6. Evaluation of discontinuities

Of course, a discontinuity is best evaluated when its size (extension) is known. The operator's wish to accurately know the "real reflector size" is understandable therefore it is expected that an nondestructive testing method, such as ultrasonic testing, give this information. However, due to the fact that on the display only the echo can be interpreted, this means the reflected sound coming from the discontinuity, it is very often difficult, and in some cases even impossible, to reliably assert the size of the reflector. In fact, the echo height plays the decisive part when evaluating discontinuities during manual Ultrasonic Testing.

6.1 Scanning method
In ultrasonic evaluation one is frequently able to come near to the true reflector size as long as the discontinuity is large compared to the diameter of the sound field. The discontinuity then reflects the complete impacting energy back, Fig. 57. By scanning the boundaries of the discontinuity, reliable information can be obtained about its extension. The ultrasonic operator normally observes the height of the discontinuity echo. The probe position on the test object at which the echo drops by exactly half indicates that the discontinuity is only being hit by half the sound beam, Fig. 58a.

Fig. 57 A large reflector in the sound beam

Fig. 58a Straight beam probe on the reflector boundry Fig. 58b Top view with reflector for extension.

This means that the acoustic axis is exactly on the boundary of the discontinuity. The probe position is marked and the operator determines further boundry points until a contour of the discontinuity is formed by joining the marked points together, Fig. 58b. Location of the reflector boundry becomes more exact the smaller the diameter of the sound beam is at the reflector position. Therefore, if the reflector extension is to be exactly measured it is recommended that a probe be selected which has its focal point at the same distance as the reflector. TR probes are especially suited which have a hose-shaped sound beam with a small diameter (1 - 3 mm) at their most sensitive depth range.

6.2 Evaluation of small discontinuities: The DGS method
A reflector which is completely contained within the sound beam is regarded as a small reflector. If such a reflector is evaluated by scanning then it is not the size of the reflector which is obtained as a result but the diameter of the sound beam! Therefore, the scanning method is not practical in this case. We have noticed previously that the height of a reflector echo will become greater the larger the sound beam area is which covers the reflector. This feasible behavior can be used on small reflectors: their echo heights increase with their areas, Fig. 59.

Fig. 59 Reflectors with different areas and their echoes

Fig. 60 Reflectors at different depths and their echoes

Under optimal conditions, e.g. drill holes with flat bottoms and at equal depths, this law can be confirmed:

The echo heights are proportional to their area or The echo heights are proportional to the square of their diameter.

Example: The flat-bottom hole with a diameter of 2 mm has an echo which is 4 times that of a 1 mm flat-bottom hole because the area has quadrupled. However, if the echoes from two drill holes at different depths are compared then an additional distance dependence of the echo heights is established, Fig. 60.

With accurate tests using flat-bottom holes at different depths a simple law can be found, at least in the far field of the applied sound beam:

The echo heights reduce to the square of their distance

This does not normally apply to the near-field of the sound beam! Here, the test results show that the echo heights within the focus reach their highest amplitude and are reduced again at shorter distances, Fig. 61.

Fig. 61 Distance amplitude curve of a 2 mm - disk reflector

Fig. 62 Evaluation of a discontinuity (F) using evaluation curves.

If such curves are put on transparent scales having the CRT format then we immediately have the possibility to comparatively evaluate echoes from unknown reflectors and those from natural reflectors, i.e. the echo height of the discontinuity is compared to that of a circular disk. The discontinuity in Fig. 62 reflects the sound waves the same as a circular disk having a diameter of 4 mm. Due to the fact that we can only assess the sound reflected from the discontinuities we must of course not equate the diameter of 4 mm with the "true size" of the discontinuity. We therefore refer to them as an equivalent disk-shaped reflector or as equivalent reflector size (ERS) . The equivalent reflector size only corresponds to the true reflector size of a discontinuity in an ideal case which is when it is circular and exactly hit vertical to the acoustic axis.

