Mechanical properties and deformation mechanisms of defect-free copper nanoparticles are investigated by combining experiments with atomistic simulations. The compressive strength of the particles ...increases with decreasing size and tends to saturate near the theoretical strength in the small-size limit. In this limit, the intrinsic size dependence of the strength is governed by the stochastic nature of dislocation nucleation near the particle surface. The particle deformation process evolves from the initial strain softening to strain hardening as the particle accumulates residual damage. The normalized strength-size relation for Cu is compared with those for Au, Ni, and Pt. The lack of universal behavior among the four FCC metals is discussed. Heavily deformed Cu nanoparticles develop polycrystalline structures and change the lattice orientation from 111 to 110. The experiments and simulations reveal the twinning mechanism of the lattice rotation leading to the new grain formation.
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Al alloy porous structures have potential application in industries such as light aerospace and some heat exchanger products. A particular advantage of selective laser melting (SLM) over conventional ...manufacturing processes is its ability to fabricate periodic lattice structures with controllable volume fractions. In this paper, 12 samples of AlSi10Mg periodic diamond lattice structures with interconnected high porosity and volume fractions from 4.5% to 22.5% were fabricated by selective laser melting. At the same time, the optimized radius at the node is introduced to reduce stress concentration. The as-built AlSi10Mg lattice was well formed without large appeared voids and cracks. A full-scale three-dimensional finite element (FE) model was established to evaluate the macroscopic deformation of the lattice structures and the microscopic stress and strain evolution in the solid struts. Local plastic stresses were found to generate near the nodes, thus forming plastic hinges, while most of the struts remain elastic. Evolution under compressive loads determines the mechanical properties of the diamond lattice structures and failure mechanism.
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•A series of diamond lattice structures with different rod diameters and optimized radius are designed and fabricated.•The introduction of the optimized radius makes the stress transition from the node to the middle of the pillar and weakens the stress concentration.•Heat treatment can’t effectively remove surface bonded particles and corrugation.•Rod diameters and optimized radius have important effects on the mechanical behavior and failure mechanism of lattice structures.
Ti–6Al–4V alloy with two kinds of open cellular structures of stochastic foam and reticulated mesh was fabricated by additive manufacturing (AM) using electron beam melting (EBM), and microstructure ...and mechanical properties of these samples with high porosity in the range of 62%∼92% were investigated. Optical observations found that the cell struts and ligaments consist of primary α′ martensite. These cellular structures have comparable compressive strength (4∼113MPa) and elastic modulus (0.2∼6.3GPa) to those of trabecular and cortical bone. The regular mesh structures exhibit higher specific strength than other reported metallic foams under the condition of identical specific stiffness. During the compression, these EBM samples have a brittle response and undergo catastrophic failure after forming crush band at their peak loading. These bands have identical angle of ∼45° with compression axis for the regular reticulated meshes and such failure phenomenon was explained by considering the cell structure. Relative strength and density follow a linear relation as described by the well-known Gibson–Ashby model but its exponential factor is ∼2.2, which is relative higher than the idea value of 1.5 derived from the model.
► Open cellular structures with porosities up to 92% were prepared by AM-EBM. ► Their microstructure and hardness were connected with these cell parameters. ► They exhibit a brittle response and fail by forming crush bands in compression. ► Crush bands have identical angle for the meshes but vary randomly for the foams. ► Relative strength and density follow linear relation with an exponential factor ∼2.2.
Sandwich structures with thick soft cores subjected to transverse loads are usually resulting in deformations with various modes, including bending, twisting and compressing. In addition, the ...compression of the core results in a significant influence of the structural response. All classical plate theories are inadequate in predicting the behavior sandwich structures with thick soft core due to the assumption of no thickness change during deformation. To overcome this issue, a hybrid first/third-order shear deformation hypothesis with consideration of compressive effect is proposed for sandwich structures with thick soft cores. The first-order shear deformation theory is applied to both the top and bottom layers, while a refined third-order shear deformation theory that releases the assumption of no thickness change is developed for the thick soft cores. An eight-node quadrilateral plate element with twelve degrees of freedom including compressible displacements is created for finite element modeling. Based on the proposed refined hybrid first/third-order theory, a finite element model is developed using the Hamilton’s principle. The numerical model is first validated by the computational results of sandwich structures using commercial software. Then, the influences of various parameters on static displacements, vibrations and compressive deformations are systematically investigated, including e.g. boundary conditions, thickness of the core and face layers, elastic modulus of the core layer and the plate size. The present finite element modeling approach provides a powerful tool to accurately investigate the mechanical response of sandwich structures.
