---

Extended finite element method (XFEM) is one of the strongest numerical methods that its basis is finite element but regardless of mesh location respect to discountinuty solves the problems. In this method, using of enreaching the nodes and increasing of their degrees of freedom (from 2 to 4 or even upto 10) virtually and without verifying the mesh and geometry of discountinuty, one can model and develop the required governing equations of the system. In this paper, fatigue crack growth of repaired aluminum panels containing a crack is studied. The cracked panels were repaired on one side with glass/epoxy composite patches in the mixed mode condition. The extended finite element method is used to study the effects of patch lay-up configuration on crack front displacement and stress intensity factor and the effect of crack angle on stress intensity factor of the repaired panels. The results show that the plate-fiber-fiber-aluminum configuration has best effect and it could reduce the stress intensity factor (k1) by upto seventy percent.

---

The present research deals with the impact response of notched aluminum plates repaired by fiber metal laminate (FML) patches under various temperatures using drop weight impact test status. Some aluminum samples repaired by FML patches were prepaired to study their impact behavior and frcture mechanisms under drop weight tests at the temperature range of -20 ℃ to 60 ℃. An Energy Profiling Diagram (EPD) was used to obtain the penetration and perforation thresholds of hybrid composites. Besides, the effect of temperature on some impact characteristics such as endurance load, contact time and permanent deflection were also studied. The results showed that the amount of force for nearly all of the samples increased by increasing of the room temperature. The ability of energy absorption of the samples was also the most at the room temperature, therefore the energy thereshold of samples increases by increasing of the room temperature. Temperature variation also affects on the impact characteristics of composites patches and in some cases results in a 20 percent reduce in impact strength of the samples. It was also shown that the most value of impact parameters reaches at -20 ℃ and 60 ℃.

---

Since aircrafts are subjected to aerodynamic and structural loads; one common defect in the aircraft fuselage and its wings is crack criterion. In most of the cases, the service life of defective parts can be increased by some sort of repairs. One of the most common types of repairs in this field is using composite patches and pasting them on damaged parts. These patches have significant advantages such as high strength, corrosion and moisture resistance, low weight and also excellent fatigue properties. In this study, base notched plates were fabricated by using of 2024 T4 aluminum alloy. Fiber metal laminate (FML) patches were made of carbon-epoxy and Phosphor – Bronze layers. These patches were attached to the base notched plate by using adhesive Arldit 2011. Specimens were subjected to tensile test and results of the tests were compared. The tested variables were chosen as lay-up, metal layer thickness and composite patch length. The results of current study indicate a dramatic increase in tensile strength of repaired parts by using these patches compared with the repaired notched parts without patches so that tensile strength is increased up to 82.4 % in the best sort of repair.

---

Today, with the development of technology, industries such as automotive and construction require products with variable cross section. Multiplicity of steps, dimensional limitation and high production costs of the components caused flexible roll forming process used to produce these products. One of the main defects in this process is the fracture phenomenon. The fracture is observed on the bending edges at transition zone that sheet thickness is large compared to the bending radius. In this research the fracture phenomenon is investigated on flexible roll forming process of channel section using ductile fracture criteria. For this purpose finite element simulation of the process using Abaqus software is done. The fracture defect in this process is investigated using six ductile fracture criteria by developing a subroutine. Experimental tests are performed on 27 specimens precut sheet of AL6061-T6, using flexible roll forming machine built in Shahid Rajaee University. By comparing simulation results with experimental results, numerical results were validated. In addition, by comparing the results of ductile fracture criteria with experimental results, the Argon ductile fracture criteria, was chosen as the most appropriate criterion to predict fracture. Also the effects of parameters as sheet thickness, bending radius and bending angle on fracture with argon selected criterion is studied.