Cast aluminium–silicon alloy, A356.0, is widely used in automotive and aerospace industries because of its outstanding mechanical, physical, and casting properties. Thermal barrier coatings can be ...applied to combustion chamber to reduce fuel consumption and pollutions and also improve fatigue life of components. The purpose of the present work is to simulate stress distribution of A356.0 under thermo-mechanical cyclic loadings, using a two-layer elastic-visco-plastic model of ABAQUS. The results of stress–strain hysteresis loop are validated by an out of phase thermo-mechanical fatigue test. Different thicknesses from 300 to 800μm of top coat and also roughness of the interfaces are simulated to get best stress gradient. Results show that increasing top coat thickness causes stress increase. The realistic interface model is useful for identifying critical areas in stress development. Two important factors having considerable effect on development of high stress in TBC, are the severity of undulations relating to amplitude and wavelength of interface waves; and the thickness of BC layer relating to mutual positioning of either interfaces. However the realistic model has some limitations including long calculation time and difficulties of generating a suitable mesh. To diminish these limitations, after recognizing critical area, in second stage of the study, a periodic unit cell is used instead . Eight models considering different mutual positioning of interfacial asperities along with different penetration in adjoining layers are simulated and compared. Results show that detachment of the thermal barrier coating system from substrate is more probable Results show that IP positioning of mutual waves produce more severe stress but contour pattern is less likely to promote crack propagation.
► Stress distribution is studied in coated A356.0 under thermo-mechanical loadings. ► Realistic interface roughness shows the critical area of tensile stress. ► Increasing TC thickness increases the stress. ► OP positioning of interface waves shows better condition for crack growth.
Most concurrent multiscale methods that expand atomistic region by continuum domain suffer from inconsistent material constitutive properties, which affect the integrity of model in the interface of ...atomistic and continuum domains. In this paper, the Generalized Particle (GP) method is employed to simulate nanoindentation and nanoscratching of a single-crystal aluminum sample. The main advantage of the GP method lies in its ability to extend the simulation model while maintaining consistent atomic properties across all scales. Coarsening of the atomic domain has been conducted through two-scale and three-scale GP model. The results showed a strong consistency between the results of full atomic simulations and those achieved through the GP method for both nanoindentation and nanoscratch simulations. Also, wave reflections were not seen at the interfaces. The study revealed that an increase in the number of distinct particle domains led to a reduction in the accuracy of multiscale simulations.
Shot peening is one of the most effective surface strengthening treatment technologies in which compressive residual stresses are induced beneath the specimen surface. Effects of various factors on ...the distribution of residual stress profile induced by shot peening have been investigated by many researchers. However, initial residual stresses are one of the important factors which affect the shot peening residual stress.
This study is aimed to present comprehensive numerical and experimental study on the effect of initial residual stresses on the shot peened specimen. Initial residual stresses were induced using a four-point bending rig and grinding. Incremental center hole drilling (ICHD) technique was employed to measure residual stresses on bent, ground, shot peened, bent plus shot peened and ground plus shot peened specimens. Numerical analyses of these processes were performed to provide quantitative comparison of different combinations of residual stresses. The comparison with experimental results helped to have a better understanding on how shot peening residual stresses were redistributed. Furthermore, the surface hardness was measured for all specimens.
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•Redistribution of residual stresses by shot peening has been studied experimentally and numerically.•Finite element analyses of random shot impacts provided were combined with bending and grinding simulations.•Interesting results were obtained on how the shot peening stresses redistribute tensile or compressive initial stresses.•Residual stresses for all cases were experimentally measured.
