Patent Description:
New product designs and manufacturing methods of structures based on polymer matrix composites are becoming more prevalent. However, composite structures may have an unacceptable level of volumetric porosity and undetected porosity and defects can lead to early failure of components.

One known method for measuring porosity in composite structures is the use of acid digestion. With acid digestion, the weight percent of matrix material and fiber material are measured separately by using acid to dissolve one of the constituents. Using these data plus mass density information for the separate materials, the percent porosity can easily be determined. However, acid digestion methods are destructive because the composite must be dissolved to measure the volume of porosity. Acid digestion is valuable as a process control tool where either entire parts or sections of parts can be sacrificed to measure the capability of the manufacturing process. For some components, however, this destructive testing method is inadequate since actual structures should be measured.

Some known methods for measuring porosity in composite structures made of fiberglass are based on the observation that pores in a composite structure cause a frequency shift in sound traveling through the structure. These known methods use multiple ultrasonic transducers to measure through transmission through a composite article in an immersion tank to determine sound attenuation to estimate the porosity content in composites. In general, these known methods require precision scanning of two transducers in an immersion tank collecting data at a plurality of frequencies. To collect the ultrasonic information needed to analyze porosity would require two or more scans of the part depending on the attenuation slope calculation method used, a serious limitation to manufacturing productivity. Additionally, two transducers are required for these measurements with their positioning axes. However, most immersion tanks designed for such inspection have only one transducer manipulator.

Another known method for measuring porosity of composite structures made of resin infused fiberglass uses a single transducer to determine sound attenuation by the composite structure in an immersion tank. In this method, sound reflected off the front wall of a composite structure in an immersion tank is compared with sound reflected off the back wall the immersion tank after passing through the composite structure. This known method typically uses a <NUM> frequency transducer.

When composite structures are made of materials other than fiberglass, some of the known methods are not as efficacious. When composite structures are very large, testing the large structure in existing immersion tanks according to the known methods may be impractical or impossible.

Various methods and apparatus for inspecting workpieces and assessing porosity thereof are known, for example, from <CIT>, <CIT>, <NPL>, <CIT>, <CIT>, and <NPL>. <CIT> relates to a method for non-destructively inspecting a composite structure with a single ultrasonic transducer, said method comprising scanning the composite structure with the single ultrasonic transducer to measure ultrasonic amplitudes for sound waves traveling through the composite structure, reflecting off the reflector plate and then traveling back through the structure to the single ultrasonic transducer; correcting the measured ultrasonic amplitudes using the calibration amplitude and other measured transmission losses; and utilizing the corrected ultrasonic amplitudes to generate at least one of a digital image showing porosity or a porosity measurement of the composite structure.

Various aspects and embodiments of the present invention are defined by the appended claims.

The present invention will now be described in connection with the accompanying drawings, in which:.

Various technical effects of the present invention include the non-destructive measurement of porosity of a composite structure, and the generation of a digital image showing cross-sectional physical characteristics of the composite structure, and may include the non-destructive detection of defects of the composite structure.

Unlike some known methods and systems for porosity determination, configurations of the present disclosure do not use an immersion tank and do not use through transmission of sound waves. Tank-less porosity measurement configurations of the present disclosure measure attenuation of the sound waves reflected back from a composite structure at which the sound waves were directed.

Porosity content of a composite structure has an effect on frequency of a sound wave as the sound travels through the composite structure. Higher frequencies generally attenuate faster as sound travels through a material, resulting in a downward shift of the peak frequency of a sound wave passing through the material. In a highly attenuative material, such as a carbon fiber composite structure, it may be beneficial to separate the attenuation of a sound wave due to scatter from the attenuation due to porosity. This can be accomplished because porosity is typically less than one wavelength in diameter whereas other scatter modes are typically greater than one wavelength in size. Scatter mechanisms greater than one wavelength in size work to either absorb or deflect energy whereas porosity less than one wavelength will work to attenuate higher frequencies and shift the returned beam frequency down. Thus, for a given ultrasonic frequency, a composite structure with a greater porosity will produce a greater attenuation of the ultrasonic sound waves. Similarly, a composite structure with a given porosity will produce greater attenuation of higher frequency ultrasonic sound waves than lower frequency ultrasonic sound waves.

