Bolt thread crack inspection device and method

Abstract

To provide bolt thread crack inspection means capable of detecting the "outer arc length of a crack" that has developed in a bolt thread section from the end face of a bolt in use.SOLUTION: In an ultrasonic beam focusing setting step S1, a phased array probe 10 is brought into close contact with the end face 1 of the bolt in use, and an ultrasonic beam S is set to focus onto a focal point 2, which is a point on a plane perpendicular to the bolt axis, on the bolt end face 1. In an inspection radius setting step S3, the focal point 2 is set at a position separated from the bolt axis by an inspection radius r on the plane where a crack M exists. In an internal arc length detection step S4, the probe 10 is rotated around the bolt axis, and a crack arc length Lb(r) at the inspection radius is detected from the received ultrasonic intensity. In an outer arc length detection step S5, the inspection radius r is varied from inside the crack M to a bolt threaded portion 3, and crack arc length detection is repeated. The outer arc length Lb of the crack at the thread root is detected from the variation in crack arc length.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an apparatus and method for inspecting a bolt thread for cracks, which detects the arc length or shape of a crack from the end face of a bolt in use. [Background technology]

[0002] When repeated stress acts on a metal material, slip bands are generated on the surface due to shear stress. The slip bands grow deeper with repeated stress, causing fatigue cracks. In bolts in use, such as foundation bolts for nuclear equipment, fatigue cracks can occur in locations that cannot be seen from the outside (for example, buried parts) due to repeated stress. Patent Document 1, for example, has proposed a means for detecting cracks in such bolts in use.

[0003] The "Phased Array Ultrasonic Flaw Inspection Method and Ultrasonic Flaw Inspection System" of Patent Document 1 uses the finite element method to simulate wave propagation in a bolt and identify the characteristic that crack echoes have high intensity at two locations: the crack opening and near the crack tip. Based on this finding, this ultrasonic flaw inspection method includes a flaw detection process, a crack inspection process, a maximum echo detection process, an axial position detection process, a tip refraction angle detection process, and a crack length detection process. In the flaw detection process, a phased array probe is disposed on one end surface of the object to be inspected, and ultrasonic waves are irradiated toward the interior of the object while changing the refraction angle to obtain flaw inspection data by digitizing the echoes from the object. In addition, in the tip refraction angle detection process, the smaller of the two refraction angles that give the peak echo intensity of the flaw inspection data is selected as the tip refraction angle. Furthermore, in the crack length detection process, the "crack length," which is the dimension of the crack in a direction perpendicular to the axial direction, is detected based on the tip refraction angle. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-184068 Summary of the Invention [Problem to be solved by the invention]

[0005] The method of Patent Document 1 described above has the following problems. (1) The intensity of the ultrasonic waves irradiated at the inspection position is weak. Patent Document 1 uses a linear array probe in which multiple (e.g., 32) elements are arranged in a line. In this case, the multiple elements are arranged only on the center line passing through the center of the circular inspection surface of the bolt being inspected, and no elements are arranged in areas away from the center line. As a result, the area of ​​the entire elements is small compared to the entire circular inspection surface, and the intensity of the ultrasound waves output from the probe is proportional to the area of ​​the entire elements, so the intensity of the ultrasound waves irradiated at the inspection position is weak. (2) The received data strength is weak. In Patent Document 1, ultrasonic waves are irradiated toward the inside of the object under inspection by changing the refraction angle, so the ultrasonic waves are not focused and the ultrasonic intensity in the irradiation surface remains weak when irradiated to the inspection position, resulting in an even weaker reception intensity of the received data. (3) It is difficult to distinguish between the received signal from the crack on the outer surface of the bolt and noise, and the outer arc length of the crack cannot be measured accurately. The outer arc length of cracks on the outer surface of a bolt has traditionally been measured by visual observation, and in relation to this, there is a demand for detecting not only the "crack length" but also the "crack outer arc length" in order to accurately predict the remaining life. However, in the method of Patent Document 1, ultrasonic waves are irradiated at the same refraction angle, so when inspecting the area around the bolt threads, most of the ultrasonic beam is reflected by the threads, resulting in a high noise level, making it difficult to distinguish between the received signal from the crack and the noise, and making it impossible to accurately measure the outer surface arc length of the crack. (4) The planar shape and size of the crack cannot be measured accurately. The planar shape and size of a crack are important for predicting the remaining strength of a bolt, but Patent Document 1 does not even suggest a means for measuring the planar shape and size of a crack.

