Where the measuring surface is coated and it is required to eliminate the coating thickness from the results, a suitable instrument that uses mode 3 should be used.
Where it is required to find the thinnest point within a given area, a scanning should be performed. For this purpose an А-scan instrument should be used.
Where many readings are to be recorded, an instrument with data logging facility should be considered.
A.2.2 Probes
Probe selection depends on equipment type, material thickness, surface condition and coating condition.
For digital display instruments the probes as specified by the manufacturer should be used. For А-scan instruments the following guidelines can be applied:
the probe frequency should be selected such that it produces at least 1,5 times the wavelength in the test object (see 8.2.2);
generally, single transducer probes should be used for thicknesses of 10 mm and above. The multiple echo technique (mode 3) should only be used with single transducer probes;
where thickness is below 10 mm, double transducer probes may be applied;
if the thickness is expected to be below 5 mm, dual element probes with special focal range should be used;
when the object is curved, consideration should be given to the selection of probe diameter;
on a coated object a single transducer probe should be used with mode 3 to allow compensation for the coating thickness.
A.2.3 Setting of the instrument
Setting of the instrument is done on a step-wedge with a thickness range covering the expected range of the object.
Material and temperature shall be equivalent to the object.
A.2.4 Measuring
Where several back-wall echoes may be read (only single transducer technique), the most accurate results are achieved by reading the nth echo and dividing the reading with n. Where this technique is used on a coated surface, the distance from echo no. 1 to echo no. n is read and divided by и-1. Hereby the coating thickness is not included in the result.
Where only one back-wall echo is used, the reading should be taken in the same position of the echo as the reading during setting of the instrument. If the surface is coated, the coating thickness multiplied with the sound velocity ratio metal/coating is included in the reading and should be subtracted before recording the result.
Where high reproducibility is essential, the exact position of the measuring point is to be documented or assured in another way. Where it is essential to detect the thinnest point within a given area, scanning should be performed. This will normally require an А-scan instrument (type 5.1 b) or type 5.1 c)).
Use of digital display instruments should strictly follow the manufacturer's instructions.
Unexpected measurements may be due to internal discontinuities. These should be verified by supplementary investigations, e.g. by using angle-beam probes.
A.3 Measurement of corrosion with pitting
A.3.1 Instrument
For thickness measurement where pitting may be expected, an А-scan instrument should be used (type 5.1 b) or type 5.1 c)).
A.3.2 Probes
For detection of pitting, a dual element probe is the most suitable. The selected probe should have a focal distance corresponding to the expected distance to the pitting.
A.3.3 Setting of the instrument
The setting of the instrument is done on a step-wedge with a thickness range covering the expected range of the object. Material and temperature shall be equivalent to the object. Where small diameter pitting is expected, detection sensitivity is to be verified on a calibration block with small diameter flat-bottom holes in the same distance range as the expected pitting.
A.3.4 Measuring
When searching for pitting, only the first back-wall echo should be used. Echoes from pitting may occur together with the back-wall echo.
Where reflector type cannot be identified as either corrosion or inclusion, supplementary investigation should be carried out using angle probes. 45° angle probes are specially suited for differentiating between inclusions and pitting.
