WO2005050178A1 - Method for measurement of composite heat damage with infrared spectroscopy - Google Patents

Method for measurement of composite heat damage with infrared spectroscopy Download PDF

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Publication number
WO2005050178A1
WO2005050178A1 PCT/US2004/031348 US2004031348W WO2005050178A1 WO 2005050178 A1 WO2005050178 A1 WO 2005050178A1 US 2004031348 W US2004031348 W US 2004031348W WO 2005050178 A1 WO2005050178 A1 WO 2005050178A1
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WIPO (PCT)
Prior art keywords
infrared
absorbance
wavenumber
around
determining
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PCT/US2004/031348
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French (fr)
Inventor
Paul H. Shelley
Jeffrey R. Kollgaard
Diane R. Lariviere
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The Boeing Company
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Publication of WO2005050178A1 publication Critical patent/WO2005050178A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/55—Specular reflectivity
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3563—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing solids; Preparation of samples therefor

Definitions

  • FIELD OF THE INVENTION This invention relates generally to measurement, and, more specifically, to measurement of heat damage to materials.
  • Resin-fiber composites are utilized in a variety of applications, such as parts of vehicles, including aircraft. Vehicles, equipment, and structures constructed utilizing resin-fiber composites may be exposed to heat in a variety of circumstances, ranging from fire to lightening strikes to weapons impacts. Heat exposure to resin-fiber composite affects the composites in a variety of ways. Chemical degradation may occur involving changes in the polymer chemical structure. This may include oxidation, material loss and either the breaking or forming of additional chemical links. Resin decomposition, charring, and ultimately fiber decomposition may occur at increasing temperatures.
  • the present invention provides a non-destructive method for efficiently and objectively determining the degree of heat exposure to a resin-fiber composite material.
  • the present invention may be utilized to quantitatively determine the degree of heat exposure of a composite.
  • a value of infrared energy reflected by a surface on a composite substrate is determined.
  • the value of infrared energy reflected, or conversely absorbed, is correlated to a degree or amount of heat exposure.
  • an infrared spectrometer is utilized to determine infrared absorbance at appropriate wavelengths of a composite substrate.
  • the infrared energy of the reflected beam is then compared with the pre- determined value of infrared energy reflected off a reference heat-damaged composite substrate to determine the amount of heat exposure.
  • a difference between infrared energy absorbance of a heat damaged composite substrate at two wavelengths is compared with a reference surface.
  • FIGURE 1 is a cross section of an exemplary heat damage measurement device in accordance with the present invention
  • FIGURE 2 is a cross section of an exemplary heat damage measurement device utilizing attenuated total reflectance in accordance with the present invention
  • FIGURE 3 is a graph of infrared absorbance parallel and perpendicular to fibers in a composite substrate
  • FIGURE 4 is a graph of infrared absorbance of a first exemplary resin-fiber composite
  • FIGURE 5 is a graph of infrared absorbance of a first exemplary resin-fiber composite
  • FIGURE 6 is a graph of absorbance differences for a first exemplary resin-fiber composite exposed to heat
  • FIGURE 7 is a graph of collected absorbance for a first exemplary resin-fiber composite exposed to heat
  • FIGURE 8 is a graph of corrected absorbance for a first exemplary resin-fiber composite exposed to heat
  • FIGURE 9 is a graph of
  • a non-destructive method for determining an amount of heat exposure to a resin-fiber composite substrate.
  • a value I s of infrared energy reflected by a surface on a composite substrate is determined at appropriate wavelengths.
  • the value I s of infrared energy reflected by the composite substrate correlates to an amount of heat exposure.
  • absorbance varies with heat exposure and thus infrared absorbance can be correlated to heat exposure to resin-fiber composites.
  • FIGURE 1 is a cross section drawing of an exemplary testing device 20 used to determine the amount of heat exposure to a resin-fiber composite substrate according to a presently preferred embodiment of the invention.
  • An infrared transmission beam 15 is.
  • the infrared source 32 suitably may include a multi-frequency infrared source 32 and the infrared detector 34 may include a single or multi-frequency infrared detector 34.
  • the infrared detector detects infrared energy passing through a filter 36 removeably held by a filter holder 38. This permits the filter 36 to be switched to allow use of the testing device 20 at different infrared frequencies.
  • the infrared levels received by the infrared detector 34 are output as an electrical signal to a processor display 40 through a conductor 35.
  • the infrared source 32 and the infrared detector 34 suitably include a hand-held infrared filter spectrometer such as the SOC410, manufactured by Surface Optics Corporation. In alternative embodiments, an infrared Fourier transform imaging spectrometer, or a portable infrared spectrometer may be utilized.
  • the infrared source 32 and infrared detector 34 are suitably enclosed within a housing 44 with feet 37. The feet 37 rest against the substrate 10 when a sample is tested.
  • the exemplary testing device 20 shown in FIGURE 1, by way of example and not limitation, also collects diffuse reflected infrared radiation diffusely reflected by the substrate 10.
  • the diffuse reflected infrared energy 18 is collected by an ellipsoidal mirror 30 and directed towards the infrared detector 34. It will be appreciated that embodiments of the present invention may suitably utilize specular (i.e., direct reflectance), diffuse reflectance, or alternative collectors, such as an attenuated total reflectance collector as described in reference to FIGURE 2 below.
  • specular i.e., direct reflectance
  • diffuse reflectance or alternative collectors, such as an attenuated total reflectance collector as described in reference to FIGURE 2 below.
  • the feet 37 of the testing device 20 hold the infrared source 32 and the infrared detector 34 at a predetermined distance and position relative to the surface 10. The mobility of the testing device 20 permits additional measurement of adjoining areas and other samples with comparable results.
  • the testing device 20 of FIGURE 1 may be utilized to non-destructively determine the degree or the amount of heat exposure of the substrate 10.
  • the infrared absorbance values of heat exposed reference composite samples are determined by measuring the I r of infrared energy reflected from a reference composite surface.
  • Absorbance A is calculated as -log 10 (I/I 0 ) where I 0 is the value of infrared energy reflected by a base material, often gold, and I is the value of infrared energy reflected by the sample under study.
  • the device 20 is then used to transmit the infrared beam 15 to the substrate 10 to be tested, and a value I s of infrared energy reflected by the substrate 10 is measured.
