A hybrid triply periodic minimal surface (TPMS) method is proposed by the implicit mathematical equation to develop a new TPMS structure. Mechanical properties of the basal TPMS structures and the hybrid TPMS structure subjected to axial crushing load are experimentally and numerically investigated. Results show that the specific energy absorption of hybrid additive and subtractive TPMS structures is up to 97.2 % and 82.4 % enhancement compared to the basal Schwarz Primitive structure, and the Undulation of load-carrying capacity of hybrid additive and subtractive TPMS structure is 60.1 % and 33.3 % lower than that of the basal Schoen IWP structure. The effect of topological shape and material distribution on mechanical properties of hybrid TPMS structures are further numerically investigated, and structural factor and wall thickness have significant influence on crashworthiness. Furthermore, the crushing behavior of hybrid additive TPMS and square honeycomb subjected to in-plane and out-of-plane impact loads are investigated, and the hybrid additive TPMS structure shows significant crashworthiness advantage in in-plane crushing condition. Furthermore, the multi-objective optimization is carried out to obtain the optimal crushing performance of the hybrid additive TPMS structure. The hybrid design can provide a good guidance for the research on crashworthiness of the TPMS structures.

在不同刮板移动速度(28.23,63.55,91.11,125.25 mm·s-1)下对IN718镍基高温合金粉末进行铺粉,粉层厚度分别为0.3,0.5,0.7,0.9 mm.采用自制铺粉均匀性测试装置收集粉层不同位置相同体积的粉末并称取其质量,通过计算质量标准差研究了刮板移动速度和粉层厚度对铺粉均匀性的影响.结果表明:粉层的质量标准差随刮板移动速度或粉层厚度的增加而增大,说明铺粉均匀性变差,刮板移动速度对铺粉均匀性的影响更大;获得最佳铺粉均匀性的工艺参数组合为刮板移动速度28.23 mm·s-1、粉层厚度0.3 mm.

Sludge drying is a crucial part of the sludge treatmentand disposalprocesses, while thermal drying has been a popular choice. In thiswork, the sludge thermal drying process is numerically simulated usinga coupled computational fluid dynamics-discrete element method(CFD-DEM) approach. The CFD-DEM model describes the flow and heattransfer in the gas-solid system, while a mass transfer approachis embedded to describe the variation in the moisture content. First,a suitable numerical model to describe the sludge water removal processis developed based on physical experimental data. Quantitative comparisonswith experimental data validate the predictive capability of the numericalmodel for the continuum gas flow field, as well as the sludge dryingprocess. Then, five key parameters including velocity, temperatureand humidity of the heat source, particle size, and sludge pile thicknessare discussed to obtain the basic laws affecting the sludge dryingprocess. The pilot dryer is subsequently simulated, and a simplifiedsolution for industrial-scale simulation is proposed. All of the abovework is supported by abundant experimental data to demonstrate thecapability of the CFD-DEM model. Finally, based on more CFD-DEM simulation,the optimal operating conditions are determined with flexibility recommendationsfor dryer design summarized. This work contributes to an in-depthunderstanding of sludge drying characteristics and also has importantpractical and theoretical engineering value.

Mechanical ball milling is used to produce multi-materials for selective laser melting (SLM). However, since different powders have different particle size distributions and densities there is particle segregation in the powder bed, which affects the mechanical properties of the printed part. Core–shell composite powder materials are created and used in the SLM process to solve this issue. Core–shell composite powder materials selective laser melting (CS-SLM) has advanced recently, expanding the range of additive manufacturing applications. Heat storage effects and heat transfer hysteresis in the SLM process are made by the different thermophysical characteristics of the core and the shell material. Meanwhile, the presence of melt flow and migration of unmelted particles in the interaction between unmelted particles and melt complicates the CS-SLM molding process. It is still challenging to investigate the physical mechanisms of CS-SLM through direct experimental observation of the process. In this study, a mesoscopic melt-pool dynamics model for simulating the single-track CS-SLM process is developed. The melting characteristics of nickel-coated tungsten carbide composite powder (WC@Ni) were investigated. It is shown that the powder with a smaller particle size is more likely to form a melt pool, which increases the temperature in the area around it. The impact of process parameters on the size of the melt pool and the distribution of the reinforced particles in the melt pool was investigated. The size of the melt pool is significantly affected more by changes in laser power than by changes in scanning speed. The appropriate control of the laser power or scanning speed can prevent enhanced particle aggregation. This model is capable of simulating CS-SLM with any number of layers and enables a better understanding of the CS-SLM process.