Determining the attenuation of a material can be very difficult. In some embodiments, an ultrasonic system including an ultrasonic transducer is first calibrated with a standard of a known attenuation, wherein examples without an ultrasonic phased array probe are not covered by the claims. The standard should typically have characteristics similar to the composite structure to be inspected. An acrylic glass standard, such as polymethyl methacrylate (PMMA) for example, has approximately the same acoustic velocity as carbon fiber composites and is highly attenuative, as are carbon fiber composites. Accordingly, in some embodiments, an acrylic glass standard of a known attenuation is used to calibrate the ultrasonic transducer prior to inspecting a carbon fiber composite structure. In other embodiments, a section of the actual production part to be inspected is used for ultrasonic calibration. The production process for composite structures often results in end products whose acoustic properties vary greatly, causing analysis to be difficult. By using the actual production material to calibrate the ultrasonic transducer, the variation may be significantly mitigated.

<FIG> graphically presents the results of ultrasonic inspection of three composite structures using the calibration with a known standard described above. Each composite structure had a known porosity determined by the acid digestion method. The composite structures were carbon fiber composite structures having porosities of <NUM>%, <NUM>% and <NUM>%. Each structure was scanned three times, each time using a different transducer. Specifically, the three transducers were a <NUM> transducer, a <NUM> transducer, and a <NUM> transducer. The standard was a <NUM> (one inch) thick piece of acrylic glass.

<FIG> plots the attenuation of each composite structure acquired using this method as a function of the frequency of the reflected sound wave. The relationship between attenuation and frequency for a composite structure is a natural logarithmic relationship. This relationship may be approximated, however, by a linear fit. As can be seen in <FIG>, when approximated by a linear fit, the porosity content of a composite will affect the slope of the line representing the frequency of the ultrasonic beam. Specifically, the greater the porosity, the greater the slope of the line.

Data acquired using this calibration method, such as that used to generate the plot in <FIG>, may be used to derive a functional relationship between attenuation of ultrasonic sound waves by a composite structure, the returned frequency of ultrasonic sound waves, and porosity content of the composite structure. The measured attenuation of each composite structure is divided by the returned frequency for that measurement. This results in an attenuation slope. <FIG> plots porosity as a function of the attenuation slope of the measurements plotted in <FIG>. Regression analysis may then be performed to determine a linear equation describing this relationship. The resulting equation has the form of: <MAT> <MAT> is the first order derivative of attenuation, or more simply the slope of the linear fit line describing the relationship between attenuation and returned frequency. If it is assumed that attenuation is zero at a zero MHz, <MAT> is the attenuation of a sound wave divided by the returned frequency. Thus, porosity percentage may be determined with a single measurement, at a single frequency.

Coefficient is a scaling term, and
Offset is a fitting term which is equal to the porosity value for zero attenuation slope measurements.

The data described above with respect to <FIG> was subjected to such a regression analysis and yielded a Coefficient value of <NUM> and an Offset value of <NUM>. These constants are generally only valid for the particular ultrasonic transducer used to collect the data. The derivation of the Coefficient and the Offset will typically need to be repeated for each particular transducer to be used.

<FIG> illustrates a block diagram of a tank-less method <NUM> for non-destructively inspecting a composite structure with an ultrasonic system including an ultrasonic transducer, wherein examples without an ultrasonic phased array probe are not covered by the claims. The ultrasonic system is calibrated <NUM> on a standard having a known attenuation. A composite structure is scanned <NUM> with the ultrasonic transducer to measure an ultrasonic amplitude and a frequency for sound waves reflected by the composite structure to the ultrasonic transducer. The measured ultrasonic amplitude is utilized <NUM> to determine an attenuation of sound waves reflected by the composite structure. The porosity percentage of the composite structure is determined <NUM> as a function of the determined attenuation and the measured frequency.