[0006] The present invention was devised to solve the above-mentioned problems. Specifically, the first object of the present invention is to provide a means for inspecting cracks in bolt threads that can detect the "outer surface arc length of a crack" that has occurred in the bolt thread from the end face of a bolt in use. The second object is to provide a means for inspecting cracks in bolt threads that can detect the "planar shape and size of a crack" that has occurred in the bolt thread. The third object is to provide a means for inspecting cracks in bolt threads that can increase the intensity of ultrasound irradiated to the inspection position from the end face of a bolt in use and increase the reception strength of the received data. [Means for solving the problem]

[0007] According to the present invention, a phased array probe having a plurality of independently excitable ultrasonic transducers and a detection surface for transmitting and receiving ultrasonic beams; a control device that controls the plurality of ultrasonic transducers and transmits and receives the ultrasonic beams from each ultrasonic transducer; a guide device that guides the phased array probe rotatably around a bolt axis while the detection surface is in close contact with an end surface of a bolt in use; a rotational position detection device that detects a rotational position of the phased array probe about the bolt axis, The phased array probe is placed in close contact with the bolt end surface, and the ultrasonic beam is focused from the bolt end surface to a focal point that is one point on a plane perpendicular to the bolt axis; On the plane where the crack exists, the focusing point is set at a position separated by an inspection radius from the bolt axis, Rotating the phased array probe around the bolt axis to detect a crack arc length at the inspection radius from the received ultrasonic wave intensity; A crack inspection device for bolt threads is provided, which repeatedly detects the crack arc length by changing the inspection radius from the inside of the crack to the bolt thread, and detects the outer surface arc length of the crack at the thread root from the change in the crack arc length.

[0008] Furthermore, according to the present invention, an ultrasonic focusing setting step is performed in which a phased array probe is brought into close contact with an end surface of a bolt in use, and an ultrasonic beam is focused on a focusing point that is a single point on a plane perpendicular to the bolt end surface and the bolt axis; an inspection radius setting step of setting the focal point at a position spaced an inspection radius from the bolt axis on the plane where the crack exists; an internal arc length detection step of rotating the phased array probe around the bolt axis and detecting a crack arc length at the inspection radius from the received ultrasonic wave intensity; A method for inspecting cracks in bolt threads is provided, which includes an outer surface arc length detection step of repeatedly detecting the crack arc length by changing the inspection radius from the inside of the crack to the bolt thread, and detecting the outer surface arc length of the crack at the thread root from the change in the crack arc length. [Effects of the Invention]

[0009] According to the present invention, a phased array probe is placed in close contact with the end face of a bolt in use, and an ultrasonic beam is focused at a focal point, which is a point on a plane perpendicular to the bolt axis from the end face of the bolt. This increases the intensity of the ultrasonic waves irradiated at the focal point (inspection position), and increases the reception intensity of the received ultrasonic data.

[0010] In addition, a focal point is set at a position on the plane where the crack exists, away from the bolt axis by the inspection radius, and the phased array probe is rotated around the bolt axis, allowing the crack arc length at the inspection radius to be detected from the received ultrasonic wave intensity.

[0011] In addition, with conventional methods, the noise level is high near the bolt thread, making it difficult to distinguish between the received signal from the crack and the noise. However, with the present invention, by changing the inspection radius from the inside of the crack to the bolt thread and repeatedly detecting the crack arc length at the inspection radius, the outer surface arc length of the crack in the bolt thread can be detected from the change in the crack arc length.

[0012] Therefore, the present invention can increase the intensity of ultrasonic waves irradiated at the inspection position (focus point) by focusing, thereby increasing the reception intensity of ultrasonic data. Also, it is possible to detect the outer surface arc length, planar shape, and size of a crack that has occurred in the bolt thread from the end face of the bolt during use. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram showing the overall configuration of a phased array flaw detector for implementing the present invention. [Figure 2] FIG. 2 is a view taken along the arrow AA in FIG. [Figure 3] FIG. 1 is a side view of a test piece (bolt) to be inspected by the present invention. [Figure 4] FIG. 4 is a cross-sectional view taken along line BB in FIG. 3, and is a schematic cross-sectional view of a crack portion. [Figure 5] 1 is a diagram showing the configuration of a crack inspection device for carrying out the method of the present invention. [Figure 6] 1 is an overall flow diagram of an ultrasonic flaw detection method according to the present invention. [Figure 7] FIG. 2 is an explanatory diagram of steps S1 to S5 of the present invention. [Figure 8] FIG. 2 is an explanatory diagram of steps S1 to S5 of the present invention. [Figure 9] This shows the test results of the outer surface arc length detection step S5 for the symbols r8, r16, and r18. [Figure 10] 10 shows test results of the outer surface arc length detection step S5 similar to FIG. 9 when the inspection radius r is at the positions indicated by symbols r8 to r18. [Figure 11] FIG. 10 is an explanatory diagram of step S6 of the present invention. [Figure 12] This shows the test results of the crack length detection step S6. DETAILED DESCRIPTION OF THE INVENTION

