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1. On the role of the microstructure in the deformation of porous solids

   https://doi.org/10.1038/s41524-022-00840-5  

   Patnaik, Sansit; Jokar, Mehdi; Ding, Wei; Semperlotti, Fabio (July 2022, npj Computational Materials)

   Abstract This study explores the role that the microstructure plays in determining the macroscopic static response of porous elastic continua and exposes the occurrence of position-dependent nonlocal effects that are strictly correlated to the configuration of the microstructure. Then, a nonlocal continuum theory based on variable-order fractional calculus is developed in order to accurately capture the complex spatially distributed nonlocal response. The remarkable potential of the fractional approach is illustrated by simulating the nonlinear thermoelastic response of porous beams. The performance, evaluated both in terms of accuracy and computational efficiency, is directly contrasted with high-fidelity finite element models that fully resolve the pores’ geometry. Results indicate that the reduced-order representation of the porous microstructure, captured by the synthetic variable-order parameter, offers a robust and accurate representation of the multiscale material architecture that largely outperforms classical approaches based on the concept of average porosity. 

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2. Advances in characterizing and understanding the microstructure of cementitious materials

   https://doi.org/https://doi.org/10.1016/j.cemconres.2019.105806  

   Monteiro, P.J. (October 2019, Cement and concrete research)

   Progress in microstructural characterization methods is summarized. Special attention is given to advanced probes, such as X-ray imaging and spectroscopy, 1H NMR relaxometry, in-situ and high-pressure X-ray diffraction, and digital holographic microscopy. Microtomography has become a mature technique and nanotography has improved its spatial resolution significantly, particularly with the use of ptychography. The review also discusses the effect of plasticizers on the microstructure of concrete and presents a critical analysis of how organic admixtures affects the hydrates obtained from pure synthesis in saturated solutions and from more realistic hydrating systems. The addition of nanomaterials into cementitious systems modifies the microstructure of the matrix so a summary of recent research is presented. It is important to integrate the impressive progress in the characterization of the micro(nano)structure with efforts developing realistic models. Therefore, the present work gives a short but critical presentation of physical chemistry approaches that aim to link the chemical composition, texture, and microstructure of the cement hydrates. 

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3. Effect of the initial microstructure on the pressure-induced phase transition in Zr and microstructure evolution

   Pandey, K. K.; Levitas, Valery I.; Park, Changyong; Shen, Guoyin (January 2023, arXivorg)

   The effect of initial microstructure and its evolution across the α→ω phase transformation in commercially pure Zr under hydrostatic compression has been studied using in situ x-ray diffraction measurements. Two samples were studied: one is plastically pre-deformed Zr with saturated hardness and the other is annealed. Phase transformation α→ω initiates at lower pressure for pre-deformed sample, suggesting pre-straining promotes nucleation by producing more defects with stronger stress concentrators. With transformation progress, the promoting effect on nucleation reduces while that on growth is suppressed by producing more obstacles for interface propagation. The crystal domain size reduces and microstrain and dislocation density increase during loading for both α and ω phases in their single-phase regions. For α phase, domain sizes are much smaller for prestrained Zr, while microstrain and dislocation densities are much higher. On the other hand, they do not differ much in ω Zr for both prestrained and annealed samples, implying that microstructure is not inherited during phase transformation. The significant effect of pressure on the microstructural parameters (domain size, microstrain, and dislocation density) demonstrates that their postmortem evaluation does not represent the true conditions during loading. A simple model for the initiation of the phase transformation involving microstrain is suggested, and a possible model for the growth is outlined. The obtained results suggest an extended experimental basis is required for better predictive models for the pressure-induced and combined pressure- and strain-induced phase transformations. 

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4. Effects of microstructure on the mechanical properties of Ti2AlC in compression

   Benitez, R. (January 2018, Acta materialia)

   This study investigates the effect of microstructure, specifically the grain size and TiAlx impurity, on the compressive strength and hysteretic behavior of Ti2AlC at room temperature. Given the plate-like nature of the MAX phase grains, the length and thicknesses of over 100 grains for each microstructure were measured. A Hall-Petch like relationship between compressive strength and the grain length was observed, but not such a relationship was observed with the grain thickness. Results from cyclic compression testing in combination with resonant ultrasound spectroscopy show that room temperature mechanical response of Ti2AlC can be divided into four stress regions regardless of the variation in grain size and/or amount of impurities. The grain size effect on the transition stresses for stress regions was also investigated. It was found that all transition stresses, between the different stress regions, also follow different Hall-Petch-type relationships. 

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5. Effect of captivity on the vertebral bone microstructure of xenarthran mammals

   https://doi.org/10.1002/ar.24817  

   Zack, Ellianna H.; Smith, Stephanie M.; Angielczyk, Kenneth D. (November 2021, The Anatomical Record)

   null (Ed.)

   Captive specimens in museum collections facilitate study of rare taxa, but the lifestyles, diets, and lifespans of captive animals differ from their wild counterparts. Trabecular bone architecture adapts to in vivo forces, and may reflect interspecific variation in ecology and behavior as well as intraspecific variation between captive and wild specimens. We compared trunk vertebrae bone microstructure in captive and wild xenarthran mammals to test the effects of ecology and captivity. We collected μCT scans of the last six presacral vertebrae in 13 fossorial, terrestrial, and suspensorial xenarthran species (body mass: 120 g to 35 kg). For each vertebra, we measured centrum length; bone volume fraction (BV.TV); trabecular number and mean thickness (Tb.Th); global compactness (GC); cross-sectional area; mean intercept length; star length distribution; and connectivity and connectivity density. Wild specimens have more robust trabeculae, but this varies with species, ecology, and pathology. Wild specimens of fossorial taxa (Dasypus) have more robust trabeculae than captives, but there is no clear difference in bone microstructure between wild and captive specimens of suspensorial taxa (Bradypus, Choloepus), suggesting that locomotor ecology influences the degree to which captivity affects bone microstructure. Captive Tamandua and Myrmecophaga have higher BV.TV, Tb.Th, and GC than their wild counterparts due to captivity-caused bone pathologies. Our results add to the understanding of variation in mammalian bone microstructure, suggest caution when including captive specimens in bone microstructure research, and indicate the need to better replicate the habitats, diets, and behavior of animals in captivity. 

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