Currently, the A-type pulse reflection ultrasonic flaw detector from YUSHI is used to evaluate the location and size of defects in the inspected workpiece based on the position and height of the defect waves on the screen. Thus, understanding the influencing factors is extremely beneficial for improving the accuracy of positioning and quantification.
Main Factors Affecting Defect Positioning
1.Influence of the Instrument
The quality of the instrument's horizontal linearity has a certain impact on defect positioning. A poor horizontal linearity can lead to deviations between the displayed defect position and the actual position.
2.Influence of the Probe
Factors such as the deviation of the probe's sound beam, the presence of double peaks, wear of the wedge, and directivity all affect defect positioning. A deviated sound beam can cause the ultrasonic wave to propagate in an unexpected direction, resulting in an incorrect position of the received reflected wave. Double peaks can lead to confusion in signal judgment, making it difficult to determine the true defect position. Wedge wear can change the refraction angle of the ultrasonic wave, causing inaccuracies in measuring the defect depth and other data. Poor directivity may result in receiving reflected waves from non-defect positions, leading to misjudgment of the defect position.
3.Influence of the Workpiece
The surface roughness, material properties, surface shape, boundary effects, temperature, and defect conditions of the workpiece all affect defect positioning. A rough surface can cause chaotic reflection of ultrasonic waves, affecting signal reception and analysis. Inhomogeneous materials can change the propagation speed of ultrasonic waves, leading to errors in defect positioning. Irregular surface shapes, such as curved surfaces, can make the path and angle of the reflected wave complex, increasing the difficulty of positioning. Boundary effects refer to the complex phenomena of reflection and refraction of ultrasonic waves when close to the workpiece boundary, which can interfere with defect positioning. Temperature changes can alter the acoustic properties of the material, thereby affecting the propagation and positioning of ultrasonic waves. The shape and orientation of the defect itself can also affect the path and time of the reflected wave, influencing positioning.
4.Influence of the Operator
Errors in parameters such as the zero point and K value (tangent of the probe refraction angle) during instrument debugging, or the use of inappropriate positioning methods, can affect defect positioning. If the zero point is set incorrectly during instrument debugging, it will make the starting position of the entire measurement wrong, resulting in inaccurate positioning of all measured defects. An error in the K value will cause deviations in the calculated defect depth and horizontal distance. Using an inappropriate positioning method, such as choosing a method that is not suitable for the workpiece and defect conditions, will also lead to inaccurate positioning.
Main Factors Affecting Defect Quatification
1.Influence of Instrument and Probe Performance
The vertical linearity of the instrument, its precision, and the probe's frequency, type, crystal size, and refraction angle all directly affect the height of the defect echo. Poor vertical linearity of the instrument can cause the same-sized defect to display different echo heights, leading to misjudgment of the defect size. The precision of the instrument determines the accuracy of the measurement. A low precision will result in large errors in the measured echo height and other data. The probe frequency affects the resolution and penetration ability of the ultrasonic wave. An excessively high or low frequency may make the defect echo unclear or inaccurate. Different probe types, such as straight probes and angle probes, have different effects on the reception and display of defect echoes.
2.Influence of Coupling and Attenuation
The acoustic impedance of the couplant and the thickness of the coupling layer have a significant impact on the echo height. If the acoustic impedance of the couplant and the thickness of the coupling layer are not suitable, the transmission efficiency of the ultrasonic wave between the probe and the workpiece will be reduced, and the echo height will decrease, resulting in errors in defect quantification. When the coupling states of the probe on the calibration block used for sensitivity adjustment and the inspected workpiece surface are different, and appropriate compensation is not carried out, the quantification error will increase, and the accuracy will decrease.
3.Influence of Workpiece Geometric Shape and Size
The shape of the workpiece bottom surface affects the echo height. A convex curved surface makes the reflected wave diverge, reducing the echo height, while a concave curved surface makes the reflected wave focus, increasing the echo height. The parallelism between the workpiece bottom surface and the detection surface, as well as the smoothness and cleanliness of the bottom surface, also have a significant impact on defect quantification. Due to the interference of the side wall, when detecting defects near the side wall of the workpiece, inaccurate quantification and increased errors will occur. The size of the workpiece also has a certain influence on quantification.
4.Influence of the Defect
Different defect shapes have a great influence on the echo height. The orientation of the defect also affects the echo height. In addition, the directivity of the defect wave is related to the size of the defect, and the difference is relatively large. Moreover, the echo height of the defect is also affected by factors such as the surface roughness of the defect, the nature of the defect, and the position of the defect. Defects with different shapes, such as spherical pores and sheet-like cracks, have different reflection and scattering characteristics of ultrasonic waves, resulting in large differences in echo heights. The relative angle between the defect orientation and the probe also affects the echo height. For example, a defect perpendicular to the probe may have a higher echo than a tilted defect. The directivity of the defect wave varies with the size of the defect. Small defects may have more scattered waves, while large defects may have more concentrated reflected waves, which will all affect the echo height and quantification. The surface roughness, nature (such as pores, inclusions, cracks, etc.), and position (depth, distance from the boundary, etc.) of the defect will also affect the echo height and quantification.
Discrimination of Non-defect echoes
In ultrasonic flaw detection, in addition to the initial wave, bottom wave, and defect wave on the screen, there may also be some other signal waves, such as late waves, triangular reflection waves, 61° reflection waves, and non-defect echoes caused by other reasons. It is very necessary to analyze and understand the causes and characteristics of common non-defect echoes.
Late Waves: These are waves that appear later due to different propagation paths of ultrasonic waves in the workpiece, resulting in some waves having a longer propagation time. Their characteristics are that they appear at specific time positions, generally related to the geometric shape and size of the workpiece.
Triangular Reflection Waves: Usually, they are formed by multiple reflections of ultrasonic waves in the angular reflection structure of the workpiece. Their waveforms and appearance positions have certain rules, related to the angle and size of the angular reflection.
61° Reflection Waves: They are waves generated by the ultrasonic wave reflecting at a specific angle under specific probe angles and workpiece structures, with specific reflection angle and propagation path characteristics.
Understanding the causes and characteristics of these non-defect echoes helps to accurately identify and exclude them during the flaw detection process, avoiding misjudgment as defect waves.