An example inspection system <NUM> suitable for performing the method <NUM> is illustrated in <FIG>. Examples without an ultrasonic phased array probe, as illustrated in <FIG>, are not covered by the claims. System <NUM> includes an ultrasonic transducer system <NUM> having an ultrasonic transducer <NUM> configured to transmit and receive ultrasonic sound waves. Ultrasonic transducer system <NUM> includes electronic equipment <NUM> configured to generate and amplify the ultrasonic sound waves. System <NUM> includes a computer-implemented data collection system <NUM> having a computer <NUM> configured to collect ultrasonic information and memory device <NUM>. Although illustrated separately from computer <NUM>, memory device <NUM> may be a part of computer <NUM>.

Memory device <NUM> is one or more devices that enable information such as executable instructions and/or other data to be stored and retrieved. Memory device <NUM> may include one or more computer readable media, such as, without limitation, dynamic random access memory (DRAM), static random access memory (SRAM), a solid state disk, and/or a hard disk. Memory device <NUM> may be configured to store, without limitation, computer-executable instructions, ultrasonic sound wave data, digital images, algorithms, scanning parameters, and/or any other type of data.

Ultrasonic transducer <NUM> generates ultrasonic sound waves at a single frequency. This single frequency may be any suitable frequency capable of sufficient penetration of the composite structure and providing a sufficient back reflection to be received by ultrasonic transducer <NUM>. Typically, the single frequency is less than the frequency used in immersion tank environments. For example, the single frequency may be between about <NUM> and about <NUM>. In some embodiments, the single frequency is between about <NUM> and about <NUM>. In some embodiments, the single frequency is about <NUM>.

In operation, a user first calibrates ultrasonic transducer system <NUM> using a standard of a known attenuation. The user then places ultrasonic transducer <NUM> adjacent a composite structure to be inspected. Electronic equipment <NUM> causes an ultrasonic sound wave to be transmitted toward the composite structure through ultrasonic transducer <NUM>. The ultrasonic sound wave passes through the composite structure and is reflected off the back wall of the composite structure. Ultrasonic transducer <NUM> receives the back reflected sound wave. Information about the transmitted and received sound wave may be analyzed when acquired and/or may be stored in data collection system <NUM> for later analysis. Data collection system <NUM> may store raw data and/or processed data concerning the received sound waves.

System <NUM> measures the frequency of the reflected sound wave and utilizes the transmitted and received ultrasonic sound waves to determine the attenuation of the ultrasonic sound wave. In some embodiments, data collection system <NUM> measures an ultrasonic amplitude and a frequency for sound waves reflected by the composite structure. Data collection system <NUM> utilizes the measured ultrasonic amplitude to determine the attenuation of the sound waves reflected by the composite structure. In some embodiments, electronic equipment <NUM> performs one or more of the measuring and determining.

Data collection system <NUM> determines a porosity percentage for the composite structure as a function of the determined attenuation and the measured frequency. Specifically, formula [<NUM>] discussed above, along with the appropriately determined Coefficient and Offset, is utilized to determine the porosity percentage as a function of the determined attenuation and the measured frequency. As discussed above, if an attenuation of zero is assumed for a frequency of zero, the determined attenuation for a single measurement divided by the measured frequency is equal to <MAT>. Thus, data collection system <NUM> can determine the porosity percentage by dividing the determined attenuation (in dB/cm) by the returned frequency (in MHz), substituting the result into equation [<NUM>] as <MAT>, and solving the equation.

