Dielectric properties and noncontact damage detection of plain-woven fabric glass fiber reinforced epoxy matrix composites using millimeter wavelength microwave

Electromagnetic wave reflections from glass fiber reinforced epoxy matrix composites with 0°/90° and ±45° fiber oriented plain-woven glass fabric (PW-GFRP-0/90, PW-GFRP-±45) at incident angles of 30°, 40° and 50° were measured in the frequency range 50–75 GHz using a free-space reflection measurement system. The complex dielectric constants of both composites were calculated using a simple transmission line theory. The complex dielectric constants of PW-GFRP-0/90 and PW-GFRP-±45 are similar and were measured to be ε′ = 4.61 ± 0.01 and ε″ = 0.16 ± 0.002, respectively. The damage stored in PW-GFRP-0/90 and PW-GFRP-±45 was also evaluated by dielectric constant changes using the same system at an incident angle of 30°. For both composites, ε′ decreased with increasing applied stress and damage parameter. The dielectric constant change is effective for detecting the damage stored in composites and can be used to quantitatively evaluate the damage.