Ultrasonic flaw detection method

Abstract

To provide an ultrasonic flaw detection method capable of performing efficient and accurate flaw detection by reducing the influence of a bottom surface multiple reflection wave.SOLUTION: In an ultrasonic flaw detection method for detecting a flaw of a material M to be flaw-detected from the magnitude of a reflected wave returned by reflecting a flaw detection ultrasonic wave U d emitted from an ultrasonic probe 1 toward the material M to be flaw-detected in the material M to be flaw-detected, a passing lower limit frequency is set in the vicinity of the center frequency of a predetermined number of times of a bottom surface multiple reflection wave Ub4 for a reflected wave appearing in a flaw detection window W set between a first time surface reflection wave signal Us1 reflected on the surface of the material M to be flaw-detected and a first time bottom surface reflection wave signal Ub1 reflected on the bottom surface of the material M to be flaw-detected.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an ultrasonic flaw detection method, and more particularly to an ultrasonic flaw detection method that reduces the influence of multiple reflected waves from the bottom surface of a material to be detected. [Background technology]

[0002] One such ultrasonic flaw detection method is disclosed in Patent Document 1, in which the receiving surface of a phased array ultrasonic probe is tilted significantly in the width direction perpendicular to the direction of movement relative to the surface of the material to be detected, and in this state, ultrasonic waves are generated that form an angle with the tilted receiving surface of the ultrasonic probe and are incident perpendicularly on the surface of the material to be detected. This causes noise-like reflected waves (hereinafter referred to as multiple reflected waves) from the surface of the material to be reflected back and forth between the tilted receiving surface and the surface of the material to be detected, causing them to quickly exit to the side of the ultrasonic probe and disappear, preventing the multiple reflected waves from affecting the reflected waves (detection signal waves) from flaws in the material to be detected. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2017-49215 Summary of the Invention [Problem to be solved by the invention]

[0004] In the conventional ultrasonic flaw detection method described above, the multiple reflected waves are quickly removed to the side and eliminated, thereby enabling flaw detection to be performed quickly and efficiently while eliminating the influence of the multiple reflected waves even when the oscillation period of the flaw detection ultrasonic waves emitted from the ultrasonic probe is shortened.

[0005] However, this method only targets surface reflected waves that are reflected from the surface of the material being inspected, and does not take into account bottom reflected waves that are reflected from the bottom surface of the material being inspected.

[0006] Therefore, the present invention is intended to solve such problems, and aims to provide an ultrasonic flaw detection method that can reduce the influence of bottom surface multiple reflected waves, etc., and perform flaw detection efficiently and with high accuracy. [Means for solving the problem]

[0007] In order to achieve the above object, in the present invention, in an ultrasonic flaw detection method for detecting flaws in a material (M) to be detected from the magnitude of the reflected wave that is reflected back within the material (M) after a flaw detection ultrasonic wave (Ud) is emitted from an ultrasonic probe (1) toward the material (M), the reflected wave signal that appears in a flaw detection window (W) set between a first surface reflected wave signal (Us1) reflected on the surface of the material (M) to be detected and a first bottom reflected wave signal (Ub1) reflected on the bottom surface of the material (M) to be detected is filtered by setting a lower pass limit frequency near the center frequency of the bottom multiple reflected wave (Ub4) of a predetermined number of times.

[0008] In the present invention, by performing filtering with a lower pass limit frequency set near the center frequency of the bottom multiple reflections of a predetermined number of times, it is possible to effectively reduce the bottom multiple reflections of the predetermined number of times or later that are generated by the previous ultrasonic waves for flaw detection and that appear after the next ultrasonic wave for flaw detection and affect the flaw detection signal wave. At the same time, multiple reflections that are a mixture of bottom reflections and surface reflections are also reduced. This shortens the oscillation period of the ultrasonic waves for flaw detection, allowing for efficient flaw detection and improved flaw detection accuracy.

[0009] The symbols in parentheses above indicate, for reference, the correspondence with specific means described in the embodiments to be described later. [Effects of the Invention]

[0010] As described above, according to the ultrasonic flaw detection method of the present invention, the influence of bottom surface multiple reflected waves and the like can be reduced, and flaw detection can be performed efficiently and with high accuracy. [Brief explanation of the drawings]

[0011] [Figure 1]FIG. 1 is an overall perspective view of a round steel bar during ultrasonic flaw detection. [Figure 2] FIG. 2 is a cross-sectional view of an ultrasonic probe. [Figure 3] 1 is a diagram showing bottom multiple reflection waves generated by ultrasonic waves for flaw detection. FIG. [Figure 4] This is a diagram comparing the frequency distribution of bottom multiple reflection waves with that of ultrasonic waves used for flaw detection. [Figure 5] FIG. 10 is a diagram showing a change in center frequency depending on the number of times a bottom-surface multiple-reflected wave is reflected. [Figure 6] 10 is a waveform diagram comparing the effect of reducing multiple reflected waves of the present invention with that of the prior art. DETAILED DESCRIPTION OF THE INVENTION

[0012] The embodiments described below are merely examples, 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.

[0013] FIG. 1 shows an example of ultrasonic testing of a round bar material M, which is the material to be tested, with an ultrasonic probe 1 placed above the round bar material M. The ultrasonic probe 1 is cylindrical, and its lower surface 11 (receiving surface) has a concave, arc-shaped center. The ultrasonic probe 1 contains a single ultrasonic vibrator, and the emitted ultrasonic waves are converged into a beam by the receiving surface 11, which has a concave center, toward the round bar material M below. In this state, the ultrasonic probe 1 or the round bar material M is moved relatively in the longitudinal direction, and flaws are detected in the longitudinal direction of the round bar material M.

