مجله مکانیک سازه ها و شاره ها
سال سوم شماره 2 (تابستان 1392)

The strength of the reinforcing fibers is the main factor affecting the strength of the composites، therefore، accurate method for measuring the strength is needed. There are two methods for determining the strength of the reinforcing fibers (e. g. carbon fiber). They are the methods of single filament and the fiber tow test (fibers impregnated with resin). The main goal of this research is to find the correct procedure of testing carbon fibers with two different methods of single filament and fiber tow test، so comparing the mechanical properties of two types of PAN based carbon fibers given by those methods. The fibers used in this research were two types of carbon fibers and their mechanical properties like tensile strength، tensile modulus، breaking strain and fracture energy were studied in both test methods. The results show that although the single filament test method is quite difficult but compared with the fiber tow test method gives higher values for tensile strength and fracture energy whereas lower value for tensile modulus and breaking strain.

Sliding mode control for wheeled mobile robots can be used due to their nonlinear dynamics and uncertainty. However، sliding mode control has the chattering problem and difficulties in finding the bounds of uncertainty which degrade the control performance. To improve the performance، this paper presents adaptive sliding mode control. The novelty includes a novel state-space model and using the voltage control strategy. The controller considers the motor dynamics whereas the previous approaches did not consider it by using torque control strategy. In the proposed design، the bounds of uncertainty are determined adaptively. As a result، the chattering problem is reduced. The proposed approach is based on the Lyapunov theory، thereby guarantees the stability of control system. In addition، the particle swarm optimization for finding the optimal design parameters is used. The proposed control approach is robust against the external disturbances and unmodeled dynamics. Simulation results show the superiority of the adaptive sliding mode control over the conventional sliding mode control.

In order to measure the volume of solid bodies، acoustic volume-meter device can be used. The reference and measuring containers of the mechanical part are separated by a speaker diaphragm. The object to be measured is placed inside the measuring container and a sinusoidal variation in the volume of the two containers is introduced by movement of the speaker diaphragm. As a result the pressure inside the containers is changed and this is measured using two microphones. Low measurement time and cost، simplicity and acceptable accuracy are the remarkable characteristics of the developed device. However، the device measurements are sensitive to the variation of temperature. This temperature sensitivity is found to be originated from the mechanical part of the device. To deal with the negative effects of temperature variations several approaches including incorporation of measurement microphones and specific components associated with them، calibration against a reference volume just before each measurement and temperature compensation، may be adopted. Among these، temperature compensation is a cost effective method that this paper is concerned with. Thus، a temperature sensor is included in the mechanical part and is connected to the interface board and the data from microphones and this sensor is recorded simultaneously. Based on the collected data and the particle swarm optimization algorithm، a function is derived that compensates the effect of temperature. The results indicate that the using the presented method، the inaccuracies due to the temperature variations can be significantly reduced and a conduct more accurate measurement

This paper presents the results of the experimental and analytical investigation carried out using a steel forged material، EA4T، subjected to impact loading conditions within the strain rate domain101-103 (1/s). The standard three point bend test specimens manufactured according to ASTM 399 standard were subjected to impact loadlevels correspondingto preset defined impact energy levels by applying the Izod impact testing equipment. The specimens were also modeled and the loading condition was simulated using the finite element commercial software، ABAQUS v6. 11. Both the experimental findings and the finite element analysis results indicated that the crack initiation conditions were satisfied when the impact energy applied to the specimen reached a specifiedlevel. It was found that for a lower level of applied impact energy، the impact loading resulted in improving the fatigue characteristic of the material. In contrast، application of the higher impact energy caused the crack to propagate further. The other finding of the study was that increasing the plastic zone size decreased the crack propagation rate (da/dE).

In this paper، differential transform method (DTM) has been used to determine natural frequencies of free vibration of variable width Euler- Bernoulli isotropic beam. The width of beam varies exponentially with arbitrary non-uniformity parameter. After extracting and nondimensionalization، the governing partial differential equation discretized to two ordinary differential equations in terms of both spatial coordinate and time based on the separation of variables method. Then، for different non-uniformity parameters and boundary conditions، natural frequencies of system have been obtained by using analytical methods and DTM. Boundary conditions are considered as: simple- simple (SS)، clamped- clamped (CC)، clamped- Simple (CS)، free- free (FF)، clamped- free (CF) and simple- free (SF). Comparison of natural frequencies obtained from DTM with analytical solution shows that DTM has a good accuracy in determination of natural frequencies، especially in the lower modes. Since that this method is based on Taylor’s series expansion، more accurate natural frequencies and also natural frequencies of higher modes can be obtain by increasing series size. Results show that for a constant series size، difference between the natural frequencies obtained from DTM with the exact solution increase totally by increasing of mode number.

In this paper، differential transform method (DTM) has been used to determine natural frequencies of free vibration of variable width Euler- Bernoulli isotropic beam. The width of beam varies exponentially with arbitrary non-uniformity parameter. After extracting and nondimensionalization، the governing partial differential equation discretized to two ordinary differential equations in terms of both spatial coordinate and time based on the separation of variables method. Then، for different non-uniformity parameters and boundary conditions، natural frequencies of system have been obtained by using analytical methods and DTM. Boundary conditions are considered as: simple- simple (SS)، clamped- clamped (CC)، clamped- Simple (CS)، free- free (FF)، clamped- free (CF) and simple- free (SF). Comparison of natural frequencies obtained from DTM with analytical solution shows that DTM has a good accuracy in determination of natural frequencies، especially in the lower modes. Since that this method is based on Taylor’s series expansion، more accurate natural frequencies and also natural frequencies of higher modes can be obtain by increasing series size. Results show that for a constant series size، difference between the natural frequencies obtained from DTM with the exact solution increase totally by increasing of mode number.

In this paper، we are considering the effect of uniform magnetic field on the velocity and temperature profile in the boundary layer and at the end the free convection heat transfer rate from vertical plates are studied. Writing energy and momentum equations with considering magnetic field effects. By help of appropriate similarity parameter and use of stream function and with Rewriting energy and momentum equations in term of stream function، all partial differential equations converted to the ordinary differential equations. At the end with the help of fourth order Range-kutta method، all equations were solved. It is revealed that with increasing magnetic field parameter، the fluid velocity in the boundary layer decreased and in other hand، the temperature in the boundary layer increased and the heat transfer rate from vertical plate decreased. It is proved that the assumed magnetic field caused to decrease about 15% in the heat transfer rate from vertical plate.

In this thesis for improving in samand behavior in besides collision، internal part of B-pillar of SAMAND modeling by use of laminated composite made up of carbon – epoxy and simulation beside collision test based on FMVSS-214 standard. This survey is performance on finite element analys and show the condition and quality of stacking sequence of difrent layers. Since change in orientation layers cause of change in structure resistance affected by collision، using of Genetic Algurithm and neural lattice get better result for minimum transfiguration of B-pillar. Also to achive the steady stacking sequence use the result of 49 Taguchi’s studys and simulation for 8 layers in -90 to 90 degree and 120 Hamersly’s tests for 8 layers in 0 to 90 degree، and 120 Hamersly test for 4 layers in 0 ti 90 degree. Based on this result and gained data from optimization algoritm and complet survey of test result، the best collision behavior of structure occure in 40 to 50 degree stacking sequence. Stracture we offer in this study own minimum transfiguration and displacement. And System in this manner، exist maximum safty and security for passengers that is the most important chalengein besides couission.

In this study، a single-purpose water heating system is experimentally compared to a dual-purpose air and water heating system at the same conditions. The system consists of a dual-purpose flat plate solar collector and a vertical water storage tank which operates at no removal condition. Air and water flow naturally and forcefully inside the collector، respectively. The aim of using dual-purpose systems is to reach greater efficiency and reduce heat loss. These systems can be used for simultaneous heating of air and water. The hot water can be used as domestic hot water and in heating systems. Besides، the hot air can be used in dryers and air conditioning systems. Therefore، application of such systems will result in high energy saving. In this study، variation of collector temperature، average storage tank temperature، and system efficiency at two different air flow rates at different times are presented. Moreover، hourly efficiency is compared for single-purpose and dual-purpose systems. The results indicate that the average efficiency was 67. 8% for the single-purpose system. Furthermore، the average efficiency was 71. 6 and 72. 3% at air velocities of 2. 8 and 3. 2m/s، respectively.