Distribution Of Pits Research Articles

A parametric study on effects of pitting corrosion on stiffened panels’ ultimate strength

Pitting corrosion commonly shaped in hull structure due to marine corrosive environment seriously causes the deterioration of structural performance. This paper deals with the ultimate strength behaviors of stiffened ship panels damaged by the pits subjected to uniaxial compression. A series of no-linear finite element analyses are carried out for three stiffened panels using ABAQUS software. Influences of the investigated typical parameters of pit degree (DOP), depth, location and distribution on the ultimate strength strength are discussed in detail. It is found that the ultimate strength is significantly reduced with increasing the DOP and pit depth and severely affected by the distribution. In addition, the pits including their distributions on the web have a slight effect on the ultimate strength. Compared with regular distribution, random one on the panel result in a change of collapse mode. Finally, an empirical formula as a function of corrosion volume loss is proposed for predicting the ultimate strength of stiffened panel.

Estimation of Cavitation Pit Distributions by Acoustic Emission

AbstractCavitation erosion in hydraulic machinery, such as in turbines and pumps, often leads to significant reduction of the service life of the affected components, with serious consequences for ...

Placobdelloides sirikanchanae sp. nov., a new species of glossiphoniid leech and a parasite of turtles from lower southern Thailand (Hirudinea, Rhynchobdellida).

A new species of glossiphoniid leech, Placobdelloides sirikanchanaesp. nov., is reported in the Asian leaf turtle (Cyclemys dentata) and the dark-bellied leaf turtle (C. enigmatica) from Songkhla Province, southern Thailand. The examination of morphological characters revealed that this new species is similar to P. siamensis (Oka, 1917), a common turtle leech species found in Thailand. Placobdelloides sirikanchanaesp. nov. demonstrates distinct morphological characters, with an elongated, narrow body, 13–17 well-developed knob papillae on each annulus, dark brown to greenish dorsal color with a crimson median line, the absence of a scarlet dot, different male and female gonopore distributions, a rough posterior sucker with a random pit distribution, and 104–115 eggs per clutch. The phylogenetic relationships of COI-ND1 genes were clarified and shown to be distinct from those of P. siamensis. Additionally, habitat preferences tended toward low oxygen conditions such as puddles or water patches on rubber plantations.

Polarization effect on optical manipulation in a three-beam optical lattice

We study the effect of polarization on optical micromanipulation in a hexagonal optical lattice formed by three equiamplitude plane waves that have their wave vectors lying equiangularly in a plane, taking into account the vectorial characteristic of the electromagnetic waves. It is demonstrated that different polarizations generate different optical force landscapes, resulting in a trapping versus detrapping phenomenon tunable by tailoring the polarization of the incident beams. The physical origin of the polarization effect on the force landscapes is then traced to the ratio between the conservative (gradient) and nonconservative (scattering) optical forces acting on a particle immersed in the three-wave optical lattice. The trapping-detrapping transition phenomenon due to the change of polarization in small particles, where the gradient force dominates, is revealed to originate from the reverse of the conservative optical force, which manifests itself by a transition of the optical potential energy landscape from one exhibiting a periodic distribution of pits to one showing a distribution of humps over space. Our results suggest an alternative handle to manipulate small particle by tuning the polarization.

Uncovering the ultrastructure of ramiform pits in the parenchyma cells of bamboo [Phyllostachys edulis (Carr.) J. Houz.

Abstract Pits are the main transverse channels of intercellular liquid transport in bamboo. Ramiform pits are a special type of simple pit with two or more branches. However, little is known about the morphology and physiological functions of ramiform pits. The anatomy of plants can provide important evidence for the role of cells. To better understand the ultrastructure and the structure-function relationship of ramiform pits, their characteristics need to be investigated. In this study, both qualitative and quantitative features of ramiform pits were studied using field-emission environmental scanning electron microscopy (FE-ESEM). The samples included the native structures and the replica structures obtained by resin castings. The results show that the ramiform pits have a diverse morphology that can be divided into main categories: type I (the primary branches) and type II (the secondary branches). The distribution of ramiform pits is different in ground parenchyma cells (GPCs) and vascular parenchyma cells (VPCs). The number, the pit aperture diameter and the pit canal length of ramiform pits in the VPCs were, respectively, greater (3-fold), larger (2–3-fold) and shorter (1.3-fold) than those in the GPCs.

