Main method
Pulse reflection method: Ultrasound waves are emitted by the probe, and the echoes reflected from defects or the bottom surface of the workpiece are received. The position and size of the defect are determined based on the time and amplitude of the echoes. This method has high detection sensitivity and can accurately locate the defect. It is the most widely used.
Penetration method: The transmitting probe and the receiving probe are placed on both sides of the workpiece respectively. The internal defect is determined based on the energy change of the ultrasonic wave after penetrating the workpiece. This method has a lower requirement for the surface finish of the workpiece, but the detection sensitivity is relatively low and it cannot locate the defect. It is often used for automated detection.
Resonance method: The thickness of the workpiece is measured by utilizing the principle that ultrasonic waves generate resonance in the workpiece. This method is particularly suitable for measuring thin plates or thin-walled tubes, but a higher requirement is placed on the surface finish of the workpiece.
The most commonly used ultrasonic testing method is the pulse reflection method.
Its core advantage lies in its high detection sensitivity, which enables accurate location of the defect's position and depth, and its wide application range, applicable to almost all materials. In contrast, the penetration method has lower sensitivity and cannot locate the defect, while the resonance method is mainly used for precise thickness measurement.
The pulse reflection method is currently the most widely used ultrasonic flaw detection method. Its core principle is: an ultrasonic pulse is emitted towards the tested workpiece. When the sound wave encounters a defect inside the material or the bottom surface of the workpiece, it will undergo reflection. The instrument receives and analyzes these reflected waves, and based on the time difference and amplitude changes, it can accurately determine whether there is a defect, its size, and its depth.
This method has extremely high sensitivity and can detect very small defects. The positioning accuracy is also very high, with an error typically less than 2%. It is applicable to various materials such as metals, plastics, and ceramics, and is widely used in industrial inspections. However, it also has limitations. For example, its detection effect is poor for near-surface defects and thin-walled workpieces, and the detection results are related to the orientation of the defect.
The size and depth of the defect are determined by the pulse reflection method. The core of this process lies in analyzing the amplitude and time of the reflected waves.
Determining depth: By measuring the time difference from the emission of the ultrasonic wave to the reception of the defect reflection wave, using the formula "depth = speed of sound × time difference / 2", the instrument usually completes the calculation automatically.
Determining size: The main factor is the amplitude of the reflected wave. The larger the defect, the more acoustic energy is reflected back, and the higher the waveform amplitude displayed by the instrument. In practical operations, the 6dB method or endpoint 6dB method is often used: Move the probe, when the height of the defect wave reduces by half (6dB), the distance the probe moves is the indicated length of the defect. It should be noted that the actual size of the defect may be larger than the measured value because factors such as orientation and surface condition of the defect can affect the height of the echo. In addition, factors such as instrument performance, probe type, and surface condition of the workpiece can also affect the measurement accuracy.
