Abstract:
Disclosed is an apparatus and methodology for structural health monitoring (SHM) in which smart devices interrogate structural components to predict failure, expedite needed repairs, and thus increase the useful life of those components. Piezoelectric wafer active sensors (PWAS) are applied to or integrated with structural components and various data collected there from provide the ability to detect and locate cracking, corrosion, and disbanding through use of pitch-catch, pulse-echo, electro/mechanical impedance, and phased array technology. Stand alone hardware and an associated software program are provided that allow selection of multiple types of SHM investigations as well as multiple types of data analysis to perform a wholesome investigation of a structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority to U.S. Provisional Application 61/088,047 having a filing date of Aug. 12, 2008, which is incorporated by reference herein. 
    
    
     GOVERNMENT SUPPORT CLAUSE 
     The present invention was developed with funding from NASA-STTR Grant number NGAI 05.0068. Therefore, the government retains certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present subject matter relates to apparatus and methodologies for monitoring structural integrity. More specifically, the present subject matter relates to apparatus and methodologies employing ultrasonic devices and associated analysis software to perform investigations of a structure. 
     BACKGROUND OF THE INVENTION 
     Structural health monitoring (SHM) is a method of determining the health of a structure from the readings of an array of permanently-attached sensors that are embedded into the structure and monitored over time. SHM can be performed in two basic ways, passively and actively. Passive SHM corresponds to monitoring a number of structure related parameters including, but not limited to, loading stress, environment action, performance indicators, acoustic emission from cracks, and inferring the state of structural health from a structural model. In contrast, active SHM performs proactive interrogation of the structure, detects damage, and determines the state of structural health from the evaluation of damage extend and intensity. Both approaches aim at performing a diagnosis or analysis of the structural safety and health, to be followed by a prognosis of the remaining life. Passive SHM uses passive sensors which only “listen” but do not interact with the structure. Therefore, they do not provide direct measurement of the damage presence and intensity. Active SHM uses active sensors that interact with the structure and thus determine the presence or absence of damage. The methods used for active SHM resemble those of nondestructive evaluation (NDE), e.g., ultrasonics, eddy currents, etc., only that they are used with embedded sensors. Hence, active SHM could be seen as a method of embedded NDE. One widely used active SHM method employs piezoelectric wafer active sensors as described, for example, in U.S. Pat. No. 7,024,315, which discloses sending and receiving Lamb waves to determine the presence of cracks, delamination, de-bonding, and corrosion. Due to its similarities to NDE ultrasonics, this approach is also known as embedded ultrasonics. 
     In order to thoroughly investigate the state of health of structural components, different methods are needed. Impedance is useful to detect corrosion, pitch-catch and pulse-echo are used to detect through the thickness damage, and phase-array is used for non-destructive evaluation methods (NDE). After data collection is performed, the data must be analyzed. Each method requires a different approach for data analysis. Different hardware and software are available for data processing, but often the data must be manually transformed to be used in the software. In order to make the SHM process smoother and quicker, both integrated hardware components and integrated software to collect, process, and display the results are needed. 
     A structural component can deteriorate or brake for different reasons. Corrosion on metallic structures reduces the thickness of the component and hence its ability to sustain load. If the thickness goes below a critical value, the structure is no longer able to resist the design load and will eventually brake. Cracks in metallic structure can lead to catastrophic failures due to either fatigue load or static load. Composite structures can fail because of delamination, de-bonding, or because of impact damages. The SHM operator must use different kinds of methods to determine whether a structure is healthy or not. 
     Wave propagation is often used for crack and delamination, but different damage locations in the thickness of the structure requires different wave modes. Electromechanical (E/M) methodologies may be used for corrosion detection and to test the integrity of the sensor-to-structure bonding. Each method requires a different hardware component with a corresponding dedicated software component. After data collection is performed the operator must format the collected data to the specific data analysis software to derive conclusions on the health of the structure. The entire process can be confusing and can easily lead to errors and missing data. Moreover it is time consuming and memory consuming because the data are often duplicated to adapt to the format of the data analysis software. 
     In order to develop any of the SHM technique several sensors are needed. During data collection several piezoelectric wafer active sensors (PWAS) may be interrogated at the same time. Automatic data collection systems that permit an operator to perform data acquisition from the all desired sensors by pressing one button are known. In addition, phase-array data processing systems are known that perform NDE through virtual beam forming. U.S. Pat. No. 6,996,480 B2 illustrates such a system. For the latter method, different PWAS array configurations are needed. The increasing number of PWAS for phase-array method means an increasing number of wiring and electronic cross-talking. 
     While various implementations of structural health monitoring devices and have been developed, and while various methodologies have been proposed to evaluate structural health, no design has emerged that generally encompasses all of the desired characteristics as hereafter presented in accordance with the subject technology. 
