Sharif Digital Repository

The reliability analysis of defective pipes, which operate under creep-fatigue loading conditions has been studied. An austenitic stainless steel pipe containing an axially external semi-elliptical surface crack, subjected to variable internal pressure at elevated temperature has been considered as the benchmark problem. Different parameters in material behavior, geometry parameters, and operational condition have been assumed as random variables in the probabilistic assessments. The reliability of the cracked pipes have been evaluated at various operation times, using different reliability methods. Sensitivity analyses have been conducted to investigate the importance measure of the random... 

In this research, the possibility of producing asphalt mixtures containing Reclaimed Asphalt Pavement (RAP) that perform well from both fatigue and rutting points of view was studied. For this, seven asphalt mixtures with varying RAP percentage and total binder content were produced. All other variables, including the gradation and binder type, were kept constant. To better control for the production variables, the mixtures were produced in the laboratory. Fatigue and rutting properties of the mixtures were evaluated using the Simplified Viscoelastic Continuum Damage (S-VECD) model and Triaxial Stress Sweep (TSS) test, respectively. Next, FlexPAVETM software was utilized to predict the... 

In this paper, the fatigue crack propagation is modeled using the Molecular Dynamics (MD) analysis based on the Embedded Atom Method (EAM). The crack growth simulation is performed for the single-crystal and poly-crystalline specimens subjected to cyclic loading/unloading conditions, and the crack growth rate is investigated at the atomistic level. The MD simulations are carried out under different model parameters, including the load pattern, maximum applied strain, temperature, loading frequency, and percentage of defects by employing more than 4000 molecular dynamics analyses to study the sensitivity of crack propagation process to the simulation parameters. The threshold values of... 

The effects of the residual stress field resulting from shot peening and the indentation technique were investigated in relation to fatigue crack closure and crack growth behaviour. Compact Specimens of 20NiCrMo2 were used in this investigation. The regions of residual stress field were located behind the fatigue crack tip. Crack closure behaviour was measured with back face strain and crack mouth opening displacement gauges. Crack length was monitored by the compliance and microscopic methods. Residual stress was measured by the incremental hole-drilling method. Subsequently the closure level, propagation rate and resulting crack growth retardation were studied. Crack closure and attendant... 

In this paper seven different multiaxial fatigue criteria based on stress, strain and energy were employed to estimate the fatigue lives of double shear lap joint specimens of aluminum 2024-T3 with bolt clamped and interference fitted-bolt clamped fastener holes. Detailed finite element (FE) simulations were conducted to obtain the stress and strain distributions in the joint to be used to as basic data in estimating the fatigue lives using multiaxial fatigue criteria. The estimated lives were compared with available experimental fatigue test results to investigate the capability of the criteria in predicting the fatigue lives. The results showed that the accuracy of life estimation for any... 

An offshore structure is exposed to environmental loading and cyclic stress variations throughout its life. This would result in the possibility of fatigue cracking in its members. Cracks in these structures usually initiate around welded areas, then grow progressively from the heat-affected zone to base metal through the thickness of the tubular members. Therefore, fatigue life improvement of offshore structures is of great importance. In this paper, structural analysis of an offshore platform is carried out and the critical joint is specified. The welding process is simulated and the residual stresses due to the welding are determined. Experimental residual stress measurement is carried... 

To improve both strength and toughness of AISI 4340 steel three microstructures, full bainite, bainite-ferrite and tempered bainite-ferrite, were produced by heat treatment of this steel. Tensile, impact and hardness properties of these microstructures were compared. The results showed that with tempering of bainite-34% ferrite dual phase steel, elongation and charpy impact energy increased significantly in comparison to bainite and bainite-ferrite microstructures. Also ductile-brittle transition temperatures of bainite-ferrite and tempered bainite-ferrite steels were measured and confirmed superior toughness properties of this microstructure. Fracture surface analysis of charpy specimens... 

In the present study, the main purpose is investigation of the coatings thickness effect on the fatigue life of AISI 1045 steel. Herein, two different coatings of warm galvanization and hardened chromium have been used on the specimens. Fatigue tests were performed on specimens with different coating thicknesses of 13 and 19 µm. In the high-cycle level, S–N curves are extracted with 13 points for each sample. The results show that the galvanized coating is the most appropriate coating with low thickness, but with significant increasing of coating thickness, the best choice is hardened chromium coating. However, artificial neural network (ANN) has been used as an efficient approach instead of... 

