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A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 0 2 0 paid to the materials used is an import- ant consideration. If not done correctly, the results may not provide an accu- rate assessment of this promising NDT technique. FUTURE DEVELOPMENTS EPRI’s major focus is on advanc- ing the capability of MW multi-frequen- cy data collection using vector network analyzers (VNA) with specifically de- signed antennae, as shown in Fig. 6. The use of a bespoke antenna system for specific materials and conditions tremendously improves the response of the device and allows for enhanced defect detection and sizing along with the capability to adjust for varying ge- ometry and material, as well as compo- sition. The antennae are also designed with broadband features that provide for expanded frequency ranges that can be used during the inspection to provide multiple benefits. First, the en- hanced range increases the probabil- ity of defect detection by expanding the possibilities for a reflected signal from specific defect types. This is par- ticularly true when compared to older single frequency systems. Second, the increased frequency range provides for finer depth increments than a smaller frequency range. One excellent example of an im- age created with an early bespoke an- tenna and VNA is of a defect sample of a high-density polyethylene plate. Figure 7 shows the ability of the multi-frequency system to image, size, and determine the depth of the defects. The part includes multiple back-drilled holes, some of which are filled with alu- minum foil, some empty, plus a top and bottom surface metal indicator. EPRI’s system was able to successfully cap- ture all defects in a single scan image as well as determine relative depths. The part measures 24 x 12 x 1 in., and was inspected in approximately 8 minutes. CONCLUSIONS Ongoing research activities into microwave techniques have resulted in numerous advances within the field that are applicable across sever- al power-related industries. The EPRI microwave inspection table currently available is one of the most advanced tools of its kind. Research continues into various supporting fields, such as mounting a microwave-based in- spection system onto a drone for up-tower inspection of wind turbine blades. Additional research is planned to continue development and refinement of software to improve the accuracy of depth detection routines. Additional- ly, future work is planned to introduce synthetic aperture focusing techniques and other advanced data analytics, such as artificial intelligence, to im- prove imaging quality and defect detec- tion and recognition. ~AM&P For more information: Christopher Nelson, senior technical leader, Electric Power Research Institute, 704.595.2533, cnelson@epri.com , www.epri.com ; Bob Stakenborghs, CEO, Advanced Micro- wave Imaging, 225.329.5815, rjstak@ advancedmwimaging.com, www. advancedmwimaging.com . References 1. R. Hochschild, Applications of Mi- crowaves in Nondestructive Testing, Nondestructive Testing, Vol 21, No. 2, p 115-120, March-April 1963. 2. R. Stakenborghs, Innovative Tech- nique for Inspection of Polyethylene Piping Base Material and Welds and Fig. 7 — HDPE flat plate (24 x 12 x 1 in., inspected in 8 minutes) demonstrating location and sizing of multiple defects using advanced MW technologies now in development.