Inspired by the hollow-tube architecture and chambers found in bamboo, we propose a design concept for mechanical metamaterials based on the hybridization of plate lattice and hollow-truss lattice (HPHL) to enhance the mechanical performance of lattice metamaterials in this paper. The design feasibility of HPHL is demonstrated through sample manufacturing using the laser powder bed fusion technique. Quasi-static compressive tests are conducted to investigate the crushing behavior of HPHL. Compared with the typical plate lattice, the HPHL exhibits a more effective crushing region, a higher mean crushing stress and a larger specific energy absorption (SEA). The SEA of HPHL is improved by 53.18% compared with that of the pure plate lattice. Based on the verified numerical simulations, the crushing mechanisms of HPHL are investigated through examining the deformation process and the stress-strain curve. It is found that the incorporation of hollow-truss lattices effectively delays the densification of the plate lattice and provides a more homogeneous and efficient stress distribution in HPHL, leading to higher crushing stress and higher energy absorption efficiency. Moreover, the effects of various design parameters, such as the hollow-truss diameter dT, hollow-truss thickness tT and plate thickness tP, on the mechanical behavior of HPHL are further explored to identify the optimal HPHL and realize the mechanical customization. The bionic hybrid design strategy proposed in this paper paves a new promising route to improve the mechanical performance of lattice metamaterials.

To support the high protection requirements of advanced equipment, honeycomb materials are encountering more demanding challenges on their flexible design and mechanical properties. Therefore, a novel hierarchical topology, namely vertex-derived strategy, is proposed to pursue a superior crushing resistance for honeycomb materials. Based on this strategy, the honeycombs with vertex-derivative characteristic are developed to investigate their energy absorption mechanism and mechanical property under the axial impact. An efficient energy-absorbing progressive folding mechanism is experimentally triggered due to the refinement of the material distribution by sub-cells. Further, a numerical study is conducted to uncover the effect of the derivative index and the thickness-length ratio of the sub-cell on the crushing resistance of honeycomb. The result shows that the derivative index and the thickness-length ratio can improve the energy dissipation capacity and crushing force efficiency of vertex-derivative honeycomb (VDH). A numerical comparison on the material utilization indicates that the vertex-derivative hexagonal topology is a superior geometry for energy dissipation than the vertex-derivative quadrilateral topology under the same geometric parameters. In addition, a theoretical solution derived by the simplified super folding element (SSFE) theory reveals the quantitative relationship of the derivative index and the sub-cell geometry parameters on the energy absorption. The present study looks forward to proposing effective suggestions to enhance the crushing resistance of the hierarchical honeycomb by the vertex-derived strategy.

Construction machinery, which is widely used in infrastructure construction, is growing rapidly all over the word. However, the complex working conditions of construction machinery lead to serious wear, particularly the wear of the bucket teeth on construction machinery. To control the wear procedure, it is essential to understand the wear mechanism and identify the wear form under variable working conditions. The modeling methods of bucket tooth wear with different wear mechanisms were reviewed. The modeling methods were divided into the analytical method and the numerical simulation method. The numerical simulation method included the discrete element method, finite element method, SPH method, and so on, which were used to simulate the bucket digging process and analyze the interaction between the material and bucket teeth during the working process. This enabled a force analysis of the bucket digging process and the identification of the location of maximum wear. By establishing a wear model, it is possible to better understand and address the wear problem in construction machinery. This article aims to summarize research methods concerning the wear of wear parts in construction machinery. It provides a theoretical foundation for future investigations in this area and aims to address challenges such as lengthy wear life testing, numerous interfering factors, and the difficulty of data collection pertaining to wear parts.

热辐射是颗粒间热量传递的基本方式之一,视角系数计算的效率和精度是颗粒热辐射数值模拟中的主要难点.本文采用斐波那契数列对非等径颗粒表面进行离散,在此基础上计算视角系数,并且对z方向的积分使用非均匀变换以提高计算精度.结果表明,在具有不同球体数的粉末床中,用该方法计算的视角系数的相对误差为5％,与传统的蒙特卡洛方法相比,计算效率提升约30％.

