A mechanical metamaterial transducer add-on (metradd) filter for enhancement of linear and nonlinear ultrasonic damage detection

Abstract

The present invention provides a technique for enhancing nonlinear ultrasonic inspections of cylindrical waveguides with a metamaterial add-on filter (101) used along with a longitudinal transducer (106) that has been proposed. It provides an advanced metamaterial add-on filter for transducers with axisymmetric metamaterial baffles that are constant axisymmetric baffles (103) or tapered axisymmetric baffles (104) enclosed in a shell which enhances linear and non-linear ultrasonic damage detection. The enhancement here is achieved by suppression of all unwanted harmonics caused due to the equipment, couplant, etc., it isolates certain guided wave modes in cylindrical waveguides which plays a key role in fluid flow and temperature measurement process. This embodiment relates to the field of linear and nonlinear ultrasonics for the inspection of wires, rods, cylindrical structures, pipelines, etc.

Description

[0001] A MECHANICAL METAMATERIAL TRANSDUCER ADD-ON (METRADD) FILTER FOR ENHANCEMENT OF LINEAR AND NONLINEAR ULTRASONIC DAMAGE DETECTION

[0002] CROSS - REFERENCE TO RELATED PATENT APPLICATION

[0003] The embodiments herein claim the priority of Indian patent application 202341040127 filed on June 12, 2023.

[0004] Field of Invention

[0005] The present invention relates generally to the field of Linear and Nonlinear Ultrasonics for the inspection of wires, rods, cylindrical structures, pipelines, and other similar structures. More specifically, the present disclosure relates to a technique using metamaterial-based mechanical filters which aims to isolate certain guided wave modes in cylindrical waveguides that plays a key role in fluid flow and temperature measurement of processes. The metamaterial-based mechanical filters selectively suppress and enhance the nonlinear ultrasonic (“NLU”) signatures thereby improving the defect detection capability.

[0006] Background of the Invention

[0007] The mechanical integrity of wires, rods, pipelines, and other cylindrical structures is a crucial factor in guaranteeing safety and reliability when they are utilized for storing high-pressure substances or transmitting them. Currently, there exist several inspection techniques aimed at identifying defects in these enclosures. However, these methods often fall short of accurately detecting micro and fatigue cracks or identifying damage in the early stages of the component's structural lifespan.

[0008] The reliability of cylindrical structures is of paramount importance, particularly in industries where the containment of high-pressure materials is involved. Wires, rods, pipelines, and similar structures serve as vital components in numerous applications, ranging from oil and gas pipelines to aerospace engineering. To ensure the safety of these structures, various inspection techniques have been developed. These methods typically involve visual inspection, non-destructive testing, or the use of advanced technologies such as ultrasonic testing or magnetic particle inspection. While these techniques have proven effective in identifying certain types of defects, they often face limitations when it comes to detecting microscopic cracks or damage in the early stages.

[0009] Ultrasonic-guided wave inspection is a non-destructive testing technique that utilizes high-frequency sound waves to inspect cylindrical structures like pipes, tubes, and rails. This method propagates sound waves along the structure, enabling the detection of defects over long distances and offering advantages such as quick and efficient inspection in complex environments. By analyzing guided wave signals, technicians can identify various defects including corrosion, erosion, cracks, and wall thickness variations. Transducers play a vital role in guided wave inspection by converting electrical energy into ultrasonic waves. Acting as emitters and receivers, they analyze reflected or scattered waves caused by structural changes, allowing for comprehensive inspections of inaccessible structures like pipelines, and ensuring integrity and safety.

[0010] Generally, ultrasonic-guided waves are extensively used in Nondestructive Evaluation (“NDE”) and Structural Health Monitoring (“SEIM”) ultrasonic systems for rapid inspection of structures such as plates, pipes, and rods due to the ultrasonic-guided wave’s property of longer range of propagation. As these guided waves are multimodal in nature, it is tricky to interpret each mode and how they interact with the different types of defects in the structures i.e., each guided wave family consists of multiple modes differing in their sensitivity to the position and nature of the defect and thus, it becomes a challenging task in ultrasound to filter out these modes.

