Sharif Digital Repository

Deployable structures have been used on small satellites since the beginning of such space programmes. Here, the design sequence for a special tensegrity structure, for use as supporting structure for deployable mesh-like antenna will be proposed. The design process is conducted to prevent structural elements from failure, and satisfy the minimum specified natural frequency. In this research, structural modeling is performed based on two external geometric parameters (overall height and diameter), and three internal geometric parameters (elements cross-section properties). The pre-stress in structural element is the last design parameter. Also, materials are pre-assigned. Internal forces of... 

Obtaining the exact elastic response of composite systems to externally applied stresses is thought to be the most important way for predicting the mechanical behavior and tailoring the properties of composites. Over the past few decades, a variety of analytical models has been developed to investigate different stress transfer mechanisms in fiber composites. Although 2-D analytical model is still highly desired. So in present research a modified analytical model is developed for 2-D elastic stress field distribution in short fiber composites in plain strain for overlapping fibers subjected to an applied axial load . Two sets of exact displacement solutions for the matrix and fiber derived... 

Nowadays, the use of advanced composite materialsin different structureshas been increased and many industriesextend application of these high-tech materials because of their very high ratio of strength to weight and variation in ply configurationsuch asdifferent fiber angles which can control the response of these planes due to design requirements.One the most recent applications of such materials inreinforcing various structures are preventing crack propagation in aircraft structures and strengthening buildings against earthquakes which have been developed considerably. But, a lot of intact topics in this criterion are available which should be studied carefully. So, according to what was... 

Damage detection by Piezoelectric Wafer Active Sensors is one of the emerging efficient and dominant Structural Health Monitoring (SHM) methods. These sensors assess the health state of the structure due two strategies: a) near-field damage detection, b) far-field damage detection. The far-field damage detection is based on the high frequency Lamb wave propagation. As wave travels in a damaged structure, phase and amplitude of the wave will be changed. So comparison of the received signal from the pristine structure with damaged structure can give useful information about the health state of the structure. As PWAS is the essential part of this method, simulation and modeling of this sensors... 

Cost-effective and reliable damage detection is critical for utilization of composite materials. This research investigates a wavelet-based approach for SHM and detection of through-thickness crack in different lengths and angles in [0/45/-45/90]s CF/EP composite plate utilizing ANSYS finite element method (FEM) software and digital signal processing technique utilizing MATLAB software. To this end, a dual-PZT actuation scheme is applied, aimed at generating the S0 mode at a square composite plate. This study contributes to the damage detection by developing an improved wavelet-based signal processing technique that enhances the visibility and interpretation of the Lamb wave signals related... 

When today’s aircrafts reach the end of their service lifes, fatigue cracks are found to have developed along rivet holes and other highly stressed regions of the aircraft. In order to extend life of these aircrafts, repairs have been made to arrest these cracks. Composite repair patches provide an innovative repair technique, which can enhance the way aircrafts are maintained. In this thesis, composite patch design is presented based on axiomatic design technique. Axiomatic design is investigated design methodology for large or complex system design. Tools to enable successful application of axiomatic design are expressed as axioms and process for axiomatic system design. The patch design... 

Nowadays, some of the major problems of the airplane industry is that airplanes’ fleet are approaching to the end of their defined life, inability to buy new planes and obtaining aircraft’s parts for repairing process. Therefore, in order to increase the life of the aging aircrafts, some essential process such as modeling processes, identify principal structure elements to strengthen and repair them should be done to all these airplanes, all these processes refer as aging process. Aging programs initiated in some countries, such as America, Australia, Britain and Canada from about thirty-five years ago, technological approaches inventing new ways to start repairing worn metallic structures... 

Dynamic properties such as the natural frequency of a structure have a significant role in the design process. The importance of this consideration is because of resonance. In this case, the amplitude of the vibration is magnified to levels that may lead to a catastrophic event. While the usual design process depends on the collected experiences and statistical data, a developing trend is to implement smart technologies to develop smart structures that are capable of self-monitoring, diagnostics and repair. Smart material components such as shape memory alloys (SMAs) and piezoelectric ceramics are increasingly being used by vibration control practitioners. Shape memory alloys are applied as... 

One of the main problems in the airline industry is that many aircrafts of today's fleet are approaching their calendaring life, and it is impossible to buy new planes and supply previous ones for most of the companies. Therefore, in order to extend the life of these aged planes, some studies have been performed. The purpose of these studies is to discover the causes of aging and removing them. Repairing, replacement and reinforcing some elements of the structure are important parts of this procedure. This process is called aging. One of the requirements of aging technology is the examination of the structural response to the applied external loads and knowledge of the structural design.... 

Lifetime extension would be a necessary preventive action to halt micro cracks expansion on the airplane external surface. To decide about whether to perform this operation, one should examine technological maturity, costs and technical parameters. Technical status in lifetime extension is one of the major drivers of this decision which should be considered in the first step of technology management process which is known as the technology attractiveness step. In this research, major technologies of rejuvenation is examined and categorized by the technology mapping. Also, key processes of rejuvenation and technological fields ranking has been determined by the procedural view tools of the... 

