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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 1 7 Fig. 1 — Microwave examination of Kevlar panel. Fig. 2 — Microwave inspection of high- density polyethylene. M icrowave (MW) nondestructive testing (NDT) is defined as non- destructive inspection of a mate- rial using electromagnetic energy in the microwave frequency range as the inter- rogating medium. The microwave fre- quency range exists between radio waves and far infrared in the 300MHz to 300 GHz frequencies. These frequencies roughly correspond to wavelengths between 1 m and 1mm in a vacuum, respectively. MW inspection operates in a sim- ilar fashion to ground penetrating ra- dar (GPR). That is, the inspection device emits an electromagnetic energy beam in the microwave frequency range and then records the reflected signals from the material under test (MUT). See Fig. 1. These reflected signals are then represented visually, and the image in- terpreted based on its appearance. The two parameters that determine electro- magnetic field propagation, as well as reflection and absorption, are the elec- trical permittivity and magnetic per- meability of the space. This means the method can be used to penetrate any dielectric material for volumetric in- spection as well to inspect the surface of metals. Today, MW NDT is common- ly used in the inspection of rubber ex- pansion joints in power plants, high density polyethylene (HDPE) butt fu- sion and electrofusion, fiberglass pip- ing and joints, ceramic materials, and other specialty applications [1-3] . MICROWAVE INSPECTION STANDARDS For any nondestructive testing method to be successfully used in in- dustrial applications, the equipment needs to be standardized via national and/or international standards. These standards are developed to govern equipment calibration as well as pro- vide information on training and certifi- cation of users and specific procedures for various materials. The following list identifies some codes that have been is- sued or are in progress to date: • ASNT Recommended Practice No. SNT-TC-1A - 2016 Edition • ASME Section III Appendix XXVI - HDPE Inspection 2017 Edition fiberglass pipe inspection and carbon fiber pipe repair inspection. However, the largest project to date is focused on developing MW approaches for inspect- ing composite wind turbine blades. INSPECTION OF WIND TURBINE BLADES Utility-scale wind turbine blades are primarily made of fiberglass rein- forced polymer (FRP) with various core materials as a small percentage of the total composition. Over time, blade • ASTM E3101-18 “Standard Practice for Microwave Examination of Poly- ethylene Butt Fusion Joints” • ASTM E3102-18 “Standard Practice for Microwave Examination of Poly- ethylene Electrofusion Joints used in Piping Application” • ISO - Microwave Inspection of HDPE Butt and Electrofusion Joints (in progress) • ASTM - Standard Practice for Micro- wave Inspection of Fiber Reinforced Polymer Products (in progress) • ASME Section V Article on Micro- wave Inspection (in progress) Many of these standards are in de- velopment or have recently been up- dated, indicating the growth of new applications in this field. HISTORICAL MICROWAVE RESEARCH The Electric Power Research In- stitute (EPRI) has been involved in ac- tive microwave research projects since approximately 2005 when inspection of HDPE pipe fusions was emerging as a best practice in the nuclear power plant arena. This was followed by ex- perimenting with microwave inspec- tion on a sampling of HDPE fusions and inspecting rubber expansion joints in power plants (Fig. 2) [4-7] . These early research efforts were followed by sev- eral new projects based on industry in- put. Many of these were associated with