BRITISH STANDARD BS 3923-2:1972

Methods for

Ultrasonic Examination of Welds —

Part 2: Automatic examination of fusion welded butt joints in ferritic steels

UDC 621.791.05:620.179.16 + 669.15–194.57:621.791.052.4:621.791.5/8:620.179.16Lice

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BS 3923-2:1972

This British Standard, having been approved by the Welding Industry Standards Committee, was published under the authority of the Executive Board on22nd August, 1972

© BSI 01-1999

First published August 1965First revision August, 1972

The following BSI references relate to the work on this standard:Committee reference WEE/34Draft for comment 70/26486

ISBN 0 580 07436 6

Co-operating Organizations

The Welding Industry Standards Committee, under whose supervision this British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations:

The Government department and scientific and industrial organizations marked with an asterisk in the above list, together with the following, were directly represented on the committee entrusted with the preparation of this British Standard:

Aluminium Federation Institution of Civil EngineersAssociated Offices Technical Institution of Electrical Engineers

Committee* Institution of Mechanical Association of Consulting Engineers

Engineers Institution of Production EngineersBritish Constructional Steelwork Institution of Structural Engineers

Association Lloyd’s Register of Shipping*

British Electrical and Allied London Transport ExecutiveManufacturers’ Association* Ministry of Defence, Combined

British Railways Board Ministry of Defence, NavyBritish Steel Industry*

Department*

Crown Agents for Oversea Shipbuilders’ and Repairers’Governments and National Association*

Administrations Society of British AerospaceDepartment of Employment Companies Limited*

Department of Trade and Industry Welding Institute*

Department of Trade and Industry, National Engineering Laboratory

British National Committee for Non-destructive Testing Society of Non-destructive Testing Great Britain

Electricity Council, the Central Process Plant AssociationElectricity Generating Board Society of Motor Manufacturers andand the Area Boards in TradersEngland and Wales Society of Non-destructive

Engineering Equipment Users ExaminationAssociation United Kingdom Atomic Energy

Institute of Physics and the Physical AuthoritySociety Water-tube Boilermakers

Ministry of Defence, Army AssociationNational Coal Board Individual firm

Amendments issued since publication

Amd. No. Date of issue Comments

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Contents

PageCo-operating organizations Inside front coverForeword ii

1 Scope 12 Definitions 13 Operators 14 Equipment 15 Surface condition 16 Parent metal examination 17 Weld examination 38 Evaluation of imperfections 49 Test plates 4

10 Presentation of results 4

Appendix A Guidance on the determination of probe characteristics 5Appendix B Notes for guidance on the D.G.S. diagram 5Appendix C Method for setting sensitivities where maximumsensitivity is required 12

Figure 1 — Relationship between ratio of diameter to thickness and angle of ultrasonic beam 2Figure 2 — Typical D.G.S. diagram for normal probes 8Figure 3 — Typical D.G.S. diagram for angle probe of 2 MHz, 20 mm × 22 mm crystal size 9Figure 4 — Typical D.G.S. diagram for angle probe of 4 MHz, 8 mm × 9 mm crystal size 10Figure 5 — Measurement of shear wave attenuation 12Figure 6 — Measurement of transfer loss 12

Publications referred to Inside back cover

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Foreword

This British Standard forms part of a series dealing with methods for thenon-destructive testing of welds. It does not state when this particular type of testing should be employed not does it give standards of acceptance, as both of these aspects should be covered in the appropriate application standard or be agreed between the contracting parties.This standard was first published in 1965 as a result of ultrasonic examination procedures being agreed between organizations interested in the manufacture and use of boilers and pressure vessels. Part 2 has now been revised to cover automatic examination of butt joints in a wide range of shapes and forms of ferritic steels.The use of a D.G.S. (Distance, Gain, Size) diagram for obtaining the appropriate sensitivity setting only has been introduced although it is not the sole method. Guidance on this method is given in Appendix B.The revised standard is not associated with any particular type of fabrication, but has been prepared to cover a wide range of products and as such lays down the broad principles of automatic ultrasonic examination. It is emphasized that a satisfactory technique can only be determined after taking into account all the relevant factors regarding the equipment to be used and the characteristics of the weld to be examined.Prior to ultrasonic examination the weld should be visually inspected and any visible flaws recorded. Automatic ultrasonic examination is usually supplemented by manual examination and both are often used in conjunction with other testing methods in order to examine completely a welded article or structure. The use of any non-destructive testing method should always be considered in relationship to inspection and testing as a whole and the full benefits of this or any other method often can only be obtained by considering the results in conjunction with those from other methods.A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunity from legal obligations.

Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 14, an inside back cover and a back cover.This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.

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1 ScopeThis Part of this British Standard deals with methods for the automatic scanning and recording of imperfections in fusion welded butt joints in ferritic steels1).Whereas the procedures apply to all thicknesses of material, the limiting parameters are:

1) whether the material traversed by the ultrasonic beam is such that the beam can be propagated through it sufficiently to achieve the agreed sensitivity;2) that with the angles of probe available, the ultrasonic beam will reflect from the under surface of the material in such a way as to cover the full cross section of the weld. For curved parts the ability of the beam to reflect from the under surface depends on the ratio of the outside diameter to the thickness (see Figure 1).

NOTE The titles of the British Standards referred to in this standard are listed on the inside back cover.

2 DefinitionsFor the purposes of this British Standard the definitions given in BS 3683-4 apply.

3 OperatorsIf required, the operator shall demonstrate to the satisfaction of the contracting parties that he is able to meet the requirements of any particular technique adopted.

4 Equipment4.1 Presentation. The examination shall be carried out by an agreed automatic orsemi-automatic scan, using a pulse echo technique with either A or B-scope presentation, and a fully automatic recording or marking system shall be used.4.2 Test frequency. The equipment shall be capable of working at a test frequency within the range 1 MHz to 6 MHz.

4.3 Probes2)

4.3.1 The area of each transmitting and/or receiving crystal shall not exceed 500 mm2. No dimension of the crystal face shall exceed 25 mm unless otherwise agreed.4.3.2 For parent metal examination, zero angle longitudinal wave probes shall be used.

4.3.3 For angle probes, the ultrasonic beam shall have an angle of refraction of between 35° and 70° within the material.4.4 Coupling medium. The coupling shall be obtained by either contact, gap or immersion scanning using a liquid or paste medium suitable for this purpose.4.5 Continuity of coupling. There shall be an indication of adequate acoustic coupling for example, by recording a compressional wave transmitted through the coupling or by a device which indicates any failure of the coupling.4.6 Recording. The recording or marking system shall clearly indicate imperfections which require further investigation by manual scanning.Any recorded imperfection shall be clearly located from a positional datum so that it can be accurately positioned along the weld. It is recommended that these positional data are located along welds at intervals of 250 mm.

5 Surface conditionThe condition of the surface that will be in contact with the probe shall be such that a satisfactory coupling between the probe and the workpiece can be maintained. Spatter and loose scale shall be removed.The surface condition of the weld and adjacent parent metal on each surface shall be such that it does not adversely influence the weld examination.NOTE Depending on the profile and surface condition, dressing of the weld area may be necessary, even when contact is only to be made with the parent metal.

6 Parent metal examination6.1 General. Whether or not the parent metal has been ultrasonically tested previously, manual ultrasonic examination shall be made after welding:

1) to locate any flaws, such as laminations, in the material through which the ultrasonic beam will pass during examination of the weld, and2) to establish the material thickness to enable the actual beam paths to be determined.

If any flaws are found, their influence on the inspection of the weld shall be considered and if possible alternative techniques of scanning shall be used to ensure complete examination of the weld. If any section cannot be so tested, this fact shall be included in the report (see 6.6).

1) For the examination of welds used in the manufacture of pipes and tubes on a continuous basis see BS 3889-1A.2) Guidance on the determination of probe characteristics is given in Appendix A.

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During the preliminary examination the operator shall assess the attenuation characteristics of the material and the influence that the surface condition will have on the coupling, in order to determine the practicability of performing an effective test (see also 6.5).6.2 Method. The material shall be examined by the pulse echo technique using a longitudinal wave probe normal to the surface of the material. (See also BS 4336-1A.)The parent metal shall be ultrasonically examined over the region covering the entire scanning zone for the subsequent weld examination.6.3 Test frequency. The frequency shall be between one and two times the frequency to be used for the weld examination so that the attenuation characteristics assessed under 6.1 are related to the subsequent shear wave test.NOTE On thin material it may be desirable to use a double crystal probe.