In the process of concrete pumping, the friction of particles on the pipe wall often leads to severe problems such as excessive pumping pressure and even pipeline rupture. In this study, a single-objective optimization of pump truck boom conveying pipe was performed to minimize pumping pressure loss by CFD-DEM and interior-point algorithm techniques. Firstly, taking the pipeline pressure loss as the index and considering the characteristics of boundary lubrication layer, the simulation of straight pipe concrete pumping is verified by the CFD-DEM coupling method. Secondly, the functional relationships between pumping pressure loss and inclination angles of different pipes were determined. Finally, the optimal spatial attitude of pipe was compared with a common used initial layout model of boom conveying pipe. The results indicated that the optimized pumping pressure loss was reduced by 2.6%, the particle distribution in the pipeline is more concentrated, and the particle movement speed is increased.

As a common defect in ceramics, the flaw-like defect will affect the fracture damage and mechanical properties of ceramics. For the current experimental equipment, it is difficult to accurately prefabricate defects in ceramics and observe the internal crack propagation when the specimen is damaged under loading, however, numerical simulation can solve this problem well. In order to explore the effect of various parameters of a single flaw on the crack evolution and bending strength of SiC ceramics under three-point bending, a SiC ceramic discrete element model with a prefabricated single-flaw defect is established and calibrated in this study, and the effects of the flaw length, inclination angle, and position on the bending strength of SiC ceramic specimens are investigated. The results show that the SiC ceramic specimen has the highest sensitivity to the existence of single-flaw defect when the flaw height ratio is 0 and inclination angle is 90°. As the height ratio of the single-flaw defect increases, the weakening effect of the flaw on the bending strength is reduced. Moreover, the influence of the flaw inclination angle on the bending strength of the specimen is weaker than that of the height ratio, but a change in the flaw inclination angle may induce the generation of secondary cracks in the specimen. In addition, the effect of the horizontal offset of the flaw on the bending strength is not obvious, but secondary cracks may also be generated in the specimen.

The roller-spreading and blade-spreading are main powder spreading methods in powder-bed addi-tive manufacturing.The discrete element method was introduced to simulate nylon powder spreading by both roller and blade spreaders.The two spreading processes were compared from several aspects including particle flow behavior,particle contact forces,forces exerted on spreaders,particle segrega-tion and powder layer density.It is found that powder spreading methods mainly affect the movement trajectory of particles,particle contact forces and forces exerted on spreaders.Complicated dispersion and circulation movement of particles occur inside the powder pile by roller-spreading,while particles have relatively weak dispersion by the blade-spreading.The normal force applied to the roller introduces a compacting effect on the powder pile and creates strong force chains that distribute uniformly in the powder pile.Therefore,the powder bed with higher density can be obtained by roller-spreading in thicker powder layer due to the compacting effect.The blade spreader sustains tangential force mainly,so the blade-spreading process limits its application to thicker powder layer.As the powder layer thickness increases,the roller-spreading is more sensitive to segregation index than that of the blade-spreading.The comprehensive comparison of two spreading processes provides criteria for selecting spreading methods.

The powder spreading process is one of the key processes in the powder-bed-based additive manufacturing (AM) technology. The roller-spreading parameters include the powder spreading layer thickness H, roller’s diameter D, roller’s rotational speed ω and translational velocity V, which have a major impact on the powder spreadability in AM processes. In this paper, the nylon powder was taken as the research object, and the discrete element method (DEM) was deployed to simulate the nylon powder spreading process by a roller. The three powder spreadability indicators including the deposition fraction, percent coverage and deposition rate were established. The central composite design (CCD) model was used to generate 30 groups of simulation cases. The regression models of three powder spreadability indicators were fitted by the response surface method (RSM). The analysis of variance was used to prove the accuracy and predicting effectiveness of regression models. In addition, the effect of process parameters on powder spreadability indicators was analyzed in detail. The results showed that the powder spreading layer thickness H was a leading influencing factor. The roller’s translational velocity V was a less important influencing factor. The roller’s diameter D and rotational speed ω had a slight influence on powder spreadability indicators. Both the H and D with V were determined as the main interactive factors on powder spreadability indicators. The three powder spreadability indicators were taken as the optimization goal, and the multi-objective optimization of roller-spreading parameters was carried out by the expectation method. The predicted optimal combination of powder spreading parameters and powder spreadability indicators were obtained. Moreover, the optimal results were verified through the experiments. The results showed that the predicted results of powder spreadability indicators were in good agreement with experimental results. The research results in this paper can provide guidance for the optimization of roller-spreading parameters in AM.

