JP3833591B2 - Ultrasonic flaw detection method - Google Patents

Description

[0001]
[Industrial application fields]
The present invention, fillet welds, which for example relates to a testing method for detecting defects in welds between the annular plate and the tank side plate of the tank bottom plate outer periphery.
[0002]
[Prior art]
A tank for storing heavy oil, liquefied gas, or the like is generally configured by placing a tank side plate on an annular plate, which is the outer peripheral portion of the tank bottom plate, and joining them together by fillet welding. A flaw detection test is performed as necessary to detect cracks in the vertical direction that occur in the surface (upper surface) of the annular plate between fillet welds between the annular plate and the tank side plate.
[0003]
A conventional general flaw detection test is to detect the presence of a defect by propagating an ultrasonic pulse to an annular plate using an ultrasonic probe and receiving an ultrasonic wave reflected by the defect. In addition, among the defects in the fillet welds between the annular plate and the tank side plate, when trying to detect cracks occurring on the surface side of the annular plate, it propagates on the surface of the annular plate as an ultrasonic probe. A longitudinal wave that transmits and receives so-called creeping waves is used. Although the fillet weld shape and the defect to be flawed are different, as described in Japanese Patent Application Laid-Open No. 2000-97919, the transverse wave emitted from the creeping wave probe is reflected on the bottom surface or back surface of the plate material. There is also a method in which the secondary creeping wave generated in the above is reflected by a defect in the fillet weld and the reflected creeping wave is received through the same path as the transmission to find the defect.
[0004]
[Problems to be solved by the invention]
However, in the conventional flaw detection method using the creeping wave type ultrasonic probe as described above, the level of the reflected wave received by the probe is low, in other words, the S / N ratio is low. There was a point.
[0005]
Therefore, an object of the present invention is to provide an ultrasonic flaw detection method capable of detecting a defect in a fillet weld with high accuracy at a high S / N ratio.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the present invention according to claim 1 is directed to ultrasonic detecting a defect of a fillet weld between a first plate and a second plate such as an annular plate and a tank side plate in a tank. In the type flaw detection method, a step of transmitting a transverse wave ultrasonic pulse from the front surface of the first plate material toward the back surface, and the transverse wave reflected by the back surface of the first plate material is reflected by a defect of the fillet weld. Receiving a creeping wave generated by the above-mentioned method and converting it into an electrical signal, and detecting the presence or absence of a defect by detecting the presence or absence of the electrical signal.
[0007]
In such a flaw detection method, the transmitted wave is a transverse wave, and only the reflected wave is a creeping wave. Transverse waves rarely undergo mode conversion during propagation, and thus energy loss associated with mode conversion is small. On the other hand, the creeping wave undergoes mode conversion into a transverse wave during propagation, resulting in a large energy loss. Therefore, in the method according to the present invention in which only one way becomes a creeping wave, the level of the received signal is large and the defect can be detected with high accuracy.
[0008]
Further, as an apparatus for effectively carrying out the flaw detection method according to the present invention, a transverse ultrasonic pulse can be transmitted from the surface of the first plate material to the back surface, and propagates through the surface of the first plate material. An ultrasonic probe that can receive an ultrasonic wave of a creeping wave and convert it into an electrical signal, a pulse generation means for generating an ultrasonic pulse from the ultrasonic probe, and an ultrasonic probe It is conceivable to include signal processing means for processing an electrical signal generated by receiving ultrasonic waves of a creeping wave and detecting a defect in the fillet weld from the presence or absence of the electrical signal .
[0009]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
[0010]
FIG. 1 schematically shows an ultrasonic flaw detector 10 according to the present invention. The flaw detection apparatus 10 in the illustrated embodiment includes a defect such as a crack generated in a fillet weld 18 between an annular plate (first plate member) 14 and a tank side plate (second plate member) 16 in a tank 12 of liquefied natural gas or the like. This is for detecting 20.
[0011]
In FIG. 1, reference numeral 22 denotes an ultrasonic probe. The ultrasonic probe 22 can transmit a transverse wave from the front surface of the annular plate 14 as the test material to the back surface, as indicated by the dotted arrow in FIG. 1, and the solid line in FIG. As indicated by the one-dot chain line arrow, the creeping wave (longitudinal wave) propagating on the surface of the annular plate 14 can be received. A creeping wave type probe can also be used because a transverse wave is generated simultaneously with the generation of the creeping wave.