     SUMMARY OF THE INVENTION 
     In view of the recognized features encountered in the prior art and addressed by the present subject matter, improved apparatus and methodologies for implementing structural health monitoring (SHM) have been provided. 
     In an exemplary configuration, software is provided for automatic control of associated hardware for providing user selectable operational environments for structural health monitoring. 
     In one of its simpler forms, an integrated software program has been developed to permit simplified selection of various structural health monitoring aspects including ability to detect and locate cracking, corrosion, and disbanding through use of pitch-catch, pulse-echo, electro/mechanical impedance, and phased array technology. 
     Another positive aspect of this type of device is that stand alone hardware and an associated single software package has been provided that permits performance of all needed SHM investigations to assess the health of a structure. 
     In accordance with aspects of certain embodiments of the present subject matter, methodologies are provided to selective evaluate collected structural health data using manually selectable multiple methodologies. 
     In accordance with certain aspects of other embodiments of the present subject matter, methodologies have been developed to analyze collected data using root mean square deviation to produce scalar numbers representing relationships between compared data points. 
     In accordance with yet additional aspects of further embodiments of the present subject matter, apparatus and accompanying methodologies have been developed to permit direct calculation of a damage index. 
     Additional objects and advantages of the present subject matter are set forth in, or will be apparent to, those of ordinary skill in the art from the detailed description herein. Also, it should be further appreciated that modifications and variations to the specifically illustrated, referred and discussed features and elements hereof may be practiced in various embodiments and uses of the invention without departing from the spirit and scope of the subject matter. Variations may include, but are not limited to, substitution of equivalent means, features, or steps for those illustrated, referenced, or discussed, and the functional, operational, or positional reversal of various parts, features, steps, or the like. 
     Still further, it is to be understood that different embodiments, as well as different presently preferred embodiments, of the present subject matter may include various combinations or configurations of presently disclosed features, steps, or elements, or their equivalents (including combinations of features, parts, or steps or configurations thereof not expressly shown in the figures or stated in the detailed description of such figures). Additional embodiments of the present subject matter, not necessarily expressed in the summarized section, may include and incorporate various combinations of aspects of features, components, or steps referenced in the summarized objects above, and/or other features, components, or steps as otherwise discussed in this application. Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the remainder of the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  schematically illustrates an exemplary hardware configuration usable in combination with operational software for implementing the present subject matter; 
         FIG. 2  is a flow chart representation of exemplary software usable with the hardware illustrated in  FIG. 1  to implement the present subject matter; and 
         FIG. 3  is a flow chart representation of an exemplary damage index diagnosis signal processing methodology in accordance with the present subject matter. 
         FIG. 4  illustrates a computer in accordance with the present subject matter. 
     
    
    
     Repeat use of reference characters throughout the present specification and appended drawings is intended to represent same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As discussed in the Summary of the Invention section, the present subject matter is particularly concerned with methods and apparatus for use in conjunction with structural health monitoring and evaluation. 
     Selected combinations of aspects of the disclosed technology correspond to a plurality of different embodiments of the present invention. It should be noted that each of the exemplary embodiments presented and discussed herein should not insinuate limitations of the present subject matter. Features or steps illustrated or described as part of one embodiment may be used in combination with aspects of another embodiment to yield yet further embodiments. Additionally, certain features may be interchanged with similar devices or features not expressly mentioned which perform the same or similar function. 
     Reference will now be made in detail to the presently preferred embodiments of the subject structural health monitoring apparatus and methodology. For a thorough structural health monitoring (SHM) investigation, several hardware components are needed including a function generator, an oscilloscope, an impedance analyzer, and an automatic signal collection unit. Most of these components are available commercially and it is possible to acquire small components directly such that it is possible to integrate all the components as a small, compact device. The automatic signal collection unit has been described in Publication No. WO/2006/017795 corresponding to International Application No. PCT/US2005/028016 published on Feb. 16, 2006 entitled “AUTOMATIC SIGNAL COLLECTION AND ANALYSIS FOR PIEZOELECTRIC WAFER ACTIVE SENSOR” and corresponding to U.S. patent application Ser. No. 11/659,071 filed Jan. 31, 2007 which is incorporated herein for all purposes. 
     Referring now to the drawings,  FIG. 1  illustrates exemplary hardware  100  that may be used in combination with operational software to be described later, for implementing the present subject matter. As illustrated in  FIG. 1 , the hardware component of the present subject matter generally corresponds to an automatic signal collection unit (ASCU)  110  which may receive signals from a number of sensors, as will be described more fully later with respect to  FIG. 3 . These signals are collected as data by ASCU  110  and sent over output lines  112  to oscilloscope  120  for display thereon. An amplifier  116  may be selectively inserted in the lines  112  by way of amplifier switch  114  prior to application to input lines  122  of oscilloscope  120 . In an exemplary configuration, amplifier  116  may be directly integrated on ASCU  110  while a switch (not illustrated) may be used to bypass the amplifier. 
