Abstract:
A moisture detection system for characterizing moisture on a sample includes a generator adapted to emit an incident beam of radiation from the terahertz spectrum of frequency onto the sample; a detector adapted to receive a reflected beam of radiation from the sample and measure radiation in the reflected beam; and a controller adapted to correlate the radiation in the reflected beam with an amount of moisture on the sample.

Description:
TECHNICAL FIELD 
     The disclosure generally relates to detection of moisture on carbon fiber composite materials. More particularly, the disclosure relates to a moisture detection system and method which utilizes terahertz (THz) radiation to detect moisture on the surface of carbon fiber composite materials. 
     BACKGROUND 
     In the fabrication of aircraft and other structures using carbon fiber composite materials, it may be necessary to apply paint, other coatings, or adhesives to the surface of the composite material. However, during the application of paint, other coatings, or adhesives to the surface of a composite material the presence of moisture may compromise the strength of the adhesion between the coating and the material. Therefore, it may be necessary to characterize the moisture levels of composite material surfaces prior to application of coatings or adhesives to the surfaces. 
     Conventional methods of measuring moisture on surfaces of composite materials include cutting and weighing samples of the material to infer water content on like-sized parts or coupons of the material. Other methods may include heating the samples and measuring the quantity of moisture which is evaporated from the samples. Infrared and microwave electromagnetic radiation can be used to interact with moisture. The terahertz regime at higher frequency offers particular sensitivity. 
     Many non-conducting, dry materials that are opaque to infrared and visible light exhibit low absorption in the terahertz (THz) frequency range. The terahertz frequency range is commonly described as 1×10 11  to 1×10 13  Hz (1 to 0.01 mm wavelength). Absorption of radiation in the THz frequency range increases with the quantity of moisture on the surface of a material. 
     Accordingly, a moisture detection system and method are needed which utilize absorption of radiation in the THz frequency range by moisture to facilitate stand-off, non-contact and non-destructive characterization of moisture levels on composite materials. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings 
    
    
     
       SUMMARY 
       Brief Description of the Illustrations 
         FIG. 1  is a schematic diagram of an illustrative embodiment of a moisture detection system which is suitable for implementation of the moisture detection method. 
         FIG. 2  is a schematic diagram which illustrates application of a focused beam of radiation in the terahertz range to the surface of a composite material at an angle of approximately 45 degrees with respect to the primary direction of the carbon fibers in the composite material. 
         FIG. 3  is a flow diagram which illustrates calibration of a moisture detection system according to an illustrative embodiment of the moisture detection method. 
         FIG. 3A  is a flow diagram which illustrates characterization of moisture on the surface of a sample using a single THz frequency in implementation of an illustrative embodiment of the moisture detection method. 
         FIG. 4  is a flow diagram which illustrates characterization of moisture on the surface of a sample in implementation of an illustrative embodiment of the moisture detection method. 
         FIG. 5  is a flow diagram of an aircraft production and service methodology. 
         FIG. 6  is a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     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 practice the disclosure and are not intended to limit the scope of the appended 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 , an illustrative embodiment of the moisture detection system, hereinafter system, is generally indicated by reference numeral  100 . The system  100  may include a generator  101 . The generator  101  may be adapted to generate an incident beam  122  having a radiation in the terahertz range of from about 1×10 12  Hz (or 100 GHz) to about 1.5×10 12  Hz (or 1.5 THz). In some embodiments, the generator  101  may be a Gunn diode oscillator with an output power of 12 mW. 