6.4 Time base calibration. The time base shall be calibrated using either the A2 or A3 block as specified in BS 2704.6.5 Sensitivity. The method of setting the sensitivity shall be agreed between the contracting parties.Methods of achieving the gain settings are described in Appendices B and C.The sensitivity of the flaw detector/probe combination shall be such that a clear signal will be obtained from the smallest defect to be detected throughout the scanning distance.The size, type and orientation of the smallest flaw to be detected shall be agreed between the contracting parties before testing is commenced.

Figure 1 — Relationship between ratio of diameter to thickness and angle of ultrasonic beam

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6.6 Reporting. A report on the parent metal examination shall be made describing any local heterogeneities (e.g. laminations, surface flaws, areas of high attenuation) which will confuse the interpretation of the weld examination at such locations. If no such heterogeneities are found, a statement that the parent metal examination was satisfactorily undertaken shall be made.

7 Weld examination7.1 Method. The examination shall be carried out with angle probes by an agreed automatic or semiautomatic scanning system, using the pulse echo technique with either A or B-scope presentation. A fully automatic recording or marking system shall be used.For imperfections predominantly parallel to the direction of welding the weld shall be scanned at right angles to the weld axis.For imperfections predominantly transverse to the direction of welding the weld shall be scanned along the weld axis.NOTE The automatic system may not facilitate the satisfactory detection of transverse imperfections. In such cases it will be necessary for an alternative automatic or manual method to be agreed between the contracting parties before testing is commenced.

7.2 Test frequency. The test frequency used shall be compatible with the requirements for sensitivity in 7.5.NOTE For guidance, the following frequencies are suitable for the beam path lengths quoted:

4 MHz to 5 MHz for up to 200 mm2 MHz to 2.5 MHz for up to 400 mm1 MHz to 1.25 MHz for over 300 mm

7.3 Scanning. Wherever possible the scanning of the weld shall be carried out from both sides of the weld on the same surface. Where the configuration is such that examination from both sides is not possible, this fact shall be included in the report (see Clause 10).The full cross section of the weld shall be scanned at the agreed sensitivity. This may be achieved, for example, by the use of

(1) a variable angle probe;or (2) a to and fro movement of a fixed angle probe;or (3) some special configuration of probes.

The distance between successive scans of the weld shall not exceed the breadth of the transmitting crystal.The speed of movement of the probe(s) shall be such that the optimum response of the recording apparatus is obtained.

7.4 Equipment calibration

7.4.1 Calibration of time base. For equipment using A-scope presentation, the time base shall be calibrated in accordance with the recommendations of BS 2704 using a type A2 or A3 test block.Where B-scope presentation is used, direct time base calibration may not be possible. In this case a special test block of the same thickness and curvature as the seam to be examined, with four slots to represent the corners of the weld, shall be used to ensure that complete coverage of the weld area is obtained.7.4.2 Position of probe index. This shall be determined in accordance with the recommendations of BS 2704. If a variable angle probe is used, the upper and lower limits of the probe index shall be determined in a similar manner to that for a fixed angle probe.7.4.3 Angle of refraction. For a fixed angle probe the angle of refraction shall be measured in accordance with the recommendations of BS 2704.If a variable angle probe is used, the upper and lower limits of the range of angle shall be determined in a similar manner to that for a fixed angle probe.7.5 Sensitivity. The method of setting the sensitivity shall be agreed between the contracting parties.Methods of achieving the gain settings are described in Appendices B and C.The sensitivity of the flaw detector/probe combination shall be such that a clear signal will be obtained from the smallest defect to be detected throughout the scanning distance.The size, type and orientation of the smallest flaw to be detected shall be agreed between the contracting parties before testing is commenced.With variable angle probes a marked loss in sensitivity may occur at the higher probe angles and therefore the sensitivity shall be determined using that angle which gives the weakest echo.7.6 Recording. If the recording system is ofthe GO/NO-GO type, the echo height shall be measured at which

1) the recorded signal appears,and

2) the recorded signal disappears,and there shall not be a difference of more than 2 dB.With a proportional type recording system, the recording indication shall be compared over its full range, with the echo height on the flaw detector.

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8 Evaluation of imperfectionsIf an imperfection is revealed by the examination described in Clause 7, a more detailed ultrasonic examination shall be carried out using an agreed manual technique which is suitable for the purpose, such as one of those given in BS 3923-1.

9 Test platesDue to the restricted width normally required, welded coupon test plates are not suitable for automatic scanning, and their examination shall be carried out by an agreed manual technique.

