Abstract:
An inventive approach is disclosed to integrate Digital Image Correlation (DIC) with the Acoustic Emission method that may be used for structural health monitoring and assessment of critical structural components in civil, mechanical, and aerospace industries. The inventive approach relies on passively recording acoustic emission across the specimen being tested and activating the DIC cameras automatically to measure deformation on the specimen&#39;s surface. The resulting acousto-optic system can be used to determine damage initiation, progressive damage development, identify critical regions and make lifetime predictions of the tested specimen.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from PCT application PCT/US2013/37252, filed on Apr. 18, 2013, which claims priority from U.S. Provisional Patent Application Ser. No. 61/635,282, which was filed on Apr. 18, 2012, and which are incorporated herein in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a nondestructive testing and evaluation (NDT &amp; E) system. 
         [0004]    2. Description of the Related Art 
         [0005]    Structural Health Monitoring (SHM) is vital in ensuring the structural integrity of critical components utilized in the aerospace, civil and mechanical industries. The development of SHM has a direct impact on public safety, primarily because it is beneficial in identifying early signs of critical failure and is related to reduced downtime and life extension of aging components and structures. 
         [0006]    NDT &amp; E systems are often a crucial part of SHM applications. Currently, no single monitoring technique has been capable of performing complete structural evaluation due to several inherent challenges including unpredictable environmental and loading conditions, limitations of the techniques themselves, lack of an adequately dense sensing network, etc. 
         [0007]    Structural integrity monitoring systems based on the Digital Image Correlation (DIC) or the Acoustic Emission (AE) methods currently exist, however they are typically implemented independently and they are operated manually when used to assess material and structural integrity. 
         [0008]    Consequently, it would be beneficial to provide an integrated approach in which multiple NDT &amp; E techniques are used to develop an effective SHM system. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    Briefly, the present invention provides a NDT &amp; E method of determining the structural integrity of a specimen or structure as external load (e.g. mechanical, thermal, environmental, etc.) is applied. The method comprises the steps of: attaching at least one acoustic sensor to the specimen; applying a contrasting pattern on the surface of the specimen; calibrating a pair of stereoscopic cameras at the contrasting pattern; passively recording acoustic stress waves propagating in the specimen using an AE system electronically coupled to the at least one acoustic sensor; automatically triggering operation of the cameras by the AE system; measuring deformation in the specimen based on load-induced movement of the contrasting pattern in a DIC system, the DIC system being electronically coupled to the cameras; and correlating acoustic stress waves and strain data to determine the structural health of the specimen. 
         [0010]    The present invention also provides a digital trigger signal (AE output) that is formed based on real time recorded and naturally occurring acoustic emission information, which is parametrized and used to extract features, based on which the signal is formed and subsequently passed into a DIC system. This trigger signal automatically activates the cameras of a DIC system for adaptive image acquisition depending only upon activity recorded nondestructively by the AE method due to changes in material and or structural integrity. 
         [0011]    Additionally, the present invention provides time-synchronization (fusion) of data obtained by the acoustic and optical nondestructive methods for cross-validation and interpretation of information related to material and/or structural integrity. 
         [0012]    Further, the present invention provides the control of a mechanical testing machine, in cases when such a machine is used in conjunction with the combined AE and DIC systems, through a digital signal (DIC output) defined in the DIC system based on full field deformation measurements that are further enabled as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The accompanying drawings, which are incorporated herein and constitute part of this invention, illustrate the presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention. In the drawings: 
           [0014]      FIG. 1  is a schematic drawing of the Acousto-Optic Sensing System (“AOSS”) integrated with a mechanical test machine according to an exemplary embodiment of the present invention; 
           [0015]      FIG. 2A  is an exemplary application of the AOSS for mechanical testing of a laboratory “dog-bone” coupon subjected to uniaxial loading. 
