The purpose of this paper is to investigate mechanical properties of Nanocomposite materials reinforced with carbon nano tubes (CNTs). This study considers the mechanical properties by three point bending test. The technique which is used in this study is one of the methods to calculate fracture toughness value. The specimens with four different mass percentages of carbon nano tubes were made. The mass percentages are 0%, 0. 01%, 0. 05%, 0. 1%, 0. 2% and 0. 3%. An ultrasonic device was used to disperse the CNTs uniformly in the epoxy matrix. So, the agglomeration of the CNT particles decreased in the matrix. In order to investigate the fracture toughness, same size cracks were created in two different directions on the specimens including low CNTs content grade and high CNTs content grade. Three point bending test was repeated at least three times for each of specimens. For each crack, there are different values of the fracture toughness and the fracture forces. Furthermore, the fracture surfaces of samples were investigated using scanning electron microscopy (SEM). The results showed that the direction of the crack, CNTs content and dispersion of the carbon nano tubes are important parameters. Also, flexural elasticity modulus that is different from tensile elasticity modulus was considered in this research.

Composite sandwich structures with grid stiffened core (SSGSC) are one of the new structural configurations applied in advanced industries such as aerospace, that are made of two thin face sheet layers attached to the top and bottom of a grid stiffened core. Due to the good advantages such as high specific strength, not only in aerospace but also they are used in other engineering applications, such as military industry, ship building, rail transport, oil platform etc. In the present study three composite SSGSC samples made of different material and thiknesses are fabricated with hand lay-up method using a silicon rubber mold and epoxy resin. Also, two metallic samples of the same dimensions as the copmposite ones, including a monolithic and a SSGSC samples made of aluminum are fabricated. In order to study their behavior subjected to the quasi-static transverse loads, the samples undergo three-point bending tests. Results of the practical tests on the composite samples showed that beyond the failure of the face sheets, the grid stiffened core will tolerate the load, also there are no delamination between the face sheet layers due to good curing process. It was found that changing the fibers of the face sheet from Glass to Carbon with the same thikness, improves strength-to-weight ratio of the SSGSC samples rather than increasing the thickness of the face sheet of the same material.

The API X65 steel (with a minimum yield strength of 65ksi equivalent to 448MPa) is one of the most common types of pipe steels in the transportation of natural gas in Iran. By studying the ductile and brittle fracture areas at the fracture surface of this steel, we can show the quality of this type of steel. In the present study, macroscopic fracture surface characteristics in three-point bending test specimen are studied (based on the geometry and standard notch of drop-weight tear test specimen). Test specimens were machined from an actual steel pipe of API X65 grade with an external diameter of 1219 mm (48 inches) and wall thickness of 14. 3 mm. Due to the quasi-static test conditions and speed of the machine’ s jaw (0. 1 mm/s), the test was carried out on base metal specimens with machine chevron notch of 15, 10, and 5. 1 mm depth, respectively, that was controlled with changing location. By applying the test load, cracking initiated from the notch root in each specimen and continued without crack specimen (ligament). At the end of the test, test specimens were cooled by liquid nitrogen and were broken in a brittle manner. In this paper, after investigation of the failure mode and the crack expansion in the standard specimen, investigation of macroscopic fracture surface characteristics was conducted by optical microscopy. By observing the fracture surface, different features such as thickness variation, shear regions (ductile fracture), cleavage fracture, shear lips, inverse fracture, and brittle fracture were studied. Having above 85% shear area, the ductile fracture of specimen was confirmed.

Composite applications are very wide in most industrial fields. Today, polymer matrix composites are used in the automotive industry, aerospace, oil, gas and petrochemical Industries and etc. Glass / epoxy composite is one of the most useful composites that has special properties such as high strength-to-weight ratio, high hardness, high corrosion resistance, low thermal expansion, resistance to nuclear radiation and absorption of energy. Composite beams may be used as flexural elements. In flexural loading, the crack initiation and failure can occur in a variety of modes that each ones has special frequencies. In this research, acoustic emission method was used to evaluate and check the different failure mechanisms of glass epoxy composite beam under three point bending loading. In order to determine different failure mechanisms, wavelet transform analysis was used for acoustic signal processing by using only one sensor. Three types of dominant failure mechanisms (matrix fracture, debonding and fiber breakage) in composite beam under bending were identified and the frequency ranges corresponding to these failure mechanisms were determined. Wavelet transform results showed that these three types of dominant failure mechanisms (matrix fracture, debonding and fiber breakage) have frequency ranges of 0-125 KHz, 125-250 KHz and 375-500 KHz respectively. Finally, the observations of scanning electron microscope from fracture surface of specimen validated the obtained results. This research showed the possibility of acoustic emission technique as a monitoring tool of glass/epoxy composite beam in flexural failure.

In this research, an influence of topology optimization in energy absorption of lattice core sandwich beams by using ABAQUS software was an investigation. Relationships between the force and displacement at the midspan of the sandwich beams were obtained from the experiments. Two types of Steel lattice cores with three cell orientation were subjected to the low-velocity impact test under threepoint bending. The core of sandwich beams was made from expanded metal sheets and a topology optimization with Solid Isotropic Microstructure with Penalization (SIMP) method was used to remove the redundant expanded metal cell. In the following, by studying the topology optimization to evaluate the impact parameters, including Specific Energy Absorption (SEA), as discussed testing purposes. The energy absorbing system can be used in the aerospace industry, shipbuilding, automotive, railway industry and elevators to absorb impact energy. Experimental and numerical results showed that topology optimization could significantly increase specific absorbed energy. Results of three-point bending crushing tests showed that the SEA of a sandwich beam with optimal core structure increased between 45% and 94% compared to the initial design structure of the core. In addition, appropriate orientation of expanded metal cell in the core of sandwich beam caused to increase the specific energy absorption by more than 90%. Finally, an appropriate optimal geometric structure with three tape of volume fraction and the best examples of criteria considered with respect to the objectives were introduced.