After the porosity percentage is determined, it may be compared to a reference value. The comparison may be performed manually and/or automatically by an operator and/or data collection system <NUM>. Thus, the determined porosity percentage may be used to determine whether or not the composite structure under inspection meets one or more standard, such as a quality control standard, defined by the reference value. A particular reference value may be applicable to an entire composite structure or only to one or more portion of the composite structure. Accordingly, a composite structure may be subject to one or more porosity threshold at one or more portion of the composite structure.

Sometimes, it may be desirable to determine the porosity percentage of a composite structure at more than one location of the composite structure. In such instances, a user may calibrate ultrasonic transducer system <NUM> using a standard of a known attenuation. The user then places ultrasonic transducer <NUM> adjacent a composite structure at a first position to be inspected. After the ultrasonic sound wave is transmitted and received, system <NUM> determines the porosity percentage at the first position as discussed above. The user may then move ultrasonic transducer <NUM> to a second position to be inspected and the process is repeated. The process may be repeated as many times as desired at as many positions as desired to determine the porosity percentage of the composite structure at the multiple positions. Information about the transmitted and received sound waves may be analyzed when acquired and/or may be stored in data collection system <NUM> for later analysis.

In the embodiment of <FIG>, system <NUM> includes a scanning system <NUM>. Scanning system <NUM> positions ultrasonic transducer <NUM> relative to the composite structure to be inspected. Scanning system <NUM> may move ultrasonic transducer <NUM>, ultrasonic transducer system <NUM>, or system <NUM>, relative to a fixed composite structure to position ultrasonic transducer <NUM> relative to the composite structure. Alternatively, or additionally, scanning system <NUM> may move a composite structure relative to a fixed ultrasonic transducer <NUM>, ultrasonic transducer system <NUM>, or system <NUM> to position ultrasonic transducer <NUM> relative to the composite structure. The positioning of the ultrasonic transducer via scanning system <NUM> may be selected manually by a user or may be automatically performed according to instructions stored in data collection system <NUM>.

Scanning system <NUM> permits automated scanning of a composite structure with system <NUM>. Scanning system <NUM> is controlled by data collection system <NUM> to position ultrasonic transducer <NUM> at a first position relative to the composite structure to determine the porosity percentage of the composite structure at the first position. Scanning system <NUM> may then reposition ultrasonic transducer <NUM> at a second position relative to the composite structure to determine the porosity percentage of the composite structure at the second position. The process may be repeated as many times as desired at as many positions as desired to determine the porosity percentage of the composite structure at the multiple positions. Information about the transmitted and received sound waves may be analyzed when acquired and/or may be stored in data collection system <NUM> for later analysis. If a composite structure is large, it may be desirable to inspect the composite structure at multiple locations along its length and/or width. Inspecting the composite structure to determine the porosity percentage at many locations along the composite structure may take a significant amount of time. Accordingly, automated scanning with system <NUM> may save time and/or permit a user to perform other tasks while the scanning is being performed.

In addition to determination of the porosity percentage of a composite structure, it is sometimes desirable to inspect a composite structure for defects, characteristics, etc. of the composite structure. As shown in <FIG>, a tank-less method <NUM> of inspecting a composite structure includes scanning <NUM> a composite structure with an ultrasonic phased array probe to measure a plurality of ultrasonic sound waves reflected by the composite structure to the ultrasonic phased array probe. The measured ultrasonic sound waves are utilized <NUM> to generate a digital image showing cross-sectional physical characteristics of the composite structure.

An example inspection system <NUM> suitable for performing method <NUM> is illustrated in <FIG>. System <NUM> includes an ultrasonic transducer system <NUM> having an ultrasonic phased array probe <NUM> configured to transmit and receive ultrasonic sound waves. Ultrasonic transducer system <NUM> includes electronic equipment <NUM> configured to generate and amplify the ultrasonic sound waves. System <NUM> includes computer-implemented data collection system <NUM> having computer <NUM> configured to collect ultrasonic information and memory <NUM>. System <NUM> includes a display device <NUM> for displaying an image generated by data collection system <NUM>.

