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Ultrasonic non-destructive testing (NDT) is a technique that detects internal flaws in materials by analyzing how ultrasonic waves propagate through them. This method relies on the differences in acoustic properties between the material and any defects it may contain. Two common approaches are the pulse-echo method, which observes reflected sound pulses, and the through-transmission method, which measures changes in the amplitude of incident waves after they pass through the material. Typical operating frequencies range from 0.5 to 5 MHz, depending on the application.
The most widely used inspection device is the A-scan ultrasonic flaw detector. This instrument displays echoes on an oscilloscope, allowing operators to determine the presence, location, and size of reflective surfaces based on the timing and amplitude of the returned signals. The basic structure and working principle of the device are illustrated in Figure 1.
Different types of ultrasonic waves can be used depending on the material and defect type. Longitudinal waves are effective for detecting internal flaws such as inclusions, cracks, and delamination in metals like ingots, plates, and forgings. Transverse waves are useful for identifying surface and subsurface defects, including weld cracks and slag inclusions. Surface waves are ideal for detecting near-surface flaws, while plate waves are suitable for thin sheets.
Advanced systems, such as B-scan and C-scan flaw detectors, build upon the A-scan technology. These instruments provide two-dimensional or three-dimensional representations of internal reflectors, offering more detailed defect visualization. While traditional ultrasonic testing uses pulsed electrical signals to excite piezoelectric transducers, eddy current testing offers an alternative for conductive materials. This method does not require direct contact with the material or coupling agents, making it suitable for inspecting rough surfaces or high-temperature environments up to 500°C.
As ultrasonic waves travel through a material, their intensity decreases due to absorption and scattering. This attenuation can be measured to assess the homogeneity of materials, especially in alloys produced in vacuum arc melting furnaces.
Compared to other NDT methods, the pulse-echo technique has several advantages: it can detect deep flaws up to several meters, has high sensitivity for small reflectors, accurately determines the shape and orientation of defects, requires access to only one side of the object, provides immediate results, and is relatively safe and portable.
However, there are some limitations. It requires skilled operators, struggles with irregular or heterogeneous materials, and still faces challenges in precisely characterizing the nature and severity of detected defects.