Measurement of moisture in composite materials with near-IR and mid-IR spectroscopy

The disclosure relates to methods for determining moisture content of composite materials. More particularly, the disclosure relates to a method for accurately assessing moisture content in a composite material using near-infrared and mid-infrared spectroscopy.

Resin-fiber composite materials are utilized in a variety of applications including the aerospace industry, for example. Composite materials may absorb atmospheric moisture over time. The integrity of composite bonded repairs may be compromised as a result of absorbing moisture. Determining the quantity of moisture, which is present in a composite material, may facilitate determination of which steps are needed prior to bonding of the composite material.

Therefore, a method for accurately assessing moisture content in a composite material using field portable and user friendly near-infrared or mid-infrared spectroscopy is needed.

The present disclosure is generally directed to a method of determining moisture content of a composite material. An illustrative embodiment of the method includes obtaining a calibration for moisture content in a series of composite material samples using the IR spectra of infrared energy reflected from the composite material samples, providing a series of moisture standards in appropriate composite materials that have increasing steps of moisture content, performing the multivariate calibration of IR spectra to moisture content with appropriate pre-processing of the IR spectra as needed and then using that calibration to predict the moisture content in the same type of composite materials where the moisture content is not known.

The disclosure is further generally directed to a method of determining a physical property of a composite material (for example weight percent moisture and/or G1c value for bond strength by the double cantilever beam bond toughness test). An illustrative embodiment of the method includes obtaining a value for a physical property of a composite material sample as a function of moisture content of the composite material sample, obtaining a calibration between moisture content of the composite material sample and a spectrum of infrared energy reflected from the composite material sample, obtaining a calibration between the spectrum of infrared energy reflected from the composite material sample and the value for a physical property of the composite material sample, and using the calibration to predict the physical properties of composite materials in question from the IR spectra of those materials in question.

FIG. 1 is a flow diagram which illustrates an illustrative method of performing a calibration between spectra of near-infrared and mid-infrared energy and moisture content or mechanical properties of composite material samples exposed to various quantities of moisture over various time periods.

FIG. 2 is a flow diagram which illustrates an illustrative method for making a series of panels with increasing moisture content and obtaining the bond strength properties with increasing moisture content.

FIG. 3 is a flow diagram, which illustrates a way to validate the calibration model for moisture measurement in composite materials.

FIG. 4 is a graph, which illustrates near infrared spectra on 100 scans of moist and dry composite material tape using first derivative data with 5 smoothing points.

FIG. 5 is a graph, which illustrates mid infrared spectra of an average of 128 scans of moist and dry composite material tape using first derivative data with 7 smoothing points.

FIG. 6 is a graph, which illustrates baseline offset corrected mid infrared data on 128 averaged scans of moist and dry composite material tape.

FIG. 7 is a graph, which illustrates raw mid infrared data on 128 averaged scans of moist and dry composite material tape.

FIG. 8 is a flow diagram of an aircraft production and service methodology.

FIG. 9 is a block diagram of an aircraft.

The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word “exemplary” or “illustrative” means “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to make or use the invention and are not intended to limit the scope of the invention, which is defined by the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.

Referring initially to FIG. 1 , a flow diagram 100 which illustrates an illustrative method of obtaining a calibration between spectra of infrared energy and moisture content of composite material samples exposed to various quantities of moisture over various time periods is shown. In block 102 , composite material samples are provided with controlled moisture content information over the range of interest for good to poor bond strength results. The composite material samples may be 2-inch composite material squares, for example and without limitation. In block 104 , the composite material sample series is measured with the hand-held spectrometer that will be used to measure material in question later. This can be a spectra with the infrared spectra range only; such as near infrared or a mid infrared device for various materials and situations.

In block 106 , the infrared data is preprocessed with an appropriate algorithm to provide the best data for the calibration of spectral data to moisture content. In block 108 , the multivariate calibration is preformed. This of often a Partial Least Squares (PLS) regression of moisture content data to the infrared spectra. In block 110 , the calibration model is saved in an appropriate format and is loaded into the hand held device that will be used to make measurements of material in question. In block 112 the calibration model in the spectrometer is used to predict moisture content on materials in question using new spectra on those materials.