[0014] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. In the drawings, common parts are designated by the same reference numerals, and duplicated explanations will be omitted.

[0015] FIG. 1 is a diagram showing the overall configuration of a phased array flaw detector 50. As shown in FIG. In this figure, a phased array flaw detector 50 includes a phased array probe 10 and a control device 20. Reference numeral 1 denotes the surface of the test piece TP (test piece surface). Hereinafter, the phased array probe 10 in FIG. 1 will be simply referred to as the "probe 10" unless otherwise necessary.

[0016] The phased array probe 10 (probe 10) has a plurality of ultrasonic transducers 12 (hereinafter referred to as "transducers 12") that can be excited independently.

[0017] In FIG. 1, a gap 11 is provided between adjacent ultrasonic transducers 12 (transducers 12), and each transducer operates independently. The transducer 12 also has a detection surface 14 that transmits and receives the ultrasonic beam S. In this example, the detection surface 14 is in direct contact with the surface of the test piece TP (test piece surface 1), but an intermediate member may be provided therebetween.

[0018] The control device 20 controls the plurality of transducers 12, and each transducer 12 transmits and receives an ultrasonic beam S. The frequency of the ultrasonic beam S is, for example, 5 MHz, which can be used in steel.

[0019] In FIG. 1, the phased array flaw detector 50 further includes a display device 22 and an image processing device 24.

[0020] The display device 22 is a display device that displays the intensity of the ultrasonic waves received by the probe 10 (reception intensity), and the like. The image processing device 24 detects foreign matter (for example, a crack M) from, for example, the reception intensity.

[0021] For example, in the case of a 64-channel control device 20, the maximum number of elements in the matrix array probe is 8×8=64, and the number of elements used in the vertical and horizontal directions is restricted, making it difficult to control the ultrasonic beam S. In particular, in order to focus the ultrasonic beam S at a position deep below the surface 1 of the test piece, the size of the entire transducer becomes large, and the influence of the limit on the number of divisions becomes significant.

[0022] The range in which the ultrasonic beam S can be focused (depth z from the test piece surface 1) is within the near field and is expressed by the following equation (1). The larger the transducer, the greater the focusing effect. z <D 2 / 4λ (1) D: nominal diameter of the vibrator to be divided, λ: wavelength Here, the nominal diameter D means the diameter of a circle equivalent to the area of ​​the vibrator.

[0023] In other words, to focus an ultrasonic beam S at a depth of 100 mm using longitudinal waves in steel, it is necessary to divide the transducer into at least 24 mm pieces (using 5 MHz, which can be used in steel), and to do this more effectively, it is desirable to divide the elements into pieces of about 35 mm. Therefore, the nominal diameter D of the divided transducer must be increased.

[0024] FIG. 2 is a view taken along the line AA in FIG. In this figure, the multiple transducers 12 are separated into concentric circles centered on a center O and positioned separately in the circumferential direction. The transducers 12 are also separated into rows in a direction perpendicular to a reference line L passing through the center O of the concentric circles, and are positioned line-symmetrically with respect to the reference line L.

[0025] The "concentric circles" are not limited to perfect concentric circles, but may be, for example, elliptical concentric circles. The reference line L is the Y axis in the figure in this example. The output of the oscillator 12 is proportional to its size. The oscillators 12 are symmetrical about the axis and are formed to have the same size.

[0026] 2, the detection surface 14 of the probe 10 is circular with a diameter D, and has a plurality of segments 15 divided into a plurality of arc-shaped portions symmetrical with respect to a reference line L. Each segment 15 corresponds to one transducer 12 described above. In this example, the diameter D is 12 mm.

