Sharif Digital Repository

Fatigue life and crack growth retardation due to periodic tensile overloads, which are superposed on constant amplitude cycles, have been investigated in the present study. In the numerical analyses, the Walker equation with the generalized Willenborg model has been used. The analyses and experiments have been performed on C(T) specimens made of 7075 aluminium. The periodic overloads have been induced at two different periodicities and the effect of overload ratio and overload periodicity on the fatigue life has been studied. The results of the experiments and analyses reveal that the normalized fatigue life versus overload ratio curve has a maximum point, which indicates that there are... 

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... 

The beneficial effects of shot peening on fatigue life of mechanical components are well known. However, there are some reports in the literature that indicate inappropriate shot peening parameters tend to reduce the fatigue life. It is therefore, the purpose of this study to find a logical quantitative justification for these observations. Using finite element method, a dynamic elasticplastic simulation of shot peening was presented. Effect of shot velocity and size on surface morphology after shot peening were examined. Fatigue crack initiation life calculation of shot peened specimens revealed that beneficial effect of shot peening significantly vanishes in the case of high velocities and... 

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... 

4340 steel bars were austenitized at 850°C for 1 hour followed by heating at 700°C (ferrite and austenite region) for 90 min and quenching into a salt bath with different temperatures of 300, 350, 400 and 450°C. The steel bars were held for 1 hour at these temperatures before air cooling to room temperature. Various ferrite-bainite microstructures with 34% volume fraction of ferrite and different bainite morphologies were obtained. The results of SEM studies showed that by increasing the austempering temperature, the morphology of bainite varies from lower to upper bainite. According to the T-T-T diagram of the studied steel, the bainite transformation will not complete for the holding time... 

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... 

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... 

Because damage identification results based on the Lamb waves propagation approach can be influenced by varying environmental and operational conditions, development of a robust monitoring system with no need of the prior measured data of the structure has gained much attention recently. The instantaneous baseline damage detection technique is one of the promising methods that overcome the mentioned obstacles. For this, the material properties, electromechanical characteristics, and the geometric features must be identical. Also, sensor distances in similar paths and the geometric dimensions of the transducers must be the same. So, implementing the instantaneous baseline damage... 

The onset of fretting fatigue is characterized by material microstructural changes in which the extent of the damage is comparable to grain size, and hence, the microstructure characteristics could have a significant effect on fatigue crack initiation. In this paper, a three-dimensional finite element crystal plasticity framework is presented for simulation of the fretting fatigue. Controlled Poisson Voronoi tessellation (CPVT) method is employed to generate the polycrystalline region. In the CPVT method, regularity parameter controls the shape of grains. In this study, the impact of grain size and regularity parameter on crack initiation life and initiation site has been investigated.... 

The aim of this study was to investigate the applicability of acoustic emission (AE) technique to evaluate delamination crack in glass/epoxy composite laminates under quasi-static and fatigue loading. To this aim, double cantilever beam specimens were subjected to mode I quasi-static and fatigue loading conditions and the generated AE signals were recorded during the tests. By analyzing the mechanical and AE results, an analytical correlation between the AE energy with the released strain energy and the crack growth was established. It was found that there is a 3rd degree polynomial correlation between the crack growth and the cumulative AE energy. Using this correlation the delamination... 

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...