Hitherto, this type of ultrasonic angle-beam flaw detection technique has been described in detail on, for example, pages 180 to 199 of "Ultrasonic Flaw Detection Technique (revised new publication)" edited by Steelmaking Committee No. 19, Japan Society for the Promotion of Science, published by THE NIKKAN KOGYO SHIMBUN LTD., a revised new edition thereof being published on Jul. 30, 1974 and the third impression of a revised new edition thereof being published on Dec. 20, 1977 (hereinafter referred to as "Literature A").
Referring to FIG. 80, conventional ultrasonic flaw detection apparatus and ultrasonic flaw detection technique will be described. FIG. 80 is a diagram illustrative of a conventional ultrasonic angle-beam flaw detection technique cited from the foregoing literature A.
In FIG. 80, a test object 1 has a base material member 2, a surface 3, a bottom 4, and a weld 5. An acoustically discontinued portion (defect) 6 is present in the weld 5 of the test object 1. The acoustically discontinued portion 6 comes in various types including a crack in a first pass weld at the time of welding, a crater crack at a welding start or end, defective fusion, poor penetration, slag contamination, a blowhole, a wormhole, a hot crack, and a difference in material from a peripheral medium due to foreign substance contamination. Further, the acoustically discontinued portions also include spots contaminated by foreign substances, cracks, flaws, etc. which already exist in materials themselves irrespective of the welding operation. For the purpose of simplicity, these acoustically discontinued portions will be referred to as the defect 6 in the following description. In the drawing, a probe 7 is rested on the surface 3 of the test object 1.
An ultrasonic pulse is transmitted into the test object 1 from the probe 7 placed on the surface 3 of the test object 1 which corresponds to a test surface. In the drawing, the propagating direction of the ultrasonic pulse is indicated by a solid line with arrows; an angle ".theta." denotes the refraction angle of the ultrasonic beam. An echo resulting from the irradiation to the defect 6 is reflected back and received by the probe 7.
The defect 6 is detected as follows. Although it is not shown, an ultrasonic flaw detector which is electrically connected to the probe 7 measures the difference between the time when the ultrasonic pulse is transmitted from the probe 7 and the time when the returning echo is received, that is, the time required for the ultrasonic pulse to propagate through the test object 1. The time required for the ultrasonic pulse to make a round trip between the probe 7 and the defect 6 is divided by two to determine the time required for one way, propagation and then the determined time for one way propagation and the one way velocity of sound in the test object 1 are used to acquire the beam path length. The beam path length is denoted by "Wy" in the drawing.
As shown in the drawing, if the thickness of the test object 1 is denoted by "t", then a horizontal distance "y" and a depth "d" from the surface 3 of the test object 1 to the defect 6 can be determined by equation 1 and equation 2 shown below. EQU y=Wy.times.sin (.theta.) Equation 1 EQU d=2t-Wy.times.cos (.theta.) Equation 2
Equation 2 for determining the depth d applies when the ultrasonic pulse transmitted from the probe 7 is reflected once on the bottom 4 of the test object 1 and applied to the defect 6. If the ultrasonic pulse transmitted from the probe 7 is directly applied to the defect 6 without using the reflection on the bottom and the echo is directly received, then the following equation 3 which holds according to a similar geometrical relationship will be used: EQU d=Wy.times.cos (.theta.) Equation 3
Although not illustrated, if the ultrasonic pulse is repeatedly reflected a few times on the bottom 4 or the surface 3 of the test object 1 before it is applied to the defect 6 before the echo is received, then an equation which also holds according to a similar geometrical relationship will be used.
In the ultrasonic angle-beam flaw detection technique, transverse waves are often employed as the ultrasonic waves propagating through the test object 1. However, there are some cases where longitudinal waves are employed as disclosed in, for examples, Japanese Examined Patent Publication No. 55-36108, Japanese Examined Patent Publication No. 56-17024, Japanese Unexamined Patent Publication No. 53-74485, Japanese Examined Patent Publication No. 57-1788, Japanese Unexamined Patent Publication No. 61-169760, Japanese Unexamined Patent Publication No. 61-239158, and Japanese Unexamined Patent Publication No. 63-261156.
