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Goldak’s ellipsoidal heat source model is widely accepted in laser powder bed fusion (LPBF) process simulations. The model is dependent on three inherent flux distribution parameters and absorptivity which need to be calibrated before application. However, the uniqueness of the calibrated heat source parameters has not been established, which is crucial to accurately represent the energy density distribution in numerical studies. This paper proposed a novel parametric optimization method that can uniquely calibrate these four heat source parameters with high accuracy. Both the simulated cross-sectional (CS) profile and the mid-front (MF) profile of the melt pool were used during calibration. The differences between the experimental and simulated profiles formed the objective function of the parametric optimization problem, which was calculated by using the least power norm method on the deviations between melt pool boundary curves. A genetic algorithm (GA) with elitism selection approach was employed to optimize the heat source parameters by minimizing this single objective function value. To theoretically verify the applicability of the method, two theoretical case studies involving two distinct sets of heat source parameters were carried out. A unique solution of Goldak’s heat source parameters with less than 2 % error can be achieved within 50 generations of the optimization process. A single-track experiment was conducted to demonstrate the application of the proposed method in a real scenario. The CS and MF melt pool profiles were extracted and the calibration procedure was applied. The profiles from simulation with the calibrated heat source parameters exhibited a good agreement with experimental results. The calibrated parameters were used to simulate multi-track printing on a single layer for validation of the proposed method. The CS melt pool profiles obtained from the multi-track simulation and multi-track experiment agreed well, which validated the accuracy of the proposed calibration method and its utility in obtaining the key Goldak heat source parameters for LPBF simulation.

This paper investigated the role of longitudinal reverse bending stress on the initiation of reverse transverse defects. The longitudinal reverse bending stress occurs due to the reverse bending of the rail between two-wheel passage leading to the generation of tensile bending stress at the railhead and the lower head areas. The longitudinal bending stress was investigated as part of a parametric study on the rail cant angle, rail stiffness, lateral-to-vertical load ratio, and rail profile. A finite element model was created by using ABAQUS to analyze the extent of reverse bending in rails with respect to the chosen set of parameters. Under different lateral-to-vertical load ratios of 0, 0.3, 0.5, and 0.7, the maximum stress at the rail lower gauge corner was found to vary between 14.57MPa and 15.47MPa under the reverse bending condition. Similarly, low values of tensile stress under the reverse bending scenario were observed with changes in the rail cant angle and axle spacing with respect to different coal and iron ore wagons. The results revealed that the magnitude of the bending stress under different conditions of reverse bending was not significant enough to initiate a crack at the lower gauge corner.

Multistage multicluster hydraulic fracturing is currently the most effective method to stimulate low‐permeability hydrocarbon reservoirs. The desired result of this stimulation technique is highly uniform growth of multiple closely spaced hydraulic fractures. In order to deepen our understanding on the competitive growth of these fractures, we have investigated the growth behaviors of multiple hydraulic fractures under different conditions. A fully coupled model with a 3‐D influence coefficient is presented for fracture growth in a rock layer, based on a combination of boundary element method and finite volume method. It is validated against classic solutions and lab experiments. The parametric analyses of dimensionless arguments to quantify the uniformity of fracture growth are performed, with varying fracture geometries and geotechnical conditions including preexisting natural fractures. The results demonstrate that fracture competition is controlled by the dimensionless toughness, the initial fracture geometric settings, and the ratio of fracture spacing to height. A higher fluid viscosity, a smaller initial length offset, and a larger spacing benefit simultaneous fracture growth for conventional bi‐wing hydraulic fractures. By contrast, single wing ones exhibit a remarkably better performance in long‐time simultaneous fracture growth at a small separation, especially at low dimensionless toughness. Under certain conditions, the closely spaced single‐wing fractures are prone to grow simultaneously and uniformly in less energy consumption. The simultaneous growth is found to be insensitive to the interference from small‐scale natural fractures. The uniform single‐wing fracture growth from the perspective of geomechanics will provide a new insight for both reservoir stimulation and formation of vein and dike swarms. Parametric analysis of simultaneous hydraulic fracture growth scenarios is performedSingle‐wing fracture geometry promotes the uniformity of multiple fracture growthThe presence of small‐scale natural fractures cannot arrest single‐wing fracture growth

