Patent Publication Number: US-11041831-B2

Title: Ultrasonic probe, ultrasonic flaw detection apparatus and method

Description:
BACKGROUND 
     The present invention relates to an ultrasonic probe, an ultrasonic flaw detection apparatus, and a method. 
     Ultrasonic testing (UT) is a typical nondestructive testing method used in a nuclear power plant. At present, in addition to a method of evaluating a flaw with a waveform called an A scope, a phased array (PA) method is used as a main technique for UT. The PAmethod is a technique of controlling the phases of ultrasonic waves generated from elements of an array sensor in order to scan, in a desired direction, an ultrasonic beam, which is a wave obtained by synthesizing the ultrasonic waves generated from the elements of the array sensor, or vary a focal point. Further, the PA method makes it possible to evaluate a test result while viewing an image. Therefore, the PA method has significantly contributed to increasing the speed and accuracy of testing. However, if the drive timing (delay time) of the elements of the array sensor is not properly controlled, correct test results are not obtained. Therefore, when a test target has a complicated shape or an ultrasonic wave is transmitted through media, delay time setup is complicated. Consequently, the application of the PA method has often become difficult. 
     Meanwhile, a full matrix capture (FMC) method is highlighted in recent years as a new ultrasonic testing method. The FMC method is a technique of acquiring a high-definition image by individually recording all waveforms corresponding to transmission/reception combinations of the elements of the array sensor and performing an appropriate software-based waveform synthesis process. The FMC method eliminates the necessity of exercising phase control during data recording. Once the waveforms are recorded, the FMC method makes it possible to generate an image off-line after changing phase matching conditions as desired. The FMC method is also able to reproduce a PA-method-based image by synthesizing waveforms in consideration of delay time. 
     The PA method controls the phases of ultrasonic waves generated from the elements of the array sensor (hereinafter referred to as the elementary waves) in order to vary the direction and focal point of an ultrasonic beam, which is a wave obtained by synthesizing the elementary waves, in accordance with a test target region. Ultrasonic waves reflected, for example, from a flaw are received again by the elements of the array sensor, converted to digital signals, and added up in consideration of delay time by a computation section such as a field-programmable gate array (FPGA) in an apparatus. In this case, the delay time should be set based on the Huygens&#39; principle so that the phases of the elementary waves are aligned to form a single envelope. However, if the linearity of waves is presumed, a focused beam is to be generated without generating a synthesized wave through an electrical circuit switching process as far as the elementary waves generated from a single oscillating element are later superimposed on each other. The FMC method, which is developed based on the above, stores, in a memory, waveforms obtained from all combinations of oscillating elements and generates a flaw detection image by synthesizing the waveforms in a subsequent software process. Pixel values of a flaw image are obtained when the amplitudes of time corresponding to path lengths between transmission/reception elements and pixels are superimposed on each other with respect to all elementary waves. Although some image generation algorithms based on different processes are proposed, they are similar to each other in basic principles. Substantially equivalent images are obtained when the proposed algorithms are used. A synthetic aperture focusing technique (SAFT) and a total focusing method (TFM) are typical methods based on the proposed algorithms. The present invention will be described on the assumption that the TFM is used as an image generation algorithm. The synthetic aperture focusing and other similar image generation algorithms are also applicable to the present invention. The FMC method is definitely a waveform data recording method and should be differentiated from the SAFT and TFM. However, the following description of the present invention assumes for the sake of simplicity that a method adopted for both recording and image generation processes is referred to as the FMC method. 
     The FMC method has some advantages. Typically, the FMC method is advantageous in that it is suitable for flaw detection of a curved object. That is to say, even if a test subject on which an ultrasonic wave is to fall has a curved surface, the FMC method is able to generate a flaw detection image more easily than the PA method. In general, when a flexible array sensor is used to detect a flaw from a curved surface with an ultrasonic wave incident on the curved surface, the array sensor is brought into close contact with the test subject or the test subject and the array sensor are both immersed in water to cause an ultrasonic wave to fall on the inside of the test subject by using the water as a mediator (this method is hereinafter referred to as the water immersion method). When the water immersion method is used, a linear array sensor having linearly arranged elements is often used. However, a flexible array sensor may also be used in the water immersion