Patent Publication Number: US-11029231-B2

Title: Assessing device, assessing system, assessing method, and computer-readable recording medium

Description:
This application is a National Stage Entry of PCT/JP2017/038020 filed on Oct. 20, 2017, which claims priority from Japanese Patent Application 2016-208663 filed on Oct. 25, 2016, the contents of all of which are incorporated herein by reference, in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to an assessing device, an assessing method, and a computer-readable recording medium. 
     BACKGROUND ART 
     Various techniques for analyzing a condition of a structure such as a building or a bridge have been developed. As one example, by analyzing a vibration characteristic concerning a structure, damage in the structure is detected. 
     PTL 1 describes a soundness assessing method for a concrete building and the like. In the soundness assessing method described in PTL 1, first, a microtremor of a concrete building is measured, and from measured data of the microtremor, temporal change of a natural frequency is acquired. Then, in the method described in PTL 1, in a case where a width of daily fluctuation of the natural frequency tends to decrease when a difference between internal and external temperatures of the concrete building increases, it is determined that damage exists in the concrete building. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2008-8810 
     SUMMARY OF INVENTION 
     Technical Problem 
     In analysis of a condition of a structure, detection of local damage in the structure is necessary in some cases. However, in the technique described in PTL 1, detection of local damage in a structure is difficult in some cases. 
     The present invention has been made to solve the above-described problem, and a main object thereof is to provide an assessing device and the like capable of assessing presence or absence of local damage in a structure. 
     Solution to Problem 
     An assessing device in one aspect of the present invention includes a dominant frequency identifying means for identifying a dominant frequency of a vibration at each of a plurality of spots in a structure, based on information indicating the vibration at each of the plurality of spots, a phase difference identifying means for identifying a phase difference at the dominant frequency between the vibrations at the plurality of spots, based on the dominant frequency and information indicating the vibrations; and an assessing means for assessing damage in the structure, based on the phase difference. 
     An assessing method in one aspect of the present invention includes identifying a dominant frequency of a vibration at each of a plurality of spots in a structure, based on information indicating the vibration at each of the plurality of spots, identifying a phase difference at the dominant frequency between the vibrations at the plurality of spots, based on the dominant frequency and information indicating the vibrations, and assessing damage in the structure, based on the phase difference. 
     A computer-readable recording medium in one aspect of the present invention non-temporarily stores a program causing a computer to perform processing of identifying a dominant frequency of a vibration at each of a plurality of spots in a structure, based on information indicating the vibration at each of the plurality of spots, processing of identifying a phase difference at the dominant frequency between the vibrations at the plurality of spots, based on the dominant frequency and information indicating the vibrations, and processing of assessing damage in the structure, based on the phase difference. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide an assessing device capable of assessing presence or absence of local damage in a structure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of an assessing device according to a first example embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a configuration of an assessing system including the assessing device according to the first example embodiment of the present invention. 
         FIG. 3  is a diagram illustrating an example of a damped free vibration targeted by the assessing device according to the first example embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating an operation of the assessing device according to the first example embodiment of the present invention. 
         FIG. 5  is a diagram illustrating an example of a test piece according to an example 1 of the present invention. 
         FIG. 6  is a diagram illustrating an example of change rates of phase differences and dominant frequencies according to the example 1 of the present invention. 
         FIG. 7  is a diagram illustrating an example of a test piece according to an example 2 of the present invention. 
         FIG. 8  is a diagram illustrating an example of change rates of phase differences and dominant frequencies according to the example 1 of the present invention. 
         FIG. 9  is a diagram illustrating an example of an information processing device that achieves an assessing device and the like according to the example embodiment of the present invention. 
     
    
    
     EXAMPLE EMBODIMENT 
     Each example embodiment of the present invention is described with reference to the accompanying drawings. In each example embodiment of the present invention, each constituent element of each device (system) indicates a block of a functional unit. A part or all of each constituent element of each device (system) are achieved by an arbitrary combination of an information processing device  500  as illustrated in  FIG. 9  and a program, for example. The information processing device  500  includes the following constituents as one example.
         a central processing unit (CPU)  501     a read only memory (ROM)  502     a random access memory (RAM)  503     a program  504  loaded in the RAM  503     a storage device  505  storing the program  504     a drive device  507  reading from and writing in a recording medium  506     a communication interface  508  connected to a communication network  509     an input-output interface  510  inputting and outputting data   a bus  511  connecting the constituent elements to each other       