Ultrasonic phased array probe <NUM> includes an array of ultrasonic transducers <NUM>. In one example embodiment, ultrasonic phased array probe <NUM> includes sixty-four ultrasonic transducers <NUM>. In other embodiments, ultrasonic phased array probe <NUM> may include an array of more or fewer ultrasonic transducers <NUM>. Use of ultrasonic phased array probe <NUM> instead of a single element probe may provide more directive energy than a single transducer probe, allowing ultrasonic phased array probe <NUM> to perform better with highly attenuative composite structures, such as carbon fiber composite structures.

Ultrasonic phased array probe <NUM> generates, via ultrasonic transducers <NUM>, ultrasonic sound waves at a single frequency, particularly phase shifted ultrasonic sound waves. This single frequency may be any suitable frequency capable of sufficient penetration of the composite structure and providing a sufficient back reflection to be received by ultrasonic transducers <NUM>. Typically, the single frequency is less than the frequency used in immersion tank environments. For example, the single frequency may be between about <NUM> and about <NUM>. In some embodiments, the single frequency is between about <NUM> and about <NUM>. More specifically, the single frequency is about <NUM>.

System <NUM> includes scanning system <NUM>, although other embodiments may not include scanning system <NUM>. Scanning system <NUM> positions ultrasonic phased array probe <NUM> relative to the composite structure to be inspected. Scanning system <NUM> may move ultrasonic phased array probe <NUM>, ultrasonic transducer system <NUM>, or system <NUM>, relative to a fixed composite structure to position ultrasonic phased array probe <NUM> relative to the composite structure. Alternatively, or additionally, scanning system <NUM> may move a composite structure relative to a fixed ultrasonic phased array probe <NUM>, ultrasonic transducer system <NUM>, or system <NUM> to position ultrasonic phased array probe <NUM> relative to the composite structure. The positioning of ultrasonic phased array probe <NUM> via scanning system <NUM> may be selected manually by a user or may be automatically performed according to instructions stored in data collection system <NUM>.

In operation, ultrasonic phased array probe <NUM> is positioned by scanning system <NUM> at a first position adjacent a composite structure to be inspected. Electronic equipment <NUM> causes ultrasonic transducers <NUM> to transmit phase shifted sound waves toward the composite structure. The ultrasonic sound waves pass through the composite structure and are reflected off the back wall of the composite structure. The ultrasonic sound waves may be scattered (e.g., deflected, absorbed, etc.) by defects in the composite structure being inspected. Ultrasonic phased array probe <NUM> receives the reflected sound waves. Information about the transmitted and received sound waves may be analyzed when acquired and/or may be stored in data collection system <NUM> for later analysis. Data collection system <NUM> may store raw data and/or processed data concerning the received ultrasonic sound waves.

The system <NUM> utilizes the ultrasonic sound wave data to generate a digital image showing cross sectional physical characteristics of the composite structure. In some embodiments, the digital image is displayed on a display device <NUM>. In some embodiments, the digital image is stored by data collection system <NUM> for later analysis, display, etc. An example of such a digital image <NUM> is shown in <FIG>. A width <NUM> of image <NUM> corresponds to the width of ultrasonic phased array probe <NUM>. A height <NUM> (or thickness) of the composite structure is the distance between a front wall reflection <NUM> from the composite structure and a back wall reflection <NUM> from the composite structure. A defect <NUM> is visible within the composite structure. In this instance, defect <NUM> is a delamination, e.g. two or more plies of carbon fiber that are not bonded together. In addition to indicating the presence of defect <NUM>, the location and approximate dimensions of defect <NUM> may be estimated from image <NUM>. Further, system <NUM> may utilize the stored ultrasonic sound wave data to determine the position of defect <NUM> within the composite structure and/or the dimensions of defect <NUM>. An example digital image <NUM> is shown in <FIG>. A defect <NUM> is visible within the composite structure. In this instance, defect <NUM> is a wrinkle, e.g. two or more plies of carbon fiber that are not perfectly flat.

