11 - Ultrasonic inspection of natural fiber-reinforced composites

Abstract

Natural fiber-reinforced composites are currently widely used in the construction, automotive, aerospace, and sporting sectors, due to their excellent mechanical properties and also low raw material and manufacturing costs. These natural fibers have unique characteristics compared to synthetic fibers, especially on their better strength and stiffness per unit weight, renewability, nontoxicity, and zero carbon dioxide fingerprint. These natural fiber-reinforced composites require adequate nondestructive testing (NDT) inspection in order to control the quality, and to fully access the structural integrity. Ultrasonic inspection is one of the most commonly applied methods for composite testing compared to other nondestructive testing methods, such as the resonance method, thermography, radiographic testing, electromagnetic testing, acoustic emission, and stereographic testing. Natural fiber-reinforced composites are basically categorized as laminate and monolithic honeycomb construction. For the honeycomb core, hammer tap testing and bond testing are suitable, as is pulse echo testing for laminate, whereas for laminate with honeycomb core, through transmission is suitable. Hammer tap test by acoustic principle is used to detect delamination and/or disbonding of the structure; however, bond testing is to inspect the quality of bonding of the composite by three types of testing modes such as pitch–catch, mechanical impedance analysis, and resonance testing. Pulse echo testing is suitable to inspect the laminate for thickness measurement and sizing of defects, while through transmission is sensitive to detect the presence of foreign objects, inclusions, interlaminar delaminations, disbonds, and porosity. The other NDT methods that are recommended for natural fiber-reinforced composites are the acoustic emission technique, infrared thermal imaging, and thermographic techniques.