The Discrete Element Method (DEM) is used to simulate the mixing and discharging processes of binary particles in a laboratory truck mixer. The contact parameters in the DEM model are calibrated by experiments and then used to simulate the particle mixing and discharging processes in truck mixer under seven groups of operation parameters. Numerical results show good agreements with the experimental data in terms of the big particle content (BPC) of discharged samples and the discharging time (T-d). It is found that the discharging speed of 8 rev/min produces the largest discharging rate (D-r) whereas the discharging homogeneity is the lowest. The highest discharging homogeneity is observed when the discharging speed is 5 rev/min. The effects of filling rate and mixing speed on the discharging rate are insignificant. The mixing homogeneity decreases with the increase of mixing time, mixing speed and filling rate, which result in the discharging homogeneity also decreasing with the increase of mixing speed and filling rate.

目的 提高激光选区熔化成形(Selective Laser Melting,SLM)Ti6Al4V合金的表面亲水性.方法 通过调控248 nm KrF准分子激光功率密度和脉冲数,对SLM成形Ti6Al4V合金进行表面改性.利用扫描电子显微镜和激光扫描共聚焦显微镜对激光辐照前后的表面形貌和粗糙度进行表征,利用EDS能谱分析仪和X射线衍射仪对激光辐照后试样的表面成分进行分析,利用接触角测量仪来表征试样表面的浸润性.结果 在50个激光脉冲数下,通过控制准分子激光功率密度为0～531 mJ/cm2,使SLM成形Ti6Al4V表面接触角由(116±4)°减小至(7.5±0.4)°.通过表面形貌观察,激光辐照后的Ti6Al4V表面变得更加平滑,表面粗化现象得到改善.激光辐照后的Ti6Al4V表面粗糙度由(40.3±3.7)μm减小至(8.3±1.7)μm,试样表面与氧气发生反应,导致氧元素质量分数增加至33.54％,且有TiO产生.通过Cassie-Baxter模型和试样三维表面形貌分析,激光辐照后的Ti6Al4V表面更加光滑,使固-液-气复合界面上固体所占的百分比增加,导致试样变得更加亲水.通过时效性测试,发现试样的表面接触角在48 h内具有稳定性.结论 通过改变准分子激光的功率密度,可以快速高效地降低SLM成形Ti6Al4V合金的表面粗糙度,改善并调控Ti6Al4V的表面亲水性.

Purpose The purpose of this paper is to extend the previous study [Computer Methods in Applied Mechanics and Engineering 340: 70-89, 2018] on the development of a novel packing characterising system based on principal component analysis (PCA) to quantitatively reveal some fundamental features of spherical particle packings in three-dimensional. Design/methodology/approach Gaussian quadrature is adopted to obtain the volume matrix representation of a particle packing. Then, the digitalised image of the packing is obtained by converting cross-sectional images along one direction to column vectors of the packing image. Both a principal variance (PV) function and a dissimilarity coefficient (DC) are proposed to characterise differences between different packings (or images). Findings Differences between two packings with different packing features can be revealed by the PVs and DC. Furthermore, the values of PV and DC can indicate different levels of effects on packing caused by configuration randomness, particle distribution, packing density and particle size distribution. The uniformity and isotropy of a packing can also be investigated by this PCA based approach. Originality/value Develop an alternative novel approach to quantitatively characterise sphere packings, particularly their differences.