     The constituent elements of each device in each example embodiment are achieved by the CPU  501  acquiring and executing the program  504  that achieves functions thereof. For example, the program  504  that achieves the functions of the constituent elements of each device is stored in advance in the storage device  505  or the RAM  503 , and is read by the CPU  501 , depending on necessity. Note that the program  504  may be supplied to the CPU  501  via the communication network  509 , or may be stored in advance in the recording medium  506  and be read and supplied to the CPU  501  by the drive device  507 . 
     There are various modification examples of a method of achieving each device. For example, each device may be achieved by arbitrary combinations of the information processing device  500  and a program which are different from each other per constituent element. Alternatively, a plurality of constituent elements included in each device may be achieved by an arbitrary combination of one information processing device  500  and a program. 
     A part or all of the constituent elements of each device are achieved by a general-purpose or dedicated circuit including a processor and the like, or a combination thereof. These may be constituted by a single chip, or may be constituted by a plurality of chips connected to each other via a bus. A part or all of the constituent elements of each device may be achieved by a combination of the above-described circuit and the like, and a program. 
     When a part or all of the constituent elements of each device are achieved by a plurality of information processing devices, circuits, or the like, a plurality of the information processing devices, the circuits, or the like may be arranged in a concentrated manner, or may be arranged in a dispersed manner. For example, the information processing devices, the circuits, or the like may be achieved as a form such as a client-and-server system or a cloud computing system in which each connection therebetween is made via a communication network. 
     First Example Embodiment 
     First, a first example embodiment of the present invention is described.  FIG. 1  is a diagram illustrating an assessing device according to the first example embodiment of the present invention. 
     As illustrated in  FIG. 1 , an accessing device  100  according to the first example embodiment of the present invention includes a dominant frequency identifying unit  110 , a phase difference identifying unit  120 , and an assessing unit  130 . Based on information indicating a vibration at each of a plurality of spots in a structure, the dominant frequency identifying unit  110  identifies a dominant frequency of a vibration at each of the plurality of spots. The phase difference identifying unit  120  identifies a phase difference at the dominant frequency between the vibrations at the plurality of spots, based on the dominant frequency and information indicating the vibrations. The assessing unit  130  assesses damage in the structure, based on the phase difference. 
     Note that in the present example embodiment, examples of damage in a structure include a crack or corrosion generated in the structure, and a phenomenon causing an insufficient joining such as a gap generated at a part where a plurality of portions constituting the structure are joined to each other. However, damage in a structure is not limited to the above-described examples, and other phenomena that may affect performance of a structure may be treated as damage in the structure. 
     As illustrated in  FIG. 2 , an assessing system  10  including the assessing device  100  according to the first example embodiment of the present invention is configured. The assessing system  10  includes the assessing device  100  and a plurality of detection units  101 . The detection unit  101  detects a vibration of a target object. 
     In an example illustrated in  FIG. 2 , as the detection units  101 , there are depicted N detection units  101  (where N is a natural number equal to or larger than two) including detection units  101 - 1  to  101 -N, but the number of the detection units  101  is not particularly limited. Depending on an assessing-target structure and the like, the required number, arrangement places, and the like of the detection units  101  are appropriately determined. The detection unit  101  is achieved by a general vibration sensor, for example. By an adhesive, a permanent magnet, a mechanical bonding, or the like, the detection unit  101  is attached to a structure that is a target to be assessed by the assessing device  100 . 
     Further, the assessing device  100  and the detection unit  101  are connected via a wireless or wired communication network or the like, for example. Note that a means for connecting the assessing device  100  and the detection unit  101  to each other is not particularly limited. By other means, the assessing device  100  may acquire and use information indicating vibrations detected by the detection units  101 . In the example illustrated in  FIG. 2 , the assessing device  100  makes assessment, based on information indicating a vibration detected by each of a plurality of the detection units  101 . 
     Next, constituent elements of the assessing device  100  according to the present example embodiment are described. 
     As described above, the dominant frequency identifying unit  110  identifies each dominant frequency of vibrations at a plurality of spots. Information indicating vibrations at a plurality of spots can be acquired, for example, by detecting vibrations by each of a plurality of the detection units  101  attached to a structure. 
     The dominant frequency identifying unit  110  identifies a dominant frequency by mainly focusing on a damped free vibration that is one of vibration responses of a structure included in information indicating detected vibrations.  FIG. 3  illustrates an example of a damped free vibration. In  FIG. 3 , the damped free vibration is represented by a portion indicated by an arrow in temporal change concerning acceleration of a vibration. In other words, the damped free vibration is a vibration in which excitation from the outside is not applied and an amplitude of acceleration decreases depending on time. A vibration that is a damped free vibration can be extracted, for example, based on a shape of an envelope curve of a time-history waveform indicating acceleration of the vibration. 
     A dominant frequency indicates a main frequency component included in a vibration. In the present example embodiment, as described below, the dominant frequency mainly indicates a frequency component at which an amplitude of a vibration becomes maximum. 