Table A.1 — Corrosion in steel - Recommended ultrasonic techniques
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No. |
Description |
Typical corrosion origin and mechanism |
Illustration |
Recommended ultrasonic technique |
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1 |
Uniform corrosion |
Occurs in corrosive environments such as:
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A.2 |
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Development in uniform corrosion |
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2 |
Pitting |
Corroded areas have clear limits while the adjacent areas are typically not attacked. Pitting can take different shapes, depending on structure and texture of the material, and on surface condition. |
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A.3 |
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Type A: Flat pitting |
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Type C: Semicircular pits |
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Ґ Л ' |
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Type B: Undermining pitting |
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Type D: Surface breaking spherical |
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Table A.1 — Corrosion in steel - Recommended ultrasonic techniques (continued)
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No. |
Description |
Typical corrosion origin and mechanism |
Illustration |
Recommended ultrasonic technique |
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2a |
Pitting |
Distribution patterns |
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See Note |
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• • |
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• • • • • • |
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• • • • • e |
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3 |
Deposit corrosion, Crevice corrosion |
Occurs under deposits and in narrow water-filled crevices |
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1 |
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See Note |
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Table A.1 — Corrosion in steel - Recommended ultrasonic techniques (continued)
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No. |
Description |
Typical corrosion origin and mechanism |
illustration |
Recommended ultrasonic technique |
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4 |
Galvanic corrosion |
Dissimilar metals |
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See Note |
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— |
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—( |
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5 |
Flow-induced corrosion |
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."Il |
_ і і 4||l |
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See Note |
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Table A.1— Corrosion in steel - Recommended ultrasonic techniques (continued)
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No. |
Description |
Typical corrosion origin and mechanism |
Illustration |
Recommended ultrasonic technique |
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6 |
Turbulence corrosion |
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4 'i IL 1 1 |
See Note |
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7 |
Mesa-type corrosion |
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See Note |
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8 |
Cavitation corrosion |
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1 ЙЖ |
See Note |
1 — Corrosion in steel - Recommended ultrasonic techniques (concluded)
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No. |
Description |
Typical corrosion origin and mechanism |
Illustration |
Recommended ultrasonic technique |
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9 |
Weld zone corrosion |
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See Note |
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NOTE These corrosion forms are shown to illustrate the possibilities and difficulties that can be encountered when achieving to detect and quantify corrosion. The illustrations are for information only. A specific recommendation regarding the technique to be applied for each case cannot be given, as it would depend on the access conditions, material thicknesses and other parameters. |
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Annex В
(informative)
Instrument settings
1 — Instrument setting on a reference block with multiple steps
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SELECT REFERENCE BLOCK ... |
OF SAME MATERIAL AND SAME SURFACE CONDITION |
OF SAME MATERIAL AND OTHER SURFACE CONDITION |
OF DIFFERENT MATERIAL AND SAME SURFACE CONDITION |
OF DIFFERENT MATERIAL AND OTHER SURFACE CONDITION |
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CALIBRATE THE EQUIPMENT |
Calibrate on a thickness above and below the thickness range to be measured. |
Calibrate on a thickness above and below the thickness range to be measured. |
Calibrate on a thickness above and below the thickness range to be measured. |
Calibrate on a thickness above and below the thickness range to be measured. |
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VERIFY LINEARITY AT INTERMEDIATE STEPS ... |
if more than 2 steps available |
if more than 2 steps available |
if more than 2 steps available |
If more than 2 steps available |
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CORRECT THE SETTING |
Not necessary |
Check and correct zero setting on the test object |
Re-calibrate on the test object if possible or use known velocity to correct the reading. |
Re-calibrate on the test object if possible or check and correct zero setting on the test object and use known velocity value. |
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UNCERTAINTY OF MEASUREMENT RELATED TO SETTING OF THE INSTRUMENT DEPENDS ON ... |
Accuracy of reference block thicknesses and, if only 2 steps used, uncertainty of linearity |
Accuracy of reference block thicknesses and the surface condition of the test object and, if only 2 steps used, uncertainty of linearity |
