CROSS REFERENCES TO RELATED APPLICATIONS
STATEMENT OF GOVERNMENTAL INTEREST
This application is a continuation of application No. 08/225,848, filed Apr. 11, 1994, now abandoned.
BACKGROUND OF THE INVENTION
 The Government has rights in this invention pursuant to Contract No. N00039-91-C-0001, awarded by the Department of the Navy.
The invention is in the field of thermal wave imaging/sensing and characterization for nondestructive/noncontact evaluation. Specifically, the invention comprises the use of microwave heating with Time Resolved Infrared Radiometry (TRIR) methods.
TRIR is a thermal characterization technique developed for the nondestructive evaluation of layered materials. In TRIR a region close to a sample's surface is heated by a source, e.g., a laser or flashlamp, with a long pulse and the sample's surface temperature is monitored as a function of time through changes in emitted infrared radiation. Specimen features which influence the production or transport of heat cause the surface temperature to change relative to areas without such features. This has allowed subsurface delaminations to be imaged.
- SUMMARY OF THE INVENTION
An infrared imaging camera allows rapid, quantitative inspection at relatively high spatial resolution. However, the visibility of the subsurface specimen features in the thermal image is determined by the magnitude of the reflected thermal signal which is determined by the depth of the defect and the ratio of the thermal effusivities of region and sample. For example, for subsurface voids filled with water this contrast is small.
In the method of the invention, a specimen/sample of a material is illuminated/heated with microwaves and then a temperature imaging/sensing means/method, such as an infrared imaging device (e.g., focal plane array), monitors the heating in the specimen due to electrical and/or magnetic property discontinuities, e.g., dielectric loss or the presence of a conducting contaminant.
BRIEF DESCRIPTION OF THE DRAWINGS
For optically opaque but microwave transparent materials containing localized absorbing regions the use of a microwave heating source, when compared with conventional laser or flashlamp sources, has distinct advantages. For particular specimen geometries and material properties, the presence of the defect region can be imaged at higher contrast and better spatial resolution than for the surface heating case of TRIR, hence, enhancing the detectability of such defect regions. Since the temperature has only to diffuse to the surface, the characteristic thermal transit times for the measurement are shorter. Moreover, since three-dimensional diffusion acts as a spatial low pass filter and reduces the image resolution of localized thermal features, the shorter path for the thermal signal allows better resolution. Finally, when the region of interest can be selectively heated by specific microwave wavelengths, the image contrast is enhanced.
FIG. 1 illustrates a schematic diagram of an example experimental setup to test the method of the invention to detect dielectric loss in a test sample.
FIG. 2 illustrates a test sample used in the experimental setup of FIG. 1.
FIG. 3, consisting of FIGS. 3(a), 3(b), 3(c) and 3(d), illustrates infrared images of the test sample of FIG. 2 taken (a) before, (b) during and (c) after a 2.7-second microwave pulse, and (d) an 8-second x-time image during a 2.7-second microwave pulse, where the temperature along a single line in FIG. 3(a) is shown as a function of time.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 4 illustrates plots of the surface temperature time response (normalized to the peak temperature) of the test sample for different lines taken from FIG. 3(d).
In the method of the invention, a specimen/sample of a material is illuminated/heated with microwaves and then a temperature imaging/sensing means/method, such as an infrared imaging device (e.g., focal plane array), monitors the heating in the specimen from microwave-absorbing regions due to electrical and/or magnetic property discontinuities, e.g., dielectric loss or the presence of a conducting contaminant.
The use of a microwave source in TRIR provides the ability to heat surface and subsurface microwave-absorbing regions directly, provided the host material is transparent or semi-transparent to microwaves. Thermal diffusion from the heated region to the surface causes a change in the surface temperature of the host and this change can be detected using temperature sensing, e.g., infrared imaging, methods. Furthermore, by varying the frequency of the microwaves, molecular species in the material can be selectively excited.
The invention uses the time-dependence of the temperature distribution to produce images of subsurface microwave absorbing features in the specimen which are much smaller than a microwave wavelength. Comparison with theoretical calculations shows that it is possible to obtain quantitative information about the electromagnetic and thermal properties of the features and their depth in the sample from the time-dependent temperature.
FIG. 1 shows a schematic diagram of an example experimental setup used to test the method of the invention to detect dielectric loss. The microwave pulse is generated by an oscillator and then amplified by a traveling wave tube amplifier. A horn antenna is placed close to the test sample surface at the desired angle, in this example 45°.
