چكيده به لاتين
According to the anisotropic nature of layered composites, the evolution of various occurred damage modes in different layers, and their viscoelastic behavior, detecting damage and/or material properties degradation becomes a challenge. In the present thesis, the use of ultrasonic guided Lamb waves, which have shown potential for detecting damage in complex composite specimens as part of a structural health monitoring system, has been investigated. Damage in a structure can be detected by analyzing the difference between the dispersion characteristics of Lamb waves on damaged versus un-damaged specimens. For this purpose, a code was developed in the MATLAB software environment. The propagation characteristics of Lamb waves are described in the form of dispersion curves, generated by solving the Lamb wave equations implementing the Global Matrix Method through the mentioned program. Furthermore, the Hashin theory, the classical lamination theory (CLT), and the inverse CLT are used for specific purposes of the damage detection process. The results of the analysis of these Lamb waves through the developed program were firstly validated by comparing the available experimental outcomes. Afterward, by examining the influences of different percentages of the applied crack density in the cross-ply composites with different values of m and n parameters, diverse mechanical properties, and various thicknesses, has been shown that Lamb wave propagation is sensitive to even the slightest difference in the characteristic of the material. It has also been observed that the greater the number of off-axis layers, the better and faster the transmission of damage information. In addition, fundamental symmetric Lamb wave S0 mode indicates the difference in the material behavior much more clearly and faster than mode A0, which is the fundamental antisymmetric mode, and also for a similar percentage of damage, rather differences in the phase velocity of S0 mode is observed. For instance, in a constant composite thickness, by inducing 10% crack density, while the m/n=1/3, 9.6% difference has been seen in the phase velocity of mode A0, but 27% conversion in the phase velocity of the mode S0, besides, while the m/n=1, 4.6% change is observed in the phase velocity of mode A0, but 13% variation in the phase velocity of the mode S0, occurs near the cut-off frequency of each composite specimen. The employment of code and the results of this study will be helpful in determining the optimal Lamb wave emission conditions to investigate the mechanical material properties and the presence or absence of defects/damage in the anisotropic multilayer composites.