[0027] In this example, the segment 15 is made up of a circular portion in the center and other arc-shaped portions. Note that the shape of the segment 15 is not limited to this example and may be other shapes. Furthermore, the size (gap width) of the gap 11 between adjacent segments 15 is preferably constant but may vary. Hereinafter, when the gap width is constant or nearly constant, the gap 11 between the segments will be referred to as a dividing line 16.

[0028] In this example, the dividing lines 16 of the segment 15 are made up of a plurality of circular dividing lines 16a and a plurality of straight dividing lines 16b.

[0029] 2, the straight dividing lines 16b are parallel to each other, perpendicular to the reference line L passing through the center O of the circle, and tangent to both ends of the circular dividing line 16a. In this case, the straight dividing lines 16b are parallel to each other and pass through the intersection of the reference line L and the circular dividing line 16a.

[0030] In this example, there are seven circular dividing lines 16a, which divide the circle of the detection surface 14 into eight annular portions (ring-shaped portions). In this example, there are fourteen straight dividing lines 16b, which divide the ring-shaped portion into 113 arc-shaped portions (including one central circle). As a result, the detection surface 14 has 113 segments 15 divided by circular dividing lines 16a and linear dividing lines 16b.

[0031] 2, the "numbers" written inside the segments 15 are segment numbers I. The segment numbers I (=1 to 64) are assigned to pairs of identical numbers that are symmetrical with respect to the reference line L.

[0032] The control device 20 has a plurality of control channels 21. Each control channel 21 controls a pair of transducers 12 that are symmetrical with respect to the axis under the same conditions. A plurality of (64 in this example) control channels 21 each control the transducers 12 of the segment 15 with segment number I (=1 to 64).

[0033] In the crack inspection method of the present invention, the above-described phased array flaw detector 50 is used to simultaneously excite a pair of transducers 12 that are symmetrical with respect to the reference line L at the same phase. In addition, in the ultrasonic focus setting step S1 described later, the probe 10 is placed in close contact with the bolt end face 1, and the ultrasonic beam S is set to be focused from the bolt end face 1 to a focal point 2, which is a point on a plane perpendicular to the bolt axis. This configuration increases the intensity of the ultrasonic waves irradiated onto the focal point 2 (inspection position), and increases the reception intensity of the received data (flaw detection waveform).

[0034] FIG. 3 is a side view of a test piece TP (bolt TP in this example) that is the subject of inspection by the present invention in an example described later. In this example, the bolt TP is a steel bolt having an outer diameter D1 of 36 mm and an overall length L1 of 200 mm, and has male threads 3 each having a length of approximately 70 mm at both the upper and lower ends. In this example, the male thread portion 3 (hereinafter referred to as "bolt thread portion 3") is an M36 metric coarse thread, with a thread outer diameter D2 of 36 mm, a thread root diameter d3 of 31.6 mm, and a thread pitch P of 4 mm. This bolt TP is fixed vertically during use, and has a crack M on a plane perpendicular to the bolt axis at an axial distance Z (Z = 50 mm) from its top surface (the surface of the test specimen, the bolt end surface 1). Crack M extends from the outer surface in a direction perpendicular to the axis. Hereinafter, the axial distance of the crack M from the bolt end face 1 will be referred to as the "crack depth Zm." This crack M simulates a fatigue crack.

[0035] FIG. 4 is a cross-sectional view taken along line BB in FIG. 3, and is a schematic cross-sectional view of a crack portion. As shown in this figure, in the present invention, the crack position closest to the bolt center O is called the "crack tip Mt," and the radial distance from the crack tip Mt to the thread root (thread root diameter) of the bolt is called the "crack length La." Furthermore, the two ends of the crack at the thread root are called the "crack start point Ms" and the "crack end point Me," and the arc length of the crack at the thread root is called the "crack outer surface arc length Lb." In the following description, the direction of the crack length La passing through the bolt center O is called the Y axis, and the direction perpendicular to this is called the X axis.

[0036] FIG. 5 is a configuration diagram of a crack inspection device 100 for carrying out the method of the present invention. In this figure, a crack inspection device 100 includes a probe 10, a guide device 30, and a rotational position detection device 40.

[0037] In this example, the probe 10 is a cylindrical member, and has on its lower surface the above-mentioned detection surface 14. The lower end (detection surface 14) of the probe 10 is positioned in close contact with the center of the bolt end surface 1, and the upper end of the probe 10 is electrically connected to the above-mentioned control device 20, display device 22, and image processing device 24 (see FIG. 1).