method. To generate an image, it is necessary to determine ultrasonic wave propagation paths that join individual elements to calculation points on the image. However, when the water immersion method is used, it is necessary to consider refraction occurring at an interface between the water and the test subject. This requires the relative coordinates of the elements with respect to the test subject and geometric information about the surface shape of the test subject. The geometric information is given in the form of discrete coordinate values or functions. However, if CAD data is available, it may be used. When the propagation paths of all elementary waves are determined in the above manner, an FMC flaw detection image can be generated in consideration of refraction. To accurately determine the relative coordinates of the elements with respect to the test subject, however, it is necessary to use an additional sensor for acquiring position information or mount a sensor on a scanner or other movable mechanical device. This poses a considerable burden in terms of both cost and labor. A method disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2012-255653 addresses the above problems by performing an electronic linear scan without controlling the delay time by using an array sensor, performing an aperture synthesis process based on the result of the electronic linear scan, and extracting the surface shape from a pixel distribution having the resulting value representing the maximum image brightness. Meanwhile, a method disclosed in Japanese Unexamined Patent Application Publication No. 2011-247649 extracts a surface echo from a waveform derived from the phased array method and estimates the surface shape by using tangent lines of circles having radiuses equivalent to beam path lengths between oscillators and the surface of the test subject. 
     SUMMARY 
     However, the above-described disclosed methods are obviously at a disadvantage in that they extract only portions existing on the propagation paths through which reflected waves return to the elements, that is, extract only a part of the surface shape. Therefore, if the method disclosed in Japanese Unexamined Patent Application Publication No. 2012-255653 or No. 2011-247649 is used to extract a convex shaped, for example, like excess weld metal, obtained signals relate to only the top of excess weld metal and neighboring flat portions. Consequently, propagation paths for incidence on the inside of the test subject from the lateral surfaces of excess weld metal do not contribute to an image or the image is generated based on wrong propagation paths. This degrades the accuracy of testing. 
     The present invention provides, for example, an ultrasonic probe that is capable of improving the accuracy of testing of a curved-surface structure. 
     According to an aspect of the present invention, there is provided an ultrasonic probe including an ultrasonic array sensor, a propagation member, and at least one ultrasonic reflection member. The ultrasonic array sensor includes oscillators and generates an ultrasonic wave. The propagation member is disposed between the ultrasonic array sensor and a test target in order to propagate the ultrasonic wave. The ultrasonic reflection member reflects the ultrasonic wave that bounces back from the surface or inside of the test target, and causes the ultrasonic wave to fall on one of the oscillators. 
     The present invention improves the accuracy of testing of a curved-surface structure. The other problems, configurations, and advantageous effects will become apparent from the following description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be described in detail based on the following figures, in which: 
         FIG. 1  is a diagram illustrating the principle of waveform recording by the FMC method; 
         FIG. 2  is a diagram illustrating elementary waves corresponding to combinations of transmission and reception elements; 
         FIG. 3A  is a diagram illustrating the behavior of an ultrasonic wave; 
         FIG. 3B  is a diagram illustrating a linear surface echo acquired from a test subject depicted in  FIG. 3A ; 
         FIG. 4  is a diagram illustrating the behavior of an ultrasonic wave in a test subject having a convex; 
         FIG. 5  is a diagram illustrating the behavior of an ultrasonic wave in the vicinity of the convex of the test subject depicted in  FIG. 4 ; 
         FIG. 6  is a diagram illustrating a surface echo acquired from the test subject depicted in  FIG. 4 ; 
         FIG. 7  is a diagram illustrating a configuration of an ultrasonic probe according to a first embodiment of the present invention; 
         FIG. 8  is a diagram illustrating virtual elements; 
         FIG. 9  is a diagram illustrating a surface echo that is acquired from a test subject having a convex by using the ultrasonic probe according to the first embodiment; 
         FIG. 10  is a diagram illustrating exemplary positions at which reflection sections are disposed; 
         FIG. 11  is a diagram illustrating other exemplary positions at which the reflection sections are disposed; 
         FIG. 12  is a diagram presenting a list of elementary waves that are used for imaging by an ultrasonic flaw detection apparatus having the ultrasonic probe according to the first embodiment; 
         FIG. 13A  is a diagram illustrating a configuration of the ultrasonic probe according to a second embodiment of the present invention; 