[0011] In contrast to linear ultrasonic, nonlinear ultrasonic guided waves (“NLU”) have the advantages of long-range inspection as well as the ability to inspect micro and fatigue cracks. NLU has the potential to detect damage much earlier in the structural life of the component when compared to conventional NDE techniques such as linear ultrasound. However, accurate measurement is hampered by the fact that the higher harmonics due to the material and equipment such as cables and couplant, etc., are difficult to be separated from the main signals and thus, such higher harmonics would lead to false positives, and it is of much interest to minimize those false positives.

[0012] Ideally, the metamaterials are artificial materials that are designed to have specific qualities that are not readily found in natural materials, and the design of the Metamaterial Transducer (“MeTradd”) is done through a study of bandgaps in the metamaterial rod. The term “bandgap” describes a low transmissivity resulting from the structure of the metamaterial.

[0013] Dispersion curves were plotted by varying the geometrical parameters of the ridges, and finally, a configuration that yields bandgaps near the expected frequency is selected for the design of the MeTradd. The metamaterial road is then fabricated with optimized design parameters and this add-on component can be designed for specific applications of harmonic suppression as well as harmonic enhancements by varying the design parameters of the metamaterial baffles.

[0014] Various prior arts have disclosed ultrasonic inspection techniques.

[0015] US patent application US20070125174A1 relates to an ultrasonic inspection system, it discloses a method and system for microstructural evaluation and degradation evaluation of an object by insonifying a structure by ultrasonic energy and receiving energy containing at least one higher harmonic frequency thereof determining a nonlinearity parameter using received energy. The nonlinearity parameter is used to determine a grain size for the subvolume of the object and a variation of the grain size within the object. The nonlinearity parameter is also used to determine fatigue damage or residual stress. The major drawback of this methodology is the inaccuracy of the harmonic measurements arising from instrument nonlinearities and the present invention reduces the instrumentation nonlinearities in the transmission side by using an add-on filter along with a transducer.

[0016] US patent application US20140358489A1 relates to a nonlinearity parameter measuring method and system consisting of two analog bandpass filters and two intermediate frequency amplifiers that are used to process a fundamental wave and a second harmonic wave, respectively. The exact actual nonlinearity parameter can be calculated as the second harmonic wave signal, and it can be differentiated from a noise signal having a similar size. The purpose of this embodiment is to remove any noise included in a second harmonic signal to obtain an exact measurement value of a nonlinearity parameter and evaluate the deterioration degree of the material. The setback in this methodology is that it uses analog filter components to process the output signal to remove unwanted noise components, but this equipment itself can act as a source of equipment nonlinearity and tamper with the exact signal and information. The present invention solves this using a mechanical add-on filter compared to the analog filter described in the above patent application, the MM transducer eliminates the unwanted instrument related to non-linearity and receives more accurate measurements.

[0017] Indian patent application IN201641024212 describes the methods and apparatus for improving the transmission amplitude of ultrasonic guided waves in a cylindrical rod waveguide attached to a rectangular block using comb transduction L (0,3) mode was chosen for the best performance at an elevated temperature of 300° C for 48 hours showing promising results. This approach develops an ultrasonic transducer based on a short waveguide suitable for applications wherein there is a space restriction for installation of the otherwise long waveguide which is conventionally used. However, this way of propagation has other wave modes (L (0,1) and L (0,2)) with lesser amplitudes. The drawback in this embodiment is the presence of other wave modes which can be disruptive and might give inaccurate measurements and patch transduction has its own limitations which might not be suitable for all the conditions that the present invention aims to solve. A filter to remove unwanted wave modes would be the most favorable case. Hence, the present invention solves this by using a mechanical add-on filter with tapered baffles which suppresses the unwanted L (0,1) and L (0,2) wave modes and allows only L (0,3) wave mode to propagate through a cylindrical rode waveguide. Through the approach of the present invention, accurate measurements at elevated temperatures can be received.

