NONCONTACTING ULTRASONIC FLAW DETECTING METHOD

Abstract: The surface of a specimen (2) to be inspected is irradiated with the light of which the intensity varies from a position distant from the material, so that ultrasonic waves are gener­ ated in the material to be inspected. A coherent light is also projected onto a fixed surface (8) provided at a given position and onto the surface (40) of said material to be inspected. Vibration generated in the surface (40) of said material (2) due to ultrasonic waves (6b) reflected by defect (7) in the material, is detected in the form of change in phase of the coherent light that is reflected by the surface of the material. The change of difference in phase between the coherent light reflected by said fixed (8) surface and the coherent light reflected by the surface (40) of the material (2) is measured with the lapse of time, in order to detect flaw.

Description: The present invention relates to a noncontacting ultrasonic flaw detecting method and particularly to a noncontacting ultrasonic flaw detecting method as described in the preamble of the patent claim 1. Such a method is known, for example, from US-A- 3 978 713. A noncontacting ultrasonic flaw detecting method known from US-A- 4 137 778 consists of instantaneously irradiating a material such as steel member to be inspected with a laser beam so that ultrasonic waves are generated in the material to be inspected by the thermal impact given to the irradiated portion, in order to utilize the ultrasonic waves for detecting flaws in the material. According to the above-mentioned noncontacting ultrasonic flaw detecting method by which the material to be inspected is simply irradiated with a laser beam, the accuracy for detecting flaw is low since ultrasonic waves are not focused in the material being inspected and the propagation direction is not controlled. With the contact-type ultrasonic flaw detecting method by which an ultrasonic probe is pressed on the surface of the material to detect flaw, on the other hand, an ultrasonic wave beam of a tilted angle or a focused ultrasonic beam is generated in the material to be inspected, in order to detect flaw therein. However, when the material to be inspected is heated at high temperatures, the ultrasonic probe may be damaged by the heat. Or, water, glycerin, oil or the like existing in space between the ultrasonic probe and the surface of the material to be inspected, evaporates to make it difficult to correctly detect flaw in the material. With this method, furthermore, it is not possible to detect flaw in the material that is heated higher than a predetermined temperature, or to detect flow in the material from a remote position. US-A- 3 978 713 discloses a noncontacting ultrasonic flaw detecting method, in which the surface of a material to be inspected is irradiated with light of which the irradiation intensity changes to generate ultrasonic waves in said material, a coherent light and another coherent light reflected by the surface of said material to be inspected are projected onto a predetermined position, the change of difference in phase between said coherent light and said another coherent light is measured with the lapse of time, and the intensity of light irradiation is changed spatially or with the lapse of time, in order to detect flaw in said material to be inspected. A certain region of said material is irradiated by a pulsed laser beam. DE-A-2707 914 describes noncontacting testing of material where ultrasonic waves, having a certain direction of propagation, are generated by a continuously moving stripe of laser light impinging the surface of said material. The object of the present invention is to provide a noncontacting ultrasonic flaw detecting method according to which the propagation direction or focusing of ultrasonic waves generated in the material to be inspected is controlled, and the condition of defects is detected relying upon the interference of light, to highly accurately detect flaw in the material. This object is accomplished with a noncontacting ultrasonic flaw detecting method as claimed. Brief Description of the Drawings Fig. 1 is a diagram of principle of a method of light irradiation for generating a vertically focusing ultrasonic wave beam; Fig. 2 is a diagram illustrating a relation between the irradiation pattern and the ultrasonic wave beam of Fig. 1; Fig. 3 is a diagram of principle of a method of light irradiation for generating an obliquely focusing ultrasonic wave beam; Fig. 4 is a diagram illustrating a relation between the irradiation pattern and the ultrasonic wave beam of Fig. 3; Fig. 5 is a diagram of principle for generating an oblique ultrasonic wave beam by moving the position of light irradiation; Fig. 6 is a diagram of principle for detecting ultrasonic waves utilizing the light; Fig. 7 is a schematic diagram for illustrating a first embodiment of an apparatus for executing the present invention; Fig. 8 is a diagram showing a signal produced by a light detector of Fig. 7; Figs. 9(a), 9(b), 9(c) and 9(d) are plan views of light choppers used in the first embodiment; Fig. 10 is a diagram showing a major portion of light irradiation means according to a second embodiment; Figs. 11(a) and 11(b) are plan views of light-shielding plates used in the second embodiment; Described below are a principle for generating an ultrasonic wave beam that plays an important role for the noncontacting ultrasonic flaw detection of the present invention, and a