CN118765540A - Ultrasonic transducer for structural health monitoring, method for producing an ultrasonic transducer for structural health monitoring, and method for structural health monitoring - Google Patents

Abstract

An ultrasound transducer (10, 12) for structural health monitoring, a method (200) for producing an ultrasound transducer (10, 12) for structural health monitoring and a method (300) for structural health monitoring are provided. An ultrasonic transducer (10, 12) includes a piezoelectric film (14), a first electrode (16) located on a first surface of the piezoelectric film (14), and a second electrode located on a second surface of the piezoelectric film (14). The piezoelectric film (14) includes polylactic acid having aligned molecular chain orientations.

Description

Ultrasonic transducer for structural health monitoring, method for producing an ultrasonic transducer for structural health monitoring, and method for structural health monitoring

Technical Field

The present invention relates generally to structural health monitoring (Structural Health Monitoring, SHM), and more particularly to an ultrasound transducer for structural health monitoring, a method for producing an ultrasound transducer for structural health monitoring, and a method for structural health monitoring.

Background

Structural Health Monitoring (SHM) is an emerging technology that directly evaluates structural integrity through the use of sensors integrated into the structure. The use of an ultrasonic guided wave SHM is advantageous for inspecting large structures with a small number of ultrasonic transducers.

However, conventional shear mode piezoelectric transducers are typically made of bulky and brittle piezoelectric ceramics. Piezoceramics require complex processing (cutting along a specific crystal direction and electrically polarizing with high voltage) to fabricate a shear mode transducer.

In view of the above, it may be desirable to provide a low cost ultrasound transducer for structural health monitoring and a simple method for producing such an ultrasound transducer.

Disclosure of Invention

Accordingly, in a first aspect, the present invention provides an ultrasound transducer for structural health monitoring. The ultrasonic transducer includes a piezoelectric film, a first electrode located on a first surface of the piezoelectric film, and a second electrode located on a second surface of the piezoelectric film. The piezoelectric film includes polylactic acid having aligned molecular chain orientations.

In a second aspect, the present invention provides a method for producing an ultrasound transducer for structural health monitoring. The method comprises the following steps: dissolving polylactic acid in a solvent to form a polylactic acid solution; forming a piezoelectric film from the polylactic acid solution, the polylactic acid in the piezoelectric film having an aligned molecular chain orientation; forming a first electrode on a first surface of the piezoelectric film; and forming a second electrode on the second surface of the piezoelectric film.

In a third aspect, the present invention provides a method for structural health monitoring. The method comprises the following steps: mechanically coupling at least one ultrasound transducer according to the first aspect to a structure; generating a guided shear wave propagating through the structure; detecting a guided shear wave; and comparing the detected ultrasonic signal of the guided shear wave with a baseline signal of the structure under reference conditions to determine the presence of a defect in the structure.

Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a first ultrasound transducer and a second ultrasound transducer for structural health monitoring according to an embodiment of the present invention;

FIG. 2 is a schematic flow chart diagram illustrating a method for producing an ultrasound transducer for structural health monitoring according to an embodiment of the invention;

FIG. 3 is a schematic flow chart diagram illustrating a method for structural health monitoring according to an embodiment of the present invention;

FIG. 4A is a photograph of a poly (L-lactic acid) (PLLA) film prepared by mechanical stretching;

FIG. 4B is an X-ray diffraction (XRD) pattern of a PLLA film without mechanical stretching and a PLLA film stretched at a stretch ratio of 5;

FIG. 4C is a schematic diagram showing an experimental test apparatus for evaluating the use of stretched PLLA films to fabricate an ultrasonic transducer according to an embodiment of the present invention;

FIG. 4D is a dispersion curve of a 1.6mm thick aluminum plate used in the experimental test apparatus of FIG. 4C under different ultrasonic modes;

FIG. 4E is a time-frequency two-dimensional (2D) plot of the ultrasonic signal detected by the ultrasonic transducer of FIG. 4C, which overlaps the dispersion curve of the aluminum plate of FIG. 4D;

fig. 4F is a graph showing changes in ultrasonic signals (SH 0 mode shear horizontal waves) observed before and after different sizes of defects are generated in the aluminum plate of fig. 4C;

FIGS. 5A and 5B are Scanning Electron Microscope (SEM) images of PLLA films in the form of fiber mats at different magnifications;

FIG. 5C is a schematic diagram illustrating the generation or detection of shear horizontal waves using an ultrasonic transducer made of PLLA film in the form of a fibrous mat, according to an embodiment of the present invention;

FIG. 5D is a graph of an ultrasonic signal in the case of using an ultrasonic transducer made of the PLLA film of FIG. 5C to detect shear horizontal waves;

FIG. 5E is a graph of an ultrasound signal in the case of using an ultrasound transducer made of the PLLA film of FIG. 5C to generate shear horizontal waves;

