Factors Affecting the Quantification of Defects in Ultrasonic Testing

Factors Affecting the Quantification of Defects in Ultrasonic Testing

Factors Affecting the Quantification of Defects in Ultrasonic Testing – In ultrasonic testing, it is necessary to find the location of the defect quickly and accurately, and judge its shape, size and type, the error plays a decisive role in the reliability and accuracy of the whole ultrasonic testing results.

1. Instrument effects

1) Horizontal linearity
Horizontal linearity, also called baseline linearity or scan linearity, refers to the extent to which the reflected wave distance displayed on the scanning line of the flaw detector is proportional to the distance of the reflector.

The level linearity of the instrument directly affects the ranging precision and the defect location.

2) Vertical linearity
Vertical linearity, also known as amplification linearity or amplitude linearity, refers to the height of the reflected wave on the flaw detector screen is proportional to the received signal voltage.

The quality of vertical linearity of the instrument directly affects the quantitative accuracy of defects.

2. The effect of the probe

1) Beam deviation

The deviation between the actual main sound beam of the probe and the axis of its theoretical geometry is called the deviation of the main sound beam.

In theory, the beam center and the geometric center of the probe wafer should coincide, whether the incident is vertical or oblique, but in practical application, the beam axis and the geometric center of the wafer will have certain deviation, therefore, the beam deviation should be measured before flaw detection, in order to correct the measurement results, in order to improve the positioning accuracy.

2) Probe Boobs

Parallel movement of the probe, the same reflector to produce two peaks of the phenomenon known as double peaks.

In general, the sound field emitted by the probe has only one main sound beam, and the sound pressure is the highest on the axis of the far field, but some probes have poor performance, and there are two main sound beams. When defects are found, it can not be determined which main sound beam discovered the defect, therefore, it is difficult to determine the actual location of defects, such probe should be selected at the time of procurement.

3) Wedge wear

Shear wave probe in the detection process, the wedge will wear, and when the operator’s uneven force, it will cause the probe wedge before and after different degrees of wear. When the front wear of wedge is large, the refraction angle and K value of probe decrease. When the wear behind the wedge is large, the refraction angle and K value increase. In addition, the wear of the probe will change the incident point of the probe, which will affect the defect location. Therefore, if it is found that the probe is worn, the parameters of the probe should be calibrated in time.

4) Directivity of probe

The probe has small semi-diffusion angle and good directivity, and the positioning error is small, whereas the positioning error is large.

5) Probe type and wafer size

For defects in different positions and directions, different types of probes should be used to reduce the quantitative error.

The size of wafer affects the length of near-field region and beam directivity, so it has some influence on the quantification.

3. The impact of the workpiece

1) Surface roughness

The roughness of the workpiece surface not only affects the coupling effect of the probe, but also makes the time of sound wave entering the workpiece different because of the uneven surface, then the phase of the sound wave entering the workpiece is just opposite, which makes the sound wave entering the workpiece interfere with each other and form a bifurcation, thus making it difficult to locate the defect.

2) Surface shape

There are two kinds of contact conditions between probe and workpiece when detecting surface workpiece. One is that the plane is in contact with the curved surface, which is point or line contact, and the probe’s refraction angle is easy to change when holding it improperly, when the probe is in contact with the surface of the workpiece, the refraction angle and the shape of the sound beam will change, which will increase the defect location and quantitative error.

3) Material of work piece

When the velocity of sound is different from that of the specimen, the K value of the probe will change.In addition, when the internal stress of the workpiece is large, the velocity and direction of sound wave will change, which will affect the location of defects.

4) Workpiece temperature

The K value of the probe is usually measured at room temperature. When the temperature of the workpiece changes, the velocity of sound in the workpiece changes, which changes the refraction angle of the probe and affects the defect location.

5) The workpiece boundary

When the defect is near the boundary of the workpiece, because of the interference between the reflected wave and the incident wave, the sound pressure distribution changes and the axis of the sound beam deviates, which increases the location error of the defect. In order to reduce the influence of sidewall, the probe with high frequency and large chip diameter and good directivity or shear wave detection should be selected. If necessary, the method of comparison of test blocks can also be used to improve the quantitative accuracy.

6) Defect condition

The defect orientation in the workpiece also affects the defect location. When the defect is inclined, the echo is higher when the diffused beam is incident on the defect, and when the defect is positioned, it will be mistaken as the defect is on the axis, which leads to inaccurate positioning.

4. The influence of the operator

1) Instrument sound speed and delay calibration

When the sound velocity of the workpiece is different from that of the test block, the baseline ratio of the instrument will change, which will affect the accuracy of defect location.

In addition, if the echo front is not aligned to the corresponding horizontal scale or the reading is not accurate, the defect location error will also increase.

2) Point of incidence, K value

When the error of incident point and K value of the probe is large, it will also affect the defect location.

3) Improper positioning

The defect location is different from that of flat plate when the cylindrical workpiece is detected by transverse wave circumferentially. If the defect is still treated as flat plate workpiece, the location error will increase. In ultrasonic flaw detection, there are many factors that affect the defect location and quantitative evaluation, which requires the flaw detection staff to constantly compare the defects detected with the theoretical defects, in the work of the accumulation of experience, so that more accurate defects of quantitative and qualitative analysis and evaluation.

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