فصلنامه علوم و فناوری دریا
سال بیست و چهارم شماره 96 (زمستان 1399)

In the present paper, an attempt has been made to improve the mechanical properties of 5083 aluminum alloy welded by metal inert gas method followed by frictional stir process using an experimental method. For this purpose, the FSP process was performed in three rotation velocities of 500, 750, and 1000 rpm and with a forward velocity of 100 cm/min. In order to achieve the effect of FSP process on the microstructure of the tested alloy, welding samples, as well as frictional stir process samples, were studied. Thus, the microhardness and tensile strength of each sample were investigated. Then, using the genetic algorithm method, the optimal value of the rotation velocity was calculated. The results showed that the FSP process makes the heat affected area finer in terms of granulation and in addition, defects such as porosity and non-wettability due to the molten weld area are reduced or eliminated by the frictional stir process. On the other hand, the results of optimization indicate that the most suitable strength is obtained at a rotational velocity of 820 rpm. At this speed, the grain size is reduced by 96% compared to the heat affected area in the initial base state. Also, the cross-sectional images of the optimal sample fracture, unlike other samples, show that the fracture did not occur in the weld area, which can be attributed to the removal of cavities in the welded area by the frictional stir process.

With the advancement of industry and the increase in the number of devices such as cars, airplanes, ships, compressors, pumps, turbines, etc., which are under repeated loading and vibration, fatigue has become more common and it is now understood that at least 90% of fractures due to mechanical reasons are due to fatigue during work. Structural failure generally occurs at stresses much lower than the ultimate strength of the material, examples of which include stairs, tanks, pipes, ships, railways, and aerospace structures. Brittle failure occurs without noticeable deformation and is caused by rapid crack propagation. The direction of movement is almost perpendicular to the direction of the tensile stress and results in a relatively flat failure level. Fracture surfaces of materials that are failure in the brittle state have their own distinct patterns. Fatigue is usually accompanied by crack at the surface of the part or areas of stress concentration and its propagation in the area under maximum stress. It is possible to use the failure mechanics method for brittle materials or to apply the appropriate reliability factor and consider the minimum allowable deformation. In the design method using fracture mechanics, three factors of applied stress, crack dimensions (however small) and toughness are the design criteria. In this paper, fatigue cracks and the mechanism of fatigue crack formation in marine instruments are investigated.

AbstractDue to the rapid growth of online interactions on the Internet, security services such as authentication, non-repudiation, integrity and Confidentiality are very essential security requirements for a secure transaction. Information encryption is one of the most important ways to protect information in cyberspace. To achieve these goals, digital signature is the most efficient tool that uses cryptography. In this paper, we conduct a comprehensive study of these digital signature schemes based on the RSA, DSA and ECDSA encryption schemes and their improved versions and compare them. In this paper checked out a review of digital signature Schemes, Based on Asymmetric cryptography and their applications.

One of the important parameters in the design of different structures is to determine the amount of failure at different levels, including the level of failure of the collapse prevention of the structure. Since researchers in recent years have studied the various problem of the design of offshore wind turbine structures, but less reference to this failure criterion under the combination of wave load and wind load, in the present study tried to determine the failure rate of collapse prevention (CP) for an offshore wind turbine with jacket platform, which is one of the most common turbine holding platforms. Attempts have been made to calculate the force applied to the offshore wind turbine under both wave and wind loading, including linear and nonlinear regular wave models and irregular waves. Since regular models do not consider the randomness and irregularity of sea level and focus the wave energy on a frequency, they are not suitable for dynamic analysis. On the other hand, the costly and time-consuming dynamics analysis calculations and the long time history of irregular waves are among the disadvantages of these methods. Therefore, in this research, endurance wave-wind analysis has been used for this important. In this paper, by determining the amount of failure of the collapse prevention of the structure and also the return period of the corresponding loading repetition, it was found that the wave height of CP failure limit for this structure is about 11.5 meters, .

In recent years, the shrimp farming industry has been one of the most influential sectors in food production and has grown rapidly into a dynamic and growing industry over the past few decades. Declining shrimp and fish stocks that are being traded and exploited on the one hand, and growing population on the other hand, have increased demand for marine protein sources. In such a situation, increasing the efficiency of aquaculture production is one of the important goals of the aquaculture industry. With the use of new technologies obtained until 2019, today it is possible to increase the density and amount of production in production units. These technologies include the implementation of modern recirculating aquaculture and biofloc systems, which is growing and developing around the world. Therefore, it is very important to provide useful aquaculture engineering solutions for use in the shrimp farming industry and the use of new technologies in this industry. In this study, various shrimp farming systems and research on the use of biofloc and recirculating systems were reviewed. The study also examines the challenges of the biofloc system and the prospects facing the shrimp industry.

Considering demographic data, area extent, and the length of coastline, all coastal cities of Iran were categorized, and were compared with each other, to find their differences and the potential capacities of each region. For this study population data of 1395(2017) was employed. Besides, satellite images and GIS software was used for the sake of analysis and calculations. The area, and coastline of cities along the Caspian Sea, Oman Sea and the Persian Gulf were calculated. Afterwards, the population growth rate for a period of ten years (2007-2017) were calculated. Results show that northern coastal cities own higher relative density in compare with the southern coastal cities; meanwhile they own shorter coastline. On the other hand, the trend of population growth, in last ten years, in the southern cities was more than that of northern cities. However, most of southern cities are still low levels in terms of Population density and Coastal congestion. The city of Abu Musa and Qeshm had a significant population growth of about 93.11% and 41.45% respectively. Meanwhile, according to the ratio of coastline both of mentioned cities own a coastal degree of just 0.73 (for Abu Musa) and 0.22 (for Qeshm).