     A parabolic mirror  102  may be positioned in beam-receiving relationship with respect to the output of the generator  101 . The parabolic mirror  102  may be adapted to focus the incident beam  122 . A beam splitter  104  may be positioned in beam-receiving relationship with respect to the parabolic mirror  102 . An optical chopper  103  may be disposed between the parabolic mirror  102  and the beam splitter  104 . A polyethylene Fresnel lens  105  may be positioned at a first beam-receiving location with respect to the beam splitter  104 . In some embodiments, the Fresnel lens  105  may have a thickness of about 5 mm. A mirror  106  may be spaced-apart with respect to the Fresnel lens  105 . A stage  120  may be disposed at a selected position with respect to the mirror  106 . The stage  120  may be adapted to support a sample  116  the surface moisture content of which is to be measured using the system  100 . The stage  120  may be a turntable and may additionally be adapted to selectively move the sample  116  along X, Y and Z axes according to the knowledge of those skilled in the art. 
     A detector  110  may be positioned at a second beam-receiving location with respect to the beam splitter  104 . A controller  111  may interface with the detector  110 . A display  112  may interface with the controller  111 . 
     In exemplary application of the system  100 , a sample  116  is placed on the stage  120 . The sample  116  has or is suspected to have moisture contamination on the surface of the sample  116 . The generator  101  emits an incident beam  122  which may have a wavelength in the terahertz range (from about 100 GHz to about 1.5 THz) against the parabolic mirror  102 . The parabolic mirror  102  focuses the incident beam  122  to a selected size (such as 4 mm, for example and without limitation) and the optical chopper  103  modulates the incident beam  122  to a selected frequency (such as 1.2 kHz, for example and without limitation). The focused and modulated incident beam  122  is transmitted through the Fresnel lens  105 , which focuses the incident beam  122  to a selected focal length (such as 204 mm, for example and without limitation). In some applications, the distance between the focus of the parabolic mirror  102  and the focus of the Fresnel lens  105  may be about 408 mm. 
     The incident beam  122  which is transmitted through and focused by the Fresnel lens  105  strikes the mirror  106 , which deflects the incident beam  122  toward and then against the surface of the sample  116 . As shown in  FIG. 2 , the stage  120  ( FIG. 1 ) may be positioned in such a manner that the electric field  126  of the incident beam  122  is oriented at an angle of approximately 45 degrees with respect to the carbon fiber direction  117  of the carbon fiber composite material sample  116 . Any moisture which may be present on the surface of the sample  116  absorbs energy from the incident beam  122 . The amount of energy which the moisture absorbs from the incident beam  122  varies with the amount of moisture on the surface of the sample  116 . 
     A reflected beam  124  is reflected from the surface of the sample  116  to the mirror  106 . The mirror  106  deflects the reflected beam  124  through the Fresnel lens  105  and to the beam splitter  104 , respectively. The beam splitter  104  deflects the reflected beam  124  to the detector  110 . The detector  110  measures the amount of reflected energy in the reflected beam  124  and transmits the measurement data to the controller  111 . The controller  111  correlates the amount of reflected energy in the reflected beam  124  with the quantity of moisture on the surface of the sample  116 . The display  112  may present the quantity of moisture on the surface of the sample  116  in a percentage, graphical or other format. Prior to analysis of the moisture content on the surface of the sample  116 , calibration of the system  100  may be accomplished by correlating degrees of moisture on the surfaces of standards with the amount of radiation reflected from the surfaces of the standards. 
     In some embodiments, after initial analysis of the reflected beam  124 , incident beams  122  of various frequencies within the THz frequency range may be transmitted against the surface of the sample  116  to obtain a spectrum of frequencies of the reflected beams  124 . The incident beams  122  may have frequencies in the range of 0.1 to 1.5 THz. The spectral reflection terahertz analysis may provide for distinct calibration of power as a function of frequency for the incident beams  122 . 
     Referring next to  FIG. 3 , a flow diagram  300  which illustrates calibration of a moisture detection system according to an illustrative embodiment of the moisture detection method is shown. In block  302 , carbon fiber composite material standards with various amounts of moisture on the surfaces of the respective standards may be provided. In block  304 , an incident radiation beam having a frequency in the terahertz range may be directed against the surfaces of the standards, respectively. In block  306 , the amount of radiation which is reflected from the surfaces of the standards may be measured. In block  308 , the amounts of moisture on the surfaces of the respective standards may be correlated with the amount of radiation which is reflected from the surfaces of the standards. 
