6. Terahertz Testing Technique for Fiber-Reinforced Composite Materials

Terahertz (THz) systems constitute an effective tool for the NDT&E community for the testing and characterization of fiber-reinforced composite materials. However, their systems are still very complicated and expensive to commercialize. Also, establishing the inspection limits for the vast majority of fiber-reinforced composite structures is still not achieved because this technique is relatively new in the area of material testing and evaluation. Nevertheless, this technique presents several advantages including the fact that it can see “through” the defects in thin composites and examine the underlying fabric of the material, overcoming the shadowing effect that is commonly observed with other NDT techniques such as ultrasonic testing and most of the radiographic testing techniques. Although the technology had been deferred for many years because of the inadequacy of its emission and detection devices, the so-called “THz gap”, this problem has recently been addressed thanks to the development of highly performing semiconductors and ultrafast electronics. To date, extremely short pulses required for the energy frequency of the THz waves can be achieved, suggesting that spatial resolution of the inspection levels higher than those of the normal microwave-based NDT techniques can be reached using THz systems. A lot has been done but much still needs to be done, particularly because there are no reported studies on the inspection of moisture uptake in fiber-reinforced composite structures nor are there any studies that confidently inspect conductive materials using THz waves. Indeed, this would be a highly valued milestone to the literature if it was achieved. In applications involving the inspection of thick composites and sandwich structures, THz systems do not, unfortunately, provide reliable inspection results owing to the attenuation and/or the scattering effects of the THz waves in thick sections.