Scanning active microwave thermography for inspection of defects in bonded fiber reinforced polymer joints

Abstract

Defects at the adhesive layer of bonded fiber-reinforced polymer (FRP) joints can compromise their structural strength and ductility, necessitating effective non-destructive evaluation (NDE) method. This study proposes scanning active microwave thermography (AMT) as an efficient and rapid NDE technique to visualize interfacial defects at bonded FRP joints. First, multi-physics numerical simulations were conducted, which agrees well with experimental results in thermal response. The simulation demonstrated the formation of an electric field concentration at the specimen edges. Second, 14 bonded FRP joints with various defect shapes, sizes, locations, and plate thicknesses were tested under AMT. Tests show that AMT can effectively detect interfacial defects in FRP joints with a diameter greater than 5 mm when carbon FRP (CFRP) is facing the AMT energy. As a comparison, when glass FRP (GFRP) are facing the AMT energy, it cannot be heated by AMT but the defect can still be detected by AMT, though it is not clear and accurate, because the bottom layer CFRP can be heated, which well explained the edge effects observed in test. Finally, a scanning AMT system was developed and used to visualize the interfacial defects at bonded FRP joints in a continuous manner, which exhibits high efficiency – defects can be inspected at a speed of 20 mm/s.