[0018] All the existing technologies and the above-mentioned prior art discloses ultrasonic inspection and other methods with the use of multiple components. Nonlinear ultrasonics have the potential to detect damage far sooner than conventional nondestructive evaluation technologies such as linear ultrasounds in a component’s structural life. The structural life of the component can be estimated by closely monitoring the higher harmonics from the material discontinuity. Further, the nonlinearity from instrumentation, transducer, and couplant creates false positives apart from material-based or micro-crack-related nonlinearity, which leads to inaccurate nonlinear measurements. Although signal filtering options are available via digital signal processing methods or via electronic filters, these have several drawbacks. Digital signal processing is limited by the digitization process wherein the unwanted signals will influence the process and reduce the effectiveness. Electronic filters will end up adding more components, thereby further adding another source of equipment nonlinearity.

[0019] For solving the above-mentioned limitations, the present invention uses metamaterials which are artificially engineered materials that exhibit unique properties that are not typically found in nature with various applications including cloaking devices, wave absorbers, seismic protection, sub-wavelength imaging, beamforming etc., The present invention counters all the limitations by suppressing the higher harmonics by selective suppression, second harmonics and enhancing all the harmonics in the cylindrical rod. The present invention is equipped with tapered baffles which can act as a guided wave mode filter to filter out L (0,2) mode and keep only L (0,3) mode which has shown to have a high potential in high-temperature applications.

[0020] Objects of the Invention:

[0021] The main object of the present invention is to provide an advanced metamaterial-based mechanical acoustic add-on filter for the transducer to enhance linear and non-linear ultrasonic damage detection.

[0022] The primary object of the present invention is to enhance the nonlinear ultrasonic inspection of cylindrical waveguides with a metamaterial add-on filter used along with a longitudinal transducer. It is another object of the present invention to isolate certain guided wave modes in cylindrical waveguides that play a key role in the fluid flow and temperature measurement process.

[0023] It is another object of the present invention to use linear and non-linear ultrasonics for the inspection of the structure of the components such as wires, rods, cylindrical structures, pipelines, etc.

[0024] It is yet another object of the present invention to use specifically engineered mechanical metamaterial architecture to achieve enhanced damage detection.

[0025] It is another object of the present invention to use two different types of mechanical metamaterial configurations, the first is using a tapered baffle mechanical metamaterial and the second with a constant height baffle mechanical metamaterial and these configurations help capture incipient defects in a cleaner background.

[0026] It is another object of the present invention to use guided waves for rapid inspection of structures due to their long-range propagation.

[0027] It is yet another object of the present invention to optimize the design parameters of the mechanical metamaterial as per the use case.

[0028] It is another object of the present invention to enable optimization of the design parameters of the mechanical metamaterial, the add-on filter can be fabricated according to the specific application.

[0029] It is another object of the present invention to suppress all the unwanted nonlinearity caused due to equipment by creating an ultrasonic bandgap near the fundamental frequency range by use of the proposed waveguide material. It is another object of the present invention to selectively suppress certain harmonics by creating an ultrasonic bandgap near harmonic frequency range with the proposed waveguide material.

[0030] Summary of the invention

[0031] The present invention is a mechanical metamaterial add-on filter for transducers in the application of linear and nonlinear ultrasonics on a guided medium. It provides an advanced add-on filter for transducers developed out of a mechanical metamaterial for enhancing linear and non-linear ultrasonic damage detection. It’s a novel technique of using metamaterial-based mechanical filters to isolate certain guided wave modes in cylindrical waveguides which plays a key role in fluid flow and temperature measurement processes. This embodiment relates to the field of linear and nonlinear ultrasonics for inspection of wires, rods, cylindrical structures, pipelines etc. A mechanical metamaterial (MM) is a specially engineered architecture that showcases exceptional mechanical characteristics arising from intentionally designed artificial structures, rather than relying solely on its constituent materials.