Coverage is one of the most important parameters which is always used in practice to characterize a shot peening process. At the same time however, it is the most missing parameter in the finite ...element simulations of this process. This study aims to relate shot peening simulation to the actual coverage that is developed during the process. Accordingly, two important models from literature are re-simulated and their capability to predict an actual coverage is assessed. Results of this study illustrate that full coverage situation is not captured by these models. Thereafter, a random finite element simulation along with a step by step examination of the treated surface is adopted in order to present an actual coverage evolution. Application of Avrami equation in order to adopt an appropriate number of impingements in simulation to achieve a given coverage for a given target area is examined. According to the result, application of this equation in simulation leads to an overestimation of impingement numbers at full coverage level unless the radius of treated are is at least ten times of the radius of a single indentation.
► Residual stress is not sufficient parameter to ensure reliability of shot peening simulation. ► Existing shot peening simulations could not reflect an actual coverage development. ► Step by step examination of treated surface can be adapted to simulate realistic peening. ► To use Avrami eq. radius of treated area should be at least 10 times of single indentation radius.
•The contribution of creep and fatigue to the total crack growth has been evaluated in deterministic calculations.•Creep-fatigue reliability analysis have been performed on defective pipes.•Various ...parameters have been considered as random variables.•Reliability of the cracked pipes has been determined at different operation times.•The effects of reducing the scattering of the random variables have been examined.•Sensitivity analysis has been performed to investigate the importance of different random variables.•Reliability analyses have been conducted using different reliability methods.
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 variables involved in the problem. Moreover, the effects of reducing the scattering of the random variables on the variations of the reliability index, β, has been examined.
•Concurrent study of grain boundary volume and grain size effect on the initial slope.•The trend of critical stress intensity factor changes at around grain size of 11.65 nm.•Studying grain size ...effect and relative crack-tip position on the observed defects.•Studying the grain boundary sliding after the crack arrest.•The grain located around the crack tip is of considerable importance.
The crack growth process in columnar nanocrystalline samples is simulated using the molecular dynamics method. The effects of grain size, grain boundary, crystallographic orientation and crack tip position on the crack growth behavior are investigated. Different sets of samples with mean grain sizes ranging from 4 nm to 14 nm are prepared. Samples with a similar number of grains and identical dimensions are considered for examining the impact of grain boundary and crystallographic orientation. To assess the effect of the grain boundary, no constraint is considered on the position and orientation of grains, while only the grain orientations are changed to examine the effect of crystallographic orientation. Also, samples with scaled dimensions are created to study the effect of grain size on the crack growth process. In such samples, the relative position of seeds and crystallographic orientation of grains remain constant. The results show that as the mean grain size is reduced, a higher level of stress is required for the initiation of crack growth. Moreover, the reduction of grain size from 14.24 nm to 11.65 nm increases the critical stress intensity factor. Further reduction of grain size decreases the critical stress intensity factor. For grain sizes smaller than 11.65 nm, the volume of stacking fault in the grains near the crack-tip is not significant. Grain size and grain boundary volume have a simultaneous impact on the initial slope which can be observed around the grain size of 11.65 nm. As a result, in nanocrystals with mean grain sizes of less than 11.65 nm, the crack grows in a brittle manner to reach the grain boundary, then progresses intergranularly with high speed. Consequently, grain boundaries and crystallographic orientations are not effective on the crack growth mechanism in nanocrystals with grain sizes less than 11.65 nm, whereas these factors can blunt or arrest the crack in nanocrystals with mean grain sizes of larger than 11.65 nm.
•Effects of uncertainties in the prediction of creep-fatigue crack growth were studied.•Random variables were considered in geometry, material behavior, and test condition.•Std of the predicted crack ...sizes, ai and ci were increased by increasing the time.•Uncertainty in the prediction of crack surfaces was more than that of crack depths.•CoV of the a0 and c0/a0 had a significant effect on these parameters.
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 crack size). It is observed also that, the uncertainty in the prediction of the half-surface crack lengths, ci, is significantly more than that of crack depths, ai. Furthermore, probabilistic evaluations are performed using different reliability methods to calculate the probabilities of exceedance of available experimental results. These evaluations clarified the importance of consideration of uncertainties in creep-fatigue crack growth prediction. Sensitivity analyses are carried out to provide useful information about the order of importance of random variables associated to the different limit state functions. It is found that, the CoV (Coefficient of Variation) of the initial crack size, a0 and c0/a0 has a significant role on the importance of these parameters; and therefore they may be important random variables in such probabilistic assessments, based on their CoV and defined limit state functions.
