What are the ultrasonic testing methods for NDT?
In this article:
- Ultrasonic Testing (UT) Is a Key Non-Destructive Evaluation Technique: UT uses high-frequency sound waves to inspect internal structures, detect flaws, and measure material thickness without damaging the test object.
- Multiple UT Techniques Serve Diverse Applications: The article outlines various ultrasonic methods including pulse-echo, through-transmission, phased array (PAUT), and time-of-flight diffraction (TOFD), each suited to specific inspection needs.
- Phased Array and TOFD Offer Advanced Imaging: PAUT enables dynamic beam steering and focusing for detailed imaging, while TOFD excels at detecting and sizing cracks with high accuracy, especially in weld inspections.
- Digital Integration Enhances Inspection Efficiency: Modern UT systems incorporate digital data acquisition, real-time visualization, and automated defect recognition, improving reliability and reducing inspection time.
- Waygate Technologies Provides Comprehensive UT Solutions: With a broad portfolio of ultrasonic instruments, probes, and software, Waygate Technologies supports industries in achieving high-precision, code-compliant inspections.
What are the ultrasonic testing methods for non-destructive testing (NDT)?
Ultrasonic testing is a vital non-destructive testing (NDT) technique that uses high-frequency sound waves to inspect the internal structure of materials.
When these waves travel through a material, they interact with its internal features—reflecting, refracting, or scattering at boundaries and imperfections. These interactions, such as the interference patterns, are key to how ultrasonic testing reveals flaws or discontinuities.
Primary ultrasonic testing methods
The fundamental effects of interference to the propagation of sound have resulted in different ultrasonic testing methods, and they include:
1. Resonance Method:
Here use is made of the reflection between two parallel limiting faces of the test specimen (fig. 1).
Fig. 1: Resonance, if the wavelength λ is just 2t (resonance method)
2. Through-transmission Method (Shadow Method):
In this method the shadowing effect of a material interface (material discontinuity) is used. Two opposing probes can be used (fig. 2) as can the “mirror-shadow-method” with the probes on one side of the test specimen (fig. 3).
Fig. 2: Shadow method (through transmission)
Fig. 3: Double disturbance of the sound wave before and after reflection on the backwall
3. Echo Method:
This method uses the signal which is reflected from a discontinuity in the material (fig. 4). Here the transmitter probe can be identical to the receiving probe, separate transmitter and receiver probes can also be used.
The most important special group of echo methods and at the same time the most important ultrasonic testing method of all is the pulse-echo method. By using ultrasonic pulses not only the size of the reflection indication (echo amplitude) can be evaluated—the echo transit time can be evaluated too.
Thus, one obtains data not only of the size of the reflector but also data regarding its position. If the position of the reflector is known (backwall) then, by using the transit time, the structure of the material can be evaluated. If the location of the reflector is not known but the properties of the material are (attenuation, sound velocity) then e. g. wall thickness measurements can be carried out.
In summary
The primary ultrasonic testing methods used with NDT equipment are the Resonance method, the Through-transmission (Shadow) method, and the Echo method, with the Pulse-Echo technique being a particularly significant and versatile application. Each method utilizes the interaction of sound waves with the material to detect flaws, measure dimensions, and assess material properties without causing damage.