10 Presentation of resultsIn the report of the automatic ultrasonic test the following information shall be included:

1) Job identification reference (including contract number and part number).2) Surface condition (as welded, chipped, ground flush).3) Date of test.4) Identity of operator.5) Instrument used (including serial number).6) Probes used, including frequency.7) Scanning techniques.8) Sensitivity of test. (This shall be recorded at the end of the test and related instrument settings shall be compared against and noted from a suitable test block.)9) Results obtained.10) Any other relevant information relating to the test.

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Appendix A Guidance on the determination of probe characteristicsA.1 Method of measurement of frequency. The usual method of measuring-probe frequency is to take the first back wall echo from a steel sample such as the A2 block of BS 2704 and to take this high frequency pulse from the end of the amplifier before rectification and present it on the calibrated time base of an oscilloscope. The number of full wave lengths in a one microsecond time interval gives the probe frequency.Alternatively, the ultrasonic operator, without any electronic knowledge, should be quite capable of measuring the probe frequency provided that the flaw detector has a fast time base expansion facility, such as 10 mm steel full screen, and sufficient resolution on each echo to indicate the high frequency pulses. In this case, the frequency is derived from the formula:

For boiler plate the velocity in compression is usually in the order of 5.93 × 103 m/s and the velocity in shear is in the order of 3.23 × 103 m/s. The velocity is known for the material and the wavelength is read from the trace as the distance covered by two consecutive half cycles for a half wave rectified display. For flaw detectors which show a full wave rectified display, it would be the distance covered by four half cycles. From this it would appear that two wavelengths were being measured but in fact normal time-base calibration for flaw detection purposes is always half of the total distance travelled; the half calibration therefore cancels out the double wavelength. The half cycles should be regularly spaced, uneven spacing indicating a distorted pulse, and probes exhibiting this feature should not be used with the D.G.S. system.

A.2 Method of measurement of near field

A.2.1 Method 1 (if an immersion tank is available). Plot the distance/amplitude curve of a small reflector along the axis of the beam. The near field length, N, is shown by the maximum amplitude position before the curve falls away into the far field. A useful target for this operation is a 2 mm or 3 mm diameter ball, mounted on the end of a rod whose diameter is less than the ball at the mounting point. The near field length for steel can be calculated from the empirical determination of N for water since the ratio of the two near fields is inversely proportional to the ratio of the velocities, i.e.:

A.2.2 Method 2 (if an immersion tank is not available). In this case, the distance/amplitude curve of the back echo is plotted from flat parallel samples of steel of similar velocity whose width would not restrict the beam. The curve should be plotted on logarithmic paper and after approximately three near field lengths, it should form a straight line. If not, attenuation has to be allowed for (see B.1.1), and when taken into account for the back wall echo a straight line should result in the far field past three near field lengths. If the straight line is then projected up to the zero decibel line, the intersection point will be or 1.57 N,where N is the near field length.

Appendix B Notes for guidance on the D.G.S. diagram

B.1 Normal probes. (See Figure 2.)

B.1.1 General. The D.G.S. diagram for single crystal normal probes was drawn by plotting amplitude in decibels from a series of disc shaped reflectors with increasing distance from the probe in water. The loss due to water attenuation was allowed for in each case and therefore the graph shows the reflection conditions for any material assuming no attenuation.The distance is given in near fields and is on a log base. If the near field length N is not known for the probe in use it may be calculated from the formula

where D is the crystal diameter.

Frequency velocitywavelength--------------------------------=

N steel( )N water( )-------------------------- Velocity water( )

Velocity steel( )-------------------------------------------=

Nπ2

--------

N D2

4 wavelength×-----------------------------------------=

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The backwall echo line indicates the maximum reflection from a large reflector with increasing distance from the probe, which becomes a straight line after three near field lengths. This confirms the radiation law for large reflectors in the far field, i.e. the amplitude is inversely proportional to the distance, so that if the distance is doubled the amplitude is halved: a 6 dB reduction. This law provides a simple method of measuring attenuation. If a difference of more than 6 dB is measured, then this will be due to attenuation in the material, leaving a simple calculation for the attenuation factor.Example 1

Plate thickness 30 mm Probe frequency 4 MHzProbe diameter 10 mmNear field length 17 mm

The second back echo at 60 mm distance is greater than 3N (51 mm), therefore, the second and fourth echoes are on the 6 dB slope.Suppose the difference in echo amplitude measured between second and fourth echo = 10 dB. Total beam path between second and fourth echo = 120 mm. Therefore, the attenuation of the plate material at 4 MHz is

Alternatively, back echoes in the near field may be used. Any difference between their amplitude in excess of what is shown in Figure 2 indicates the amount of attenuation over the total distance travelled between the two echoes.NOTE The attenuation factor in the example 1/30 dB/mm means that the sound is attenuated by 1/30 dB for every millimetre of sound path travelled, bearing in mind that the sound in a reflection technique has to travel there and back.