FIG. 5F is a graph comparing signals from the ultrasonic transducer of FIG. 5C as a receiver for shear horizontal waves when the aluminum plate of FIG. 5C mechanically coupled to the ultrasonic transducer is in air and immersed in water;

FIG. 5G is a graph comparing signals obtained with an ultrasonic transducer made of a piezoelectric poly (vinylidene fluoride) (PVDF) film and bonded to an aluminum plate in air and immersed in water;

FIGS. 5H-5K are graphs of ultrasonic signals obtained by an ultrasonic transducer made of PLLA films in the form of fiber mats detecting shear horizontal waves in an aluminum plate with defects of different sizes when the structure is in air and water;

fig. 5L is a graph comparing DI values calculated based on ultrasonic signals obtained when the structure is in air and in water;

FIG. 5M is a schematic diagram showing PLLA films in the form of a fibrous mat bonded to an aluminum plate and square silver electrodes deposited on the fibrous mat to form an ultrasonic transducer according to an embodiment of the present invention;

FIG. 6A is a schematic top plan view of a method of manufacturing PLLA films in the form of fiber mats by centrifugal electrospinning using a rotating collector;

FIG. 6B is a schematic top plan view of an ultrasound transducer made of the polylactic acid film of FIG. 6A for generating and detecting omni-directional guided shear waves in a structure, according to an embodiment of the present invention; and

Fig. 7 is a schematic diagram illustrating a method of monitoring the integrity of a tubular structure by generating and/or detecting torsional waves in the tubular structure using an ultrasonic transducer made of PLLA membrane and electrodes, according to an embodiment of the present invention.

Detailed Description

The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention and is not intended to represent the only forms in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the scope of the invention.

The term "piezoelectric film" as used herein refers to a thin layer of a substance capable of generating electricity when subjected to mechanical stress.

The term "aligned molecular chain orientation" as used herein refers to an arrangement of linear polymer molecules such that the chains of the molecules are aligned along a common direction or plane to obtain anisotropic properties that vary depending on the direction of measurement.

The term "shear wave" as used herein refers to a transverse wave that occurs in an elastic medium when the elastic medium is subjected to a periodic shear strain created by a force. Thus, the term "guided shear wave" as used herein refers to a shear wave that propagates along a structure while being guided by the boundaries of the structure, wherein the particle motion direction is parallel to the surface of the structure and perpendicular to the propagation direction of the shear wave.

The term "stretch" as used herein refers to making a material or substance longer or wider by pulling. Thus, the term "mechanically stretched" as used herein refers to a process in which a material is subjected to a tensile force, causing the material to elongate and align the molecular chain orientation of the material along the direction of the applied force.

The term "stretch ratio" as used herein refers to an increase in dimension along the stretch direction. For example, a stretch ratio of two (2) refers to a length increase of twice the original length, and a stretch ratio of five (5) means a length increase of five (5) times the original length.

The term "coupled" as used herein refers to the physical attachment of one object to another object. Thus, the term "mechanically coupled" as used herein refers to bonding a material to a structure by adhesion and applying a mechanical force through strain at the interface between the material and the structure.

The term "about" as used herein refers to two numbers within a range of numbers and is also used to indicate that a value includes the standard deviation of the error of the device or method used to determine the value. The term "about" as used herein may allow for some degree of variability in a value or range, for example, within 10%, within 5% or within 1% of a specified value or specified range limit.

Referring now to fig. 1, a first ultrasound transducer 10 and a second ultrasound transducer 12 for structural health monitoring are shown. Each of the first ultrasonic transducer 10 and the second ultrasonic transducer 12 includes a piezoelectric film 14, a first electrode 16 located on a first surface of the piezoelectric film 14, and a second electrode (not shown) located on a second surface of the piezoelectric film 14. The piezoelectric film 14 includes polylactic acid having aligned molecular chain orientations.

An isomer of polylactic acid may be used to form the piezoelectric film 14 of the first ultrasonic transducer 10 and the second ultrasonic transducer 12. Accordingly, the polylactic acid may be selected from the group consisting of poly (L-lactic acid) (PLLA) and poly (D-lactic acid) (PDLA).

The piezoelectric film 14 comprises polylactic acid having aligned molecular chain orientations. The aligned molecular chain orientation of the polylactic acid may be parallel to the propagation direction of the guided shear wave 18 to be generated or detected, or perpendicular to the propagation direction of the guided shear wave 18 to be generated or detected. In one or more alternative embodiments, the piezoelectric film 14 may be circular in shape, and the aligned molecular chain orientation of the polylactic acid may be along the circumferential direction of the piezoelectric film 14. When the piezoelectric film 14 including polylactic acid is in a circular shape and molecular chains are oriented along the circumferential direction of the piezoelectric film 14, the ultrasonic transducer can generate or detect an omnidirectional shear horizontal wave.

