel for example as happens in my experimental tests)?

: : : In some book I've found that such a transition would produce
: : : a sort of further focalization, but there is no trace of
: : : quantitative evaluation of this phenomenon.

: : : Any suggestion ?( bibligraphic references are also welcome)

: : : Massimiliano Pau





 
 Reply 
 
Massimiliano Pau
Teacher, -
Univ. of Cagliari, Dept. of Mechanical Engineering, Italy, Joined Sep 2002, 25

Massimiliano Pau

Teacher, -
Univ. of Cagliari, Dept. of Mechanical Engineering,
Italy,
Joined Sep 2002
25
09:49 Mar-02-2001
Re: Sound field in water/steel transition
: If I understand clearly, you mean to inquire about the characterstic of a wave entering a steel test piece immersed in water.v=0.23 in/msec
: I such a case the sound wace is focussed nearer to the transducer in the test piece than would be focussed in water.
: There is a formula to calculate the depth of focalpoint within the test part and the water path distance.

Not exactly. The problem (still unresolved) is to calculate the beam diameter size
after a water->steel transition
The only sure thing is a reduction of the size. But how much?

Massimiliano



 
 Reply 
 
M.W. Moyer
M.W. Moyer
00:48 Mar-02-2001
Re: Sound field in water/steel transition
The beam diameter doesn't change between water and steel. Because the steel has a higher velocity, the focal length is shortened by the same amount.
The estimate of -6db beam diameter is given by the equation:
BD= 1.02 * Focal Length * vel /( frequency * diameter)
If the velocity doubles, then the focal length halves so the beam diameter remains the same.
In special cases with high velocity materials and short focal lengths, the longitudinal energy from the edge of the transducer can be
mode converted to shear which effectively reduces the diameter of the transducer, increasing the effective beam diameter.

M. W. Moyer

: : If I understand clearly, you mean to inquire about the characterstic of a wave entering a steel test piece immersed in water.v=0.23 in/msec
: : I such a case the sound wace is focussed nearer to the transducer in the test piece than would be focussed in water.
: : There is a formula to calculate the depth of focalpoint within the test part and the water path distance.
.
: Not exactly. The problem (still unresolved) is to calculate the beam diameter size
: after a water->steel transition
: The only sure thing is a reduction of the size. But how much?
.
: Massimiliano
.



 
 Reply 
 
S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
02:01 Mar-02-2001
Re: Sound field in water/steel transition
Theoretical calculations may be difficult, but a geometric ray tracing
technique can be attempted to try and find the spot size. Another laborious
but practical way is to drill different size reflectors (Flat-Bottomed
Holes) of different diameters but to the same depth in a block of steel and
see where the echo height becomes constant, since once the area of the
reflector is equal or more than the beam diameter, the echo height will
remain constant. Though laborious, the technique is simple to implement and
is certain to give you the desired information. The problem is to first
estimate the depth of focussing but that is relatively simple - Swamy

---------------

: : If I understand clearly, you mean to inquire about the characterstic of a wave entering a steel test piece immersed in water.v=0.23 in/msec
: : I such a case the sound wace is focussed nearer to the transducer in the test piece than would be focussed in water.
: : There is a formula to calculate the depth of focalpoint within the test part and the water path distance.
.
: Not exactly. The problem (still unresolved) is to calculate the beam diameter size
: after a water->steel transition
: The only sure thing is a reduction of the size. But how much?
.
: Massimiliano
.



 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1282

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1282
08:57 Mar-21-2001
Re: Sound field in water/steel transition

Dear Forum members:

On 2000.11.02 Massimiliano Pau posted a question about a beam spot size. Paul Meyer suggested a ray tracing option as in optics and I used an old software programme to calculate the spot sizes. On March 1 2001, Bhanu suggested that there is a formula for such calculations. Dr. Fred Hotchkiss from Panametrics then contacted me to point out that the formulae relating to this were posted on the Panametrics website and the results seemed to differ from what I had posted in the forum.

I returned to the problem and made some further observations. The biggest problem I made was the way in which I interpreted the output from the software I used. I provided the programme output for the 6dB drop for the one-way path. When measuring the 6dB drop in pulse-echo this is equivalent to the 3dB drop in the one-way path. Dr. Hotchkiss pointed out that my calculations for spot diameter were closer to the 12dB drop and that the formulae would indicate that the focal spot size will remain the same in steel and water (although occur at a different path position).

To correct my error I have modelled the beam and made "amplitude slices" to measure the beam dimensions for the 3dB drops at the appropriate focal spot axial positions. The outputs are provided in the attached document (http://www.ndt.net/wshop/attach/model.htm) and agree well with Dr. Hotchkiss instructions.

My apoilogies to all (especially Massimiliano) for the mis-information.

Ed



 
 Reply 
 

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