Damage classification of cnt/cnc-reinforced pu foam-cored sandwich panels through acoustic emission testing

Abstract

Aerospace companies are making significant attempts to design highly-efficient lightweight aircraft while satisfying the required mechanical properties and allowing increased volume for the payload. Composite structures, specifically sandwich composites, have widespread use in the aviation industry owing to their high strength-to-weight ratio when compared to standard metal-aluminum construction materials. Although honeycomb sandwich composites have high impact resistance, they have disadvantages such as moisture absorption, water accumulation, and debonding due to the open-cell structure of the honeycombs. Accordingly, rigid polymeric foams with a closed-cell core structure are capable of overcoming these disadvantages and replacing honeycombs in future applications. Closed-cell and rigid polyurethane (PU) foams are valuable since they have an easy manufacturing process, multifunctionality, and capacity for reinforcement studies. Carbon nanotubes (CNTs) frequently serve as reinforcement materials in PU foams owing to their superior strength and light weight. However, when the concentration of CNT reinforcement is relatively high, disadvantages such as agglomeration and nonuniform dispersion may occur in the foam structure, leading to poor mechanical properties of the materials. Therefore, reinforcing CNTs into PU foams by hybridizing them with auxiliary materials such as cellulose nanocrystals (CNCs) might be an efficient alternative approach to the dispersion problem. The hybridization with CNCs can prevent the formation of strong Van der Waals bonds between CNTs. Ensuring passenger safety in aircraft requires prompt detection of any potential damage. Despite several benefits of foam-filled sandwich composites, they also have particular damage mechanisms that may occur under high loadings. The non-destructive testing method called acoustic emission (AE) testing allows real-time identification of transient elastic waves that are released from the defect source by sensors and classification of these failure mechanisms. Unlike other non-destructive testing techniques such as electromagnetic, radiographic, and ultrasonic, AE analyzes the failure tendency and prevents potentially irreversible damage by using data released directly from the defect source. Nevertheless, it is necessary to use a clustering method to categorize AE data into appropriate clusters that are each associated with a possible damage mechanism. Among manual clustering, signal processing, supervised clustering and unsupervised clustering; unsupervised clustering arises due to their great performance in first-time analysis of AE hits without having a historic data of the experiment. The performance of the k-means algorithm, which is one of the most efficient unsupervised clustering methods, heavily depends on the first cluster centers, and it often gets stuck at local minima. Hence, the k-means genetic algorithm can resolve this challenge through improved searching capacity that can provide an optimum solution in a reasonable amount of time. Previous research in the literature mainly studied the AE analysis of sandwich composites with honeycomb and foam cores, but there is a lack of research on sandwich composites with nanomaterial reinforcement of core materials. The thesis investigated the impact of reinforcement studies on the damage mechanisms of CNT/CNC-reinforced foam-cored sandwich composites through the AE test system. Damage mechanisms were classified using peak frequency, amplitude, and cumulative energy of AE data by the k-means genetic algorithm. In addition, as a subsection, the bending properties of CNT-reinforced PU foam-filled 3D woven I-beam composites were studied. Thus, a preliminary study was performed for CNT reinforcement studies on PU foam and its combination with sandwich composites. Furthermore, this preliminary study was beneficial for understanding the influence of PU foams on sustaining structural integrity and avoiding damage mechanisms in composite structures.