The second electrode is disposed on the second surface of the piezoelectric film 14 at a side of the piezoelectric film 14 opposite the first electrode 16. At least one of the first electrode 16 and the second electrode may be a layer having a comb-shaped pattern. In the illustrated embodiment, the first electrode 16 is comb-shaped. The fingers or teeth 20 of the comb electrode layer 16 may have a periodicity corresponding to the wavelength of the guided shear ultrasonic waves 18. The material of the first electrode 16 and the second electrode may be silver, carbon or gold. In one or more alternative embodiments, the first electrode 16 and/or the second electrode may take other shapes, such as a circle or square.

Having described the elements of the first and second ultrasound transducers 10, 12, a method for producing the first and second ultrasound transducers 10, 12 for structural health monitoring will now be described below with reference to fig. 2.

Referring now to fig. 2, a method 200 for producing an ultrasound transducer for structural health monitoring is shown. The method 200 begins at step 202 with dissolving polylactic acid in a solvent to form a polylactic acid solution. The solvent may be chloroform, tetrahydrofuran or dioxane, and the polylactic acid solution may be prepared by dissolving polylactic acid in a solvent.

At step 204, a piezoelectric film is formed from the polylactic acid solution, the polylactic acid in the piezoelectric film having aligned molecular chain orientations. The polylactic acid film having piezoelectricity may be manufactured by mechanically stretching a polylactic acid film made of a chemical solution. Alternatively, the polylactic acid film having piezoelectricity may be made into a fiber mat form by electrospinning a polylactic acid solution using a rotary spinneret or a rotary collector.

More specifically, in the mechanical stretching embodiment, the polylactic acid film may be made of a polylactic acid solution by casting, spin coating, or spray coating. In such an embodiment, the step 204 of forming a piezoelectric film from the polylactic acid solution may include: the piezoelectric film is mechanically stretched to a predetermined multiple of the original length of the piezoelectric film. The predetermined multiple of the original length of the piezoelectric film may be between about 2 times and about 7 times. The piezoelectric film may be mechanically stretched at a temperature between about 120 degrees celsius (°c) to about 155 ℃. In one or more embodiments, the piezoelectric film may be mechanically stretched at a temperature between about 130 ℃ and about 155 ℃. The polylactic acid film may be mechanically stretched to several times (preferably 2 to 6 times) the original length of the polylactic acid film at a high temperature of 120 ℃ or more (preferably between about 130 ℃ and about 155 ℃), with aligned molecular chain orientations.

In an electrospinning embodiment, the step 204 of forming a piezoelectric film from a polylactic acid solution may include: applying an electric field to a tip of a spinneret comprising a polylactic acid solution; spraying polylactic acid solution from the spinneret, the sprayed polylactic acid solution passing through an electric field; and depositing the sprayed polylactic acid solution on a collector to form polylactic acid fibers. More specifically, the polylactic acid fiber may be manufactured by applying a strong electric field to the tip of a spinneret containing a polylactic acid solution. A continuous fine jet of polylactic acid solution may be ejected from a spinneret and moved through an electric field to deposit on a collector to form polylactic acid fibers. In such an embodiment, the step 204 of forming a piezoelectric film from the polylactic acid solution may further include: the polylactic acid fibers are spun using a spinneret or collector, which is capable of rotating, to form a polylactic acid fiber mat. More specifically, the polylactic acid film having aligned molecular chain orientation in the form of a fiber mat may be formed by spinning the polylactic acid fiber using a rotary spinneret or collecting the fiber using a rotary collector, and then annealing at a high temperature of 120 ℃ for 1 hour.

Accordingly, a piezoelectric film including polylactic acid with aligned molecular chains can be produced by mechanically stretching the polylactic acid film at a high temperature. Alternatively, the piezoelectric film comprising polylactic acid with molecular chain alignment may be in the form of a polylactic acid fiber mat by fabrication of electrospinning (in which a spinneret or collector rotates).

At step 206, a first electrode is formed on a first surface of the piezoelectric film, and at step 208, a second electrode is formed on a second surface of the piezoelectric film. The first electrode and the second electrode may be formed by depositing an electrode layer on a piezoelectric film comprising polylactic acid via spraying, printing, or vacuum deposition. Accordingly, the first electrode layer may be disposed on the first surface of the piezoelectric film, and the second electrode layer may be disposed on the second surface of the piezoelectric film at a side opposite to the first electrode layer.

Referring again to fig. 1, the use of a first ultrasound transducer 10 and a second ultrasound transducer 12 for structural health monitoring is shown. In use, as shown in fig. 1, the first ultrasonic transducer 10 and the second ultrasonic transducer 12 are mechanically coupled to the structure 22 to monitor the integrity of the structure 22, the first ultrasonic transducer 10 being operable to generate a guided shear wave 18 propagating in the structure 22, the second ultrasonic transducer 12 being operable to detect the guided shear wave 18 propagating in the structure 22. The piezoelectric film 14 generates and/or detects the guided shear wave 18 propagating in the structure 22, thereby indicating the health of the structure 22. The molecular chain orientation in the piezoelectric film 14 may be parallel to the plane of the surface of the structure 22 and perpendicular to the direction of propagation of the guided shear wave 18 to be generated and detected, or parallel to the direction of propagation of the guided shear wave 18 to be generated and detected.