principle for detecting, by using light, ultrasonic waves reflected by defects in the material that is to be inspected. First, described below is a principle for generating an ultrasonic wave beam, which can be divided into the one based upon a light irradiation pattern by spatially changing the irradiation intensity, and the one based upon the irradiation of light by changing the irradiation intensity with regard to time. An example is described below to determine the propagation direction of focused ultrasonic wave beam using a pattern of light irradiation having intensity that changes spatially. As shown in Fig. 1, the surface of a specimen 2 (or a material to be inspected) is instantaneously irradiated with the light emitted from a source of light 1. The distribution of light intensities is expressed as a concentric Fresnel ring pattern 3a. Black portions of the Fresnel ring pattern 3a represent areas where intense light is received, and white portions represent areas where weak light is received. Strong thermal impact is applied to the black portions of the Fresnel ring pattern 3a to generate ultrasonic waves. Ultrasonic waves generated in each of the portions are interfered by each other in the specimen 2, and form a vertically focusing ultrasonic wave beam 4a having a propagation direction which focuses at a point on an extension at the center of the Fresnel ring pattern 3a, as indicated by a dot-dash line in Fig. 2. As shown in Fig. 3, furthermore, the distribution of light intensities may also be expressed as a ring pattern 3b which does not contain the center of Fresnel ring pattern 3a of Fig. 1. In this case, the ultrasonic waves which are generated form an obliquely focusing ultrasonic wave beam 4b having a propagation direction that focuses at a point on an extension at the center of the ring pattern 3b, as indicated by a dot-dash line in Fig. 4. Described below is an example for determining the propagation direction of ultrasonic wave beam by the irradiation of light of which the intensity change in regard to time. The light to be irradiated may be moved. Namely, as shown in Fig. 5, the source of light 1 is moved to move the position of light irradiation from a point PA to a point PB at a speed v. In this case, there are generated ultrasonic waves spherically from the surface of the specimen 2 that is successively irradiated with the light. The ultrasonic waves spherically generated from each of the positions along the way from the position PA to the position PB are interfered by each other to form phase planes 5, whereby the ultrasonic waves propagate in a direction indicated by arrow 6. An angle (incident angle) ϑ between the propagation direction and a normal drawn to the surface of the specimen 2 is given by, where V a denotes the speed of sound in the specimen 2. Thus, the position of light irradiation is moved to change the incident angle of ultrasonic waves. In other words, an ultrasonic wave beam having a propagation direction in the specimen 2 is generated by irradiating the surface of the specimen 2 with the light of which the intensity changes with the lapse of time. Described below is a principle for detecting the ultrasonic waves reflected by defects in the specimen by using light. As shown in Fig. 6, a first coherent light 12 emitted from a source of light 10 is partly reflected by a half mirror 8 and is permitted to be incident upon a light detector 11 via a path 13. On the other hand, a second coherent light 12a which has passed through the half mirror 8 is reflected by the surface of the specimen 2 and is allowed to be incident upon the light detector 11 via a path 14. As the ultrasonic waves reflected in the direction of arrow 6 by a defect 7 in the specimen 2 reaches the surface of the specimen 2, the surface of the specimen 2 undergoes the displacement as indicated by a dotted line 9a. The displacement with the lapse of time if indicated by h(t). The coherent light 12a is reflected by the displaced surface 9a, and is made incident upon the light detector 11 via a path 15. Therefore, the light detector 11 measures the change in the intensity of light depending upon the displacement h(t) of the surface of the specimen 2 with the lapse of time, i.e., depending upon the difference in phase between the first coherent light 12 and the second coherent light 12a that is reflected, which changes with the lapse of time. The reasons will be described below. If the distance of the path 13 is denoted by ℓ r and the distance of the path 14 by ℓ o , then the distance ℓ(t) of the path 15 is given by, where β denotes an incident angle of light upon the surface of the specimen 2. If the coherent light reaching the light detector 11 via path 13 is denoted by Ø r and the coherent light reaching the light detector 11 via path 15 by Ø o , then the wave motions of these light rays are expressed as follows: where k denotes a wave number of the light. The light detector 11 detects the intensity Ø i of interfering light of Ø r and Ø o . The intensity Ø i of interfering light is given by the following equation, where symbol * denotes a complex conjugate. As represented by the equation (5), it will be recognized that the intensity Ø i of interfering light detected by the light detector 11 changes depending upon the displacement h(t) of the surface of the specimen 2 caused by the ultrasonic waves with the lapse of time, i.e., changes depending upon the difference in phase between the coherent light Ø r and the coherent light Ø o , that varies with the lapse of time. An embodiment (first embodiment) of the present invention will be described below. As shown in Fig. 7, a ray of light 30 which is a portion of light flux emitted from the source of light 1 passes through a light chopper 17 that is a light-shielding member, and falls on the surface of the specimen 2. The position of irradiating light 30 is successively changed with the lapse of time by the light chopper 17. The ultrasonic wave beam generated in the surface of the specimen 2 irradiated with light propagates in a direction 6a, and is reflected by the internal defect 7 in a direction 6b. The reflected ultrasonic wave beam changes a region 40 on the surface of the specimen 2. The light emitted from a laser device 10 is transformed into a parallel beam through two lenses 16a, 16b, and is guided to the half mirror 8. A coherent light 32 reflected by the half mirror 8 is incident directly upon the light detector 11, and a coherent light 31 which has transmitted through the half mirror 8 is reflected by the region 40 and is incident upon the light detector 11 which sends an interfering intensity of the coherent light 32 and coherent reflected light 31 to a waveform observation instrument 19. In this embodiment, a photomultiplier tube is used as the light detector 11, and a synchroscope is used as the waveform observation instrument 19. A trigger generator 18c sends a trigger signal to the waveform observation instrument 19 just as the light chopper 17 permits the ray of light 30 to pass through. That is, as the ray of light 30 is to be transmitted, the light emitted from a photodiode 18a is received by a light sensor 18b via an aperture 42 (see Fig. 9) formed in the light chopper 17, and the trigger generator 18c produces a trigger signal responsive to the received signal. The waveform observation instrument 19 displays the input signal from the light detector 11 which changes with the lapse of time, with the trigger signal from the light chopper 17 as a starting point (reference value for time measurement). Fig. 8 shows an example of signal waveform displayed on the waveform observation instrument 19, wherein the ordinate represents the voltage V and the abscissa represents the time t. At a time t₁, a pulse signal 50 is reflected by the defect 7, and an ultrasonic wave pulse signal is produced. The light detector 11 detects the phase difference between the coherent light reflected by the half mirror 8 and the coherent light reflected by the surface of the specimen 2, as the intensity of light, Therefore, the reflected light should desirably be detected as a spot light via lenses. However, the reflected light may once be focused on the screen to detect the intensity of the image. A variety of ultrasonic wave beams can be produced through the light chopper 17 by selecting openings 41 that are formed in the light chopper 17 and that permit the passage of light. Plan views of Fig. 9 illustrate representative examples of the openings 41 formed in the light chopper 17. An obliquely directed ultrasonic wave beam is generated by using an opening 41a shown in Fig. 9(a), a vertically directed ultrasonic wave beam is generated by using an opening 41b shown in Fig. 9(b), an obliquely focusing ultrasonic wave beam is generated by using openings 41c shown in Fig. 9(c), and a vertically focusing ultrasonic wave beam is generated by using openings 41d shown in Fig. 9(d). Another embodiment (second embodiment) of the present invention will be described below. This embodiment is different from the first embodiment with regard to the setup of light irradiation means which generates ultrasonic wave beam in the specimen 2. Fig. 10 shows a major portion of the second embodiment, i.e., shows the setup of this light irradiation means only. The light emitted from the light source 1 passes through a light switch 20, and only the light which has passed through a lattice 23 of a shielding lattice plate 22 is allowed to fall on the surface of the specimen 2. Here, the light-shielding member consists of the light switch 20 and the shielding lattice plate 22. The light switch 20 permits the light to pass through instantaneously in synchronism with a signal from the pulse generator 21. The obliquely focusing ultrasonic wave beam and the vertically focusing ultrasonic wave beam can be generated if lattices 23a, 23b shown in Figs. 11(a.) and 11(b) are used for the shielding lattice plate 22 in the light irradiation means.

Claims: A noncontacting ultrasonic flaw detecting method, in which the surface of a material to be inspected is irradiated with light of which the irradiation intensity, changes to generate ultrasonic waves in said material, in which a first coherent light beam is reflected from a fixed surface at a given position and is incident upon a light measuring means but not upon said material, in which a second coherent light beam is reflected from the surface of said material to be inspected and incident upon said light measuring means and in which the change of difference in phase between said first coherent light beam and said second coherent light beam is measured with the lapse of time, in order to detect flaw in said material to be inspected, characterized in that the intensity of light irradiation is spatially changed by rotating a light-shielding plate having openings that permit the passage of light.

Inventors: TAKAHASHI, FUMINOBU
Applicants: HITACHI, LTD.
Application year: 1984
Main classification: G01N29
IPC codes: G01N29