[0038] The guide device 30 guides the probe 10 rotatably about the bolt axis with the detection surface 14 in close contact with the end face 1 of the bolt in use. In this example, the guide device 30 comprises a probe jig 32 and a bolt jig 34 .

[0039] The probe jig 32 is a hollow cylindrical member, inside which the probe 10 can be fixed. In this example, the probe jig 32 can be divided into two halves on a plane passing through the axis, and the two halves are connected to each other with screws or the like, thereby fixing the probe 10 inside. The outer surface of the probe jig 32 is cylindrical with the same diameter, and one end 42a of a string-like member 42 is fixed to a part of the outer surface and wound around the circumference of the cylinder.

[0040] The bolt jig 34 is a member having a hollow cylindrical portion 34a centered on the bolt axis, and has a female threaded portion at the lower end of the hollow cylindrical portion 34a, which can be threaded into the threaded portion at the upper end of the bolt TP and fixed to the upper end of the bolt TP. The upper end of the hollow cylindrical portion 34a is positioned in close contact with the cylindrical outer surface of the probe jig 32, and guides the probe jig 32 so that it can rotate around the bolt axis while positioning the detection surface 14 of the probe 10 at the center of the bolt end face 1. For example, the bolt jig 34 may be a nut that screws onto the male thread portion (bolt thread portion 3).

[0041] In this example, the rotational position detection device 40 comprises a string member 42 and an encoder 44 (for example, a linear encoder). The other end 42b of a string member 42 extending horizontally from the outer surface of the probe jig 32 is fixed to the encoder 44, and the rotational position of the probe 10 is detected from the amount of horizontal movement of the string member 42. This configuration is not essential, and it may be a combination of, for example, a gear that rotates together with the probe 10 and a rotation detection device (for example, a rotary encoder) that meshes with the gear and detects its rotation.

[0042] The configuration of the crack inspection device 100 described above allows the detection surface 14 of the probe 10 to be positioned in close contact with the center of the bolt end face 1, while the probe jig 32 and the probe 10 can be rotated around the bolt axis. In the embodiment described below, this rotation is performed manually, but it may also be performed using a motor or the like. Furthermore, by measuring the horizontal movement amount of the string-like member 42 with the encoder 44, the rotational position (rotational angle) of the probe jig 32 can be measured accurately.

[0043] FIG. 6 is an overall flow diagram of the ultrasonic flaw detection method of the present invention. In this figure, the ultrasonic flaw detection method of the present invention includes steps S1 to S5 using the above-mentioned crack inspection device 100. Fig. 7 is an explanatory diagram of steps S1 to S5 of the present invention.

[0044] In the ultrasonic focusing setting step S1, as shown in FIG. 7(A), the probe 10 is brought into close contact with the end face 1 of the bolt in use, and the ultrasonic beam S is set to be focused from the bolt end face 1 to a focal point 2, which is a point on a plane perpendicular to the axis of the bolt TP (the "bolt axis"). The axial distance Z from the bolt end face 1 where the focusing point 2 is located will be referred to as the "focusing depth Za" hereinafter. The focusing depth Za is set by the control device 20 within a range in which the crack M can exist.

[0045] In the crack position detection step S2, the inspection radius r from the bolt axis is fixed to the inside of the bolt thread portion 3 where the reflection of ultrasonic waves from the bolt thread portion 3 is small. Next, the focal depth Za at which the focal point 2 is located is changed within the range where a crack M may exist, and the probe 10 is rotated around the bolt axis. From the ultrasonic intensity received during this process, the crack depth Zm, which is the axial distance of the plane where the crack M exists, is detected. The means for detecting the crack depth Zm is not limited to this example, and other well-known methods (for example, the method of Patent Document 1) may be used.

[0046] In the inspection radius setting step S3, as shown in FIG. 7(B), a focal point 2 is set at an axial position (crack depth Zm) where a crack M exists, at a position spaced apart by an inspection radius r from the bolt axis. The inspection radius r should be set to a value smaller than the thread radius R (=D2 / 2=18 mm) of the thread outer diameter D2. In actual flaw detection, even if the inspection radius r is set to exceed the screw radius R, the location beyond the screw radius R will only receive echoes from the screw part, so no particular problems will arise in terms of flaw detection. In addition, since it is necessary to check the concentric echo at the root of the thread when calculating the arc length, the inspection radius r must at least exceed the radius of the root of the thread (= d3 / 2).