         FIG. 13B  is a diagram illustrating a configuration of an exemplary modification of the ultrasonic probe according to the second embodiment; 
         FIG. 14  is a diagram illustrating a configuration of the ultrasonic probe according to a third embodiment of the present invention; 
         FIG. 15  is a diagram illustrating a configuration of the ultrasonic probe according to a fourth embodiment of the present invention; 
         FIG. 16  is a diagram illustrating a configuration of the ultrasonic flaw detection apparatus; and 
         FIG. 17  is an exemplary flowchart illustrating the ultrasonic flaw detection apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     Configurations and operations of an ultrasonic flaw detection apparatus including an ultrasonic probe according to first to fourth embodiments of the present invention will now be described with reference to the accompanying drawings. 
     First Embodiment 
     The following first describes the procedures for recording a waveform by the FMC method according to the present invention and generating an image by suing the recorded waveform. 
       FIG. 1  illustrates the principle of waveform recording by the FMC method. It is assumed for ease of explanation that the total number of elements of a sensor is four. The principle remains unchanged even if the number of elements is increased. Waveform signal recording is started by exciting only a first element (element  101 ) in an array sensor  100  so that an ultrasonic wave is incident. The ultrasonic wave  106  propagated in a test subject  107  is reflected or scattered from a flaw or other reflection source  105 , returned toward the elements, and received by the first and other elements (elements  101 ,  102 ,  103 ,  104 ). 
     The received wave may be recorded on an individual element basis. An alternative is to store signals simultaneously received by the elements  101 - 104  in a hardware memory and sequentially read the stored signals by switching through, for example, a multiplexer. The order in which the elements are switched over is not restricted. The elements may be randomly switched over. The received wave is converted to an electrical signal and stored in the hardware memory as elementary waves W 11 , W 12 , W 13 , W 14 . Similarly, a second element (element  102 ) is excited, and the resulting reflected wave is received by the first to fourth elements (elements  101 ,  102 ,  103 ,  104 ). 
     When recording is repeated while the element to be excited is sequentially changed in the above manner, elementary waves W mn  (m, n=1, 2, 3, 4) corresponding to all combinations of transmission and reception elements can be obtained as depicted in  FIG. 2 . If the total number of array sensor elements is N, there are N 2  patterns of combinations. In theory, the equation W mn =W nm  (m, n=1, 2, . . . , N) is established due to reciprocity of wave motion. Therefore, all elementary waves need not be recorded, and (N 2 +N)/2 patterns of combinations will do. In reality, however, reciprocity is not perfectly established in most cases because of circuit and element characteristics. Consequently, the present invention will be described on the assumption that all patterns of elementary waves are to be recorded. In some cases where, for example, a propagation path oriented at a specific angle is to be used or waveform signals of faulty elements are to be excluded, certain elementary waves may be selectively used as needed for imaging. 
     A method of generating a flaw detection image from the above-mentioned elementary waves will now be described with reference to  FIG. 3A . For the sake of simplicity, the following description assumes that there are two propagation substances. The same idea applies even if there are more than two propagation substances. Further, for ease of explanation, the following description assumes that water and steel are used as the propagation substances. In  FIG. 3A , a propagation substance  301  is water, and a propagation substance  302  is steel. It is obvious that the same idea applies even if some other propagation substances are used. 
     For example, an ultrasonic wave starting from an element  303   a  of an array sensor  300  is rectilinearly transmitted along a propagation path  304   a  in water  301 . A portion of the ultrasonic wave is then reflected at a point  307   a  on an interface  306  between the water  301  and the steel  302 , and some other portion of the ultrasonic wave is oriented at a refraction angle satisfying the Snell&#39;s law and proceeds into the steel  302  along a propagation path  304   b . If it is assumed here that a reflection source  305  is, for example, in the steel  302 , the ultrasonic wave is reflected from the reflection source  305 , proceeds along a propagation path  304   c , and returns to the interface  306 . Further, the ultrasonic wave is refracted at a point  307   b  on the interface  306  in such a manner as to satisfy the Snell&#39;s law, proceeds along a propagation path  304   d  in the water  301 , and is eventually received by an element  303   b.    
     When the coordinates of the element  303   a  are (xm, zm), the coordinates of the reflection source  305  are (xi, zi), and the coordinates of the point  307   a  on the interface are (xb1, zb1), the time of propagation τmi from the element  303   a  to the reflection source  305  is given by the equation below. 
     
