Spherical Defects Research Articles

Investigating the orientation dependence of local fields around spherical defects using crystal plasticity simulations

The presence of a void or secondary particle plays a crucial role in both the mechanical response and damage evolution of metals. This work presents local stress and strain field predictions in a single crystalline matrix that contains a spherical void or hard particle using crystal plasticity finite element method (CP-FEM) simulations. Simulations demonstrate highly heterogeneous orientation dependent local fields near defects. In particular, we show that matrix decohesion around hard particles will occur first before void growth in pre-existing voids under strain-controlled uniaxial tension and isochoric loading. Furthermore, CP-FEM simulations predict that the [1̄11]-oriented grain is most susceptible for failure, while grains oriented toward the [001] orientation are more resistant to failure. This work provides insights into how grain-scale microstructure with volumetric defects influence the local damage and failure behavior in metal alloys.

Intelligent identification method for pipeline ultrasonic internal inspection

ABSTRACT In pipeline ultrasonic testing, the accurate identification of defect types is a prerequisite for quantifying defect sizes. To this end, this paper proposes a defect identification algorithm based on the combination of Wavelet Packet Transform (WPT), Distance Evaluation Technique (DET) and GA-SVM to precisely identify defect boundaries. First, the ultrasonic echo signals are processed using WPT to more accurately extract defect information. Then, DET is employed to eliminate redundant feature parameters, which are sorted by sensitivity to determine the input feature parameters for the GA-SVM. On this basis, a self-developed ultrasonic testing experimental system is used to collect ultrasonic echo signals from Q235 steel pipes with cylindrical, spherical, and conical defects at different spatial orientations of the probe and the pipe. After processing and identifying the signals, it was found that the proposed algorithm achieves an average identification accuracy of 99% for different types of defects, thereby realising automatic identification of pipeline defects. The results provide new ideas and methods for the intelligent identification of volumetric defect types in pipeline ultrasonic internal testing and promote the development of industrial-grade ultrasonic internal detectors.

Effect of aluminum particles on the insulating properties of C4F7N/CO2 mixed gas

The C4F7N/CO2 mixed gas is most expected to replace SF6 in high-voltage power equipment, and the generation of free metal particles in gas-insulated equipment is unavoidable and highly susceptible to inducing insulation failures. In this paper, the effects of flaky, spherical, and mixed shape aluminum particle defects on the breakdown characteristics of C4F7N/CO2 mixed gas are investigated and compared to those of SF6 under the same conditions. The experimental results show that the effect of particle defects on the insulating properties of C4F7N/CO2 gas mixtures is visualized by the fact that particle defects reduce the breakdown voltage of C4F7N/CO2 gas mixtures, increase their stochasticity, and reduce the enhancement of the insulating properties of C4F7N/CO2 gas mixtures by increasing the air pressure. The greater the number of particles, the greater is the degree of influence. Comparison of the SF6 and C4F7N/CO2 gas mixtures reveals that, in the case of flake defects, increasing the air pressure improves the insulation performance of the SF6 gas better than that of the C4F7N/CO2 gas mixture. Under the condition of the same number of particles, the larger the proportion of spherical particles in the hybrid particles, the smaller is the influence of metal particles on the C4F7N/CO2 mixed gas and on SF6 gas.

Mathematical Modeling of the Heat Transfer Process in Spherical Objects with Flat, Cylindrical and Spherical Defects

This paper proposes a method for determining the optimal parameters for the thermal testing of plant tissues of fruits and vegetables containing surface and subsurface defects in the form of areas of plant tissues with different thermophysical characteristics. Based on well-known mathematical models for objects of predominantly flat, cylindrical and spherical shapes containing flat, spherical and cylindrical regions of defects, numerical solutions of three-dimensional, non-stationary temperature fields were found, making it possible to measure the power and time of the thermal exposure of the sample surface to the radiation from infrared lamps using the finite element method. This made it possible to ensure the reliable detection of a temperature contrast of up to 4 °C between the defect and defect-free regions of the test object using modern thermal imaging cameras. In this case, subsurface defects can be detected at a depth of up to 3 mm from the surface. To determine the parameters of mathematical models of temperature fields, such as thermal conductivity and a coefficient of the thermal diffusivity of plant tissues, a new method of a pulsed heat flux from a flat heater is proposed; this differs in the method of processing experimental data and makes it possible to determine the required characteristics with high accuracy during the active stage of the experiment in a period not exceeding 1–3 min.