[0032] Ultrasonic-guided waves are extensively used in non-destructive evaluation (NDE) and structural health monitoring (SHM). Non-Destructive Evaluation (NDE) is employed to assess the characteristics of a structure and detect any imperfections or flaws resulting from welding or discontinuities, all without causing harm to the structure. Structural Health Monitoring (SHM), on the other hand, is utilized to observe and analyze the evolution of a structure over time. This is achieved by periodically collecting response measurements to track alterations in the material and geometric properties of the structures. These guided waves are extensively used for the rapid inspection of structures such as plate pipes, and rods because of their long-range propagation. In contrast to linear ultrasonic, nonlinear ultrasonic (NLU) guided waves have the advantages of long-range inspection as well as the ability to inspect micro and fatigue cracks. NLU has the potential to detect damage much earlier in a component’s structural life compared to NDE techniques such as linear ultrasound. The design of the present invention (MeTradd) is done through a study of bandgaps in the metamaterial (MM) rod. The MM rod is fabricated with optimized design parameters. The MM add-on component can be designed for specific applications of harmonic suppression as well as harmonic enhancement by varying the design parameters of the metamaterial baffles. Two types of mechanical metamaterial configurations using a tapered and another with constant height baffle features are proposed. The first one is a tapered baffle mechanical metamaterial that is capable of selective suppression of certain mode L (0,2) along with enhancement of another mode L (0,3). The second one is a constant baffle mechanical metamaterial for mode suppression L (0,2) as well as enhancement of nonlinear ultrasonic signatures for non-destructive evaluation thereby capturing incipient defects in a cleaner background.

[0033] In the tapered mechanical metamaterial, the tapered baffles placed on the rod slow down the wave and convert the L (0,2) mode to L (0,3) mode. In the constant baffle mechanical metamaterial’s case, selective filtering and enhancing the harmonic components are achieved which consists of an array of flat axisymmetric ridges arranged periodically on the surface of the rod designed to suppress and enhance specific frequency components. The suppression modes L (0,2) and harmonics are due to the band gap occurring in that region while enhancement is due to local resonance.

[0034] Other features and advantages of embodiments will be apparent from the accompanying drawings and from the detailed description that follows.

[0035] Brief Description of the Drawings

[0036] Figure 1 illustrates (a) Metamaterial Transducer Add-On (MeTradd) filter placed along a cylindrical rod and MeTradd with (b) constant baffles and (c) tapered baffles represented in an encapsulated shell which is transparent for demonstration purpose.

[0037] Figure 2 illustrates Schematic of the monotonous input signal to a cylindrical structure and the multi-frequency output received. Figure 3(a) illustrates Schematic showing the suppression of all the higher harmonics.

[0038] Figure 3(b) illustrates Schematic showing the suppression of fundamental harmonics.

[0039] Figure 3(c) illustrates Schematic showing the selective suppression of second harmonics.

[0040] Figure 3(d) illustrates Schematic showing the enhancement of all the higher harmonics.

[0041] Figure 4 (a) represents schematic illustrating the axisymmetric of finite element model used to generate dispersion with (b) and (c) showing the blow-up of a unit cell and its corresponding internal mesh.

[0042] Figure 5 illustrates dispersion Curves for a normal cylindrical rod waveguide and ridged rod with optimal geometrical parameters showing band gap from 480 kHz to 550 kHz.

[0043] Figure 6 illustrates an experimental setup showing the MM rod clamped between the transducers with the help of spring loaded mechanism.

[0044] Figure 7 shows the experimentally obtained frequency responses for (a) the plane rod, (b) the rod with MM as add-on filter and (c) the joined MM (JMM) filter rod assembly with a discontinuity - Frequency spectrum of the transmitted wave obtained from experiments for without MM (black), with MM (red), and with Joined MM (blue).

