LASER ULTRASONIC FLAW DETECTION METHOD AND APPARATUS

Abstract: A laser beam for ultrasonic generation is emitted to a reflector/vibrator to generate an ultrasonic wave, and the ultrasonic wave is transmitted to an object to be inspected. The ultrasonic wave transmitted to the object is reflected at a flaw inside the object, returns to the reflector/vibrator and vibrates it. The vibration is detected using a laser beam for ultrasonic detection to find flaws inside the object. According to the present invention, ultrasonic flaw detection can be made for reflecting materials having corrugations on their surface or having a low reflection factor of light, and for objects made of materials which cannot easily generate the ultrasonic wave by the laser beam. For such materials, the conventional method has not been useful because it adopts the direct application of the laser beam to the surface of the object to generate the ultrasonic wave.

Description: TECHNICAL FIELD The present invention relates to an inspection for advanced thermal reactors, fast breeders, light water reactors, plant equipment and other equipment in general, and more particularly it relates to a laser ultrasonic detection method and apparatus therefor which can be applied to in-service inspection as well as to continuous monitoring of specimens to be monitored under operation. BACKGROUND ART According to prior art ultrasonic flaw inspection methods, defects such as flaws and the like is inspected by directly irradiating laser beams from ultrasonic generation laser apparatus and ultrasonic detection laser apparatus upon the surface of an inspection object of metal material or the like. In Figure 4 which shows an example of the prior art ultrasonic flaw detection, the system has an ultrasonic generation laser apparatus 1, an ultrasonic detection laser apparatus 2, an ultrasonic generation laser beam 3, and an ultrasonic detection laser beam 4. In Figure 4, reference numeral 5 represents an object of inspection, 6 a flaw, and 7 an ultrasonic wave, respectively. In Figure 4, an ultrasonic generation laser beam 3 in a pulse form having a high energy of several tens mJ to several hundreds mJ is emitted from the ultrasonic generation laser apparatus 1 to irradiate and heat repeatedly the surface of the object 5 to be inspected so as to generate the ultrasonic wave 7 by causing thermal strain therein. Then, a continuous ultrasonic detection laser beam 4 with a low energy of several mJ is emitted from the ultrasonic detection laser apparatus 2 onto the surface of the object 5 to detect a minute vibration appearing on the surface of the inspection object 5, the vibration being produced when the ultrasonic wave 7 generated by the ultrasonic generation laser beam 3 reaches the surface of the object 5 after reflection from a flaw 6 and the like. In the prior art laser ultrasonic flaw detection methods in which both the ultrasonic generation laser beam and the ultrasonic detection laser beam are directly irradiated on the surface of an inspection object, there are the following serious problems before the methods are put into practice, thus presently impeding wide application thereof. One of the problems is that in detection of ultrasonic signals returning from an object by the ultrasonic detection laser apparatus, when the object has irregularities or undulations on its surface as is the case with most objects, or when reflectivity of light is poor, the reflected laser beam from the object is scattered or attenuated, thereby hampering sufficient light quantities necessary for effective measurement from reaching the ultrasonic detecting laser apparatus, thus eventually rendering the ultrasonic detection thereof impossible. Another problem is that when it is required to generate an intense ultrasonic wave in an object to be inspected by utilizing an ultrasonic generation laser beam, the power of the ultrasonic generation laser beam must also be increased which, however, possibly result in that the surface of the object is burned at the increase of the power. Still another problem is that at whatever angle of irradiation an ultrasonic generation laser beam is projected on to the surface of the inspection object, most of the ultrasonic wave thus induced in the inspection object propagates in the direction perpendicular to the surface thereof, thereby allowing only a vertical flaw detection, but inhibiting the angle beam detection which is important in the ultrasonic flaw detection. Further, there is such a problem that when there exists on the surface of the inspection object an opaque liquid, for example, molten metallic sodium and the like which prevents transmission of the laser beam therethrough, or when the inspection object is made of a material which prevents easy generation of an ultrasonic wave with a laser beam, the ultrasonic detection cannot be applied thereto. DISCLOSURE OF THE INVENTION The present invention is directed to solve the hereinabove stated problems associated with the prior art. A main object of the invention is to provide a laser ultrasonic flaw detection method and an apparatus therefor which can prevent burning of the surface of the inspection object and provide the angle beam method ultrasonic flaw detection as well which is particularly preferable for such an inspection object as having irregularities on the surface, poor reflectivity of light, or to such cases where an opaque liquid which prevents transmission of laser, beams therethrough is present on the surface of