     Generally, in a damped free vibration generated in a structure, a natural vibration depending on a dynamic characteristic of the structure becomes dominant. For this reason, generally, a dominant frequency identified by the dominant frequency identifying unit  110  approximately corresponds to a natural vibration of an assessing-target structure. From this, behavior of a vibration at the dominant frequency approximately corresponds to a dynamic characteristic of the structure. Accordingly, paying attention to the above-described dominant frequency enables a dynamic characteristic of a structure to be grasped. 
     When damage or the like is generated in a structure, a dynamic characteristic of the structure changes. In other words, in this case, a dominant frequency may change. Accordingly, for example, continuously identifying a dominant frequency of a structure enables damage or the like in a structure to be detected. 
     The dominant frequency identifying unit  110  identifies, as a dominant frequency, a frequency at which an amplitude becomes maximum, among frequency components included in a damped free vibration. Information indicating a vibration that is acquired by the detection unit  101  or the like generally indicates temporal change of the vibration. In this case, for example, the dominant frequency identifying unit  110  transforms temporal change of a vibration into frequency components, and identifies a dominant frequency, based on an amplitude at each frequency. 
     The dominant frequency identifying unit  110  transforms temporal change of a vibration into frequency components by the discrete Fourier transform, the fast Fourier transform, or the like, for example. Further, by using an autoregressive model or the like, the dominant frequency identifying unit  110  may model a damped free vibration represented by information indicating a detected vibration, and may acquire a dominant frequency, based on a frequency characteristic of the model. As described above, for example, the dominant frequency identifying unit  110  identifies, as a dominant frequency, a frequency having the largest amplitude. 
     The phase difference identifying unit  120  identifies a phase difference at a dominant frequency between vibrations at a plurality of spots in a structure. More specifically, the phase difference identifying unit  120  identifies a phase difference at a dominant frequency between vibrations at two spots among a plurality of spots in a structure. 
     Note that it is assumed in the present example embodiment that information indicating vibrations at three or more spots in a structure can be acquired, such as the detection units  101  of the assessing system  10  are installed at three or more spots in a structure. In this case, the phase difference identifying unit  120  identifies a phase difference for at least one set of two spots selected from the three or more spots. In this case, the two spots included in each set are preferably adjacent spots. 
     As described above, in some cases, a natural frequency of a structure changes depending on damage or the like generated in the structure. Accordingly, in some cases, a condition of a structure such as presence or absence of damage in the structure, can be assessed based on change in the natural frequency of the structure, by detecting change in a dominant frequency, or the like, for example. 
     However, in a method in which assessment on damage in a structure is performed based on change in a natural frequency of the structure, assessment of presence or absence of damage or the like in the structure is difficult in some cases. Cases where it is difficult to assess presence or absence of damage or the like, based on change in a natural frequency of a structure include a case where local damage is generated in a structure. Examples of the case where local damage is generated in a structure include a case where damage is generated at a joined part that is a part where two or more members are joined to each other. 
     In some cases, a structure is configured with two or more members being joined to each other. For example, when a structure is a bridge, the structure is configured with a floor plate and a main girder being joined to each other. In a structure including such a configuration, force concentrates at a joined part where members are joined to each other. Accordingly, in some cases, at the joined part, local damage may be generated. Then, when damage is generated at the joined part, a rigidity of the joined part decreases, and a natural frequency of the structure including the joined part changes. 
     However, when local damage such as damage at a joined part is generated, in some cases, change in a natural frequency is relatively small as compared with the case where large damage is generated in a structure. For this reason, in the method in which assessment on damage in a structure is performed based on change in a natural frequency of the structure, assessment on presence or absence of damage or the like in a structure is difficult in some cases. 
     Meanwhile, when damage is generated at a joined part, constraint between joined members declines. In other words, a condition of a mechanical joint between joined members changes. As a result of this change, vibration responses including that of the joined part change. For example, at constituent members sandwiching a joined part, phases of vibrations at a dominant frequency change. In other words, a phase difference at the dominant frequency between vibrations at the constituent members sandwiching the joined part changes. 
     In view of it, in the present example embodiment, the phase difference identifying unit  120  identifies a phase difference at a dominant frequency between vibrations at two spots among a plurality of spots in a structure. By identifying a phase difference, damage at a spot between the two spots can be assessed in the below-described assessing unit  130 . 
     Positions of two spots are determined in such a manner that the above-described joined part exists between the two spots (i.e., the positions of the two spots sandwich the joined part), thereby enabling damage-related assessment on presence or absence of damage or the like at the joined part. As one example, when a structure is a bridge, one of a plurality of the detection units  101  is provided at a floor plate, and another of a plurality of the detection units  101  is provided at a main girder, whereby local damage generated at a joined part between the floor plate and the main girder can be detected. 