[0045] Figure 8 illustrates (a) Numerical and (b) experimental demonstration of selective suppression of second harmonics by using a metamaterial with band gap at 500 kHz and exciting at 250 kHz. Figure 9 illustrates (a) Numerical and (b) experimental demonstration of selective suppression of fundamental frequency by using a metamaterial with band gap at 500 kHz and exciting at 500 kHz.

[0046] Figure 10 illustrates enhancement of all the higher harmonics by the usage of metamaterial shown by comparison of the normalized frequency spectrum of experimental results.

[0047] Figure 11 illustrates schematic showing the use of proposed Metradd filter with tapered baffles illustrating that the selective generation of enhanced L(0,3) mode in the rod leads to the effective generation of bulk longitudinal waves in a target sample at a higher temperature.

[0048] Figure 12 illustrates Frequency-wavenumber dispersion plot of bare rod of 10mm diameter overlaid on 2D FFT plot (a) without Tapered Baffles, (b) with Tapered baffles, clearly showing the up-conversion of L(0,2) mode to L(0,3) mode at 0.5 MHz.

[0049] Figure 13 illustrates Time spectrogram plot obtained from simulation (a) with Tapered baffles, (b) without Tapered baffles.

[0050] Figure 14 illustrates Time spectrogram plot obtained from experiment (a) with Tapered baffles, (b) without Tapered baffles.

[0051] While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention.

[0052] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms, and the inventor should appropriately interpret the concept of the term appropriately to describe its own invention in the best way. The present invention should be construed as meaning and concept consistent with the technical idea of the present invention based on the principle that it can be defined. Therefore, the embodiments described in this specification and the configurations shown in the drawings are only the most preferred embodiments of the present invention and do not represent all the technical ideas of the present invention. Therefore, it should be understood that equivalents and modifications are possible.

[0053] Detailed Description of the Invention with Respect to the Drawings

[0054] The present invention as embodied by "A Mechanical Metamaterial Transducer Add-On (MeTradd) Filter for enhancement of Linear and Nonlinear Ultrasonic Damage Detection" succinctly fulfils the above-mentioned need(s) in the art. The present invention has objective(s) arising as a result of the above-mentioned need(s), said objective(s) being enumerated below. In as much as the objective(s) of the present invention are enumerated, it will be obvious to a person skilled in the art that, the enumerated objective(s) are not exhaustive of the present invention in its entirety, and are enclosed solely for the purpose of illustration. Further, the present invention encloses within its scope and purview, any structural alternative(s) and / or any functional equivalent(s) even though, such structural alternative(s) and / or any functional equivalent(s) are not mentioned explicitly herein or elsewhere, in the present disclosure. The present invention therefore encompasses also, any improvisation(s) / modification(s) applied to the structural alternative(s) / functional alternative(s) within its scope and purview. The present invention may be embodied in other specific form(s) without departing from the essential attributes thereof.

[0055] Throughout this specification, the use of the word "comprise" and variations such as "comprises" and "comprising" may imply the inclusion of an element or elements not specifically recited.

[0056] The present invention provides a novel Mechanical Metamaterial Transducer Add-On (MeTradd) Filter (101) for enhancement of Linear and Nonlinear Ultrasonic (NLU) Damage Detection. The proposed MeTradd can be specifically used for two different applications for linear and nonlinear ultrasonic guided waves as mentioned below.

[0057] NLU has the potential to detect damage far sooner than conventional NDE technologies such as linear ultrasounds in a component's structural life. The structural life of the component can be estimated by closely monitoring the higher harmonics generated from the material discontinuity. However, as shown in Figure 2 nonlinearity from instrumentation, transducer, and couplant creates false positives apart from material based or micro-crack related nonlinearity, which leads to inaccurate nonlinear measurements. Although signal filtering options such as Butterworth, Chebyshev, etc., are available via digital signal processing methods, or via analog electronic filters, these have several drawbacks. Digital signal processing is limited by the digitization process wherein the unwanted signals will influence the process and reduce the effectiveness. Electronic filters will end up adding more components, thereby further adding another source of equipment nonlinearity.