Novel W-doped MoS2electrocatalysts have been successfully fabricated through a facile one-pot solvothermal method and employed for the hydrogen peroxide reduction reaction (HPRR) in emerging alkaline H2O2-based fuel cells. The wide composition stoichiometry of W-doped MoS2is obtained in molar fractions of 30% and 15%. It has been found that 30% W-doped MoS2presents superior catalytic activity with a high peak current density of 2.83 mA cm−2at −0.86 V vs.Hg/HgO, owing to the heteroatom doping and the defect sites that emerged in the nanostructure. Furthermore, the influence of alkali concentration, H2O2concentration and temperature on the HPRR is systematically investigated. The mechanism of HPRR is illustrated using a rotating disk electrode as a direct two-electron electroreduction pathway. Thereby, a new insight into heteroatom doping in transition metal dichalcogenides for HPRR applications is provided.

Laser cladding (LC) is a powder deposition technique which is used to deposit layers of clad material on a substrate to improve its surface properties. It has widespread application in structural repair, in particular, the repair of dies and molds used in the automobile industry. These molds and dies are subjected to cyclic thermo-mechanical loading and therefore undergo localized damage and wear. The final clad quality and integrity is influenced by various physical phenomena, namely, melt pool morphology, microstructure evolution and residual stress generation. Consequently, it is imperative to understand the physical phenomena influencing the process variables so as to develop a knowledge base for the usage of this process in repair based applications. The current study is focused on the development of a 3D finite element thermal model for powder injection laser cladding of CPM 9 V powder on H13 tool steel used extensively in repair of dies and molds. The thermal model incorporates deposition of clad elements via element birth technique along with uniform moving heat source and Gaussian powder distribution. The thermal model also takes into account, the attenuation of laser power due to laser-powder interaction and heat partition between the substrate and the powder. The temperature field predicted from thermal analysis was used to calculate the initial melt pool dimensions, taking into account the vaporization of clad elements. Furthermore, the thermal model predicts the final clad layer dimensions (clad height and width) by considering molten metal spreading via Tanner's Law. The clad characteristics (clad geometry, clad dilution and heat affected zone) are predicted with reasonable accuracy with prediction errors lying within ∼14% for most of the cases. The thermal model indicates a relatively strong dependence of interaction time on the heat penetration than that of laser intensity. The thermal model further predicts that the clad width increases with an increase in the laser power till it reaches a critical power and then it saturates. This critical laser power is capable of melting the entire injected powder material; hence, the width of the clad does not increase with power beyond this point.

A novel electrochemical chromium oxidation strategy by in situ electro-generated reactive oxygen species was developed in this study. Activated multi-walled carbon nanotubes modified electrode was employed to produce H2O2 via two-electron-pathway oxygen reduction reaction with onset potential of [?]0.06 V (vs Hg/HgO) and peak current density of 1.54 mA*cm[?]2. The H2O2 formation was confirmed by selective luminescent probes with UV-vis spectroscopy, and the Cr(III) can be efficiently oxidized by the as formed H2O2. Interestingly, *OH can also be obtained by the Cr(III)-induced Fenton-like reaction, and offer further oxidative stress to enhance the Cr(III) oxidation. Consequently, a significantly enhanced Cr(III) conversion of 98.5 +- 1.0% was achieved in this indirect oxidation as compared to 29.8 +- 1.3% in the corresponding direct electrochemical oxidation. Besides, indirect pathway exhibited a considerable improvement for the Cr(III) oligomers oxidation (from 17.6 +- 1.0% to 93.5 +- 1.5%), indicating a potential solution to overcome the polymerization bottleneck of Cr(III) oxidation. Moreover, the electrolysis current well maintained the original value after stability test, suggesting favorable alkaline tolerance. The proposed method provided a cleaner approach for Cr(III) oxidative mobilization in alkaline media, and new insight on the redox cycle of electro-generated reactive oxygen species.

