Weak bond detection in composites using solitary waves

Abstract

A composite material is composed of fibers embedded in a matrix and is called Fiber Reinforced Plastic (FRP). Particularly, Carbon Fiber Reinforced Plastic (CFRP) has carbon fiber as the reinforcement. It has a high strength-to-weight ratio and is not less susceptible to fatigue or corrosion compared to metallic materials. Therefore, composite materials like CFRP find applications in aerospace industry due to their low weight and advanced mechanical properties. For large scale composite structures, smaller parts often need to be connected together. The connection of these parts could be done either by mechanical joints such as fasteners or by gluing them together using adhesives. This research focuses on bonded composite structures. A prevailing problem in bonded joints is the indeterminate nature of the strength of the adhesive bond. Low strength bonds, i.e., weak bonds, are very difficult to detect and may become the cause of catastrophic failure. Hence, this work aims to develop a technique based on highly nonlinear waves called solitary waves to detect weak bonds. Nonlinear solitary waves can be generated in a 1-D chain of granular crystals, which consists of discrete particles having nonlinear interactions. Solitary waves are high energy packets, which maintain a finite spatial width while traveling. These waves, traveling in the granular chain, can be transmitted into the specimen by making direct contact with the chain. The reflected wave pattern from the specimen can give information about the localized stiffness of the specimen. In this study, two kinds of CFRP specimens are fabricated - one with pristine bond and the other with a weak bond. For making a weak bond specimen, mold release agent is used to coat the surface of CFRP laminate prior to bonding. This coating acts as an impurity at the bondline and is an example of poor surface preparation. First, standard tests such as the three point bending and the end notched flexure tests are conducted to demonstrate that the strength of the bond is reduced due to the presence of the release agent coating. Further, impact test by solitary wave injection is conducted to cause weak bonds to develop delamination and obtain the response of solitary waves reflected from the specimens. We study the reflected wave characteristics for weak bond detection. Two cases are considered; in the first case, two specimens - pristine and weak bond samples, both without having an initial crack - are tested. In the second case, two specimens - pristine and weak bond samples, both with an initial crack - are tested. The results differentiate the weak bond with the pristine bond based on reflected wave characteristics. This study shows promising results indicating the potential of solitary wave-based detection of weak bond for hot spot monitoring of composite-based structures.