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WesDyne is an engineering and technical services firm dedicated to providing standard and custom nondestructive examinations (NDE) products and systems
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Technical Discussions
Charles
Charles
13:17 Apr-09-2017
General query on Linear scan vs Dynamic depth focusing

Hi,

I'm pretty new to NDT and I have a question that needs answering!

When comparing Linear scan (Plane B-scan) against Dynamic depth focusing (DDF or Focused B-scan), why is it that the side lobes are significantly reduced in DDF?

Regards,
Charles

 
 Reply 
 
Charles
Charles
13:19 Apr-09-2017
Re: General query on Linear scan vs Dynamic depth focusing
In Reply to Charles at 13:17 Apr-09-2017 (Opening).

Also, why is it that sectorial scans have significant presence of side lobes too?

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1300

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1300
17:21 Apr-09-2017
Re: General query on Linear scan vs Dynamic depth focusing
In Reply to Charles at 13:19 Apr-09-2017 .

Charles, DDF is a processing algorithm that sums the received signal, therefore it is not actually eliminating "side-lobes" but is instead limiting the contributions of off-axis paths.
As for your second question, perhaps you are confusing "side-lobes" with "grating-lobes". Side-lobes are a function of the aperture to wavelength ratio so not usually a consideration in S-scans any more than other scan-types. However, if you attempt to over-extend the recommended steering limits of a good probe design, you could start to observe artefacts that result from grating lobes. In NDT.net, there is a paper and associated video that explains the formation of grating lobes at http://www.ndt.net/article/ndtnet/2015/2_Ginzel.pdf

 
 Reply 
 
Charles
Charles
11:05 Apr-10-2017
Re: General query on Linear scan vs Dynamic depth focusing
In Reply to Ed Ginzel at 17:21 Apr-09-2017 .

Hi Ed,

Thank you for your reply. But I think I did not clarify my question properly. What I meant was for the B-scan results from Linear Scan against DDF.

Here's a link for the B-scans I simulated in MATLAB: http://imgur.com/VSBr3Hv

Are the areas represented in light blue and green side lobes? If so, why is there such a difference between the two algorithms?

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1300

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1300
14:22 Apr-10-2017
Re: General query on Linear scan vs Dynamic depth focusing
In Reply to Charles at 11:05 Apr-10-2017 .

Charles, I did not see much evidence for "lobes" in your model, however, I did see evidence of reduced off-axis sensitivity in the DDF so the beam width appears to be reduced. Your modelled example is of a single point in space with the array passing over it with a lateral motion. A good example of how the DDF function is applied over multiple distances is seen on the NDT.net lexicon website http://www.ndt.net/ndtaz/content.php?id=693
The wavefront emitted from a probe is an arc so when you move along the soundpath beyond the near zone, the lobe contributions are minimal and the main reason for the beam width is beam divergence. The light blue and green components are therefore just the off-axis beam dimensions.

 
 Reply 
 
Charles
Charles
12:11 Apr-13-2017
Re: General query on Linear scan vs Dynamic depth focusing
In Reply to Ed Ginzel at 14:22 Apr-10-2017 .

Thanks Ed, I'll note this down.

I'm currently working on an article done by Holmes et. al. "Post-processing of the full matrix of ultrasonic transmit-receive array data for non-destructive evaluation. I'm facing a huge dilemma with the calculation for Near field distance.

Here's the paper: https://1drv.ms/b/s!AgU5dK8FuGgCnw6GI_QtUtJdSDhZ

With the transducer details in table 1, in Section 3.6 she mentions about near field distance for Figure 5. I've calculated the near field distance to be approximately 3.226e-4m which is about 0.32256mm. Does this value make sense to you? Because it seems kind of weird for the Near field to be at such a short distance with the analysis she made for the Plane B-scan contour map.

Regards,
Charles

 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1300

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1300
15:25 Apr-13-2017
Re: General query on Linear scan vs Dynamic depth focusing
In Reply to Charles at 12:11 Apr-13-2017 .

In the paper you reference, the passive dimension of the array is not provided so I would assume that the estimate of near field is made using the active array aperture. In the case of 8 elements with a pitch of 0.63mm and assuming 6300m/s as the velocity for aluminium, the near zone in the active aperture direction is about 0.5mm. When I model it with a passive width of 6mm the near field distance based on the last maximum extends to about 3.5mm.

There is an odd statement in the paper "The output of each element was a five cycle, Gaussian windowed tone burst with a centre frequency of 5 MHz...". Perhaps this is OK for modelling and it may even be possible with some phased-array electronics. But a 5 cycle toneburst to linear array elements would have significant resolution issues and attempting to obtain good and useful phasal interference of the adjacent pulses could prove tricky too. However, Ms. Holmes identified that these were the "simulated" parameters.

 
 Reply 
 
Henry Herrera
R & D,
UT Quality, Canada, Joined Jun 2000, 19

Henry Herrera

R & D,
UT Quality,
Canada,
Joined Jun 2000
19
18:03 Apr-13-2017
Re: General query on Linear scan vs Dynamic depth focusing
In Reply to Charles at 12:11 Apr-13-2017 .

Charles,
Regarding the calculation of the near field for the experiments in the paper by Holmes et al., 2005.

I think you are just missing a 10^3 product there.
The test you did using the 64 elements in an unfocused transducer will be approximately:
d64 = 64*.63 = 40.32 mm
N64 = (d64^2)/(4*1.26) = 322.56 mm

In section 3.5, they are targeting a reflector at 20*lambda = 25.2 mm, using 16 elements. The unfocused near field is at:
d16 = 16*.63 = 10.08 mm
N16 = (d16^2)/(4*1.26) = 20.16.

These are approximated values, you need to apply correction factors for a rectangular unfocused array.

 
 Reply 
 

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