System for detection of flaws by use of microwave radiation

Targeted portions of material layered structure is probed by microwave radiation focussed onto the targeted portion by adjustment of antenna position and orientation establishing a single oblique incidence path for reflection of antenna emitted probing radiation. Signal measurements of the radiation along the oblique incidence path is obtained to provide for evaluation and detection of defects in the targeted portion of the structure being probed.

BACKGROUND OF THE INVENTION
 The detection and evaluation of structural flaws by non-destructive use of
 radiation is generally known in the art, including use of microwave
 radiation exhibiting certain advantages over other forms of radiation such
 as x-ray, ultrasound and thermography radiation. Such uses of microwave
 radiation include emission and reflection of the radiation after
 interaction with the targeted material for detecting the presence or
 absence of a defect therein. Microwave radiation types of defect detection
 systems heretofore involved one or more antennas for emission of the
 radiation and reception of reflective radiation. Some of the advantages
 over the use of other forms of radiation include, avoiding use of a
 couplant and heat diffusion means, increasing depth of detection and
 avoiding the provision of radiation hazard prevention. It is therefore an
 important object of the present invention to enlarge evaluation of
 structural flaws by the advantageous use of microwave radiation as
 disclosed in connection with the embodiment covered in the aforementioned
 prior copending parent application, Ser. No. 08/798,683 now U.S. Pat. No.
 5,859,535. According to the disclosure in such prior copending parent
 application, detection location and sizing of an internal defect in a
 thick non-metallic material or composite is achieved by isolating in time
 reflections of microwave radiation from external surfaces using Fourier
 transformation of frequency domain data applied in a straight forward
 manner.
 SUMMARY OF THE INVENTION
 In accordance with the present invention, microwave radiation is both
 transmitted from and received by one antenna positioned and orientated to
 transmit and receive microwave energy along an oblique angle from the
 surface normal of a targeted material at a focus location from which
 reflected radiation originates in order to identify and evaluate defects
 from measurement of the microwave radiation. Evaluation of structural
 flaws is thereby achieved through calculations based on readings of
 radiation measurement data with respect to a single radiation path,
 pursuant to techniques constituting an extension of the approach set forth
 in the aforementioned prior copending parent application, wherein
 reflection from a defect is not completely separated in time with respect
 to reflection originating from some interfaces in the material structure
 being targeted. By use of gating techniques, generally known in the art,
 the data on reflections from boundaries or interfaces are excluded from
 data on reflections originating from targeted defects of interest in
 accordance with the present invention. The new approach or technique of
 the present invention involves utilization of frequency domain response
 from either an internal thin layer or an external surface of the targeted
 structure after a reverse Fourier transformation is applied to the time
 domain for return of gated reflection to the frequency domain. Such new
 technique is based on the defect induced changes in the resonances of the
 material layer in question in the frequency domain. This new approach is
 also applicable to cases wherein the signals originated from material
 interfaces near the defect are not completely excluded from the defect
 signal by time gating.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Referring now to the drawing in detail, one embodiment of the invention is
 illustrated in FIG. 1 which depicts a single antenna 10 from which a
 microwave radiation beam is emitted along a normal surface incidence path
 12 toward a focus location 14 in a non-metallic type of target 16
 involving for example dielectric materials or composites. Such outgoing
 microwave radiation from antenna 10 focused onto the target 16 is
 reflected from the target surface spaced by a stand-off distance from the
 antenna 10 along the path 12. When disposed in another position and at a
 different angular orientation relative to the target 16, the antenna 10
 emits or receives radiation along another path 18 at an oblique angle
 (.theta.) to the path 12 as denoted in FIG. 1.
 Also depicted by way of example in FIG. 1, is a support arrangement for the
 antenna 10 so as to accommodate its repositioning and reorientation on a
 platform 30 for establishment of the different radiation paths 12 and 18.
 An optical beam splitter 20 shown in FIG. 1 is angularly reoriented about
 a bearing axis 26 on scattering surface plane 22 of the platform 30, which
 is supported for angular adjustment about the incidence path 12 on a
 suitable fixed stand 32. As diagrammed in FIGS. 2A and 2B, the platform 30
 also supports an optical mirror 24 which is utilized in cooperation with
 the beam splitter 20 in a manner generally known in the art to angularly
 adjust the antenna 10 on the platform 30 along a translation path 28 to
 establish the radiation path 18 focused on the same target location 14 at
 the oblique angle (.theta.) to path 12.
 FIG. 1 also diagrams an operative connection of the antenna 10 to a
 calibrated microwave network analyzer 34 which is per se known in the art.
 Such a microwave network analyzer 34 includes for example a wide frequency
 band microwave energy source, multi-frequency transmitter and coherent
 receiver from which signal measurement data is received and coupling
 circuits through which the transmitter and receiver are operatively
 connected to the antenna 10. A signal separation means is also included
 for independently processing outgoing and returned scattering signals.