     Further, two spots for which a phase difference is acquired by the phase difference identifying unit  120  are neighboring spots, among a plurality of spots in a structure, whereby a position where local damage is generated can be more accurately identified. 
     The phase difference identifying unit  120  identifies a phase difference by various procedures. For example, the phase difference identifying unit  120  calculates a difference between phase values that are acquired by performing the Fourier transform on a free damped vibration represented by information indicating vibrations at spots, and thereby identifies a phase difference at a domain frequency between the vibrations at the two spots. 
     The phase difference identifying unit  120  may identify a phase difference by using time-history waveforms in a band of a dominant frequency of vibrations at spots. In this case, the phase difference identifying unit  120  derives the time-history waveforms indicating temporal change in the band of the dominant frequency by performing band limitation on information indicating vibrations at spots, by a band-pass filter or the like. Then, the phase difference identifying unit  120  acquires, for each of the time-history waveforms, a time at which an amplitude of the time-history waveform becomes maximum, and further acquires a difference between the times, thereby identifying a phase difference. 
     When information indicating vibrations at three or more spots in a structure is acquired, similarly, the phase difference identifying unit  120  identifies a phase difference for each set of the two spots appropriately selected from the three or more spots. 
     The assessing unit  130  assesses damage in a structure, based on a phase difference acquired by the phase difference identifying unit  120 . When a phase difference at a dominant frequency between vibrations at two spots in a structure indicates that damage is generated in the structure, the assessing unit  130  assesses damage as being generated at a portion between the two spots. 
     As one example, the assessing unit  130  assesses damage in a structure by comparing, with a phase difference at a reference timing, each phase difference acquired by the phase difference identifying unit  120 . 
     For example, when a change rate of a phase difference between two spots acquired by the phase difference identifying unit  120  and a phase difference at the reference timing for the same spots satisfies a predetermined requirement, the assessing unit  130  assesses damage as existing at a portion between the two spots in a structure. As one example, when a change rate of a phase difference between two spots and a phase difference at the reference timing for the same spots exceeds a threshold value, the assessing unit  130  assesses damage as being generated at a portion between the two spots. In this case, the threshold value is appropriately determined depending on a structure, positions of two spots in the structure, whether or not a joined part is included in a portion between the two spots, or the like. Further, a change rate of phase differences, which is a value standardized by a phase difference or the like at the reference timing, for example, is used rather than an absolute value of a phase difference, whereby, setting of the threshold value depending on a portion in a structure becomes unnecessary, or the like, and therefore setting of the threshold value becomes easy. 
     The assessing unit  130  may assess damage in a structure, based on another method. For example, the assessing unit  130  may assess damage in a structure, based on a variation of phase differences acquired by the phase difference identifying unit  120 . 
     In this case, for example, vibrations are detected a plurality of times at two target spots by the detection units  101  or the like of the assessing system  10 . Concerning information indicating vibrations that are acquired a plurality of times at timings in a predetermined fixed range by the detection units  101  or the like, the dominant frequency identifying unit  110  identifies a dominant frequency concerning each of the vibrations. Further, based on the identified dominant frequency and the like, the phase difference identifying unit  120  acquires each phase difference. 
     Then, the assessing unit  130  assesses damage in a structure, based on a variation of a plurality of phase differences acquired as described above for vibrations. As one example, the assessing unit  130  assesses damage as existing at a portion between two spots in a structure, based on a change rate between a variation of a plurality of phase differences acquired by the phase difference identifying unit  120  and a variation of a plurality of phase differences at the reference timing. When a variation of a plurality of phase differences satisfies a predetermined requirement such as a requirement that the variation exceeds a threshold value, the assessing unit  130  may assess damage as existing at a portion between the two spots in the structure. 
     Note that the assessing unit  130  may assess damage in a structure by an appropriate combination of a plurality of the above-described methods, the combination being made depending on necessity. 
     Subsequently, an operation of the assessing device  100  in the first example embodiment of the present invention is described by using a flowchart illustrated in  FIG. 4 . 
     First, the dominant frequency identifying unit  110  identifies a dominant frequency (a step S 101 ). The dominant frequency identifying unit  110  identifies a dominant frequency for each of vibrations at a plurality of spots in a structure. As information indicating vibrations at a plurality of spots in a structure, the dominant frequency identifying unit  110  acquires and uses information indicating vibrations detected by the detection units  101  in the assessing system  10 , for example. 
     Next, the phase difference identifying unit  120  identifies a phase difference between vibrations at two spots in the structure, based on the dominant frequency acquired at the step S 101  and the information indicating the vibrations (a step S 102 ). Note that when information indicating vibrations at three or more spots in the structure is acquired, as one example, the phase difference identifying unit  120  successively identifies a phase difference for each of a plurality of sets of two spots among the three or more spots. 
     Next, the assessing unit  130  assesses damage in the structure, based on the phase difference identified at the step S 102  (a step S 103 ). In the case where a change rate, a variation, or the like of the phase difference between the two spots in the structure exceeds a predetermined threshold value for example, the assessing unit  130  assesses damage as being generated at a spot between the two spots in the structure. When a phase difference is acquired for a plurality of sets of two spots in the structure at the step S 102 , the assessing unit  130  may assess damage for each of a plurality of sets of two spots. 