Referring next to FIG. 2 , a flow diagram 200 which illustrates an illustrative method of making moisture content standards for the multivariate calibration and prediction method of FIG. 1 . In block 202 , a set of dried composite panels is provided and carefully weighted. In block 204 , the dry standards are measured with a hand-held infrared spectrometer in order to obtain the dry material spectra for all the dry standards. In other applications, the surface of the composite material may be irradiated with near-spectrum or mid-spectrum infrared energy. In some applications, the infrared spectra may be obtained using a hand-held spectrometer system such as that which is available from the Polychromix corp. or A2 Technologies, for example and without limitation.

In block 206 , the dry standards are put into a heat and humidity chamber to increase their moisture content. In block 208 , the standards are weighed periodically until an increase of about 0.2%. With each increase of 0.2% weight a pair of standards are measured with a hand-held spectrometer and then the pair are bonded together in order to test the bond strength at the increased moisture content level. In block 210 , the bonded panels are cut to an appropriate size for the bond strength test and the bond strength test is performed. In block 212 steps 208 and 210 are repeated until there is no further weight change or until the bond strength test results are consistently poor.

Referring next to FIG. 3 , a flow diagram 300 , which illustrates an illustrative method for validating the multivariate model that was made for moisture content measurement is shown. In block 302 , the methods shown in FIG. 1 and the standards made in FIG. 2 are used to make an appropriate model of moisture content or reduction of bond strength with increasing moisture content versus changes in the infrared' spectra. In block 304 , a new set of standards is created using the same method that was illustrated in FIG. 2 and this new set panels is predicted for moisture content and bond strength with the method created in FIG. 1 . In block 306 , the multivariate model is validated by calculating the root mean square error of prediction from the prediction results in step 304 and the actual moisture content by weight percent increase. In block 308 the model is optimized by using various data preprocessing and calibration methods to obtain the lowest possible prediction error in this validation step.

Referring next to FIGS. 4-7 , graphs which illustrate infrared spectra obtained from composite materials having various degrees of moisture content are shown. In FIG. 4 , a graph, which illustrates near infrared spectra on 100 scans of moist and dry composite material tape using first derivative data with 5 smoothing points is shown. In FIG. 5 , a graph, which illustrates mid infrared spectra of an average of 128 scans of moist and dry composite material tape using first derivative data with 7 smoothing points is shown. In FIG. 6 , a graph, which illustrates baseline offset corrected mid-TR data on 128 averaged scans of moist and dry composite material tape is shown. In FIG. 7 , a graph, which illustrates raw mid-IR data on 128 averaged scans of moist and dry composite material tape is shown.

Referring next to FIGS. 8 and 9 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method 78 as shown in FIG. 8 and an aircraft 94 as shown in FIG. 9 . During pre-production, exemplary method 78 may include specification and design 80 of the aircraft 94 and material procurement 82 . During production, component and subassembly manufacturing 84 and system integration 86 of the aircraft 94 takes place. Thereafter, the aircraft 94 may go through certification and delivery 88 in order to be placed in service 90 . While in service by a customer, the aircraft 94 may be scheduled for routine maintenance and service 92 (which may also include modification, reconfiguration, refurbishment, and so on).

Each of the processes of method 78 may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on.

As shown in FIG. 9 , the aircraft 94 produced by exemplary method 78 may include an airframe 98 with a plurality of systems 96 and an interior 100 . Examples of high-level systems 96 include one or more of a propulsion system 102 , an electrical system 104 , a hydraulic system 106 , and an environmental system 108 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry.

The apparatus embodied herein may be employed during any one or more of the stages of the production and service method 78 . For example, components or subassemblies corresponding to production process 84 may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft 94 is in service. Also one or more apparatus embodiments may be utilized during the production stages 84 and 86 , for example, by substantially expediting assembly of or reducing the cost of an aircraft 94 . Similarly, one or more apparatus embodiments may be utilized while the aircraft 94 is in service, for example and without limitation, to maintenance and service 92 .

Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.