Sharif Digital Repository

Failure in sheet metal forming can occur by necking, fracture or wrinkling. By using a forming limit diagram (FLD) as a powerful tool to prevent sheets metal failures in the forming process, provides parameters controlling throughout forming. There are different developed methods for predicting FLDs, which estimate sheet metal forming strains limits. Assessment of FLD estimation reveals that there is a dependency between the effect of several factors containing normal stress, shear stress, sheet thickness, mechanical properties, metallurgical properties, yield function, strain path, and bending with formability. In this research, the effects of bending via two finite element models are... 

The eccentric bracing system equipped with vertical links is capable of providing high levels of stiffness, strength and ductility, and therefore, can be efficiently used for seismic retrofit of existing structures. This study aims to investigate the seismic reliability of steel moment-resisting frames retrofitted by this system using a novel combined series–parallel system approach. The seismic response of 4, 8 and 12-storey steel moment-resisting frames (MRFs) are evaluated under a set of design basis earthquakes (DBE) before and after retrofitting intervention. Adopting an engineering demand parameter approach (EDP-Based) for reliability assessment and development of analytical models for... 

The eccentric bracing system equipped with vertical links is capable of providing high levels of stiffness, strength and ductility, and therefore, can be efficiently used for seismic retrofit of existing structures. This study aims to investigate the seismic reliability of steel moment-resisting frames retrofitted by this system using a novel combined series–parallel system approach. The seismic response of 4, 8 and 12-storey steel moment-resisting frames (MRFs) are evaluated under a set of design basis earthquakes (DBE) before and after retrofitting intervention. Adopting an engineering demand parameter approach (EDP-Based) for reliability assessment and development of analytical models for... 

Flow stress and ductility behaviors of the annealed and severely deformed Al were investigated at warm deformation temperatures. Constrained groove pressing (CGP) method as a severe plastic deformation process was used. The tensile test was carried out at the temperature range of the 298-573 K and strain rate range of 0.001-0.1 s−1 to present the elevated temperature deformation behavior utilizing hyperbolic sine constitutive equation. The flow stress of the CGPed sample is increased with the number of CGP passes and decreased with temperature. Dynamic recovery and strain softening are found as main restoration mechanisms. Flow stress amounts are not remarkably affected by the strain rate.... 

According to previous studies on the fracture surface morphologies of bulk metallic glasses, the stable crack growth region width and vein pattern size increase with the plasticity at room temperature. In the present work, the fracture surface morphologies of Ti- and Zr-based bulk metallic glasses bent over a wide temperature range (0.1–0.8 glass transition temperature) are systematically analyzed. According to our finding, the stable crack growth region width increases while the vein pattern size decreases as the ductility improves by varying temperature. This observation is in contrast to the common thought that the ductility is proportional to the stable crack growth region width and vein... 

In this research, the samples of a low carbon steel sheet were rolled up to a thickness prestrain of 67% at three different temperatures consisted of room, blue brittleness, and subzero temperature. Microhardness, SEM, and tensile tests were carried out to evaluate the static recrystallization kinetics defined by the Avrami equation, microstructural evolution, and mechanical properties. It was found that the Avrami exponent is altered with change in prestrain temperature and it achieves the value of 1 to 1. 5. Moreover, it was indicated that prestraining at subzero temperature followed by annealing at 600 °C leads to considerable enhancement in tensile properties and kinetics of static... 

The diagrid structural systems are mainly used for their structural capabilities and architectural aesthetic possibilities which are provided by the unique geometric configurations of these systems. However, the seismic performance factors of these structural systems are not yet explicitly recommended in the existing building codes. In this study, the seismic performance factors (SPFs) of 6- to 24-story steel diagrid structures are determined considering the post-buckling behavior of diagonal members in compression. Also, the effect of change in span length and the diagonal angles on the SPFs of diagrid structures is studied. The ATC-19 coefficient method is used for calculating the SPFs... 

In this work, similar butt joints of AA6061-T6 alloy prepared by underwater friction stir welding (UWFSW) and friction stir vibration welding (FSVW) processes were examined. The characteristics of joints were compared with the joints obtained by conventional friction stir welding (CFSW). The different kinds of microstructural modifications that occurred during CFSW, FSVW, and UWFSW processes were analyzed. The results are employed to analyze the different behaviors in strength, ductility, weldability, and hardness of the joints in different processes at different traverse speeds, rotational speeds, and vibration frequency. It was found that mechanical vibration decreases the grain size in... 

In this work, similar butt joints of AA6061-T6 alloy prepared by underwater friction stir welding (UWFSW) and friction stir vibration welding (FSVW) processes were examined. The characteristics of joints were compared with the joints obtained by conventional friction stir welding (CFSW). The different kinds of microstructural modifications that occurred during CFSW, FSVW, and UWFSW processes were analyzed. The results are employed to analyze the different behaviors in strength, ductility, weldability, and hardness of the joints in different processes at different traverse speeds, rotational speeds, and vibration frequency. It was found that mechanical vibration decreases the grain size in... 

Mechanical properties of slab tracks on a foundation with nonlinear stiffness are accounted for. At first, the cracking stages were inspected in FEM models, and it was learned that slab tracks have one-way exural behavior. Secondly, experimental full-scale models were made, and the accuracy of analyses was verified by comparing the FEM loadde ection curves with those of previous studies and validating the cracking and ultimate loads with those obtained from experiments. Finally, the effects of several parameters on the cracking and ultimate loads and the energy absorption of steel fiber-reinforced slab tracks were investigated by examining the real behavior of slab tracks on elastic... 