       
         
           
             
               
                 
                   
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     where c1 and c2 are the speeds of sound of water  301  and steel  302 , respectively. 
     The time of propagation τni from the reflection source  305  to the element  303   b  can be determined based on the same idea. Thus, the time of propagation from the element  303   a  to the element  303   b  is given by the equation τmi+τni. This also holds true for combinations of the other transmission and reception elements. No matter whether there are actual reflection sources, the aperture synthesis process and the TFM generate an image on the assumption that all pixels are reflection sources. Therefore, the reflection source  305  may be regarded as a pixel. Consequently, when all elementary waves are added together at the pixel, a pixel value Si is given by the equation below. 
                     S   i     =       ∑     m   =   0       N   -   1       ⁢       ∑     n   =   0       N   -   1       ⁢       W   mn     ⁡     (       τ     m   ⁢           ⁢   i       +     τ   ni       )                   (   2   )               
where N is the number of elements of the array sensor  300 .
 
     The elementary waves include not only reflected signals resulting from the reflection source  305  in the steel  302 , but also those reflected from the interface  306 . As mentioned earlier, the ultrasonic wave transmitted from the element  303   a  propagates rectilinearly along the propagation path  304   a  in the water  301 , partly bounces off the point  307   a  on the interface  306 , and proceeds along a propagation path  304   e  until it is received by an element  303   c . An ultrasonic wave transmitted from an element other than the element  303   a  is also partly reflected from the interface  306  (boundary) and received by a certain element except when it is outside an element array region of the array sensor  300 . 
     Consequently, a linear surface echo  308  depicted in FIG.  3 B is formed in the vicinity of the interface  306  between the imaged water  301  and steel  302 . Analyzing the position of the surface echo  308  makes it easy to determine the relative distance and angle between the array sensor  300  and the steel  302 . 
     A case where a weld zone or other subject having a convex surface is the test subject will now be described with reference to  FIG. 4 . In this case, too, for ease of explanation, the following description assumes that water and steel are used as the propagation substances. That is to say, in  FIG. 4 , a propagation substance  401  is water, and a propagation substance  402  is steel. It is obvious that the same idea applies even if some other propagation substances are used. 
     First of all, the behavior of an ultrasonic wave propagated to the vicinity of the top of a convex  405  on the surface of the test subject will be described. Here, it is assumed, for example, that the ultrasonic wave transmitted from an element  403   a  of an array sensor  400  propagates rectilinearly along a propagation path  404   a  and reaches the convex  405 . The ultrasonic wave is partly reflected from the convex  405 , partly refracted at a point  407   a  on the boundary, and propagated to the inside of the steel  402 . In this instance, the angle of reflection and the angle of refraction are such that the angle of incidence and the angle of reflection or the angle of refraction satisfy the Snell&#39;s law with respect to a normal line at the point  407   a  on the convex. The ultrasonic wave propagated along a propagation path  404   b  is reflected from a reflection source  408 , refracted again at the boundary, and received by an element of the array sensor  400 , such as an element  403   c.    
       FIG. 4  indicates that the ultrasonic wave reflected from the reflection source  408  propagates again through a point  407   b  on the boundary between the convex  405  and the water  401  and returns to the array sensor  400 . However, it is also conceivable that the ultrasonic wave may return to the array sensor, for example, through a flat portion  406   b  of the boundary. Further, the ultrasonic wave transmitted from the element  403   a  is partly reflected from the point  407   a  on the convex  405  and received by an element  403   b . Propagation paths for reflection and refraction through flat portions  406   a ,  406   b  other than the convex are the same as described with reference to  FIGS. 3A and 3B . 
     The behavior of an ultrasonic wave propagated to the vicinity of an end of the convex  405  on the test subject surface will now be described with reference to  FIG. 5 . Here, it is assumed that the ultrasonic wave transmitted from an element  501  propagates through the water  401  and reaches the vicinity of an end of the convex  405  on the test subject surface, for example, a point  503   a . The ultrasonic wave is partly reflected from the point  503   a  and partly refracted and propagated into the steel  402 . The ultrasonic wave refracted at the point  503   a  and propagated into the steel  402  is reflected from a reflection source  504  and returned toward the interface. In such an instance, the ultrasonic wave partly reaches an element surface as depicted in  FIG. 4  and partly propagates to the outside of the element surface along propagation paths  502   c ,  502   d  depicted in  FIG. 5 . 
       FIG. 5  depicts, as an example, only a path for propagation to the outside of the element surface. As a waveform signal resulting from such propagation is not recorded as an elementary wave, information about a reflection source is lost. Meanwhile, as regards the ultrasonic wave reflected from the point  503   a , a tangent line in the vicinity of an end of the convex has a great gradient for the element surface of the array sensor  400 . Therefore, the ultrasonic wave propagates toward the outside of the element surface of the array sensor  400 , for example, along a propagation path  502   e . This results in the loss of information about surface reflection from a relevant part. 