In the present study three composite sandwich structures with grid stiffened core (SSGSC) samples made of different materials and thicknesses are fabricated with hand lay-up method using a silicon rubber mold and epoxy resin. Also, two metallic samples of the same dimensions as the composite ones, including a monolithic and a SSGSC samples made of aluminum are fabricated. In order to study their behavior subjected to the quasi-static transverse loads, the samples undergo three-point bending tests. Results of the practical tests on the composite samples showed that beyond the failure of the face sheets, the grid stiffened core will tolerate load, also there are no delamination between the face sheet layers due to good curing process. The experimental modal testing is achieved on all the samples. The frequency response, mode shape as well as damping coefficients are obtained from each experiment. Finally, numerical modal analysis is done and the results are compared with the experiments.

Equi-atomic Nickel-Titanium (NiTi or Nitinol) has the ability to return to a former shape when subjected to an appropriate thermo-mechanical regime. Pseudoelastic and shape memory effects are some of the behaviors presented by these alloys. The unique properties concerning these alloys have encouraged many investigators to look for applications of NiTi for biomedical applications. One of the most successful applications of Nitinol is orthodontic archwire. The best features of these wires are super-elasticity, the phenomena that causes easy engagement (loading conditions). Superelastic nitinol wires deliver clinically desired light continuous force during deactivation (unloading conditions), enabling effective tooth movement with minimal damage for periodontal tissues. Superelasticity is characterized by a load-deflection plot with a horizontal region [plateau] during unloading, implying that a constant force may be exerted over that particular range of tooth movement. It is known that the NiTi alloy wire undergoes a phase transformation from an austenitic to a martensitic phase as the load increases during the loading process. Metallurgical studies have attributed these characteristics to a reversible phase transformation from the body centered cubic structure to the monoclinic structure of nickel-titanium when the stress reaches a certain level during deformation.The increasing amount of energy stored inside the NiTi wire during this process is consumed during the unloading process as the transformation is reversed, and the martensite structure reverts to austenite. Superelasticity is only exhibited by wires showing high endothermic energy in the reverse transformation from the martensitic phase to the parent phase and with low Load/deflection ratios. These wires show nearly constant forces in the unloading process, a desirable physiological property for orthodontic tooth movement. NiTi archwires have gained acceptable by orthodontists as initial alignment wires. Most of the information about the behavior of orthodontic wires is based on mechanical laboratory three point bending tests to study load-deflection characteristics. A three-point bending test allowed load-deflection curves offer reproducibility. Variations in model design have been shown to affect load- deflection plots. The load deflection performance of NiTi wires depends on the design of the test model. Modified three point bending test which simulates wire force on the teeth in the oral conditions has more correct results than ordinary three point bending test. The purpose of this study is to investigate the load-deflection characteristics of superelastic nickel titanium wires with a new model design trough modified bending tests. In this research a new three point bending fixture was invented and designed to determine the superelastic property in clinical conditions, and the wire samples were held in the fixture similar to oral cavity. By means of this instrument the three point bending test simulates wire force on the teeth in the oral configuration. The lower section of fixture is a rail fabricated from steel, a special movable base with a curved canal assembled over the rail. The upper section designed to simulate the teeth arrangement and curvature: A stainless steel disk (316L,   Ø80 mm, h10 mm) selected; and twelve rods (316L, Ø 5 mm, h10 mm) welded to the points representing center of teeth on the disk. The center points of teeth were located on the medium upper standard arc. Distance between the center points of teeth (interbracket distance) was similar to Wilkinson model.To achieve deflections 1, 2 and 4 mm, teeth 5,3 (right) and 2 (left) were selected respectively and rods in these points were movable. A fillet face machined on the rod surface parallel to the standard arc. Brackets fixed on the flat face of rod by superglue and orthodontic wire attached to fixed appliance. The superelastic behavior has been investigated through load-deflection tests. By rotating steel disk, different locations of teeth with different wire curvature were selected for loading. Commercial arch wires (Force I-American Orthodontics, Ø0.016 in) were loaded to 1, 3, 4 mm deflections. Each bending test was carried out three times. The results show that the constant force for commercial NiTi archwire (Force I) used in this study is more than 150 grams and minimum deflection required for superelastic property is more than 0.7 millimeters. In deflections lower than 0.7 millimeters, these archwires did not show plateau at unloading sequence, because martensitic transformation have not performed at low deflections. In these conditions (lower tooth deflections) the superelastic property is not available.

A Finite Element Analysis has been applied to a type of four-point bending specimen with S/W=3 to determine which condition a pure mode II can be constructed. The ANSYS simulation results have demonstrated that conditions l1=l4 and l2=l3 could not guarantee a pure mode II case be generated. The ratio W/D and ratio a/W have a remarkable contribution to the formation of pure mode II. At W/D=3.75 and a/W from 0.25 to 0.35, an almost perfect shear mode II can be achieved at the single crack tip region. When a/W varies 0.2 to 0.6, the specimen can also be treated as in pure shear mode II case if a 6 percent of the ratio KI/KII is acceptable. A KII expression in fifth-degree polynomials has been calibrated.

This study investigated the effects of atmospheric aging through three summer months on the mechanical properties of polyester reinforced with different mass rates of alfa fibers (Stipa Tenacissima). For this purpose, three-point bending tests were performed on pure polyester and polyester /alfa fiber composite specimens. A finite element model of flexural testing was developed to analyze the mechanical behavior of the Alfa/polyester composite. The test results showed that the alfa coarse fibers with 30 wt % were capable of enhancing the mechanical properties of the polyester/alfa composite.