فهرست مطالب f. ahmadzadeh

In the friction stir welding (FSW) process, a special rotating tool is used for welding different parts without utilizing any electrode as filler material. By rotating the cylindrical tool and sinking it into the line of connection between two parts, friction and disturbance are created, and at the same time, heat and pressure also rise to prepare the conditions for the welding. During the last two decades, a large number of research has been carried out to investigate the mechanical properties and microstructure of joints made from several similar and dissimilar alloys. Crack growth resistance of such joints under tensile, shear and mixed loading is an important design parameter to evaluate the lifetime of welded parts in friction stir welding, while there is limited research on combined mode I/II in FSW welding. To fill this research gap, in this research tensile and fracture strength in dissimilar FSW welding of AA5083/AA5052 aluminum alloys are investigated experimentally. For this purpose, two pin diameters, three pin rotation speeds and welding speeds have been selected as welding variable parameters. After carrying out the welding, the tensile and cracked mixed mode fracture tests were performed to determine the effect of considered parameters on the tensile and fracture strength of the joint. The results showed that the biggest tensile and fracture strength happens in different tool rotation and welding speeds for different loading angles (fracture modes).

One of the most effective ways to preserve biodiversity is to convert conventional systems into organic ones. The organic farming system reduces the negative effects of intensive management. The biomass of the soil ecosystem has different responses in management methods including tillage, fertilizer and defense inputs, and organic modifiers. Understanding the role and responses of biomass in different cropping systems is essential to support sustainable horticultural practices in managing plant-parasitic nematode.

Sampling has been done from two organic farming systems, conventional and pasture, in order to identify the morphology of soil plant parasitic nematodes, soil physicochemical characteristics, and microbial respiration. Plant parasitic nematodes were extracted, killed, fixed, and transferred to glycerin and permanent slides were prepared. The effect of the type of cultivation system on the abundance and diversity of plant parasitic nematodes in common and organic apple and peach orchards compared to pasture were investigated by multivariate analysis of variance (MANOVA) in Xlastat 2020 software.

20 Genera belonging to 11 families of plant-parasitic nematodes were identified, which had different frequencies based on the type of cropping systems and crops. The comparison of the type of organic cultivation system with the common showed that it had a significant effect on the frequency and diversity of plant parasitic nematodes; the highest frequency of the nematode genus in organic peach cultivation system is Gracilacus and the lowest frequency is Scutylenchus genus in common apple cultivation. The abundance of nematode family in organic system is more than common and the abundance of nematode family in peach organic system is more than apple organic system. It should be mentioned that the common apple system is close to pasture in terms of abundance. The abundance of nematode family in the common system of peach is more than the common system of apple. The type of cultivation system had a significant effect on the frequency and diversity of Pratylenchus, Helicotylenchus, Tylenchus, and Rotylenchus genera, while it did not have a significant effect on the  Gracilacus genus. Type of host plant had a significant effect on the frequency and diversity of plant parasitic nematodes definitely in all soil condition variables. The type of host plant was significant on the frequency of all plant nematodes except Tylenchus and a significant effect on the diversity of all except Tylenchus and Rotylenchus. Among all soil factors, microbial respiration, EC, OC, K, P, and texture (percentage of soil particles including sand, silt, and clay) showed a significant effect on all lineages of nematodes.

Significant differences in community structure in plant parasitic nematode communities in three systems were recorded. The results of the present study have shown that the frequency and diversity nematodes are different from the results of other researchers in apple and peach orchards, that plant types and cultivars, basic genotype, or soil management practices affect the composition of plant parasitic nematode community groups. In this study, the abundance and diversity of nematode in the organic system was more than in pasture, and conventional agriculture with more than 10 years of history, including tillage and chemical inputs, and organic matter loss, erosion, and low in the soil agroecosystem. The reduction of the plant parasitic nematode in the conventional system compared to the organic system is caused by the reduction of organic matter, tillage, or chemical inputs. The results showed that the type of product and management practices affect nematode communities. The composition of soil nematode communities is significantly different in organic, conventional, and pasture farming systems. The organic peach and apple system is facing an increase in nematodes more than conventional. It seems that plant parasitic nematodes are sensitive to soil management practices. Soil nematodes are useful indicators to evaluate the intensity of management and sustainable management of horticultural ecosystems on soil ecosystem performance; because they have several feeding habits in soil, and micronutrient networks and play an important role in the food cycle, pest suppression, and regulation of microbial communities. There were high pest pressure of nematodes in the organic peach farming system in comparison to the apple farming system. In this research, apple is better than the peach for organic production in Damavand orchards. In addition, growers consider peaches better than apples due to market and price considerations. With the increase in the demand and price of organic products, the organic system is facing high pest pressure due to fewer management options compared to the conventional system, which requires the development of integrated plant nematode management strategies.

Acoustic waves, like electromagnetic waves, can be converged at a focal point, which can be achieved with the help of negative refraction of waves and structures with gradient refractive indices. In this study, with the help of a fluid-fluid phononic crystal of methyl nonafluorobutyl ether (MNE) scatterers in an ethanol matrix, a gradient index structure at its homogenization frequency (i.e..l³a) designed and simulated. Gradual changes in structure with gradual changes in the radius of cylinders (or changes in filling factor), a hyperbolic secant profile for the refractive index is obtained to force the waves to follow curved paths in the structure. In most phononic crystal lenses, the focal point is fixed after it is made. Due to the temperature sensitivity of ethanol and methyl nonafluorobutyl ether, in this paper, an attempt is made to shift the focal point with the help of temperature effect, and two temperatures of 10 °C and 40 °C were investigated. The focal point from 199 mm at a temperature of 25 °C reaches 184 mm at a temperature of 40 °C, while at a temperature of 10 °C the focal point is almost unchanged. The finite element method has been used to calculate the band structure of the unit cell and also to investigate the wave propagation inside the two-dimensional gradient refractive index structure.