Modeling the influence of microstructure on the stress distributions of corrosion pits

This work explores how microstructure influences stress distributions around corrosion pits through computational modeling that captures crystallographic orientation dependencies in corrosion potential and mechanical response. Several sites are considered for initial corrosion pits to investigate the resultant variation in pit geometry and stress quantities. Comparisons to a homogeneous, semi-circular model show that microstructure-sensitive cases can have much larger maximum stress magnitudes, but the stress intensity factors from the homogeneous case are quite congruous to those from the microstructure-sensitive cases. This study illustrates how microstructure impacts local quantities, whereas global quantities are more consistent and comparable to a fully homogeneous assumption.

Theoretical and experimental studies of passivity breakdown of Aermet 100 ultra‐high stainless steel in chloride ion medium

AbstractPassivity breakdown and pitting behavior in the presence of aggressive Cl− on ultra‐high strength steel Aermet 100, commonly used for landing gear, has been studied, the data are explained in view of the point defect model (PDM). Cyclic potentiodynamic polarization (CPP), electrochemical impedance spectroscopy (EIS), and X‐ray photoelectron spectroscopy (XPS) were examined to observe corrosion progress and corrosion products in the salt spray tests at 4, 8, 12, 16, and 20 days intervals. The breakdown behavior was studied using CPP, whereas scanning electron microscopy (SEM) and digital confocal microscopy were used to explore the micromorphology and depth distribution of pits. The cumulative distribution basically satisfies a normal probabilistic distribution at the different concentrations of chloride and pH, which is consistent with PDM assuming a near‐normal distribution of potential breakdown sites regarding to the cation vacancy diffusivity.

Size Distribution Model and Development Characteristics of Corrosion Pits in Concrete under Two Curing Methods.

In this paper, the effect of chloride ions on the development of corrosion pits in different reinforced concrete under different environmental conditions is studied. A fitting model for the size distribution of pits in seawater and sea-sand concrete (SSC) under different curing modes is established, and time-dependent fractal features are examined. Under wet/dry chloride cycles, the weight loss rate and corrosion rate of steel bars are higher, and the number of corrosion pits appears to increase on a small scale within 60-day. A majority of the corrosion is metastable pitting, and we propose a model to describe the size distribution of pitting in different periods. The good agreement between the proposed model and the available data illustrates that the proposed model is reliable and accurate. Macroscopic pitting occurs first in wet/dry chloride cycles. With the increase of concrete age, the size distribution of pits under wet/dry chloride cycles is uneven, and the pit sizes in submerged concrete tend to be equal.

Simulation of Damage Evolution and Study of Multi-Fatigue Source Fracture of Steel Wire in Bridge Cables Under the Action of Pre-Corrosion and Fatigue

A numerical simulation method for the damage evolution of high-strength steel wire in a bridge cable under the action of pre-corrosion and fatigue is presented in this paper. Based on pitting accelerated crack nucleation theory in combination with continuum mechanics, cellular automata technology (CA) and finite element (FE) analysis, the damage evolution process of steel wire under pre-corrosion and fatigue is simulated. This method automatically generates a high-strength steel wire model with initial random pitting defects, and on the basis of this model, the fatigue damage evolution process is simulated; thus, the fatigue life and fatigue performance of the corroded steel wire can be evaluated. A comparison of the numerical simulation results with the experimental results shows that this method has strong reliability and practicability in predicting the fatigue life of corroded steel wire and simulating the damage evolution process. Based on the method proposed in this paper, the fatigue life of steel wires with different degrees of corrosion under the action of different stress levels is predicted. The results show that as the degree of corrosion increases, the fatigue properties of steel wire gradually decrease, and the influence of existing pitting corrosion on fatigue life is far greater than that on mass loss.Stress concentration is the main cause of fatigue life of corroded steel wire in advance attenuation. In addition, the fracture process of steel wire with multi-fatigue sources and the effect of the number and distribution of pits on the fatigue life of steel wire are studied. The results show that, compared with a stepped pitting distribution, a planar pitting distribution has a greater impact on the damage evolution process. The fatigue life of steel wire is positively correlated with the number of pits and the angle and distance between pits.

