KR101380491B1 - Non-destructive inspection using laser-ultrasound and infrared thermography - Google Patents

Abstract

An inspection system 200 is provided to inspect the internal structure of the target material 216. The inspection system includes a generation laser 210, ultrasonic detection systems 220, 226, 228, 230, thermal imaging system 234, and a processor / control module 232. The generating laser 210 produces a pulsed laser beam 212 that induces ultrasonic displacement and thermal transients in the target material 216. The ultrasonic detection system detects ultrasonic surface displacements in the target material 216. The thermal imaging system 234 detects thermal transients in the target material 216. The processor 232 may analyze both the detected ultrasonic displacement and the thermal image of the target material 216 to produce information about the internal structure of the target material. The target material 216 preferably includes a composite material.

Description

NON-DESTRUCTIVE INSPECTION USING LASER-ULTRASOUND AND INFRARED THERMOGRAPHY}

FIELD OF THE INVENTION The present invention relates to nondestructive testing, and more particularly to thermal imaging and ultrasonic testing for examining the internal structure of materials.

In recent years, the use of improved composite structures in aerospace, automotive and many other commercial industries has seen significant growth. Composite materials provide a significant improvement in performance, but require stringent quality control procedures both during the manufacturing process and after the material has been serviced as a finished product. In particular, the non-destructive evaluation (NDE) method should evaluate the structural integrity of the composite material. Appropriate assessments should be able to detect internal defects, delamination and porosity in both the near surface and deep interior regions.

Various methods and apparatus have been proposed for evaluating the structural integrity of composite structures. In one solution, an ultrasonic source is used to generate an ultrasonic surface displacement in the target material. The ultrasonic surface displacement is then measured and analyzed. The source of ultrasound may be a pulsed generation laser beam directed to the target. Laser light from the individual detection lasers illuminates the ultrasonic surface displacement and is scattered by the workpiece surface. The collection optics then collect the scattered laser energy. The collected optical element is connected to an interferometer or other device, and the structural integrity of the complex structure can be obtained through analysis of the scattered laser energy. Laser ultrasound has been known to be very effective for inspecting parts during the manufacturing process.

Typically, while the probe laser beam connected to the interferometer detects surface displacement or velocity, thermal expansion at localized areas on the surface causes the laser source to generate sound. Thermal expansion due to absorption of the generated laser produces a displacement, which is demodulated by a laser-ultrasonic detection system, leading to a pulse at the beginning of the laser-ultrasonic signal. This echo is usually referred to as surface echo. The surface echoes can block any echoes created by defects located close to the sample surface. The duration of the surface echo depends on the laser pulse duration and the frequency bandwidth of the detection system. Typically, CO ₂ for detection Using a generated laser and confocal Fabry-Perot, the surface echo can last up to several microseconds. Thus any defect that produces an echo during this time can be blocked. For this reason, laser-ultrasound inspection is sensitive to defects located deep inside and less sensitive to defects located close to the surface.

Another NDE method, transient infrared (IR) thermography, does not detect defects located more than a few millimeters deep in the polymer-substrate portion, and thus does not enable efficient inspection of polymer-substrate composites.

Embodiments of the present invention relate to systems and methods that fully address the aforementioned and other needs. Embodiments of the invention are further described in the following detailed description and claims. Advantages and features of embodiments of the present invention will become apparent from the detailed description, together with the accompanying drawings and the claims.

Embodiments of the present invention combine laser laser techniques with thermal imaging techniques to sufficiently address the aforementioned and other needs. Laser ultrasonic generation techniques can be used to provide the transient heat source. Thus, transient infrared (IR) thermography can be combined with laser ultrasound to provide a more complete non-destructive inspection of the polymer-substrate portion (i.e., composite material).

One embodiment provides an inspection system for inspecting internal structures near the surface and deep internal structures of the target material. The inspection system includes a generation laser, an ultrasonic detection system, a thermal imaging system, and a processor / control module. The generating laser produces a pulsed laser beam that is operative to guide both the ultrasonic displacement portion and the thermal transient portion in the target material. The ultrasonic detection system detects ultrasonic surface displacement in the target material. The thermal imaging system detects thermal transients in the target material. The processor / controller may analyze and correlate both the detected ultrasonic displacements and thermal images of the target material to produce information about the internal and deep internal structures near the surface of the target material.