This article is intended to highlight the effectiveness of longitudinal Glass Fiber Reinforced Polymer (GFRP) bars in combination with GFRP sheets on the flexural capacity of Reinforced Concrete (RC) columns. Seven half-scale RC columns including five strengthened and two control unstrengthened specimens were experimentally tested under axial and cyclic lateral loads. The strengthened columns with two different longitudinal GFRP bar ratios were tested under three different axial load levels. The flexural strength and ductility parameters of the specimens were calculated by obtaining their deformations and measuring the loads from load cells. The experimental results indicate significant... 

This paper presents a new method for optimization of the dynamic response of structures subjected to seismic excitation. This method is based on the concept of uniform distribution of deformation. In order to obtain the optimum distribution of structural properties, an iterative optimization procedure has been adopted. In this approach, the structural properties are modified so that inefficient material is gradually shifted from strong to weak areas of a structure, This process is continued until a state of uniform deformation is achieved. It is shown that, in general, for a MDOF structure, there exists a specific pattern for distribution of structural properties that results in an optimum... 

This article investigates the effect of T6 heat treatment on microstructure and mechanical properties of hot extruded in-situ Al-15wt%Mg 2Si composite. This composite has already been introduced as a new class of light materials but the brittle structure of the primary Mg 2Si which is formed during solidification limits its application. As-cast composite was directly extruded as rod by using three different dies. Microstructure after this type of extrusion was studied by optical and scanning electron microscopy. Results demonstrated that extruded and heat treated composite possesses considerably higher strength and enhanced ductility in comparison with the as-cast samples. It was also found... 

In order to analyze the flow behavior and workability of Ni–42Cu in cast and wrought conditions, hot deformation tests were performed at temperatures and strain rates within the ranges of 900–1150 °C and 0.001–1 s−1, respectively. Tensile tests showed a “hot ductility trough” at 950 °C for both alloys. The drop in hot ductility was more considerable in the cast alloy because of the sluggish dynamic recrystallization. The hot ductility drop and grain boundary cracking, particularly in the cast alloy, were attributed to the segregation of detrimental atoms to the boundaries. It was shown that the hot ductility of the wrought alloy could be improved with increasing strain rate. It was... 

The present article focuses on the high-temperature mechanical properties of the magnesium alloy AZ91. The addition of rare-earth (RE) elements up to 2 wt pct improves both yield and tensile strengths at 140 °C by replacing the Mg17Al12 phase with RE-containing intermetallic compounds. This intermetallic phase is thermally and metallurgically stable and is expected to boost the grain-boundary strengthening. It also increases the resistance of grain boundaries to flow at high temperatures. Further increases of RE additions reduce strength and ductility due to growth of the Al11RE3 brittle phase, which has sharp edges. Still, at a 3 wt pct RE addition, the strength of the alloy at high... 

This paper deals with the effects of three low-carbon steel filler metals consisting of ferritic and austenitic phases on the weld joints of the tungsten inert gas (TIG) welding of Hardox 500 steel. The correlation between the microstructure and mechanical properties of the weld joints was investigated. For this purpose, macro and microstructure were examined, and then microhardness, tensile, impact, and fracture toughness tests were carried out to analyze the mechanical properties of joints. The results of optical microscopy (OM) images showed that the weld zones (WZ) of all three welds were composed of different ferritic morphologies, including allotriomorphic ferrite, Widmanstätten... 

Extensive recent researches have been underway to model the fracture mechanics degradation based on continuum damage mechanics (CDM) technique. CDM theory is a powerful tool for solving problems such as large plastic deformations that the fracture mechanics is unable to solve. This model is derived by means of the thermodynamics internal variable theory and based on the experimental results on material properties. In this paper, the reliability of rectangular plates containing a central circular hole under static tensile load using the CDM approach for ductile fracture has been studied. To investigate the initiation and evolution of damages, anisotropic damage expressed by second order... 

In this research, a parametric study is carried out on the effect of soil-structure interaction on the ductility and strength demand of buildings with embedded foundation. Both kinematic interaction (KI) and inertial interaction effects are considered. The sub-structure method is used in which the structure is modeled by a simplified single degree of freedom system with idealized bilinear behavior. Besides, the soil sub-structure is considered as a homogeneous half-space and is modeled by a discrete model based on the concept of cone models. The foundation is modeled as a rigid cylinder embedded in the soil with different embedment ratios. The soil-structure system is then analyzed subjected... 

Polylactic acid (PLA) and thermoplastic starch (TPS) are known as bio-based and biodegradable thermoplastic polymers that can be used in different applications owing to their inherent physical and mechanical properties. In order to reduce the higher costs of PLA and tuning its physical and mechanical properties suitable for short life packaging applications, blending of PLA with the TPS, more economical biodegradable polymer, has been considered in academic and industrial researches. However, melt blending of PLA with TPS without compatibilization process caused some drawbacks such as coarsening morphology and declining mechanical properties and ductility because of thermodynamic... 

Austempered ductile iron (ADI) is one of the most widely used types of ductile iron produced by austempering heat treatment. ADI heavy section parts are employed in different industries owing to their unique mechanical properties. Cooling rate in thick parts is significantly low, so heavy section ductile iron parts should have an adequate austemperability for preventing pearlite formation in the middle of the casting. In order to achieve the proper austemperability and fully ausferritic structure, alloying elements like nickel are added to the melt. The objective of this work is to study the role of austempering parameters on nickel alloyed ADI specimens fabricated from 75-mm-thick Y-block....