The core working mechanism of ultrasonic flaw detector is essentially to use the propagation characteristics of high-frequency sound waves (usually exceeding 20kHz) in solid materials to detect internal defects. Its physical basis is the principles of reflection, transmission, and diffraction of waves.

The entire process begins with the ultrasonic waves emitted by the probe. The piezoelectric chip inside the probe generates high-frequency mechanical vibration, namely ultrasonic waves, under the excitation of electrical pulses. This sound wave is transmitted to the inspected workpiece at a specific speed and direction. The speed of sound depends on the elastic modulus and density of the material, and is a fundamental key parameter for the positioning calculation of the flaw detector.

When ultrasonic waves propagate in a material, if the material is uniformly dense, the sound waves will proceed smoothly until they are reflected back by the bottom or the other end. However, once defects (such as pores, cracks, inclusions) are encountered, reflection will occur at the interface due to the significant difference in acoustic impedance (density multiplied by sound velocity) between the defect and the workpiece substrate material. The ultrasonic waves reflected back from this part (called defect echoes) are received by the probe and converted into electrical signals again by the piezoelectric chip.

The core function of a flaw detector is to accurately process these echo signals. It amplifies and filters the electrical signal before displaying it on the screen. The horizontal axis of the screen represents the time (or distance converted) of sound wave propagation, and the vertical axis represents the amplitude of the echo. The instrument operator determines defects by observing the position and height of the echo signal appearing on the screen:

Defect localization: Based on the time difference between the emitted wave and the defect echo, as well as the known material sound velocity, the instrument can accurately calculate the depth of the defect from the detection surface.

Defect quantification: The height of defect echoes is usually related to the size of the defect's reflection area. By comparing the echo height of known sized "artificial defects" (such as flat bottomed holes on standard test blocks), the equivalent size of the defect can be evaluated.

Defect recognition: Defects of different properties (such as point inclusions and plane cracks) will produce waveforms with different characteristics. Experienced operators can use waveform features and defect locations to assist in judgment.

In summary, ultrasonic flaw detectors are like doctors who perform "B-ultrasound" for industrial materials. By actively emitting sound waves and "listening" to echoes from internal structures, based on solid wave physics principles, they achieve precise positioning, quantification, and evaluation of invisible defects, thereby ensuring the safety and integrity of critical structures.