a. davar

Composite structures are prone to reduced performance due to defects occurring over time during the manufacturing and loading process. In this study, the effect of longitudinal and circumferential cracks alone as well as their combined (longitudinal and circumferential together) effect on vibrational properties of composite cylindrical shells was investigated simultaneously using experimental and numerical modal testing. Also, the effect of crack length and depth in longitudinal and circumferential directions on the natural frequencies of composite cylindrical shells was investigated. Initially, in orderto validate the finite element (FE) model, an intact shell and a defective shell were modeled and analyzed, and the results were compared with the reference. After validating the FE model, first, modeling and frequency analysis were carried out for a composite cylindrical shell consisting of four layers of woven glass fibers (300 g/m2) impregnated with an epoxy resin with a 0.73-mm thickness for each layer. Then, the defects (cracks) in both longitudinal and circumferential directions in different dimensions and sizes were modeled and after analyzing all samples (intact and defective), naturalfrequencies and shape modes were obtained. To perform the experimental modal testing (EMT), several shell samples, similar to the simulated samples, were made and the natural frequencies were extracted for all samples. The natural frequencies obtained from the two methods were compared and the estimates show that the results of both methods are in a good agreement. The results also show that longitudinal cracks affect the natural frequencies and the mode shape more than circumferential cracks do.

A composite grid sandwich panel consists of a core with a composite grid structure and two faces on both sides of the core. This study investigated low-velocity impact in grid sandwich panels with grid cores experimentally and numerically by constructing and performing experimental tests using Abaqus finite element software. In the experimental part, three sandwich panels with grid cores were made for the low- velocity impact test. In the numerical part, three-dimensional elements were used, and damage was solved via programming in the Fortran language in Abaqus software. The results showed that the use of foam in the core of these structures reduced deflection due to impact despite a slight increase in the final weight of the structures.

In this study, the transient dynamic analysis of grid-stiffened composite conical shells is discussed. The transient dynamic response of the composite conical shell with simply supported boundary conditions under the lateral impact load, which is applied extensively and uniformly on a certain surface, is obtained using the convolution integral and based on the method of addition of modes. The validation of the obtained results has been done with the help of references and ABAQUS finite element software. The effects of various parameters such as fiber angle, geometric ratios, type, etc. have been investigated in forced vibrations. Finally, the effect of reinforcing the conical shell with the help of grid-stiffened structures has been studied.

In this paper, the low-velocity impact response of composite plates in the presence of pre-loads is investigated using three new models for contact force estimation. The boundary conditions are considered as simply supported and the behavior of the material is linear elastic. The equations are based on both classical and first order shear deformation theory and the Fourier series method is used to solve the governing equations. The mass of the impactor is considered to be large mass and therefore the impact response is categorized as quasi-static. In the first impact model, the contact force history is first considered as a half-sine and then the maximum contact force and contact duration are calculated.  In the second model, an improved two degree of freedom (ITDOF) spring-mass system is expressed by calculating the effective contact stiffness using a fast-iterative scheme. In the third model, which is expressed for the first time in this paper, the plate is considered as a series of masses and springs constructing a multi degree of freedom (MDOF) spring-mass system and the average forces applied to springs is introduced as the contact force. Validation of these models is done by comparing the results with the analytical, numerical and experimental results and shows good agreement. Results show that the new MDOF spring-mass system is more accurate for calculating the contact force rather than the ITDOF spring-mass system.

The present study analyzes the free vibration of multi-layered composite cylindrical shells and perforated composite cylindrical shells via a modified version of Reddy’s third-order shear deformation theory (TSDT) under simple support conditions. An advantage of the proposed theory over other high-order theories is the inclusion of the shell section trapezoidal form coefficient term in the displacement field and strain equations to improve the accuracy of results. The non-uniform stiffness and mass distributions across reinforcement ribs and the empty or filled bays between the ribs in perforated shells were addressed via a proper distribution function. For integrated perforated cylindrical shells, the results were validated by comparison to other studies and the numerical results obtained via ABAQUS. The proposed theory was in good consistency with numerical results and the results of previous studies. It should be noted that the proposed theory was more accurate than TSDT.

In this paper, the low-velocity impact response of laminated composite cylindrical shells subjected to the combined pre-loads is investigated. The pre-load is applied as the mechanical pre-load (axial force and radial pressure) and the thermal pre-load. The boundary conditions are considered as simply supported and the behavior of the material is linear-elastic. The equations are based on the first-order shear deformation theory and the Fourier series method is used to solve the analytical equations. The impactor studied as a large mass and therefore the impact response is considered to be quasi-static. The results show that regardless of the type of the axial pre-load (tensile or compressive), changes in contact parameters during the impact are linearly related to the temperature changes. Furthermore, these variations with respect to the radial pressure is almost linear for the tensile axial pre-load, but it is nonlinear for the compressive axial pre-load.

In this paper, the free vibrations of a composite cylindrical grid shell reinforced with carbon nanotubes are investigated. Equilibrium equations are derived based on first-order shear deformation theory. The orientation of carbon nanotubes is assumed to be uniaxial in the direction of the assumed thickness and the elastic modulus of the polymer composite reinforced with carbon nanotubes is calculated using the Rule of mixture. In order to achieve the stiffness parameter equivalent to the shell of grid cylinders, Smeared method has been used to suppress the effect of ribs. Abaqus software and valid references have been used to validate the obtained results. According to the results, the presence of peripheral ribs in the structure increases the frequency and reduces the radial displacement. Also, the thickness and angles of the ribs are important in improving the frequency and the presence of carbon nanotubes plays an important role in strengthening the structure and Increases the frequency and decreases the radial displacement due to the applied load.