Each of the first and second ultrasonic transducers 10, 12 may be mechanically coupled to the structure 22 by being bonded to a surface of the structure 22 or embedded in the structure 22 using an adhesive (not shown). The structure 22 may be mechanically coupled to the piezoelectric film 14 by an adhesive.

An ultrasound guided shear wave 18 is a wave that propagates along a structure 22 while being guided by the boundaries of the structure 22, where the particle motion direction is parallel to the surface of the structure and perpendicular to the propagation direction of the guided shear wave 18. Although in the illustrated embodiment the guided shear wave 18 is shown as being generated and detected by the first and second ultrasonic transducers 10, 12, in alternative embodiments the guided shear wave 18 may alternatively be generated or detected by other modalities, such as lasers or discrete transducers, in combination with an ultrasonic transducer made of a piezoelectric film and electrode layer comprising polylactic acid.

The strain in the piezoelectric film 14 of the first ultrasonic transducer 10 may be caused by an electric field applied through the first electrode 16 and the second electrode to generate a guided shear wave 18 that propagates in the structure 22. More specifically, the electrical excitation signal may be applied across the thickness of the piezoelectric film 14 of the first ultrasonic transducer 10. The electric field applied through the first electrode 16 and the second electrode induces a strain in the piezoelectric film 14 to generate a guided shear wave 18 that propagates in the structure 22.

The guided shear wave 18 in the structure 22 may be detected from electrical signals output from the first electrode 16 and the second electrode of the piezoelectric film 14 of the second ultrasonic transducer 12. The signal receiving the guided shear wave 18 may be obtained by measuring the charge generated from the first electrode 16 and the second electrode at the top and bottom surfaces of the piezoelectric film 14 of the second ultrasonic transducer 12. The signal from the second ultrasonic transducer 12 is used as an indicator of structural health. This signal may be compared to a predetermined baseline signal from the structure 22 under previous conditions, and deviations from the baseline signal may be used to indicate changes in the structure 22, such as the presence or development of defects in the structure 22.

The structure 22 to be monitored may be metal, plastic or composite. In the illustrated embodiment, the structure 22 is a flat plate structure. When the structure 22 is planar, the guided shear waves 18 generated and detected in the structure 22 by the first and second ultrasonic transducers 10, 12 may be shear horizontal waves. In a tubular structure, the guided shear wave 18 generated and detected in the tubular structure may be a torsional wave or in a torsional wave mode.

Referring now to fig. 3, a method 300 for structural health monitoring is shown. The method 300 begins at step 302 with mechanically coupling at least one ultrasonic transducer to a structure. At least one ultrasonic transducer on the structure may be operable to generate or detect a guided shear wave in the structure.

At step 304, a guided shear wave propagating through the structure is generated, and at step 306, the guided shear wave is detected. The at least one ultrasonic transducer may be operable to generate and/or detect a guided shear wave propagating in the structure. For example, the guided shear wave may be generated by a laser, a discrete transducer, or an ultrasonic transducer made of a piezoelectric film comprising polylactic acid and an electrode layer mechanically coupled to the structure, and the guided shear wave may be detected by another ultrasonic transducer made of a piezoelectric film comprising polylactic acid and an electrode layer mechanically coupled to the structure. Alternatively, the guided shear wave may be generated by an ultrasonic transducer made of a piezoelectric film comprising polylactic acid and an electrode layer mechanically coupled to the structure, and the guided shear wave may be detected by a laser, a discrete transducer, or another ultrasonic transducer made of a piezoelectric film comprising polylactic acid and an electrode layer mechanically coupled to the structure.

In one or more embodiments in which the guided shear wave is generated by at least one ultrasonic transducer, the step 304 of generating the guided shear wave propagating through the structure may comprise: an electric field is applied across the first electrode and the second electrode of the first ultrasonic transducer to induce strain in the piezoelectric film of the first ultrasonic transducer to generate a guided shear wave.

In one or more embodiments in which the guided shear wave is generated by at least one ultrasonic transducer, the step 306 of detecting the guided shear wave may comprise: electrical signals output from the first electrode and the second electrode of the second ultrasonic transducer are measured.

At step 308, the detected ultrasonic signal of the guided shear wave is compared to a baseline signal of the structure under reference conditions to determine the presence of a defect in the structure. In one or more embodiments, the signal output from the ultrasound transducer may be compared to a predetermined baseline signal from the structure under previous conditions, and deviations from the baseline signal may be used to indicate changes in the structure, such as the presence or development of defects in the structure.

Example

Example 1

Now, an example of monitoring the structural integrity using an ultrasonic transducer including a piezoelectric film containing polylactic acid (PLLA) and molecular chain orientation obtained by mechanically stretching the PLLA film at high temperature will be described.

