DGS OF ULTRASONICS Evulation

8/10/2019 DGS OF ULTRASONICS Evulation

1/28

DGS Evaluation


2/28

DGS Evaluation

The DGS (Distance/Gain /Size) system of discontinuity size

evaluation is a technique for estimating the equivalent size of a

reflector, when the reflector is smaller than the ultrasonic

beam.

Consider a probe scanning over three similar discontinuities ofdifferent sizes:


3/28

Reflector Size and Screen Height


4/28

Reflector size and screen height


5/28

Reflector size and screen height


6/28

DGS Evaluation

Reflectors of different size, at the same beam path distance,

will have an echo height proportional to their area, assuming

that the reflectors are in the far zone.

As the area of a circle is proportional to the square if its

diameter, if the reflector is a circular disc, the echo height willbe proportional to the square of the diameter.

This can be confirmed experimentally on area/amplitude test

blocks.

A 6 mm disc should have an amplitude 4 times that of a 3 mm

disc. Now consider a probe scanning over three identical

discontinuities at varying beam path lengths:


7/28

Reflector Depth and Screen Height


8/28



9/28



10/28

Reflectors of the same size will have an echo height inversely

proportional to the square of the beam path distance, assuming

that the reflectors are in the far zone.

This can be confirmed experimentally with distance/amplitude

blocks.

A reflector having an amplitude of 100% at 50 mm will have an

amplitude of 25% at 100 mm.

This of course assumes that the reflectors are in the far zone

and are smaller than the beam diameter.


11/28

When this concept was first established, the general

DGS diagramwas developed as shown below:


12/28

DGS Evaluation

The horizontal axis was the distance (D) measured in near

zones.

The vertical axis was the amplitude of the signal (expressed as

% screen height left hand scale) or Negative Gain, G (right hand

scale).

Each curve represented the characteristic of a particular

reflector diameter (S) with the curves being calculated

mathematically and confirmed experimentally. The size (S) was

expressed as a fraction of the transducer diameter.


13/28

The Generalized DGS Diagram is Useful

And Interesting, but not a Practical Tool

In the early days of development of the DGS approach to DGS

sizing, it was necessary to employ this generalised approach. It

is much more useful, however, to use direct reading scales

which can be fixed to the UFD screen for immediate size

estimation.


14/28



These types of scales are made specifically for a particular

probe (the B4S 4 MHz, 24 mm diameter zero compression

probe) and shows a characteristic curve for 6 mm, 4 mm and 3

mm reflectors.

It also shows a backwall echo (BE) to which the technician sets

the backwall echo as the reference sensitivity.

In the example shown above:

1. The technician sets the screen height from a backwall echo

at 350 mm to the BE point.

2. The echo appearing in the example touches the 4 mm line,so is estimated to have equivalent reflectivity to a 4 mm

diameter disc reflector.


15/28



This technique has

one limitation it

assumes that there

is a backwall

reflector available at

350 mm.

This is not realistic,

so the final

modification to the

DGS scales is to

include acharacteristic curve

for a backwall

reflection.


16/28



This curve shows a family of characteristic curves from 2 mm

to 10 mm equivalent flat bottomed hole (EFBH). There are also

two other curves RE1 and RE2.

These are called the reference echo curves, and are used to set

the correct sensitivity from the backwall.

At shorter beam paths, set the backwall to RE1 and add 16 dB,

then read the equivalent flaw sizes directly from the screen.

At longer beam paths, set the backwall to RE2 and add 8 dB.

See the left hand vertical axis for extra gain required for RE1 an

RE2. This is a typical scale, and the shape of the curves and

reference reflectors will depend on the test range, as well as

the probe diameter and frequency.

DGS scales are specific to each probe specification and test

range.


17/28

Points to Ponder

Why are some DGS curves shown as dotted and wavy lines at

shorter beam paths?

Why are the RE curves a different shape to the EFBH curves?

If you set the backwall on RE1, what extra gain should you

need to increase the screen height to RE2?

If you wanted to increase the range of this curve from 100 mm

to 200 mm, how would you do it?

If you wanted to measure down to 1 mm EFBH using this scale,

how would you do it?


18/28

DGS on Digital Instruments

The advent of digital instruments has made it possible to givethe DGS system much greater flexibility, and most of the newer

digital instruments will have a DGS capacity.


19/28

DGS on Digital Instruments

The benefits of using digital technology include:

greater flexibility in evaluating echoes

the ability to program DGS settings and reuse identical

settings for each probe

the usual advantages of recording raw data, equipmentsettings, beam path and trace details.


20/28

Application of DGS evaluation

DGS evaluation is a very convenient and reproducible

technique, with a number of specific applications. The features

of DGS are:

The sensitivity is set using a back wall echo from the test

material negating the need for a separate reference block

and compensating for transfer/attenuation losses.

The discontinuity must be smaller than the beam. For

convenience, it is generally applied to reflectors less than

the transducer diameter.

Once the reflector is bigger than the beam, it is seen as aninfinite reflector, and DGS measurements are meaningless.


21/28


The technique records the equivalent EFBH reflectivity. The

EFBH is the area of the ideal disc reflector. Reflectivity of a

discontinuity depends on:

size (area)

orientation texture

shape (aspect ratio)

Because the EFBH is an ideal reflector, real reflectors will

always be larger than the EFBH size. The EFBH represents

the absolute minimum size of any reflector.


22/28


DGS allows reflectors to be compared over a range of sizes and

beam paths.

DGS is internationally recognised and employed in a number of

product acceptance standards.

The system is rapid and convenient if the correct scales areavailable for the probe and range.

If the correct scales are not available, the generalised diagram

may be used to establish a specific curve.


23/28

Check Your Progress

QB


24/28

Check Your Progress

1. The main advantage of the DGS system is that:

a. it measures the true size of a defect

b. it measures discontinuities bigger than the beam

c. it measures the equivalent size of discontinuities smaller

than the beamd. it does not depend on vertical linearity and can be used

with the suppression on

Answer: c - It measures the equivalent size of discontinuities

smaller than the beam.


25/28

Check Your Progress

2. If the reflected screen height from a 2 mm disc is 50%, what

would you expect the screen height from a 4 mm disc at the

same beam path length to be?

a. Greater than 100%

b. 100%c. 25%

d. Too small to measure

Answer: a - Greater than 100%


26/28

Check Your Progress

3. Discontinuity A is a 12 dB stronger reflector than discontinuity

B at the same beam path length. Discontinuity A is a 6 mm flat

bottomed hole (disc). Discontinuity B is also a flat bottomed

hole, and its diameter is therefore:

a. 2 mm

b. 3 mm

c. 4 mm

d. 5 mm

Answer: b - 3 mm


27/28

Check Your Progress

4. Why is it important to ensure that the suppression is off when

using the DGS system?

a. To allow bigger reflectors to be measured.

b. To allow smaller reflectors to be measured.

c. To maintain horizontal linearity.d. To maintain vertical linearity.

Answer: d - To maintain vertical linearity.


28/28

Check Your Progress

5. You have established the EFBH of a forging discontinuity is 3

mm. Which of the following conclusions is most likely to be

correct?

a. The discontinuity has an area at least as large as a 3 mm

diameter disk.

b. The discontinuity is unacceptable.

c. The discontinuity is no larger than 3 mm diameter.

d. The discontinuity is a 3 mm inclusion.

Answer: a - The discontinuity has an area at least as large as a

3 mm diameter disk.

Date post:	02-Jun-2018
Category:	Documents
Upload:	lram70
View:	227 times
Download:	0 times

DGS OF ULTRASONICS Evulation

Documents