A Computational Investigation of the Group Effect of Pits on the Ultimate Strength of Steel Plates

In this work, we focus on assessing the group effect of localized corrosion on the ultimate strength of the marine structural plates and study the load-deformation behaviors of plates of various slenderness and uniaxial compression. Meanwhile, we investigate different corroded patterns from a single circular pit to 25 circular pits distributed over the plate and carry out hundreds of nonlinear finite element simulations by combining the number, depth, distribution of pits with imperfections and slenderness of plate. The distribution of multiple pits causes scattering of stress concentration on the plate, then the plastic section of plate changes with wider distribution of damage simultaneously. The ultimate strength arises when un-loading zone comprised of the yielding strips and holes extends across the plate. It can be concluded that the corroded condition defined as group effect of pits manipulates the deformation state and the loading capacity of plate at the ultimate strength mode that coincides with the proportion of effective loading area and section in the process of post-buckling. To validate the effect of pits group, we perform the numerical experiments of the post-buckling of steel plates containing pits in a row with different orientation.

Analysis on anti‐wear mechanism of bionic non‐smooth surface based on discrete phase model

In order to study the lubrication and anti-wear mechanism of the pit-type bionic non-smooth surface used in the low-speed and high-torque seawater hydraulic motor valve plate pair, the discrete phase models of the four pits are simulated under different working conditions. In this study, the trajectories of different diameters particles in the hemispherical pits are analysed, which can reflect the movement of different sizes and masses wear debris in the pits. The discrete phase concentration distributions of the four-kind pits, hemispherical pits, cylindrical pits, four-prism pits and tri-prism pits, are simulation under the same working conditions, which reflects the effect of pit geometry on the movement of wear debris. The discrete phase concentration distributions of four pits moving at different rotation speeds and different rotation radii are calculated, which indicates that the rotation speed of the motor and the distribution of pits on the valve plate will affect the ability of the pit to store wear debris.

A study on corrosion behaviors of laser cladded Fe−Cr−Ni coating in as‐cladded and machined conditions

This paper investigates the effect of turning process on surface quality and corrosion behaviors of laser cladded coatings on the 1045 steel. Corrosion resistance of the 1045 steel is improved by laser cladded Fe−Cr−Ni coatings. The as‐cladded coating behaves nobler and more resistant to corrosion compared to the 1045 steel substrate. By using turning as a finish process, surface roughness Ra of the laser cladded coating is decreased to 0.517 µm to meet the surface roughness requirement (i.e., Ra ≤ 0.8 µm) of a single‐telescopic hydraulic column. Also, surface defects including laps, cavities, and adhered material particles occur on the cladded coatings after machining. The machined coatings exhibit comparable polarization resistance compared to the as‐cladded condition, and the polarization resistance of the machined surface increases with feed rate increasing. The corrosion pits show sensitivity on feed marks induced by machining. This concentrated distribution of corrosion pits is a result of the fluctuation of local electron work function (EWF), which promotes the formation of local corrosion cells in adjacency to feed marks.

Reducing the residual stress in micro electroforming layer by megasonic agitation

In order to reduce the large residual stress in micro elelctroforming layer, megasonic assisted electroforming is proposed here. Micro electroforming experiments were performed with and without megasonic agitation, respectively. Four different megasonic power densities were applied to investigate the influence of megasonic agitation on reducing the residual stress. The residual stress was measured by X-ray diffraction (XRD) method. Experiment results show that the residual stresses fabricated with megasonic agitation are less than that fabricated without megasonic. When the megasonic power density is 2 W/cm2, the residual stress can be the minimum value of −125.7 MPa, reduced by 60% in comparison with the value of −315.1 MPa electroformed without megasonic agitation. For exploring the mechanism of megasonic agitation on reducing the residual stress, the dislocation density and crystal orientation were calculated by the single-line Voigt profile analysis and Relative Texture Coefficient (RTC) method, respectively. The diameters and distributions of pits on the surface of electroforming layer were observed by the STM-6 tool microscope and counted by the Image-Pro Plus software. It reveals that one hand of the mechanism is the acoustic streaming produced by megasonic can strengthen the motion of dislocation in crystal lattice and makes the crystal lattices grow towards the equilibrium shape, which is benefit to crystallization with low residual stress. When the megasonic power density is 2 W/cm2, the dislocation density increases to be the maximum value of 8.09 × 1015 m−2 and the difference between RTC(1 1 1) and RTC(2 0 0) decreases to be zero, which is consistent with the residual stress results. The other hand is that the stable cavitation produced by megasonic can provide residual stress release points during the electroforming process.