  • FIGURE 2 is a cross section of an alternative exemplary testing device 60 that may be utilized to determine the amount of heat damage to a resin-fiber composite substrate 10 according to an alternate embodiment of the invention.
  • An infrared beam 15 is transmitted by an infrared source 32.
  • the beam 15 is transmitted through a crystal 70 placed against the substrate 10.
  • the beam 15 is reflected off the lower surface of the crystal 70, but due to an interaction of the infrared energy with the crystal 70 in contact with the substrate 10, a portion of the infrared energy is absorbed.
  • the reflected attenuated beam 16 is the detected by an infrared detector 74.
  • the use of a crystal 70 in this manner is referred to as attenuated total reflectance (ATR).
  • the reflected beam 16 passes through a filter 76 removeably held in a filter holder 78.
  • An infrared detector 74 detects the infrared energy of the reflected beam at the frequency passed by the filter 76.
  • a number of suitable filters may be successively placed in the filter holder 78, thereby permitting measurement of the attenuated total reflectance of the substrate 10 at a variety of frequencies.
  • Output from the infrared detector 74 is transmitted through a conductor 75 to a processor 80 for recording or analysis.
  • the infrared source 72, crystal 70, and collector 74 are suitably enclosed in a housing 84.
  • the housing 84 may be successively placed against the substrate 10 thereby permitting measurements to be taken at a variety of locations and surfaces.
  • the device 60 thus may detect attenuated infrared reflectance suitably utilizing one or more narrow pass filters 76, and a broad band infrared source 72.
  • an infrared spectrometer may be incorporated in the device 60 to permit measurement of attenuated total reflectance of the substrate 10 over a continuous frequency range.
  • the reflectance at infrared frequencies by composite substrates may vary depending upon the orientation of the reflected infrared beam with respect to alignment of the reinforcing fibers in the resin-fiber composite being measured.
  • FIGURE 3 is a graph of different reflectances resulting from differences in fiber orientation.
  • the spectra taken with reflectance generally in alignment with the fibers in the resin-fiber composite show more pronounced absorbance peaks than the spectra (perpendicular 1 and perpendicular 2) taken generally oblique or perpendicular to the fiber alignment in the fiber metal composite.
  • the absorbances of the parallel spectra, parallel 1 and parallel 2 are less than that of the perpendicular 1 and perpendicular 2 spectra by an absorbance amount of approximately 0.3. It will be appreciated that at varying angles to the fiber alignment in the fiber metal composite, substantial variability of specular reflectance of infrared energy may be obtained even for the same sample at the same frequency.
  • reflectance is measured in a direction in alignment with the fibers of the resin-fiber composite. It will be appreciated that when the surface coatings or contamination of a composite surface are removed for infrared measurements, the fibers are typically visible. Thus, in non-destructive testing of the substrate an operator may conveniently align the measurement device (not shown) to perrnit the reflectance measurement to be made parallel to the fiber alignment, suitably facilitating uniformity in measuring comparable surfaces.
  • FIGURE 4 is a graph of six infrared absorbance specra over a range of wavenumbers from six resin-fiber composite substrates (not shown) exposed to various amounts of heat. Absorbance spectra A-F show absorbance versus wavenumbers from approximately 4600 to approximately 400 cm-1.
  • the resin-fiber substrate tested here includes the first exemplary resin-fiber composite BMS 8-212 with no heat exposure for spectrum A, with 200 degrees heat exposure for 1 hour for spectrum B, 300 degrees for 1 hour for spectrum C, 400 degrees for 1 hour for spectrum D, 500 degrees for 1 hour for spectrum E, and 600 degrees for 1 hour for spectrum F. All of the specra show absorbance peaks near 3500 wavenumbers, 3000 wavenumbers, 1600 wavenumbers, and 1300 wavenumbers. However, variability of the spectra is not always linear, with heat exposure at specific wavenumbers not always directly correlated with heat exposure. In FIGURE 5, three of the spectra of FIGURE 4 are shown, and three are deleted, for clarity.
  • Spectrum B shows heat exposure of 200 degrees for 1 hour
  • spectrum D shows 400 degree exposure for 1 hour
  • spectrum F shows 600 degrees exposure for 1 hour for the same first exemplary resin-fiber composite BMS 8-212 over the same range of wavenumbers as FIGURE 4.
  • reference lines at 6.57 ⁇ m or 1522 wavenumbers, and 5.96 ⁇ m or 1678 wavenumbers are shown.
  • 5.96 ⁇ m absorbance increases with heat exposure while at 6.57 ⁇ m absorbance is relatively constant. Similar correlations may be made at different wavenumbers as well, as discussed in connection with FIGURES 6, 7 and 8 below. This permits correlation of infrared absorbance at single and multiple wavenumbers with heat exposure of the resin-fiber composite.
  • composite heat damage to the exemplary resin-fiber composite BMS 8-212 may be indicated subtracting absorbances at specified wavenumbers. For example, damage may be indicated when absorbance at 1678 cm-1 subtracted from absorbance at 1522 cm-1 equals less than around 0.2, or becomes negative.' Temperature exposures of over 300 degrees show a difference of less than .1 and becoming negative as exposure to temperatures increases from 400 to 600 degrees.
  • heat damage to the exemplary composite BMS 8-212 may be indicated by corrected absorbance values at 1678 cm-1.
  • the absorbance by way of example but not limitation, may be corrected by subtracting a reference absorbance at 2000 wavenumbers.
  • heat damage of exposure over 1 hour to 400 degrees F or higher is indicated by corrected absorbance greater than 0.07.
  • Table B Table B below:
  • heat damage may be indicated by absorbance values at around 1767 wavenumbers, or 5.66 ⁇ m, for the exemplary resin-fiber composite the BMS 8-212.
  • the heat damage is indicated by corrected absorbance greater than 0.4 where absorbance at 1767 cm-1 is corrected by subtracting a reference absorbance at 5 ⁇ m, or 2000 wavenumbers. Corrected absorbance increases above 0.04 with exposures to greater than 400 degrees Fahrenheit.
