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Ultrasonic non-destructive testing (NDT) relies on the differences in acoustic properties between materials and their internal defects to detect flaws without causing damage. This technique utilizes ultrasonic waves, which travel through the material and reflect or attenuate when encountering discontinuities. The most common methods include pulse-echo and through-transmission, where the reflection of sound pulses or changes in wave amplitude are analyzed. Typical operating frequencies range from 0.5 to 5 MHz, depending on the application.
The most widely used instrument is the A-scan ultrasonic flaw detector, which displays reflected signals on an oscilloscope. By analyzing the presence, timing, and amplitude of these reflections, operators can determine the location, size, and nature of internal defects. The basic structure of such a device includes a transducer, a pulser/receiver, and a display unit, as illustrated in Figure 1.
Different types of ultrasonic waves are used based on the material and defect type. Longitudinal waves are ideal for detecting internal flaws like cracks, voids, and inclusions in metals. Transverse waves are effective for surface and near-surface defects, while surface waves are suitable for identifying surface-level issues. Plate waves are used for thin sheets. Advanced systems like B-scan and C-scan provide two- and three-dimensional representations of the internal structure, enhancing defect visualization.
Modern ultrasonic systems often use piezoelectric transducers, which convert electrical pulses into sound waves. However, eddy current transducers can also be employed for conductive materials, offering advantages such as no need for coupling agents and the ability to inspect rough or high-temperature surfaces.
As ultrasonic waves travel through a material, they experience attenuation due to absorption and scattering. This property is useful in assessing the homogeneity of materials, particularly in alloys produced using vacuum consumable melting techniques.
Compared to other NDT methods, pulse-echo ultrasonic testing offers several benefits: it can penetrate deep into materials, detect very small defects, accurately locate and characterize flaws, and requires access from only one side. It provides immediate results and is safe and portable. However, it requires skilled operators and may not be suitable for complex, thin, or heterogeneous materials. Additionally, quantifying the exact nature of detected flaws remains challenging.
Overall, ultrasonic testing is a powerful and versatile method, widely used across industries for ensuring material integrity and safety.