     Switch  132  may be used to activate the impedance analyzer  130  or the function generator  140 . The various hardware components can be connected to a computer through a USB port as described later with respect to  FIG. 3 . Oscilloscope  120  and function generator  140  are connected through a parallel port for triggering of the signal. In exemplary configurations, ASCU  110  may be connected to the sensors through a cable or they may be connected wireless to ASCU  110 . To perform structural health monitoring (SHM) with the hardware described above, the following software program modules, which may be combined into a single software program, may be provided: pitch-catch data collection, pulse-echo data collection, round-robin data collection, impedance data collection, DI analyzer for pitch-catch and pulse-echo data, DI analyzer for impedance data, embedded ultrasonics structural radar (EUSR) for phase-array (round robin) data, and capacitance measurement. 
     Depending on the particular software implementation, in exemplary embodiments of the present subject matter, an initial window may be displayed to illustrate possible selection types with a subsequent window opened showing the selected data collection type. Those of ordinary skill in the art will appreciate that further windows illustrating various selection options as well as other displays including, but not limited to, output displays may be provided again depending on the particular software implementation selected. The remainder of the description of the various exemplary embodiments of the present subject matter will be discussed under the assumption that various windows providing selection and other options are employed. In an exemplary embodiment, software usable with the present subject matter may be developed using LabVIEW®, however such is not a limitation of the present subject matter as fully functional software may be implemented by those of ordinary skill in the art using any suitable programming language. 
     The interface program is divided into five different environments. The first three, Pitch Catch, Phase Array, and Impedance, are for data collection while the second two are for data analysis. An initial window may be provided illustrating each of these environments using, for example, a push button type display where a user may make a selection using, for example, a mouse type pointing device. It is also possible to possible to stop the Shell program through a similarly displayed Stop button. Run and Pause buttons may also be provided. A self-explanatory flow chart  200  is presented in  FIG. 2  showing the general steps involved in the various selectable environments as described herein after. 
     By pressing the Pitch Catch button, a pitch-catch/pulse-echo environment window opens. This window may be configured to display several Tabs offering additional options. 
     In data collection windows, options are provided to select which transducers the user wants to use. It is possible to perform data analysis right after data collection or to continue with data collection with another method or the same as before. All the data and results are saved in a folder specified by the user with the name specified by the user. 
     The first tab of the Pitch Catch environment is a SETUP tab. The set up for this window depends on the kind of specimen and frequency you are using. The tabs that need to be checked and eventually to be changed before doing data collection are: Post Sample; Trigger value; Sample Frequency. Knowing the frequency and the speed of the excited wave, it is possible to select the sample frequency such as to be able to record the desired wave. 
     A second tab in the Pitch Catch environment is a Control Panel tab. By selecting this tab, it is possible to select transmitter sensors and receiver sensors. It is possible to load an external file were a recorded interrogation sequence or it is possible to add the sequence manually. Other options are: selection of folder where the data will be saved and specify the name of the sub folder in a Name tab. The user can set the number of average to compute before saving the data collected. 
     Within the Pitch Catch environment, data collection progress may be visualized by selecting a third tab, a Display Panel tab. The Display Panel may show a number of different graphical displays displaying waveform graphs illustrating, for example, signal amplitude vs. time. When data collection is finished, the program will return to the initial window of the shell program. 
     From the initial window of the shell program, a Round Robin environment may be selected by pressing a Round Robin button where upon a phase array environment opens. Here again, a number of different option tabs may be presented including, for example, SETUP, Control Panel, and Display Panel options. The setup window is the same or similar to that for the Pitch Catch environment. For data processing with the EUSR program, the user should check that post-sample is set to 1. To perform round robin data collection, the user must enter the total number of sensors connected or the last number of the wiring connection. Each of the sensors provided in the configuration may be associated with an indicator light/control on the setup window. If the user does not want to take data from some of the sensors connected, it is possible to deselect them by turning off the indicator light associated with the corresponding sensor. When the program has finished collecting data, it will close the Phase Array window and return to the initial Shell window. The Display Panel window is the same or similar to that for the Pitch Catch environment. 
     From the initial window of the shell program, an Impedance environment may be selected by pressing an Impedance button where upon an impedance environment window opens. In this environment window, the user can specify the total number of sensors available and select or deselect the sensors the user wants to interrogate. It is possible to specify also the total number of points to record in a selected frequency range. Selecting the Start button the program will display the real and imaginary impedance graphs, store the data, and display the PWAS capacitance. After the program has finished it will return to the initial shell window. 