  • FIGURE 8 is a graph of the corrected absorbances reflected in Table C as follows:
  • FIGURE 9 includes 6 absorbance spectra over a range of wavenumbers from six samples of a second exemplary resin-fiber composite substrate (denoted BMS 8-256) exposed to various amounts of heat.
  • Spectrum A reflects no heat treatment.
  • Spectrum B reflects 200 degrees heat exposure for 1 hour.
  • Spectrum C reflects 300 degrees heat exposure for 1 hour.
  • Spectrum D reflects 400 degree heat exposure for 1 hour.
  • Spectrum E reflects 500 degree exposure for 1 hour.
  • Spectrum F reflects 600 degree heat exposure for 1 hour. Because BMS 8-256 has a different chemical composition than BMS 8-212, its absorbance peaks arid responses to heat exposure are somewhat different than that for BMS 8-212 as described in connection with FIGURES 3 and 4 above.
  • FIGURE 10 is a simplified version of FIGURE 9 with only spectra B, D and F of FIGURE 9 shown.
  • absorbance increases with increasing heat exposure.
  • absorbance decreases with increasing heat exposure. This difference is more pronounced when absorbance at a baseline point at 5.00 ⁇ m is subtracted from the absorbance at 4.60 ⁇ m. See Table E and discussion of FIGURE 12, below.
  • the spectra reflected in FIGURES 9 and 10 permit the correlation of heat exposure to absorbance.
  • FIGURE 11 shows heat damage to the exemplary composite BMS 8-256 indicated by absorbance differences.
  • Absorbance at 2174 cm-1 is subtracted from absorbance at 1692 cm-1. According to this exemplary embodiment, heat damage is indicated by absorbance difference greater than around 0.075. Absorbance for temperature exposures of 400 degrees and greater for 1 hour exceed 0.15, while those for 300 degrees or less are approximately 0.5.
  • corrected absorbance at 4.60 ⁇ m or approximately 2174 wavenumbers may be utilized to indicate heat exposure.
  • Absorbance in this example may be equalized by subtracting a reference absorbance at 5 ⁇ m or 2000 wavenumbers. Absorbance values less than 0.015 or negative indicate heat damage of 400 degree exposure or greater for 1 hour.
  • FIGURE 12 is a graph of the data of Table E below:
  • FIGURE 13 shows a further exemplary embodiment of the present invention as applied to BMS 8-256.
  • FIGURE 13 is a graph of corrected absorbance values at 5.61 ⁇ m or 1783 wavenumbers over a range of temperature exposures. At temperature exposures of 400 degrees and greater, absorbance values above 0.04 indicate such exposure, while absorbance values for exposures of 300 degrees F or less are typically between 0.02 and 0.04.
  • the data graphed in FIGURE 13 are that shown in Table F below: TABLE F
  • absorbance heat damage or a degree of heat exposure for the exemplary composite BMS 8-256 may be correlated to corrected absorbance at 5.91 ⁇ m or 1692 wavenumbers.
  • Heat exposure of 400 degrees F or greater is indicated by an absorbance value greater than 0.125, while exposures of 300 degrees or less are typically between 0.05 and 0.1.
  • Absorbance is again corrected by subtracting absorbance at 5 ⁇ m or 2000 cm-1. Exposures to 400 degrees F or greater demonstrate absorbances at 0.15 or higher.
  • FIGURES 15 and 16 show spectra for heat exposed samples for a third exemplary resin-fiber composite designated BMS 8-276.
  • Figure 17 is a plot of a resulting correlation of heat exposure to absorbance.
  • FIGURE 15 is a graph of six infrared absorbance spectra over a range of wavenumbers from 4000 wavenumbers to approximately 400 wavenumbers, with no heat exposure for spectrum A, 200 degree heat exposure for 1 hour for spectrum B, 300 degrees for 1 hour for spectrum C, 400 degrees for 1 hour for spectrum D, 500 degrees for 1 hour for spectrum E, and 600 degrees for spectrum F.
  • the spectra for BMS 8-276 typically show less variation at the middle heat ranges of 300 and 400 degrees than BMS 8-212 and BMS 8-276.
  • FIGURE 16 shows absorbances for three of the six spectra from FIGURE 15.
  • Spectrum B reflects 200 degree heat exposure
  • spectrum D reflects 400 degree heat exposure
  • spectrum F reflects 600 degree heat exposure.
  • Reference lines at 5.0 ⁇ m or 2000 wavenumbers and 5.79 ⁇ m or 1727 wavenumbers are drawn for reference. By way of example but not limitation, variation at these two wavenumbers permits correlation of heat exposure to absorbance.
  • FIGURE 17 is a graph of corrected absorbance at 5.79 ⁇ m or 1727 wavenumbers of BMS 8-276 resulting from heat exposure.
  • the corrected absorbance is suitably obtained by subtracting absorbance at a reference wavelength of 5 ⁇ m or 2000 wavenumbers from absorbance at 5.79 ⁇ m or 1727 wavenumbers.
  • a heat damage threshold of exposure of 400 degrees at 1 hour is indicated in FIGURE 17 by corrected absorbances of greater than around 0.1. Heat exposures of 400 degrees F or greater are reflected by corrected absorbances of approximately 0.14 and higher, while 300 degree and less heat exposure is typically approximately 0.075.
  • the absorbance charted on FIGURE 17 is that of Table H below reflecting increasing absorbance with increasing temperature.
  • an exemplary routine 90 of the present invention is illustrated in flow chart form.
  • the infrared absorbance Ai of the sample at a first wavelength is measured.
  • the infrared absorbance A 2 of a sample at a second wavelength is measured.
  • the sample measurements are reviewed for validity. For example, with attenuated total reflectance, if absorbance of certain frequencies does not reach a certain minimum level, then insufficient pressure may have been applied to the testing device holding it against the sample. Alternately, spurious measurements may be indicative of contamination or remaining coatings. If the readings are not within feasible ranges, then new readings are taken by returning to block 92.
  • the routine 90 continues at a block 96 where the data may be equalized.
  • Equalization of the data suitably may include subtraction of more variable or active readings at alternate frequencies, or subtraction by absorbance or readings at more stable reference frequencies.
  • the equalized data including Ai and Ai may then be compared to a reference at a block 98 to determine a degree or amount of heat exposure of the sample or substrate, such as by comparing the equalized data to graphs or tables of the reference sample at exposures to specific degrees or amounts of heat.
