Saudi Cultural Missions Theses & Dissertations
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Item Restricted Damage identification in plate structures using dynamic response(Saudi Digital Library., 2026) Alshammari, Yousef Lafi Ateeq; Khan, MuhammadStructural Health Monitoring (SHM) extensively relies on dynamic response measurements to assess the condition of engineering structures and detect damage. Cracks in plate structures represent a significant concern in engineering, as they can affect the integrity and performance of various mechanical components. The propagation of these cracks can take several forms, including straight, curved, and random paths. Most previous studies have mainly focused on straight cracks; however, they often assumed a constant damping ratio in their analytical models. In practice, the damping ratio is likely to change with the crack path, particularly with variations in crack length and orientation, which can reduce the accuracy and reliability of these models. Furthermore, in engineering structures, cracks rarely propagate along idealised straight paths. Instead, they develop along curved or irregular paths. These curved crack paths can significantly alter the patterns of stiffness degradation, the mechanisms of energy dissipation, and the overall dynamic response. Despite their practical importance, the effects of these paths on vibration characteristics remain unexplored. In this thesis, the influence of both straight and curved crack paths on the dynamic response of cantilever fused deposition modelling acrylonitrile butadiene styrene (FDM ABS) and aluminium plate structures was investigated using experimental modal analysis (EMA), finite element analysis (FEA) and machine learning. These substantially different materials were selected to examine whether crack path effects yield consistent trends in vibration response across dissimilar material systems, thereby supporting the broader applicability of the findings. A novel methodology for modelling both straight and curved crack paths was developed. Unlike most previous studies that mainly idealised cracks as straight paths or focused on crack length alone, the novelty of this methodology lies in its ability to systematically model and identify both straight and curved crack paths using dynamic response measurements. The straight cracks are defined using coordinate pairs, while curved cracks are described by second-order polynomial equations. The proposed curved crack modelling scheme reduced the number of possible crack path configurations from 7,140 to 252, corresponding to an approximate 96.5% reduction, while preserving representative geometric diversity for analysis. The effects of crack length and orientation on damping ratio were examined for aluminium and fused deposition modelling (FDM) printed ABS plates. The influence of polynomial coefficients (quadratic, linear, and constant) on the dynamic response of aluminium plates was examined. Finally, both forward and inverse predictive models were developed using linear regression (LR) and artificial neural networks (ANNs) to relate vibration features to crack paths. The results show that crack path has a greater influence on the dynamic response than crack length alone. Numerical results showed that a 50 mm Y-axis crack reduced the first natural frequency to 761.36 rad/s, compared with 771.67 rad/s for a longer 70.71 mm crack oriented at 45°, confirming that crack orientation can dominate over crack length. The same Y-axis crack produced the highest maximum stress and strain, reaching 13.2 MPa and 4.93 × 10⁻⁴, respectively, compared with 8.84 MPa and 3.31 × 10⁻⁴ for the 45° crack and 4.20 MPa and 1.57 × 10⁻⁴ for the X-axis crack. Experimental results further demonstrated that damping ratio increased with crack growth, from approximately 0.0080 to 0.01728 in FDM ABS, representing an increase of about 110%, and from 0.0071 to 0.0117 in aluminium, representing an increase of about 65%. For curved cracks, the quadratic and linear polynomial coefficients had the greatest influence on natural frequency, vibration amplitude, and damping ratio, whereas the constant coefficient had a negligible effect. The proposed prediction models were experimentally validated, with the straight crack polynomial regression model achieving an overall average damping-ratio prediction error of 1.79%. For curved cracks, the forward LR and ANN models predicted natural frequency, amplitude, and damping ratio with an overall average error of 2.28%, while the inverse models estimated final crack path coordinates with an average error of 8.26%. These quantitative findings demonstrate that incorporating crack path geometry improves vibration-based damage assessment and provides a validated framework for crack path identification in plate structures8 0