[0012]
Although not shown, the ultrasonic probe 22 has one piezoelectric element that serves as both an ultrasonic pulse transmitting member and a receiving member. Connected to this piezoelectric element are a pulse generation circuit 24 for generating a pulse and a signal processing circuit 28 for processing the received signal and displaying it on the cathode ray tube 26.
[0013]
A sweep generation circuit 30 is connected to the pulse generation circuit 24. The sweep generation circuit 30 generates a sawtooth sweep signal, and generates a trigger pulse having a constant period from the sweep signal. The pulse generation circuit 24 is driven in response to this trigger pulse, and generates an ultrasonic pulse by applying large energy to the piezoelectric element of the ultrasonic probe 22. This ultrasonic pulse from the piezoelectric element is propagated to the annular plate 14 as a creeping wave and a transverse wave.
[0014]
The sweep signal generated by the sweep generation circuit 30 is horizontally amplified and applied to the X-axis polarizing plate of the cathode ray tube 26, and functions to swing the spot from left to right.
[0015]
The signal processing circuit 28 connected to the piezoelectric element of the ultrasonic probe 22 includes a high-frequency amplifier circuit 32 that amplifies an electrical signal generated when the piezoelectric element receives a creeping wave, and the amplified signal. And a video amplification circuit 34 for detecting the video signal and amplifying the video signal, and a vertical amplification circuit 36 for vertically amplifying the output from the detection / video amplification circuit 34. The signal that has passed through the vertical amplifier circuit 36 is applied to the Y-axis deflection plate of the cathode ray tube 26 so that the waveform of the received signal is displayed on the cathode ray tube 26.
[0016]
The circuit shown in FIG. 1 includes a known circuit such as a detection switching circuit 38 for switching the detection method and a gate circuit 40 for taking out a signal in a limited time.
[0017]
The ultrasonic flaw detection apparatus 10 of the illustrated embodiment further includes a carriage 42 that supports the ultrasonic probe 22. The carriage 42 is configured to automatically travel on the annular plate 14 in the circumferential direction of the tank while maintaining a constant distance from the tank side plate 16.
[0018]
Next, a method for detecting a defect 20 such as a crack generated in the fillet weld portion 18 between the annular plate 14 and the tank side plate 16 using the flaw detection apparatus 10 having the above-described configuration will be described.
[0019]
First, as a pretreatment, the shape and thickness of the annular plate 14 of the opened tank 12 are confirmed. Next, the surface region of the annular plate 14 adjacent to the fillet weld portion 18 and a range in which a defect 40 such as a crack can occur is specified. Further, the thickness of the annular plate 14 and the installation position of the ultrasonic flaw detector 10 with respect to the inner surface of the tank side plate 16 (that is, the distance from the incident position of the ultrasonic pulse of the ultrasonic probe 22 to the tank side plate 16). From this, the propagation distance of the ultrasonic pulse is calculated, and the scanning range is calculated from the range in which the crack 20 can occur. Based on this calculated value, the time axis and flaw detection sensitivity of the flaw detection apparatus 10 are calibrated.
[0020]
Further, a treatment such as applying an appropriate adhesive medium on the annular plate 14 is performed so that the adhesion of the ultrasonic probe 22 to the surface of the annular plate 14 is improved and a better flaw detection test is possible. .
[0021]
Thereafter, the carriage 42 of the flaw detection apparatus 10 is set at a predetermined position on the annular plate 14, and the flaw detection test is performed by emitting ultrasonic pulses while moving the carriage 42 in the circumferential direction of the tank.
[0022]
When electrical energy is applied from the pulse generation circuit 24 to the piezoelectric element of the ultrasonic probe 22, an ultrasonic pulse is generated. Of the longitudinal wave component of this ultrasonic pulse, the one incident on the annular plate 14 at the first critical angle becomes a creeping wave and propagates outward along the tank radial direction. Simultaneously with the generation of creeping waves, a transverse wave of ultrasonic pulses is transmitted from the front surface of the annular plate 14 to the back surface. This transverse wave is reflected by the back surface of the annular plate 14, and a part thereof goes to the fillet weld 18.
[0023]
When a creeping wave and a transverse wave are generated in this way, if a crack 20 is present in the fillet weld 18 at a position facing the ultrasonic probe 22, first, the creeping wave is reflected by the crack 20. Then, the creeping wave is returned to the ultrasonic probe 22 and converted into an electrical reception signal by the piezoelectric element (see the dashed line arrow in FIG. 1). Then, after being processed by the signal processing circuit 28, the signal is displayed on the cathode ray tube 26. In FIG. 2, the waveform indicated by symbol a represents the received reflected wave.
[0024]