     Consequently, in the vicinity of the boundary between the imaged water  301  and steel  302 , only surface echoes  602   a ,  602   b  from flat portions and a surface echo  601  from the vicinity of the top of the convex are formed as depicted in  FIG. 6  so that a surface shape cannot accurately be extracted. Therefore, even if an attempt is made to image the inside of the steel  402  by the TFM or the aperture synthesis method, refracted propagation paths cannot accurately be determined. Thus, distortion may occur to displace a reflection source, such as a flaw, or decrease an echo intensity, thereby adversely affecting test results. 
     In view of the above circumstances, the first embodiment is configured as depicted in  FIG. 7  so that reflection sections  705   a ,  705   b  are disposed between the array sensor and the surface of the steel. This makes it possible to receive, without fail, an ultrasonic wave reflected from the steel surface and propagated to the outside of the scope of the element surface of the array sensor  400  and an ultrasonic wave reflected from a reflection source and propagated to the outside of the scope of the element surface of the array sensor  400 . 
     The above will be explained in detail with reference to  FIG. 7 . For example, an ultrasonic wave transmitted from an element  701  of the array sensor  400  and reflected from a point  703   a  is propagated along a propagation path  702   f , reflected toward the element surface from a point  704   a  on the inner surface of the reflection section  705   a , propagated along a propagation path  702   g , and received by an element  701   b . Therefore, if the reflection section  705   a  does not exist, an ultrasonic wave propagated directly to the outside of the scope of the element surface can also be received. 
     Further, an ultrasonic wave reflected from the reflection source  504  and returned from the steel  402  to the water  401 , for example, through propagation paths  702   c ,  702   d  is also reflected, for example, from a point  704   b  on the inner surface of the reflection section  705   b , propagated along a propagation path  702   e , and received by an element  701   c . Therefore, if the reflection section  705   b  does not exist, an ultrasonic wave propagated to the outside of the scope of the element surface can also be received. This makes it possible to acquire an increased amount of information about the reflection source. 
     Stated differently, the array sensor  400  (ultrasonic array sensor) includes multiple elements (oscillators) and generates an ultrasonic wave. The water  401  (propagation member) is disposed between the array sensor  400  and the steel  402  (test target) to propagate the ultrasonic wave. At least one reflection section  705   a ,  705   b  (ultrasonic reflection member) reflects an ultrasonic wave that is reflected from the surface or inside of the steel  402  and returned, and causes the ultrasonic wave to fall on a certain element. 
     The reflection sections  705   a ,  705   b  reflect an ultrasonic wave as far as they are formed of a substance that differs in acoustic impedance from a propagation substance. Thus, the reflection sections  705   a ,  705   b  may be formed, for example, of resin or metal. If the propagation substance is the water  401 , its acoustic impedance is approximately 1.5×10 6  kg/m 2 s. The greater the difference from this numerical value, the higher the reflectance. Therefore, it is preferable that the reflection sections  705   a ,  705   b  be formed of metal, such as stainless steel. Obviously, however, the reflection sections  705   a ,  705   b  may be formed of metal other than stainless steel. 
     As reflection occurs at a surface, the influence exerted by the thickness of a plate is small. However, if the plate is excessively thin, a plate wave is generated. When the oscillation of the plate wave propagates through water and reaches an element, it may become a noise source. Therefore, it is preferable that the plate thickness be several millimeters or greater. 
     It is preferable that the reflection sections  705   a ,  705   b  be planar and perpendicular to the element surface. The reason is that the use of such reflection sections makes it easy to calculate a propagation path (e.g., propagation path  702   g ) at the time of imaging. That is to say, the reflection sections  705   a ,  705   b  (ultrasonic reflection members) have a flat surface perpendicular to a plane on which the elements (oscillators) of the array sensor  400  are disposed, and reflect an ultrasonic wave from the flat surface. 
     However, the reflection sections  705   a ,  705   b  need not always be shaped like a plate although the use of reflection sections other than plate-like reflection sections complicates the calculation of propagation paths. Even if the reflection sections have a curved surface, the same advantageous effects can be obtained as far as the propagation paths can be calculated. 
     The reflection sections should preferably be disposed so that an end face  1003  of an endmost element  1001   a  coincides with the inner surface of the reflection section  705   a  as depicted in  FIG. 10 . The reflection section  705   b  is based on exactly the same idea, and should preferably be disposed so that its inner surface coincides with an element end face. That is to say, there are two reflection sections (ultrasonic reflection members), and the reflection sections  705   a ,  705   b  are disposed at opposing ends of an array of elements (oscillators) of the array sensor  400 . 
     The reason is that image generation can be accomplished without making substantial changes to a program as far as processing is performed for imaging by the TFM or the aperture synthesis method with the reflection sections disposed as described above on the assumption that the elements virtually exist at positions symmetrical with respect to the end face  1003  as depicted in  FIG. 8 . 
     For example, as regards the propagation path  702   g , processing may be performed on the assumption that an ultrasonic wave is propagated along a propagation path  802   a  and received by a virtual element  801   a . This also holds true for the propagation path  702   e . That is to say, as regards the propagation path  702   e , processing may be performed on the assumption that an ultrasonic wave is looped back symmetrically with respect to the inner surface of the reflection section  705   b , propagated along a propagation path  802   b , and received by a virtual element  801   b .  FIG. 12  lists elementary waves that are used for imaging in the above case. 