Friction and wear on titanium alloy surface machined by ultrasonic vibration-assisted milling

With the development of the feasibility study on ultrasonic vibration-assisted machining in difficult cutting materials, its surface microstructure and the use performance get more and more attention. In this paper, the 3D roughness and surface residual stress are analyzed based on the experiments of common milling (CM) and ultrasonic vibration-assisted milling (UVAM) TC4 titanium alloy. Besides, the surface friction and wear properties machined by CM and UVAM are compared. The results show that, although arithmetic average deviation ($${s_\mathrm{a}}$$) and root-mean-square deviation ($${s_\mathrm{q}}$$) increase slightly because of the high-frequency vibration, the uniformity and consistency of the surface texture improve obviously. Moreover, the surface residual compressive stress increases with the increasing of the ultrasonic current. Additional, the friction coefficient and wear volume are reduced obviously by the surface microstructure machined by UVAM under oil lubrication. Because there are many continuous pits distribution on the surface, which can not only store the debris but also generate the hydrodynamic effect. However, there is no obvious difference in surface tribological properties machined by CM and UVAM under dry friction, due to the disappearance of hydrodynamic effect.

The Underoil Droplet Corrosion Behavior of X100 Pipeline Steel

Canadian natural bitumen production will reach a peak of 112 million barrels per day in 2078 [1]. Pipelines are primarily used to transport this bitumen. Bitumen must be diluted in order to be pumped through a pipeline effectively because it is otherwise too viscous to flow [2]. Underoil droplet corrosion of diluted bitumen (dilbit) pipelines may lead to the loss of pipe wall thickness, which deteriorates the structural reliability and increases the risk of leaking of pipelines. The current models for atmospheric droplet corrosion, where no limitation to oxygen access is imposed, cannot be applied to underoil droplet corrosion in dilbit pipeline conditions. In this study, a NaCl droplet on X100 pipeline steel covered by paraffin oil is used to simulate the corrosive environment encountered in diluted bitumen pipelines. A new distinctive corrosion mechanism is proposed to explain the corrosion of X100 pipeline steel beneath an underoil salty droplet. This mechanism accounts for the depleted oxygen levels in the droplet environment. In the initial stage (1 h), the distribution of corrosion pits was heterogeneous with one area under the droplet presenting a higher pit density. As the corrosion proceeded (24 h), the localized corrosion was governed by pH development due to initial pit density. Regions of initially higher pit density switched to general corrosion, while regions with lower pit densities continued pitting. The effects of droplet volume, chloride ion concentration, and temperature on the underoil droplet corrosion behavior of X100 pipeline steel were also studied by scanning electron microscopy, Raman spectroscopy, and profilometry. At 23 °C, lepidocrocite and hematite were observed in the region where pitting was initially prevalent. However, as this region switched to uniform corrosion, goethite, lepidocrocite, and hematite were formed. The same distribution of crystal structures was identified for the droplets with varying volume and chloride ion concentration. When the temperature was increased to 60 °C, hematite was detected in both regions. Decreased droplet volumes, increased chloride concentration, and raised temperatures increase corrosion penetration when uniform corrosion is prevalent. This study provides important findings to better understand the mechanisms behind droplet corrosion underoil and informs future predictions of the corrosion rate of pipeline steels.

Discovering cortical sulcal folding patterns in neonates using large-scale dataset.

The folding of the human cerebral cortex is highly complex and variable across individuals, but certain common major patterns of cortical folding do exist. Mining such common patterns of cortical folding is of great importance in understanding the inter-individual variability of cortical folding and their relationship with cognitive functions and brain disorders. As primary cortical folds are mainly genetically influenced and are well established at term birth, neonates with minimal exposure to the complicated postnatal environmental influences are ideal candidates for mining the major patterns of cortical folding. In this paper, we propose a sulcal-pit-based method to discover the major sulcal patterns of cortical folding. In our method, first, the sulcal pattern is characterized by the spatial distribution of sulcal pits, which are the locally deepest points in cortical sulci. Since deep sulcal pits are genetically related, relatively consistent across individuals, and also stable during brain development, they are well suited for representing and characterizing the sulcal patterns. Then, the similarity between the distributions of sulcal pits is measured from the spatial, geometrical, and topological points of view. Next, a comprehensive similarity matrix is constructed for the whole dataset by adaptively fusing these measurements together, thus capturing both their common and complementary information. Finally, leveraging the similarity matrix, a hierarchical affinity propagation algorithm is used to group similar sulcal folding patterns together. The proposed method has been applied to 677 neonatal brains, and revealed multiple distinct and meaningful sulcal patterns in the central sulcus, superior temporal sulcus, and cingulate sulcus.