[0014] In order for the single-transducer ultrasonic probe 1 to receive and emit ultrasonic waves well, its receiving surface 11 is oriented parallel to the top surface (surface) M1 of the round steel bar M in the depth direction as shown in Figure 2, so that the flaw detection ultrasonic waves Ud emitted from the center of the ultrasonic probe 1 are incident perpendicularly on the top surface M1 of the round steel bar M (thick arrow in Figure 2). However, in this case, the bottom-reflected waves Ub from the lower surface (bottom surface) M2 of the round steel bar M are reflected back and forth between the receiving surface 11 and the probe, generating multiple reflected waves that act as noise (thin arrow in Figure 2).

[0015] 3, if the oscillation period T of the flaw detection ultrasonic wave Ud from the ultrasonic probe 1 is short, Ub4 of the bottom multiple reflected waves Ub1 to Ub4 caused by the immediately preceding flaw detection ultrasonic wave Udf appears after the next flaw detection ultrasonic wave Udr to affect as noise the flaw detection signal wave appearing in the flaw detection window set in this region. This effect becomes more severe as the oscillation period T becomes shorter.

[0016] By the way, if we observe the bottom multiple reflected waves Ub when detecting flaws in a round steel bar M, we can see that its frequency distribution shifts to the lower frequency side as shown in Figure 4 compared to the ultrasonic waves for flaw detection Ud emitted from the ultrasonic probe 1. This is because when the material to be detected is steel, the higher the frequency, the greater the attenuation within the steel due to Rayleigh scattering, as shown in the following formula (1):

[0017]

number

[0018] In the above equation (1), Cray is a constant, Pa is the sound pressure of the ultrasound before passing through the steel, P is the sound pressure of the ultrasound after passing through the steel, D is the distance traveled in the steel, d is the grain size of the steel, and f is the frequency of the ultrasound (the reciprocal of the wavelength λ), where in the case of steel, λ >> d, and attenuation due to Rayleigh scattering (above equation (1)) is dominant.

[0019] Figure 5 shows an example of the number of reflections of bottom-reflected waves and the change in the center frequency of their frequency distribution. It shows the change in the center frequency of each reflected wave for each reflection when a 6 MHz ultrasonic wave is incident perpendicularly on the circumferential surface of a round steel bar from an ultrasonic probe during water immersion testing, and is reflected back from the bottom surface of the steel.

[0020] For example, if the oscillation period T (see Figure 3) of the flaw detection ultrasonic wave Ud is shortened from 1 ms (frequency 1 KHz) to approximately 0.38 ms (frequency 2.6 KHz), the bottom multiple reflection wave Ub generated by the previous flaw detection ultrasonic wave Udf will appear after the next flaw detection ultrasonic wave Udr, and it will be the multiple reflection waves Ub4, Ub5, ... after four reflections that will affect the flaw detection signal wave.

[0021] The flaw detection window W (Figure 6) for capturing the flaw detection signal wave Ui is set between the first surface reflected wave signal Ud1 and the first bottom reflected wave signal Ub1, and the center frequency of the flaw detection signal wave Ui is between the center frequency of the flaw detection ultrasonic wave Ud (e.g., 6 MHz) and the center frequency of the first bottom reflected wave Ub1 (e.g., 4 MHz, see Figure 5). On the other hand, the center frequency of the fourth bottom multiple reflected wave Ub4 is 2.5 MHz, as shown in Figure 5. Therefore, if a high-pass filter of, for example, 3 MHz is set in the receiving circuit 21 of the ultrasonic flaw detector 2 (Figure 2) connected to the ultrasonic probe 1, the fourth and subsequent bottom multiple reflected wave signals Ub4, Ub5, ... that affect the detection of the flaw detection signal Ui can be reduced. In this case, not only the bottom multiple reflected wave signals Ub4, Ub5, ... but also the multiple reflected waves that are a mixture of these and the surface reflected wave signals are simultaneously reduced.

[0022] This effect is explained using Figure 6. Figure 6(1) shows the case where a 3MHz high-pass filter is set, and the signal strength of the flaw detection signal Ui is 78% of the signal strength of the flaw detection ultrasonic wave Ud, while the signal strength of the reflected signal (noise signal) Sn due to the bottom multiple reflected wave is 27% of the signal strength of the flaw detection ultrasonic wave Ud, which is sufficiently small and results in a large S / N ratio. In contrast, when a 1MHz high-pass filter is set, the signal strength of the noise signal Sn is 41% of the signal strength of the flaw detection ultrasonic wave Ud, as shown in Figure 6(2), and the S / N ratio is small.

[0023] Although the above embodiment has been described with reference to a single transducer type ultrasonic probe, the present invention is not limited to this and may be, for example, a phased array type. In the above embodiment, the material to be inspected is a round bar, but this is not limitative and it may be a square bar or a flat plate. A band-pass filter may be used instead of the high-pass filter. [Explanation of symbols]

[0024] 1...ultrasonic probe, 2...ultrasonic flaw detector, 21...receiving circuit, M...round steel bar (material to be inspected), Ud...ultrasonic waves for flaw detection, Ui...flaw detection signal, Us1...first surface reflected wave signal, Ub1...first bottom reflected wave signal, Ub4...predetermined number of bottom multiple reflected waves, W...flaw detection window.

Claims

[Claim 1] An ultrasonic flaw detection method for detecting flaws in a material to be detected from the magnitude of the reflected waves that return after ultrasonic waves are emitted from an ultrasonic probe toward the material to be detected and reflected within the material, in which a filter is applied to the reflected wave signals that appear in a flaw detection window that is set between a first surface reflected wave signal reflected on the surface of the material to be detected and a first bottom reflected wave signal reflected on the bottom surface of the material to be detected by setting a lower pass frequency near the center frequency of the bottom multiple reflected wave of a predetermined number of times.

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