[0047] In the internal arc length detection step S4, the probe 10 is rotated around the bolt axis, and the crack arc length Lb(r) at the inspection radius r is detected from the received ultrasonic wave intensity (reception intensity).

[0048] In the outer surface arc length detection step S5, as shown in Figure 7(C), the inspection radius r is changed from the inside of the crack M to the bolt thread portion 3, and the inner arc length detection step S4 is repeated, and the outer surface arc length Lb of the crack at the thread root is detected from the change in the crack arc length Lb(r).

[0049] FIG. 8 is an explanatory diagram of step S5 of the present invention. In this figure, symbol r16 is the position where the inspection radius r coincides with the thread root, r=d3 / 2=15.8 mm, symbol r8 is r=8 mm, and symbol r18 is r=18 mm. The inspection radii r from r8 to r18 are spaced at 1 mm intervals, and are not shown except for r8, r16, and r18. In other words, the inspection radius r and the number in the symbol r match, r0 is the bolt center, r16 is the thread root (r = root diameter / 2 = 31.6 / 2 ≒ 16 mm), and r18 is the outer diameter of the threaded portion (r = outer diameter / 2 = 36 / 2 = 18 mm). [Example]

[0050] FIG. 9 shows the test results of the outer surface arc length detection step S5 for the symbols r8, r16, and r18. In this figure, the horizontal axis represents the circumferential rotation distance of the probe jig 32, with 100 mm corresponding to one rotation (360 degrees) of the probe 10. The vertical axis represents the intensity of the received ultrasonic waves (reception intensity).

[0051] In Figure 7(C), if the focal point 2 of the ultrasonic beam S does not coincide with the crack M and the ultrasonic beam S is not reflected by the crack M, the ultrasonic beam S passes through the focal point 2 and is diffused downward, resulting in a weak reception intensity. On the other hand, when the focal point 2 of the ultrasonic beam S coincides with the crack M and the ultrasonic beam S is reflected by the crack M, the received intensity becomes strong. For this reason, as shown by symbol r8 in Figure 9, the reception intensity is weak in the rotation range where there is no crack M, and is strong in the rotation range of the crack arc length Lb(r) where there is a crack M. From this, the crack arc length Lb(r) where there is a crack M can be easily detected.

[0052] On the other hand, in Figure 7(C), even if the focal point 2 of the ultrasonic beam S does not coincide with the crack M and the ultrasonic beam S is not reflected by the crack M, if the ultrasonic beam S that passes through the focal point 2 is reflected by the bolt thread portion 3, the received intensity will be strong. If crack M can be identified from the reception intensity at the thread root r16 (r = 16 mm) in Figure 9, it is best to determine the crack outer surface arc length Lb from this. However, as with r16 in Figure 9, there are cases where it is difficult to distinguish between the rotation range where crack M is present and the rotation range where crack M is present. In such cases, it is best to determine the crack arc length Lb from, for example, r15, which is inside the thread root r16. That is, cracks are identified using the following image. OK: Crack M can be identified, NG: Crack M cannot be identified Example 1: r16NG → r15OK Example 2: r16NG → r15NG → r14OK

[0053] Also, in Figure 7(C), when the focal point 2 of the ultrasonic beam S is close to the bolt thread portion 3 and when the focal point 2 is at the bolt thread portion 3, the ultrasonic beam S that passes through the focal point 2 is reflected by the bolt thread portion 3, resulting in a stronger received intensity. Therefore, as shown by the symbol r18 (r=18 mm) in FIG. 9, it becomes almost impossible to distinguish between a rotation range without a crack M and a rotation range with a crack M.

[0054] Figure 10 shows the test results of the outer surface arc length detection step S5, similar to Figure 9, when the inspection radius r is at the positions indicated by symbols r8 to r18. In this figure, (A) shows the case where the focal point 2 is located at the bolt thread portion 3, and (B) shows the case where the focal point 2 is located inside the bolt thread portion 3.

[0055] From Figure 10(A), it can be seen that when the focal point 2 is located at the bolt thread portion 3, the reception intensity is strong over the entire rotation range equivalent to one rotation of the probe 10, and it is not possible to identify the rotation range in which the crack M is located. Furthermore, from Figure 10(B), when the focal point 2 is located inside the bolt thread portion 3, the rotation range of the crack arc length Lb(r) where the crack M is located can be identified when the inspection radius r is small (for example, symbols r8 to r15). However, when the inspection radius r is large (for example, symbol r16), it becomes difficult to identify the rotation range of the crack arc length Lb(r).

