32 Questions to Help You Understand Nondestructive Testing—Ultrasonic Testing

01 What is nondestructive testing?

(1) Nondestructive testing is a testing method used to inspect the surface and internal quality of inspected components without causing any damage to the workpiece or the raw material in its service condition.

(2) Nondestructive Testing: NondestructiveTesting (abbreviation NDT)

02 What are the commonly used nondestructive testing methods?

There are numerous nondestructive testing methods; according to a survey and analysis conducted by NASA, they can be categorized into six major groups comprising more than 70 specific techniques. However, in practical applications, the following methods are relatively common:

Conventional nondestructive testing methods include:
– Ultrasonic Testing (abbreviated as UT);
– Radiographic Testing (RT);
– Magnetic particle Testing (abbreviated as MT);
– Penetrant Testing (abbreviated as PT);
– Eddy current testing (abbreviated as ET);

Non-conventional nondestructive testing techniques include:
– Acoustic Emission (abbreviated as AE);
– Leak Testing (abbreviated as UT);
– Optical Holography;
– Infrared Thermography;
– Microwave Testing

03 What is the basic principle of ultrasonic flaw detection?

There are numerous types of ultrasonic flaw detectors; however, in practical nondestructive testing applications, pulse-echo ultrasonic flaw detectors are by far the most widely used. In generally homogeneous materials, the presence of a defect leads to material discontinuity, which in turn often results in an impedance mismatch. According to the law of reflection, when an ultrasonic wave encounters the interface between two media with different acoustic impedances, part of the wave is reflected. The magnitude of the reflected energy depends on the difference in acoustic impedance between the two media at the interface, as well as on the orientation and size of the interface. Pulse-echo ultrasonic flaw detectors are designed precisely on this principle.

Currently, most portable pulse-echo ultrasonic flaw detectors employ A-scan display. In an A-scan, the horizontal axis of the display represents the propagation time or travel distance of the ultrasonic wave within the test material, while the vertical axis indicates the amplitude of the reflected ultrasonic echo. For example, if a defect is present in a steel workpiece, the presence of this defect creates an interface between two different media—the defect and the surrounding steel. Because the acoustic impedance at this interface differs from that of the surrounding material, the incident ultrasonic wave is partially reflected upon encountering the interface (see Figure 1). The reflected energy is then received by the transducer and displayed on the screen as an echo waveform at a specific position along the horizontal axis. This position corresponds to the depth of the defect within the test material. The height and shape of the echo vary depending on the nature of the defect, thereby providing information about its characteristics.

04 What are the advantages and disadvantages of ultrasonic testing compared with X-ray testing?

Ultrasonic testing offers several advantages over radiographic testing, including higher detection sensitivity, shorter inspection cycles, lower costs, greater flexibility and convenience, higher efficiency, and non-harmfulness to humans. However, it also has certain limitations: it requires smooth test surfaces, relies on the expertise of experienced inspectors to identify defect types, and does not provide direct visual evidence of defects. Ultrasonic testing is particularly suitable for inspecting components with substantial thickness.

05 What are the main characteristics of ultrasonic flaw detection?

(1) When ultrasonic waves propagate through a medium, they exhibit reflection at interfaces between different materials. If a defect is encountered whose size is equal to or greater than the wavelength of the ultrasonic wave, the wave will be reflected back from the defect, and the flaw detector can display this reflected wave. However, if the defect’s size is even smaller than the wavelength, the sound wave will diffract around the defect and no reflection will occur.

(2) Ultrasonic waves exhibit excellent directivity; the higher the frequency, the better the directivity. They radiate into the medium in a very narrow beam, making it easy to pinpoint the location of defects.

(3) Ultrasonic waves propagate with substantial energy; for example, the energy carried by an ultrasonic wave at a frequency of 1 MHz (100,000 Hz) is one million times greater than that of a sound wave with the same amplitude but a frequency of 1,000 Hz.

06 When performing ultrasonic flaw detection on a plate with a thickness of 14 mm, what is the relationship among the three primary curves on the distance-amplitude curve?

Long-range measurement cable Ф1 × 6 –12 dB
Quantitative line Ф1 × 6 –6 dB
Cut-off frequency Ф1 × 6 −2 dB

07 During ultrasonic flaw detection, what factors might cause the bottom echo to disappear?

(1) Large defects near the surface;
(2) Absorption defects;
(3) Large inclined defects;
(4) The oxide scale does not adhere well to the steel plate.

