Summary of Ultrasonic Testing Sensitivity Calibration and Defect Quantification Methods

I. Summary of Ultrasonic Testing Sensitivity Calibration Methods

In ultrasonic flaw detection, in order to better locate and quantify defects, we need to understand the following methods:

1. Distance-Amplitude Curve (DAC) Comparison Method
Applicable for: No bottom echo, workpiece thickness X < 3N
The DAC method calibrates the testing sensitivity by using a reference block with reflectors of the same size but at different distances to generate a distance-amplitude curve, which serves as the reference sensitivity. In actual testing, the reflected echo from a defect is compared with this curve to determine the equivalent size of the defect.

2. Test Block Comparison Method in Amplitude Comparison

Applicable for: Reference blocks with material, sound path, and acoustic properties similar to the test workpiece.

A reference block needs to be prepared. According to standard requirements, the echo amplitude from the reflector within the block is adjusted to a specified height to determine the testing sensitivity.

2. Large Flat-Bottom Hole Amplitude Comparison Method in Amplitude Comparison

Applicable for: Workpieces with flat or curved bottom surfaces, X ≥ 3N

When the sensitivity for detecting large flat-bottom reflectors in the workpiece is known, the testing sensitivity for artificial reflectors is determined by the decibel difference between the back-wall echo of the workpiece and the echo from an artificial defect at the same depth.

II. Summary of Defect Quantification Methods

3. Equivalent Size Method

Applicable for: X ≥ 3N, and defect size is smaller than the ultrasonic beam cross-sectional area

This method includes both the calculation method and the AVG curve method, both based on the theoretical echo sound pressure formula. The former involves manually substituting values into the formula for calculation, while the latter uses an AVG curve drawn on the instrument, which then automatically calculates the equivalent defect size.

4. Length Measurement Method

Applicable for: When the ultrasonic beam cross-sectional area is larger than the defect size

The defect size is determined based on the echo amplitude of the defect and the distance the probe is moved.

Relative Sensitivity Length Measurement Method:
Taking the highest echo as a relative reference, the probe is moved along the defect’s length while reducing the amplitude by a certain decibel value (commonly 6 dB) to determine the defect length.

Half-Amplitude Method (also called the 6 dB method):
When the echo amplitude is reduced by 6 dB, it becomes exactly half of the original height.

End-Point Half-Amplitude Method (also called End-Point 6 dB Method):
When there are large variations in echo amplitude across different parts of the defect or multiple echo peaks, the end-point half-amplitude method is commonly used.

Absolute Sensitivity Length Measurement Method:
When the instrument sensitivity is fixed, the probe is moved parallel along the length of the defect. The distance the probe travels while the defect echo amplitude decreases to a specified level (usually the evaluation line) is taken as the indicated defect length.