Another embodiment provides a method for examining the internal structure of a target. The method includes inducing both an ultrasonic displacement portion and a thermal transient in the target material. Using a single pulsed generation laser beam, these ultrasonic displacements and thermal transients can be generated. Ultrasonic displacements and thermal transients generated by the generated laser beam directed towards the surface of the target can be detected and analyzed. Generation and analysis may include synchronization and correlation of both ultrasound and thermal information to yield a more complete understanding of the structure of the target. For example, by analyzing the ultrasonic displacements, it is possible to produce information about the deep internal structure in the composite material. Thermal images can produce information about the internal structure near the surface of the composite material. By correlating the ultrasound information with the thermal image information, a better understanding of the overall internal structure of the target can be obtained.

Yet another embodiment provides a composite material inspection system. The composite material inspection system includes a generation laser for generating a pulsed laser beam that guides the ultrasonic displacement portion and the thermal transient portion in the composite material. An ultrasonic detection system is provided for detecting ultrasonic surface displacement in a composite material. A thermal imaging system is provided for detecting thermal transients in composite materials. The control module may match the thermal imaging frame acquisition rate to the pulse rate of the generating laser beam. In order to produce information about the overall internal structure of the target, a processor is provided for analyzing and correlating the thermal image with the detected ultrasonic displacement.

1 illustrates the use of a generation laser beam and a detection laser beam for generating and detecting ultrasonic displacements and thermal transients, in accordance with an embodiment of the invention.

2 provides a block diagram for illustrating the basic configuration of a laser ultrasound / thermal imaging system.

3 provides a block diagram, or functional diagram, of a laser ultrasound and IR imaging system in accordance with an embodiment of the present invention.

4 depicts the processing of an IR image used to collect information about an internal structure near the surface of a target, in accordance with an embodiment of the invention.

FIG. 5 shows infrared results obtained by scanning a polymer plate with a planar bottom hole with a pulsed CO 2 laser beam, in accordance with an embodiment of the invention.

6 provides a logic flow diagram in accordance with one or more embodiments of the present invention.

7 shows a block diagram of a generation laser for generating an ultrasonic displacement portion and a thermal transient in accordance with an embodiment of the invention.

Preferred embodiments of the invention are shown in the drawings, wherein like reference numerals are used to refer to corresponding like parts of the various figures.

Embodiments of the present invention combine a laser-ultrasound technique with a thermal imaging technique to provide a more complete non-destructive examination of a target material (e.g., without limitation, polymer-substrate portion (i.e., composite material)). Can provide. One embodiment of the present invention provides an inspection system operative to inspect the internal structure of a target material. The inspection system includes a generation laser, an ultrasonic detection system, a thermal imaging system, and a processor / control module. The generating laser produces a pulsed laser beam that functions to induce both ultrasonic displacement and thermal transients in the target material. The ultrasonic detection system detects ultrasonic surface displacements in the target material. The thermal imaging image detects thermal transients in the target material. The processor can analyze and correlate both the detected ultrasonic displacements and thermal images of the target material to generate information about the overall internal structure of the target material. Embodiments of the present invention are provided for faster inspection speeds, improved system reliability, and lower operating costs.

1 illustrates the use of a generation laser beam and a detection laser beam to generate and detect laser ultrasonic displacement and thermal transients, in accordance with an embodiment of the invention. During the test, the illumination (detection) laser beam 104 detects ultrasound at a target 106, such as a composite material, while the laser beam 102 generates ultrasound and thermal transients. As can be seen, these lasers can be applied coaxially to the target 106. The generated laser beam 102 causes thermal elastic expansion 112 at the target 106, which leads to the shape or waveform 108 of the ultrasonic deformation. The deformation, or ultrasound, 108 propagates at the target 106 and modulates, scatters, and reflects the detection laser beam 104 to produce phase modulated light 110 in a direction away from the target 106. . The lights 110 may be collected and processed to obtain information describing the internal structure of the target 106.

