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Technical Discussions
S. Gripp
S. Gripp
00:03 Jul-08-1999
Squirters on steel

Hi Friends,

Recently I have been struggling with water jet coupling on steel
samples.

Using a near field length distance between probe and surface,
and about 30 mm free water path, it worked pretty well on
very smooth surfaces (4 MHz).
On a surface like rolled or roughly machined state,
I had tremendous reverberations in the water delay, such that I
was unable to examine the bulk of the material.
At a distance of only a few millimeteres between the nozzle outlet
and the specimen surface the ghost echoes suddenly ceased.

Where in hell do these echoes come from,
and how do I get them away ??

Many thanks for any inspiration or reference,
Sebastian


    
 
 
David Hermanutz
Consultant,
Hbndt.com, China, Joined Jul 2012, 85

David Hermanutz

Consultant,
Hbndt.com,
China,
Joined Jul 2012
85
04:22 Jul-11-1999
Re: Squirters on steel
: Hi Friends,

: Recently I have been struggling with water jet coupling on steel
: samples.

: Using a near field length distance between probe and surface,
: and about 30 mm free water path, it worked pretty well on
: very smooth surfaces (4 MHz).
: On a surface like rolled or roughly machined state,
: I had tremendous reverberations in the water delay, such that I
: was unable to examine the bulk of the material.
: At a distance of only a few millimeteres between the nozzle outlet
: and the specimen surface the ghost echoes suddenly ceased.

: Where in hell do these echoes come from,
: and how do I get them away ??

: Many thanks for any inspiration or reference,
: Sebastian

Consider the backpressure from the escaping water out of the pressure stream afer it hits the surface roughness.
At a distance of 3 mm there is enough head pressure to push the back interference "reverb" pressure waves away from the soundpath.
Try buffing the surface (if practicable), awetting agent applied to the test surface, or increased pressure in your water jet.

Please write back with any positive results as I haven't tried my solutions yet.

David Hermanutz
Pilbara NDT
Melbourne, Australia


    
 
 
tadahiro nomura
tadahiro nomura
00:50 Jul-14-1999
Re: Squirters on steel
: Hi Friends,

: Recently I have been struggling with water jet coupling on steel
: samples.

: Using a near field length distance between probe and surface,
: and about 30 mm free water path, it worked pretty well on
: very smooth surfaces (4 MHz).
: On a surface like rolled or roughly machined state,
: I had tremendous reverberations in the water delay, such that I
: was unable to examine the bulk of the material.
: At a distance of only a few millimeteres between the nozzle outlet
: and the specimen surface the ghost echoes suddenly ceased.

: Where in hell do these echoes come from,
: and how do I get them away ??

: Many thanks for any inspiration or reference,
: Sebastian




    
 
 
tadahiro nomura
tadahiro nomura
00:54 Jul-14-1999
Re: Squirters on steel
Dear.Mr Hermanutz,
Let me interrupt halfway.
I am suffering from the problem which is the same, too.

What kind of thing is as for wetting agent, for example there?
Doesn't it disappear soon with water jet?
/--
Tadahiro Nomura
t-nomura@tokimec.co.jp
--/

: : Hi Friends,

: : Recently I have been struggling with water jet coupling on steel
: : samples.

: : Using a near field length distance between probe and surface,
: : and about 30 mm free water path, it worked pretty well on
: : very smooth surfaces (4 MHz).
: : On a surface like rolled or roughly machined state,
: : I had tremendous reverberations in the water delay, such that I
: : was unable to examine the bulk of the material.
: : At a distance of only a few millimeteres between the nozzle outlet
: : and the specimen surface the ghost echoes suddenly ceased.

: : Where in hell do these echoes come from,
: : and how do I get them away ??

: : Many thanks for any inspirationor reference,
: : Sebastian

: Consider the backpressure from the escaping water out of the pressure stream afer it hits the surface roughness.
: At a distance of 3 mm there is enough head pressure to push the back interference "reverb" pressure waves away from the soundpath.
: Try buffing the surface (if practicable), a wetting agent applied to the test surface, or increased pressure in your water jet.

: Please write back with any positive results as I haven't tried my solutions yet.

: David Hermanutz
: Pilbara NDT
: Melbourne, Australia





    
 
 
John Rutter
John Rutter
08:05 Jul-15-1999
Re: Squirters on steel
Is the angle of incidence (of the water jet) exactly 90 degrees? Certainly the air:water interface will give more and more reflections as the angle varies from perpendicular, especially as distance increases and diameter of water jet decreases.

For what it's worth, you might want to try a wetting agent in order to assure rapid wetting out of rough surfaces. This probably won't affect reverberation though. Sonotech has a Boeing approved wetting agent called Echowet. (Fastest way to navigate to individual product information is through the site index.)

Sorry I don't have much else to suggest. . .

-John



    
 
 
S. Gripp
S. Gripp
03:27 Jul-21-1999
Re: Squirters on steel
First, many thanks for your attention !

In fact, the angle of incidence was NOT exactly 90 deg.,
that's why I used squirters.

With other parts such as airplane structures I didn't encouter this problem.
And from where does the "ghost echo" energy come from ?
In case it is reflected from the sides of the water jet,
why is the surface condition of such high importance ?

For flat smooth parts (CFK panels) I never encountered strong reverberations
from the interface with a strong dependence of the incidence angle
or surface roughness.

Pulse repetition rate or echoes from the second round trip through the water
are also excluded.
So I still don't know where the signals come from.

Any suggestions ?


    
 
 
S. Gripp
S. Gripp
03:43 Jul-21-1999
Re: Squirters on steel
Dear.Mr Hermanutz and Mr. Nomura,

first, thanks for your attention !

in the present application, surface treatment is not possible (oh dear! Always the same story!!)

wetting agent and high pressure seem quite promising to me, so I will inform you about the results!

What I still don't understand is the fact that I never
had these problems on CFK, which is usually also of the kind of "not-too-smooth" surface (due to the
ply-off-skin)and also with more or less oblique angles of incidence (a few degrees do not really matter)

Any more ideas ?


    
 
 

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