[0056] Based on the above findings, in the outer surface arc length detection step S5, the crack outer surface arc length Lb at the thread root is detected from the change in the crack arc length Lb(r), as shown by symbols r8 to r16 in FIG. 10(B).

[0057] 6, the ultrasonic flaw detection method of the present invention further includes steps S6 and S7. FIG. 11 is an explanatory diagram of step S6 of the present invention.

[0058] In the crack length detection step S6 in FIG. 11, first, the rotational position of the probe 10 is fixed within the range of rotational angle of the probe 10 that detected the crack arc length Lb(r) in the internal arc length detection step S4. This fixed position (angle) is preferably in the direction of the center of the crack arc length Lb(r) at the smallest inspection radius r among the inspection radii r at which the crack arc length Lb(r) is detected. This fixed position is the Y axis in FIG. Next, in the crack length detection step S6, the inspection radius r is changed in the diameter direction within the range from the inside of the crack M to the bolt thread portion 3 while keeping the rotation position fixed, and the crack length La is detected from the intensity of the received ultrasonic waves. [Example]

[0059] FIG. 12 shows the test results of the crack length detection step S6. In this example, the inspection radius r is changed along the Y axis in FIG. 12 from the top end (r=D1 / 2) to the bottom end (r=-D1 / 2) of the bolt. 12 corresponds to the bolt diameter direction, and Shot number 1, 37 corresponds to the bolt outer diameter D1 (=36 mm). The vertical axis represents the received ultrasonic wave intensity (received intensity). In this figure, point A is the thread root, point B is the crack tip Mt, and the crack length La can be detected from the distance between AB. In this figure, the areas with low reception strength and shot numbers of approximately 13 to 33 are due to the bolt axis center where there is little reflection from the bolt thread portion 3, and the areas with high reception strength and shot numbers of approximately 1 to 3 and 34 to 36 are due to reflection from the bolt thread portion 3.

[0060] In the crack shape detection step S7, the planar shape and size of the crack M occurring in the bolt thread portion 3 are detected from the change in the crack arc length Lb(r) obtained in the outer surface arc length detection step S5 and the crack length La obtained in the crack length detection step S6.

[0061] According to the above-described embodiment of the present invention, the phased array probe 10 is brought into close contact with the end face 1 of a bolt in use, and the ultrasonic beam S is set to be focused on the focal point 2, which is a point on a plane perpendicular to the bolt axis from the bolt end face 1. This increases the intensity of the ultrasonic waves irradiated to the focal point 2 (inspection position), and increases the received intensity of the ultrasonic waves.

[0062] Furthermore, on the plane where the crack M exists, a focal point 2 is set at a position separated by an inspection radius r from the bolt axis, and the phased array probe 10 is rotated around the bolt axis, so that the crack arc length Lb(r) at the inspection radius r can be detected from the received ultrasonic wave intensity.

[0063] Furthermore, with conventional means, the noise level is high near the bolt thread 3, making it difficult to distinguish between the received signal from the crack and the noise. However, with the present invention, by changing the inspection radius r from the inside of the crack to the bolt thread 3 and repeatedly detecting the crack arc length Lb(r) at the inspection radius r, it is possible to detect the outer surface arc length Lb of the crack in the bolt thread 3 from the change in the crack arc length Lb(r).

[0064] Therefore, the present invention can increase the intensity of the ultrasonic waves irradiated at the inspection position (focus point 2) by focusing, thereby increasing the received intensity of the ultrasonic waves. Also, it is possible to detect the outer surface arc length Lb of a crack that has occurred in the bolt thread portion 3 from the end face 1 of the bolt during use, the planar shape of the crack M, and the size of the crack.