     As described above, the assessing device  100  according to the first example embodiment of the present invention assesses damage in a structure, based on a phase difference at a dominant frequency between vibrations at a plurality of spots in the structure. More specifically, in the assessing device according to the present example embodiment, for vibrations detected at two spots in a structure, a phase difference at a dominant frequency identified by the dominant frequency identifying unit  110  is identified by the phase difference identifying unit  120 . Then, based on the identified phase difference, the assessing unit  130  assesses damage in the structure. 
     A natural frequency of a structure may change when damage is generated in the structure. However, when damage generated in a structure is local damage, change in a natural frequency of the structure is small in some cases. For this reason, when damage in a structure is assessed based on a natural frequency of the structure, there is a possibility that local damage in the structure is not detected. 
     In contrast to this, when local damage is generated in a structure, change in a phase difference between vibrations at spots around a portion where damage is generated occurs in some cases. In other words, paying attention to a phase difference between vibrations enables local damage in a structure to be detected in some cases. 
     In the assessing device  100  according to the present example embodiment, a phase difference at a dominant frequency between vibrations at a plurality of spots in a structure is used in assessing damage in the structure. Accordingly, the assessing device  100  according to the present example embodiment enables assessment of local damage in the structure, such as presence or absence of local damage in the structure. 
     Example 1 
     Next, description is made on an example in which the assessing device  100  and the assessing system  10  according to the first example embodiment of the present invention are applied to assessment of presence or absence of local damage in a target object. 
     In a first example, the assessing system  10  targeted a concrete block, and assessed presence or absence of damage generated in the concrete block. Concretely, a crack was formed as simulated damage on a surface of the concrete block that was a test piece. Then, based on vibrations before and after the formation of the crack, the damage was assessed by the assessing system  10 . 
       FIG. 5  illustrates an example of a concrete block as a target of assessment on damage. In the present example, a rectangular-shaped concrete block  180  was used as a test piece. On one surface of the concrete block  180 , five detection units  101  including detection units  101 - 1  to  101 - 5  were attached. As each of the detection units  101 , a vibration sensor was used. The vibration sensor used as the detection unit  101  is of a piezoelectric type, and is of a voltage output type in which a signal amplification circuit is incorporated. Between the detection units  101 - 1  and  101 - 2 , a crack  181  was formed. 
     In each of cases before and after the crack  181  was formed, the concrete block  180  was excited by hammering that uses a hammer  190 . In each of the cases, each of the detection units  101 - 1  to  101 - 5  detected a vibration caused by the excitation. 
     The assessing device  100  assessed damage in the concrete block  180 , based on information indicating the vibrations detected by the detection units  101 - 1  to  101 - 5 . 
     First, the dominant frequency identifying unit  110  acquired a Fourier spectrum by performing the Fourier transform on the information indicating the vibrations in each of before and after the crack  181  was formed. Then, the dominant frequency identifying unit  110  identified, as a dominant frequency, a frequency at which an amplitude becomes maximum in each of the Fourier spectra before and after the crack  181  was formed. 
     According to the information indicating the vibrations detected by the detection units  101 - 1  to  101 - 5  at a time point before the crack  181  was formed, the dominant frequency was common to the detection units  101 - 1  to  101 - 5 . A vibration shape at the dominant frequency was a shape similar to that of a flexural primary vibration mode. 
     According to the information indicating the vibrations detected by the detection units  101 - 1  to  101 - 5  at a time point after the crack  181  was formed, the dominant frequency was common to the detection units  101 - 1  to  101 - 5 . A vibration shape at the dominant frequency was a shape similar to that of the flexural primary vibration mode. 
     Subsequently, assuming that a phase amount is a value of a phase at the dominant frequency in each of the Fourier spectra before and after the above-described crack  181  was formed, the phase difference identifying unit  120  acquired, as a phase difference, each difference between the phase amounts. In the present example, the phase difference identifying unit  120  identified, as the phase difference, the difference between the phase amounts, at the dominant frequency, of the vibrations detected by the detection units  101 - 1  and  101 - 2 . The detection units  101 - 1  and  101 - 2  are two detection units close to a place where the crack  181  was formed. 
     Subsequently, the assessing unit  130  acquired a change rate of the phase differences at the dominant frequencies between the vibrations detected by the detection units  101 - 1  and  101 - 2  before and after the crack  181  was formed, and assessed damage. The change rate of the phase differences at the dominant frequencies between the vibrations detected by the detection units  101 - 1  and  101 - 2  was acquired by using the following equation (1). Note that in the equation (1), ΔΦ 1  represents the change rate of the phase differences at the dominant frequencies between the vibrations detected by the detection units  101 - 1  and  101 - 2  before and after the crack  181  was formed. Further, Φ 1before  represents the phase difference at the dominant frequency between the vibrations detected by the detection units  101 - 1  and  101 - 2  before the crack  181  was formed. In addition, Φ 1after  represents the phase difference at the dominant frequency between the vibrations detected by the detection units  101 - 1  and  101 - 2  after the crack  181  was formed. 
     