Scanning system <NUM> permits automated scanning of a composite structure with system <NUM>. Scanning system <NUM> is controlled by data collection system <NUM> to position ultrasonic transducer array <NUM> at a first position relative to the composite structure to scan with ultrasonic sound waves from the ultrasonic transducer array <NUM>. Scanning system <NUM> may then reposition ultrasonic transducer <NUM> at a second position relative to the composite structure. This process may be repeated as many times as desired, at as many positions as desired. Information about the transmitted and received sound waves may be analyzed when acquired and/or may be stored in data collection system <NUM> for later analysis. If a composite structure is large, it may be desirable to inspect the composite structure at multiple locations along its length and/or width. Inspecting the composite structure to determine the porosity percentage at many locations along the composite structure may take a significant amount of time. System <NUM> may scan a composite structure at many locations automatically to acquire ultrasonic sound wave data. A user can review the data, and/or the digital images generated from such data contemporaneously with the scan or after a scan of the entire composite structure is complete. Accordingly, automated scanning with system <NUM> may save time and/or permit a user to perform other tasks while the scanning is being performed.

System <NUM> is used to determine porosity percentage of a composite structure. The embodiments acquire ultrasonic data for determining porosity percentage while also acquiring ultrasonic data for generating digital data showing cross-sectional characteristics of a composite structure being inspected. In some such embodiments, the entire ultrasonic transducer array <NUM> may be used or less than all ultrasonic transducers <NUM> of ultrasonic transducer array <NUM> may be used to acquire data for determination of the porosity percentage of the composite structure being inspected. In other embodiments, such as illustrated in <FIG>, a system <NUM> may include ultrasonic transducer system <NUM> and ultrasonic transducer system <NUM>.

Thus, it has been shown that various configurations of the present invention provide nondestructive, tank-less methods for inspecting composite structures during the manufacturing process and that this method is advantageous for designing these components. Configurations of the present invention use only one scan for porosity determinations instead of multiple scans as was utilized for some prior art techniques. Configurations of the present invention do not utilize an immersion tank, thereby reducing costs and allowing inspection of materials too large for many existing immersion tanks. Method configurations of the present invention are relatively simple and straightforward and utilize skills that most ultrasonic inspectors possess.

Claim 1:
A method (<NUM>, <NUM>) for non-destructively inspecting a composite structure not using an immersion tank with an ultrasonic system (<NUM>, <NUM>) including an ultrasonic probe (<NUM>, <NUM>), said method comprising:
scanning (<NUM>, <NUM>) the composite structure with an ultrasonic phased array probe (<NUM>) of the ultrasonic system (<NUM>, <NUM>) by using a single frequency of sound waves emitted by the ultrasonic phased array probe to measure a plurality of ultrasonic sound waves reflected by the composite structure to the ultrasonic phased array probe (<NUM>),
wherein said scanning (<NUM>, <NUM>) the composite structure comprises scanning (<NUM>) the composite structure with the ultrasonic system (<NUM>, <NUM>) to measure an ultrasonic amplitude and a frequency for sound waves reflected by the composite structure to the ultrasonic array probe (<NUM>), and wherein the sound waves reflected by the composite structure comprise a back reflection from the composite structure;
utilizing (<NUM>) the measured amplitude of the measured ultrasonic sound waves to determine an attenuation for sound waves reflected by the composite structure; and
determining (<NUM>, <NUM>) a physical characteristic of the composite structure based at least in part on the measured ultrasonic sound waves, wherein determining a physical characteristic comprises
determining (<NUM>) a porosity percentage for the composite structure as a function of the determined attenuation and the measured frequency of the ultrasonic sound waves reflected by the composite structure, and
utilizing the measured ultrasonic sound waves to generate a digital image (<NUM>, <NUM>) showing cross-sectional physical characteristics of the composite structure.