 Pursuant to the present invention, the processed signal measurement data
 from the analyzer 34 undergoes signal processing in a reflectometer
 control system 37 through which control data is generated and fed to an
 adjuster 40 for adjustment of position and orientation of the antenna 10
 on the scattering plane 22 as aforementioned.
 The geometry of the antenna and scattering plane arrangement corresponding
 to that of FIG. 1 is shown in FIGS. 2A and 2B with respect to the focus
 location 14 in the radiation targeted material 16. As diagrammed in FIG.
 2A, the scattering plane 22 corresponding to paths 12 and 18 is depicted
 for both the position of the antenna 10 focussed along the path 12, and
 the other incidence position to which it is displaced along the
 translation path 28. The front view of FIG. 2B shows the antenna 10
 positioned on the scattering plane 22 at an azimuthal angle (.beta.) to
 horizontal axis 46. Based on the arrangement depicted in FIGS. 1, 2A and
 2B, measurements of microwave radiation is utilized to identify and
 evaluate defects or flaws in the target 16 in accordance with different
 embodiments, including the embodiment disclosed in the aforementioned
 prior copending parent application.
 FIG. 3 illustrates a radiation targeted structure 100 subject to microwave
 radiation transmitted and reflected along an incidence path 42 for
 detection, location and sizing of interface defects near an internal thin
 layer 106 in accordance with one embodiment of the present invention. The
 structure 100 includes a rigid foam slab 102 bonded to a composite plate
 104 by the thin layer 106 formed of interface bonding glue having a hole
 108 therein as the simulated interface defect. Based on radiation
 measurements graphically depicted in FIG. 4, the frequency response varied
 along the ordinate for normal incidence is reflected by curve 44
 indicating a typical resonance and anti-resonance type structure 100 with
 a dip at a frequency of 26.9401 GHz. As the angle (.theta.) of incidence
 of the radiation varied, the such frequency of the dip at point 46 on
 curve 44 shifted to higher frequencies. As a consequence, the radiation
 signal amplitude along the ordinate at this frequency shown in FIG. 5 for
 the graphical plot 48, reflects a sharp increase as the angle of
 incredence (.theta.) increases. In the absence of any defect, the
 systematic shift in frequency response does not occur. Instead, there is
 an overall decrease in signal amplitude for all frequencies as the angle
 of incidence increases. FIG. 6 graphically depicts the variations in
 signal amplitude of the measured radiation vs angle of incidence (.theta.)
 for targeted multi-layered type structures 100 having different void
 sizes. Curve 50 reflects no defect in the glue layer 106. Curves 52, 54
 and 56 respectively reflect voids 108 having dimensions of
 0.5".times.0.06", 0.75".times.0.06" and 1".times.0.06". Curve 58 reflects
 a void 108 forming a complete airgap. It is apparent from the graphical
 plots 50, 52, 54, 56 and 58 in FIG. 6 that the frequency selected
 measurement at an angle of incidence (.theta.) between 8 and 10 degrees
 along the abscissa provides for defect detection within a signal amplitude
 range of 8 db along the ordinate. Thus, such graphically depicted data
 indicates that by recording only oblique incidence angle reflections, a
 defect is present when a certain signal amplitude value is exceeded for a
 small oblique incidence angle (.theta.) so as to discriminate against
 effects of target surface conditions. Also at oblique incidence,
 reflections unrelated to the defect and its immediate surroundings do not
 reach the antenna 10 so to reduce the need for Fourier transformation and
 time gating.
 FIGS. 7 and 8 relate to other embodiments of the invention through which
 defects located at the interfaces between two materials are detected and
 evaluated and wherein at least one of such materials is non-metallic. As
 shown in FIG. 7A, a single antenna 10 is adjustably positioned and
 orientated for emission of probing microwave radiation at an incidence
 angle (.theta.) to the surface 78 of a slab 74 of non-metallic material
 having a metallic backing 76 to form a planar interface 80 underlying the
 surface 78. The radiation emission beam thus enters the surface 78 of slab
 74 along the adjustably angled oblique incidence path 82 and is propagated
 through the non-metallic slab for reflection from the interface 80 and a
 90 degree corner interface segment 84 at which slab 74 terminates. The
 reflected radiation accordingly emerges from surface 78 of slab 74 in
 spaced relation to the incidence path 82 along a reflective radiation path
 86 for reception by the same antenna 10, as shown in FIG. 7A. Where the
 terminal end of the non-metallic slab 74 is remote from the incidence path
 82, the antenna 10 is angularly adjusted relative to the interface 80 for
 travel of the microwave radiation through slab 74 as shown in FIG. 7B so
 as to provide for emergence of the reflected radiation along path 86
 without repositioning of antenna 10 for recognition of reflected
 radiation.