A solution of PLLA in 10% by weight was prepared by dissolving PLLA in chloroform in a silicone oil bath at 50 ℃. The solution was cast onto a smooth glass substrate by using a doctor blade. Then, the glass substrate was placed on a heating plate of 50 ℃ to evaporate the solvent. The resulting PLLA film was peeled from the glass and cut to the desired dimensions for the subsequent stretching step. The film was cut into rectangles of 3cm by 5 cm. The two sides of the rectangular film were clamped, leaving a length of 3cm to be stretched. The film was heated at 150 ℃ for 10 seconds using a heat gun prior to stretching. The films were stretched at different stretch ratios of 2 to 7. The heat gun continued to heat the film until 10 seconds after the desired draw ratio was reached. The membrane is then removed from the fixture and cut to the dimensions required for characterization and manufacture of the transducer.

Referring now to fig. 4A, a photograph of a poly (L-lactic acid) (PLLA) film prepared by mechanical stretching is shown.

Referring now to fig. 4B, X-ray diffraction (XRD) patterns of PLLA films not mechanically stretched and PLLA films stretched at a stretch ratio of 5 are shown. As can be seen from fig. 4B, the PLLA film that was not stretched was in an amorphous phase, whereas the PLLA film with a stretch ratio of 5 exhibited high crystallinity, with the crystallographic orientations along (200) and (110) directions. XRD patterns of stretched PLLA films showed: the stretched PLLA film has a beta-type (beta-type) crystal structure, which is the piezoelectric phase of PLLA.

The stretched PLLA films were used to fabricate ultrasonic transducers and the feasibility of using ultrasonic transducers to detect surface fracture cracks on structures was evaluated.

Referring now to fig. 4C, a schematic diagram of an experimental test apparatus 400 for evaluating the use of stretched PLLA membrane 402 to fabricate ultrasonic transducer 404 is shown, in accordance with an embodiment of the present invention. A 1.6mm thick aluminum plate 406 was used. The integrity of the aluminum plate 406 is monitored using an ultrasonic transducer 404 made by mechanically stretching the PLLA membrane 402 to detect the shear horizontal wave 408 (one of the plate structures directs the shear horizontal wave).

Referring now to fig. 4D, a dispersion curve for a 1.6mm thick aluminum plate in different ultrasonic modes is shown. Zero mode shear horizontal wave (SH 0) is chosen because the mode is non-dispersive-its group velocity does not change with frequency, as can be seen from the corresponding dispersion curve SH0 in fig. 4D. Furthermore, SH0 is more sensitive to defects oriented along its wave propagation direction due to the in-plane particle displacement motion of SH 0.

Referring again to fig. 4C, a piece of PLLA film 402 with 2.7mm x 2.7mm electrode pattern 410 was bonded to a 1.6mm thick aluminum plate 406 by epoxy to act as a receiver of ultrasonic waves 408, with molecular chains aligned along the propagation direction of SH0 mode waves. The signal used to detect shear horizontal wave 408 is obtained by measuring the charge generated from electrode 410 at the top and bottom surfaces of PLLA film 402. The discrete shear mode transducer probe 412 is used to generate ultrasonic waves 408 at a distance of 60mm from the receiver 404.

Referring now to fig. 4E, a time-frequency two-dimensional (2D) plot of the ultrasonic signal detected by the ultrasonic transducer 404 is shown, which overlaps the dispersion curve of the aluminum plate 406. From the spectrum diagram shown in fig. 4E, the experimental results show that: ultrasound transducer 404 made of PLLA membrane 402 is capable of detecting shear horizontal waves in SH0, SH1, and SH2 modes.

Referring again to fig. 4C, horizontal crack 414 was simulated using thin notches of increasing depth, followed by a through hole at the location of the notch.

Referring now to fig. 4F, SH0 mode is monitored and ultrasound signals are collected at different defect states, as shown. As can be seen from fig. 4F, a change in the ultrasonic signal (SH 0 mode shear horizontal wave) is observed before and after the creation of the differently sized defect 414. More specifically, the signal amplitude is highest in the original state, decreases with increasing defect depth, and eventually disappears as the shear horizontal wave 408 is blocked by the via defect 414.

Example 2

In this example, the piezoelectric film containing PLLA is in the form of a fibrous mat that is manufactured by electrospinning a PLLA solution onto a rotating collector. During electrospinning, a strong electric field is applied to the tips of a spinneret containing a PLLA solution. A continuous fine jet of solution is ejected from the spinneret and moved through an electric field to deposit on a collector. A constant feed rate of 0.5mL/h and an electric field strength of 1.5kV/cm were used.

Referring now to fig. 5A and 5B, scanning Electron Microscope (SEM) images of PLLA films in the form of fiber mats at different magnifications are shown. As can be seen from fig. 5A and 5B, PLLA fibers having a diameter of several microns are highly aligned and densely packed to form a mat.