To investigate the relationship between micro-defects in ceramic materials and macro mechanical properties and behaviours, a computational model of SiC ceramics with randomly oriented elliptical pores was established using the discrete element method (DEM). The effects of pore defect content and its aspect ratio on the failure mode, stress-strain curve and mechanical properties of specimen were investigated under uniaxial compression. The effective Young's modulus which was obtained from DEM simulations was compared with the predictions of Mori-Tanaka scheme (MTS) and Self-Consistent scheme (SCS) at various pore defect densities. The results showed that the compressive strength and crack initiation stress decrease nonlinearly as the pore defect content increases. Furthermore, the smaller the aspect ratio of the elliptical pore defects was, the more obvious the weakening trend was. As the pore defect content increases, the failure mode of the specimen changed from brittle fracture to tensile-shear mixing and then to axial splitting. The stress-strain curves showed a certain “softening” period during the loading process. The effective Young's modulus obtained from the DEM simulations coincides with the approximations of MTS and SCS at low pore densities. However, when the pore defect density became larger, the DEM simulation results were slightly lower than the theoretical results of the Mori-Tanaka scheme, which only considers the weak interaction between defects.

Discrete element (DE) simulation of a ball mill with a large number of particles is challenging when each particle is considered. Similarity principle could be adopted to reduce the number of particles in a simulation whilst still maintaining the accurate flow behaviour of particles. This paper presents a scaling relationship between particle gravitational acceleration, mill diameter and mill rotational speed. A series of scaled simulations of particle motion with different mill diameters are carried out. Consistent motion of a single particle and multiple particles in ball mills with different diameters and rotational speeds verifies the proposed relationship, which could be an effective approach to reduce the size of simulations for ball mills.

A metal matrix is an indispensable component of metal-bonded diamond tools. The composition design of a metal matrix involves a number of experiments, making costly in terms of time, labor, and expense. The discrete element method (DEM) is a potential way to relieve these costs. The aim of this work is to demonstrate a methodology for establishing and calibrating metal matrix’s DEM model. A Co-based metal matrix with WC and Ni additives (CoX–WC–Ni) was used, in which the Co-based metal was Co–Cu–Sn metal (CoX). The skeletal substances in the metal matrix were treated as particles in the model, and the bonding substances were represented by the parallel bond between particles. To describe the elasticity of the metal matrix, a contact bond was also loaded between particles. A step-by-step calibration procedure with experimental tests of three-point bending and compression was proposed to calibrate all microcosmic parameters involved during the establishment of DEM models: first for the CoX matrix, then for the CoX–WC matrix and CoX–Ni matrix, and finally for the CoX–WC–Ni matrix. The CoX–WC–Ni DEM model was validated by the transverse rupture strength (TRS) of two new compositions and the results indicated that the model exhibited a satisfactory prediction ability with an error rate of less than 10%.

The percent TRS reduction, DTRS, which is the percent reduction of the transverse rupture strength of metal matrix diamond segments with or without diamonds, is a key metric for evaluating the bonding condition of diamonds in a matrix. In this work, we build, calibrate, and verify a discrete-element simulation of a metal matrix diamond segment to obtain DTRS for diamond segments with various diamond-grain sizes, concentrations, and distributions. The results indicate that DTRS increases with increasing diamond-grain concentration and decreases with increasing diamond-grain size. Both factors can be explained by the total diamond contact length, the increase of which causes the increase in DTRS. The distribution of diamond grains in segments also strongly influences the increase of DTRS. The use of DTRS as a metric to assess the bonding condition of diamonds in matrixes is not valid unless the diamond-grain size, concentration, and distribution and total diamond contact length are the same for all diamond segments under consideration.

A model of a SiC ceramic containing a single pre-existing flaw was established based on the discrete element method. The effects of the flaw inclination angles, which ranged from 0° to 75°, on the mechanical properties of the specimen under uniaxial compression were studied. The evolution of the force-chain field, displacement field and stress field around the pre-existing flaw in the process from the load to failure was also analysed. The results showed that the flaw inclination angle affected the mechanical properties of the specimen as well as the initiation and propagation of the first crack. Based on the investigation of the force chain field, it was found that the distribution curve of the normal force carried by the parallel bond in the specimen with the corresponding angles under compression is similar to the “peanut” rose diagram, while the shear force distribution curve is similar to the "butterfly wings" rose diagram. In addition, in the analysis of the displacement field and the stress field, the displacement field around the flaw can be divided into four types in the process from specimen loading to its failure. Meanwhile, it was found that initiation of the first crack was affected by tensile stress. With the propagation of the first crack, the tensile stress concentration region at the flaw tip moved and dissipated correspondingly.