  • FIGURE 19 an alternate routine 100 of the present invention is shown in flow chart form.
  • absorbance Aj at a first wavelength is measured utilizing a first filter.
  • FIGURE 20 shows a flow chart of an exemplary method 200 of the present embodiment utilizing absorbance at a single wavelength.
  • infrared absorbance of the sample Ai is measured.

Abstract

A non-destructive method determines an amount of heat exposure to a resin-fiber composite substrate. A value of infrared energy reflected by a composite substrate is determined. The value of infrared energy reflected, or conversely absorbed, is correlated to a degree or amount of heat exposure. According to an aspect of the present invention, one method utilizes an infrared spectrometer to determine infrared absorbance of a composite substrate. The infrared energy of the reflected beam is then compared with the pre-determined value of infrared energy reflected off a reference heat damaged composite substrate to determine the amount of heat exposure.

Description

METHOD FOR MEASUREMENT OF COMPOSITE HEAT DAMAGE WITH INFRARED SPECTROSCOPY
FIELD OF THE INVENTION This invention relates generally to measurement, and, more specifically, to measurement of heat damage to materials. BACKGROUND OF THE INVENTION Resin-fiber composites are utilized in a variety of applications, such as parts of vehicles, including aircraft. Vehicles, equipment, and structures constructed utilizing resin-fiber composites may be exposed to heat in a variety of circumstances, ranging from fire to lightening strikes to weapons impacts. Heat exposure to resin-fiber composite affects the composites in a variety of ways. Chemical degradation may occur involving changes in the polymer chemical structure. This may include oxidation, material loss and either the breaking or forming of additional chemical links. Resin decomposition, charring, and ultimately fiber decomposition may occur at increasing temperatures. Repair or removal of heat damaged resin-fiber composite materials involves determining the degree of harmful heat damage to the composite substrate. Ascertaining the degree of heat damage to composite materials is typically performed by visual inspection, but heat damage to resin-fiber composites is not always visually apparent. Thus, current visual heat damage inspections of resin-fiber composites involve a substantial degree of subjectivity. Therefore, there currently exists an unmet need in the art for a non-destructive, quantitative, objective determination of a degree of heat damage to resin-fiber composite materials. SUMMARY OF THE INVENTION The present invention provides a non-destructive method for efficiently and objectively determining the degree of heat exposure to a resin-fiber composite material. The present invention may be utilized to quantitatively determine the degree of heat exposure of a composite. According to one embodiment of the invention, a value of infrared energy reflected by a surface on a composite substrate is determined. The value of infrared energy reflected, or conversely absorbed, is correlated to a degree or amount of heat exposure. According to one aspect of the present invention, an infrared spectrometer is utilized to determine infrared absorbance at appropriate wavelengths of a composite substrate. The infrared energy of the reflected beam is then compared with the pre- determined value of infrared energy reflected off a reference heat-damaged composite substrate to determine the amount of heat exposure. According to another aspect of the present invention, a difference between infrared energy absorbance of a heat damaged composite substrate at two wavelengths is compared with a reference surface. BRIEF DESCRIPTION OF THE DRAWINGS The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. FIGURE 1 is a cross section of an exemplary heat damage measurement device in accordance with the present invention; FIGURE 2 is a cross section of an exemplary heat damage measurement device utilizing attenuated total reflectance in accordance with the present invention; FIGURE 3 is a graph of infrared absorbance parallel and perpendicular to fibers in a composite substrate; FIGURE 4 is a graph of infrared absorbance of a first exemplary resin-fiber composite; FIGURE 5 is a graph of infrared absorbance of a first exemplary resin-fiber composite; FIGURE 6 is a graph of absorbance differences for a first exemplary resin-fiber composite exposed to heat; FIGURE 7 is a graph of collected absorbance for a first exemplary resin-fiber composite exposed to heat; FIGURE 8 is a graph of corrected absorbance for a first exemplary resin-fiber composite exposed to heat; FIGURE 9 is a graph of infrared absorbance of a second exemplary resin-fiber composite; FIGURE 10 is a graph of infrared absorbance of a second exemplary resin-fiber composite; FIGURE 11 is a graph of absorbance difference for a second exemplary resin- fiber composite exposed to heat; FIGURE 12 is a graph of corrected absorbance for a second exemplary resin-fiber' composite exposed to heat; FIGURE 13 is a graph of corrected absorbance for a second exemplary resin-fiber composite exposed to heat; FIGURE 14 is a graph of corrected absorbance for a second exemplary resin-fiber composite exposed to heat; FIGURE 15 is a graph of infrared absorbance of a third exemplary resin-fiber composite; FIGURE 16 is a graph of infrared absorbance of a third exemplary resin-fiber composite exposed to heat; FIGURE 17 is a graph of corrected absorbance for a third exemplary resin-fiber composite; FIGURE 18 is a flow chart of an exemplary testing method of the present invention; FIGURE 19 is a flow chart of a further exemplary testing method of the present invention; and FIGURE 20 is a flow chart of a third exemplary testing method of the present invention. DETAILED DESCRIPTION OF THE INVENTION By way of overview, a non-destructive method is provided for determining an amount of heat exposure to a resin-fiber composite substrate. A value Is of infrared energy reflected by a surface on a composite substrate is determined at appropriate wavelengths. The value Is of infrared energy reflected by the composite substrate correlates to an amount of heat exposure. Within the infrared spectrum, absorbance varies with heat exposure and thus infrared absorbance can be correlated to heat exposure to resin-fiber composites. FIGURE 1 is a cross section drawing of an exemplary testing device 20 used to determine the amount of heat exposure to a resin-fiber composite substrate according to a presently preferred embodiment of the invention. An infrared transmission beam 15 is. transmitted by an infrared source 32. The beam 15 is reflected off a sample surface 10, and the reflected beam 16 is detected by an infrared detector 34. The infrared source 32 suitably may include a multi-frequency infrared source 32 and the infrared detector 34 may include a single or multi-frequency infrared detector 34. In the embodiment shown in FIGURE 1, the infrared detector detects infrared energy passing through a filter 36 removeably held by a filter holder 38. This permits the filter 36 to be switched to