08 Briefly describe the causes of ultrasonic attenuation during wave propagation in a medium in ultrasonic testing.

(1) As the propagation distance of ultrasonic waves increases, the beam cross-section becomes progressively larger, resulting in a reduction of energy per unit area.

(2) Material attenuation arises from two mechanisms: first, absorption caused by the viscosity of the medium; second, scattering resulting from diffuse reflection at medium interfaces.

09 What is the primary function of test blocks?

(1) Verify sensitivity;
(2) Calibrate scan linearity.

10 When using ultrasonic testing to inspect large, disc-shaped forgings, what requirements are imposed on the working bottom surface if the initial sensitivity for flaw detection is adjusted based on the bottom echo?

(1) The bottom surface must be parallel to the inspection surface;
(2) The bottom surface must be flat and have a certain degree of smoothness.

11. What are the three principles for selecting the K-value of an ultrasonic probe?

(1) The ultrasonic beam scans the entire weld cross-section;
(2) The sound beam should be as perpendicular as possible to the primary defect;
(3) Sufficient sensitivity.

12. What are the main components of an ultrasonic flaw detector?

It mainly consists of a synchronization circuit, a power-generation circuit, a receiving circuit, a horizontal scanning circuit, a display, and a power supply.

13. What is the primary function of the transmitting circuit?

The synchronization pulse signal input by the synchronization circuit triggers the transmitting circuit to operate, generating a high-frequency electrical pulse that excites the piezoelectric transducer, inducing high-frequency vibrations and thereby generating ultrasonic waves in the medium.

14. In ultrasonic flaw detection, why is a couplant used between the transducer surface and the surface of the test workpiece?

An air gap between the chip surface and the surface of the test workpiece will cause complete reflection of the ultrasonic waves, leading to inaccurate flaw detection results and rendering flaw detection impossible.

What are the three criteria for defect identification specified in GB 1150-73?

(1) No bottom wave, only multiple-reflection waves from defects.
(2) No bottom wave, only multiple chaotic defect waves.
(3) Both defect echoes and bottom echoes are present simultaneously.

What is the purpose of the distance-amplitude curves specified in JB1150-73:1973?

The distance–amplitude curve is primarily used to determine defect size and provide a basis for acceptance criteria; it consists of three curves: the rejection line, the quantification line, and the length-measurement line.
Reject line—determine the maximum permissible equivalent of the defect;
Quantitative line—control line for determining the size and length of defects;
Long-range measurement line—initial sensitivity control line for flaw detection.

17. What is an ultrasonic field?

The space filled with ultrasonic field energy is called the ultrasonic field.

18. What are the main parameters that characterize the ultrasonic field?

Key physical quantities that characterize the ultrasonic field include sound intensity, sound pressure, acoustic impedance, beam divergence angle, and the near-field and far-field regions.

19. What is the most important performance indicator of an ultrasonic flaw detector?

Resolution, dynamic range, horizontal linearity, vertical linearity, sensitivity, and signal-to-noise ratio.

20. How many types of near-display modes are there for ultrasonic flaw detectors?

(1) In an A-scan display, the horizontal axis of the oscilloscope screen represents ultrasonic travel time (or distance), while the vertical axis represents the amplitude of the reflected echo.
(2) In B-mode display, the horizontal axis of the oscilloscope screen represents ultrasonic travel time (or distance), and this type of display provides a cross-sectional image in the depth direction of the probe scan.
(3) In C-scan display instruments, the oscilloscope screen represents the projection plane of the test workpiece. This type of display can depict the horizontal projection position of defects, but it cannot indicate the burial depth of the defects.

21. What is the primary function of an ultrasonic transducer?

1. The probe is an electroacoustic transducer that converts the returning sound waves into electrical pulses.
2. By controlling the propagation direction and the degree of energy concentration of ultrasonic waves, adjusting the probe’s incident angle or the ultrasonic beam’s divergence angle allows the main acoustic energy to be directed into the medium at different angles or to modify the wave’s directivity, thereby enhancing resolution.
3. Implement waveform conversion;
4. Control the operating frequency; suitable for different operating conditions.