This study investigates the effects of uncertainties in the prediction of creep-fatigue crack propagation in 316L(N) austenitic stainless steel plates containing a semi-elliptical surface defect. Different parameters in geometry, material behavior and test condition are considered as random variables in probabilistic assessments. Monte-Carlo sampling method is employed to estimate the probability distribution of desired outputs such as propagated crack sizes, stress intensity factors and creep rupture life. It is shown that, the standard deviation of the predicted crack sizes in both through-wall direction and along the surface of the plate will be increased by increasing the time (hence the... 

Dynamic compaction (DC) is a common soil improvement technique, used extensively worldwide. DC treatment design is usually based upon empirical relations and past experience. The common problem with all empirical relations is oversimplification of the mechanisms, and the use of parameters that are highly dependent on engineering judgement. In this paper, a developed finite-element code is used for modelling the impact behaviour of dry and moist sandy soil. The code is verified against the results of centrifuge tests. Then the validity of the popular Menard empirical relation for determination of improvement depth in DC design is investigated. The effect of initial relative density, tamper... 

Influences of rare earth (RE) elements addition on thermal fatigue behaviors of AZ91 alloy were studied. Repeated heating and cooling cycles were applied on the samples at 170 and 210 °C to develop thermal fatigue cracks. Crack growth mechanisms and microstructural influences were investigated by optical and scanning electron microscopy (SEM) as well as energy dispersive X-ray spectroscopy (EDS). Thermal fatigue behaviors were observed to improve successively by addition of the RE up to 2wt.%. This improvement was attributed to the consummation of aluminum in melt by precipitation of the needle shaped Al11RE3 phases. This process was attributed to the reduction of Mg17Al12 phase volume... 

Different microstructures were produced by heat treatment of 4340 steel. These microstructures are bainite, martensite, ferrite-martensite and ferrite-bainite. Mechanical tests were carried out at room temperature. The results showed that steel with bainite-ferrite microstructure has better ductility and charpy impact energy than steels with martensite-ferrite and full bainite microstructures. But yield and tensile strengths of this steel are less than the yield and tensile strengths of the other two steels. Hardness measurements showed that their hardness is the same. Fracture surface observations of tensile specimens showed increase in toughness of bainite-ferrite in comparison to... 

This research explored the corrosion effect of NaCl solution on the surface quality of AZ31 magnesium alloy. Cubic-shaped specimens were immersed in standard 3.5% NaCl solution for 1, 2, 3, and 4 hours. The size of corrosion pits, fatigue crack propagation area, and surface topology were characterized. Fatigue tests were carried out on pre-corroded AZ31 specimens under different stress amplitudes. The correlation between the corrosion time and the virtual crack size was obtained. The virtual crack size is a new parameter that can relate the corrosion to the fatigue life. A new model was proposed to map the surface topology on the virtual crack size of pre-corroded AZ31 magnesium alloy. The... 

This research explored the corrosion effect of NaCl solution on the surface quality of AZ31 magnesium alloy. Cubic-shaped specimens were immersed in standard 3.5% NaCl solution for 1, 2, 3, and 4 hours. The size of corrosion pits, fatigue crack propagation area, and surface topology were characterized. Fatigue tests were carried out on pre-corroded AZ31 specimens under different stress amplitudes. The correlation between the corrosion time and the virtual crack size was obtained. The virtual crack size is a new parameter that can relate the corrosion to the fatigue life. A new model was proposed to map the surface topology on the virtual crack size of pre-corroded AZ31 magnesium alloy. The... 

Structural health monitoring (SHM) has been adopted in the aircraft and civil industries over the last two decades. The ultrasonic guided wave propagation technique is a promising damage detection approach in airplane thin wall structures because of their long distance traveling ability and low attenuation. In fact, by efficiently designing the pattern of sensors one can effectively pick up the changes that happen in propagated signals due to presence of any kind of structural damage. This detection approach could be effectively assisted by numerical simulation techniques like finite-element modeling. To obtain a damage detection algorithm based on instantaneous baseline data, the...