allow use of the testing device 20 at different infrared frequencies. The infrared levels received by the infrared detector 34 are output as an electrical signal to a processor display 40 through a conductor 35. In one presently preferred embodiment, by way of example and not limitation the infrared source 32 and the infrared detector 34 suitably include a hand-held infrared filter spectrometer such as the SOC410, manufactured by Surface Optics Corporation. In alternative embodiments, an infrared Fourier transform imaging spectrometer, or a portable infrared spectrometer may be utilized. In the exemplary testing device 20, the infrared source 32 and infrared detector 34 are suitably enclosed within a housing 44 with feet 37. The feet 37 rest against the substrate 10 when a sample is tested. The exemplary testing device 20 shown in FIGURE 1, by way of example and not limitation, also collects diffuse reflected infrared radiation diffusely reflected by the substrate 10. The diffuse reflected infrared energy 18 is collected by an ellipsoidal mirror 30 and directed towards the infrared detector 34. It will be appreciated that embodiments of the present invention may suitably utilize specular (i.e., direct reflectance), diffuse reflectance, or alternative collectors, such as an attenuated total reflectance collector as described in reference to FIGURE 2 below. The feet 37 of the testing device 20 hold the infrared source 32 and the infrared detector 34 at a predetermined distance and position relative to the surface 10. The mobility of the testing device 20 permits additional measurement of adjoining areas and other samples with comparable results. The testing device 20 of FIGURE 1 may be utilized to non-destructively determine the degree or the amount of heat exposure of the substrate 10. Initially, the infrared absorbance values of heat exposed reference composite samples (not shown) are determined by measuring the Ir of infrared energy reflected from a reference composite surface. Absorbance A is calculated as -log 10 (I/I0) where I0 is the value of infrared energy reflected by a base material, often gold, and I is the value of infrared energy reflected by the sample under study. The device 20 is then used to transmit the infrared beam 15 to the substrate 10 to be tested, and a value Is of infrared energy reflected by the substrate 10 is measured. Absorbance is derived as described above and comparison is made between the absorbance of the substrate 10 and the absorbance of the heat exposed reference samples (not shown) to calculate an amount of heat exposure. It will be appreciated that surface coatings, or other contaminants, may affect infrared absorbance of the composite substrate measured. Thus, or surface coatings to the composite are typically removed prior to such infrared measurements. It will also be appreciated Is may be compared with Ir without calculating absorbance. However calculating and utilizing absorbance provides graphing and calculating convenience. FIGURE 2 is a cross section of an alternative exemplary testing device 60 that may be utilized to determine the amount of heat damage to a resin-fiber composite substrate 10 according to an alternate embodiment of the invention. An infrared beam 15 is transmitted by an infrared source 32. The beam 15 is transmitted through a crystal 70 placed against the substrate 10. The beam 15 is reflected off the lower surface of the crystal 70, but due to an interaction of the infrared energy with the crystal 70 in contact with the substrate 10, a portion of the infrared energy is absorbed. The reflected attenuated beam 16 is the detected by an infrared detector 74. As is known in the art, the use of a crystal 70 in this manner is referred to as attenuated total reflectance (ATR). In this exemplary embodiment, the reflected beam 16 passes through a filter 76 removeably held in a filter holder 78. An infrared detector 74 then detects the infrared energy of the reflected beam at the frequency passed by the filter 76. It will be appreciated that a number of suitable filters may be successively placed in the filter holder 78, thereby permitting measurement of the attenuated total reflectance of the substrate 10 at a variety of frequencies. Output from the infrared detector 74 is transmitted through a conductor 75 to a processor 80 for recording or analysis. The infrared source 72, crystal 70, and collector 74 are suitably enclosed in a housing 84. The housing 84 may be successively placed against the substrate 10 thereby permitting measurements to be taken at a variety of locations and surfaces. The device 60 thus may detect attenuated infrared reflectance suitably utilizing one or more narrow pass filters 76, and a broad band infrared source 72. It will be appreciated that in alternative embodiments, by way of example but not limitation, an infrared spectrometer may be incorporated in the device 60 to permit measurement of attenuated total reflectance of the substrate 10 over a continuous frequency range. Through testing, it has been determined that the reflectance at infrared frequencies by composite substrates may vary depending upon the orientation of the reflected infrared beam with respect to alignment of the reinforcing fibers in the resin-fiber composite being measured. FIGURE 3 is a graph of different reflectances resulting from differences in fiber orientation. Four specular infrared spectra are measured over a range of wavenumbers (cm-1), two spectra are measured approximately parallel to fibers in the resin-fiber composite, and two spectra are measured approximately perpendicular to fibers in the resin-fiber composite. Absorbance spectra denoted parallel 1, parallel 2, perpendicular 1, and perpendicular 2 in FIGURE 3 show absorbance versus wavenumbers (in cm-1) from approximately 4000 cm-1 to approximately 400 cm-1 of a first exemplary fiber metal composite designated BMS 8-212 (Boeing Materials Services 8-212). Absorbance in this instance is graphed on a range of approximately 0.85 to 1.45. As shown in FIGURE 3, the spectra taken with reflectance generally in alignment with the fibers in the resin-fiber composite (parallel 1 and parallel 2) show more pronounced absorbance peaks than the spectra (perpendicular 1 and perpendicular 2) taken generally oblique or perpendicular to the fiber alignment in the fiber metal composite. Further, the absorbances of the parallel spectra, parallel 1 and parallel 2, are less than that of the perpendicular 1 and perpendicular 2 spectra by an absorbance amount of approximately 0.3. It will be appreciated that at varying angles to the fiber alignment in the fiber metal composite, substantial variability of specular reflectance of infrared energy may be obtained even for the same sample at the same frequency. Thus, in one presently preferred embodiment, reflectance is measured in a direction in alignment with the fibers of the resin-fiber composite. It will be appreciated that when the surface coatings or contamination of a composite surface are removed for infrared measurements, the fibers are typically visible. Thus, in non-destructive testing of the substrate an