     Only sixteen elementary waves listed at the center and enclosed by a thick outline are actually recorded. However, when the virtual elements are taken into consideration,  FIG. 12  indicates that the amount of information available for imaging is increased nine-fold. A portion outside the thick outline represents information that was formerly discarded. The virtual elements should be positioned symmetrically with respect to an end face  1103  as depicted in  FIG. 11 . There may be a gap between the elements and the end face  1103 . In this case, an ultrasonic wave arriving at the gap is not received. However, a greater amount of information is acquired than in a case where no virtual elements are taken into consideration. 
     Further, when reflection occurs at the reflection sections, mode conversion may occur to newly generate a longitudinal wave and a transverse wave and cause a false echo. However, the false echo can be avoided to a certain extent by imposing a restriction such that when, for example, a longitudinal-wave critical angle is exceeded by the angle of incidence on the reflection sections, the resulting elementary waves are not to be used for generating an image with a longitudinal wave. 
     When the reflection sections are disposed as descried above, a surface echo  901  is formed in the vicinity of the boundary between the imaged water  301  and steel  302  and positioned over not only the flat portions and the vicinity of the top of the convex but also the vicinity of an end of the convex as depicted in  FIG. 9 . Therefore, the surface shape can be accurately extracted. Consequently, refracted propagation paths can be accurately determined when the inside of the steel  402  is to be imaged by the TFM or the aperture synthesis method. This makes it possible to generate an undistorted image, prevent flaws and other reflection sources from being displaced and the echo intensity from being lowered, and provide an ultrasonic flaw detection method capable of accurately and easily performing nondestructive testing on a flaw in a curved-surface structure. That is to say, the present embodiment improves the accuracy of testing of a curved-surface structure. 
     As illustrated in  FIG. 16 , the ultrasonic flaw detection apparatus includes, for example, an ultrasonic array sensor, a pulsar, a receiver, a recorder, a computer, and a display. The computer includes, for example, a central processing unit (CPU) or other processor, a memory, a hard disk drive (HDD), and an interface (I/F). 
       FIG. 17  is an exemplary flowchart illustrating the ultrasonic flaw detection apparatus. The pulsar (transmitter) generates an ultrasonic wave from each element of the ultrasonic array sensor in accordance with a command from the CPU (step S 15 ). The receiver receives a signal that is reflected from a test target and received by each element, and then records the received signal in the recorder (step S 20 ). In this instance, the receiver and the recorder function as a receiving/recording section. The CPU (imaging section) in the computer images the reflected signal recorded in the recorder by the aperture synthesis method, the total focusing method, or other similar method (step S 25 ). 
     More specifically, the CPU (processor) in the computer generates an image of the test target in accordance with an ultrasonic wave that is reflected and returned from the surface or inside of the test target and incident on the elements (oscillators) of the ultrasonic array sensor through the reflection sections  705   a ,  705   b  (ultrasonic reflection members) and with an ultrasonic wave that is reflected and returned from the surface or inside of the test target and incident on the elements without passing the reflection sections  705   a ,  705   b . The CPU displays the image of the test target on the display. 
     Second Embodiment 
     A second embodiment of the present invention will now be described with reference to  FIG. 13A . The second embodiment is configured so that an ultrasonic wave emitted from the elements of an array sensor  1305  is incident on a test subject  402  through a wedge  1300  (shoe) and a propagation substance  1304 . As is the case with the first embodiment, the test subject  402  is steel having a convex shaped, for example, like excess weld metal. The wedge  1300  functions as a propagation member for propagating an ultrasonic wave. The wedge  1300  is a solid body. 
     The wedge  1300  has a concave that sufficiently covers the top of the convex. The propagation substance (ultrasonic propagation substance)  1304  is filled into a gap between the wedge  1300  and the convex. For example, glycerin paste, which is frequently used as a contact medium, is used as the propagation substance  1304 . It is obvious that a substance other than glycerin paste may be used as far as it is capable of propagating ultrasonic waves. An alternative method is to supply water from the outside or fill a gel-like substance into the gap. 
     The wedge  1300  is formed of a resin material. The wedge  1300  may be formed of a material generally used as a wedge for angle beam flaw detection, such as polystyrene or acrylic. However, the wedge  1300  according to the present embodiment includes reflection sections  1306   a ,  1306   b . Inner surfaces of the reflection sections  1306   a ,  1306   b  reflect an ultrasonic wave. The reflection sections  1306   a ,  1306   b  (ultrasonic reflection members) may be formed of any material that differs in acoustic impedance from the wedge  1300  (propagation member). However, if the wedge  1300  is formed of resin, it is preferable that the reflection sections  1306   a ,  1306   b  be formed of metal such as stainless steel. An alternative method is to provide a gap instead of embedding a plate-like substance as the reflection sections  1306   a ,  1306   b . Stated differently, the reflection sections  1306   a ,  1306   b  (ultrasonic reflection members) are formed of a gaseous substance. 
     A method based on an idea similar to the above one is to shape the wedge  1300  in such a manner that its end face coincides in position with the reflection sections  1306   a ,  1306   b  as depicted in  FIG. 13B . This method provides the same advantageous effects as the other methods. 