           [0016]      FIG. 2B  is an exemplary application of the AOSS for mechanical testing of a laboratory “compact tension” coupon subjected to uniaxial loading that induces cracking; 
           [0017]      FIG. 2C  is an exemplary application of the AOSS for structural testing of a partially reinforced concrete masonry wall subjected to cyclic lateral loading simulating earthquake excitations; 
           [0018]      FIG. 3A  is an exemplary demonstration of a DIC camera setup in the AOSS for structural testing of a partially reinforced masonry wall subjected to cyclic lateral loading simulating earthquake excitations; 
           [0019]      FIG. 3B  is an exemplary demonstration of the AE sensor setup in the AOSS for structural testing of the partially reinforced masonry wall shown in  FIG. 3A ; 
           [0020]      FIG. 4A  is a graph overlaying stress values obtained by the AOSS with the average full field vertical strain (true strain) calculated by the DIC system and the AE amplitude distribution extracted by real-time recorded AE waveforms of voltage versus time, with the true stress recorded by the AOSS using real time information from the mechanical testing machine; 
           [0021]      FIG. 4B  is a graph overlaying load values obtained by a mechanical test machine with the crack length measured by the DIC system and the cumulative absolute energy computed by the recorded AE as loading is applied to a compact tension coupon similar to  FIG. 3B ; 
           [0022]      FIGS. 5A-C  is an exemplary demonstration of triggering the DIC image acquisition by using real time recorded AE information in the partially reinforced concrete masonry wall application shown in  FIG. 2C  and  FIG. 3 , wherein 
           [0023]      FIG. 5A  shows the specific AE feature (cumulative energy) as a function of time and correlated with load recorded by a mechanical test machine and also read by the AOSS trends data; 
           [0024]      FIG. 5B  shows representative images with full field surface strain distribution showing crack initiation; 
           [0025]      FIG. 5C  shows similar surface strain distribution on the masonry wall that corresponds to the second time instance marked as “Critical Crack Growth” in  FIG. 5A ; 
           [0026]      FIG. 6A  is a graph demonstrating time-synchronization of information collected by the AOSS by using data obtained by the DIC (strain), AE (Partial Power  3 ) and an added infrared thermography system (surface temperature) in an exemplary application in the case of the compact tension coupon test shown in  FIG. 2B ; 
           [0027]      FIG. 6B  is an enlargement of a portion of the graph of  FIG. 6A  marked by the broken line that shows the time synchronization of the three different datasets achieved by the AOSS; 
           [0028]      FIG. 7  is an exemplary application of integrating information (data fusion) obtained by the AOSS for damage diagnosis in the case of the compact tension coupon test shown in  FIG. 2B , with the Mahalanobis distance being defined as a parameter using information from both DIC and AE in the AOSS; 
           [0029]      FIG. 8A  is a graph highlighting the correlation between DIC (residual stiffness) and AE (cumulative energy) data with lifetime fractions of a laboratory coupon similar to the laboratory coupon shown in  FIG. 2A ; and 
           [0030]      FIG. 8B  is another graph highlighting the correlation between DIC (energy density) and AE (cumulative energy) data with lifetime fractions of a laboratory coupon. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    In the drawings, like numerals indicate like elements throughout. Certain terminology is used herein for convenience only and is not to be taken as a limitation on the present invention. The terminology includes the words specifically mentioned, derivatives thereof and words of similar import. The embodiments illustrated below are not intended to be exhaustive or to limit the invention to the precise form disclosed. These embodiments are chosen and described to best explain the principle of the invention and its application and practical use and to enable others skilled in the art to best utilize the invention. 
         [0032]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
         [0033]    As used in this application, the word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the word exemplary is intended to present concepts in a concrete fashion. 
         [0034]    Additionally, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. 
         [0035]    The present invention includes an optical and an acoustic method to adaptively obtain full field deformation measurements on a material&#39;s or structure&#39;s surface based on a signal formed by extracting information from volume-related measurements of naturally occurring acoustic emission. This invention, referred to here as “Acousto-Optical Sensing System (AOSS)”, describes the communication between its constituents and basis of operation using the Digital Image Correlation (DIC) and Acoustic Emission (AE) methods. The inventive system and method features an approach to activate and trigger recordings in cameras related to DIC based on signals formed and exported by the AE method. This approach can be used to time-synchronize full field deformation measurements with emission of acoustic stress waves occurring in the inspected volume of a material and/or structure by natural causes such as evolving damage due to the application of external loadings, such as, for example, mechanical, thermal, environmental, and other types of loadings. 