[0058] A possible way to address this problem is by using metamaterials. Metamaterials are artificially engineered materials that exhibit unique properties that are not typically found in nature with various applications including cloaking devices, wave absorbers, seismic protection, subwavelength imaging, beamforming etc. The present invention relates to the suppression of all the higher harmonics, selective suppression of fundamental and second harmonics and enhancement of all harmonics on a cylindrical rod.

[0059] In the preferred embodiment of the present invention, a Metamaterial based mechanical acoustic add-on filter (101) designed for suppression and enhancement of higher harmonic components to improve non-linear ultrasonic detection and also the up-conversion of L(0,2) to L(0,3) mode is proposed.

[0060] In the preferred embodiment of the present invention, wherein a cylindrical rod waveguide (102) is provided, and a Mechanical Metamaterial Transducer Add-On (MeTradd) Filter (101) placed along the cylindrical rod (102) wherein the waveguide metamaterial consists of an array of flat axisymmetric baffles (103) arranged periodically on the surface of the rod (102) for harmonic filtering of signals.

[0061] In the preferred embodiment of the present invention, wherein the waveguide metamaterial consists of an array of tapered axisymmetric baffles (104) arranged periodically on the surface of the rod for mode up-conversion.

[0062] The proposed waveguide metamaterial can suppress all the unwanted nonlinearity caused due to equipment, couplant etc., by creating an ultrasonic bandgap near fundamental frequency range. The waveguide metamaterial can selectively suppress certain harmonics by creating by creating an ultrasonic bandgap near harmonic frequency range.

[0063] The proposed waveguide metamaterial can enhance all the harmonics and can detect defects like discontinuity in the cylindrical waveguide from the nonlinear responses.

[0064] In another embodiment of the present invention, wherein the geometry or shape of the baffles in the waveguide metamaterial rod can be rectangular or cylindrical.

[0065] In the preferred embodiment of the present invention, wherein the system for generating and directing ultrasonic waves for inspections of specimens using MM add-on filter comprises of a custom-made holder setup to attach transducer and add-on assembly to the workpiece for proper ultrasonic wave propagation.

[0066] In the preferred embodiment of the present invention, wherein the system further comprises of a pulser receiver device to generate the electric pulses for the transducer. An ultrasound pulser receiver device is used for receiving the ultrasonic waves and an oscilloscope is used to display the signals. The signals are transmitted to the computer which processes the signals. Figure 2 shows the schematic of the monotonous input signal to a cylindrical structure and the multi-frequency output received - Factors that influence the output received including the sources of unwanted (red) and required (green) nonlinearities.

[0067] 4 different cases are considered to demonstrate the effective usage of the MM add-on filter component for various NLU applications as shown in Table 1. Case 1 as shown in Figure 3(a) shows the suppression of all harmonics by slightly placing band gap nearer to fundamental frequency and thus suppressing all higher harmonics. Case 2 as shown in Figure 3(b) shows the complete suppression of fundamental frequency by overlapping the band gap over the entire fundamental frequency range. Case 3 as shown in Figure 3(c) shows the selective suppression of second harmonic frequency components by having stop band in the second harmonic frequency range. Case 4 as shown in Figure 3(d) shows the complete enhancement of all the frequency components by designing MM.

[0068] For each case the MM filter is designed to have bandgap in specific frequency ranges. The parameters for the baffles are finalized by performing band gap study of a unit cell by varying the dimensional parameters height (h), radius (r), thickness (t), and unit cell length (L) as shown in Figure 4. The values of different parameters chosen for different cases are tabulated in Table 2.