operator may conveniently align the measurement device (not shown) to perrnit the reflectance measurement to be made parallel to the fiber alignment, suitably facilitating uniformity in measuring comparable surfaces. FIGURE 4 is a graph of six infrared absorbance specra over a range of wavenumbers from six resin-fiber composite substrates (not shown) exposed to various amounts of heat. Absorbance spectra A-F show absorbance versus wavenumbers from approximately 4600 to approximately 400 cm-1. The resin-fiber substrate tested here includes the first exemplary resin-fiber composite BMS 8-212 with no heat exposure for spectrum A, with 200 degrees heat exposure for 1 hour for spectrum B, 300 degrees for 1 hour for spectrum C, 400 degrees for 1 hour for spectrum D, 500 degrees for 1 hour for spectrum E, and 600 degrees for 1 hour for spectrum F. All of the specra show absorbance peaks near 3500 wavenumbers, 3000 wavenumbers, 1600 wavenumbers, and 1300 wavenumbers. However, variability of the spectra is not always linear, with heat exposure at specific wavenumbers not always directly correlated with heat exposure. In FIGURE 5, three of the spectra of FIGURE 4 are shown, and three are deleted, for clarity. Spectrum B shows heat exposure of 200 degrees for 1 hour, spectrum D shows 400 degree exposure for 1 hour, and spectrum F shows 600 degrees exposure for 1 hour for the same first exemplary resin-fiber composite BMS 8-212 over the same range of wavenumbers as FIGURE 4. In this example, reference lines at 6.57 μm or 1522 wavenumbers, and 5.96 μm or 1678 wavenumbers are shown. At 5.96 μm absorbance increases with heat exposure while at 6.57 μm absorbance is relatively constant. Similar correlations may be made at different wavenumbers as well, as discussed in connection with FIGURES 6, 7 and 8 below. This permits correlation of infrared absorbance at single and multiple wavenumbers with heat exposure of the resin-fiber composite. Turning to FIGURE 6, composite heat damage to the exemplary resin-fiber composite BMS 8-212 may be indicated subtracting absorbances at specified wavenumbers. For example, damage may be indicated when absorbance at 1678 cm-1 subtracted from absorbance at 1522 cm-1 equals less than around 0.2, or becomes negative.' Temperature exposures of over 300 degrees show a difference of less than .1 and becoming negative as exposure to temperatures increases from 400 to 600 degrees. FIGURE 6 is a plot of this net absorbance difference for 1 hour temperature exposures at the temperatures listed in the following Table A ("ABS" = Absorbance):
Figure imgf000009_0002
In an alternate embodiment of the present invention, heat damage to the exemplary composite BMS 8-212 may be indicated by corrected absorbance values at 1678 cm-1. The absorbance, by way of example but not limitation, may be corrected by subtracting a reference absorbance at 2000 wavenumbers. In FIGURE 7 heat damage of exposure over 1 hour to 400 degrees F or higher is indicated by corrected absorbance greater than 0.07. The values graphed in FIGURE 7 are listed in Table B below: TABLE B
Figure imgf000009_0001
Figure imgf000010_0001
In a- further exemplary embodiment as shown in FIGURE 8, heat damage may be indicated by absorbance values at around 1767 wavenumbers, or 5.66 μm, for the exemplary resin-fiber composite the BMS 8-212. In this embodiment, the heat damage is indicated by corrected absorbance greater than 0.4 where absorbance at 1767 cm-1 is corrected by subtracting a reference absorbance at 5 μm, or 2000 wavenumbers. Corrected absorbance increases above 0.04 with exposures to greater than 400 degrees Fahrenheit. FIGURE 8 is a graph of the corrected absorbances reflected in Table C as follows:
Figure imgf000010_0002
FIGURE 9 includes 6 absorbance spectra over a range of wavenumbers from six samples of a second exemplary resin-fiber composite substrate (denoted BMS 8-256) exposed to various amounts of heat. Spectrum A reflects no heat treatment. Spectrum B reflects 200 degrees heat exposure for 1 hour. Spectrum C reflects 300 degrees heat exposure for 1 hour. Spectrum D reflects 400 degree heat exposure for 1 hour. Spectrum E reflects 500 degree exposure for 1 hour. Spectrum F reflects 600 degree heat exposure for 1 hour. Because BMS 8-256 has a different chemical composition than BMS 8-212, its absorbance peaks arid responses to heat exposure are somewhat different than that for BMS 8-212 as described in connection with FIGURES 3 and 4 above. FIGURE 10 is a simplified version of FIGURE 9 with only spectra B, D and F of FIGURE 9 shown. By way of example at approximately 1700 wavenumbers or 5.91 μm, absorbance increases with increasing heat exposure. However, by way of example, at 4.6 μm or 2174 wavenumbers absorbance decreases with increasing heat exposure. This difference is more pronounced when absorbance at a baseline point at 5.00 μm is subtracted from the absorbance at 4.60 μm. See Table E and discussion of FIGURE 12, below. The spectra reflected in FIGURES 9 and 10 permit the correlation of heat exposure to absorbance. By way of example but not limitation, FIGURE 11 shows heat damage to the exemplary composite BMS 8-256 indicated by absorbance differences. Absorbance at 2174 cm-1 is subtracted from absorbance at 1692 cm-1. According to this exemplary embodiment, heat damage is indicated by absorbance difference greater than around 0.075. Absorbance for temperature exposures of 400 degrees and greater for 1 hour exceed 0.15, while those for 300 degrees or less are approximately 0.5.
\\ \\
The absorbance differences graphed on FIGURE 11 are those of Table D below:
Figure imgf000011_0001
Figure imgf000012_0001
In an alternate embodiment, corrected absorbance at 4.60 μm or approximately 2174 wavenumbers may be utilized to indicate heat exposure. Absorbance in this example may be equalized by subtracting a reference absorbance at 5 μm or 2000 wavenumbers. Absorbance values less than 0.015 or negative indicate heat damage of 400 degree exposure or greater for 1 hour. FIGURE 12 is a graph of the data of Table E below:
Figure imgf000012_0002
FIGURE 13 shows a further exemplary embodiment of the present invention as applied to BMS 8-256. FIGURE 13 is a graph of corrected absorbance values at 5.61 μm or 1783 wavenumbers over a range of temperature exposures. At temperature exposures of 400 degrees and greater, absorbance values above 0.04 indicate such exposure, while absorbance values for exposures of 300 degrees F or less are typically between 0.02 and 0.04. The data graphed in FIGURE 13 are that shown in Table F below: TABLE F
Figure imgf000013_0001
In FIGURE 14, in a further exemplary embodiment, absorbance heat damage or a degree of heat exposure for the exemplary composite BMS 8-256 may be correlated to corrected absorbance at 5.91 μm or 1692 wavenumbers. Heat exposure of 400 degrees F or greater is indicated by an absorbance value greater than 0.125, while exposures of 300 degrees or less are typically between 0.05 and 0.1. Absorbance is again corrected by subtracting absorbance at 5 μm or 2000 cm-1. Exposures to 400 degrees F or greater demonstrate absorbances at 0.15 or higher.
