Stiffeners are widely used as building blocks of a wide range of engineering structures because they provide better resistance without significantly increasing the overall weight of the structure. However, stiffened composite structures are subjected to multi-field loading in extremely complex operating environments, which will degrade the strength and may eventually lead to failure of the structure. The presence of a discontinuity in the form of interlayer delamination or interfacial debonds in stiffened composite plates detrimentally affects the structure’s safety and durability. These defects may grow to a critical size, which becomes unstable, causing the entire structure's catastrophic failure. On the other hand, the plates provided with cutouts are extensively used in transport vehicle structures. Cutouts are provided to lighten the structure, for ventilation, and to provide accessibility, which degrades the strength of the structure. This will show a completely different behaviour of the structure, so it becomes necessary to investigate. Through the Finite Element technique, the 9-noded isoparametric heterosis plate elements have been used to model the panel and the stiffener’s flange, and the 3-noded isoparametric beam element is used to model the web of the stiffener. The debond is introduced at the plate-stiffener interface by creating dummy nodes at the interface, and the fictitious springs are employed to prevent the interpenetration of the nodes. 

Understanding the vibration performance of these structures helps in designing components that can withstand static and dynamic loads and aids in investigating phenomena such as flutter. Furthermore, analysing the panel's vibration behaviour can be utilised as a valuable tool for structural health monitoring purposes, especially for identifying and locating defects like interfacial debonding 

Buckling behaviour of stiffened composite panels is a critical area for ensuring structural integrity in aerospace, civil, marine, mechanical, and automotive industries. The work primarily focuses on understanding the influence of interfacial debonding and hygrothermal conditions on structural stability under various non-uniform in-plane edge loadings. Further, understanding dynamic instability is crucial as the phenomenon can manifest as an increase in transverse vibration and may arise even below the critical buckling load, especially when damage like delamination is present. Overall, this work provides crucial insights for tailoring the design of composite structures and integrity assessment.  

Ultrasonic-guided wave techniques offer an accurate and efficient procedure for damage monitoring in structures. To develop reliable damage monitoring systems, it is essential to have a thorough understanding of ultrasonic-guided waves' quantitative nature that can be transmitted in composite laminates.

The Semi-Analytical Finite Element (SAFE) method is employed for the analysis of guided waves' dispersion behaviour in composite laminates by accounting for the in-plane load effect. An infinite-width plate is considered such that the cross-section of the waveguide is modelled using 3-noded isoparametric 1-D elements representing the plate's thickness. The equation of motion is formulated by using Hamilton’s equation.

Apart from the study related to understanding the dispersive behaviour of the wave in the prestressed panel, a study will be conducted to assess and quantify the damage in the delaminated stiffened plate subject to in-plane edge loading under a hygrothermal environment. Various signal features are analysed through different signal processing techniques to comprehend wave characteristics. The outcome shows that the presence of a stiffener causes a conversion of an A0 mode to an S0 mode, which propagates and can aid in detecting debonding in the panel. 

Furthermore, this study can serve as a valuable tool for structural health monitoring purposes, facilitating the identification and localisation of debond in composites, ultimately contributing to the assessment of the structural integrity of such composite structures