FIG. 2 provides a block diagram with basic components for performing ultrasonic laser testing and infrared (TR) thermography. Generation laser 210 generates generation laser beam 212, with optical assembly 214 directing generation laser beam 212 to target 216. As shown, the optical assembly 214 includes a scanner that moves the laser beam 212 along a scan (or test) plan 218, or other similar means. The optical assembly 214 can be a visual camera, depth camera, IR camera, range detector, narrowband camera, or other optics known to those skilled in the art. It may include a sensor. Each of these optical sensors may require calibration prior to performing the inspection. This calibration verifies the system's ability to integrate the information collected by the various sensors. The generating laser 210 generates an ultrasonic wave 108 and a thermal transient in the target 216. Thermal imaging system 232 captures a thermal image of the target. These images can be processed to generate information about the near surface internal structure of the target 216. This process will be described in more detail below with reference to FIG.

The ultrasonic wave 108 and the thermoelastic expansion 112 generating thermal transitions are the result of the composite material absorbing the generated laser beam. Composite material 216 readily absorbs the generated laser beam 212 without corrosion or break down. In order to overcome the signal-to-noise ratio (SNR) problem, higher power generating lasers are not necessarily preferred because they cause material corrosion on the surface of the workpiece and potentially damage components. Because it can. In another embodiment, depending on the material being tested, some corrosion may be allowed to increase the SNR of the detected signal. The generating laser beam 212 has a pulse duration, power, and frequency suitable for inducing ultrasonic surface displacement and appropriate thermal transients. For example, TEA laser (transverse-excited atmospheric) CO ₂ can produce a beam having a wavelength of 10.6 microns to 100 nanoseconds (nanosecond) pulse width. The power of the laser must be sufficient to deliver, for example, 0.25 joule pulses to the target, which may require a 100 watt laser operating at a pulse repetition rate of 400 Hz. The generated laser beam 212 is absorbed as heat to the surface of the target, whereby corrosion-free thermoelastic expansion occurs.

The detection laser 220 operating in the pulse mode or the CW mode does not guide the ultrasonic displacement portion. For example, an Nd: YAG laser may be used. The power of this laser should be sufficient to deliver 100 microsecond pulses of 100 milli-joules, which may require, for example, a 1 kilowatt (KW) laser. The detection laser 220 generates the detection laser beam 222. The detection laser 220 may include a filtering means 224 or may be optically connected to the filtering means 224 to remove noise from the detection laser beam 224. The optical assembly 214 directs the detection laser beam 224 to the surface of the composite material 216, which scatters and / or reflects the detection laser beam 224. The final phase modulated light is collected by the collecting fiber 226. As shown here, scattered and / or reflected detection laser light passes back through the optical assembly 214. Optional optical processor 228 and interferometer 230 may process the phase modulated light to generate a signal containing information indicative of the ultrasonic displacement at the surface of composite material 216. The data processing and control system 232 may coordinate the operation of the laser ultrasound system component and the thermal imaging component to generate information about the internal structure of the target.

The data processing and control system 232 may be one single processing device, or multiple processing devices. Such processing devices may include microprocessors, microcontrollers, digital signal processors, microcomputers, central processing units, field programmable gate arrays, programmable logic devices, digital circuits, and memories. It may be one or more of any device that manipulates (analog and / or digital) signals based on operational instructions. The memory may be a single memory device or multiple memory devices. Such a memory device may be one or more of read only memory, random access memory, volatile memory, non-volatile memory, static memory, dynamic memory, flash memory, cache memory, any device that stores digital information. The memory stores the operation instructions corresponding to some or all of the steps and / or functions described later, and the data processing and control system 232 executes.

3 provides a block, or functional diagram, of a laser ultrasound and IR imaging system 300 in accordance with an embodiment of the present invention. The laser ultrasound and IR imaging system 300 includes a generation laser 302, a control module 304, a laser ultrasound detection system 306, a thermal imaging system 308, a processing module 310, and an optical system. 312. The generating laser 302 generates a generating laser beam which is directed by the optical system 312 to a target 314 made of a material (eg, a composite material), as described above, In this material, ultrasonic displacements are induced. The laser ultrasonic detection system 306 generates a detection laser beam, which is directed to the target by the optical system 312, whereby the detection laser beam is detected by the ultrasonic displacement at the surface of the target 314. The beam may be phase modulated. The detection laser beam is scattered by the surface of the target. Optical system 312 also collects this scattered phase modulated light. The laser ultrasonic detection system 306 may process the collected phase modulated light to develop a signal including information about an ultrasonic displacement portion. This signal is provided to the processing module 310.