Referring now to fig. 5C, the use of an ultrasonic transducer 500 made of PLLA membrane 502 in the form of a fiber mat to generate or detect shear horizontal waves 504 is shown. To evaluate the performance of an ultrasonic transducer 500 made from PLLA membrane 502 in the form of a fiber mat in generating and detecting guided shear waves in a structure, ultrasonic transducer 500 was bonded to an aluminum plate 506 and tested using a shear mode transducer probe 508. The ultrasonic transducer 500 is designed to specifically generate or detect a shear horizontal wave 504 (SH 0 mode) having a wavelength of 6.2mm and a frequency of 500 kHz. The electrodes 510 of the ultrasonic transducer 500 have a comb pattern, wherein the periodicity of the electrode comb fingers is the same as the wavelength of the shear horizontal wave 504 (i.e., 6.2 mm). The molecular chain orientation direction is the same as the propagation direction of SH0 mode wave 504.

Referring now to fig. 5D, an ultrasound signal is shown with an ultrasound transducer 500 made of PLLA membrane 502 used to detect shear horizontal waves 504. An electrical excitation signal (3 cycles of sine wave, 100vpp,500 khz) is applied to the shear mode transducer probe 508 and the SH0 mode wave 504 is detected by the ultrasound transducer 500.

Referring now to fig. 5E, an ultrasound signal is shown with an ultrasound transducer 500 made of PLLA membrane 502 used to generate shear horizontal waves 504. The same electrical excitation signal is applied along the thickness direction of PLLA membrane 502 to generate SH0 mode wave 504 and the wave is detected by shear mode transducer probe 508.

The hatched areas in fig. 5D and 5E both indicate wave packets of the signal corresponding to SH0 mode wave 504.

To demonstrate the use of the ultrasonic transducer 500 for monitoring the performance of underwater structures, an aluminum plate 506 mechanically coupled to the ultrasonic transducer 500 as described above was immersed in water and the ultrasonic signal was compared to the signal when the aluminum plate 506 was in air.

Referring now to fig. 5F, a comparison of signals from an ultrasonic transducer 504 as a receiver for shear horizontal waves 504 is shown when an aluminum plate 506 mechanically coupled to the ultrasonic transducer 500 is in air and immersed in water. Ultrasonic transducer 500 is made of PLLA membrane 502 in the form of a fiber mat and is bonded to aluminum plate 506. The wave packet of the signal corresponding to the selected mode of the guided shear wave 504 is shaded in fig. 5F. As can be seen from fig. 5F, the amplitude and arrival time of the signal corresponding to SH0 mode wave 504 does not change before and after aluminum plate 506 is immersed in water.

Referring now to fig. 5G, a comparison of signals obtained with an ultrasonic transducer made of a piezoelectric poly (vinylidene fluoride) (PVDF) film and bonded to an aluminum plate in air and immersed in water is shown. In this comparative example, an ultrasonic transducer is used to generate and detect Lamb waves (Lamb waves) instead of shear horizontal waves. As can be seen from fig. 5G, in contrast to fig. 5F, for the lamb wave ultrasonic transducer made of the piezoelectric PVDF film, substantial changes in both amplitude and arrival time were observed before and after immersing the aluminum plate in water.

To investigate the performance of an ultrasonic transducer made of PLLA membrane in the form of a fiber mat for Structural Health Monitoring (SHM) of underwater structures, the ultrasonic transducer was bonded to a 1.6mm thick aluminum plate and ultrasonic signals were obtained when the structure was in air and immersed in water. The ultrasonic transducer is designed with comb-shaped electrodes for specifically detecting shear horizontal waves of SH0 mode at a frequency of 500 kHz. A shear mode piezoelectric ceramic at a distance of 100mm from an ultrasonic transducer made of PLLA film was used as a transmitter of SH0 wave. A circular defect of 3mm diameter was machined to different depths (i.e., through holes) from 0.3mm to 1.6mm, 50mm from the emitter.

Referring now to fig. 5H-5K, there are shown ultrasonic signals obtained by an ultrasonic transducer made of PLLA membrane in the form of a fiber mat detecting shear horizontal waves in an aluminum plate with defects of different sizes when the structure is in air and water. More specifically, fig. 5H shows ultrasonic signals obtained from defects of different depths while in air, fig. 5I is an enlarged view of ultrasonic signals in a circular portion shown in fig. 5H, fig. 5J shows ultrasonic signals obtained from defects of different depths while in water, and fig. 5K is an enlarged view of ultrasonic signals in a circular portion shown in fig. 5J.

A Damage Index (DI) is calculated to correlate the changes in the ultrasound signal with defects of different sizes. The DI value is calculated based on a comparison of the signal amplitude obtained at the original stage and the signal amplitude obtained using a defect of a specific size. The calculation of the damage index DI is based on a residual signal energy method that measures the energy ratio between the different signals using the following formula:

Wherein DI represents the damage index; x represents the signal amplitude at the original stage; y represents the signal amplitude when a defect of a specific size is used; t1 and t2 represent the beginning and end of the selected period.