       
         
           
             
               
                 
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     Note that for comparison, a change rate of dominant frequencies of the vibrations detected by the detection unit  101 - 1  was acquired by using an equation (2). Note that in the equation (2), ΔF 1  represents the change rate of the dominant frequencies of the vibrations detected by the detection unit  101 - 1  before and after the crack  181  was formed. Further, F 1before  represents the dominant frequency of the vibration detected by the detection unit  101 - 1  before the crack  181  was formed. In addition, F 1after  represents the dominant frequency of the vibration detected by the detection unit  101 - 1  after the crack  181  was formed. Note that in the present example, as described above, the dominant frequencies of the vibrations detected by the detection units  101 - 1  to  101 - 5  are common at each of the time points before and after the crack  181  was formed. 
     
       
         
           
             
               
                 
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       FIG. 6  illustrates the change rate ΔΦ 1  of the phase differences and the change rate ΔF 1  of the dominant frequencies described above. The change rate ΔΦ 1  of the phase differences was −20% (percent). Meanwhile, the change rate ΔF 1  of the dominant frequencies was −5%. In other words, a change rate of ΔΦ 1  to the change rate ΔF 1  was four times. The change rate of the phase differences at the dominant frequencies was a relatively large change rate, as compared with the change rate of the dominant frequencies. 
     In the present example embodiment, it is considered that a rigidity of the concrete block  180  as a test piece decreased due to the formation of the crack  181  in the concrete block  180 , and as a result, a natural frequency of the concrete block  180  decreased. Then, it is considered that the dominant frequency corresponding to the natural frequency decreased following the decrease of the natural frequency of the concrete block  180 . Further, it is considered that the formation of the crack  181  between the detection units  101 - 1  and  101 - 2  decreased a rigidity of the portion that is in the concrete block  180  and that is between the detection unit  101 - 1  and the detection unit  101 - 2 . It is considered that as a result of this, the phase difference at the dominant frequency between the vibrations detected by the detection units  101 - 1  and  101 - 2  increased. Then, it is considered that since the crack  181  was local, the above-described change rate of the phase differences at the dominant frequencies became relatively large, as compared with the change rate of the dominant frequencies. 
     As demonstrated in the present example, for the local damage in the structure, the phase difference at the dominant frequency between the vibrations detected by the two detection units  101  changes remarkably as compared with the dominant frequency. Therefore, it is confirmed that even when assessment is difficult in a method based on a change rate of a dominant frequency (i.e., a natural frequency of a structure), the assessing device  100  or the like can assess generation of damage, for example, by appropriately setting a threshold value used in the assessing unit  130 . 
     Example 2 
     Next, description is made on another example in which the assessing device  100  and the assessing system  10  according to the first example embodiment of the present invention are applied to assessment of presence or absence of damage in a target object. 
     In the present example, the assessing system  10  targeted a concrete block to which a metal plate was bonded, and assessed presence or absence of damage to a bonded part between the metal plate and the concrete block. Concretely, a gap was formed as simulated damage at the bonded part between the metal plate and the concrete block, which was in the concrete block to which the metal plate was bonded. Then, based on vibrations before and after the formation of the gap, the damage was assessed by the assessing system  10 . 
       FIG. 7  illustrates an example of a concrete block as a target of assessment on damage. In the present example, a rectangular-shaped concrete block  180  and a rectangular-shaped metal plate  182  were used as a test piece. To one surface of the concrete block  180 , the metal plate  182  was bonded with an adhesive. The metal plate  182  and the concrete block  180  were bonded to each other in such a manner that center points of the bonded surfaces coincided with each other. 