In practise this almost never occurs which means that the true size of a discontinuity is normally larger than the equivalent reflector size. A law for this cannot be derived because the echo height is strongly dependent on the characteristics of the discontinuity, this means its geometry, orientation to the sound beam and the surface quality. For example, a pore (spherically shaped gas inclusion) with a diameter of 2 mm has an equivalent reflector size of 1 mm; an angled flat reflector 5 mm long gives, according to orientation, a result of ERS 0 (not detectable) to perhaps ERS 2.

This uncertainty in the evaluation of the discontinuity is however neutralized when other possibilities and techniques in ultrasonic testing are used to inspect detected discontinuities closer. An experienced ultrasonic operator can, without additional expense, accurately give information about the discontinuity which he has detected. Scanning the discontinuity from different directions, assessing the echo shape and the behavior of the display when moving the probe (echo dynamics) are just a few techniques which can be successfully applied.

Despite the remaining uncertainty with evaluation of natural discontinuities the above method of discontinuity evaluation is applied in many countries due to the fact that the method is based on well proven laws in the sound field. It is therefore reproducible, i.e. the evaluation results are independent of testing device and operator.

The socalled DGS scales or discontinuity evaluation can be obtained from the probe manufacturer for many probes and various calibration ranges. DGS means that the scale is allocated an echo at the Distance, with correctly set Gain and (equivalent reflector) Size. However, the modern version of the DGS scale would need some explanation because it was developed to fulfill the requirements of the most common specificationsin practical testing: If, on a certain test object whose purpose and therefore stress values are known, an ultrasonic test is to be carried out then firstly, if necessary with destructive testing, it should be established how large the permitted material flaw should be. Of course, the position of such a flaw in the material and its rate of occurance play a part.

If a permitted flaw size has been determined then this size is multiplied with the safety factor which, amongst others, also takes the evaluation uncertainty of the ultrasonic test into account. The corresponding echo amplitude curve for this size is now of importance for the ultrasonic test. The ultrasonic operator scans the test object with the probe and only needs to record the indications which exceed this recording curve, Fig. 63.

Fig. 63 DGS scale for the probe B 4 S.

Fig. 64 Discontinuity evaluation with a DGS scale

Consequently, only one curve is necessary for the evaluation. Due to the fact that, depending on the application, different recording limits occur, it must be possible to allocate other equivalent reflector sizes to this curve. This allocation is shown by a table positioned at the top right of the scale: starting from a defined default setting of the instrument, the auxiliary gain is taken from the table which belongs to the required recording value and added to the gain controls. If the correct range calibra tion has been made then test object scanning can now begin. When an indication from the test object exceeds the recording curve then this result is to be recorded in writing and evaluated. If required, the test instructions provide the following measures: rejection, repair or further tests for exact assessment of the discontinuity (diagnosis) .

Fig. 64 shows testing of a forged part. The recording curve corresponds to Equivalent Reflector Size 3. The detected discontinuity, at a depth of 110 mm, exceeds the curve, i.e. all reflector data must now be recorded into a predetermined form.

6.3 Sound attenuation
In addition to the laws which establish the behaviour of disk shaped reflectors within the sound beam of a probe (distance and size laws) another effect can be observed: The sound attenuation. The sound attenuation is caused by the structure of the test object but is also strongly dependent on the frequency and the wave mode of the applied probe. Only when these effects are known can they be considered by the discontinuity evaluation. However, the evaluation becomes more difficult, timeconsuming and more unreliable so that DGS evaluation can be burdened with tolerances which are too great.

6.4 The reference block method
These uncertainties in evaluation can be reduced when there is a socalled reference block available which is made of the same material as the object to be tested and which also contains artificial reflectors whose echoes can be directly compared to the discontinuity echoes from the test object. The application of the reference block method is, in practise, made in two different ways:

6.4.1 Comparison of echo amplitudes
The test object is tested with a high gain setting by which the smallest detectable reflector is displayed. An echo indication is peaked, i.e. the maximum echo indication is achieved by careful movement of the probe and the echo peak set by adjustment of the gain to a predetermined height, e.g. 80% CRT screen height (reference height) , Fig. 65.