The generation laser 302 also generates a thermal transient for thermographic measurements of the target 314. A thermal imaging system, such as an IR camera 308, acquires a thermal image, or a frame of thermal transients within the target 314. One image is obtained for each generated laser pulse. Additional images may be obtained at a designated time after each generating pulse. These different images can be processed to generate a complete area of thermography inspection which is examined by laser-ultrasound.

The results of the thermography secure the laser-ultrasonic results and in this way provide a more complete and more reliable inspection. Transient IR thermography alone does not provide efficient inspection of composite parts such as, for example, polymer matrix composites. Due to the low thermal conductivity on the polymer substrate, transient IR thermography reacts only on the upper surface of the composite portion. Thus IR thermography cannot be used to detect and identify polymer substrates, ie, defects located deep within the composite portion.

The laser ultrasound and IR imaging system 300 includes both laser ultrasound providing a deep internal inspection system and thermal imaging that resolves the area near the surface of the target 314. This solves the problem associated with laser ultrasound that is less responsive to defects in the near surface area. Combining these two techniques allows for a more complete non-destructive inspection of the composite portion, or composite material, than was possible when using only laser ultrasound or IR thermography alone.

4 depicts the processing of an IR image to gather information about the internal structure of the target 314. Each time a generated laser beam is fired, or after a designated time after it is fired, a thermal image can be collected. As the generating laser beam is fired or pulsed at the target 314A, the generating laser beam will be scanned along the scan path 316. Each point 318 to which the generated laser beam is directed will have a thermal transition 320. Target 314N shows the scanning of path 316 by repeatedly illuminating target 314 using a generated laser beam and collecting multiple thermal images. These thermal transients can be used to determine the thermal properties associated with the target material. For example, by analyzing the thermal image over time at the target, quantitative thermal wall thickness can be determined. This may be provided as a composite visual image, as shown in FIG. 5. This processing approach analyzes infrared images (more specifically, temperature changes as a function of time in different ranges). Relative temperature change curves were built from all IR images for each point of the IR camera.

Another embodiment may provide a scanned IR thermography technique to inspect material for near surface defects. This may limit the peak thermal load of the target, where only a small portion of the target is heated at a time. Such systems use a scanned laser to induce thermal transients.

5 shows the infrared results obtained by scanning a polymer plate with a flat bottom hole with a pulsed CO ₂ laser beam. Defects in the target 502 are clearly visible in the gray scale image 500. Image 500 includes various points 504 within material 502. This image can be generated using the imaging method described in US Pat. No. 6,367,969 “Synthetic reference thermal imaging method”, which is incorporated herein by reference. An IR transient thermography analysis approach can be used to accurately measure the thickness of the target and provide a visually coded display indicative of the cross-sectional thickness in the desired area of the target.

Basically, IR transient thermography uses an inflection point in the temperature-at-time (Tt) response analysis of the surface of a rapidly heating target, preferably obtained from “front-side” IR camera observations. . This inflection point occurs relatively early in the T-t response and is essentially independent of the transverse heat loss mechanism. (This consideration may be particularly relevant when working with metals, because of the high thermal conductivity of the metal, the thermal response of the metal target is quite fast, and therefore the time to obtain thermal data measurements is usually short. An inflection point is extracted from the thermal data obtained for a specified time period from successive IR camera image frames. This time period is preferably somewhat longer than the expected characteristic time, based on an estimate of the thickness of the target being evaluated.

Thermal reference data is calculated for each (x, y) pixel position of the imaged target and then used to determine the contrast as a function of time for each pixel. The computer system may control the imaging system and record and analyze surface temperature data obtained through the IR camera to provide color or gray pattern keyed images that accurately correspond to the thickness of the target. This information can be combined with the laser ultrasound data to produce a more detailed internal picture of the target.