 Travel of radiation from antenna 10 through a wedge-shaped slab 74' is
 shown in FIGS. 7C and 7D respectively corresponding to the slab
 arrangements shown in FIGS. 7A and 7B involving angular orientation and
 repositioning of the same antenna 10 for emission and reception of the
 microwave radiation from the boundaries of the interfaced slab 74' and
 metallic backing 76. The angular adjustment of the antenna 10 and
 repositioning thereof, as well as the supply of probing radiation for
 emission from the antenna is effected by electronic processing control
 through a reflectometer control system 37 and analyzer 34, as diagrammed
 in FIG. 1. A simplified system involves an arrangement as diagrammed in
 FIG. 8 for detection and evaluation of interface defects between the
 material layers depicted in FIGS. 7A, 7B, 7C and 7D.
 In regard to defects located at the boundaries between two materials having
 a right angle geometry as shown in FIGS. 7A and 7C, the detection
 technique involving use of the single antenna 10 is particularly
 simplified since data collection at multiple frequencies or use of Fourier
 transform into the time domain for signal separation is not needed. Two
 factors for implementation of the defect detection technique are however
 important. One is selection of the incidence angle (.theta.) influenced by
 dielectric properties of the materials. Such incidence angle is dependent
 on the distance (c) as depicted in FIG. 7A, having its largest allowed
 value when the incidence angle reaches 90.degree.. For materials having a
 dielectric constant between 2 and 3, incidence angles between 45.degree.
 and 35.degree. are used as a compromise between reduced specular
 reflection and adequate material penetration, where the height of the
 vertical segment in the 90.degree. reflector was 2 inches as denoted in
 FIG. 7A.
 With reference now to FIG. 8, microwave radiation is fed from a source 88
 to a hybrid coupler 90 for transmission through a signal circulator 92 to
 the antenna 10 for both measured emission therefrom of transmitted
 radiation and measured reception of reflected radiation. A reflected
 radiation signal is accordingly fed from the signal circulator to a mixer
 system 94 from which an output signal is obtained for defect evaluation by
 evaluator 96. Part of the output of signal mixer system 94 is also fed to
 the antenna adjuster 40 through which the antenna 10 is angularly
 orientated and positioned so as to meet the requirements for defect
 evaluation at the interface boundary locations specified with respect to
 FIGS. 7A-7D. Electronic processing of transmitted radiation measurement
 signals and reflected radiation measurement signals by the mixer system 94
 enables a signal adjuster 98 in the form of a phase controller or
 frequency controller to receive the output of mixer system 94 so as to
 provide a feedback thereto and a control input to the mixer system 94 in
 order to detect components of the reflected radiation in different phase
 relationships to the transmitted radiation, thereby maintaining a
 predetermined signal parameter such as a maximum signal amplitude or a
 selected signal frequency in response to the reflected radiation signal
 output fed to the evaluator 96. Continuous measurement of radiation
 reflection and detection of defects is thereby maintained. Furthermore,
 through the adjuster 40 the condition of maximum signal amplitude or
 selected signal frequency is made to coincide with proper orientation and
 positioning of the antenna 10 for reception of retroreflected radiation
 from the interface 80 being evaluated.
 The foregoing described single antenna technique for non-destructive defect
 evaluation is applicable to structural arrangements such as an acoustic or
 thermal absorbing coating corresponding to non-metallic slab 74 on flat or
 wedged-shaped backings such as metallic hulls of Naval vessels, without
 prior knowledge of material thickness or electrical properties. The
 electronic feedback from the signal adjuster 98 diminishes potential
 effects of misalignment variations during reflectometric scanning of
 interface surfaces for mapping defect locations. Signal amplitude
 maximization or maintenance of selected frequency response as hereinbefore
 referred to enables maintenance of the correct angle of incidence
 (.theta.) of the antenna 10 during emission of probing radiation for the
 occurrence of retroreflection and to compensate for small material and
 geometric variations.
 The operating frequencies for the system 37 diagrammed in FIG. 8, are
 selected near the midfrequency between peak and valley values in the
 frequency domain, based on the natural resonance and anti-resonance of the
 target structure dependent on its thickness and material properties.
 Because of interference between the signal originating from the defect and
 the resonating response of its nearby structure, the oblique incidence
 signal is observed to be larger when a defect is present as compared to
 the absence of a defect at frequencies midway between natural resonance
 nodes and antinodes of the target structure. At an oblique incidence angle
 (.theta.) of 10 degrees for example, the adverse effects of material
 surface roughness is also substantially diminished.
 Obviously, other modifications and variations of the present invention may
 be possible in light of the foregoing teachings. It is therefore to be
 understood that within the scope of the appended claims the invention may
 be practiced otherwise than as specifically described.