The surface temperature of the test sample is monitored in this case by an infrared scanner but the invention, in general, could use any temperature sensing method/device. For example, in addition to the devices already mentioned, the surface temperature could be sensed by monitoring either the variations in the optical reflectivity of the surface of the material or the deflection of a probe beam skimming over the material's surface. The microwave pulse and the scanner frame rate are synchronized and images constructed using a computer. The test sample shown in FIG. 2 is a structured multilayer formed from a Teflon layer of varied thickness, d, a water layer of constant thickness, L, below the Teflon and a Plexiglass backing.
FIG. 3 shows x-y images of the sample taken (a) before, (b) during and (c) after a 2.7 s microwave pulse. The three water layers appear in the images in a temporal order corresponding to the different Teflon layer thicknesses. This is shown more clearly in the x-time image in FIG. 3(d), where the temperature along a single line in FIG. 3(a) is shown as a function of time. The total time for the x-time image was 8 s and the pulse length was 2.7 s. The start of the x-time image coincides with the beginning of the microwave pulse. The different times for the maximum temperature rise are easy to recognize.
Plots of the temperature time response are shown in FIG. 4 for different lines taken from FIG. 3(d) for positions over each of the three water layers. The data has been normalized to its peak amplitude to correct for the nonuniform microwave distribution obtained, since the work was done in the near field due to power considerations. These results compare very well to theoretical calculations of the surface temperature in a one-dimensional geometry which are shown by the thick solid lines in FIG. 4.
Subsurface features in materials which can be detected using the method of the invention include boundaries/edges; nonconducting discontinuities, i.e., a dielectric loss caused by water; and conducting contaminants such as wires or graphite fibers. For the detection of fibers, a range of angles of incidence of microwaves to the sample and a range of microwave frequencies can be selected and employed to improve detectability. For example, a specific microwave frequency can cause resonant absorption of the microwaves in a conducting contaminant. The conducting contaminant is more easily detected under resonant absorption because microwave absorption is higher resulting in a greater temperature rise in the contaminant. The optimum microwave frequency to achieve resonant absorption is determined by the length and dielectric properties of the fiber and can be calculated or measured experimentally.
It is also important in detecting fibers that the polarization of the microwaves match the orientation of the fiber contaminants, i.e., the polarization of the microwaves must be aligned (e.g., by physically orienting the microwave horn by rotation) parallel to the long axis of the contaminating fiber. There will be no excitation and, hence, no detection of the contaminating fiber if the polarization is transverse to the fiber.
Also, for some classes of contaminants the microwave magnetic field interaction is as important as the electric field interaction with the microwave angle of incidence and polarization impacting the magnetic field interaction as well. The sensitivity of this technique to the presence of discrete fibers provides the opportunity for development of smart materials with embedded fibers as sensors which are interrogated using the microwave thermographic technique.
The invention's use of microwave induced TRIR imaging as a quantitative nondestructive/noncontact evaluation method for characterizing subsurface defects containing water and for detecting conducting contaminants has tremendous potential. Broad classes of microwave and radio frequency absorbers should be accessible to such measurements and microwave sources should extend the area of application of thermal wave imaging using infrared detection. The analytical model for the time dependence of the surface temperature allows extraction of quantitative data including the depth of the defect. The measurements can be extended to other microwave absorbing systems and can be further developed by the selection of characteristic microwave wavelengths.
As with other infrared radiation imaging techniques, large area inspection is possible but high spatial resolution imaging can also be achieved at lower image sizes. The resolution is determined by the infrared optical system, not by the microwave wavelength; subwavelength imaging has been demonstrated to 30 μm resolution. Since the imaging process is wavelength independent with a high depth resolution, microwave photothermal measurements potentially allow spectroscopic measurements to be made even for strongly absorbing materials.
Possible applications of microwave-induced infrared imaging include:
a. detecting water in composites or beneath coatings;
b. detecting fiber breakage (impact damage), such breakage showing up as an anomaly or hot spot;
c. evaluating low observable materials for contaminants including the development of real time manufacturing process control;
d. substituting for a strain gauge when used to illuminate and detect movement in prearranged fibers attached to a structure, e.g, a bridge;
e. studying cure rates for composites;
f. providing a new approach for measurements on disordered materials as well as materials of biological and medical interest; and
g. in general, for high spatial resolution imaging of microwave absorbing regions on and in a range of materials.