[0069] Table 1 Different cases of application of MM add-on filter in NLU measurements

[0070] Table 2 Baffle parameter for the MM design for selected cases

[0071] Figures 3(a), (b), (c) & (d) represent Schematic showing the suppression of all the higher harmonics, suppression of fundamental harmonics, selective suppression of second harmonics and enhancement of all the higher harmonics.

[0072] Figure 4 shows (a) Schematic illustrating the axisymmetric of finite element model used to generate dispersion with (b) and (c) showing the blow-up of a unit cell and its corresponding internal mesh. The dispersion curve is obtained by solving an eigen-frequency problem on a unit cell applying the periodic boundary conditions at the interfaces between the adjacent units according to the Bloch-Floquet theorem using Solid mechanics module in COMSOL. Suppression of higher harmonics is due to the Bragg scattering phenomenon occurring in the waveguide metamaterial rod. The baffles placed on the surface of the rod strongly reflect the higher harmonics. The dispersion curve obtained for the case 1 indicating band gap from 490 kHz to 550 kHz is as shown in Figure 5. Figure 5 shows Dispersion Curves for a normal cylindrical rod waveguide and ridged rod with optimal geometrical parameters showing band gap from 480 kHz to 550 kHz.

[0073] Figure 6 illustrates an experimental setup showing the MM rod clamped between the transducers with the help of spring loaded mechanism. Figure 7 shows the experimentally obtained frequency responses for (a) the plane rod, (b) the rod with MM as add-on filter and (c) the joined MM (JMM) filter rod assembly with a discontinuity. It has been demonstrated that the complete suppression of all the harmonics has been achieved. The presence of discontinuity identified as a higher harmonic component can be distinctively identified. Figure 7 shows Frequency spectrum of the transmitted wave obtained from experiments for without MM (black), with MM (red), and with Joined MM (blue).

[0074] Figure 8 shows the numerical and experimental results for the selective suppression of second harmonic components. Here, the excitation frequency was 250 kHz and the second harmonics component arising at 500 kHz has been suppressed by the MM due to the band gap created at that range. Thus, the second harmonics suppression was achieved in simulation as well as in experiments. Similarly, the fundamental frequency suppression was demonstrated using the same MM filter. Figure 9 depicts the numerical and experimental results showing same respectively.

[0075] The last case shown in Figure 10 demonstrates a complete enhancement of all the harmonics with the MM add-on filter whose parameters as specified in Table 2.

[0076] Ultrasonics in guided medium plays a vital role in NDE and SHM of critical structures. Because of multimodal nature, careful investigation is required to identify and interpret the guided wave mode signals accurately. However, a metamaterial -based MeTradd filter is proposed with tapered baffles for the up-conversion of longitudinal modes within the family of guided ultrasonic rod waves (from L(0,2) to L(0,3)), which is of interest to certain waveguide transducer applications, as illustrated in Figure 11, to selectively enhance the L(0,3) mode in circular rod waveguides.

[0077] Figure 11 - Schematic shows the use of proposed Metradd filter with tapered baffles illustrating that the selective generation of enhanced L(0,3) mode in the rod leads to the effective generation of bulk longitudinal waves in a target sample at a higher temperature. The mode up-conversion is demonstrated using numerical simulations and experiments as discussed below. Table 3 below shows the baffle parameters considered for this purpose.

[0078] Table 3 provides Baffle parameters for different cases of application of MM add-on filter in Linear Ultrasonics

[0079] Numerical simulations were used in order to understand wave propagation in the MeTradd. Two 3-D FE models were prepared using a commercially available FE package Abaqus. One model was with tapered baffles and the other one was without any baffles. An excitation was given at one end with central frequency at 0.5 MHz, and the displacements were monitored at the other end of the rod. The time traces (A-scans) obtained from these simulations were further investigated using several frequency-time analysis techniques such as 2D FFT (Fast Fourier transform) and STFT (Short-time Fourier transformation). The corresponding plots for with and without tapered baffles are provided in the Figure 12. It can be easily noticed that the E(0,2) mode is missing from the tapered baffled model which justifies our findings. The same observations are verified with STFT plots as well for the numerical simulations as well as via experiments as shown in Figures 13 and 14 respectively. Dispersion curves are overlaid to give clarity in understanding wave modes.