     On the surface of the concrete block  180  to which the metal plate  182  was bonded, five detection units  101  including detection units  101 - 1  to  101 - 5  were attached. As the detection unit  101 , a vibration sensor similar to that in the first example was used. Note that the detection unit  101 - 3  was attached to the metal plate  182 . Then, to the bonded part between the concrete block  180  and the metal plate  182 , a tensile load is applied in a direction opposite to a mutually bonded direction, whereby a gap  183  was formed. Note that the gap  183  was formed at a portion that was in the bonded part between the metal plate  182  and the concrete block  180  and that was between the detection units  101 - 2  and  101 - 3 . 
     In each of cases before and after the gap  183  was formed, the concrete block  180  was excited by hammering that uses a hammer  190 . In each of the cases, each of the detection units  101 - 1  to  101 - 5  detected a vibration caused by the excitation. 
     Based on information indicating the vibrations detected by the detection units  101 - 1  to  101 - 5 , the assessing device  100  assessed damage in the bonded part between the metal plate  182  and the concrete block  180 . First, the dominant frequency identifying unit  110  acquired a Fourier spectrum by performing the Fourier transform on the information indicating each of the vibrations before and after the gap  183  was formed. Then, the dominant frequency identifying unit  110  identified, as a dominant frequency, a frequency at which an amplitude becomes maximum in each of the Fourier spectra before and after the gap  183  was formed. 
     Similarly to the example 1, according to the information indicating the vibrations detected by the detection units  101 - 1  to  101 - 5  at a time point before the gap  183  was formed, the dominant frequency was common to the detection units  101 - 1  to  101 - 5 . A vibration shape at the dominant frequency was a shape similar to that of the flexural primary vibration mode. 
     According to the information indicating the vibrations detected by the detection units  101 - 1  to  101 - 5  at a time point after the gap  183  was formed, the dominant frequency was common to the detection units  101 - 1  to  101 - 5 . A vibration shape at the dominant frequency was a shape similar to that of the flexural primary vibration mode. 
     Subsequently, assuming that a phase amount is a value of a phase at the dominant frequency in each of the Fourier spectra before and after the above-described gap  183  was formed, the phase difference identifying unit  120  acquired, as a phase difference, each difference between the phase amounts. In the present example, the phase difference identifying unit  120  identified, as the phase difference, the difference between the phase amounts, at the dominant frequency, of the vibrations detected by the detection units  101 - 2  and  101 - 3 . The detection units  101 - 2  and  101 - 3  are two detection units close to a place where the gap  183  was formed. 
     Subsequently, similarly to the example 1, the assessing unit  130  acquired a change rate of the phase differences at the dominant frequencies between the vibrations detected by the detection units  101 - 2  and  101 - 3  before and after the gap  183  was formed, and thereby assessed damage at the joined part. The change rate of the phase differences at the dominant frequencies between the vibrations detected by the detection units  101 - 2  and  101 - 3  were acquired by using the following equation (3). Note that in the equation (3), ΔΦ 2  represents the change rate of the phase differences at the dominant frequencies between the vibrations detected by the detection units  101 - 2  and  101 - 3  before and after the gap  183  was formed. Further, Φ 2before  represents the phase difference at the dominant frequency between the vibrations detected by the detection units  101 - 2  and  101 - 3  before the gap  183  was formed. In addition, Φ 2after  represents the phase difference at the dominant frequency between the vibrations detected by the detection units  101 - 2  and  101 - 3  after the gap  183  was formed. 
     