     That is to say, the reflection sections  1306   a ,  1306   b  (ultrasonic reflection members) are disposed in the propagation member or on its surface. It is preferable that opposing ends be shaped in the above-described manner. However, the advantageous effects are provided to a certain extent even if only one end is shaped. The role of the reflection sections and the contribution of the reflection sections to imaging are the same as described in conjunction with the first embodiment and will not be redundantly described. 
     Third Embodiment 
     A third embodiment of the present invention will now be described with reference to  FIG. 14 . The third embodiment differs from the second embodiment in the shape of the wedge. Accordingly, an array sensor  1400  and the angle of the element surface with respect to the test subject surface also differ from the counterparts in the second embodiment. In general, array sensors are capable of generating the highest-intensity ultrasonic wave in the forward direction of the element surface. Therefore, the third embodiment is suitable for a case where a strong ultrasonic wave is to be obliquely incident from a lateral surface of a weld zone. 
     More specifically, a surface on which the elements (oscillators) of the array sensor  1400  are disposed is perpendicular to the normal line N of a curved surface of an end portion of excess weld metal. Here, the end portion of the excess weld metal is, for example, a portion that is within a predetermined distance from the end of the excess weld metal. As the end portion of the excess weld metal has a curved surface, the gradient of the normal line N varies within a predetermined range. Therefore, one normal line N is determined, for example, from the average in the range of the gradient of the normal line N. 
     In the above case, too, reflection sections  1403   a ,  1403   b  are included in a wedge  1401 . These reflection sections  1403   a ,  1403   b  play the same role as in the first and second embodiments. The role of the reflection sections and the contribution of the reflection sections to imaging are the same as described in conjunction with the first embodiment and will not be redundantly described. 
     Fourth Embodiment 
     A fourth embodiment of the present invention will now be described with reference to  FIG. 15 . The fourth embodiment is configured so that a weld zone  1506  of a piping  1502  is surrounded by a cylindrical wedge  1503 , and that an array sensor  1501  disposed outside the wedge  1503  tests the inside of the weld zone  1506  by the FMC method. The wedge  1503  is cylindrical in shape. That is to say, the wedge  1503  (propagation member) is shaped like a cylinder to cover the weld zone  1506  of the piping  1502 . The axial cross-section of the wedge  1503  has the same structure as described in conjunction with the second embodiment. Accordingly, although not depicted in  FIG. 15 , the wedge  1503  includes the reflection sections. 
     Further, for example, a groove  1504  is formed in the outer surface of the wedge  1503  in order to facilitate the circumferential motion of the array sensor. That is to say, the wedge  1503  (propagation member) has a groove for guiding the array sensor  1501  (ultrasonic array sensor) along the circumference of the piping  1502 . Any other structure may be adopted instead of the grove as far as it facilitates the array sensor&#39;s circumferential motion above the weld zone. 
     Moreover, the wedge  1503  is divided so that it is easily attachable to the piping. When tests are to be performed, the wedge  1503  may be disposed to pinch the piping and then secured by connection section  1505 . A propagation substance, such as glycerin paste, is filled into a gap  1507  between the wedge  1503  and the weld zone  1506 . As the propagation substance, a substance commonly used for ultrasonic flaw detection may be adopted. An alternative is to adopt a structure for supplying water from the outside. 
     Using a cylindrical wedge including reflection sections as described above in conjunction with the present embodiment makes it possible to provide an ultrasonic flaw detection method that is not only capable of precisely extracting the surface shape of a piping weld zone, but also capable of accurately and easily performing nondestructive testing on a flaw in the piping weld zone by imaging its inside by the TFM or the aperture synthesis method. 
     The present invention is not limited to the foregoing embodiments, but includes various modifications. For example, the foregoing embodiments have been described in detail in order to facilitate the understanding of the present invention, and the present invention is not necessarily limited to embodiments including all the described elements. Some elements of one embodiment may be replaced by the elements of another embodiment. Further, the elements of one embodiment may be added to the elements of another embodiment. Furthermore, some elements of each embodiment may be deleted, subjected to the addition of other elements, or replaced by other elements. 
     Moreover, for example, the above-described elements and functions may be partly or wholly implemented by hardware by designing, for example, with an integrated circuit. Additionally, for example, the above-described elements and functions may be implemented by software by allowing a processor to interpret and execute programs that implement the respective functions. The programs, tables, files, and other items of information for implementing the functions may be stored in a memory, a recording device such as a hard disk or a solid-state drive (SSD), or a recording medium such as an IC card, an SD card, or a DVD. 
     The embodiments of the present invention may include the following aspects. 
     (1). There is provided an ultrasonic flaw detection method that causes an ultrasonic wave emitted from each of elements of an ultrasonic array sensor to reach a test target through a propagation medium and images a signal reflected from the test target by the aperture synthesis method, the total focusing method, or other similar method, wherein an ultrasonic reflection section is positioned in the inside of or on the surface of the propagation medium in such a manner that the ultrasonic wave reflected from the surface of the test target falls back on any of the elements.
 