         [0036]    The present invention also provides a novel approach of integrating in real testing time the DIC with the AE method both at the hardware and at the post processing/analysis levels. The hardware integration between the two techniques is based on user-defined, multi-parametric criteria in the AE system that are used to form a digital output signal. Upon receiving this signal, the DIC cameras are automatically activated and triggered to acquire images based on a user-built script. Consequently, the unique advantage of this hardware integration is an adaptive recording and therefore also memory-storage effective NDE approach that can time-synchronize acoustic with optical information related to material and/or structural failure. 
         [0037]    An exemplary AOSS system  100  according to the present invention is shown in  FIG. 1 . A mechanical test stand  110  is used to apply mechanical load to a test specimen  112 , which simulates an object under load. Such object may be a bridge beam, a building column, an airframe, or other structural element that may be under load by its normal operational environment. The load may be a compressive load, a tensile load, a torsional load, a bending load, environmental load, fatigue load or any other load that may be experienced by structural elements. In this particular embodiment, test stand  110  provides axial tensile/compressive loads to a test specimen  112 . 
         [0038]    Exemplary test specimen  112  is shown in  FIG. 2A . Test specimen  112  is an aluminum alloy designed according to (ASTM) standards, although those skilled in the art will recognize that test specimen  112  can be other materials as well. Test specimen  112   a  in  FIG. 2B  is another example of the same material with a different specimen geometry based on American Society for Testing and Materials (“ASTM”) standards. Test specimen  113  shown in  FIG. 2C  is a partially reinforced concrete masonry wall to demonstrate the applicability of the AOSS in several materials and at different length scales. 
         [0039]    An exemplary AE data acquisition system includes the four-channel DiSP system  114  developed by Physical Acoustics Corporation, schematically shown in  FIG. 1 . AE system  114  is equipped with four piezoelectric transducers  115 - 118  and associated preamplifiers  119 - 122 . While a single transducer  115  may be used, additional transducers  116 - 118  may be used as receivers that passively receive acoustic stress waves propagating through test specimen  112 ,  113  during testing. Further, while piezoelectric transducers are used, other sensors technologies, such as fiber bragg and MEMS sensors can be used. Such sensing technologies are referred to as “acoustic sensors” in the remainder of this text. The system  114  shown in  FIG. 1  has analog inputs that allow load and displacement/strain recordings from the mechanical test stand  110 . 
         [0040]    The exemplary DIC data acquisition system  123  is a GOM ARAMIS 3D 5-megapixel camera system with analog inputs that allow load recording from the mechanical test stand  110 . In addition, the data acquisition of the DIC system  123  also supports input/output ports that can trigger the cameras to activate and record images based on an external trigger. Exemplary 5-megapixel cameras are Baumer TGX15  124 - 125 , shown in  FIG. 3A . 
         [0041]    The AE system  114  is electronically connected to the DIC system using BNC cables and an external parametric box, shown at  101  in  FIG. 1 , so that information in the form of input/output signals between the AE system  114  and the DIC system  123  can be exchanged in real experimental time. An exemplary 
         [0042]    The direct connection between the AE system  14  and the DIC system  123  enables the DIC system  123  to be automatically activated and triggered to acquire images based on a TTL signal generated by the AE system  114 . Additionally, the AE system  114  is connected using BNC cables  102  to test stand  110  and to a load cell  105  using BNC cables  103 . Thus, the AE system  114  is also equipped to receive load/displacement or any other parametric input in real time through a parametric box. Further, the DIC system  123  is connected to the load cell  105  by BNC cables  104 . A closed loop is formed between test stand  110 , DIC system  123 , and AE system  114  so that information (for example, load) recorded from one system can be passed to other systems (for example AE and DIC) and synchronized at both the time and loading stages. 
         [0043]    For DIC measurements, a contrasting speckle pattern must be present on the surface of test specimens  112 - 113 . In this case, a random speckle pattern is applied on the surface of test specimens  112 - 113  for tracking deformation, and pretest images of test specimens  112 - 113  are taken to determine the sensitivity of system  123  for a particular field of view. The random speckle pattern is used to identify the relative displacement of test specimens  112 - 113  by correlating the acquired images to a known reference image under load. 