Acquisition of surface temperature data is initiated by igniting a generating laser to illuminate and heat a portion of the surface of the target. Then a thermal image frame is recorded for a period of time after each generating laser pulse, which is used to form a T-t history (e.g., history associated with a thermal transient).

Then, heat flow analysis of the T-t history may be performed on each pixel of the acquired image frame to determine the thickness of the target at the location of each resolution element. Conventionally, the analysis of the transient heat flow through the solid portion of the target is characterized by the characteristic time required for a “pulse” of thermal energy to penetrate the target at the first surface and be reflected at the opposite surface, returning to the first surface. It needs a step to determine. Since this characteristic time is related to the spacing between the two surfaces, it can be used to determine the thickness of the target between the two surfaces at the desired point. A contrast-versus-time curve is determined for each (x, y) pixel location corresponding to each resolution element of the target surface.

6 provides a logic flow diagram illustrating a method for inspecting a material, such as a composite material, in accordance with an embodiment of the invention. Task 600 applies both laser ultrasound techniques and thermal imaging or infrared thermography techniques to examine and examine the internal structure of the material under test. Operation 600 begins at step 602 where ultrasonic displacements and thermal transients are induced in the target material. These can all be accomplished using a generated laser beam in conjunction with the laser ultrasound system. As described with reference to FIGS. 1 and 4, this generating laser beam generates both an ultrasonic displacement portion and a thermal transient when directed to the surface of the target material. In step 604, ultrasonic displacements and thermal transients are detected. The ultrasonic displacement portion may be detected using an ultrasonic system, for example a laser ultrasonic system. Thermal transients can be detected by obtaining a thermal image of the target material. As mentioned above, the generation of thermal transients and ultrasound can be synchronized or correlated. This information can be used to match the results of the analysis performed at step 606. In step 606, both detected ultrasonic displacements and thermal images are analyzed. The detected ultrasonic displacement will provide information regarding the deeply located internal structure of the target material, while the thermal image of the thermal transition can be processed to determine the near surface structure in the surface material. Since the ultrasonic displacement portion and the thermal transient portion are generated by the same generated laser beam, this information can be used to easily correlate the detected ultrasonic displacement portion with the thermal image. In step 608 this enables a detailed and complex understanding of both the near surface structure and the deep internal structure of the target material.

By applying a time stamp to the obtained thermal image, the correlation can be partially made. The frame rate of thermal image acquisition can also be matched to the pulse rate of the generated laser beam. By thermography, a composite image can be determined as another representation of the target material. This may involve a determination of the quantitative thermal thickness reached by analyzing the thermal image. The change in quantitative heat wall thickness may indicate that there is a flaw located near the surface of the target material at a point where an unexpected change in quantitative heat wall thickness occurs. This information can be visualized by the contrast display, in which the sudden change in contrast represents a discontinuity, or variation, of the quantitative thermal wall thickness.

The generating laser beam may be a mid-IR ultrasonic generating laser. This generating laser provides a compact high-average power mid-IR laser to generate ultrasonic and thermal transients. As shown in FIG. 7, the generation laser 700 includes a pump laser head 702 having a fiber laser therein, wherein the fiber is a generation laser head ( 704). By using a fiber laser, the laser pump can be located remotely from the generating laser head 704. The pump laser head may be connected to the generating laser head 704 via an optical fiber 702.

By positioning the pump laser head 702 away from the generating laser beam delivery head 704, the overall payload is reduced, the stability requirements for the robotic system delivering the generated laser beam and obtaining thermal images are reduced. The compact mid-IR generating laser head is enabled. A compact light-weight module comprising a generating laser beam delivery head and an IR camera needs to be mounted in the inspection head of the robotic system. This allows the form of a mid-IR laser source using a smaller robot. Thus, using portable laser ultrasound systems and IR thermography systems, new complex inspection opportunities for in-field complex NDEs are created. This approach is described in US patent application No.1 " FIBER LASER TO GENERATE ULTRASOUND ", which is incorporated herein by reference.

In short, embodiments of the present invention provide an inspection system capable of inspecting the internal structure of a target material. Such inspection systems include generating lasers, ultrasonic detection systems, thermal imaging systems, and processor / control modules. The generating laser generates a pulsed laser beam capable of inducing ultrasonic displacement and thermal transients in the target material. The ultrasonic detection system detects ultrasonic surface displacements in the target material. The thermal imaging system detects thermal transients in the target material. The processor may analyze both the detected ultrasonic displacement portion and the thermal image of the target material to produce information about the internal structure of the target material.