This paper presents the out-of-phase thermo-mechanical stress analysis of thermal barrier coating (TBC) system in real working conditions used as thermal barrier in diesel engine cylinder heads. The ...coating system in this research comprises 350μm zirconium oxide top coat (TC) and 150μm metallic bond coat (BC). These layers were deposited on the substrate, aluminum A356 alloy, by the aid of air plasma spray (APS) method. Afterwards, the specimen was subjected to thermo-mechanical fatigue (TMF) loadings. Based on the experimental conditions, FE simulations were performed by both time-independent and time-dependent substrate material properties in ABAQUS software. Simulation results related to heat transfer analysis demonstrate only about 10.5% comparative error compared to experimental results. Moreover, defining time-dependent properties, which were obtained from two-layer visco-plastic model, yields results with 15% less comparative error in comparison to the results based on time-independent material properties. In addition, the effects of roughness and porosity in coating layers and substrate were studied on three different models by the aid of a scanning electron microscopy image. Obtained results based on real geometry illustrate that consideration of porosity in TC layer has an effective role in the stress distribution of this layer. However, BC layer stress distribution is much more dependent on interface morphology.
•Study of out-of-phase thermo-mechanical stress analysis in TBC based on the experiment•Consideration of the real geometry (roughness and porosity) on stress distribution•Effect of using time-dependent material properties for substrate on stress results•Accumulation of axial stresses in all layers and its effect on total axial force•Comparison of both heat transfer and stress results (simulation and experiment)
Ultrasonic Testing (UT) is one of the well-known Non-Destructive Techniques (NDT) of spot-weld inspection in the advanced industries, especially in automotive industry. However, the relationship ...between the UT results and strength of the spot-welded joints subjected to various loading conditions is unknown. The main purpose of this research is to present an integrated search system as a new approach for assessment of tensile strength and fatigue behavior of the spot-welded joints. To this end, Resistance Spot Weld (RSW) specimens of three-sheets were made of different types of low carbon steel. Afterward, the ultrasonic tests were carried out and the pulse-echo data of each sample were extracted utilizing Image Processing Technique (IPT). Several experiments (tensile and axial fatigue tests) were performed to study the mechanical properties of RSW joints of multiple sheets. The novel approach of the present research is to provide a new methodology for static strength and fatigue life assessment of three-sheets RSW joints based on the UT results by utilizing Artificial Neural Network (ANN) simulation. Next, Genetic Algorithm (GA) was used to optimize the structure of ANN. This approach helps to decrease the number of tests and the cost of performing destructive tests with appropriate reliability.
Large-scale molecular dynamics simulations are carried out to investigate the friction behavior of nanocrystalline nickel (Ni) at low temperature. Grain sizes ranging from 4–11nm are used to explore ...the behavior of nanocrystalline Ni near the Hall-Petch breakdown. Effects of grain size, grain morphology and scratch depth on coefficient of friction (COF) and deformation mechanism of nanocrystalline Ni are investigated. Reduction of the COF with the refinement of grain size in nanocrystalline nickel is observed, however, for a grain size of 5nm this trend is observed to be reversed. It is also found that scratch depth can influence the estimation of friction behavior of nanocrystalline nickel.
•Study of MD simulation of nanoscratching of columnar nanocrystalline Ni substrate.•Study of grain size, grain morphologies and scratch depth on COF of NC Ni.•COF does not change significantly with the grain size and varies in a limited range.•When grain size decreases, COF reduces down to a grain size of 5nm, after COF increases.•Scratch depth has a significant effect on COF for all grain sizes in NC Ni.
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