Ultrasonic defect detection method

Abstract

To provide an ultrasonic defect detection method that can be easily installed at a manufacturing site, and can reliably detect defects such as unmelted residues in an object to be inspected.SOLUTION: An ultrasonic probe 1 is scanned parallel to a surface of an object to be inspected M with a certain thickness; a reception time from the emission of an ultrasonic wave to the reception of a bottom reflected wave is measured at each scanning position; in a histogram having the most frequent reception time as its apex, it is assumed that there is a defect at the scanning position indicating the reception time in a region X where the bilateral symmetry of the histogram is destroyed; and planar spread of the object to be inspected is detected from the coordinates of each scanning position indicating the reception time in the region X.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an ultrasonic defect detection method, and more particularly to an ultrasonic defect detection method which is useful for detecting defects such as unmelted residues in a base material. [Background technology]

[0002] When using ultrasonic waves to detect defects in an object to be inspected, the reflection of ultrasonic waves from these defects is often utilized if the defect is a gap, inclusion, etc. This involves setting a detection window between the reflected waves from the surface and bottom of the object to be inspected, and when a reflected wave appears within this detection window, it is detected as a reflection from a defect.

[0003] Patent Document 1 proposes a method for inspecting a solidified portion in a slab produced during continuous casting, in which ultrasonic waves are generated in the slab, and the time difference between the generation of the ultrasonic waves and the generation of surface vibrations caused by the ultrasonic waves depends on the decrease in sound speed due to the solidified portion in the slab. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2006-343203 Summary of the Invention [Problem to be solved by the invention]

[0005] However, for defects such as residual melting in the base material, the reflectivity at the defect surface is low because the defect and the base material are of the same material, and for this reason, the conventional method that uses defect reflection does not obtain sufficient reflected wave intensity and the defect is buried in noise, making it difficult to detect the defect. Although the use of X-rays for defects such as residual melting in the base material is also considered, the equipment is large-scale, requiring structures to prevent exposure, and it is difficult to easily install it at the manufacturing site.

[0006] Therefore, the present invention is devised to solve such problems, and has an object to provide an ultrasonic defect detection method that can be easily installed at a manufacturing site and can reliably detect defects such as unmelted residues in an object to be inspected. [Means for solving the problem]

[0007] In order to achieve the above object, in the first invention, an ultrasonic probe (1) is scanned parallel to the surface of an object to be inspected (M) having a constant thickness, and the reception time from the oscillation of the ultrasonic wave to the reception of the bottom reflected wave at each scanning position is measured. In a histogram having the most frequent reception time as its apex, a defect is determined to exist at the scanning position indicating the reception time in a region (X) where the left-right symmetry of the histogram is lost.

[0008] According to the first invention, the ultrasonic speed in a defective area is different from that in a normal area, so the histogram with the most frequent value of the reception time as the peak becomes asymmetric. Therefore, it is possible to determine that there is a defect at the scanning position that indicates the reception time in the area where the left-right symmetry is lost. Since the most frequent value of the reception time is used as the basis, it is possible to reliably determine the defect even if the thickness of the object to be inspected changes.

[0009] In the second invention, the planar extent of the object to be inspected is detected from the coordinates of each scanning position indicating the reception time in the region (X) where the left-right symmetry is lost.

[0010] According to the second invention, by comparing the difference (or ratio) of reception times (sound speeds) with the mode and the collapsed area (each point of the defect), the planar extent of the defect can be reliably detected.

[0011] In the third invention, the thickness of the in-plane defect is detected from the difference between the reception time at each scanning position in the region (X) where the left-right symmetry is lost and the reception time showing the mode.

[0012] According to the third aspect of the present invention, the difference in reception time corresponds to the thickness of the defect, so that the thickness of the defect can be detected reliably.

[0013] The reference characters in parentheses indicate, for reference, the correspondence with specific means described in the embodiments described later. Effect of the Invention

[0014] As described above, according to the ultrasonic defect detection method of the present invention, it is possible to reliably detect defects such as unmelted residues in an object to be inspected by simply installing the method at a manufacturing site. [Brief description of the drawings]

[0015] [Figure 1] FIG. 2 is a perspective view of the main part of an apparatus for carrying out the method of the present invention. [Diagram 2] FIG. 2 is a plan view showing a scanning path of an ultrasonic probe. [Diagram 3] This is an A-scan image of ultrasonic reflected waves when a void defect and a residual defect were detected. [Figure 4] This is an A-scan image of ultrasonic reflected waves when a residual melting defect was detected. [Diagram 5] 13 is a histogram of reflected wave reception times when a melting residue defect is detected. [Figure 6] 1 is a defect detection image of a residual melt defect detected by the method of the present invention. [Figure 7] This is an image of a defect detected by X-rays. [Figure 8] 13 is a defect detection image of a residual defect caused by defect reflection. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] It should be noted that the embodiment described below is merely an example, and various design improvements made by those skilled in the art without departing from the gist of the present invention are also included in the scope of the present invention.

[0017] Figure 1 shows the main parts of an apparatus for carrying out the method of the present invention. In Figure 1, motors 13, 14 for changing position are provided on an inverted L-shaped stay 12 attached to the lower end of a support column 11, and a probe 1 for receiving and emitting ultrasonic waves is disposed facing downward via these motors 13, 14. An object to be inspected M is positioned below the probe 1, and in this embodiment, the object to be inspected M is a disk of a fixed thickness made of Ti-50Al alloy whose upper and lower surfaces are parallel to each other.