Referring now to fig. 5L, a comparison of DI values calculated based on ultrasonic signals obtained when the structure is in air and in water is shown. The signal was obtained by detecting shear horizontal waves in an aluminum plate with defects of different sizes by means of an ultrasonic transducer made of PLLA film in the form of a fiber mat. By comparing the DI values as shown in fig. 5L, it can be seen that: the sensitivity of detecting defects when the structure is in water is comparable to the sensitivity of detecting defects when the structure is in air. This demonstrates the advantage of using an ultrasound transducer made of PLLA membrane for underwater SHM.

Referring now to fig. 5M, PLLA membrane 520 in the form of a fiber mat was bonded to aluminum plate 522, and square silver electrodes 524 (2.7 mm x 2.7 mm) were deposited on fiber mat 520 to form ultrasonic transducer 526 as shown to study the directionality of ultrasonic transducer 526 as a receiver for guided shear waves 528. The guided shear waves or shear horizontal waves 528 are generated by the discrete shear mode probes 530 at different angles, as shown in fig. 5M. The signal from the ultrasonic transducer 526 is compared to the guided shear wave 528 at a different angle of incidence. By comparing the signal from the ultrasound transducer 526 with the shear horizontal wave 528 at a different angle of incidence, it can be seen that: the ultrasound transducer 526 is sensitive to the guided shear wave 528 when the angle of incidence is 0 °, 90 °, 180 °, and 270 ° (i.e., when the molecular chain is aligned parallel to the direction of propagation of the ultrasound waves in the structure, or perpendicular to the direction of propagation of the ultrasound waves in the structure).

Example 3

Referring now to fig. 6A, a method of manufacturing PLLA membrane 600 in the form of a fiber mat by centrifugal electrospinning using rotating collector 602 is shown. In a centrifugal electrospinning apparatus, the collector 602 or spinneret 604 can be rotated. The resulting PLLA membrane 600 is circular in shape, with molecular chains oriented along the circumferential direction of the PLLA membrane 600.

Referring now to fig. 6B, a shear mode ultrasound transducer 606 is shown, the shear mode ultrasound transducer 606 being made of the polylactic acid film 600 obtained in fig. 6A for generating and detecting an omni-directional guided shear wave 608 in a structure 610. PLLA membrane 600 with circular electrode 612 is bonded to plate structure 610 by an adhesive to form ultrasonic transducer 606 coupled to structure 610. As can be seen from fig. 6B, an ultrasonic transducer 606 may be used to generate or detect an omni-directional shear level wave 608.

Example 4

Referring now to fig. 7, a method of monitoring the integrity of a tubular structure 700 by generating and/or detecting a torsional wave 712 (one of the tubular structures directs shear waves) in the structure 700 using an ultrasonic transducer 702 made of PLLA membrane 706 and electrodes 708, 710 is shown. Ultrasound transducers 702 and 704 made of PLLA containing piezoelectric film 704 and electrode layers 708, 710 are mechanically coupled to tubular structure 700. The ultrasonic transducers 702 and 704 are designed to effectively generate or detect a torsional wave 712. As shown in fig. 7, the molecular chains are oriented along the circumferential or axial direction of the tubular structure 700, and the electrode layer 708 is a circular pattern oriented along the circumferential direction of the tubular structure 700. The electrode layer 708 of the ultrasound transducer 702 for generating the torsional wave 712 may be circular in shape to effectively generate the torsional wave 712, while the electrode layer 710 of the ultrasound transducer 704 for detecting the torsional wave 712 may be in the form of an array along the circumferential direction of the tubular structure 700. By using these transducers 702 and 704 in a pitch-catch mode, the severity and location of defects in the tubular structure 700 can be detected.

The experimental results show that: piezoelectric induced deformations caused by the piezoelectric effect of certain isomers of polylactic acid may be operatively coupled to a structure to generate guided shear waves, detect the guided shear waves in the structure and indicate structural integrity of the structure.

Experimental results also show that: structural Health Monitoring (SHM) using guided shear waves generated and/or detected by an ultrasound transducer according to embodiments of the present invention, wherein the ultrasound transducer comprises a piezoelectric film comprising polylactic acid, has significant advantages without being substantially affected by the presence of a liquid. As the experimental results demonstrate, when the structure to be monitored is in a surrounding medium (e.g. liquid), the guided shear wave propagating in the structure and the indication of the health of the structure are not substantially affected by the surrounding medium.