Since the flaw detector calibration is always for the distance in front of the probe and not the total distance travelled, it may be more convenient to express the factor in the same way, i.e. 1/15 dB/mm or 66 dB/m. It is important to indicate in which form the factor is being used.The lines below the back echo line show the amplitude of reflectors which are smaller than the beam width. These values are shown in relation to the crystal diameter and, for small reflectors within the near field, the nodes and anti-nodes of sensitivity are clearly indicated. They do not arise with a large reflector since all the incident energy is totally reflected.Well into the far field all these lines indicate that reflections from small reflectors follow the inverse square law, so that if the distance is doubled then the amplitude is reduced by a factor of 4 (12 dB).

The term “small reflectors” is related to the beam width and not to the crystal diameter, since reflectors greater than the probe diameter behave as small reflectors when far enough away to be encompassed within the spreading beam width. This is shown in Figure 2 by the lines with S values of 1 and 2 times the crystal diameter.B.1.2 Setting the sensitivity. In order to use Figure 2 to determine the sensitivity setting, the operator first needs to know the smallest disc shaped flaw or equivalent reflector size that has to be detected and, secondly, at what amplitude he wishes to record it. The following plate testing example illustrates the method:Example 2

Plate thickness 100 mmProbe frequency 4 MHzProbe diameter 25 mmNear field length 104 mm

Smallest flaw to be detected equivalent to flaw 5 mm diameter (0.2S). Defect echo height 2 screen divisions.The first back echo will be at a distance of approximately 1 near field. It can be seen from Figure 2 that close to the surface an 0.2S flaw will give an echo approximately 26 dB below the back echo set to full screen height.Near the bottom of the specimen such a flaw will only be 17 dB below but at the worst position, 0.5N distance (52 mm), it will be 29 dB below.In order to find the flaw at 2 screen divisions at the worst point (29 dB below), the back echo has first to be set to 2 screen divisions and then increased by 29 dB. This calibration ensures that all flaws that have to be detected will give a signal of not less than 2 screen divisions height, whether or not there is attenuation present.It is obvious when the minimum record level is drawn on the diagram that reflections from certain depths could give echoes greater than that level and yet still be acceptable. To determine therefore whether or not the echo is greater or less than the minimum size or in fact to determine its actual equivalent flaw size, the gain should be reduced by the decibel difference between the 0.2S line and the record level at the defect depth. The 2 screen divisions is then the minimum record level at the distance.

10 6–120

---------------- 130------ dB/mm or 33 dB/m=

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This example assumes no attenuation in the material at the frequency in use, although by taking the back echo as a reference any attenuation present would only make the sensitivity too high rather than too low. If the attenuation has been measured it may turn out to be insignificant

e.g. (5 dB/m) in view of the thickness

involved, If the attenuation was significant, however, as shown in Example 1 (1/30 dB/mm) then this should be taken into account as follows:

Draw in the attenuation curve as shown. In this case, since the back echo has been taken as the reference reflector, the allowance for attenuation is greater the nearer the reflector is to the surface. The difference at the surface in this case is 6.7 dB i.e. (100 mm × 2 × 1/30).