Furthermore, FIG. 80 illustrates a single probe technique wherein the single probe 7 transmits and receives the ultrasonic waves. However, there is also a double probe technique wherein separate probes are used for transmitting and receiving, respectively, as disclosed in, for example, Japanese Unexamined Patent Publication No. 62-222160, Japanese Unexamined Patent Publication No. 60-73453, Japanese Unexamined Patent Publication No. 64-59152, Japanese Unexamined Patent Publication No. 5-322857, Japanese Unexamined Patent Publication No. 7-120439, Japanese Examined Patent Publication No. 57-51062, Japanese Unexamined Patent Publication No. 55-13845, and Japanese Unexamined Patent Publication No. 5-288722.
Furthermore, there has been known a technique wherein an array-shaped probe is employed for the probe 7 to change electronic scanning or the refraction angle .theta. as disclosed in, for example, Japanese Unexamined Patent Publication No. 57-141549, Japanese Examined Patent Publication No. 1-46027, Japanese Examined Patent Publication No. 5-84464, Japanese Examined Patent Publication No. 6-64017, Japanese Examined Patent Publication No. 6-64027, Japanese Examined Patent Publication No. 3-50989, Japanese Examined Patent Publication No. 4-16174, Japanese Unexamined Patent Publication No. 60-66159, Japanese Unexamined Patent Publication No. 64-57165, or Japanese Unexamined Patent Publication No. 7-229879, instead of mechanically scanning the surface 3 of the test object 1 by the probe 7 longitudinally and laterally manually or automatically to perform the detection of flaws.
There has been known still another method called a tandem probe technique which is a double probe technique based on mechanical scanning which employs two probes having the same refraction angle .theta. as disclosed in, for example, Japanese Unexamined Patent Publication No. 5-288722, Japanese Examined Patent Publication No. 62-28870, Japanese Unexamined Patent Publication No. 64-9361, or Japanese Unexamined Patent Publication No. 56-67750.
Every one of the techniques described above, however, ignores the fact that an ultrasonic beam diverges due to diffraction. All the foregoing methods carry out the detection of the defect 6 according to the above equations established from the geometrical relationships based only on the central axis of an ultrasonic beam to estimate the size of the defect 6 from the height of an echo. This has posed a problem with the accuracy of measurement of the shape, size, position, etc. of the defect 6.
There have been attempts to take advantage of the spread of the ultrasonic beams caused by diffraction in order to improve the measurement accuracy. Such attempts include, for instance, the methods utilizing a synthetic aperture signal processing which has been disclosed, for example, in Japanese Unexamined Patent Publication No. 2-278149, Japanese Unexamined Patent Publication No. 2-248855, or Japanese Unexamined Patent Publication No. 5-172789.
In these methods based on the synthetic aperture signal processing, however, the length of only one beam path based on direct scanning is considered for the ultrasonic beam which is emitted from the probe 7 to the defect 6 and reflected from the defect 6 back to the probe 7, and signals are processed according thereto. Hence, the problem with the accuracy of the measurement of the shape, the size, the position, etc. of the defect 6 still remains unsolved.
The conventional ultrasonic flaw detection apparatuses and ultrasonic flaw detection techniques described above ignore the fact that an ultrasonic beam diverges due to diffraction. They are designed to carry out the detection of the defect 6 according to the above equations established according to the geometrical relationships based only on the central axis of an ultrasonic beam to estimate the size of the defect 6 from the height of an echo, posing a problem in that the accuracy of measurement and detection of shape, size, position, etc. of the defect 6 is not very good.
In other conventional ultrasonic flaw detection apparatuses and ultrasonic flaw detection techniques, although the divergence attributable to the diffraction is considered, the length of only one beam path based on direct scanning is considered for the ultrasonic beam which is emitted from the probe 7 to the defect 6 and reflected from the defect 6 back to the probe 7, and signals are processed according thereto. Hence, there has been a problem in that the accuracy of measurement and detection of the shape, size, position, etc. of the defect 6 is not very good.
The present invention has been made with a view toward solving the problems described above, and it is an object of the present invention to provide an ultrasonic flaw detection apparatus and an ultrasonic flaw detection technique which permit higher accuracy of the examination performed by ultrasonic waves through a test object so as to permit improved capability of detection and higher accuracy of measurement of the shape, size, position, etc. of a detect or the like.