JOURNAL OF NEW MATERIALS
Year:2015 | Volume:6 | Issue:1 (21)|Papers Count:12

In this research, the transient shear stress distribution in an adhesive joint due to fiber breakage has been investigated. Transient stress is a dynamic response of the joint to the fiber discontinuities till their static equilibrium state. To study this behavior, equations governing the motion of fibers in the matrix, due to their breakage, are derived and the effect of number of broken fibers is studied on transient response of the structure. Shear lag model is used to extract fiber displacement. The equilibrium equations are solved using the explicit finite difference method. The effect of fiber materials and adhesive thickness is also studied on stress distribution. Results show that for an increase in the number of broken fibers, the stress concentration in the composite structure increases. Moreover, the shear stress created in the matrix and the adhesive layer is reduced with an increase in fiber elastic modulus, such that for glass and graphite fibers (E=74 and 130 GPa respectively), the maximum shear stresses in the adhesive are 0.861 and 0.461 MPa, while in the adherends, they are equal to 3.192 and 2.409 MPa respectively.

This study was carried out to determine the optimum of boron quantity affect on low temperature impact energy of the mild steel weld metals fabricated by submerge arc welding (SAW). For this reason some plates of ST 37 in 1×0150×200 mm dimensions prepared and welded by SAW. Boron as the oxide powder, added and mixed with flux powder. After welding the OM, SEM, impact and micro hardness tests were carried out for different samples of the weld metals.The results of the chemical analysis of different samples confirmed presence of 20, 30, 60, 70 and 110 ppm boron in weld metals. The OM result shows that in sample containing 20 ppm boron, acicular ferrite was increased and polygonal ferrite, grainboundary and widmenstatten ferrite was decreased. Relative to original samples by increasing of the boron from 20 to 110 ppm often polygonal, grainboundary and widmenstatten ferrite were formed. The microhardness test result showed that sample contained 20 ppm boron has the highest hardness compared to other samples. The impact test results showed that the high impact energy for sample contains 20 ppm boron at the three ambient, zero and -20 °C. The fractography result showed that, brittle fracture share for sample contains 20 ppm boron with the highest impact energy was 10 percent and for sample contains 110 ppm boron with the lower impact energy about 50 percent.

Metal foams as an advanced material show many interesting properties which is the reason for their wide use in industries. The properties and technology of metal foams are developing and improving. In this investigation aluminum-A356 foam was produced by melting process at first step and then it was prepared for aging process by making an alloy of 4%wt. Cu. The results were shown that, solution process at 510 °C for 12 hours and then quenching in cold water, carrying on the process by aging at 160 °C for 3 hours gave rise to best mechanical properties for A356-4%wt. Cu. The yield strength of the A356-4%wt. Cu and the aged A356-4%wt. Cu were compared which presented a change from 10 to 27 MPa, about 170% yield strength increase. The energy absorption, was measured by uniaxial compressive loading, it was shown an increase of about 185% for the aged alloy in comparison to non-heat-treated specimen. Elastic modulus characteristic for the above mentioned alloy had not considerably changed in comparison to other cycles with lower solution and aging time and was between 10 to 11 GPa. Comparison of foam cell structure before and after the compression test was shown that 4% Cu was contribute to make spherical factor to be almost equal to 1 and also the cell structure was more homogeneous rather than the A356 foam. As a final result the internal surface shrinkage was disappearing by adding 4%wt. Cu.

The role of a kind of plasticizer (OLA8) and two types of nucleating agents (LAK301 and PDLA) on the physical, mechanical and thermal behaviors of poly- lactic acid (PLA) were studied by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and tensile test. The results show that the OLA8 is an efficient plasticizer for PLA. this is because in the presence of 20wt% of OLA8 the elongation at break of PLA promoted. Also adding plasticizer to PLA causes to drop down both enthalpy and crystallinity unlike the presence of nucleating agent. For example the enthalpy and crystallization of a sample with composition of 20% OLA8-5%PDLA-2.8%LAK is half of than that of pure PLA. At constant plasticizer content, addition nucleating agents in different types and quantities causes to promote a little bit in enthalpy and crystallinity. in the presence of both plasticizer and nucleating agent (20%OLA8-5%PDLA-2.8%LAK) half time crystallization as 0.46" min was recorded. In the same composition the glass transition temperature reduces 10 °C relation to pure PLA.