22 Why is it necessary to strengthen the integrated recording and reporting of ultrasonic flaw detection?

After ultrasonic testing of any workpiece, an inspection report must be issued as evidence of the quality of the work. A correct ultrasonic test report depends not only on the establishment of reliable testing methods and accurate results, but also, to a large extent, on the completeness and accuracy of the original records and the final report itself. If a workpiece is inspected but no records are kept and no report is issued, the ultrasonic testing becomes entirely meaningless.

23. What are the applications of nondestructive testing?

Applicable stages: design phase; manufacturing process; finished-product inspection; in-service inspection.
Application scope: various materials (metals, non-metals, etc.); various workpieces (welded components, forgings, castings, etc.); and various engineering projects (road construction, dam construction, bridge construction, airport construction, etc.).

24. During ultrasonic weld inspection, what are the methods for adjusting the instrument time-scan line used for defect localization?

There are three methods: horizontal positioning, vertical positioning, and sound-path positioning.

25. In ultrasonic flaw detection, how are defects in welds classified, and what criteria are used for such classification?

In ultrasonic testing of welds, defects in the weld are generally classified into three categories:
Point defects:
In the classification, defects with a length of less than 10 mm are referred to as point-like defects; generally, their length is not measured, and such defects are counted as 5 mm.
Linear defects:
Defects with a length greater than 10 mm are referred to as linear defects.
Surface defects:
Defects with a length greater than 10 mm and a height greater than 3 mm are referred to as planar defects.

26. What is the function of ultrasonic test blocks?

The purpose of ultrasonic test blocks is to verify the performance of the instrument and probe, establish the initial inspection sensitivity, and calibrate the scan linearity.

27 What is the correct value of the refraction angle β for an angled probe?

The correct value of the refraction angle for an angled probe is referred to as the K-value, which is equal to the ratio of the horizontal distance from the probe’s beam exit point to the reflection point to the corresponding depth.

28. What should be done if unacceptable defects are found in welds that have undergone local nondestructive testing?

Additional radiographic inspection shall be performed in the direction of defect extension or at suspected areas. If, following the additional inspection, there are still doubts about the weld quality, the entire weld shall be subjected to radiographic examination.

29. What is the primary function of the synchronization signal generator in an ultrasonic flaw detector? Which two circuit sections does it primarily control?
The synchronization circuit generates synchronous pulse signals to trigger the coordinated operation of all circuit sections within the instrument, primarily controlling the synchronous transmission and synchronous scanning circuits.

30. What is the purpose of nondestructive testing?

1. Improve manufacturing processes;
2. Reduce manufacturing costs;
3. Enhance product potential;
4. Ensure the safe operation of equipment.

31. Functions and Main Application Industries of the Ultra-Exploration Instrument

The ultrasonic flaw detector is a portable, industrial nondestructive testing instrument that enables rapid, convenient, nondestructive, and highly accurate detection, localization, evaluation, and diagnosis of various internal defects in workpieces, including welds, cracks, inclusions, laps, porosity, and blowholes. It is suitable for both laboratory use and on-site engineering applications. This instrument finds extensive application in manufacturing, the iron and steel metallurgy industry, metal processing, and the chemical industry—sectors that require defect detection and quality control—and is also widely used for in-service safety inspections and service-life assessments in aerospace, rail transportation, and boiler and pressure vessel industries. It is an indispensable tool in the nondestructive testing field.

32 National and Industry Standards Related to Ultrasonic Testing

The national and industry standards for ultrasonic flaw detection include:
1. QB/T 12604.1–90 Nondestructive Testing Terminology—Ultrasonic Testing
2. JB/T 10061-1999 General Technical Requirements for Type A Pulse-Echo Ultrasonic Flaw Detectors
3. JJG 746-91 Ultrasonic Flaw Detectors—National Metrological Verification Procedure of the People’s Republic of China

Source: Internet
Article Layout: A Quality Life, Quality Starts Here. This layout is original; please credit the source when reprinting!
Disclaimer: This article is reprinted from the internet, and the copyright belongs to the original author. Due to the large number of reprints, it is often impossible to identify the true original author; therefore, only the source of the reprint is indicated. If there are any issues regarding the copyright of the work, please contact us promptly, and we will remove the content to protect your rights!