     As previously noted, the initial shell window provides two options for data analysis, Damage Index (DI) for Pitch Catch and Damage Index for Impedance. The DI software is used to assess the severity of the damage in a structure. The damage metric is a scalar quantity that results from the comparative processing of the signal under consideration. The damage metric should reveal the difference between readings (impedance spectrum or wave packets) due to the presence of damage. Ideally the damage index would be a metric which captures only the spectral features that are directly modified by the damage presence, while neglecting the variations due to normal operation conditions (i.e., statistical difference within a population of specimens, and expected changes in temperature, pressure, ambient vibrations, etc.). Several damage metrics may be used to compare impedance spectra or wave packages and assess the presence of damage. In this program, root mean square deviation (RMSD) is used and the mathematical expressions, given in terms of the signal, is as follows: 
               R   ⁢           ⁢   M   ⁢           ⁢   S   ⁢           ⁢   D     =           ∑   N             ⁢       [       Re   ⁡     (     S   i     )       -     Re   ⁡     (     S   i   0     )         ]     2           ∑   N             ⁢       [     Re   ⁡     (     S   i   0     )       ]     2                 
where, the superscript 0 signifies the pristine state of the structure and N is the number of data points. This equation yields a scalar number, which represents the relationship between the compared readings. The advantage is that the data do not need any preprocessing, i.e., the data obtained from the measurement equipment can be directly used to calculate the damage index.
 
     With reference to  FIG. 3 , there is illustrated a flow chart representation of an exemplary damage index diagnosis signal processing methodology in accordance with the present subject matter. Signals from a PWAS array are collected by way of an oscilloscope associated with an automatic signal collection unit (ASCU) and the data are recorded on a computer. 
     Computer  412  may generally include such components as at least one memory/media element or database for storing data and software instructions as well as at least one processor. In the particular example of  FIG. 4 , a processor(s)  422  and associated memory/media elements  424   a ,  424   b  and  424   c  are configured to perform a variety of computer-implemented functions (i.e., software-based data services). At least one memory/media element (e.g., element  424   b  in  FIG. 4 ) is dedicated to storing software and/or firmware in the form of computer-readable and executable instructions that will be implemented by the one or more processor(s)  422 . Other memory/media elements (e.g., memory/media elements  424   a ,  424   c ) are used to store data which will also be accessible by the processor(s)  422  and which will be acted on per the software instructions stored in memory/media element  424   b . The various memory/media elements of  FIG. 4  may be provided as a single or multiple portions of one or more varieties of computer-readable media, such as but not limited to any combination of volatile memory (e.g., random access memory (RAM, such as DRAM, SRAM, etc.) and nonvolatile memory (e.g., ROM, flash, hard drives, magnetic tapes, CD-ROM, DVD-ROM, etc.) or any other memory devices including diskettes, drives, other magnetic-based storage media, optical storage media and others. Although  FIG. 4  shows three separate memory/media elements  424   a ,  424   b  and  424   c , the content dedicated to such devices may actually be stored in one memory/media element or in multiple elements, Any such possible variations and other variations of data storage will be appreciated by one of ordinary skill in the art. 
     In one particular embodiment of the present subject matter, a first portion of memory/media  424   a  is configured to store input data for the subject structural health monitoring system. Input data stored in memory/media element  424   a  may include raw measurement data exported from the ASCU from the PWAS array. Data in memory  424   a  may also include input parameters provided from a user. Although such user-established limits and other input data may be pre-programmed into memory/media element  424   a , they may also be entered as input data from a user accessing an input device  426 , which may correspond to one or more peripheral devices configured to operate as a user interface with computer  412 . Exemplary input devices may include but are not limited to a keyboard, touch-screen monitor, microphone, mouse and the like. 
     Second memory element  424   b  includes computer-executable software instructions that can be read and executed by processor(s)  422  to act on the data stored in memory/media element  424   a  to create new output data (e.g., Damage Index data) for storage in a third memory/media element  424   c . Such output data may be provided to a peripheral output device  428 , such as monitor, printer or other device for visually depicting the output data, or as control signals to still further components. Computing/processing device(s)  422  may be adapted to operate as a special-purpose machine by executing the software instructions rendered in a computer-readable form stored in memory/media element  424   b . When software is used, any suitable programming, scripting, or other type of language or combinations of languages may be used to implement the teachings contained herein. In other embodiments, the methods disclosed herein may alternatively be implemented by hard-wired logic or other circuitry, including, but not limited to application-specific circuits. 
     The program will automatically read all history data for all sensors in the network at various frequency and process with DI algorithm. Denoising is by default in the pre-processing part, but it can be deselected if needed. After that, the DI algorithm will analyze all history data for all PWAS pairs and generate a DI data file stored in the same folder. The program will also provide a diagnosis finding out which PWAS pair is suffering the maximum change (as damaged) and which one is suffering the minimum change (as undamaged). The user can also choose to observe, by way of a DI Display Panel, the DI curve for any single PWAS or PWAS pair in the program. 
     While the present subject matter has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art.