Abstract:
A method for detecting suspended discontinuity densities in a material is provided. The method includes transmitting terahertz electromagnetic radiation toward a surface of the material. This radiation is received at an expected location after interacting with the material. The power level of the received radiation is measured and deviation from the expected value is used to determine a suspended discontinuity density gradient in the material. The method can be used with either reflected radiation or transmitted radiation. Embodiments of the method can calculate the index of refraction in the material and correlate this with the suspended discontinuity density of the material.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention is directed generally towards a non-destructive method for detecting densities of components of a composite material (referred to herein as discontinuity densities and suspended discontinuity densities). More specifically it is directed toward determining micro-inclusion gradients and micro-inclusion gradient boundaries in micro-inclusion-impregnated materials utilizing sub-centimeter wavelength electromagnetic radiation. This method also detects micro-inclusion concentration levels in micro-inclusion-impregnated materials in a nondestructive, contactless fashion. 
     (2) Description of the Prior Art 
     Terahertz Time-Domain Spectroscopy (THz-TDS) was first developed in research institutions in the early 1990&#39;s. The technology began to see first commercial development later that decade with very large (the size of a refrigerator), very expensive apparatuses for niche research application. Over the following decade, THz-TDS systems became much smaller and operated with much greater dynamic range. For example, the current state of the art system is roughly the size of a suitcase and can image across a bandwidth of 0.03 to 3.5+THz (corresponding to a free space wavelength of about 1 cm to about 100 microns) with a dynamic range of about 100 dB. 
     Terahertz (THz) imaging combines aspects of optics, such as high resolution imaging, and radio-frequency electromagnetics to yield an imaging technology that can take sub-millimeter resolution pictures of objects hidden within and on the other side of materials. THz waves may not pass through all materials equally well; however, differences in material transparency allow for identification of one material hidden behind another, much like an electromagnetic version of ultrasound. 
     THz scanners are finding wider usage outside the research community. NASA has employed THz-TDS scanners to detect defects within the tiles on the outside of the Space Shuttle fuel tank. The Transportation Safety Administration (TSA) has employed THz scanners at airport security checkpoints nation-wide to detect concealed objects on people attempting to board planes. 
     Micro-inclusions are microscopic chunks of material manufactured for a wide variety of uses in research, medicine, consumer goods and various industries. Micro-inclusions are usually between 10 to 300 micrometers in diameter. They are used as lightweight filler in composite materials such as lightweight concrete. Micro-inclusions can impart the following qualities reduced weight, reduced thermal conductivity, and increased resistance to compressive stress that far exceeds that of other similar materials. These properties are exploited in high pressure environments where other similar materials would implode. Micro-inclusions having internal hollows create materials having different properties. 
     A material for testing can have a desired concentration of micro-inclusions. In making this material, a liquid (uncured) version of the material is put into a curing mold after being mixed with sufficient micro-inclusions to achieve the desired concentration. The buoyancy of the micro-inclusions in the curing liquid causes them to migrate towards the surface of the liquid opposite the pull of gravity while the material/micro-inclusion mixture cures. In other words, the liquid is denser than the micro-inclusions causing the liquid to sink while pushing the micro-inclusions closer to the upper surface. Thus, when the mixture is finished curing, there is a region in the resulting composite material at the lowest point of the material referenced to gravity where most of the micro-inclusions have floated away, and there is a region at the highest point of the material referenced to gravity having an excess of micro-inclusions. Naturally, there is a gradient in the micro-inclusion concentrations at the boundaries where the micro-inclusion concentration shifts due to the aforementioned gravitational/buoyancy effect. As a matter of quality control, it is desirable to maintain these concentrations within the desired limits. 
     SUMMARY OF THE INVENTION 
     Accordingly it is an object of the present invention to provide non-destructive method for measuring elements of a composite material having differing distributions. 
     A further object is determining micro-inclusion densities in a composite material having micro-inclusions. 
     There is provided a method for detecting suspended discontinuity densities in a material. The method includes transmitting terahertz electromagnetic radiation toward a surface of the material. This radiation is received at an expected location after interacting with the material. The power level of the received radiation is measured and deviation from the expected value is used to determine a density gradient in the material. The method can be used with either reflected radiation or transmitted radiation. Embodiments of the method can calculate the index of refraction in the material and correlate this with the micro-inclusion density of the material. 
     Other objects and advantages of the present invention will become apparent from the following description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein like reference numerals and symbols designate identical or corresponding parts throughout the several views and wherein: 
         FIG. 1  is a diagram of first embodiment of the invention having a colocated source and sensor for a reflection terahertz radiation system; 
         FIG. 2  is a diagram of second embodiment of the invention having an offset source and sensor for a reflection terahertz radiation system; 
         FIG. 3  is a diagram of third embodiment of the invention having an offset source and sensor for a transmission terahertz radiation system; 
         FIG. 4  is a diagram of third embodiment of the invention having an offset source and sensor for a transmission and reflection terahertz radiation system; 
         FIG. 5A  is a diagram showing an image of a sample taken with a transmission terahertz radiation system; and 
         FIG. 5B  is a diagram showing an image of a sample taken with a reflection terahertz radiation system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-4  show embodiments of the invention with reference to a test sample  10 . Reference numbers for the test sample are the same throughout. Test sample  10  is shown in cross-section. It includes a substrate  12  having micro-inclusions  14  suspended therein. Test sample  10 , substrate, and micro-inclusions  14  are not shown to scale. In actual use, micro-inclusions  14  will be much smaller and present at a much greater concentration. Region  16  of sample  10  has a micro-inclusion gradient having many more micro-inclusions  14  at a surface  18  of the sample  10 . While application of this technology is particularly applicable to micro-inclusions dispersed in a polymer matrix, it is equally applicable to any group of material agglomerates in suspension in a bulk material where the agglomerates are intended to be arranged with a certain uniformity, absence of uniformity, or gradient. 
