Fracture monitoring of textile reinforced cementitious sandwich panels using non-contact millimeter wave spectrometry

Textile-reinforced cementitious sandwich are lightweight, slender, non-corrosive, and cost-effective composites that have been recently proposed for structural applications. However, its composite nature depicts a complex fracture behavior, and manufacturing defects, such as a deficient interlaminar bond, can produce premature debonding. Lateral debonding can reduce up to 70% of the maximum load. Early detection of damage is key to reduce the cost and increase the impact of repair. Nondestructive testing (NDT) techniques have widely shown their ability to characterize and detect damage in cementitious composite materials. Microwave and millimeter wave (MMW) are electromagnetic (EM) NDTs that, besides not being attenuated by the insulating core, provide non-contact and real-time measurements. However, EM-based NDTs, besides Ground Penetrating Radar (GPR), have not been sufficiently explored for damage monitoring of cementitious media. Additionally, the complex fracture behavior of the composite makes the EM wave response difficult to interpret. In this manuscript, reference and artificially debonded TRC sandwich beams are tested in three-point bending and monitored with MMW spectrometry. Digital image correlation (DIC) is employed to monitor surface strain and interpret the EM wave response of the material. Results showed the sensitivity and characterization power of MMW spectrometry to different damage mechanisms such as matrix cracking, interfacial debonding, and changes in the angle of incidence, among others.