H13 tool steel with excellent hot working properties is commonly used for manufacturing dies. However, the damage of die surface due to cyclic thermo-mechanical loading is detrimental to the service life. In order to enhance the die life, it has been observed that cladding based repair is superior to welding or thermal spraying repair techniques. In this paper, experimental study of laser cladding of H13 has been carried out. CPM 9V steel powder has been deposited on H13 tool steel plate for repairing the die surface damage using a CW CO2laser in conjunction with powder injection system. The effect of laser parameters on clad geometry and clad quality has been investigated. The microstructure of laser cladded samples has been characterized using optical microscope (OM) and scanning electron microscope (SEM). The phases and the residual stresses present in the clad have been determined via X-ray diffraction. The micro-hardness profiles obtained in the clad–substrate system and the hardness change due to cyclic thermal loading have also been characterized. Optical micrographs of the clad microstructure shows existence of vanadium carbide particles embedded in martensite and retained austenite. The hard vanadium carbide particles increase the clad hardness to an average of four times greater than the substrate hardness. It has been observed that compressive residual stresses are generated in clad which is desirable for repair applications as it will impede the crack propagation resulting in enhanced die life.

Abstract: Instrumented indentation test has been extensively applied to study the mechanical properties such as elastic modulus of different materials. The Oliver–Pharr method to measure the elastic modulus from an indentation test was originally developed for single phase materials. During a spherical indentation test on shape memory alloys (SMAs), both austenite and martensite phases exist and evolve in the specimen due to stress-induced phase transformation. The question, “What is the measured indentation modulus by using the Oliver–Pharr method from a spherical indentation test on SMAs?” is answered in this paper. The finite element method, combined with dimensional analysis, was applied to simulate a series of spherical indentation tests on SMAs. Our numerical results indicate that the measured indentation modulus strongly depends on the elastic moduli of the two phases, the indentation depth, the forward transformation stress, the transformation hardening coefficient and the maximum transformation strain. Furthermore, a method based on theoretical analysis and numerical simulation was established to determine the elastic moduli of austenite and martensite by using the spherical indentation test and the Oliver–Pharr method. Our numerical experiments confirmed that the proposed method can be applied in practice with satisfactory accuracy. The research approach and findings can also be applied to the indentation of other types of phase transformable materials. [Copyright &y& Elsevier]

The interface area in laser cladding, microstructures and mechanical properties of which determine the structural integrity of the laser-deposited coating, consists of two distinctive zones: the dilution and heat-affected zones (HAZs). The dilution region is the region where melted substrate forms a mixture with coating material and possesses microstructure similar to the coating. The HAZ is the region that lies in the substrate but possesses different microstructures compared with original substrate due to heat transfer from melt pool during processing. There are limited data available on mechanical properties of the dilution and HAZ and, in this study, the mechanical properties of the two regions have been evaluated using nanoindentation. This testing technique has the ability to resolve the mechanical properties at fine spacings, unlike microhardness testing and conventional mechanical testing. Stellite 6 powder was deposited onto a round bar AISI 4130 steel using a continuous wave Nd:YAG laser. The effect of laser cladding process on the microstructure, hardness, and elastic modulus of the coating, at the interface has been examined by nanoindentation testing. The elastic modulus of the substrate measured was found to be 198.76 ± 24.96 GPa, which is in a good agreement with the standard elastic modulus of AISI 4130 reported in literature. For the laser clad specimen, dilution area showed elastic modulus of 192.62 ± 7.67 GPa, slightly higher than the average elastic modulus of the HAZ, 189.94 ± 14.75. This is due to the dilution area containing a mixture mainly of Fe, Cr and Co. The increased level of Fe in this dilution area leads the tendency of this region to behave as substrate.

Abstract

The fatigue failure of a super-elastic NiTi alloy was observed by uniaxial stress-controlled cyclic tests. During the cyclic loading a hysteresis loop with a varied but stabilized size after certain cycles was obtained, which is similar to plastic shakedown. The material exhibits unique brittle fracture with a large transformation strain. The fatigue life of the material greatly depends on the applied peak nominal stress, the nominal stress amplitude and the mean nominal stress. A relation between the dissipation energy at the stabilized stage of cyclic loading and the number of cycles at failure was derived from the experimental results. Based on the obtained experimental results, a uniaxial fatigue failure model based on the energy approach was proposed to predict the fatigue life. It was shown that the proposed model provides good predictions to the uniaxial fatigue lives of super-elastic NiTi alloys with different types of cyclic stressing.