In this study, the effects of aspect ratio (AR) on the magneto-rheological elastomers (MREs) in tensile-compressive loading have been investigated considering magnetic field intensity, deformation ...rate, and amplitude in a moderately wide range. For this purpose, first, several MREs with different ARs (0.268-0.539) have been constructed. Then, using the Instron device, a test setup has been designed to identify the behavior of MREs in different magnetic fields in tensile-compressive mode. To provide the magnetic field, six permanent magnets made of neodymium 42 have been used. The MREs have been harmonically actuated with different deformation amplitudes (8%-14%) and deformation rates (1-8 Hz). The results illustrated a moderate impact of the AR on the hysteretic response of the MREs and also on its storage modulus. Results showed that the compressive and tensile storage moduli of MREs can increase up to 39.87%, and 52.70%, respectively, by decreasing the value of AR from 0.539 to 0.268. Furthermore, the MR effects of 29.95% and 21.66% for the compressive and tensile storage moduli, respectively, have been achieved. Finally, considering field intensity and mechanical deformations, an empirical mathematical model has been suggested to predict the compressive-tensile hysteretic response of MREs in the presence of AR and validated by the experimental results.
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Ti–6Al–4V reticulated meshes with different elements (cubic, G7 and rhombic dodecahedron) in Materialise software were fabricated by additive manufacturing using the electron beam ...melting (EBM) method, and the effects of cell shape on the mechanical properties of these samples were studied. The results showed that these cellular structures with porosities of 88–58% had compressive strength and elastic modulus in the range 10–300MPa and 0.5–15GPa, respectively. The compressive strength and deformation behavior of these meshes were determined by the coupling of the buckling and bending deformation of struts. Meshes that were dominated by buckling deformation showed relatively high collapse strength and were prone to exhibit brittle characteristics in their stress–strain curves. For meshes dominated by bending deformation, the elastic deformation corresponded well to the Gibson–Ashby model. By enhancing the effect of bending deformation, the stress–strain curve characteristics can change from brittle to ductile (the smooth plateau area). Therefore, Ti–6Al–4V cellular solids with high strength, low modulus and desirable deformation behavior could be fabricated through the cell shape design using the EBM technique.
Developments in selective laser melting (SLM) have enabled the fabrication of periodic cellular lattice structures characterized by suitable properties matching the bone tissue well and by fluid ...permeability from interconnected structures. These multifunctional performances are significantly affected by cell topology and constitutive properties of applied materials. In this respect, a diamond unit cell was designed in particular volume fractions corresponding to the host bone tissue and optimized with a smooth surface at nodes leading to fewer stress concentrations. There were 33 porous titanium samples with different volume fractions, from 1.28 to 18.6%, manufactured using SLM. All of them were performed under compressive load to determine the deformation and failure mechanisms, accompanied by an in-situ approach using digital image correlation (DIC) to reveal stress-strain evolution. The results showed that lattice structures manufactured by SLM exhibited comparable properties to those of trabecular bone, avoiding the effects of stress-shielding and increasing longevity of implants. The curvature of optimized surface can play a role in regulating the relationship between density and mechanical properties. Owing to the release of stress concentration from optimized surface, the failure mechanism of porous titanium has been changed from the pattern of bottom-up collapse by layer (or cell row) to that of the diagonal (45°) shear band, resulting in the significant enhancement of the structural strength.
•Investigating the specimen size effect on the mechanical response of PVC foams.•The influence of the specimen size was negligible under quasi-static loading rates.•The dynamic specimen size effect ...was investigated experimentally for the first time.•In contrast with the quasi-static regime, the dynamic specimen size is significant.•The influence of the specimen shape (rectangular or cylindrical) was negligible.