     A gradient in micro-inclusion concentration can be detected by its effect on electromagnetic waves reflecting from a material surface having a gradient or traveling through a gradient in the material if the gradient in micro-inclusion concentration also causes a gradient in refractive index of the resulting composite material.  FIG. 1  shows an embodiment using a surface-normal reflection of electromagnetic waves.  FIG. 2  shows an embodiment using an offset reflection of electromagnetic waves.  FIG. 3  shows an embodiment using refraction of transmitted electromagnetic waves to determine material characteristics.  FIG. 4  shows an embodiment of the invention using both transmitted and reflected electromagnetic waves. 
     The embodiment shown in  FIG. 1  includes a combined electromagnetic radiation source and sensor  20  capable of transmitting electromagnetic radiation shown at  22 . Radiation  22  contacts surface  18  of sample  10  and reflects directly back to combined source and sensor  20  along the same path  22 . An analysis system  24  is joined to combined source and sensor  20 . 
     In operation, source  20  transmits electromagnetic radiation  22 , preferably in the frequency range of about 0.03 to 3.5 THz. The radiation transmission can be a pulse of radiation having a duration assuring reception at source/sensor  20 . Radiation  22  contacts surface  18  of sample  10  and is reflected. Reflected radiation along path  22  is received at source/sensor  20 . Analysis system  24  monitors the intensity of the received signal. This embodiment allows easy monitoring of reflected signal changes. These changes indicate a refractive index change if no other sources of reflected signal variation (i.e.—surface roughness) are present. 
       FIG. 2  shows an embodiment having an offset source  26  and reflected radiation sensor  28 . This type of arrangement is known in the art as “pitch-catch.” Reflected radiation sensor  28  is positioned to receive reflections according to Snell&#39;s law. Source  26  transmits electromagnetic radiation along path  30 . Electromagnetic radiation  30  reflects from surface  18  of the sample  10 , and the reflected radiation follows path  32 . Reflected radiation is received at sensor  28 . As before, sensor  28  is joined to an analysis system  24 . Analysis system  24  monitors the intensity of the received signal to indicate a refractive index change if no other sources of reflected signal variation (i.e.,—surface roughness) are present. 
       FIG. 3  shows an embodiment having an offset source  26  with a transmitted radiation sensor  34  located on the opposite side of sample  10 . Sensor  34  is positioned at an expected transmission location to receive transmitted electromagnetic radiation according to predicted refraction characteristics of sample  10 . As before, source  26  provides electromagnetic radiation along path  30 . At surface  18 , electromagnetic radiation  30  is refracted along path  36 . Refracted electromagnetic radiation  36  exits sample  10  and is refracted along path  38  as transmitted electromagnetic radiation. (If a micro-inclusion concentration gradient exists in the material path  36  may be a curved path.) Dashed path  36 ′ shows the expected path of the refracted radiation in the absence of a micro-inclusion gradient. Dashed path  38 ′ shows the expected path of the transmitted electromagnetic radiation in the absence of a micro-inclusion gradient. Sensor  34  is joined to an analysis system  40  for analyzing the transmitted radiation. 
     In one embodiment, transmitted radiation sensor  34  can be a single sensor positioned such that a lower intensity or an absence of received radiation indicates a material defect. In a more sophisticated embodiment, transmitted radiation sensor  34  can be an array of sensors having each sensor positioned at a different position with respect to the back surface of sample  10 . This transmitted radiation sensor array can provide a transmitted radiation reception location to the analysis system. The analysis system can then use this information to calculate the refracted angle and determine various micro-inclusion properties such as the volume micro-inclusion concentration gradient rate of change. 
     The embodiment shown in  FIG. 4  includes analysis of both reflected electromagnetic radiation and transmitted electromagnetic radiation. Source  26  transmits electromagnetic radiation along path  30 . A portion of electromagnetic radiation  30  reflects from surface  18  of the sample  10 , and the reflected radiation follows path  32 . Reflected radiation sensor  28  is positioned to receive reflections according to Snell&#39;s law. Reflected radiation sensor  28  is joined to analysis system  40 . Analysis system  40  monitors the intensity of the received signal to indicate a refractive index change if no other sources of reflected signal variation (i.e.,—surface roughness) are present. Another portion of electromagnetic radiation on path  30  is refracted into the sample  10  on path  36 . Refracted electromagnetic radiation  36  exits sample  10  and is refracted along path  38  as transmitted electromagnetic radiation. Dashed path  36 ′ shows the expected path of the refracted radiation in the absence of a micro-inclusion gradient. A transmitted radiation sensor  34  or sensor array  42  is provided to receive the transmitted radiation  38 . Sensor array  42  is joined to analysis system  40  for analyzing the position and intensity of the transmitted radiation. In yet another adaptation of this embodiment, source  26  and sensor  28  could be positioned perpendicular to surface  18 , and sensor  34  or sensor array  42  could be perpendicular to the opposite surface of sample  10 . 