[0018] The upper end of the support column 11 is connected to an XY slider device (not shown), which allows the probe 1 to freely scan in a horizontal plane parallel to the surface of the object to be inspected M below while maintaining a constant distance therefrom. In this embodiment, the scanning trajectory is turned back and forth at constant intervals on the surface of the object to be inspected M, as shown by L in Fig. 2, and thereby ultrasonic waves are received and oscillated in the thickness direction of the object to be inspected M (perpendicular to the paper surface of Fig. 2) at each scanning position (flaw detection position) at a predetermined interval over the entire surface of the object to be inspected M, making it possible to detect the presence or absence of defects.

[0019] In this case, if the defect inside the object M to be inspected is a void or the like, the reflected wave Rd from the defect will appear within a detection window W set between the surface reflected wave Ra and the bottom reflected wave Rb in an A-scope as shown in Figure 3 (1).

[0020] However, when the defect is a residual melting part, for example, the base material part (healthy part) of the test object M is a Ti-50Al alloy, while the defective part is pure Ti and they are the same material, so the reflectance R at the boundary between the healthy part and the defective part, which is determined by the difference between the acoustic impedances Z1 and Z2 shown in the following formula (1), is extremely small, and the reflected wave Rd from the defect hardly appears within the detection window W, as shown in Figure 3 (2). In formula (1), Z1 and Z2 are the acoustic impedances of the healthy part and the defective part, respectively.

[0021] JPEG2025002280000002.jpg30144

[0022] On the other hand, the sound velocity (longitudinal wave) CL of ultrasonic waves is determined by the following formula (2). When paying attention to the sound velocity, it is considered that even if the material of the healthy part and the defective part is the same, there may be a difference in the sound velocity between the two parts. If this is the case, the arrival time of the reflected wave should be different between the healthy part and the defective part of the above-mentioned inspection object M, which is a plate-like part of the same thickness. In addition, in formula (2), E is Young's modulus, ρ is density, and ν is Poisson's ratio. When actually observing the A-scope, the arrival time of the bottom reflected wave Rb at the defective part (see FIG. 4(2)) is delayed by Hd compared to the arrival time of the bottom reflected wave Rb at the healthy part (see FIG. 4(1)) because the sound velocity changes when passing through the defective part (the sound velocity decreases in this embodiment).

[0023] JPEG2025002280000003.jpg32153

[0024] FIG. 5 shows a histogram in which the horizontal axis indicates the reception time (zero crossing point) of the bottom reflected wave Rb in the A-scope at all flaw detection positions of the object M to be inspected by the above-mentioned probe 1, and the vertical axis indicates the frequency. The histogram shows the most frequent value at the zero crossing point of the bottom reflected wave Rb in the healthy part, which occupies the majority of the object M to be inspected. The reason why the histogram has a symmetrical mountain shape as a whole is that the sound speed may change due to grain boundary scattering even in the healthy part, and the probability of the change usually shows a normal distribution. The reason why the mountain shape of the histogram shown in FIG. 5 is not symmetrical is because there is a sound speed fluctuation due to the presence of a defective part, and in this embodiment, the symmetric shape of the mountain shape is broken in the area where the reflected wave reception time is long (shaded area X in FIG. 5). Note that in this embodiment, the frequency is maximum at the shaded area X, but there are also cases where it does not show a maximum.

[0025] Therefore, if the inspection positions where the reflected wave reception time in the A-scope is in the range where the peak shape of the histogram is broken are displayed with, for example, different brightness or color so as to be distinguishable from other inspection positions, and an image (C-scope) is created in which the brightness level or color density is changed according to the difference between the reflected wave reception time at the inspection position and the reflected wave reception time showing the mode, the spread and thickness of the defect can be visually shown. An example is shown in Figure 6. In Figure 6, the area with high brightness can be visually identified as defect D, and the intensity of the brightness corresponds to the thickness of defect D. This Figure 6 is in good agreement with Figure 7, which shows defect D obtained by X-raying the same inspection object M. Incidentally, with the conventional method using the reflected wave from the defect, no defect can be detected at all, as shown in Figure 8.

[0026] In the above embodiment, a C-scope image is drawn, but it is also possible to store the reflected wave reception time and its difference as data corresponding to the position coordinates of the inspection position without drawing it, and perform the necessary calculations. The object to be inspected does not have to be a disk, and may be of any shape, such as a rectangle, so long as it has a certain thickness. [Explanation of symbols]

[0027] 1: Probe, L: Scanning trajectory, M: Test object, Ra: Surface reflected wave, Rb: Bottom reflected wave, Rd: Defect reflected wave, X: Area where the left-right symmetry is lost.

Claims

1. An ultrasonic defect detection method in which an ultrasonic probe is scanned parallel to the surface of an object to be inspected of a constant thickness, and the reception time from the oscillation of the ultrasonic wave to the reception of the bottom reflected wave at each scanning position is measured, and a defect is determined to exist at the scanning position indicating the reception time in a region where the left-right symmetry of the histogram is broken, with the most frequent value of the reception time at the peak.

2. 2. The ultrasonic defect detection method according to claim 1, wherein the planar extent of the object to be inspected is detected from the coordinates of each scanning position indicating the reception time in the region where the left-right symmetry is lost.

3. 3. The ultrasonic defect detection method according to claim 2, wherein the thickness of the in-plane defect is detected from the difference between the reception time at each scanning position in the region where the left-right symmetry is lost and the reception time showing the mode.

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