Accuracy of reference block thicknesses and accuracy of thicknesses of the test object or validity of known value of velocity and, if only 2 steps used, uncertainty of linearity |
Accuracy of reference block thicknesses and accuracy of thicknesses of the test object and the surface condition of the test object or validity of known value of velocity and, if only 2 steps used uncertainty of linearity |
2 — Instrument setting on a reference block with one thickness or without a reference block
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REFERENCE BLOCK |
OF SAME MATERIAL AND SAME SURFACE CONDITION |
OF SAME MATERIAL AND OTHER SURFACE CONDITION |
NO REFERENCE BLOCK OF SAME MATERIAL AVAILABLE |
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CALIBRATE THE EQUIPMENT |
Set the velocity and zero to agree with the known value and thickness |
Set the velocity and zero to agree with the known value and thickness |
Set the velocity to a known value for the test object and Set zero by using a known value or by using mode 3 or by using automatic probe recognition |
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VERIFY LINEARITY AT INTERMEDIATE STEPS |
Not possible |
Not possible |
Not possible |
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CORRECT THE SETTING |
Not necessary |
Check and correct zero setting on test object |
Not possible |
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UNCERTAINTY OF MEASUREMENT RELATED TO SETTING OF INSTRUMENT DEPENDS ON |
Accuracy of reference block thickness and uncertainty of linearity |
Accuracy of reference block thickness and uncertainty of linearity and surface condition of the test object |
The validity of the known values |
Annex C
(informative)
Parameters influencing accuracy
C.1 Parameters influencing accuracy
The following Table C.1 lists the parameters influencing accuracy:
1 — Table of parameters influencing accuracy
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Item |
Parameter |
Result |
Possible improvements |
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Test object |
Material |
Composition |
Attenuation, absorption, scattering and local variation of velocity |
Setting of instrument on the same material as test object |
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Structure |
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Anisotropy |
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Surface condition |
Cleanliness |
Local variations of surface conditions lead to variations of couplant thickness |
Cleaning |
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Roughness |
Grind surface as required |
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Surface profile |
Using of small diameter probe |
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Coating |
Coating |
Coating velocity different from base material velocity resulting in inaccuracy |
Removing coating or using mode 3 |
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Paint |
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Surface treatment |
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Geometry |
Non-parallelism |
Back-wall echo can disappear or can be distorted |
Parallelism should be within the probes beam divergence angle (± 1,22 arcsin A/d) |
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Curvature |
Loss of coupling efficiency |
Use a smaller diameter probe |
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Range |
Distortion of back-wall echo caused by attenuation |
Using mode 1 and a lower probe frequency using mode 4 |
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(continued)
Table C.1 — Table of parameters influencing accuracy (concluded)
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Reference |
Method |
Uncertainty of calibration method |
Inaccurate readings |
Using block representative of part, steps thinner and thicker than expected thickness, choice of calibration method see Annex В |
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Reference block |
Thickness and velocity uncertainty |
Accuracy cannot be better than block uncertainties |
Accurate measurement of block thickness and sound velocity |
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Measuring |
Equipment |
Resolution |
Accuracy cannot be better than system resolution |
Using higher accuracy instrument, higher probe frequency and broadband probes |
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Cable length |
Excessive cable length distorts the signals |
Using shorter cable and calibrate with the same cable |
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Drift of instrument |
Inaccurate readings |
Warming-up the unit and wait for stable reading or use stable equipment |
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Time of flight |
Accuracy cannot be better than time of flight measurement accuracy |
Using higher accuracy instrument |
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Linearity |
Inaccurate readings |
Ensuring linearity of system |
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Trigger point |
Inaccurate readings |
Selecting best trigger point |
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Operation |
V-path |
Wrong reading because thickness differs from ultrasonic path |
Using a thickness gage with V-path correction or taking into account the roof angle and separation Using a single element probe |
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Phase shift |
Erroneous reading |
Taking the phase shift into account |
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Repeatability |
Use of unit |
Method |
Improper operation |
Providing correct procedure or instructions. Conducting repeatability tests |
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Coupling |
Bad coupling introduces dispersion in the readings. |
Selecting couplant to suit the surface conditions Using mode 3 if possible |
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User training |
Error on reading |
Operator training |
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Miscellaneous |
Temperature |
Variation of sound I velocity |
Error on reading |
Calibrating at the same temperature as test object or correcting calibration for change of sound velocity |