w w The data
Figure imgf000013_0002
Figure imgf000014_0001
FIGURES 15 and 16 show spectra for heat exposed samples for a third exemplary resin-fiber composite designated BMS 8-276. Figure 17 is a plot of a resulting correlation of heat exposure to absorbance. FIGURE 15 is a graph of six infrared absorbance spectra over a range of wavenumbers from 4000 wavenumbers to approximately 400 wavenumbers, with no heat exposure for spectrum A, 200 degree heat exposure for 1 hour for spectrum B, 300 degrees for 1 hour for spectrum C, 400 degrees for 1 hour for spectrum D, 500 degrees for 1 hour for spectrum E, and 600 degrees for spectrum F. The spectra for BMS 8-276 typically show less variation at the middle heat ranges of 300 and 400 degrees than BMS 8-212 and BMS 8-276. FIGURE 16 shows absorbances for three of the six spectra from FIGURE 15. Spectrum B reflects 200 degree heat exposure, spectrum D reflects 400 degree heat exposure, and spectrum F reflects 600 degree heat exposure. Reference lines at 5.0 μm or 2000 wavenumbers and 5.79 μm or 1727 wavenumbers are drawn for reference. By way of example but not limitation, variation at these two wavenumbers permits correlation of heat exposure to absorbance. FIGURE 17 is a graph of corrected absorbance at 5.79 μm or 1727 wavenumbers of BMS 8-276 resulting from heat exposure. The corrected absorbance is suitably obtained by subtracting absorbance at a reference wavelength of 5 μm or 2000 wavenumbers from absorbance at 5.79 μm or 1727 wavenumbers. A heat damage threshold of exposure of 400 degrees at 1 hour is indicated in FIGURE 17 by corrected absorbances of greater than around 0.1. Heat exposures of 400 degrees F or greater are reflected by corrected absorbances of approximately 0.14 and higher, while 300 degree and less heat exposure is typically approximately 0.075. The absorbance charted on FIGURE 17 is that of Table H below reflecting increasing absorbance with increasing temperature.
Figure imgf000014_0002
Figure imgf000015_0001
Referring now to FIGURE 18, an exemplary routine 90 of the present invention is illustrated in flow chart form. At a block 92 the infrared absorbance Ai of the sample at a first wavelength is measured. At a block 94 the infrared absorbance A2 of a sample at a second wavelength is measured. At a decision block 95 the sample measurements are reviewed for validity. For example, with attenuated total reflectance, if absorbance of certain frequencies does not reach a certain minimum level, then insufficient pressure may have been applied to the testing device holding it against the sample. Alternately, spurious measurements may be indicative of contamination or remaining coatings. If the readings are not within feasible ranges, then new readings are taken by returning to block 92. Otherwise, with valid data the routine 90 continues at a block 96 where the data may be equalized. Equalization of the data, as described above, suitably may include subtraction of more variable or active readings at alternate frequencies, or subtraction by absorbance or readings at more stable reference frequencies. The equalized data including Ai and Ai may then be compared to a reference at a block 98 to determine a degree or amount of heat exposure of the sample or substrate, such as by comparing the equalized data to graphs or tables of the reference sample at exposures to specific degrees or amounts of heat. Turning to FIGURE 19, an alternate routine 100 of the present invention is shown in flow chart form. At a block 105, absorbance Aj at a first wavelength is measured utilizing a first filter. At a block 110, the infrared absorbance A2 of the sample at a second wavelength is measured utilizing a second filter. At a block 120 A2 is subtracted from Ai to form a difference. At a block 130, the resulting difference is compared to a reference to determine a degree of heat exposure of the sample. It will also be appreciated that, for some resin-fiber composites, heat exposure may directly conflate with absorbance at a single frequency without utilization of alternate absorbances. FIGURE 20 shows a flow chart of an exemplary method 200 of the present embodiment utilizing absorbance at a single wavelength. At a block 205, infrared absorbance of the sample Ai is measured. At a block 210, that absorbance is compared to a reference to determine a degree or amount of heat exposure of the sample or substrate While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.

Claims

What is claimed is:
1. A non-destructive method for determining an amount of heat exposure to a resin-fiber composite substrate, the method comprising: non-destructively determining a value Is of infrared energy reflected by a surface on a composite substrate; and correlating the value Is of the infrared energy reflected to an amount of heat exposure.
2. The method of Claim 1, further comprising determining a value It of infrared energy reflected from a reference composite surface.
3. The method of Claim 2, further comprising comparing Is with L.
4. The method of Claim 1, wherein determining Is includes utilizing an infrared spectrometer.
5. The method of Claim 4, wherein the infrared spectrometer includes an infrared filter spectrometer.
6. The method of Claim 4, wherein the infrared spectrometer includes an ellipsoidal mirror collector.
7. The method of Claim 4, wherein the infrared spectrometer includes an attenuated total reflectance collector.
8. The method of Claim 4, wherein the infrared spectrometer includes at least two filters.
9. The method of Claim 8, wherein the at least two filters include narrow bandpass infrared filters.
10. The method of Claim 1, wherein determining Is includes determining absorbance at at least one wavenumber corresponding with increased infrared absorbance by a composite exposed to heat.
11. The method of Claim 10, wherein the at least one wavenumber is around 2174 cm-1.
12. The method of Claim 10, wherein the at least one wavenumber is around 2000 cm-1.
13. The method of Claim 10, wherein the at least one wavenumber is around 1783 cm-1.
14. The method of Claim 10, wherein the at least one wavenumber is around 1727 cm-1.
' 15. The method of Claim 10, wherein the at least one wavenumber is around 1767 cm-1.