     Referring next to  FIG. 3A , a flow diagram  300   a  which illustrates characterization of moisture on the surface of a sample using a single THz frequency in implementation of an illustrative embodiment of the moisture detection method is shown. In block  302   a , a sample is provided. In some embodiments, the sample may be carbon fiber composite material sample. In block  304   a , a radiation beam having a frequency in the tetrahertz range is directed against a surface of the sample. In some embodiments, a radiation beam may be directed against the surface of the sample at about a 45-degree angle with respect to a primary direction of carbon fibers in the sample. In some embodiments, a radiation beam in the terahertz range of about 100 GHz to 1.5 THz may be directed against the surface of the sample. In block  306   a , the amount of radiation reflected from the surface of the sample is measured. In block  308   a , the amount of energy in the radiation reflected from the sample is correlated to the amount of moisture on the surface of the sample. In some embodiments, the method may include directing radiation beams in the terahertz range of about 0.1 to about 0.5 THz against the surface of the sample, measuring radiation from the radiation beams reflected from the sample and correlating amounts of energy in the radiation beams reflected from the sample to amounts of moisture on the sample. 
     Referring next to  FIG. 4 , a flow diagram  400  which illustrates characterization of moisture on the surface of a sample such as a composite material, for example and without limitation, in implementation of an illustrative embodiment of the moisture detection method is shown. In block  402 , various amounts of moisture on different standards may be correlated with the amounts of radiation in the terahertz range which is reflected from the respective standards. In block  404 , a sample with an unknown level of moisture on its surface may be provided. In some embodiments, the sample may be a carbon fiber composite material. In block  406 , a radiation beam having a first frequency in the terahertz range may be directed against the surface of the sample. In embodiments in which the sample is a composite material, the radiation beam may be directed against the composite material at 45 degrees relative to the primary direction of carbon fibers in the material. In some applications, the radiation beam may have a frequency in the range of 100 GHz to 1.5 THz. In block  408 , the amount of radiation in the first frequency reflected from the surface of the sample in block  406  may be measured. In block  410 , a radiation beam having subsequent frequencies in the terahertz range of 0.1 to 1.5 THz may be directed against the surface of the sample. In embodiments in which the sample is a composite material, the radiation beam may be directed against the material at 45 degrees relative to the primary direction of carbon fibers in the material. In block  412 , the amount of radiation in the subsequent frequencies which were reflected from the surface of the sample in block  410  may be measured. In block  414 , the amount of energy which was reflected from the sample at each frequency may be correlated with the amount of moisture on the surface of the sample. 
     Referring next to  FIGS. 5 and 6 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  500  as shown in  FIG. 5  and an aircraft  600  as shown in  FIG. 6 . During pre-production, exemplary method  500  may include specification and design  502  of the aircraft  600  and material procurement  504 . During production, component and subassembly manufacturing  506  and system integration  508  of the aircraft  600  takes place. Thereafter, the aircraft  600  may go through certification and delivery  510  in order to be placed in service  512 . While in service by a customer, the aircraft  600  may be scheduled for routine maintenance and service  514  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of method  500  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. 6 , the aircraft  600  produced by exemplary method  500  may include an airframe  610  with a plurality of systems  608  and an interior  612 . Examples of high-level systems  608  include one or more of a propulsion system  614 , an electrical system  606 , a hydraulic system  602 , and an environmental system  604 . 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  500 . For example, components or subassemblies corresponding to production process  506  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  600  is in service. Also one or more apparatus embodiments may be utilized during the production stages  506  and  508 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  600 . Similarly, one or more apparatus embodiments may be utilized while the aircraft  600  is in service, for example and without limitation, to maintenance and service  514 . 
     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.