In this study, properties of Ni-P/nano PTFE coating deposited on aluminum substrate using commercial electro less nickel bath were studied. To carry out the research, micro hardness, thickness, roughness and morphological properties of the coating due to different PTFE solution densities and temperatures were studied. Effects of heat treatment on micro hardness and morphology of the obtained coating was investigated also. Vickers micro hardness and Rz and Ra parameters were measured for hardness and roughness, respectively. Particle distribution and chemical composition of the nano composite coatings were evaluated using SEM/ EDS and XRD techniques. results showed that presence of nano PTFE particles in the solution decreased the micro hardness of Ni-P from 520 HV to 250 HV and increased the roughness (Rz) from 1.49 to 4.6mm by changing the PTFE content from 2 to 14 g/l. Increasing the solution temperature from 75 to 90°C leaded to increase the hardness from 125 to 210 HV. Also it was found that heat treatment at 300°C for 4h increased the coatinghardness to 375 HV because of formation of Ni3P phases.

In this research, nickel- free austenitic stainless steel samples with the chemical composition of ASTM F2581 were produced by powder metallurgy (mechanical alloying and liquid- phase sintering by a Mn-Si eutectic alloy additive) for medical applications. Afterwards, their structure, hardness and wear behavior were investigated. The structure of the powders and sintered samples was studied by scanning electron microscopy, optical microscopy, image analyzer method, density measurements and ferritometry. Also, the effect of the sintering aid content on density, hardness and wear behavior was focused on. By investigating the samples with 0 to 8% additive, the lowest porosity level and the highest hardness and wear resistance were obtained for the sample having 6% sintering aid. The optimum mechanical properties of this sample, in comparison with the others, are attributed to the lowest porosity level (about 10%), the nanocrystalline structure and the presence of nitrogen and carbon in the structure. In the presence of this amount additive, liquid-phase sintering was activated, the mechanical properties were improved compared to the additive-free sample.

Fatigue failure analysis is one of the most important issues in different industries such as oil, gas, and petrochemical. On the other hand welding operation is a prevalent and inevitable method for connecting elements and components. Because of presence of inherent defects in these welded joints, fatigue cracks originate and propagate, and failures occur in these zones more than others. Weld toe region is one of the crack and fracture source in welded structures due to stress concentrations and high residual stresses zones. Different methods can be used to improve mechanical behavior of welded structures; such as: peening, dressing and grinding methods, which are known as post weld treatments. Ultrasonic impact treatment is one of the modern and promising technologies which can enhance fatigue behavior of structures by residual stress and geometry effects. It also has some positive effects on corrosion resistance of metals. In this paper, effect of ultrasonic impact treatment on corrosion fatigue life of A106-B carbon steel pipes has been investigated. Evaluation of effect of this post- weld treatment bymicro- hardness test, residual stresses analysis, and weld geometry measurement have been studied as supplementary tests. Improvement in fatigue life due to ultrasonic impact treatment was 99.4 percent. Also fatigue strength has increased by 19.8 percent at 20000 cycles.

In this research, cyclic oxidation and hot corrosion behavior of nanostructure CoNiCrAlYSi coatings, deposited by HVOF thermal spray process, have been investigated. To achieve nanosized particles, the initial CoNiCrAlYSi powder was cryomilled and then HVOF sprayed to form a nanostructure coating. The coating’s performance has been evaluated via cyclic oxidation and hot corrosion tests. Microstructure changes and phase analysis of the coatings during tests have been studied. The coatings endured on the substrate for 115 cycles and 700 hours during cyclic oxidation and hot corrosion tests, respectively. The increased lifetime of nanostructure coatings with respect to conventional coatings is attributed to some a-Al2O3 particles dispersed in the structure, created by cryomilling, and high-diffusivity paths as a result of nanocrystallization, which favors the formation of a continuous and protective a-Al2O3 barrier layer on the top of the coating.

