Application of composite patches in repair of damaged/aged aircraft structures is one of the most popular repairing methods in aerospace engineering. Since running experiments are difficult, time consuming, expensive, and also require high level of expertise, simulation of the behavior of the patch and also the faulty components after repair can assist designers and engineers in optimization of their designs. In this article, full-scale simulation of a damaged panel that is experimentally repaired with a composite patch will be considered using ABAQUS, a commercial finite element code. The crack growth process is modeled with the extended finite element method and the cohesive zone model (CZM) is used to model the progressive damage in the adhesive of the composite patch repair. Also, sensitivity analysis is performed on the CZM parameters and it is shown that the three parameters i.e. the shear toughness, maximum first shear traction, and penalty parameter for the elastic stiffness are important in the simulation of adhesively bonded composite patch repairs. The calibrated cohesive properties are successfully used to predict the response of the composite patch to the strengthened in damaged structure with considering the linear and nonlinear stage of failure process. The simulation results obtained in different stages have been verified with the existing experimental results.

Molecular surface electrostatic potential was used to predict the solvent effect on the reaction rate, endo/exo selectivity and diastereomeric excess of a Diels-Alder reaction. It is shown that these quantities can be expressed in terms of molecular surface electrostatic potentials of solvents which are obtained computationally by the HF/6-31þþG* procedure. Regression analyses and an experimental database are used to obtain analytical representation of rate constant, endo/exo selectivity and diastereomeric excess. The models obtained show that the hydrogen bond donor ability of solvents on the above mentioned properties is substantial, whereas solvophobicity only affects the reaction rate and endo/exo selectivity of the reaction.

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In this paper, the configuration parameters of pre-designed composite patch repair are optimized with the aim of achieving the highest level of stability of crack growth in aluminum in the presence of some constraints such as weight, load sustainability, shear stress in the adhesive layer and maximum stress in the patch. For this purpose, the patch is modeled in full scale by ABAQUS, a commercial finite element code. The crack growth process is simulated with the extended finite element method under uniaxial tensile loading, and the Cohesive Zone Model is used to model the progressive damage in the adhesive of the composite patch repair. Also, sensitivity analysis is performed on the configuration parameters and it is shown that three parameters, i.e. width, stiffness ratio, and height of the patch are more important. Nonlinear fracture mechanics concepts have been used in calculating the stability of crack in the cracked aluminum plate. The results show that optimization based on the method proposed in this paper causes the stability of crack growth to increase by 21% while the patch weight is reduced by 52%.

Hydrogen bonding effects of protic solvents, apart from bulk properties, on the reaction rate of the cycloaddition of cylopentadiene and vinyl acetate in the presence of water and methanol in the gas phase were investigated. The results showed that methanol increases the reaction rate in the gas phase more than the water. This is attributed to the stronger hydrogen bonding effect of methanol in this phase. Ab initio and semi-empirical calculations show that methanol stabilizes the transition state of the reaction more than water. This arises from two different origins, distribution of charge and geometry of the hydrogen bond.