On Defect Evolution in EBM Additively Manufactured Ti-6Al-4V via In Situ Investigations.

This study concerned the in situ investigation of the defect evolution and fracture mechanism of additively manufactured (AM) Ti-6Al-4V under uniaxial tensile tests. In order to achieve this, microstructure characterization was initially carried out in order to identify the defects within the matrix of the candidate material. In situ testing was then performed, focusing on the spherical defect to observe its evolution under tensile loading. It was found that, before the fracture stage, the geometric evolution of the spherical defect towards an ellipse shape was dominated by the load in the tensile direction. In addition, the slip band density was found to be aggravated near the spherical defect due to the geometric discontinuity-induced stress concentration. During the fracture process, the defect geometry evolved as an irregular shape, which was mainly attributed to the micro-void-induced localized multi-axial stress state. The fracture analysis indicated that defects play a key role in crack initiation, leading to the fracture of LPBF materials.

Isotropic Pinning Enhancement: Comparison Between Spherical and Point Defects

Isotropic Pinning Enhancement: Comparison Between Spherical and Point Defects

Spherical image acquisition and defect detection of nuclear fuel based on the line scan

Graphite balls are used as matrix materials of nuclear fuel. To ensure the safe use of nuclear energy, it is necessary to detect the surface defects of graphite balls to screen out the balls that do not meet safety requirements. However, due to the small surface defects and complex background of graphite balls, the detection accuracy based on traditional detection methods is difficult to meet engineering requirements. To solve the above problems, this paper designs an image acquisition device for graphite balls based on line scanning to collect spherical images and designs an improved algorithm based on the U2-Netp network to solve the problem of low accuracy of defect detection. Firstly, the algorithm replaces the up-sampling in the RSU module with cubic linear interpolation to enhance the smoothness of the image. Secondly, spatial channel attention and multi-feature fusion modules are built between the encoder and the decoder to improve the accuracy of the model to obtain interesting information. Finally, the prediction output of Sigmoid and Threshold fusion is designed to solve the problems caused by the gradient disappearance and the uneven polarity of positive and negative samples. Tested on the actual collected data set, the improved U2-Netp algorithm improved the average accuracy of detecting surface defects on graphite balls by 8.1% compared to the original U2-Netp algorithm, reaching 85.3%.

Understanding melting of Ti crystals with spherical voids from molecular dynamics simulations

Titanium (Ti) is one of the most important metals used in several industrial applications, and the presence of spherical defect reduces its strength and stability. We simulate the melting of Ti crystals with a spherical void of radii 0.6, 0.8, 1.0, and 1.5 nm and also of the crystal without it. Ti is modeled using embedded atom method ,and all crystals are heated at 1 atm from 300 to 2200 K till it melts completely. All molecular dynamics trajectories are analyzed using radial distribution functions, bond-orientational order parameters, Voronoi tessellation, and velocity auto-correlation functions. The results show that 0.6, 0.8, 1.0, and 1.5 nm voids fill before the crystals melt and they fill immediately within few picoseconds; thereafter, atoms rearrange/order to crystal like arrangements, wherein overall crystallinity remains hcp for crystals with 0.6 and 0.8 nm void and changes to bcc for the crystals with 1.0 and 1.5 nm voids. For all crystals with and without void, melting takes place with the loss of both long- and short-range orders and not from liquid like nuclei as proposed by classical nucleation theory.