Structural Health Monitoring (SHM) is a multidisciplinary emerging research field. SHM assesses the health state of the structure through processing and interpreting of signals and can predict remain life of the structure. Damage detection with Piezoelectric Wafer Active Sensors (PWAS) is one of the effective and emerging methods of SHM. One of the most critical parts of aerial structures is lap joints in which SHM could be very effective. In this research, damage detection of lap joints using lamb wave propagation has been studied. A 3D model of an aluminum lap joint presented. The PWAS are modeled separately and bonded to the host structure and coupled electromechanically with the... 

Nowadays, the structural health monitoring is considered as an essential element in improving the performance of the structures. The role and importance of the issues in this area have been reached to the point that different ideas and policies are developed for the safety margin over the useful life of the structures. To analyze more accurately, a real model of a turbine blade is required. This issue would be taken into more consideration when the scale of the turbine gets larger. Changing of the design philosophies over time results in effective changes in the design process and maintenance procedures. Any single of these inspections has different importance according to users' demands. In... 

Popular usage of composite materials in aerospace, civil and defense industries in the last decades has been the cause for paying more attention to optimization of composites tailoring. Due to inherent complexity of composite problems, i.e. discrete nature, complex interrelationship of design variables, existence of so many local optimum points, and etc., gradient base methods of optimization are found to be incapable and as a result using other powerful methods seems inevitable. Nowadays Genetic algorithm (GA) as an evolutionary technique is used for tackling composite problems. The success of a genetic algorithm can be quantified by estimating the cost, time required and the quality of... 

In the aerospace industry, there are a lot of high temperature environments, including engine parts and heat shields, in which the damages under these working conditions may be so destructive and even in some cases; they may result in sudden catastrophic failure of the structure. Nowadays, to prevent the occurrence of such failures and to provide the appropriate materials capable of working in such harsh environments and being able to protect the other adjacent structural components, new generation of thermal barrier coating materials so called Functionally Graded Materials (FGMs), with the controllable material properties in one or more directions were introduced. Recently, FGM applications... 

Metal and inter-metallic matrix composites offer high performance materials suitable for many low weight applications which are of growing interest in high tech industries. However, to submit their entire potential applications, their complex mechanical behavior has to be understood. Nevertheless, one of the major setbacks of designing with these materials has been the delamination phenomenon. So far, this phenomenon has been considered from macro-mechanical point of view regarding laminates to be semi-isotropic or orthotropic; which indicates the importance of micro-mechanical research to be done. Furthermore, the fact that macroscopic response and failure mechanisms of these composites are... 

As aircraft fleets get older, the challenge of Flight Safety gets greater. This Thesis focus on management strategies and principles to keep long-life fleet safe and effective into old age. But the safety and efficacy are not the only ones who are considered at long-life fleet. Here the question arises why the aircraft prior to exposure to the effects of rising life expectancy, are not replaced by other aircraft.The reason is the high cost of the replacement. Aircraft are expensive to design and manufacture. In commercial aviation, extreme competition and miniscule profit force pressures to delay replacement of older aircraft with expensive new types. But in Military aircraft reason for... 

In recent years world countries faced with the ageing aircraft fleet problem which increased the repair and maintenance costs of these aircrafts. The non-destructive inspection methods are common inspection methods of old aircrafts. To do this, the aircraft must test with a certain time scheduled inspection programme in a hangar. This method not only increased the repair and maintenance costs but also affects the flight schedule. To overcome this problem, the structural health monitoring can use to online monitoring the structure and when it needs for repair, the aircraft send to hanger. So, repair and maintenance costs minimize while the flight schedule less affected. One of the structural... 

This study presents the best layout with optimal number of sensors for an arbitrary monitoring area based on reference free damage detection and localization. In recent years, damage detection approaches with the concept of instantaneous baseline measurements in Structural Health Monitoring (SHM) with lamb wave propagation are being developed. An array of spars transducers is obligatory in SHM with lamb wave propagation, so that designing the best one is a concern for many researchers. To obtain the best sensors layout, damage localization process should be done for all definable sensors layouts in many different crack positions. Obviously, doing this seems not practical as wave propagation... 

The phenomenon of aging causes a great deal of damage to the airborne structures for which there is a specific maintenance instruction, or if an unexpected damage occurs in specific area of a structure, this damage must be repaired and a special repair process must be designed for it. However, due to the diversity of damages and also variations of the implemented loads, the load is transferred to other structural elements, hence, it is necessary to extend the life of the structure in order to identify these loaded elements and reinforce the applied forces. First, this phenomenon involves a design of a large number of metal patches, where a large number of drills have to be made in the body... 

In this research it is aimed to have an investigation on the thermo-mechanical analysis of functionally graded skew plates. In order to analyze the behavior of the skew FGM plates, a constant pressure load as a mechanical load is distributed through the plate. Also the thermo-mechanical load is a combination of this mechanical load and a temperature gradient in upper and lower surface applied as in-phase . Various boundary conditions considered in analysis are combination of simply support and clamped edges including CCCS, CCSS, CCCC. The analysis are based on CPT and the Von-Karman theory is used in the solution. The graded materials are distributed through the thickness according to P-FGM...