An application of stochastic modeling to pitting of Spent Nuclear Fuel canisters

Chloride induced stress corrosion cracking (CISCC) is one of the main factors affecting the integrity of used nuclear fuel in dry storage canisters, especially at coastal sites. CISCC has complex interactions associated with environment, stress and materials properties. This paper is focused on the development of probability distribution functions for maximum pit depth based on experimental data created at Lawrence Livermore National Laboratory (LLNL) that have not been fully analyzed before. The LLNL report outlined the key variables of pit initiation, termination and growth rate that may lead to CISCC, with the final aim of prediction of canister life and optimization of the time interval inspection of the canisters. The key parameters characterizing the probability distributions of pits at each stage depend on environment, material susceptibility conditions, and residual stress intensity. A basic stochastic approach to estimate probability distributions based on the median and maximum pit depths observed in experimental data succeeds in reproducing the experimental results, and can be used to estimate the distribution of maximum depth at future times.

A New Corrosion Mechanism for X100 Pipeline Steel Under Oil-Covered Chloride Droplets

A 1.7 mM NaCl droplet on X100 pipeline steel covered by paraffin oil is used to simulate the corrosive environment encountered in heavy oil or bitumen pipelines. The development of corrosion under the droplet was monitored and explored in two stages. In the initial stage (1 h), the distribution of corrosion pits was heterogeneous with one area under the droplet presenting a higher pit density. As the corrosion proceeded (24 h), the localized corrosion in the area under the droplet with the higher pit density switched to general corrosion, while the other region of the droplet continued to pit. The mechanisms driving this new distinctive corrosion form developed beneath an underoil droplet are explained.

Crystal Dissolution Kinetics Studied by a Combination of Monte Carlo and Voronoi Methods

Kinetic Monte Carlo (kMC) methods have been used extensively for the study of crystal dissolution kinetics and surface reactivity. A current restriction of kMC simulation calculations is their limitation in spatial system size. Here, we explore a new and very fast method for the calculation of the reaction kinetics of a dissolving crystal, capable of being used for much larger systems. This method includes a geometrical approach, the Voronoi distance map, to generate the surface morphology, including etch pit evolution, and calculation of reaction rate maps and rate spectra in an efficient way, at a calculation time that was about 1/180 of the time required for a kMC simulation of the same system size at one million removed atoms. We calculate Voronoi distance maps that are based on a distance metric corresponding to the crystal lattice, weighted additively in relation to stochastic etch pit depths. We also show how Voronoi distance maps can be effectively parameterized by kMC simulation results. The resulting temporal sequences of Voronoi maps provide kinetic information. By comparing temporal sequences of kMC simulation and Voronoi distance maps of identical etch pit distributions, we demonstrate the opportunity of making specific predictions about the dissolution reaction kinetics, based on rate maps and rate spectra. The dissolution of an initially flat Kossel crystal surface served as an example to show that a sequence of Voronoi calculations can predict dissolution kinetics based on the information about the distribution of screw defects. The results confirm that a geometrical relationship exists between the material flux from the surface at a certain point and the distance (or, when considering anisotropy, a function of distance) to the nearest defect. In this study, for the sake of comparability, the calculations are made using input parameters directly derived from the kMC models operating at the atomic scale. We show that, using values of v(rpit) and weighting factors obtained by kMC, the resulting surface morphologies and material flux are almost identical. This implies that discrete Voronoi calculations of starting and end points of the dissolution are sufficient to calculate material flux maps, without the time-consuming overhead of computing the interim reactions at the atomic-scale. This opens a promising new venue to efficiently upscale full-atomic kMC models to the continuum macroscopic level where reactive transport and Lattice Boltzmann calculations can be applied.

Parametric and nonparametric description of the surface topography in the dry and MQCL cutting conditions

This paper focuses on the parametric and nonparametric description of surface topography after turning in the dry and MQCL conditions. The carried out research involved the longitudinal turning tests of duplex stainless steel, conducted in a range of variable cutting speeds vc and feeds f. Surface topography measurements were conducted with the application of Infinite Focus G4 microscope. In the first stage of research, the parametric analysis of the machined surfaces has been conducted. It included the evaluation of amplitude surface roughness parameters. In the next stage the nonparametric description of machined surfaces was carried out. This analysis has been focused on the presentation of machined surfaces in the form of contour maps and isometric images. The last stage of experiment involved the evaluation of surface isotropy and formulation of material ratio charts. The conducted research shows that the application of MQCL method can lead to the reduction of 3D surface roughness parameters compared to values reached after the dry machining. Moreover, the better distribution of irregularity peaks and pits under the MQCL method, comparing to that obtained after dry cutting implies that surfaces machined with the use of MQCL technique can be characterized by a high wear resistance.