Mechanical responses and stress distribution of rock-like specimen containing a spherical defect under uniaxial compression

Spherical defect is often seen in rock material, and the size and position of the defect seriously affect the stability of rock mass. In this study, a locating device for preparing spherical defects was designed. A series of uniaxial compression tests and simulations were conducted to study the influence of the defect size and location on the mechanical, micro-cracking, failure characteristics, and stress distribution of the sample containing a spherical defect. The results indicate that the uniaxial compressive strength (UCS) and Young’s modulus both decrease with increasing defect size. The location variation of the same-sized spherical defect has a significant influence on the UCS but hardly influences Young’s modulus. The closer the defect is to the boundary, the lower the UCS is. The acoustic emission (AE) monitoring results indicate that the damage point aggregates and expands around the defect, forming a failure surface in the later stage of specimen loading. The failure modes of the specimen are divided into tensile failure, shear failure, and tensile-shear failure. Laboratory test results match well with the numerical simulation results obtained through the PFC3D program. The numerical stress monitoring results indicate that there is significant stress concentration near the defect. And the concentrated stress at the defect boundary drops before the the model reaches its peak stress, which can be used to predict the failure of the defected rock mass. The research findings could supply a certain reference for stability analysis and control of rock material containing similar types of defects.

Thermodynamic study of molten pool during selective laser melting of alsi10mg alloy

A study of the thermodynamic behavior during the Selective Laser Melting process of AlSi10Mg alloy was conducted using a combined numerical simulation and experimental approach. A multiphase flow model incorporating gas-liquid-solid phases was established. The numerical simulation results aligned well with experimental findings, affirming the reliability of the numerical model. Insufficient and excessive laser power densities respectively led to incomplete fusion defects and spherical pore defects. The evolution of the melt pool was primarily influenced by recoil pressure, Marangoni force, and surface tension, resulting in keyhole formation, convective flow, and liquid surface oscillation as characteristic features of melt pool evolution. Solidification process and mechanical analyses revealed that the central region of the melt pool exhibited smaller temperature gradients and higher solidification rates. Properly increasing the scanning speed under suitable laser power conditions is more conducive to optimizing the solidification structure and enhancing the mechanical properties of SLM components. This study elucidates the thermodynamic evolution mechanism during the SLM process of aluminum-based alloys, providing theoretical support and guidance for optimizing the SLM process.

Numerical investigation of the influence of defects on the multiaxial fatigue strength of additively manufactured alloys

Laser Powder Bed Fusion (L-PBF) is one of the Additive Manufacturing (AM) techniques that have revealed salient advantages in enabling the fabrication of 3D parts with intricate shapes and added functionalities. Despite many advances, scientific challenges still exist. One of them, especially where the additively manufactured industrial components undergo fatigue loading, is the defects (gas pores and Lack of Fusions (LoFs)) that are induced during the fabrication process. The present work aims to investigate the influence of AM defects on fatigue strength under multiaxial loading conditions. From numerical simulations, the key question addressed is the impact of defects’ morphology on their criticality under multiaxial loadings. Finite Element (FE) elastic simulations at load ratio R = -1 under multiaxial loading conditions (tension, torsion and tension-torsion) have been performed on numerically generated spherical defect and LoFs obtained from micro-CT scans of additively manufactured TA64 alloy. The Crossland criterion (stress-based) was used to evaluate the fatigue strength and the obtained numerical results were compared with the available experimental results.

Pressure-based process monitoring of direct-ink write material extrusion additive manufacturing

As additive manufacturing (AM) has become a reliable method for creating complex and unique hardware rapidly, the quality assurance of printed parts remains a priority. In situ process monitoring offers an approach for performing quality control while simultaneously minimizing post-production inspection. For extrusion printing processes, direct linkages between extrusion pressure fluctuations and print defects can be established by integrating pressure sensors onto the print head. In this work, the sensitivity of process monitoring is tested using engineered spherical defects. Pressure and force sensors located near an ink reservoir and just before the nozzle are shown to assist in identification of air bubbles, changes in height between the print head and build surface, clogs, and particle aggregates with a detection threshold of 60–70% of the nozzle diameter. Visual evidence of printed bead distortion is quantified using optical image analysis and correlated to pressure measurements. This methodology provides an ability to monitor the quality of AM parts produced by extrusion printing methods and can be accomplished using commonly available pressure-sensing equipment.

RG flows on two-dimensional spherical defects

We study two-dimensional spherical defects in d-dimensional Conformal Field Theories. We argue that the Renormalization Group (RG) flows on such defects admit the existence of a decreasing entropy function. At the fixed points of the flow, the entropy function equals the anomaly coefficient which multiplies the Euler density in the defect’s Weyl anomaly. Our construction demonstrates an alternative derivation of the irreversibility of RG flows on two-dimensional defects. Moreover, in the case of perturbative RG flows induced by weakly relevant deformations, the entropy function decreases monotonically and plays the role of a C-function. We provide a simple example to explicitly work out the RG flow details in the proposed construction.