[0080] The MeTradd with tapered baffles can act as a guided wave mode filter to filter out E(0,2) mode and only keeps E(0,3) mode which have shown to have high potential for high temperature applications.

[0081] Although the proposed concept has been described as a way of example with reference to various models, it is not limited to the disclosed embodiment and that alternative designs could be constructed without deviating from the scope of invention as defined above.

[0082] It will be apparent to a person skilled in the art that the above description is for illustrative purposes only and should not be considered as limiting. Various modifications, additions, alterations, and improvements without deviating from the scope of the invention may be made by a person skilled in the art.

Claims

We Claim,1. A Mechanical Metamaterial Add-On (MeTradd) Filter (101) for transducers (106) for enhancement of Linear and Nonlinear Ultrasonic Damage Detection, comprising of: a cylindrical rod waveguide (102), characterized by: a Mechanical Metamaterial Transducer Add-On (MeTradd) Filter (101) placed along the cylindrical rod (102) with an array of axisymmetric metamaterial baffles (103, 104) arranged periodically on the surface of the rod (102) for harmonic filtering of signals, wherein the axisymmetric metamaterial baffles are constant height axisymmetric baffles (103) or tapered height axisymmetric baffles (104) enclosed in a shell (105).

2. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter as claimed in claim 1, wherein an array of flat constant height axisymmetric baffles (103) is arranged periodically on the surface of the rod for selective and harmonic filtering of signals for enhancement of NLU (Non-Linear Ultrasonic) signatures for non-destructive evaluation (NDE).

3. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter as claimed in claim 1, wherein an array of tapered axisymmetric baffles (104) is arranged periodically on the surface of the rod for mode up-conversion which enables selective suppression of certain mode L(0,2) along with enhancement of another mode L(0,3) for high-temperature NDE applications.

4. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter as claimed in claim 1 , wherein the geometry or shape of the baffles in the waveguide metamaterial rod are cylindrical.

5. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter as claimed in claim 1, wherein the waveguide metamaterial (101) suppresses all the unwanted nonlinearity by creating an ultrasonic bandgap near the fundamental frequency range, wherein all unwanted nonlinear higher harmonic components from the transmission side of the NDE ultrasonic pitch-catch configuration are removed with the help of MeTradd with constant baffles (102).

6. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter as claimed in claim 1 , wherein the waveguide metamaterial (101, 103) selectively suppresses the secondharmonics by varying the dimensional parameters of the baffles such as height (h), radius (r), thickness (t) and unit cell length (L).

7. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter as claimed in claim 1, wherein the waveguide metamaterial (101, 103) enhances all the harmonics by varying the dimensional parameters of the baffles such as height (h), radius (r), thickness (t) and unit cell length (L).

8. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter as claimed in claim 1, wherein a system for generating and directing ultrasonic waves for inspections of specimens using the add-on filter comprising of:A holder set-up to attach the transducer and the add-on filter assembly to the workpiece for proper ultrasonic wave propagation;A device to generate the electric pulses for the transducer;An ultrasound receiver for receiving the ultrasonic waves;An instrument to display the signals;A computer to process the signal.

9. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter of claim 8, wherein the device to generate electrical pulses into the transducer is a pulser receiver.

10. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter of claim 8, wherein the device to receive the ultrasonic waves is a pulser receiver.

11. The mechanical Metamaterial Transducer Add-On (MeTradd) Filter of claim 8, wherein the instrument to display the signals is an oscilloscope.

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