Assuming now that a reflector in excess of the record level was found at a depth of 0.3N, two corrections should be made to the gain: 4 dB for the difference between the 0.2S line and the record level, and 5 dB for the record level to attenuation line, i.e. the total correction to be made is a reduction in echo height of 9 dB after which the decision as to whether or not the flaw is rejectable is directly related to the record level of 2 screen divisions.If it is necessary to determine the actual equivalent flaw size for the flaw echo at 0.3N, its amplitude in decibels above the 2 screen divisions should be measured and reduced by the decibel value, record level to attenuation line. For example:

Flaw depth 62 mm = 0.6NEcho height above 2 screen division = 19 dBRecord level to attenuation line = 3 dBTherefore, final position above record level = 16 dB (on 0.4S line)From Figure 2, S × crystal diameter = equivalent flat-bottomed hole diameterTherefore: 0.4 × 25 = 10 mm diameter

NOTE 1 In the example used the back echo happened to be at 0.96N but it could have been 0.4N or 5.5N or any other value, depending on the material thickness and the probe near field length. Furthermore, the S values and the distance values in near fields could both be given directly in millimetres once the probe crystal diameter and near field length are known.NOTE 2 The D.G.S. diagram shown in Figure 2 is based on the established sound radiation laws. It is essential, therefore, that the amplifier and probe characteristics in use do not give conflicting results from those shown on the diagram. In other words, the flaw detector and probe combination should be entirely compatible with the D.G.S. diagram in use.

B.2 Angle probes. (See Figure 3 and Figure 4.)

B.2.1 General. The D.G.S. diagram for angle probes is essentially the same as for normal probes, except that distance is measured from the probe index and therefore most of the near field is fortunately lost in the probe. The horizontal axis as before gives the distance along the beam path; corresponding equivalent surface distances are shown for the different angle probes. Note that in the case of the two D.G.S. diagrams shown in Figure 3 andFigure 4, the probe size is stated and therefore the equivalent flat-bottomed hole sizes and the horizontal axis have been calibrated directly in millimetres.In order to use these diagrams to determine the sensitivity setting the operator first needs to know the smallest disc-shaped flaw or equivalent reflector size that has to be detected and secondly, at what amplitude he wishes to record it.To set the sensitivity for weld testing with an angle probe however, is more complex than using a simple normal probe. Because there is no backwall echo to use as a standard reference, a separate reference echo has to be used, such as the 100 mm radius of the A2 calibration block (see BS 2704). This means that an allowance has to be made for the different surface conditions (transfer loss) between the test block and the plate, the different attenuation factors between the two materials and also the difference in distance between the 100 mm radius reflector and the farthest defect distance in the material.The procedure to arrive at the correct gain setting to ensure that the smallest defect to be detected throughout the scanning distance will at least give an echo at the appropriate screen height, taking into account the factors mentioned above, is as follows:

1) Measure attenuation of material (see B.2.2).2) Measure transfer loss between A2 test block and plate (see B.2.3).3) Read difference in decibels between back echo from 100 mm radius on A2 block to minimum defect size at worst position on D.G.S. diagram which should be available for the probe in use.4) Set back echo from 100 mm radius to 2 divisions, plus difference in decibels from (3) plus transfer loss from (2) plus difference in attenuation from 100 mm radius on A2 block to maximum testing distance in plate.

1200---------- dB/mm

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Figure 2 — Typical D.G.S. diagram for normal probesLice

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10©

BS

I 01-1999

Figure 4 — Typical D.G.S. diagram for angle probe of 4 MHz, 8 mm × 9 mm crystal size

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Example 3Probe of 4 MHz, 8 mm × 9 mm crystal sizeSmallest defect to be detected equivalent to 2 mm diameter disc-shaped flawMaximum defect distance 150 mmTransfer loss 4 dBAttenuation of A2 block 40 dB per metre beam path in front of probeAttenuation of plate 80 dB per metre beam path in front of probe

The setting required to bring a 2 mm diameter equivalent flaw to the record level at the worst distance (150 mm) would be an echo from the A2 block 100 mm radius to 2 divisions, plus 30 dB from (3) plus 4 dB from (2), plus 8 dB difference in attenuation from 100 mm radius on A2 block to maximum testing distance in plate, i.e. an A2 back echo to 2 divisions plus 42 dB.With this sensitivity, the minimum equivalent flaw size to be recorded will at least reach 2 screen divisions, wherever it appears throughout the scanning distance. It is possible as with the normal probe test that echoes in excess of 2 screen divisions could be acceptable. To determine whether or not this is so, and at the same time to evaluate the equivalent flaw size for any such echo, proceed as follows:

a) Draw on the diagram the attenuation curve as shown.b) Locate the defect depth on the diagram in distance and amplitude above the record level.c) Reduce the defect position by the difference in decibels between the record level and the attenuation line.

Example 4Defect echo 50 mm beam path from screenEcho height 34 dB greater than record level

of 2 screen divisionsRecord level to attenuation line 8 dBTherefore, new position ondiagram = 34 – 8 = 26 dB above record level and hence equivalent flaw size is 4 mm diameter.