The skilled artisan will appreciate that the term " substantially ", or " substantially ", when used herein, provides industry acceptable tolerances for corresponding terms. These industrially acceptable errors range from less than 1% to 20% and correspond to one or more of component values, integrated circuit process variations, temperature variations, rise and fall times, and thermal noise. As used herein, the term " functionally linked " as used herein includes, but is not limited to, a direct connection and an indirect connection through another component, element, circuit or module, , An element, a circuit, or a module does not modify the information in the signal, but one or more of the current level, the voltage level, and the power level may be adjusted. For those skilled in the art, logical connections (ie, connections in which one element is logically connected to another) include direct and indirect connections between two elements. The skilled artisan will appreciate that the term " preferred comparison " when used herein, indicates that a comparison between two or more elements, items, signals, etc. provides a desirable relationship. For example, when the preferred relationship is that signal 1 has a magnitude greater than signal 2, when the magnitude of signal 1 is greater than the magnitude of signal 2, or when the magnitude of signal 2 is less than the magnitude of signal 1, the desired comparison Can be obtained.

Claims (25)

A method for examining a target, the method comprising: Generating a ultrasonic displacement and thermal transient to the target by firing a generation laser beam onto the surface of the target, Detecting an ultrasonic displacement at the target, Obtaining a thermal image of the target, detecting a thermal transient at the target, Analyzing the ultrasonic displacement portion detected at the target, and Analyzing the detected thermal transient of the target, Obtaining an image frame of the target such that each frame has an array of pixels; Assigning each frame a frame number corresponding to the passage of time, and Analyzing the sequential frames of the thermal image to determine a quantitative thermal wall thickness. The method according to claim 1, And the target comprises a composite material. The method according to claim 1, Obtaining information about the deep internal structure of the target by analyzing the detected ultrasonic displacement; Obtaining information about the internal structure near the surface of the target by analyzing the detected thermal transients of the target; Correlating the information about the deep internal structure of the target with the information about the internal structure near the surface of the target. The method for testing a target, characterized in that it further comprises. The method according to claim 1, Matching the thermal image acquisition rate to a pulse rate of a generated laser beam The method for testing a target, characterized in that it further comprises. The method of claim 1, wherein quantitative thermal wall thickness is detected by analyzing a thermal image at the target. 6. The method of claim 5, wherein an unexpected change in the quantitative thermal wall thickness indicates a defect in the target at the unexpected change portion. delete The method of claim 1, wherein analysis of the thermal image at the target includes the use of infrared (IR) transient thermography. A test system for testing a target, the test system comprising: A generation laser for generating a pulsed laser beam to induce ultrasonic displacement and thermal transient at the target, An ultrasonic detection system for detecting an ultrasonic surface displacement at the target; A thermal imaging system for detecting a thermal transient at the target, and A processor for analyzing both the ultrasonic displacement portion detected at the target and the thermal image of the target to produce information about the internal structure of the target, And a control module for matching the thermal image frame acquisition rate to the pulse rate of the generated laser beam. The method of claim 9, wherein the ultrasonic detection system A detection laser for generating a detection laser beam for illuminating the ultrasonic surface displacement at the target, An acquisition optical element for collecting light phase-modulated by an ultrasonic surface displacement portion from the detection laser beam scattered at the target surface; An interferometer for processing the phase modulated light and generating one or more output signals; Processing unit processing one or more output signals to obtain data indicative of ultrasonic surface displacement at the target Inspection system comprising a. 10. The inspection system of claim 9, wherein the thermal imaging system comprises an infrared (IR) transient thermography system. 12. The inspection system of claim 11, wherein the IR transient thermography system comprises an IR sensitive camera for obtaining an image frame of a target illuminated by the generated laser beam. 10. The inspection system of claim 9, wherein the processor correlates the detected ultrasonic displacement with a thermal image. delete 10. The method of claim 9, And the target comprises a composite material. delete delete delete delete delete delete delete delete delete delete

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