Structural adhesives in aeronautical applications are routinely exposed to complex loading histories that generate time-dependent deformation, making accurate prediction of their viscoelastic response essential for reliable assessment of joint integrity. This work presents an integrated experimental and modeling study of the aerospace-grade epoxy adhesive 3M Scotch-Weld EC-2216 using multi-step creep and stress-relaxation tests performed at room temperature and controlled loading rates, combined with fractional viscoelastic modeling. Unlike traditional single-step characterizations, the multi-step protocol employed here captures the cumulative loading effects and fading-memory dynamics that govern the adhesive’s mechanical response. The experimental data were analyzed using fractional Maxwell, Voigt–Kelvin, and Zener formulations. Statistical evaluation based on the Bayesian Information Criterion (BIC) consistently identified the Fractional Zener Model (FZM) as the most robust representation of the stress-relaxation behavior, effectively capturing both the unrelaxed and relaxed modulus. The results demonstrate that EC-2216 exhibits hierarchical relaxation mechanisms and history-dependent viscoelasticity that cannot be accurately described by classical integer-order models. Overall, the study validates the use of fractional operators to represent the broad and hierarchical relaxation spectra typical of toughened aerospace epoxies and provides a rigorous framework for durability assessment and predictive modeling of adhesively bonded structures.

This work presents a teaching approach for postgraduate aeronautic engineering students, fo-cusing on the analysis and understanding of structural dynamics in complex, real-world applications rather than simplified models. Using a commercial quadrotor as a case study, students investigate critical issues affecting lateral stability—specifically, abnormal vibrations and unintended sliding during autonomous self-leveling. These stability challenges prompt students to explore whether the issues stem from control limitations or structural deficiencies. Students employ the frequency response function (FRF) through impact hammer testing to characterize the natural frequencies of the quadrotor's arms and propellers, identifying potential resonances caused by the propulsion system. The experimental analysis offers students insights into structural solutions and control system reprogramming to counteract resonance-induced vibrations, improve stability, and enhance autonomous functionality. This hands-on approach allows students to directly apply concepts of structural dynamics to complex aerospace systems, preparing them for advanced prob-lem-solving in real-world engineering environments.

In this work, the structural dynamics of a commercial quadrotor was investigated, which presents problems of lateral stability or sliding instead of remaining self-leveling through its autonomous control system. The possible existence of vibrations motivated the characterization of the frequency response of the arms and propellers by means of the frequency response function (FRF) using the impact hammer technique. The experimental results provided sufficient information for the future implementation of structural solutions or through reprogramming in the autonomous control algorithm that compensates or mitigates the negative effect of vibration levels due to resonance phenomenon and improves the stability in self-leveling condition of the quadrotor.

En este trabajo se presenta un panorama general de la estrategia de Monitoreo de Integridad Estructural, sus componentes de hardware y escala o niveles de aplicación, así como los elementos relacionados con el análisis, diagnóstico y pronóstico. Se describe el amortiguamiento interno estructural como una medida de la disipación de energía y su potencial para ser utilizado como parámetro de medición de la integridad estructural. Se realizó un proceso experimental en una probeta de unión adhesiva estructural sometida a cargas de bajo ciclaje donde se evaluó la evolución de las energías disipadas. Los resultados experimentales muestran cambios significativos en la capacidad de disipación de energía de la unión adhesiva que se relacionan directamente con la evolución del daño. Se discuten los resultados de la integración de la estrategia MIE utilizando la medición de amortiguamiento por histéresis para la evaluación del daño, la resistencia residual o la vida útil de un sistema estructural.

Damage in single lap joints under shear load was assessed. Specimens were tested in accordance with ASTM D1002-10(2019). Controls were taken on adhesive and surface preparation, adhesive thickness, and load-specimen alignment. Digital Image Correlation was used to measure stresses over the adhesive, and shear modulus variation and percent of damage were assessed. Results show shear modulus losses against shear stress. Finally, early damage evolution was determined up to 10% of the shear load rate and later, increasing up to 70%-96%.