JPS6318263A - Ultrasonic flaw detection for bolt - Google Patents

Abstract

PURPOSE:To detect a crack flaw generated in a bolt accurately by ultrasonic flaw detection, by using a longitudinal wave vertical probe having such a characteristic that an ultrasonic beam is focused eccentric from the center of the beam thereof. CONSTITUTION:A special ultrasonic probe 1 is made up of a vibrator 4 formed in a curved surface and a delay material 5 made of acrylic resin. When a crack flaw 7 generated at the thread bottom of a bolt 2 is irradiated with a tilted ultrasonic beam 6, a sound pressure of the ultrasonic beam reflected from the crack flaw becomes much larger because of a shape characteristic of the crack flaw. The crack flaw generated at the screw bottom of the bolt generally covers from that occurs near the thread tip 3 to that occurs at a relatively distant point. The crack flaw generated at a desired point of the thread section can be detected by scanning the thread tip 3 with the probe 1. The inspection of the entire area of the thread bottom is possible by changing the direction of scanning with the probe 1 totally in the diametrical direction on the surface of the thread tip section 3.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for accurately detecting cracks occurring in bolts used in steel structures such as bridges using ultrasonic flaw detection.

(Prior Art) Bolts used in steel structures may break during use due to cracking occurring at the bottom of the bolt thread due to corrosion or fatigue loading. Such damage to bolts may reduce the safety of the structure or cause bolts to break and fall from the structure, causing damage to vehicles passing under the PiZ, creating a safety problem.

Therefore, bolts in use are regularly inspected, and currently used methods include tapping inspection using a hammer or inspection using ultrasonic waves.

(Problems to be Solved by the Invention) However, identifying the sound caused by the tapping of a hammer is difficult and ineffective unless the bolt is severely damaged by cracking.

In addition, in the ultrasonic method, inspection is carried out by bringing a conventional vertical probe into contact with the threaded tip of the bolt or the bolt head, but the ultrasonic beam propagates uniformly throughout the bolt. It is difficult to detect small scratches that occur from the bottom of the screw because reflected echoes from the screw appear.

The present invention has been made to solve these problems. Figure 1 shows the arrangement of the special ultrasonic probe used for inspection and the bolt that is the object to be inspected. Cracks that occur on bolts during use generally start from the bottom of the thread of the bolt. Therefore, it is necessary to effectively irradiate the ultrasonic beam to the bottom of the screw.

(Means for Solving the Problems) That is, the special ultrasonic probe 1 used in the present invention has the beam of a conventional focusing probe (vertical type) eccentric or eccentric from the central axis by a number of lO. It has a characteristic of having a focal point at a certain point, and is generally inspected by contacting an ultrasonic probe from the threaded tip 3 of the bolt 2.

(Function) The special ultrasonic probe 1 includes a vibrator 4 processed into a curved surface,
It is composed of a delay material 5 made of acrylic resin. The beam focusing range of this special ultrasonic probe is generally about 20 to 0000 m deep from the screw tip, but other ranges can be designed and manufactured.

When the ultrasonic beam 6 tilted in this way is irradiated to the crack 7 generated from the threaded bottom of the bolt 2, the sound pressure reflected from the ultrasonic crack can become significantly large due to the shape characteristics of the cracked metal. found. The reason for this is, firstly, that the main beam of ultrasonic waves cannot be effectively applied to the screw tip when f' scratches are used with conventional ultrasonic probes, which reduces the detection ability. It has been removed. Second, the bottom of the bolt thread is generally curved.
When a crack occurs in the screw bottom, the hot water part forms a part of a nearly right-angled corner.

Therefore, when using the special ultrasonic probe according to the present invention, the main beam of ultrasonic waves is irradiated onto the hot water part at a part of the corner.
Moreover, since it is a part of the corner, the reflection characteristics of the ultrasonic beam become large. This is because the sound pressure reflected from the scratches is significantly greater than the sound pressure reflected from the threaded part when there is no crack, and the threaded part and the crack can be easily distinguished.

The cracks that occur from the bottom of the bolt screw are generally cracks 7' that occur near the screw tip 3, as shown in Figure 2.
Since there are cracks up to 7" that occur relatively far from the thread, by scanning the special ultrasonic probe 1 over the surface of the screw tip 3 in the direction of the arrow 8, it is possible to detect cracks that occur at any location on the thread. In addition, the entire area of the threaded bottom of the bolt can be inspected by changing the scanning direction of the special ultrasonic probe 1 on the threaded tip surface in all directions in the diametrical direction.

(Example) The effects of the present invention will be illustrated in more detail with reference to Examples below.

The bolt shown in Figure 3(a) (diameter 22mm, length 105+s
m) at a distance of approximately 3 hm from the screw tip to a depth of approximately 1
Regarding a port with a crack of 1, the flaw detection results were compared between a special ultrasonic probe according to the method of the present invention and a conventional vertical probe. The special ultrasonic probe used this time has a flaw detection frequency of 5MHz, a transducer diameter of 15m1, an eccentric angle of 15°, and an ultrasonic beam focal range of 15~Him, while the conventional probe has a flaw detection frequency of 511I)lz and a transducer. The respective flaw detection waveforms with a diameter of 10 mm are shown in FIGS. 3(b) and (C).

As shown in FIG. 3(c), in the conventional ultrasonic probe, the reflected echo from the crack is 1ff in the reflected echo 9 from the screw part! The wound echoes could not be detected effectively. - In the case of the special ultrasonic probe invented by Rikiki, see Figure 3 (
As shown in b), the reflected echo 10 from the crack appears large and can be detected by sufficiently distinguishing it from the reflected echo 9 from the threaded portion.

(Effects of the Invention) As described above, when bolts are inspected using the special ultrasonic probe according to the method of the present invention, even minute cracks that could not be detected conventionally can be detected. In addition to ports, it can also be applied to inspecting threaded parts such as vibrator threaded joints, which has a significant advantage in terms of maintenance and inspection.

[Brief explanation of drawings]

Figure 1 shows the inspection status of a bolt, which is an object to be inspected, using a special ultrasonic probe, Figure 2 shows the scanning inspection status of the special ultrasonic probe, and Figure 3 shows the flaw detection waveform using the special ultrasonic probe. be. 1... Special ultrasonic probe, 2... Bolt, 3...
Screw tip, 4... vibrator, 5... delay material, 6...
Ultrasonic beam, 7...Crack, 7'...Crack near the screw tip, 7N...Crack far from the screw tip, 8.
...Scanning direction of special ultrasonic probe, 9...Reflection echo from bolt thread, 10...Reflection echo from bolt crack.

Claims (1)

[Claims] Inspecting cracks that occur in bolts used in steel structures such as bridges using a longitudinal wave vertical probe that has the characteristic that the ultrasonic beam focuses eccentrically from the beam center of the probe. An ultrasonic flaw detection method for bolts featuring: