نشریه مهندسی مکانیک و ارتعاشات
سال نهم شماره 2 (پیاپی 31، تابستان 1397)

The purpose of this paper is to introduce a new approach for troubleshooting of gas pipelines using mechanical waveforms. In this paper, an attempt has been made to determine the defects by using natural frequency characteristics and changes. Due to the relationship between mass and stiffness in determining the natural frequency, a mass increase has been used instead of the reduction of stiffness. The proposed method involves modeling a 2-inch pipe with a length of 2 m in the Abaqus software and examining the natural frequencies of the pipe in a state of intact and defective (cracking). Then, an equivalent mass value was applied to model the defects. Subsequently, the pipe was subjected to experimental modal analysis and various conditions were tested by creating artificial defects (adding mass). Then, the obtained information was used in the study of the perceptron neural network in MATLAB software, and the program output is the defect location and its severity (mass value).

Today, with advances in medical, using robotic rehabilitation systems for treating incapability movements have been taken into consideration so much. One of the most important means of rehabilitation systems are used for auxiliary lower leg motion systems that the main factor in design and construction of these systems is moving leg according to the correct walking pattern. In this paper, design of an active device with one degree of freedom is provided for rehabilitating the weak leg of disable people. The proposed model is based on the structure of a four-bar mechanism. Its design and construction parameters with use of genetic optimization algorithm are selected in such a way that leg motions have the least difference with the pattern of walking. After the extraction of the kinematic equations of the closed-loop system and solving them, optimization with focusing on each joint of hip and knee is done separately with appropriate difference with weighting coefficients of objective function (W1, W2). Due to the defined optimization objective function, with increasing W1 respect to W2, the ability of system will increase in approximating the angle of hip joint to the optimal time function and by increasing W2 respect to W1, the ability of device will increase in approximating the angle of knee joint to the corresponding time function. The results indicated that the device ability for empowerment of leg motion of the disable people is in the way to rehabilitate the desired walking movement for them.

In this paper the influence of various properties of materials on the natural frequencies and vibration mode shapes in a differential casing for the heavy car is studied. Mechanical properties play an important role in determining the output frequencies and vibrational deformations. For this purpose, the differential model was designed in software CATIA and entered in simulation software ABAQUS which has the high ability to estimate the dynamic behavior of different systems. Materials such as gray cast iron with high vibration damping, steel with high density and rigidity, aluminum and magnesium alloys with low density were evaluated for the differential casing. The results showed that the natural frequency of the first five vibrational modes differ in the materials and by choosing gray cast iron can prevent a resonance phenomenon. Another part of the numerical simulation is dedicated to the effect of connection type between two parts of differential on the dynamic behavior. For this purpose, one of the screws for connecting the two parts of the differential was not secure, a defect that may be caused by an operator when repairs. For one bolt loosened condition natural frequency decreases about 3 to 4 percent. This can increase the chance of a resonance effect.

Ultrasonic Phased Arrays are an emerging technology in nondestructive testing and evaluation. The type of linear phased array probe is a prevailing type in which elements are placed side by side and longitudinally. In this paper ultrasonic phased array testing (UPAT) is introduce and in order to modeling and design UPAT consideration of wave equation solution by Finite Element is proposed. ABAQUS 6.14 is used as finite element software. Wave propagation in two form is simulated, straight and inclined. Direct and sectorial scanning is based on these forms of waves propagation. Time delay is the most important issue that should be considered in UPAT modeling. Results of simulation is verified by comparing previous research in UT field. Finally, results in additional to proposed the obvious view of UPAT, it's so useful for properly design probes and defect detection UPAT devices. Using finite element methods in this field is cost and time effective.

Nowadays, accurate prediction of micro structure vibration and dynamic behavior have been considered by many researchers. In this work, vibration analysis and control of cantilever micro-beam integrated with piezoelectric actuator and sensor layers considering surface effects have been investigated. Hamilton’s principle and Energy method are used to derive governing partial differential equation of the motion. GDQM is applied to discretize the PDE into the ordinary differential equation. The effects of system geometry, Young’s modules of surface stress, surface residual stress and surface mass density on natural frequency are analyzed. Also, the effect of linear optimal controller design on dynamic the response time and piezoelectric control voltage has been evaluated. Results indicate that respond on model increases and vibration amplitude decreases faster.

In this article we aim to study the modal behaviour of rotary equipment via predictive method with two examinations. Exam 1: identifying of a complete system without need to measuring of one row of FRF matrix while one column of this matrix is identified. The whole examination is simulated with rotor at four freedom degree. Exam 2: identifying the available parameters in an industrial system which was not considered in exam 1. The experimental system is a continuous vibration system, so in order to identify such a system, we need to use a method that describe it into a discrete system to illustrate the turbulences in data measuring. After that, we can explain the continuous system with a short model at a certain frequency band. Furthermore, we can estimate the modal parameters. It is also showed that the accuracy which special left vectors were determined can actually affected by the accuracy of modal constants related to the measured data. As a result, the special left vectors are affected by turbulences of the measured data. The effects of turbulences on freedom degrees with small response amplitude at bond frequency in the vicinity of closed modes, are more distinguished. Despite of the vibrations, the predicted special left vectors for determining one row of FRF matrix is suitable with one measured column.

In this research, the electron beam welding process of P91 steel plates was simulated by the finite element method in two dimensions and in a short time with acceptable accuracy in two steps (non-coupled thermal and mechanical analysis). The simulation results were validated with experimental and numerical results presented by other researchers. Moreover, by applying post weld heat treatment, the residual stresses were reduced. Then, the effect of heat treatment cycle parameters such as heat input power, heating time, holding time and cooling rate were investigated on the quantity of residual stress reduction. By examining the results of the effects of the parameters, an optimum heat treatment cycle is introduced to maximum reduction of the residual stress. Using this optimum heat treatment cycle, the residual stress value was reduced to 55 percent.