Bulk metallic glasses are a new type of advanced materials. They are characterized by their topologically disordered atomic structures. The lack of long-range translational symmetry in the atomic arrangement in bulk metallic glasses contributes to a range of unique and outstanding mechanical properties. For example, the yield strength of metallic glasses can be as large as twice that of corresponding crystalline alloys. However, the major issue to hinder metallic glasses application is the apparent brittleness. Unlike crystalline alloys, metallic glasses show abrupt failure with zero macroscopic plasticity. Various methods are being investigated in material engineering to improve the plasticity of bulk metallic glasses. The most recent progress is to develop shape memory bulk metallic glass composites, a combination of metallic glass and shape memory alloy. The large stress-induced transformation strain in shape memory alloy leads to the increase in the plasticity of the new composite material. The stress-strain behaviour of shape memory bulk metallic glass composites was investigated in this paper by using the finite element method. A unit cell model, which includes shape memory alloy phase and metallic glass phase, under uniaxial tension were numerically simulated. The effects of phase volume fraction, transformation stress and strain on the stress-strain behaviour of this new composite material were examined in this research.

Not Available

A computational approach is proposed to predict the sliding wear caused by a loaded spherical pin contacting a rotating disc, a condition typical of the so-called pin-on-disc test widely used in tribological studies. The proposed framework relies on the understanding that, when the pin contacts and slides on the disc, a predominantly plane strain region exists at the centre of the disc wear track. The wear rate in this plane strain region can therefore be determined from a two dimensional idealisation of the contact problem, reducing the need for computationally expensive three dimensional contact analyses. Periodic unit cell techniques are used in conjunction with a ratchetting-based failure criterion to predict the wear rate in the central plane strain region. The overall three dimensional wear rate of the disc is then determined by scaling the plane strain wear rate with a conversion factor related to the predicted shape of the wear track. The approach is used to predict pin-on-disc test data from an Al–Si coating using a tungsten carbide pin. The predicted results are found to be consistent with measured data. [Copyright &y& Elsevier]

In this work, the wear behaviour of coated components subjected to sliding contact conditions is investigated using a multiscale micromechanics approach. Periodic unit–cell–type continuum mechanics models are used to predict localized deformation patterns at the scale of the coating thickness (mesoscale) and the rate of material removal due to repeated sliding contact. To that purpose, realistic contact loads determined at the component level (macroscale) are applied at the mesoscopic level. The results indicate that the deformation of the coating is controlled by the cyclic accumulation of plastic deformation, or ratchetting, at the coating subsurface. Based on a ratchetting failure criterion, a wear equation is proposed and applied to investigate parametrically the influence of the principal material, loading and surface roughness parameters on the wear rate. The results reveal that the wear rate increases with contact pressure and depends on coating thickness and the roughness of the counterpart surface. It was also found that a reduction in the friction coefficient and an increase in the coating strain hardening behaviour can considerably improve the wear resistance of coated components.

The residual stress induced by rapid temperature cycling in a component during a selective laser melting (SLM) process has a significant detrimental effect on the component’s quality and may cause component failure during or after the printing process. Predicting and therefore actively controlling the residual stress are important research topics in SLM. Numerical simulation is an effective tool to predict the residual stress and distortion throughout the printing process. Goldak heat input model has been commonly used to model the heat source in an SLM process. However, it incurs an unpractically high computational cost when simulating building a part with dimensions in centimetres due to the huge scale gap between the component size and the laser spot size. In the present work, a characteristic time-based heat input (CTI) model has been developed and implemented in a finite-element thermal-mechanical model to significantly reduce the process simulation time while satisfactorily predicting the temperature and residual stress state of SLMed parts. This characteristic time-based heat input model speeds up the computation by applying the integrated energy along the scan path over a characteristic heating time, which is defined as the ratio of the axis of the ellipsoidal heat source in the laser scanning direction to the scanning speed. The characteristic heating time ensures that the peak temperature and subsequent heat transfer of each deposited track can be satisfactorily captured in just one numerical step, respectively. The present model was calibrated and validated by identified isothermal curves on the cross section of a printed Ti-6Al-4V track. Case studies demonstrate that the simulated temperature profile and stress field are in a good agreement with Goldak model. The maximum distortion of a 32 mm single-track sample predicted by CTI model is within 6% of that predicted by Goldak heat input model while CTI’s computational time is just 0.4% of Goldak’s.

