ICONIC 2007 St. Louis, MO, USA June 27-29, 2007 A NOVEL NEAR-FIELD MILLIMETER WAVE NONDESTRUCTIVE INSPECTION TECHNIQUE FOR DETECTING AND EVALUATING ANOMALIES IN POLYMER JOINTS Sergey Kharkovsky 1, Emilio A. Nanni 1, Reza Zoughi 1, Johnny Yu 2 and Rod Wilson 2 1 Applied Microwave Nondestructive Testing Laboratory (amntl) Department of Electrical and Computer Engineering University of Missouri-Rolla, Rolla, MO 65409 Tel: (573) 341 4728, Fax: (573) 341 4352, E-mail: sergiy@umr.edu 2 ATK Energetic Systems Division, Radford, VA E-mail: Johnny.Yu@atk.com Abstract. Complex geometry of structures (joints, tubes, cylinder, etc.) often limits the ability to evaluate their integrity using standard nondestructive testing and evaluation methods. A novel near-field millimeter wave bi-static technique, employing dielectric waveguides, was developed to provide high sensitivity to anomalies in the interface of two polymer members. In this paper, a description of this technique and the results of detection and evaluation of the anomalies in polymer joints are presented. Keywords: polymer joints, disbond, millimeter waves, dielectric waveguides 1 INTRODUCTION Many adhesively bonded interfaces between materials (dielectric-dielectric, dielectric-metal, etc.) presently can not be effectively tested and evaluated with standard nondestructive testing and evaluation (NDT&E) methods. For example, optical methods can not be used for nontransparent materials or for samples with complex geometry (joints, tubes, cylinder, etc.). Ultrasonic methods often can not be used since ultrasonic energy is usually not transmitted through soft materials such as foam or polymers, and they also have problems inspecting structures with complex geometry. Microwave and millimeter wave methods have certain advantages over ultrasonic and optical methods when detecting defects (e.g., disbonds, delaminations, etc.) inside dielectric structures [1]. The utility of microwave and millimeter wave NDT&E methods and techniques has been effectively demonstrated for a diverse array of applications including detection and sizing of minute changes (in the few micrometer range) in thickness of dielectric sheet materials (including conveyed products), detection of small voids and thin disbonds and delaminations in stratified (sandwich) composites and estimation of their locations [1-10]. However, similar to other methods microwave and millimeter wave methods often can not be used in conjunction with structures possessing complex geometries, particularly for the structures with joints where it is difficult for the waves to readily access the joint. A novel near-field millimeter wave nondestructive inspection technique, utilizing dielectric waveguides, was developed to effectively inspect the interface region between two polymer materials (the 375
joint) in a manner significantly different than those commonly employed. Although dielectric waveguides have been used as inspecting probes for some applications, for example for probing the fields in the vicinity of a millimeter wave circuit or antenna [11] and millimeter wave imaging [12-13], it has not been used as a common probe for detection of anomalies. This paper presents the technique and its application for inspecting the interfaces of polymer joints possessing subtle anomalies such as localized and small air bubbles (i.e., localized disbond) produced during the manufacturing process. 2 APPROACH AND EXPERIMENT The complex geometry of the polymer structure of interest tends to produce measurements with relatively low sensitivity to the presence of disbonds in interfaces of these structures. To overcome this issue a new bi-static measurement technique for detection and evaluation of disbonds, incorporating dielectric waveguide probes, was designed and applied. Figure 1 shows the schematic of the bi-static measurement apparatus with dielectric waveguides. It consists of a millimeter wave transmitter (1), two hollow waveguide sections (2), two dielectric waveguide sections (3) and a receiver (4). The hollow waveguides couple the signal into and direct the signal out of their corresponding dielectric waveguides. Moreover, the dielectric waveguide sections must be inserted into the hollow waveguides in such a way to minimize unwanted reflections within. The receiver could be a phase or a magnitude (power level) detector or a combination of both depending on the required measurement sensitivity. Sample under test Figure 1: Schematic of the bi-static measurement set up with dielectric waveguides. One of the main features of the novel technique is that the ends of the dielectric waveguides are particularly shaped to provide an effective signal coupling into the sample under test. The shape of the dielectric waveguide probes depends on geometry of the sample and the dielectric property of its materials. For instance, Figure 2a shows the apparatus with the shaped dielectric waveguides probes inspecting a cylindrical polymer joint consisting of an adhesively bonded hollow polymer cylinder and ring. The dielectric waveguide probes are shaped in such a way that the reflections from the air-cylindrical polymer interface are minimized and the transmitted signal locally illuminates the inspected interface between the cylinder and ring. Another important issue of the technique is that the angle between the incident (transmitted) wave and the inspected interface is arranged to be smaller than the critical angle for a total internal reflection 376
