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1. Brittle to ductile transition during compression of glassy nanoparticles studied in molecular dynamics simulations

   https://doi.org/10.1063/5.0151127  

   Akl, Marx; Huang, Liping; Shi, Yunfeng (July 2023, Journal of Applied Physics)

   Understanding how nanoparticles deform under compression not only is of scientific importance but also has practical significance in various applications such as tribology, nanoparticle-based probes, and the dry grinding of raw materials. In this study, we conducted compression tests on model brittle glassy nanoparticles using molecular dynamics simulations. We found that during the early stages of plastic deformation, shear bands formed in a similar pattern regardless of the nanoparticle size. However, as the deformation continued, dominant cracks emerged in large nanoparticles while being suppressed in smaller ones. This size-dependent brittle-to-ductile transition can be explained by a simple model based on Griffith's theory. We also investigated the effect of the surface stress state on fracture using thermally tempered nanoparticles. We observed that the presence of compressive surface stress strengthened the nanoparticle by suppressing crack formation, even when a pre-notch was present. On the other hand, tensile surface stress had the opposite effect. Interestingly, nanoparticles with both tensile and compressive surface stress promoted shear deformation, which could potentially compromise the mechanical performance of tempered glass despite delayed crack formation. 

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2. Micromechanics based discrete damage model with multiple non-smooth yield surfaces: Theoretical formulation, numerical implementation and engineering applications

   https://doi.org/10.1177/1056789517695872  

   Jin, Wencheng; Arson, Chloé (May 2018, International Journal of Damage Mechanics)

   null (Ed.)

   The discrete damage model presented in this paper accounts for 42 non-interacting crack microplanes directions. At the scale of the representative volume element, the free enthalpy is the sum of the elastic energy stored in the non-damaged bulk material and in the displacement jumps at crack faces. Closed cracks propagate in the pure mode II, whereas open cracks propagate in the mixed mode (I/II). The elastic domain is at the intersection of the yield surfaces of the activated crack families, and thus describes a non-smooth surface. In order to solve for the 42 crack densities, a Closest Point Projection algorithm is adopted locally. The representative volume element inelastic strain is calculated iteratively using the Newton–Raphson method. The proposed damage model was rigorously calibrated for both compressive and tensile stress paths. Finite element method simulations of triaxial compression tests showed that the transition between brittle and ductile behavior at increasing confining pressure can be captured. The cracks’ density, orientation, and location predicted in the simulations are in agreement with experimental observations made during compression and tension tests, and accurately show the difference between tensile and compressive strength. Plane stress tension tests simulated for a fiber-reinforced brittle material also demonstrated that the model can be used to interpret crack patterns, design composite structures and recommend reparation techniques for structural elements subjected to multiple damage mechanisms. 

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3. A modified mixed-mode Timoshenko-based peridynamics model considering shear deformation

   https://doi.org/10.1016/j.ijmecsci.2024.109802  

   Bautista, Victor; Shahbazian, Behnam; Mirsayar, Mirmilad (January 2025, International Journal of Mechanical Sciences)

   The original two-dimensional bond-based peridynamic (BBPD) framework, which only considers the pairwise forces (compression and tension) between two material points, is extended by incorporating the effect of shear deformation in the calculations and its influence on the failure of the bonds. To this end, each bond is considered as a short Timoshenko beam, and by doing so, the traditional BBPD is enhanced into a more comprehensive model known as multi-polar peridynamic (MPPD). The proposed novel approach explicitly considers the shear influence factor used in Timoshenko beams and introduces a strain-based shear deformation failure criterion. The model is then validated against two benchmark experimental tests (i.e., a standard pure mode I edge crack, and a Kalthoff-Winkler configuration) reported in the literature under in-plane dynamic loading and plane stress conditions. In most cases, the developed model is shown to be more accurate in predicting the crack paths obtained from the experimental results when compared to other theoretical methods delineated in the literature. Furthermore, a noticeable change in crack branching and crack path is observed in a study on the effects of Poisson’s ratio and the loading rate. This investigation also demonstrated that the proposed MPPD model can accommodate materials with Poisson’s ratios up to 1/3, expanding the range beyond the traditional BBPD limitations. 

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4. Kink bands promote exceptional fracture resistance in a NbTaTiHf refractory medium-entropy alloy

   https://doi.org/10.1126/science.adn2428  

   Cook, David H.; Kumar, Punit; Payne, Madelyn I.; Belcher, Calvin H.; Borges, Pedro; Wang, Wenqing; Walsh, Flynn; Li, Zehao; Devaraj, Arun; Zhang, Mingwei; et al (April 2024, Science)

   Single-phase body-centered cubic (bcc) refractory medium- or high-entropy alloys can retain compressive strength at elevated temperatures but suffer from extremely low tensile ductility and fracture toughness. We examined the strength and fracture toughness of a bcc refractory alloy, NbTaTiHf, from 77 to 1473 kelvin. This alloy’s behavior differed from that of comparable systems by having fracture toughness over 253 MPa·m^1/2, which we attribute to a dynamic competition between screw and edge dislocations in controlling the plasticity at a crack tip. Whereas the glide and intersection of screw and mixed dislocations promotes strain hardening controlling uniform deformation, the coordinated slip of <111> edge dislocations with {110} and {112} glide planes prolongs nonuniform strain through formation of kink bands. These bands suppress strain hardening by reorienting microscale bands of the crystal along directions of higher resolved shear stress and continually nucleate to accommodate localized strain and distribute damage away from a crack tip. 

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5. Atomistic simulations of superplasticity and amorphization of nanocrystalline anatase TiO2

   https://doi.org/DOI:10.1016/j.eml.2018.05.009  

   X. Zhang, H.J. Gao (July 2018, Extreme mechanics letters)

   As an important type of functional material, nanocrystalline TiO2 with anatase phase has been used for solar energy conversion and photocatalysis. However, there have been only a few limited studies on the mechanical behaviors of nanocrystalline anatase. We performed a series of large-scale atomistic simulations to investigate the deformation of nanocrystalline anatase with mean grain sizes varying from 2 nm to 6 nm and amorphous TiO2 under uniaxial tension and compression at room temperature. The simulation results showed that for uniaxial tension, the fracture strains of simulated samples increase as the mean grain size decreases, and a superplastic deformation occurred in the nanocrystalline sample with a grain size of 2 nm. Such superplasticity of nanocrystalline anatase is attributed to the dominance of grain boundary sliding and nanoscale cavitation during deformation. The simulation results also showed that during uniaxial compression, the amorphization induced by high local compressive stress is the controlling plastic deformation mechanism, resulting in a good compressibility of nanocrystalline TiO2. During both tension and compression, nanocrystalline TiO2 exhibited good deformability, which is attributed to the fact that the grain boundaries with high volume fractions and disordered structures accommodated large plastic strains. Our present study provides a fundamental understanding of the plastic deformation of nanocrystalline anatase TiO2, as well as a route for enhancing the tensile and compressive deformability of nanostructured ceramics. 

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