16. The method of Claim 10, wherein the at least one wavenumber is around 1692 cm-1.
17. The method of Claim 10, wherein the at least one wavenumber is around 1678 cm-1.
18. The method of Claim 10, wherein the at least one wavenumber is around 1522 cm-1.
19. The method of Claim 10, wherein correlating the infrared absorbance to an amount of heat exposure of the sample includes determining a difference between infrared absorbance of the sample at at least two wavenumbers.
20. The method of Claim 19, wherein the at least two wavenumbers are around 1522 cm-1 and around 1678 cm-1.
21. The method of Claim 19, wherein the at least two wavenumbers are around 1629 cm-1 and around 2174 cm-1.
22. A non-destructive method for determining an amount of heat damage to a resin- fiber composite sample, the method comprising: transmitting an infrared beam onto a sample of a resin-fiber composite; detecting a reflected infrared beam reflected by the sample; determining infrared absorbance of the sample; and ' correlating the infrared absorbance to an amount of heat damage to the sample.
23. The method of Claim 22, wherein transmitting an infrared beam includes transmitting the infrared beam in a direction approximately parallel with fibers in the sample.
24. The method of Claim 22, wherein determining the infrared absorbance includes using an infrared spectrometer.
25. The method of Claim 22, wherein correlating the infrared absorbance includes determining absorbance at at least one wavenumber corresponding with an infrared spectra of a heat damaged resin-fiber composite surface.
26. The method of Claim 25, wherein the at least one wavenumber is around 2174 cm-1.
27. The method of Claim 25, wherein the at least one wavenumber is around 2000 cm-1.
28. The method of Claim 25, wherein the at least one wavenumber is around 1783 cm-1.
29. The method of Claim 25, wherein the at least one wavenumber is around 1727 cm-1.
30. The method of Claim 25, wherein the at least one wavenumber is around 1767 cm-1.
31. The method of Claim 25, wherein the at least one wavenumber is around 1692 cm-1.
32. The method of Claim 25, wherein the at least one wavenumber is around 1678 cm-1.
33. The method of Claim 25, wherein the at least one wavenumber is around 1522 cm-1.
34. The method of Claim 35, wherein correlating the infrared absorbance to an amount of heat damage of the sample includes deriving a difference between infrared absorbance at at least two wave numbers.
35. The method of Claim 34, wherein deriving a difference between infrared absorbance of the sample at at least two wavenumbers includes deriving a difference between infrared absorbance at around 1522 cm-1 and at around 1678 cm-1.
36. The method of Claim 34, wherein deriving a difference between infrared absorbance of the sample at at least two wavenumbers includes deriving a difference between infrared absorbance at around 1692 cm-1 and at around 2174 cm-1.
37. The method of Claim 22, wherein detecting a reflected infrared beam reflected by the sample includes filtering the reflected infrared beam.
38. The method of Claim 37, wherein filtering the reflected infrared beam includes utilizing at least two filters.
39. A non-destructive method for determining an amount of heat exposure of a resin-fiber composite sample, the method comprising: transmitting an infrared beam onto a sample of resin-fiber composite; detecting a reflected infrared beam reflected by the sample; determining a first infrared absorbance of the sample from the reflected infrared beam at a first wavenumber; determining a second infrared absorbance of the sample from the reflected infrared beam at a second wavenumber; deriving a first difference between the first infrared absorbance and the second infrared absorbance; and quantitatively determining a amount of heat exposure by correlating the first difference to a reference sample.
40. The method of Claim 39, wherein correlating the first difference to a reference sample includes comparing the first difference with a second difference of infrared absorbance at the first wavenumber and the second wavenumber for the reference sample.
41. The method of Claim 39, wherein determining at least one of the first infrared absorbance and the second infrared absorbance includes using an infrared spectrometer.
42. The method of Claim 41, wherein the infrared spectrometer includes an infrared filter spectrometer.
43. The method of Claim 42, wherein the infrared spectrometer includes at least two filters.
44. The method of Claim 43, wherein the filters include narrow bandpass infrared filters.
45. The method of Claim 41, wherein the infrared spectrometer utilizes an ellipsoidal mirror.
46. The method of Claim 41, wherein the infrared spectrometer utilizes attenuated total reflectance.
47. The method of Claim 39, wherein the first wavenumber and the second wavenumber correspond with an infrared spectra of a heat damaged composite surface.
48. The method of Claim 47, wherein the first wavenumber is around 1522 cm-1.
49. The method of Claim 47, wherein the second wavenumber is around 1678 cm-1.
50. The method of Claim 47, wherein the first wavenumber is around 1692 cm-1.
51. The method of Claim 47, wherein the second wavenumber is around 2174 cm-1.
52. A non-destructive method for determining a degree of heat exposure of a resin- fiber composite substrate, the method comprising: transmitting an infrared beam onto an resin-fiber composite substrate; filtering with a first filter a reflected infrared beam reflected by the substrate; detecting a first filtered portion of the reflected infrared beam; and determining a first infrared absorbance of the substrate.
53. The method of claim 52, further comprising correlating the first infrared absorbance a degree of heat exposure.
54. The method of claim 52 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 2174 cm-1.
55. The method of claim 52 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 2000 cm-1.
56. The method of claim 52 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 1783 cm-1.
57. The method of claim 52 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 1727 cm-1.
58. The method of claim 52 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 1767 cm-1.
59. The method of claim 52 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 1692 cm-1.
60. The method of claim 52 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 1678 cm-1.
61. The method of claim 52 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 1522 cm-1.
62. The method of Claim 52, further comprising: filtering with a second filter a reflected infrared beam reflected by the substrate; detecting a second filtered portion of the reflected infrared beam; and determining a second infrared absorbance of the substrate.
63. The method of Claim 62, further comprising subtracting the second infrared absorbance from the first infrared absorbance.
64. The method of claim 62 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 1522 cm-1, and determining a second infrared absorbance includes determining absorbance at a wavenumber of around 1678 cm-1.
65. The method of claim 62 wherein determining a first infrared absorbance includes determining absorbance at a wavenumber of around 1692 cm-1, and determining a second infrared absorbance includes determining absorbance at a wavenumber of around 2174 cm-1.
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