         [0044]    In an alternative embodiment, such as, for example, determining strain in a bridge beam (not shown), if natural surface contrasts are readily present on the beam, such as, for example, dirt, paint chips, or any other random pattern, then the random speckle pattern does not necessarily need to be applied to the beam. In such a situation, a load is already present in the beam and the piezoelectric transducers  115 - 118  are attached to the beam after the load has been applied to the beam. 
         [0045]    As an exemplary method, true stress (calculated by using data recorded by the load cell of the test stand) versus true strain (calculated by using the DIC system), while AE data (AE waveform amplitude distribution) has been synchronized and added to the true stress-true strain curve in  FIG. 4A . As an additional exemplary method, crack length monitoring (computed by the DIC system) as a function of applied (by the test stand) load and the cumulative absolute energy (computed by the AE system) is shown in  FIG. 4B . The AE system and the features extracted or computed by it were used in the examples shown to activate and record data by the DIC system. 
         [0046]    Additional exemplary correlations between DIC, AE and mechanical test data are shown in  FIG. 5A  in case of cyclic loading of a structural component. The cumulative energy computed by the AE system is synchronized with load information and is used to trigger the cameras of the DIC system that record full field images of surface deformation (in this case strain) as shown in  FIG. 5 . 
         [0047]      FIG. 5A  shows the specific AE feature (cumulative energy as a function of time and correlated with load recorded by the mechanical test machine and also read by the AOSS trends data. Note the two time instances denoted by the two vertical broken lines and labeled as “Crack Initiation” and “Critical Crack Growth”. Changes in the AE features at this time instances were used to trigger the DIC system and record full field surface strain information as shown in  FIGS. 5B and 5C . 
         [0048]      FIG. 5B  shows representative images with full field surface strain distribution showing crack initiation, noted by the box “B” on the bottom left corner of the Figure, which corresponds to the first time instance marked in  FIG. 5A .  FIG. 5C  shows similar surface strain distribution on the masonry wall that corresponds to the second time instance marked as “Critical Crack Growth” in  FIG. 5A . The lines marked “C” correspond to regions on the wall with more pronounced crack formation and grown in a staircase pattern. 
         [0049]    In  FIGS. 6A and 6B , the measured strain by the DIC system after receiving feedback from the AE system is plotted against a calculated AE feature (partial power, which is an AE parameter that can be extracted from digital signal processing) as well as the surface temperature change of the specimen measured by an addition to the exemplary AOSS which in this case also comprises an infrared thermography camera (not shown). 
         [0050]    In  FIG. 7 , a parameter based on what is known as Mahalanobis distance was calculated by using features recorded by both the AE and DIC system, after the DIC system was triggered by the AE system; this parameter is used to detect the extent of damage (in this case length of crack in an experiment similar to the experiment illustrated in in  FIG. 2B ). 
         [0051]    In  FIG. 8  an exemplary application of how the demonstrated AOSS is used to predict the life fraction of a material or structural component is presented by plotting parameters computed by using DIC data (residual stiffness in  FIG. 8A  and energy density in  FIG. 8B ) with the cumulative energy computed based on AE data. Note that DIC data were recorded after triggering of the DIC system using an input signal based on changes of the AE data. 
         [0052]    The advantages of the inventive Acoustic-Optical Sensing system include seamless integration of AE with DIC, in that AE and DIC are now capable of communicating with each other without manual intervention. Consequently, this novel setup provides deformation measurements through the DIC system based on criteria defined in the AE system. Further, the novel system provides the capability to acquire DIC imagery only when identified by the operator as “critical” AE information is recorded, thereby enabling digital memory savings in the DIC system. This aspect could be particularly useful in long-term SHM applications. 
         [0053]    Additionally, the novel AOSS provides the opportunity to integrate full field mechanical parameters such as in- and out-of-plane deformation measurements and material properties including Poisson&#39;s ratio with time, frequency and joint time-frequency AE features such as amplitude, peak frequency, partial powers, and other known parameters. This combination of information enables a cross-validated evaluation of material and/or structural integrity which increases the reliability of the measurements recorded by each of the two systems independently. 
         [0054]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.