Fig. 65 Test object with a flaw: echo at 80% (reference height)

Fig. 66 Reference block: reference echo at 30%.

Fig. 67 References block: reference echo to reference height

Using the same settings, the reflector from the reference block is scanned which is approximately positioned at the same distance as the discontinuity, Fig. 66. The quantative unit for evaluation is now the gain change of the ultrasonic instrument which is necessary to set the reference echo to the reference height, Fig. 67.

Result: The discontinuity echo is 8 dB higher than the reference echo because the gain must be increased by 8 dB (from 34 dB to 42 dB).
The recording limit normally corresponds to the echo height of the reference reflector whose size is to be determined, the same as the DGS method, before the ultrasonic test.

6.4.2 Distance amplitude curve


Fig. 68 Reference block wiht side drilled holes and resulting echoes

Fig. 69 DAC of the reference echoes (top) and with time corrected gain (bottom).

All reflectors in the reference block are scanned before the test, their maximum echo heights marked on the attachment scale of the display and joined by a curve, Fig. 68.

The curve produced is called the Distance Amplitude Curve, or DAC for short. When a discontinuity echo appears, an immediate assessment can be made whether or not the discontinuity echo exceeds the DAC. In addition to this a determination is made, by a corresponding gain change, to see by how many dBs an echo exceeds the curve. This excess recording echo height (EREH) is our reproducible measure for the evaluation and reporting of the discontinuity.
The advantages of the reference block method with a DAC are:

1. that it is no longer necessary to compare each discontinuity echo with the corresponding reference echo from the reference block but to directly make the evaluation with the DAC.
2. that the heavy reference block need not be transported to the testing location.
3. that the recording of a DAC for certain applications is only required once because the curve is documented on a transparency or in the memory of a modern ultrasonic test instrument.

By recording the curve using reflectors in a test object comparable to the work piece, this curve contains all the influences in the test object (distance law, sound attenuation, surface losses). Corresponding corrections are therefore not necessary. Regarding the evaluation results, we must understand here that the effect of the discontinuity (geometry, orientation and surface quality) is not taken a great deal into account the same as the DGS method. Therefore, the result of a discontinuity evaluation with the reference block method has the same uncertainty as the DGS method.

The preference regarding which method to use is subjective. The corresponding national test specifications normally state the test method to be used so that the operator is not able to make his own decision. If no data is available, the test situation should be analyzed in order to decide which method be best used:

Firstly, it must be established whether a reference block exists which corresponds to the test object. If yes, then the test can be carried out simply and reproducibly with the reference block method. If no reference block is available then the DGS method can be used, or a reference block must be subsequently produced comparable to the test object.

However, in many cases the DGS method can be used without difficulty, namely when the test object is made of low alloy steel, has a simple geometry, a low sound attenuation and an even surface quality. The test should be carried out with a narrow band standard probe with a frequency between 1 MHz and 6 MHz for which there is a DGS diagram or a DGS scale.

The new computer controlled instruments normally support the program controlled recording of DACs. With the USD 10 the recorded DAC is automatically converted to a horizontal line. This is known as time corrected gain (TCG) , Fig. 69.

The recording curve is therefore an horizontal line so that the evaluation can be visually and acoustically supported using a monitor gate (flaw alarm), Fig. 70a-c. At the same time for each echo, the excess recording echo height is displayed in dB (DBR value in the measurement line of the USD 10) in addition to the data for discontinuity location.

Fig. 70a Weld testing with the USD 10

Fig. 70 b A discontinuity to be recorded, DBR 14,4 dB

Fig. 70c A discontinuity not to be recorded; DBR -9.2 dB.

Of course, the recorded curves, including the complete instrument settings, can be stored. It is therefore guaranteed that any later tests can be carried out with the same settings. By storing the instrument settings, including the active A-Scans with the discontinuity echo, the operator has all the data available for producing a test report at any time.

© NDT.net - info@ndt.net

|Top|