       FIG. 5A  shows a representation of transmitted electromagnetic energy  38  received by transmitted radiation sensor  34 . In regions having constant concentration of micro-inclusions, electromagnetic radiation is transmitted in the expected manner such as indicated by  44 . In regions having a micro-inclusion gradient such as that shown at  16  of  FIG. 1 , electromagnetic radiation is transmitted to the expected location on sensor  34  or sensor array  42  with a reduced intensity as shown at  46 . 
       FIG. 5B  shows a representation of reflected electromagnetic radiation  32  received by reflected electromagnetic radiation sensor  28 . Regions having a higher or lower concentration of micro-inclusions at the surface  18  of sample  10  have different refractive indices. This results in a comparatively different amount of radiation being reflected from the surface. A surface having a uniform distribution of micro-inclusions is shown at  48 , and a surface having a different, uniform concentration of micro-inclusions is shown at  50 . A concentration gradient of micro-inclusions exists at the boundary between regions  48  and  50 . 
     The concentration of micro-inclusions in a composite material will affect the effective refractive index of the material directly proportionally to the weighted percentage of the volume ratio between micro-inclusions and material across the imaged material surface or volume. If the refractive index of the micro-inclusions  14  is higher than the refractive index of the bulk material  12  they are suspended in, then the refractive index of the composite material will increase. If the refractive index of the micro-inclusions  14  is lower than the refractive index of the bulk material  12  they are suspended in, then the refractive index of the composite material will decrease. Electromagnetic reflection from a surface increases with the disparity of the refractive indices of the materials comprising the two sides of an interface. For a constant material on one side of the surface (e.g., air) and a varying refractive index material on the other side of the surface (e.g., a material with a varying micro-inclusion gradient), the reflected electromagnetic signal magnitude will be greater from the regions of the material surface where the refractive index is higher. 
     The surface reflection of electromagnetic radiation in the 100 micron to 1 cm wavelength range will also show the location, magnitude, and boundaries of the micro-inclusion gradient as a change in magnitude of the surface reflection of the electromagnetic radiation in accordance with Snell&#39;s Law. The reflection will show greater reflected signal energy from the material surface in the region of the material where the micro-inclusion concentration causes a greater disparity between the material&#39;s refractive index and the refractive index of the medium in direct contact with the material surface. As the micro-inclusion concentration causes the composite material&#39;s refractive index to become more similar to that of the surrounding media, the surface reflection will decrease. This effect allows examining of large material volumes from a surface analysis if the cured composite material is homogenous along one axis perpendicular to the direction of micro-inclusion buoyancy drift. 
     The change with micro-inclusion concentration in refractive index and absorption coefficient across the sub-centimeter wavelength range allows calculation of the micro-inclusion concentration in the material. As the micro-inclusion concentration in a material increases, the refractive index and absorption coefficient in the material will decrease (increase) if the micro-inclusions have a lower (higher) attenuation coefficient and absorption coefficient than the matrix material in the micro-inclusion composite material. Parenthetical statements in this paragraph are meant to show that the converse statement is also true. As the volume-percentage of the micro-inclusions increases, the percentage of the material having a lower (higher) refractive index and lower (higher) attenuation coefficient increases and the overall attenuation through the material and reflection coefficient at the surface of the material decreases (increases). Thus, the micro-inclusion concentration can be determined by the attenuation of the electromagnetic radiation through the material. Micro-inclusion concentration at the surface of a composite material can be determined by the magnitude of the reflected electromagnetic radiation from the material&#39;s surface. 
     This imaging method will allow imaging of the micro-inclusion gradient which occurs in the gravitational top and bottom volume of micro-inclusion impregnated materials due to gravitational drift of the micro-inclusions during the curing process. The detection of this micro-inclusion gradient edge will allow manufacturers of micro-inclusion-impregnated materials to accurately determine where the dividing line exists between the gravity-induced micro-inclusion gradient and the regions where the micro-inclusion concentrations are constant. The regions of the material with constant levels of micro-inclusions are useful for applications such as those described in [0007], whereas the regions with variable concentrations of micro-inclusions will not provide the expected application performance response if consistent micro-inclusion concentrations are expected in design. Regions of non-homogeneous micro-inclusion concentrations in acoustic materials will not perform as desired, and this method allows non-destructive inspection to determine such non-homogeneity without exposure to an acoustic environment or as a supplemental diagnostic tool. 
     This imaging method also allows for determination of micro-inclusion concentration levels in micro-inclusion impregnated materials in a contactless, nondestructive way. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive nor to limit the invention to the precise form disclosed; and obviously many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.