The effect of periapical bone defects on stress distribution in teeth with periapical periodontitis: a finite element analysis

BackgroundApical periodontitis directly affects the stress state of the affected tooth owing to the destruction of the periapical bone. Understanding the mechanical of periapical bone defects/tooth is clinically meaningful. In this study, we evaluate the effect of periapical bone defects on the stress distribution in teeth with periapical periodontitis using finite element analysis.MethodsFinite element models of normal mandibular second premolars and those with periapical bone defects (spherical defects with diameters of 5, 10, 15, and 20 mm) were created using a digital model design software. The edges of the mandible were fixed and the masticatory cycle was simplified as oblique loading (a 400 N force loaded obliquely at 45° to the long axis of the tooth body) to simulate the tooth stress state in occlusion and analyze the von Mises stress distribution and tooth displacement distribution in each model.ResultsOverall analysis of the models: Compared to that in the normal model, the maximum von Mises stresses in all the different periapical bone defect size models were slightly lower. In contrast, the maximum tooth displacement in the periapical bone defect model increased as the size of the periapical bone defect increased (2.11–120.1% of increase). Internal analysis of tooth: As the size of the periapical bone defect increased, the maximum von Mises stress in the coronal cervix of the tooth gradually increased (2.23–37.22% of increase). while the von Mises stress in the root apical region of the tooth showed a decreasing trend (41.48–99.70% of decrease). The maximum tooth displacement in all parts of the tooth showed an increasing trend as the size of the periapical bone defect increased.ConclusionsThe presence of periapical bone defects was found to significantly affect the biomechanical response of the tooth, the effects of which became more pronounced as the size of the bone defect increased.

The effect of root canal treatment and post-crown restorations on stress distribution in teeth with periapical periodontitis: a finite element analysis

AimTo evaluate the effects of root canal treatment (RCT) and post-crown restoration on stress distribution in teeth with periapical bone defects using finite element analysis.MethodologyFinite element models of mandibular second premolars and those with periapical bone defects (spherical defects with diameters of 5, 10, 15, and 20 mm) were created using digital model design software. The corresponding RCT and post-crown restoration models were constructed based on the different sizes of periapical bone defect models. The von Mises stress and tooth displacement distributions were comprehensively analyzed in each model.ResultsOverall analysis of the models: RCT significantly increased the maximum von Mises stresses in teeth with periapical bone defects, while post-crown restoration greatly reduced the maximum von Mises stresses. RCT and post-crown restoration slightly reduced tooth displacement in the affected tooth. Internal analysis of tooth: RCT dramatically increased the maximum von Mises stress in all regions of the tooth, with the most pronounced increase in the coronal surface region. The post-crown restoration balances the internal stresses of the tooth and is most effective in periapical bone defect − 20-mm model. RCT and post-crown restoration slightly reduced the tooth displacement in all regions of the affected tooth.ConclusionsRoot canal treatment seemed not to improve the biomechanical state of teeth with periapical bone defects. In contrast, post-crown restoration might effectively balance the stress concentrations caused by periapical bone defects, particularly extensive ones.

Sputtering from rough tungsten surfaces: Data-driven molecular dynamics simulations

The sputtering of tungsten surfaces caused by hot plasma particles is an important process in fusion reactors where divertors are typically made of tungsten sheets. In this study, we present a molecular dynamics simulation strategy to investigate the sputtering yields of tungsten surfaces with geometrical defects. This should serve as a model for non-monocrystalline surfaces in general and could also be a rough model for nanoscale “fuzzy” layers, which are known to be formed by surface bombardment with energetic particles. Using a non-cumulative approach, we simulate the irradiation of tungsten surfaces with cone-shaped, cylindrical, and spherical defects by argon atoms. We analyze the sputtering yields as functions of particle energy and defect sizes. As a result, we find that surfaces with distinctly shaped defects always exhibit reduced sputtering yields, compared to smooth ones. We also investigate the angular distributions of sputtered particles and find them mostly to be in accordance with prior experimental and computational results.