the inspection object, or the inspection object is made of such materials that inhibit generation of ultrasonic wave with laser beam. The laser ultrasonic flaw detection method of the present invention comprises the steps of:irradiating a reflector/vibrator plate with an ultrasonic generation laser beam to generate an ultrasonic wave in the reflector/vibrator plate; transmitting thus generated ultrasonic wave to an object to be inspected; receiving at said reflector/vibrator plate a reflected ultrasonic wave which is reflected at a defect such as flaws or the like inside the inspection object; and detecting with an ultrasonic detection laser beam a vibration induced responsive to the reflected ultrasonic wave in the reflector/vibrator plate. The laser ultrasonic flaw detection apparatus according to the present invention comprises: an ultrasonic generation laser apparatus for emitting an ultrasonic generation laser beam; a reflector/vibrator plate for generating an ultrasonic wave responsive to the laser beam irradiation as well as for inducing a vibration therein responsive to an ultrasonic wave returning thereto; and an ultrasonic detection laser apparatus for irradiating the reflector/vibrator plate with a laser beam as well as detecting a reflected laser beam which is modulated by the vibration induced in the reflector/vibrator plate, wherein ultrasonic wave generated by the vibration of the reflector/vibrator plate is applied to the inspection object and the reflecting ultrasonic wave returning from the inspection object is received by the reflector/vibrator plate. In the present invention, the reflector/vibrator plate is irradiated with a laser beam in a pulse form emitted from the ultrasonic generation laser apparatus, an ultrasonic wave is generated in the reflector/vibrator plate and the thus generated ultrasonic wave is transmitted to the inspection object. The reflector/vibrator plate receives the reflected ultrasonic wave reflected by the defect such as flaws or the like, and the induced vibration therein is detected by a continuous laser beam from the ultrasonic detection laser apparatus. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described further in detail with reference to the accompanying drawings, in which; Figure 1 is a schematic block diagram illustrating the principle of the invention; Figure 2 is a diagram illustrating a schematic arrangement of a laser ultrasonic flaw detection apparatus embodying the invention as applied to inspection of a pressure tube in an advanced thermal reactor; Figure 3 is an enlarged view of a portion encircled with two-dot chain line in Figure 2; and Figure 4 is a diagram which illustrates a prior art ultrasonic flow detection method. BEST MODE FOR CARRYING OUT THE INVENTION A preferred embodiment of the invention will be described with reference to the drawings. Figure 1 illustrates the principle of the invention, in which the same reference numerals as in Figure 4 are used for same or similar parts, and further, numeral 8 represents a reflector/vibrator plate. The reflector/vibrator plate 8 in Figure 1 has a mirror-finished portion for receiving an ultrasonic detection laser beam 4 and a mat-finished portion for receiving on its surface an ultrasonic generation laser beam 3. The mirror-finished portion improves laser beam reflection thereon, and the mat-finished portion is made of a material which readily causes thermal strain by a laser beam irradiation and generates an ultrasonic wave therein. By the ultrasonic generation laser apparatus 1, a high-energy ultrasonic generation laser beam 3 in pulse form is irradiated onto the surface of the reflector/vibrator plate 8 and, responsive to a repeated heating on the surface by the irradiation, a resulting thermal strain therein generates an ultrasonic wave 7. The thus generated ultrasonic wave 7 propagates in the direction perpendicular to the surface of the reflector/vibrator plate 8, and is transmitted into an inspection object 5, and, when a defect such as a flaw 6 is present in the object 5, the ultrasonic wave 7 is reflected therefrom and returns to the reflector/vibrator plate 8 to cause a minute vibration therein. The presence of the defect of flaw 6 can be detected by detecting the thus induced vibration with an ultrasonic detecting laser beam 4 emitted from an ultrasonic detection laser apparatus 2. Figure 2 illustrates a schematic arrangement of a laser ultrasonic flaw detection apparatus of the present invention as applied to the inspection of a pressure tube in an advanced thermal reactor. Figure 3 further shows in detail an enlarged view of a portion pertaining to the invention indicated by the two-dot chain line in Figure 2. The same reference numerals in the drawing as in Figure 1 are used for the same or similar portions and elements. Further, the illustrated system has a laser ultrasonic flaw detector 11, a pressure tube 12, mirrors 13, 14, a sealed tube 15, and a screw rod 16. In Figure 2, inside the pressure tube 12 there flows a high temperature, high pressure water from the bottom to the upward simulating an inspection condition under operation of the advanced thermal reactor. The ultrasonic generation laser apparatus 1 and ultrasonic detection laser apparatus 2 are contained inside the sealed tube 15 filled with a gas such as air or the like in order to isolate them from water, and a reflector/vibrator plate 8 constitutes an ultrasonic transmitter/receiver member. The sealed tube 15 is vertically movable by a screw rod 16 