الفهرس 
LITERATURE REVIEWOnly 14 pages are availabe for public view
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Abstract
Composite materials are widely used in automotive, naval, and aerospace structures, where they are often subjected to fatigue loading which is one of the most common failure modes in all structural materials. The repetitive loading of a composite material causes gradual degradation of strength and stiffness due to the accumulation of discrete micro-damage (e.g. fiber fractures, fiber/matrix debondind, matrix cracks).The main objective of the present work is to investigate the capability of experimental modal analysis as a nondestructive tool to characterize and quantify the fatigue behavior of laminated composite beam with different lamina orientations and cantilevered boundary condition. In the present work, experimental modal analysis was conducted on the specimens previously subjected to fatigue loading to determine the modal parameters (natural frequency, damping ratio and mode shape). This was achieved through studying the response of modal testing with different specimens of different lamina orientations as a main factor affecting fatigue life. This correlates modal parameters such as: damping ratio, natural frequency and mode shape to fatigue behavior. The composite material used in experiments is glass fiber reinforced polyester (GFRP) laminate. Plane bending fatigue tests were performed on standard fatigue specimens. The fatigue test was interrupted at different fatigue life ratios (n/Nf) and modal testing was conducted to determine the change in modal parameters. The results showed that the changes of modal parameters provide a proper means for predicting the fatigue behavior of composite structures. from the experimental results of both dynamic and fatigue tests, curve fitting technique was used to correlate modal parameters to fatigue life. An exponential and quadratic equations have been obtained which correlate fatigue life ratio to damping ratio and resonant frequency respectively.It was noticed from the curves representing exponential and quadratic equations that the value of damping ratio ξ is more noticeable than the value of frequency, which means that the effect of ξ on the fatigue life is predominant hence the damping ratio is sufficient to express the effect of natural frequency. A numerical study was made to predict fatigue life using the normalized residual stiffness and the normalized fatigue life models. The degradation rate in stiffness with the number of cycles was used as damage metric to predict the fatigue life of composite materials.Statistical analysis of fatigue life data based on two parameter Weibull distribution model was employed. It has been found that the probabilistic distribution of fatigue-life of glass-fiber reinforced composites, at a particular stress level, can be modeled by two-parameter Weibull distribution, with high statistical co-relation coefficient. For all considered composite specimens with different lamina orientations, different modes of failure were found at low and high fatigue stress levels, which establish different damage mechanisms. The results revealed that higher shape parameters were observed at higher stress levels with less scatter in the fatigue-life data indicating a more uniform damage mechanism. Two methods have been used to obtain the parameters of Weibull distribution. The two-parameter Weibull distribution has also been employed to incorporate failure probability into S-Nf relationships which can be used by designers to obtain the fatigue strength of GFRP at the desired level of failure probability.