GB/T 11344-2021Non-destructive testing - Ultrasonic thickness measurement (English PDF)

4.1 By measuring the time for the ultrasonic signals transmitted by the probe to pass

through the measured material for one, two or multiple times, the thickness of the measured material is determined.

4.2 The sound velocity of the measured material is a function of the physical properties

of the material. In terms of a given material, it is usually assumed that the sound velocity of the material is a constant.

4.3 Ultrasonic thickness measurement can be achieved through the following four

methods shown in Figure 1.

5.1 Instruments

5.1.1 The instruments shall be able to easily adjust and display the thickness value

within the measurement range. The time baseline adjustment function has different names on different instruments, for example, range, scan, material calibration or sound velocity.

5.1.2 The digital direct-reading ultrasonic thickness gauge converts the sound path

time between the initial pulse and the first echo, or multiple echoes into a meter indication or digital display. This type of instruments are used for the thickness measurement of the product within a specific material and thickness range, and directly digitally display the thickness.

5.2 Probes

5.2.1 Ultrasonic thickness measurement shall adopt dual-element or single-element

longitudinal-wave probes.

5.2.2 The other specific requirements for the probes are shown in 6.3.

5.3 Coupling Agent

5.3.1 Ultrasonic thickness measurement satisfies the acoustic coupling between the

probe and the material by applying fluid or gel. The coupling agent shall have good sound permeability and have no adverse effects on the tested materials, equipment and operating personnel.

5.4 Calibration Blocks

The sound velocity and thickness of the calibration blocks shall be already-known, and identical or similar to the material of the tested product. The calibration blocks can be in a set or a step. The thickness of the blocks should cover the thickness range of the tested product.

5.5 Testing Personnel

6.1 Surface Condition

6.1.1 The surface of the tested product shall be clean and flat, and free of loose surface

or non-sticky coatings. The measurement contact area shall be not less than 2 times the diameter of the probe. Poor contact may cause the loss of acoustic energy and changes in signal and sound beam propagation paths.

6.2 Selection of Methods

6.2.1 General requirements

6.2.1.1 Ultrasonic thickness measurement is usually applied to the following two fields.

6.2.1.2 In accordance with the material, geometrical shape and thickness of the tested

product, as well as the requirements for measurement accuracy, appropriate instruments and measurement methods shall be selected. See Appendix C and

6.2.2 Measurement during manufacture

6.2.2.1 When the pulse echo method is adopted for measurement (Method 1, Method

2 and Method 3), see Appendix C (Figure C.1 and Figure C.2).

6.2.3 In-service measurement of residual wall thickness

6.2.3.1 In-service measurement is mainly applied to the residual thickness

measurement of corroded and abrasive products. It is recommended to use dual-element probe; the measurement of low reflection echo area may appropriately increase the gain.

6.2.3.2 When measuring many data points with location information, instruments with

data recording function should be used.

6.3 Selection of Probe

6.3.1 Broadband probes excite a shorter pulse than narrowband probes and have a

better resolution. They are generally used for the thickness measurement of coatings or thin plates. When measuring high-attenuation materials, the broadband probes can obtain more stable echoes.

6.3.4 When measuring products with a relatively small thickness, the delay block probe

and Method 2 or Method 3 may be used for the measurement. When the acoustic impedance of the delay block material is relatively low, for example, plastic delay block being placed on the metal for measurement, the interface echo may generate a phase shift, which shall be corrected to obtain accurate measurement results. Some thickness gauges have the function of automatic correction.

6.3.5 When the surface temperature of the material is relatively high, and the delay

block is used as a thermal barrier, the delay block shall be able to withstand the temperature of the tested product.

6.3.6 In accordance with different thicknesses and materials, the frequency range of

the probes may be between 100 kHz for the measurement of high-attenuation materials and 50 MHz for the measurement of thin metals sheets.

6.4 Thickness Measurement under Special Conditions

6.4.1 General requirements

6.4.2 Measurement below the temperature of 0 °C

6.4.3 Measurement in high-temperature environment

6.4.3.1 When the temperature is higher than 60 °C, high-temperature probe shall be

used, and the coupling agent shall satisfy the requirements for use at the testing temperature.

6.4.3.2 When using ultrasonic flaw detector with A-scan display, the instrument should

have a screen-locking function to facilitate the testing personnel’s evaluation of the response signal. The contact time of the probe shall be limited to the shortest time

6.4.4 Measurement in hazardous environment

6.4.4.1 When measuring in hazardous environment, the current laws and regulations

on safety shall be strictly abided by.

7.1 Settings of Ultrasonic Flaw Detector with A-scan display

7.1.1 Combination of ultrasonic flaw detector with A-scan display and direct-

contact single-element probe

7.1.2 Combination of ultrasonic flaw detector with A-scan display and delay

block single-element probe

7.1.2.1 When using the delay block single-element probe, the instrument shall be able

to correct the time of passing the delay block, so that it can correspond to zero thickness at the end of the delay. The instrument shall have “delay” control or electronic automatic zero adjustment function.

7.1.2.2 If the sound velocity can be adjusted in advance to the sound velocity of a given

material, then, it can be calibrated through the method of adjusting the delay control, until the instrument displays the correct thickness value. If the sound velocity cannot be set, the following methods may also be used to set the instrument.

7.1.2.3 Another method for the adjustment of the delay block probe is a variation of the

method specified in 7.1.2.2.

7.1.3 Combination of ultrasonic flaw detector with A-scan display and dual-

element probe

7.1.4 Instrument settings for high-precision thickness measurement of thick

parts

7.1.4.1 The basic setting is performed in accordance with the method specified in 7.1.1.

The calibration block shall accurately calibrate the thickness value of the entire scanning distance, which is, about 10 mm or 25 mm for full screen.

7.2 Settings of Digital Direct-reading Ultrasonic Thickness Gauge

7.2.1 The instrument shall have the functions of “sound velocity setting” (or “material

selection” or “sound velocity correction”) and “zero correction”.

7.2.2 Usually, the blocks with the same material as the tested product are used. One

with a thickness not smaller than the maximum value of the measured thickness, and the other with a thickness not greater than the minimum value of the measured thickness.

7.2.3 The probe is placed on the thicker block; adjust the “sound velocity setting” of the

instrument, so that the reading displayed by the thickness gauge is close to the known value.

7.3 Settings of Ultrasonic Thickness Gauge with A-scan display

The settings of A-scan thickness gauge that directly displays thickness are shown in

7.1 and 7.2.

7.4 Instrument Settings for Thickness Measurement under Special Testing

Conditions The instrument settings for thickness measurement under special testing conditions are shown in Appendix E.

7.5 Verification of Instrument Settings

When the following situations occur, the instrument settings shall be verified.

8.1 Testing Reports

The relevant information and data of the measurement process shall be recorded in detail in accordance with the actual situation of operation on the site. In addition to the information required by the contract, at least the following contents shall be included.

8.2 Testing Records

Generally, the following contents are included.