Split Hopkinson Pressure Bar (SHPB) apparatuses were extensively utilized in the dynamic testing of polymeric foams in previous studies; however, there are many technical limitations on the specimen size when utilizing SHPBs. Consequently, most research on the influence of specimen size on the mechanical response was limited to the quasi-static loading regime. The specimen size effect was reported to be negligible in the quasi-static regime. This observation was traditionally extrapolated to the dynamic regime; however, the hypothesis has not been thoroughly verified by experimental testing. The present study represents the most thorough investigation (to date) on the influence of specimen shape, cross-sectional area and thickness on the dynamic mechanical response of Polyvinyl Chloride (PVC) foams. A drop tower testing machine equipped with a massive 45.45 kg dropping entity and a novel sacrificial energy dissipation system was employed for the dynamic testing. The findings from this work revealed a significant sensitivity to the specimen profile on the dynamic mechanical response of PVC foams in contrast to the negligible effects in the quasi-static regime. Investigating the effect of specimen thickness on the mechanical response further demonstrated that the engineering strain rate is not a suitable index for reporting PVC foams’ rate sensitivity. Instead, the impact velocity is more appropriate since specimens with different thicknesses exhibited a similar compressive stress/strain response when tested at the same impact velocities while experiencing different engineering strain rates. The influence of specimen shape (rectangular versus cylindrical) was negligible in both the quasi-static and dynamic regimes.
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Broken gangue from the collapsing roof of a longwall coal panel can be used as the backfilling material for goaf backfilling. The particle size gradation has important influence on the compressive ...deformation of the gangue backfilling material and particle breakage. The compressive deformation of the gangue backfilling materials at 4 different particle size grades is simulated by PFC3D in this paper. The law of the compressive deformation of the gangue backfilling material, the particle cluster distribution and the variation of the gangue block shape are analyzed. The microscopic mechanism of the compressive deformation of the gangue backfilling material is investigated based on the force-chain distribution. The results indicate that when the gangue backfilling material has a small particle size, the specimen has low porosity and a few amounts of the broken particles can fill the pores of the material. When the particles have relatively large sizes, the specimen has relatively high porosity and the broken particles are unable to completely fill the pores. The loading conditions of the particles can only be changed by the frame structure formed by the large particles. If the proportion of the particle size grade is reasonable, a frame structure forms due to the large particles and the small particles with various particle sizes can fill the pores of the backfilling material. This enhances the deformability of the gangue and decreases the compressive deformation of the backfilling material. After backfilled into the goaf, the backfilling material bears the overburden pressure, which means the process of roof convergence is the process of compressive deformation of the backfill body. That is to say, the stiffer the backfilling material, the better the control effect. The importance of the reasonable proportion of the particle size grade is explained from the aspect of microscopic breakage of the particles, which provides a scientific basis for backfill mining.
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•Influence of particle size on compressive deformation of gangue was studied.•Large particles formed skeleton structure and small particles filled the pores.•Different particle size after crushing led to different compression performance.•Particle breakage was studied from a macroscopic and microscopic point of view.•Small particles increased the coordination number of large particles.
While the microstructure at the grain boundary of hot-deformed nanocrystalline Nd-Fe-B magnets is intensively studied, limited works have been devoted to the deformation behavior of the Nd2Fe14B ...matrix grain, especially for micrometer scale grains and at high temperatures. In this study, the plastic deformation behavior of the Nd2Fe14B matrix grains was studied by uniaxial compression from 1050°C to 1120°C using both anisotropic and isotropic sintered Nd-Fe-B magnets. The anisotropic magnets were orientated with the c-axis 0°, 45° and 90° to the loading axis. According to the Arrhenius-type power law relationship, the stress exponent n increased from 1.5 to 2.7 when the c-axis changed from 0° to 90° to the loading direction and the corresponding apparent activation energies Q increased from 384 to 437 kJ/mol. Although the observed n > 1 coincides with the interface-reaction controlled solution-precipitation creep mechanism, the determined Q is much larger than those reported in the nanocrystalline magnet. Our study conferred the first demonstration of short straight dislocations array on the (001) plane in the 45°-orientated anisotropic specimen after 10% height reduction. Grain elongation and local misorientation inside the grain were most obvious in the 45°-oriented specimen after compression. The extent of the local misorientation agreed well with the spacing corresponding to geometrically necessary dislocation. The results reveal that grain rotation via grain boundary sliding and dislocation-mediated deformation were operable depending on the loading direction when the micro-sized magnets were deformed above 1000°C.
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