       
         
           
             
               
                 
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     Note that also in the present example, for comparison, a change rate of the dominant frequencies of the vibrations detected by the detection unit  101 - 2  was acquired by using the equation (4). Note that in the equation (4), ΔF 2  represents the change rate of the dominant frequencies of the vibrations detected by the detection unit  101 - 2  before and after the gap  183  was formed. Further, F 2before  represents the dominant frequency of the vibration detected by the detection unit  101 - 2  before the gap  183  was formed. In addition, F 2after  represents the dominant frequency of the vibration detected by the detection unit  101 - 2  after the gap  183  was formed. Note that in the present example, as described above, the dominant frequencies of the vibrations detected by the detection units  101 - 1  to  101 - 5  are common at each of the time points before and after the gap  183  was formed. 
     
       
         
           
             
               
                 
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       FIG. 8  illustrates the change rate ΔΦ 2  of the phase differences and the change rate ΔF 2  of the dominant frequencies described above. The change rate ΔΦ 2  of the phase differences was −30%. Meanwhile, the change rate ΔF 2  of the dominant frequencies was −5%. In other words, a change rate of ΔΦ 2  to the change rate ΔF 2  was six times. Similarly to the example 1, also in the present example, the change rate of the phase differences at the dominant frequencies was a relatively large change rate, as compared with the change rate of the dominant frequencies. 
     In the present example, it is considered that a rigidity of the test piece constituted by the concrete block  180  and the metal plate  182  decreased due to the formation of the gap  183  at the bonded part between the concrete block  180  and the metal plate  182 . It is considered that as a result of this, the natural frequency of the test piece decreased. Then, it is considered that the dominant frequency corresponding to the natural frequency decreased following the decrease of the natural frequency of the test piece. Further, it is considered that the formation of the gap  183  at a portion between the detection units  101 - 2  and  101 - 3  decreased a mechanical characteristic of the portion that was in the test piece and that was between the detection unit  101 - 2  and the detection unit  101 - 3 . It is considered that as a result of this, the phase difference at the dominant frequency between the vibrations detected by the detection units  102 - 2  and  101 - 3  increased. Then, it is considered that since the gap  183  was local, the above-described change rate of the phase differences at the dominant frequencies became relatively large, as compared with the change rate of the dominant frequencies. 
     Similarly to the example 1, also in the present example, for the local damage at the joined part in the structure, the phase difference at the dominant frequency between the vibrations detected by the two detection units  101  changes remarkably as compared with the dominant frequency. Therefore, it is confirmed that even when assessment is difficult in a method based on a change rate of a dominant frequency, the assessing device  100  or the like can assess generation of damage at a joined part in a structure, by appropriately setting a threshold value used in the assessing unit  130 , or the like. 
     As described above, it is confirmed that the assessing device  100  in the example embodiment of the present invention can assess presence or absence of the local damage in the structure. 
     Further, the change rate of the phase differences in the present example is larger than the change rate of the phase differences in the example 1. In other words, the change rate of the phase differences when local damage is generated at a joined part between constituent members of the structure is larger than the change rate of the phase differences when local damage is generated in one constituent member of the structure. In other words, it is confirmed that the assessing device  100  in the example embodiment of the present invention can be used for assessing the local damage generated at the joined part between the constituent members of the structure. 
     Although the present invention is described above with reference to the example embodiment and the examples, the present invention is not limited to the above-described example embodiment and examples. Various modifications that can be understood by those skilled in the art can be made on a configuration and details of the present invention within the scope of the present invention. Further, configurations in the example embodiments can be combined with each other without departing from the scope of the present invention. 
     This application claims priority based on Japanese Patent Application No. 2016-208663 filed on Oct. 25, 2016, the disclosure of which is incorporated herein by reference in its entirety. 
     REFERENCE SIGNS LIST 
     
         
           10  Assessing system 
           100  Assessing device 
           101  Detection unit 
           110  Dominant frequency identifying unit 
           120  Phase difference identifying unit 
           130  Assessing unit 
           180  Concrete block 
           181  Crack 
           182  Metal plate 
           183  Gap 
           190  Hammer