(2). There is provided the ultrasonic flaw detection method as described in (1), wherein the ultrasonic reflection section is shaped like a flat plane, and wherein the flat plane is perpendicular to the array surface of the elements.
 
(3). There is provided the ultrasonic flaw detection method as described in (1) or (2), wherein the ultrasonic reflection section is positioned at opposing ends of the array of the elements of the ultrasonic array sensor.
 
(4). There is provided the ultrasonic flaw detection method as described in any one of (1) to (3), wherein the propagation medium is formed of a solid body, and wherein the ultrasonic reflection section is formed of a substance that differs in acoustic impedance from the solid body.
 
(5). There is provided the ultrasonic flaw detection method as described in (4), wherein the solid body is formed of resin, and wherein the ultrasonic reflection section is formed of metal.
 
(6). There is provided the ultrasonic flaw detection method as described in (4), wherein the solid body is formed of resin, and wherein the ultrasonic reflection section is formed of a gaseous substance.
 
(7). There is provided the ultrasonic flaw detection method as described in anyone of (1) to (3), wherein the propagation medium is formed of a solid body, and wherein at least one end face of the propagation medium coincides with an end of the array of the elements of the ultrasonic array sensor.
 
(8). There is provided the ultrasonic flaw detection method as described in anyone of (1) to (7), wherein the propagation medium is disposed on the outer surface of a weld zone of a piping, and wherein ultrasonic waveform data is recorded while the ultrasonic array sensor is moved along the outer surface of the propagation medium.
 
(9). There is provided an ultrasonic flaw detection apparatus including an ultrasonic array sensor, a transmitter, a propagation medium (propagation substance), a receiving/recording section, an imaging section, and an ultrasonic reflection section, wherein the ultrasonic array sensor is disposed on the surface of a test target, the transmitter transmits an ultrasonic wave from each of elements of the ultrasonic array sensor, wherein the propagation medium causes the ultrasonic wave to reach the test target, wherein the receiving/recording section receives and records a signal reflected from the test target, wherein the imaging section images the reflected signal by the aperture synthesis method, the total focusing method, or other similar method, and wherein the ultrasonic reflection section is positioned in the inside of or on the surface of the propagation medium in such a manner that the ultrasonic wave reflected from the surface of the test target falls back on any of the elements.
 
(10). There is provided the ultrasonic flaw detection apparatus as described in (9), wherein the ultrasonic reflection section is shaped like a flat plane, and wherein the flat plane is perpendicular to the array surface of the elements.
 
(11). There is provided the ultrasonic flaw detection apparatus as described in (9) or (10), wherein the ultrasonic reflection section is positioned at opposing ends of the array of the elements of the ultrasonic array sensor.
 
(12). There is provided the ultrasonic flaw detection apparatus as described in any one of (9) to (11), wherein the propagation medium is formed of a solid body, and wherein the ultrasonic reflection section is formed of a substance that differs in acoustic impedance from the solid body.
 
(13). There is provided the ultrasonic flaw detection apparatus as described in (12), wherein the solid body is formed of resin, and wherein the ultrasonic reflection section is formed of metal.
 
(14). There is provided the ultrasonic flaw detection apparatus as described in (12), wherein the solid body is formed of resin, and wherein the ultrasonic reflection section is formed of a gaseous substance.
 
(15). There is provided the ultrasonic flaw detection apparatus as described in any one of (9) to (11), wherein the propagation medium is formed of a solid body, and wherein at least one end face of the propagation medium coincides with an end of the array of the elements of the ultrasonic array sensor.
 
(16). There is provided the ultrasonic flaw detection apparatus as described in any one of (9) to (15), wherein the propagation medium is disposed on the outer surface of a weld zone of a piping, and wherein ultrasonic waveform data is recorded while the ultrasonic array sensor is moved along the outer surface of the propagation medium.
 
     According to aspects (1) to (16) above, the ultrasonic reflection section is positioned in the propagation medium in such a manner that the ultrasonic wave reflected from a curved surface of the test target propagates back onto the element surface of the ultrasonic array sensor. Therefore, an ultrasonic wave propagating along a path not leading to the ultrasonic array sensor can also be used for imaging. Consequently, the overall shape of excess weld metal can be extracted. Imaging can be achieved simply by virtually increasing the number of elements by the TFM.