[0065] It should be noted that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. [Explanation of symbols]

[0066] D nominal diameter, D1 bolt outer diameter, D2 thread outer diameter, d3 thread root diameter, I segment number, L reference line, L1 bolt overall length, La crack length, Lb crack outer surface arc length, Lb(r) crack arc length, M crack, Mt crack tip, Ms crack start point, Me crack end point, P thread pitch, S ultrasonic beam, O center, r inspection radius, TP test piece (bolt), z depth, Z axial distance, Za focusing depth, Zm crack depth, λ wavelength, 1 test piece surface (bolt end face), 2 focusing point, 3 male thread portion (bolt thread portion), 10 phased array probe (probe), 11 gap, 12 ultrasonic vibrator (vibrator), 14 detection surface, 15 segment, 16 division line, 16a circular division line, 16b straight division line, 20 Control device, 21 control channel, 22 display device, 24 image processing device, 30 guide device, 32 probe jig, 34 bolt jig, 34a hollow cylindrical portion, 40 rotation position detection device, 42 string-like member, 42a one end, 42b other end, 44 encoder (linear encoder), 50 phased array flaw detector, 100 crack inspection device

Claims

1. a phased array probe having a plurality of independently excitable ultrasonic transducers and a detection surface for transmitting and receiving ultrasonic beams; a control device that controls the plurality of ultrasonic transducers and transmits and receives the ultrasonic beams from each ultrasonic transducer; a guide device that guides the phased array probe rotatably around a bolt axis while the detection surface is in close contact with an end surface of a bolt in use; a rotational position detection device that detects a rotational position of the phased array probe about the bolt axis, The phased array probe is brought into close contact with the bolt end surface, and the ultrasonic beam is set to be focused on a focal point that is one point on a plane perpendicular to the bolt end surface and the bolt axis; On the plane where the crack exists, the focusing point is set at a position separated by an inspection radius from the bolt axis, Rotating the phased array probe around the bolt axis to detect a crack arc length at the inspection radius from the received ultrasonic wave intensity; A crack inspection device for bolt threads that repeatedly detects the crack arc length by changing the inspection radius from the inside of the crack to the bolt thread, and detects the outer surface arc length of the crack at the thread root from the change in the crack arc length.

2. The plurality of ultrasonic transducers are separated into concentric circles, separated into rows perpendicular to a reference line passing through the center of the circle, and positioned symmetrically with respect to the reference line, 2. The bolt thread crack inspection device according to claim 1, wherein the control device has a plurality of control channels that control the axisymmetric pair of ultrasonic transducers under the same conditions.

3. the guide device has a hollow cylindrical shape, and a probe jig capable of fixing the phased array probe inside the probe jig; 2. The bolt thread crack inspection device according to claim 1, further comprising: a bolt jig that can be fixed to the upper end of a bolt in use and that guides the probe jig rotatably around the bolt axis with the detection surface in close contact with the bolt end face.

4. 2. The bolt thread crack inspection device according to claim 1, further comprising: a display device that displays an intensity of ultrasonic waves received by the phased array probe; and an image processing device that detects the cracks from the intensity of the ultrasonic waves.

5. an ultrasonic focusing setting step of bringing a phased array probe into close contact with an end surface of a bolt in use and setting the ultrasonic beam to focus from the end surface of the bolt to a focusing point that is a single point on a plane perpendicular to the bolt axis; an inspection radius setting step of setting the focal point at a position spaced an inspection radius from the bolt axis on the plane where the crack exists; an internal arc length detection step of rotating the phased array probe around the bolt axis and detecting a crack arc length at the inspection radius from the received ultrasonic wave intensity; an outer surface arc length detection step of repeatedly detecting the crack arc length by changing the inspection radius in the range from the inside of the crack to the bolt thread portion, and detecting the outer surface arc length of the crack at the thread root from the change in the crack arc length.

6. 6. The method for inspecting cracks in a bolt thread portion according to claim 5, further comprising a crack length detection step of fixing the rotational position of the phased array probe within a range of rotation angles of the phased array probe at which the crack arc length is detected, varying the inspection radius from the inside of the crack to the bolt thread portion, and detecting the crack length from the intensity of the received ultrasonic waves.

7. 7. The method for inspecting cracks in a bolt thread according to claim 6, wherein the planar shape and size of the crack occurring in the bolt thread are detected from the change in the crack arc length and the crack length.

8. The inspection radius is fixed to the inside of the bolt thread portion where the reflection of ultrasonic waves from the bolt thread portion is small, The axial distance from the bolt end surface to the plane perpendicular to the bolt axis and on which the convergence point is located is changed within a range in which the crack may exist, 6. The method for inspecting a bolt thread for cracks according to claim 5, further comprising a crack position detection step of rotating the phased array probe around the bolt axis and detecting the axial distance of the plane on which the crack exists from the intensity of received ultrasonic waves.

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