and inserted into the pressure tube 12, so that the sealed tube 15 is set at a predetermined position therein. With reference to Figure 2 and 3, an ultrasonic generation laser beam 3 emitted from the ultrasonic generation laser apparatus 1 is reflected by the mirror 13 and then irradiated onto the reflector/vibrator plate 8. Some of the ultrasonic generating laser beam 3 reflected by the reflector/vibrator plate 8 is scattered and attenuated to a null, while most of the ultrasonic generation laser beam 3 is absorbed and repeatedly heats up the reflector/vibrator plate 8 to produce a thermal strain therein, thereby generating an ultrasonic wave 7 resulting from the thermal strain. The ultrasonic 7 wave generated in the reflector/vibrator plate 8 propagates in water in the direction perpendicular to the surface thereof, then is transmitted into the material of the pressure tube 12, and is reflected by a defect such as a flaw or the like if there exists any in the pressure tube 12. Then the reflected wave returns to the reflector/vibrator plate 8 causing a small vibration therein. On the other hand, an ultrasonic detection laser beam 4 emitted from the ultrasonic detection laser apparatus 2 is reflected by the mirror 14 and then irradiated onto the reflector/vibrator plate 8. The ultrasonic detection laser beam 4 is reflected by the reflector/vibrator plate 8 after being modulated therein, and returns via the mirror 14 to the ultrasonic detection laser apparatus 2. Then through analytical processing in the laser ultrasonic flaw detector 11 ultrasonic signals are detected to enabling the flaw inspection to be carried out. The ultrasonic generation laser beam 3 is in a pulse form having energy of several tens to hundreds mJ, and generally a YAG (yttrium aluminum garnet) laser is utilized for the oscillation thereof. Further, the ultrasonic detection laser beam 4 is a continuous wave form having energy of several mJ and generally a He-Ne (helium neon) laser is utilized for the oscillation thereof. As described above, the reflector/vibrator plate 8 is prepared such that it has a mirror-finished surface portion where the ultrasonic detection laser beam 4 irradiates and a mat-finished surface portion where the ultrasonic generation laser beam 3 irradiates. The mirror-finished surface portion has a surface roughness within 1 µm so as to provide a improved laser beam reflection, and the mat-finished surface portion facilitates absorption of heat energy and inhibits reflection therefrom. The material of the reflector/vibrator plate 8 is preferably made of a material that reflects laser beams, and generates an ultrasonic wave due to thermal strain when irradiated with a laser beam. The material is preferably metal selected from the group of stainless steel, iron, copper alloys, chromium and the like in view of availability, cost, machinability, resistance to high temperature/pressure, radiation and the like. Although in the illustrated embodiment of the invention the reflector/vibrator plate 8 is disposed obliquely with respect to the surface of the pressure tube 12 so as to enable an angle beam ultrasonic inspection, it may be disposed parallel to the surface of the pressure tube 12 so as to carry out a vertical ultrasonic inspection. Further, an appropriate optical fiber laser transmission medium may be employed for transmission of the ultrasonic generation laser beam 3 and ultrasonic detection laser beam 4 between the ultrasonic generation laser apparatus 1/ultrasonic detection laser apparatus 2 and the reflector/vibrator plate 8. In a practical use of the present invention, it is preferable to fill a gap between the reflector/vibrator plate and an inspection object with such a substance as water, oil, molten metallic sodium or the like which allows an easy propagation of ultrasonic wave. As set forth hereinabove, the present invention, provides many advantages and merits as follows. The laser ultrasonic flaw detection method has become applicable even to such inspection objects having irregularities on their surfaces or poor reflectivity to the ultrasonic detection laser beams, which heretofore have been the cause for impeding the inspection by the prior art, since according to the present invention the reflected ultrasonic wave reflected by the defect such as flaws in the inspection object is further transmitted to the reflector/vibrator plate 8, then the mechanical vibration thus produced therein is detected by the ultrasonic detection laser beam. Further, in the case of an inspection object having different surface reflectivity depending on the place of inspection, even through an ultrasonic detection laser beam has a sufficient reflection intensity, the ultrasonic detection level would change, which causes a failure in a stable inspection in the prior art. However, according to the present invention the ultrasonic detection laser beam is applied to the same position on the reflector/vibrator plate, and, therefore, stable data of inspection can be obtained. In addition, there is no possibility of burning down the inspection object although the reflector/vibrator plate 8 itself is burnt, even when an intense ultrasonic generation laser beam is utilized. Burning of the reflector/vibrator plate, if not too excessive, will not affect the inspection itself, and if too excessive, it may be replaced as expendables. Further, according to the prior art method, it is quite difficult to carry out a stable inspection even with a constant energy ultrasonic generation