National Aerospace Laboratories

This study aims to experimentally characterise the mechanical behaviour of a single lap joint thermoplastic (TP) adherend bonded with a thermoset (TS) adhesive. Understanding the interface behavior of these joints is crucial, given the lower surface energies of thermoplastics compared to thermosets, making adhesion a challenging task over the TP surfaces. Therefore, prior surface modification of TP adherends is essential for effective adhesive bonding. Plasma treatment (PT), a method being explored here, alters the surface roughness, improves the contact angle, and increases the surface free energy. Surface roughness induced by PT is quantified using an atomic force microscope (AFM). The correlation between NDT assessment, surface roughness parameters, and single lap shear strength (SLSS) is being investigated. Fractography analysis and other findings contribute to a comprehensive understanding of the factors influencing the joint strength and integrity of such bonded systems, thereby establishing an appropriate bonding scheme for greater structural integrity towards aircraft applications.

The use of fibre optic sensors for structural health monitoring (SHM) has been persuaded by the aircraft industry and research organizations for over three decades. The Fibre Bragg Grating (FBG) sensors, because of their inherent advantages, are considered the best choice for the aircraft industry for structural health monitoring applications. These sensors are fragile in nature and are still being used in lab-scale experiments. A methodology is developed to package the sensors using a glass-carbon/epoxy composite to make the sensor robust without altering its sensitivity. Different arrangements are established for the protection of the sensing region and the non-sensing region of the sensor. Sensor packaging is carried out in such a way that the sensor quality and its strain transfer capability are unaltered. Based on spectrum comparison and ease of installation, the sensing region of FBG is packaged between composite layers, and the non-sensing region is protected using Teflon and other flexible tubes. The temperature and strain response of the sensors are studied experimentally and compared with bare FBG sensor and resistance strain gauge (RSG) respectively. These studies showed that packaging has the same response to strain and temperature and is very reliable.

In this work, a linear fractional transformation–fluid structural interaction (LFT-FSI) approach is developed by coupling a low-fidelity robust flutter method with high-fidelity FSI solutions and applied on the Advisory Group for Aerospace Research and Development (AGARD) wing for transonic flutter studies. Here, the uncertain aeroelastic equation is formulated in modal coordinates, considering low-fidelity aerodynamics based on the unsteady doublet lattice method. Uncertainties in unsteady aerodynamic parameters of the low-fidelity system are modeled in the LFT framework. A high-fidelity FSI approach based on coupled computational structural dynamics–computational fluid dynamics (CSD-CFD) technique is employed to accurately estimate the flutter behavior of the wing at limited transonic Mach numbers. The magnitudes of aerodynamic uncertainties are then estimated by a structured singular value (μ) based model updation technique, accounting for the aeroelastic damping obtained from high-fidelity FSI analysis. Finally, robust flutter analyses are performed based on the μ method to estimate the transonic flutter boundary of the wing using the low-fidelity model with updated aerodynamic uncertainty. The capability of the proposed LFT-FSI technique is demonstrated by comparing the flutter boundary of the wing with the available wind tunnel data in the whole transonic regime. It is observed that the proposed LFT-FSI technique provides an accurate prediction of flutter boundary in the transonic regime by reasonably estimating the bounds of aerodynamic uncertainty using limited subcritical (lower Mach) high-fidelity FSI data.