Effect of Interfacial Defects on Mechanical and Electrical Properties of Composite Materials Fatigue

Formation of defects during the manufacturing, adhesive bonding, and repair process can be very detrimental to the performance of a composite structure. Specifically, interfacial defects can severely affect the durability of fiber reinforced laminated composite materials and their joints. Despite progress in detection of defects (micro-scale void and micro-to-macro scale disbond) using popular nondestructive methods, significant challenges still remain in quantifying the distribution of defects and the resulting loss of properties. The objective of this study is to explore the effects of interfacial defects on the strength of adhesively bonded lap joint made of laminated composite substrate and epoxy adhesive. Nano-micro-scale interfacial defects (surface features and impurities) can cause a weak joint called “Kissing bond” where the substrates may remain in contact, but unable to provide the required bond strength. This is often difficult to characterize and a major concern for reliability. The proposed concept relies on the fact that heterogeneity at the material interfaces changes charge polarization inside a material when subjected to an AC electric field and this fundamental material behavior can be quantified using Broadband Dielectric Spectroscopy (BbDS) technique. Both bulk and spatial distribution of dielectric properties can be measured nondestructively to represent the state of weak interface. Surface contamination which causes weak bond can be identified prior to bonding and hence potentially allowing for corrective measure. Results show that changes in dielectric properties (e.g., complex impedance, permittivity, capacity) can be utilized as a quantitative measure of the residual or remaining strength of a weak joint under tensile loading.