laser beam being applied onto the inspection object since the intensity of an ultrasonic wave to be generated therein changes depending on a changing energy absorption coefficient with changes with varying surface conditions. By contrast, according to the present invention, irrespective of the surface conditions of the inspection object, a constant intensity ultrasonic wave can be obtained, thus ensuring a stable inspection. Further, when the surface of the reflector/vibrator plate 8 which receives the ultrasonic generation laser beam has a surface condition that reflection is minimized and energy absorption is maximized to improve an ultrasonic exchange coefficient, a desired intensity of an ultrasonic wave can be generated with a minimum input of laser energy. Simply by disposing the reflector/vibrator plate 8 obliquely with respect to the surface of the inspection object, an ultrasonic angle beam inspection method, unrealized by the prior art, can be realized by the present invention. Considering that there flows opaque liquid metallic sodium instead of water inside the pressure tube 12 of the illustrated embodiment of the invention, it is not possible by the prior art method to carry out the ultrasonic inspection thereof since the ultrasonic generation laser beam is blocked by the liquid metallic sodium. However, provision of the reflector/vibrator plate 8 according to the present invention enables the laser ultrasonic flaw detection to be applied even to such an inspection object having on the surface thereof a liquid which will not pass the ultrasonic generation laser beam. Still further, since any appropriate material for generating therein an ultrasonic wave with an ultrasonic generation laser beam can be selected for the reflector/vibrator plate 8, a laser ultrasonic flaw detection can be applied to any inspection object made of a material which presents difficulties in generation of an ultrasonic wave with an ultrasonic generation laser beam. With such varieties of the advantages and merits being accomplished by the invention, the laser ultrasonic flaw detection method the application of which was restricted to a very limited type of inspection objects can be applied to extended fields of applications. In particular, it has become possible for a nuclear reactor, its associated equipment, piping, specimens under monitoring and the like to be monitored continuously and inspected during operation, thereby contributing significantly to the improvement of safety in the operation of the nuclear power generation plant.

Claims: A laser ultrasonic flaw detecting method comprising the steps of: generating an ultrasonic wave in a reflector/vibrator means by irradiating said reflector/vibrator means with an ultrasonic generation laser beam; transmitting thus generated ultrasonic wave to an object to be inspected; receiving by said reflector/vibrator means a reflected ultrasonic wave reflected by a defective position as a flaw and the like inside said object; and detecting with an ultrasonic detection laser beam a vibration generated in said reflector/vibrator means in response to the reflected ultrasonic wave received by said reflector/vibrator means. A laser ultrasonic flaw detection apparatus comprising: an ultrasonic generation laser apparatus for emitting an ultrasonic generation laser beam; a reflector/vibrator means for generating an ultrasonic wave upon irradiation of a laser beam thereon and vibrating upon reception of an ultrasonic wave thereon; and an ultrasonic detection laser apparatus for emitting a laser beam to irradiate said reflector/vibrator means, and detecting a reflected laser beam from said reflector/vibrator means, said reflected laser beam being modulated by a vibration in said reflector/vibrator means, wherein an ultrasonic wave generated by the vibration of said reflector/vibrator means is applied to an object to be inspected; and a reflected ultrasonic wave from said object to be inspected is received by said reflector/vibrator means. A laser ultrasonic flaw detection apparatus according to claim 2, wherein said ultrasonic generation laser beam and said ultrasonic detection laser beam are transmitted to said reflector/vibrator means by means of reflection mirrors, respectively. A laser ultrasonic flaw detection apparatus according to claim 2, wherein said ultrasonic generation laser beam and said ultrasonic detection laser beams are transmitted to said reflector/vibrator means through an optical fiber. A laser ultrasonic flaw detection apparatus according to claim 2, wherein said reflector/vibrator means is made of a metal or ceramic material. A laser ultrasonic flaw detection apparatus according to claim 2, wherein said reflector/vibrator means has a mat-finished portion where an ultrasonic generation laser beam irradiates, and a mirror-finished portion where an ultrasonic detection laser beam irradiates. A laser ultrasonic flaw detection apparatus according to claim 2, wherein: said object to be inspected is a pressure tube; said reflector/vibrator means comprises an ultrasonic transmitter/receiver member of a sealed tube disposed movably in an axial direction in said pressure tube; and wherein a laser beam is applied to said ultrasonic transmitter/receiver member.

Inventors: ARAI KEISHO, NARUO KAZUTERU - TENJINMAE, ARAI, KEISHO, NARUO, KAZUTERU, NARUO, KAZUTERU, 1606-11, TENJINMAE
Applicants: DORYOKURO KAKUNENRYO, DORYOKURO KAKUNENRYO KAIHATSU JIGYODAN
Application year: 1991
Main classification: G01N29
IPC codes: G01N29