As is apparent from the foregoing discussion, the present invention provides an ultrasound transducer for structural health monitoring, a method for producing an ultrasound transducer, and a method for structural health monitoring using guided shear waves by integrating at least one ultrasound transducer into a structure to be monitored, wherein the at least one ultrasound transducer is made of a piezoelectric film and an electrode layer comprising polylactic acid. Advantageously, the ultrasound transducer of the present invention can be made of low cost piezoelectric polymers and the method for producing the ultrasound transducer is scalable for mass production. More advantageously, the ultrasound transducer of the present invention has piezoelectric properties, without requiring an electrical polarization treatment. The flexible and low profile ultrasound transducer of the present invention can also be advantageously applied to structures having complex shapes. The method for structural health monitoring of the present invention advantageously requires simpler signal interpretation by using non-dispersive, zero-mode guided shear waves, experiences inherently low attenuation, and minimizes the environmental impact of structures in contact with liquids or solids.

Ultrasonic transducers for structural health monitoring and methods for structural health monitoring may be applied to monitoring the integrity of pipes, underground structures, etc. used to transport chemicals.

While the preferred embodiments of the present invention have been described, it is to be understood that: the invention is not limited to only the described embodiments. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the scope of the invention as described in the claims.

Furthermore, unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive rather than an exclusive or exhaustive sense; that is, it is interpreted in the meaning of "including, but not limited to".

Claims (16)

1. An ultrasonic transducer for structural health monitoring, the ultrasonic transducer comprising:

A piezoelectric film comprising polylactic acid having aligned molecular chain orientations;

a first electrode on a first surface of the piezoelectric film; and

And a second electrode on a second surface of the piezoelectric film.

2. The ultrasound transducer of claim 1, wherein the polylactic acid is selected from the group consisting of poly (L-lactic acid) (PLLA) and poly (D-lactic acid) (PDLA).

3. The ultrasound transducer according to claim 1 or 2, wherein the aligned molecular chain orientation of the polylactic acid is parallel to or perpendicular to a propagation direction of a guided shear wave to be generated or detected.

4. The ultrasonic transducer according to claim 1 or 2, wherein the piezoelectric film is circular in shape, and the aligned molecular chains of the polylactic acid are oriented along a circumferential direction of the piezoelectric film.

5. The ultrasound transducer of any of the preceding claims, wherein at least one of the first electrode and the second electrode is a layer having a comb pattern.

6. The ultrasonic transducer of claim 5, wherein the fingers of the comb electrode layer have a periodicity corresponding to the wavelength of the guided shear wave.

7. A method for producing an ultrasound transducer for structural health monitoring, the method comprising:

Dissolving polylactic acid in a solvent to form a polylactic acid solution;

Forming a piezoelectric film from the polylactic acid solution, the polylactic acid in the piezoelectric film having an aligned molecular chain orientation;

Forming a first electrode on a first surface of the piezoelectric film; and

A second electrode is formed on the second surface of the piezoelectric film.

8. The method for producing an ultrasonic transducer for structural health monitoring according to claim 7, wherein the step of forming a piezoelectric film from said polylactic acid solution comprises:

The piezoelectric film is mechanically stretched to a predetermined multiple of the original length of the piezoelectric film.

9. The method for producing an ultrasonic transducer for structural health monitoring according to claim 8, wherein said predetermined multiple of said original length of said piezoelectric film is between about 2 times to about 7 times.

10. The method for producing an ultrasonic transducer for structural health monitoring according to claim 8 or 9, wherein said piezoelectric film is mechanically stretched at a temperature between about 120 degrees celsius (°c) to about 155 ℃.

11. The method for producing an ultrasonic transducer for structural health monitoring according to claim 10, wherein said piezoelectric film is mechanically stretched at a temperature between about 130 ℃ and about 155 ℃.

12. The method for producing an ultrasonic transducer for structural health monitoring according to claim 7, wherein the step of forming a piezoelectric film from said polylactic acid solution comprises:

applying an electric field to a tip of a spinneret comprising the polylactic acid solution;

ejecting a polylactic acid solution from the spinneret, the ejected polylactic acid solution passing through the electric field; and

The sprayed polylactic acid solution is deposited on a collector to form polylactic acid fibers.

13. The method for producing an ultrasonic transducer for structural health monitoring according to claim 12, wherein the step of forming a piezoelectric film from said polylactic acid solution further comprises:

spinning the polylactic acid fibers using the spinneret or the collector to form a polylactic acid fiber mat, wherein the spinneret or the collector is rotatable.

14. A method for structural health monitoring, the method comprising:

Mechanically coupling at least one ultrasonic transducer according to any one of claims 1 to 6 to a structure;

Generating a guided shear wave propagating through the structure;

Detecting the guided shear wave; and

The detected ultrasonic signal of the guided shear wave is compared to a baseline signal of the structure under reference conditions to determine the presence of defects in the structure.

15. The method for structural health monitoring of claim 14, wherein generating a guided shear wave propagating through said structure comprises:

an electric field is applied across the first and second electrodes of the first ultrasonic transducer to induce strain in the piezoelectric film of the first ultrasonic transducer to generate the guided shear wave.

16. A method for structural health monitoring according to claim 14 or 15, wherein the step of detecting said guided shear wave comprises:

electrical signals output from the first electrode and the second electrode of the second ultrasonic transducer are measured.