of the wave from the inspected interface when disbond (air) occurs between the cylinder and the ring. The cylindrical polymer joint is rotated as shown in Figure 2b, while a DC output voltage is recorded at each measurement point. The presence of a disbond is detected when the output voltage significantly differs from the measured average voltage. As a result, this technique is inherently very sensitive to the presence of localized anomalies and particularly those near the dielectric waveguide probes. Cylinder Ring Inspected interface (a) (b) Figure 2: (a) Side view and a (b) top view of a hollow cylindrical polymer joint and dielectric waveguide probes. In this investigation, a set of individual and adhesively bonded cylindrical polymer samples, consisting of a hollow polymer cylinder and a ring, were manufactured. The bonded samples contained localized air bubbles (e.g., localized disbonds) on the interface of the polymers, produced during the manufacturing process. Using the dielectric constant of the polymer, the critical angle for total reflection of electromagnetic waves from the polymer-air interface was calculated. The critical angle was used for the placement of the dielectric waveguides so that total internal reflection would occur for polymer-disbond (air) interface. The measurements were conducted at several frequencies in V-band (50-70 GHz) and W-band (90-110 GHz). 3 RESULTS One of the samples was a cylindrical polymer joint similar to that shown in Figure 2 with three locally bonded areas in the interface. The bottom view schematic of the interface is shown in Figure 3a. The bonded areas were provided by using three patches of double sided tape (shown by white squares in Figure 3a) with dimensions of 4 mm by 4 mm and thickness of 0.5 mm, in such a way that the rest of the interface (marked by black color in Figure 3a) was air (disbonded areas). Figure 3b shows the output voltage, measured at 32 evenly spaced points around the circumference of the joint as the sample was rotated. Figure 3b demonstrates three noticeable drops in the measured output voltage, which also correlated exactly with the locations of the double sided tape patches representing good bonding. This result clearly shows that the millimeter wave signal incident at the interface is reflected significantly differently by a disbonded area of the interface than by a bonded area. 377
Figure 4a shows another joint sample that was produced using normal adhesive from the manufacturing process but in such a way that some areas around the joint were bonded well and some were not. This graphical representation was obtained from a picture of the interface using a digital camera. The percentage of disbond, calculated as the length of a disbonded region along a radial line across the joint, at each of 32 measurement points was determined from Figure 4a and is plotted in Figure 4b as the solid line (percent scale is not shown). The measured output voltage of the bi-static setup for this sample is also presented in Figure 4b. 0.02 0.018 0.016 0.014 0.012 0.01 0.008 0 4 8 12 16 20 24 28 32 Position on Sample (a) Figure 3: (a) Bottom view schematic of the sample with three locally bonded areas in the interface and (b) output voltage of the setup with dielectric waveguide probes vs. angular position of the probes. (b) (a) (b) Figure 4: (a) Bottom view schematic of the sample with bond and disbond areas and (b) angular distribution of disbond based on visual inspection (solid) and dependency of the output voltage on the angular position of the probes (dotted). It can be seen from Figure 4b that changing the position of the probes with respect to the bonded area of the sample interface causes the output voltage to increase in disbonded areas and decrease in bonded areas. The most important observation is that there is a very good correlation between the scaled percentage disbond distribution and the output voltage changes. 378
4 SUMMARY A novel near-field millimeter wave bi-static technique with dielectric waveguide probes has been presented. It was shown that the technique provides detection of the anomalies in polymers interfaces. When the bi-static technique was applied to samples with manufacturing defects a direct correlation was observed between percent disbond and variation output voltage. By using dielectric waveguides to increase signal strength and the reflection from a polymer to disbond (air) boundary, defects embedded in polymer joints were successfully detected. ACKNOWLEDGEMENT This work was supported by a Research and Development grant from ATK Energetic Systems Division, Radford, Virginia. REFERENCES 1. Zoughi R., Microwave Non-Destructive Testing and Evaluation, Kluwer Academic Publishers, The Netherlands, 2000. 2. Bakhtiari, S., and R. Zoughi, Microwave thickness measurement of lossy layered dielectric slabs using incoherent reflectivity, Research in Nondestructive Evaluation, vol. 2, no. 3, pp. 157-168, 1990. 3. Bakhtiari, S., S. Ganchev and R. Zoughi, Open-ended rectangular waveguide for nondestructive thickness measurement and variation detection of lossy dielectric slabs backed by a conducting plate, IEEE Transactions on Instrumentation and Measurement, vol. 42, no. 1, pp. 19-24, February 1993. 4. Bakhtiari, S., S. Ganchev, N. Qaddoumi and R. Zoughi, Microwave non-contact examination of disbond and thickness variation in stratified composite media, IEEE Transactions on Microwave Theory and Techniques, vol. 42, no. 3, pp. 389-395, March 1994. 5. Ganchev, S., N. Qaddoumi, E. Ranu and R. Zoughi, Microwave detection optimization of disbond in layered dielectrics with varying thicknesses, IEEE Transactions on Instrumentation and Measurement, vol. IM-44, no. 2, pp. 326-328, April 1995. 6. Ganchev, S., N. Qaddoumi, S. Bakhtiari and R. Zoughi, Calibration and measurement of dielectric properties of finite thickness composite sheets with open-ended coaxial sensors, IEEE Transactions on Instrumentation and Measurement, vol. 44, no. 6, p. 1023-1029, December 1995. 7. Bakhtiari, S., S. Gopalsami and A.C. Raptis, Characterization of delamination and disbonding in stratifies dielectric composites by millimeter wave imaging, Materials Evaluation, vol. 53, no. 4, pp. 468-471, April 1995. 8. Qaddoumi, N., R. Zoughi and G.W. Carriveau, Microwave detection and depth determination of disbonds in low-permittivity and low-loss thick sandwich composites, Research in Nondestructive Evaluation, vol. 8, no. 1, pp. 51-63, 1996. 379
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