Industries. Resistance to corrosion and welding should be taken into consideration. Austenitic stainless steels AISI321 are greatly employed in various industries because of their great resistance to corrosion and oxidation. In the present study, resistance to corrosion and oxidation in welding lines in Austenitic stainless steel AISI321 AND AISI347 in an oxidizing dried condition in 1050 °C was investigated. The resistance was analyzed by the resistance was analyzed by applying a thermogravimetry and scanning electron microscope. The results revealed that oxidation in base metal occurred in two phases formed an un-protective oxide layer on base and filler metal. The formed layer on the base metal was an Iron oxide layer, while there was a porous oxide layer on the filler metal. Oxidation in welding line is very complicated. The oxidation speed in the base metal is more than the filler metal. Therefore, the resistance of oxidation of this welding line is low.

In this work, production of composite layer contains silicon carbide particles in ASTMA106 with GTAW (gas tungsten arc welding) was investigated. Deffrent valume fractions of silicon carbide was deposit on the surface of steel slab which was melted upon the contact with GTAM gases. The structure of layer produced was investigated with (OM), (SEM) and the phases was determined with (XRD). The coating has dendritic structure with silicon carbide reinforcement which improved hardness and wear behaviors. It was observed that presence of sic increased the hardness of samples from 650 to 1200 Vickers ) and also improved wear resistance drastically.

In this paper, a 3D micromechanical model is presented to study the effect of randomly distributed fiber within the matrix on the mechanical characterizations of unidirectional fiber reinforced composites. The Representative Volume Element (RVE) of the model consists of r×c unit cells in which fibers are distributed randomly within the matrix cells. The presented model is general and can be used to extract the stiffness properties of a fibrous composite subjected to thermal and mechanical, normal and shear loading and also to investigate the initiation of matrix plastic deformation. Different statistical distribution models including normal, uniform and beta distribution are used to study the fiber arrangement. The obtained results for the elastic properties in which fiber distributed at random show good agreement with both experimental data and available models. Statistical analysis of dependency of stiffness and strength properties with fiber array within the matrix is carried out by applying hypothesis testing based on the central limit theorem in each normal, uniform and beta distribution. According to the obtained numerical results, the necessary interpretation is done in each case. Numerical results of hypothesis testing show that fiber distribution has no effect on the stiffness properties of a fiber composite, while the strength properties of the composites are highly affected by the fiber arrangement.

In the present effort, the nano quasicrystalline Al-Ni-Cr compounds were successfully synthesized by mechanical alloying and subsequent annealing and quenching. The effect of annealing temperature on the formation of crystalline and quasicrystalline phases was studied by an X-ray diffractometer and a scanning tunneling microscope. The crystallite size of quasicrystalline phase was investigated by high resolution transmission electron microscopy (HRTEM). Furthermore, the corrosion behavior of the produced compounds was evaluated by using 3-electrode potentiodynamic polarization test in both Na2SO4 1M and NaCl 3.5% solutions. It was found that, the structure of the compounds is affected significantly by the annealing temperature in which the quasicrystalline phase can only be created at considerably high annealing temperatures. Accordingly, the ternary z-phase (with hexagonal structure), two binary phases designated g2 and d (with rhombohedral and hexagonal structures, respectively) and D3 decagonal quasicrystalline phase were formed at 900 °C, 1000 °C and 1100 °C, respectively and it was observed that the nano-quasicrystalline phase developed. It was also found that, the quasicrystalline phase can be improved significantly the corrosion resistance of the produced samples in both NaCl and Na2SO4 solutions.