B.2.2 Measurement of shear wave attenuation. The shear wave attenuation of the plate can be measured as follows:

1) Use one probe of the same angle and frequency as will be used for the test and calibrate the screen for beam path.2) Use two probes, separate transmitter and receiver, and locate as shown in Figure 5 so that signal A. B1 appears at 50 mm and signal A. B2 at 100 mm on the screen.

3) Measure the difference between through transmission signal height A. B1 and A. B2.4) Subtract the difference in decibels indicated on the D.G.S. diagram, the remainder being attenuation over half skip distance on the screen or 100 mm of beam path.

Example 5

B.2.3 Measurement of transfer loss. The transfer loss between the A2 block and the plate under test can be measured as follows:

1) Use one probe of the same angle and frequency as will be used for the test and calibrate the screen for beam path.2) Use two probes, separate transmitter and receiver, and locate as shown in Figure 6.3) Measure difference between through transmission signal height of A2 block and plate.4) Subtract the difference indicated on the D.G.S. diagram, the remainder being transfer loss.

Example 6

Probes of 4 MHz, 8 mm × 9 mm crystal size

skip distance 50 mm

Signal height A. B1 44 dBSignal height A. B2

Difference according to D.G.S.diagram, 5 dB (50 mm to 100 mm)Therefore,

of

beam path on screen.

Probes of 4 MHz, 8 mm × 9 mm crystal sizeThrough transmission signal

( skip on screen), A2 block 56 dB

Through transmission signal

( skip on screen), plate

Difference according to D.G.S.diagram, 3 dB (50 mm to 80 mm)Attenuation difference according to attenuation curve for A2 block at 50 mm and attenuation curve for plate at 80 mm, 5 dB.Therefore, transfer loss = 12 – 3 – 5 = 4 dB.

12--

5410------dB

dB

attenuation 10 dB 5 dB–50 mm

----------------------------------- 1 10 dB mm⁄⁄= =

12--

12-- 68

12------ dB

dB

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Appendix C Method for setting sensitivities where maximum sensitivity is requiredAfter following the requirements of the standard for a particular probe selection and time-base calibration the sensitivity settings should be made as follows:

1) The probe is applied to the test surface and the gain controls are adjusted so that the “grass level” of the material grain structure is raised 2 mm high above the base-line at the appropriate testing distance. Any reduction or increase in the sensitivity to facilitate the exploration of defects is to be noted in decibels.

2) The probe is next placed on the A2 calibration block (see BS 2704) and a record made of the difference in the attenuator reading required to bring the echo from the 100 mm radius for angle probes, or the 100 mm parallel section for normal probes, down to full screen height.3) The attenuator difference between the 100 mm radius for angle probes, or the 100 mm parallel section for normal probes, of the A2 block and that required to raise an echo from a 1.5 mm drilled hole at an appropriate beam path distance, is recorded.It should be stated whether this drilled hole is parallel to or at right angles to the test surface.

Figure 5 — Measurement of shear wave attenuation

Figure 6 — Measurement of transfer loss

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Example of maximum sensitivity record

Allowance can then be made for the transfer loss and the average attenuation of the test piece as described in Appendix B.

Calibration: Screen calibrated to 250 mm full path length

Probe selection: 2 MHz, 45°, single crystalTesting sensitivity:

To give “grass level” up to 250 mm path

A2 calibration block:

(100 mm radius) echo requires 28 dB inserted to bring it down to full screen height

Test block with 1.5 mm hole:

Hole was drilled at 125 mm path length parallel to the test surface; requires 16 dB inserted from testing sensitivity level to give full screen height from this hole

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BSI publications referred to in this standard:

This standard makes reference to the following British Standards:BS 2704, Calibration blocks and recommendations for their use in ultrasonic flaw detection.BS 3683, Glossary of terms used in non-destructive testing. Part 4. Ultrasonic flaw detection.BS 3889, Methods for non-destructive testing of pipes and tubes. Part 1A. Ultrasonic testing of ferrous pipes (excluding cast).BS 3923, Methods for ultrasonic examination of welds. Part 1. Manual examination of fusion welded butt joints in ferritic steels.BS 4336, Methods for non-destructive testing of plate material. Part 1A. Ultrasonic detection of laminar imperfections in ferrous wrought plate.

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