Sharif Digital Repository

Usage of concrete-filled pultruded glass fiber-reinforced polymer (GFRP) tubes (CFPGT) as columns can increase the service life of structures. However, marine structures such as oil platforms are always prone to fire because of the low resistance to the elevated temperatures. The purpose of this investigation is to evaluate the effects of concrete core strength (30 and 60 MPa), and exposure temperature (25, 100, 200, 300, and 400 °C) and time (60 and 120 min) on the compressive and bond behavior of CFPGTs. The properties of unexposed and exposed concrete core, pultruded GFRP hollow tubes, and CFPGTs were determined via compressive and disk-split tests. Also, the push-out test was used to... 

Concrete-filled polyethylene (PE) tubes (CFPT) are composite systems using the polyethylene tubes as confinement for the marine structures in the splash zone to extend the service life of structures subjected to harsh environments. However, thermal cycles in marine environments can affect the behavior of such composite systems. This paper evaluates the effect of concrete infill strength (30 MPa and 60 MPa), the number of thermal cycles (50, 100, and 150 cycles) ranging from 25 °C to 60 °C, and thermal cycle type (type A and B) on the compressive and bond response of CFPTs. Characteristics of control and conditioned concrete infill, PE tubes, and CFPTs were obtained by means of compression... 

This paper investigates the influence of collision angle on the interface characteristics and mechanical properties of magnetic pulse welded AA4014 tubes/Cu rods. It is shown that the collision angle, which affects the collision velocity, plays a crucial role in controlling the thickness of intermetallic compounds (IMC) layer and defect formation at the weld interface. The creation of a thin and continuous IMC layer at the weld interface is responsible for the higher load-bearing capacity of the sample welded under a collision angle of 4°. The welds made at higher collision angles are featured by a thick IMC layer containing defects such as micro-cracks, pores and cavities, which reduced the... 

While for shallow waters the use of old offshore jackets still seems efficient and justifiable, for deep and ultra-deep waters such platforms cannot be used. During the past few decades the old generation of fixed offshore platforms was succeeded by the new floating platforms and new designs such as FPSOs. A new family of floating offshore platforms has been developed. These should be able to respond to size, weight and space for operating equipment, i.e. they can be constructed to have a wide range of load-bearing capacities. Use is made of the old concept of Life Saving Tubes, which in their simplest form can be a toroidal shaped tyre inner tube. The Torus-shaped idea can be further... 

The fracture toughness and deformation mechanism of PP/CaCO3 (15 wt.%) composites were studied and related to load-bearing capacity of the particles. To alter the load-bearing capacity of the particles, different particle sizes (0.07-7 μm) with or without stearic acid coating were incorporated. The fracture toughness of the composites was determined using J-Integral method and the deformation mechanism was studied by transmission optical microscopy of the crack tip damage zone. It was observed that the load-bearing capacity of the particles decreased by reduction of particle size and application of coating. A linear relationship between normalized fracture toughness and inverse of... 

Transformation-induced plasticity (TRIP) steel resistance spot welds are delicate to low-energy interfacial failure via crack propagation through martensitic fusion zone during cross-tension (CT) loading. This paper addresses the effect of three different types of in situ postweld heat treatment (PWHT) on the mechanical properties of TRIP steel resistance spot welds. Depending on the post weld second pulse current level, three different strengthening mechanisms were found including (i) martensite tempering with reduced hardness, (ii) refining of martensite packets with improved toughness and (iii) nugget re-melting/enlargement combined with possible reduction of grain boundary impurity... 

This paper addresses the microstructure and tensile-shear mechanical performance of MS1200 Giga-grade martensitic advanced high strength steel resistance spot welds. The key phase transformations in MS1200 welds were lath martensite formation in the fusion zone (FZ) and upper-critical heat affected zone (HAZ), new ferrite formation in the inter-critical HAZ and martensite tempering in the sub-critical HAZ. The MS1200 welds were featured by a near matching hardness in the fusion zone and under-matching hardness in the heat affected zone (HAZ) compared to the base metal. At certain process window a complete nugget pullout and separation was observed with high post-necking tearing energy. The... 

Martensitic stainless steel (MSS) welds are notorious for their susceptibility to low-energy failure due to the formation of brittle martensitic structure in the fusion zone. The unique approach to enhance the mechanical properties of MSS resistance spot welds during both the tensile-shear and the cross-tension loading is to improve the fracture toughness of the fusion zone. In the present study, the effect of double-pulse welding on the microstructure-mechanical properties relationship of the AISI420 MSS resistance spot welds is investigated. Depending on the second pulse current level, various metallurgical phenomena was observed including (i) rapid tempering of martensite in the fusion... 

An axial load-transfer analysis for energy piles is presented in this study that incorporates empirical models for estimating the side shear resistance and end bearing capacity in rock along with associated normalized stress-displacement curves. The analysis was calibrated using results from field experiments involving monotonic heating of three 15.2 m-long energy piles in sandstone. Analyses of the field experiments indicates that poor cleanout of the excavations led to an end restraint smaller than that expected for a clean excavation in sandstone. Specifically, end bearing parameters representative of cohesionless sand were necessary to match the load-transfer analysis to the field... 

Purpose - The purpose of this paper is to analyze a squared lattice cylindrical shell under compressive axial load and to optimize the geometric parameters to achieve the maximum buckling load. Also a comparison between buckling loads of a squared lattice cylinder and a solid hollow cylinder with equal weight, length and outer diameter is performed to reveal the superior performance of the squared lattice cylindrical shells. Design/methodology/ approach - A cylindrical lattice shell includes circumferential and longitudinal rods with geometric parameters such as crosssection areas of the rods, distances and angles between them. In this study, the governing differential equation for buckling... 

This paper aims to study the behavior of UPVC-confined columns reinforced with polypropylene fibers under axial compression. A sum of 42 column specimens has been tested, including 18 Unconfined Concrete Cylinders (UCC) and 24 confined Concrete-Filled UPVC Tube (CFUT) specimens. To investigate the sensitivity of these columns to various parameters, different levels of material volumes have been used, which include two concrete strengths of 40 and 50 MPa, two tube grades of PN6 and PN10, and three polypropylene fiber content of 0, 1 and 3 kg/m3 of the concrete mixture. The influence of each variable on the ultimate strength, ductility and confinement effectiveness of the specimens have been...