To overcome the previous limitations imposed by standard laser cladding alloys, a novel iron-based laser cladding alloy, SS415, is developed and investigated. The wear and rolling contact fatigue resistance properties of the novel SS415 laser cladded on a premium railway hypereutectoid rail are experimentally investigated using a custom-made roller-on-disc test machine. The obtained results are compared with our previously published results of non-clad rail and the standard iron-based cladding alloys, namely, 410L and SS420, specimens under the same conditions. The accumulated wear rate and the pileup area were quantified with the aid of an optical profilometer. The obtained results were correlated to in-depth wear surface characterization using optical microscopic images. The experimental results showed that the novel SS415 exhibits favorable hardness variation, more ductile behavior, promising work hardenability, and a 95% reduction in the wear rate, in comparison to the non-cladded samples. Moreover, the SS415 cladding resulted in less surface damage and the lowest degree of third-body abrasive wear modes, after the completion of the rolling contact cycles. Based on this investigation, the novel SS415 is a promising iron-based laser cladding alloy that offers the best wear and fatigue performance compared to both 410L and SS420 cladded samples. Additionally, this novel alloy offers significantly improved mechanical and tribological properties compared to the two standard alloys.

A physician palpates a tissue to detect an embedded tumor nodule by sensing an increase in local tissue stiffness and nodule size. The Hertz contact model, however, is unable to predict the material or physical properties of a tumor nodule embedded in a healthy tissue of finite thickness. In this study, utilizing a hyperelastic material model, we propose a general methodology to analyze the extent to which the stiffness, size, and depth of a nodule embedded in a tissue affect its detectability. Using dimensional analysis, we generate simple power-law relations to predict physical and material properties of tumor nodules embedded in healthy tissue during indentation. Our results indicate that indenter radius and indentation depth are critical parameters in nodule detection and a thin indenter and large indentation depth increase detection sensitivity of an embedded tumor nodule. Our results also show that anisotropic material properties of either a tissue or an embedded nodule render the embedded tumor nodule undetectable using indentation. We define palpation sensitivity maps that can be used to predict material and physical properties of tumor nodules in healthy tissues. The analysis and results presented in this study might increase accuracy and precision in instrumented probe-based laparoscopic or robotic surgeries.

Carbon nanotubes (CNTs) were coated onto glass fibres (GFs) using a one-step flame synthesis method. The GF/epoxy resin matrix interfacial bonding strength was investigated by fibre bundle pull-out tests. Results showed that, compared to the as-received GF/epoxy, significant interfacial bonding strength enhancement of ~33% was obtained for CNT-coated GFs/epoxy. Moreover, the wettability of CNT-coated GF by epoxy was increased significantly evidenced by the reduction of CNT-coated GF/epoxy contact angles. The flame synthesis method possesses the potential of efficient high-performance composites fabrication.

The wheel-rail contact is an open system contact, which is subjected to various environmental conditions, such as temperature, humidity, water, and even leaves. All these environmental factors influence wheel-rail wear. Classical wheel-rail wear has been extensively studied under dry and clean conditions previously. However, with changes in environmental conditions, the wear rate and wear mechanism can change. This paper reviews recent contributions to wheel-rail wear with a special focus on the influence of environmental conditions. The main part includes the basics of wheel-rail wear, experimental methodology, wear and rolling contact fatigue (RCF), and some measures to counter these degradation mechanisms.

A new variant of the Element-Free Galerkin (EFG) method, that combines the diffraction method, to characterize the crack tip solution, and the Heaviside enrichment function for representing discontinuity due to a crack, has been used to model crack propagation through non-homogenous materials. In the case of interface crack propagation, the kink angle is predicted by applying the maximum tangential principal stress (MTPS) criterion in conjunction with consideration of the energy release rate (ERR). The MTPS criterion is applied to the crack tip stress field described by both the stress intensity factor (SIF) and the T-stress, which are extracted using the interaction integral method. The proposed EFG method has been developed and applied for 2D case studies involving a crack in an orthotropic material, crack along an interface and a crack terminating at a bi-material interface, under mechanical or thermal loading; this is done to demonstrate the advantages and efficiency of the proposed methodology. The computed SIFs, T-stress and the predicted interface crack kink angles are compared with existing results in the literature and are found to be in good agreement. An example of crack growth through a particle-reinforced composite materials, which may involve crack meandering around the particle, is reported. [ABSTRACT FROM AUTHOR]