Effect of internal defects on tensile strength in SLM additively-manufactured aluminum alloys by simulation

This research investigates the effect of internal defects on the tensile strength of Selective Laser Melting (SLM) additively-manufactured aluminum alloy (AlSi10Mg) test parts used for civil aircraft light weight design. A Finite Element Analysis (FEA) model containing internal defects was established by combining test data and the stress concentration factor comparison method. The effect of variation in the number, location and shape of defects on the finite element results was analyzed. Its results show that it is reasonable to use spherical defect modeling. The finite element modeling and analysis methods are also applied to the study of the effect of internal defects on tensile strength in additive manufacturing of other metallic materials. According to the FEA results of single defects at different scales, the formula for calculating the weakening degree of tensile strength applicable to the defective area of less than 15% was established. The result of the procedure is reliable and conservative. This research results can guide the selection of process parameters for the additive manufacturing of aluminum alloys. Further, the research results can promote the application of metal additive manufacturing in designing light-weight civil aircraft structures.

Fatigue crack growth behavior in additive manufactured Ti6Al4V alloy with intentionally embedded spherical defect

The present study aims at elucidating the impact of unavoidable defects in selective laser melting (SLM) additive manufactured alloys on fatigue performances. The novel idea is to reveal the relationship between the printing defect and fatigue crack growth, by intentionally embedding a 500 μm spherical defect in an SLM-Ti6Al4V alloy exerted to low-cycle fatigue (LCF). The results show that the cyclic softening of the specimen was not affected by the embedded defect and surprisingly its fatigue life was nearly the same as the virgin specimens free of embedded defects at low strain amplitudes. The performance is attributed to the single crack initiation site and only small change in the stress intensity factor range (ΔK). In contrast, when subjected to higher strain amplitudes during LCF, the embedded defect dominates the crack initiation and crack propagation occurs readily. It is shown that the experiments support a model description based on the competitive failure mechanisms between the surface defects and internal defects.

Structural Design and Simulation of a Multi-Channel and Dual Working Condition Wafer Defect Inspection Prototype.

Detecting and classifying defects on unpatterned wafers is a key part of wafer front-end inspection. Defect inspection schemes vary depending on the type and location of the defects. In this paper, the structure of the prototype is designed to meet the requirements of wafer surface and edge defect inspection. This prototype has four inspection channels: scattering, reflection, phase, and contour, with two working conditions: surface and edge inspection. The key structure of the prototype was simulated using Ansys. The simulation results show that the maximum deformation of the optical detection subsystem is 19.5 μm and the fundamental frequency of the prototype is 96.9 Hz; thus, these results meet the requirements of optical performance stability and structural design. The experimental results show that the prototype meets the requirements of the inspection sensitivity better than 200 nm equivalent PSL spherical defects.

Defect regulation in bimetallic oxide cathodes for significantly improving the performance of flexible aqueous Zn-ion batteries

Bimetallic oxides have received extensive attention as cathode materials in aqueous zinc-ion batteries (AZIBs) due to their effective multi-electron storage mechanism. However, their capacity still falls short of the current demand. In this study, through reasonable structural design, we synthesized spherical glucose-derived carbon-regulated defect state Zn3V3O8 (ZVO) bimetallic oxide (CS@ZVO) cathode. Such a design avoids the layered accumulation of electrode materials, which prevents the structural collapse during the cycle process, leading to a significant improvement in the cycle stability of AZIBs. Additionally, the introduction of glucose-derived carbon can shift the valence state of vanadium in the ZVO bimetallic oxide cathode towards reduction, thereby significantly improving its Zn-ion storage capacity. The resulting CS@ZVO cathode manifests a high discharge capacity of 154 mAh g−1 at 1 A g−1 after 100 cycles and a stable lifespan of 2000 cycles at 5 A g−1. We also constructed a flexible and foldable AZIB using a hydrogel electrolyte, which operates with a specific capacity of up to 230 mAh g−1 at 0.5 A g−1 and can be bent and folded arbitrarily when working. This strategy for regulating the valence state of bimetallic oxides offers a promising path for the development of cathode materials in flexible AZIBs.