On the other hand, the transverse wave emitted from the ultrasonic probe 22 is reflected by the back surface of the annular plate 14 and then reaches the crack 20 (see the dotted arrow in FIG. 1). The transverse wave that has reached the crack 20 changes its mode to become a creeping wave, and travels backward toward the ultrasonic probe 22 (see the solid line arrow in FIG. 1). This reflected creeping wave is also converted into an electrical reception signal by the piezoelectric element, processed by the signal processing circuit 28, and then displayed on the cathode ray tube 26. The signal is indicated by symbol b in FIG.
[0025]
Here, when the reception level of the signal a obtained in the case of the creeping wave in both transmission and reception is compared with the reception level of the signal b obtained in the mode conversion from the transverse wave to the creeping wave, the latter is more than the former. It can be seen from FIG. 2 that it is much larger. This is because the creeping wave is mode-converted into a transverse wave during propagation, so that the energy loss is remarkably large, the transverse wave has almost no mode conversion during propagation, and the energy loss during propagation is extremely small. That is, when both the transmitted wave and the reflected wave are creeping waves, the energy loss is more than twice that of the case where only the reflected wave is a creeping wave. Therefore, the method of detecting the crack 20 by mode conversion from a transverse wave to a creeping wave has a higher S / N ratio, and the crack 20 can be reliably detected.
[0026]
As described above, the crack 20 can be detected from the presence or absence of the received signal b by using only the ultrasonic flaw detection method using the transverse wave / creeping wave. However, the position and depth of the crack 20 or the path of the ultrasonic wave is being detected. One of the signals may not appear due to various causes such as the presence of an obstacle. Therefore, when at least one of the signals a and b is received, it is preferable to handle it as a suspicion of the existence of the defect 20 such as a crack.
[0027]
In addition, the material of the annular plate 14 and the speed of ultrasonic waves in the material are known. For example, when the annular plate 14 is SM400, the sound velocity of the transverse wave is 3230 m / s, and the longitudinal wave is 5920 m / s. Therefore, if the thickness of the annular plate 14 and the distance from the inner surface of the tank side plate 16 to the ultrasonic probe 22 are measured in advance, the defect 20 detected from the time when the signals a and b appear is identified. be able to. Thereby, more reliable defect detection becomes possible.
[0028]
In addition, when there is no defect 20 in the fillet welded portion 18, no reflected wave is generated, and neither of the signals a and b is generated, so that it can be determined that the state is normal.
[0029]
As mentioned above, although preferred embodiment of this invention was described in detail, it cannot be overemphasized that this invention is not limited to the said embodiment.
[0030]
For example, in the above embodiment, the received signal is processed in an analog manner, but it can be digitally processed and digitally displayed by a digital circuit, or a defect position can be specified and a signal can be recorded from the digital signal. Can do.
[0031]
Further, the carriage 42 may not be of the automatic traveling type. Alternatively, the carriage 42 is not necessary when a flaw detection test is performed only on a specific portion.
[0032]
Furthermore, in the above embodiment, the defect 20 of the fillet weld 18 between the annular plate 14 and the tank side plate 16 is detected. However, the present invention is not limited to the SM400, and other materials such as 9% nickel steel. The present invention is also applicable to flaw detection of fillet welds.
[0033]
【The invention's effect】
As described above, according to the present invention, a transversal ultrasonic pulse is transmitted and a creeping wave formed by being reflected by a defect such as a crack is received. As a result, it is possible to detect a defect generated in the fillet weld between the plate members with high accuracy.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory view showing an embodiment of an ultrasonic flaw detector according to the present invention.
FIG. 2 is a graph showing a waveform of a signal obtained by the ultrasonic flaw detector shown in FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Ultrasonic flaw detector, 12 ... Tank, 14 ... Annular board (1st board | plate material), 16 ... Tank side board (2nd board | plate material), 18 ... Fillet weld part, 20 ... Defect, 22 ... Ultrasonic probe , 24 ... pulse generation circuit (pulse generation means), 26 ... cathode ray tube, 28 ... signal processing circuit (signal processing means), 42 ... bogie.

Claims (1)

An ultrasonic flaw detection method for detecting a defect (20) of a fillet weld (18) between a first plate (14) and a second plate (16),
Transmitting a transverse ultrasonic pulse from the front surface to the back surface of the first plate (14);
The creeping wave generated by reflecting the transverse wave reflected by the back surface of the first plate member (14) by the defect (20) of the fillet weld (18) is received as an electrical signal. Converting, and
Detecting the presence or absence of a defect (20) by detecting the presence or absence of the electrical signal;
Ultrasonic flaw detection method including

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