Publisher Correction: Damage-tolerant architected materials inspired by crystal microstructure.

In Fig. 4a of this Article, owing to an error in the production process, the scale bar inadvertently read 1 mm instead of 1 m. This error has been corrected online.

Damage-tolerant architected materials inspired by crystal microstructure.

Architected materials that consist of periodic arrangements of nodes and struts are lightweight and can exhibit combinations of properties (such as negative Poisson ratios) that do not occur in conventional solids. Architected materials reported previously are usually constructed from identical 'unit cells' arranged so that they all have the same orientation. As a result, when loaded beyond the yield point, localized bands of high stress emerge, causing catastrophic collapse of the mechanical strength of the material. This 'post-yielding collapse' is analogous to the rapid decreases in stress associated with dislocation slip in metallic single crystals. Here we use the hardening mechanisms found in crystalline materials to develop architected materials that are robust and damage-tolerant, by mimicking the microscale structure of crystalline materials-such as grain boundaries, precipitates and phases. The crystal-inspired mesoscale structures  in our architected materials are as important for their mechanical properties as are crystallographic microstructures in metallic alloys. Our approach combines the hardening principles of metallurgy and architected materials, enabling the design of materials with desired properties.

Added value of the pulmonary vein pulsatility index and its correlation to neonatal umbilical artery pH in fetal growth restrictions: a Vietnamese matched cohort study.

BACKGROUND: In clinical obstetrics, many guidelines recommended the use of Doppler fetal ductus venosus blood flow to monitor and to manage fetal growth restriction (FGR). The ductus venosus and the pulmonary venous flow pattern of fetuses are similar. Umbilical artery pH (UA pH) is essential in identifying adverse pregnancy outcomes, particularly in fetal growth restriction cases. Nevertheless, the literature indicates that the relationship between pulmonary vein pulsatility index (PVPI) and UA pH in FGR cases has not been well investigated. This study aimed to identify the alteration in PVPI in FGR cases and evaluate the correlation between PVPI and UA pH in FGR newborns. METHODS: This matched cohort study of singleton pregnancies from 28+ 0 to 40+ 0 weeks of gestation without congenital abnormalities included 135 cases of FGR (disease group) and 135 cases of normal growth (control group). The PVPI was measured at the proximal segment of the right or left pulmonary vein, approximately 5 mm from the left atrium wall. The umbilical artery pulsatility index (UAPI) was measured on the free umbilical cord. An elective cesarean section or labor induction are both options for ending the pregnancy, depending on the condition of the mother or fetus. Umbilical artery blood samples were collected within 5 min of delivery for UA pH measurement. SPSS version 20 and Medcalc version 20.1 were used for data analysis. RESULTS: FGR cases had a significantly higher mean fetal PVPI than the control group (1.16 ± 0.26 vs. 0.84 ± 0.16; p < 0.01), and PVPI and UAPI were positively correlated (r = 0.63; p < 0.001). PVPI and UA pH were negatively correlated in FGR patients, with r = -0.68; p < 0.001. The PVPI value on the 95th percentile had a prognostic value of UA pH < 7.20 with a sensitivity of 88.2%, specificity of 66.3%, positive predictive value of 46.9%, and negative predictive value of 94.3%. CONCLUSIONS: There was a statistically significant difference in PVPI values in FGR cases compared to the normal growth group, a positive correlation between PVPI and UAPI, and a negative correlation between PVPI and UA pH. PVPI might have a prognostic meaning in predicting UA pH at birth.

Spatially Programmable Architected Materials Inspired by the Metallurgical Phase Engineering.

Programmable architected materials with the capabilities of precisely storing predefined mechanical behaviors and adaptive deformation responses upon external stimulations are desirable to help increase the performance and the organic integration of materials with surrounding environments. Here, a new approach inspired by the physical metallurgical principles is proposed to allow the materials designers to not only enhance the global strength but also precisely tune mechanical properties (such as strength, modulus, and plastic deformation) locally in architected materials to create a new class of intelligent mechanical metamaterials. Such programmable materials not only have high strength and plastic deformation stability but also the ability to regulate the local deformation states and spatially control the internal propagation of deformation. This innovative approach also provides new and effective ways to enhance the adaptivity of the materials thanks to responsive strengths that not only make the materials increasingly stronger but also allow threshold-based adaptive responses to external loading.

Damage-programmable design of metamaterials achieving crack-resisting mechanisms seen in nature.

The fracture behaviour of artificial metamaterials often leads to catastrophic failures with limited resistance to crack propagation. In contrast, natural materials such as bones and ceramics possess microstructures that give rise to spatially controllable crack path and toughened material resistance to crack advances. This study presents an approach that is inspired by nature's strengthening mechanisms to develop a systematic design method enabling damage-programmable metamaterials with engineerable microfibers in the cells that can spatially program the micro-scale crack behaviour. Machine learning is applied to provide an effective design engine that accelerate the generation of damage-programmable cells that offer advanced toughening functionality such as crack bowing, crack deflection, and shielding seen in natural materials; and are optimised for a given programming of crack path. This paper shows that such toughening features effectively enable crack-resisting mechanisms on the basis of the crack tip interactions, crack shielding, crack bridging and synergistic combinations of these mechanisms, increasing up to 1,235% absorbed fracture energy in comparison to conventional metamaterials. The proposed approach can have broad implications in the design of damage-tolerant materials, and lightweight engineering systems where significant fracture resistances or highly programmable damages for high performances are sought after.

The origin of the boundary strengthening in polycrystal-inspired architected materials.

Crystal-inspired approach is found to be highly successful in designing extraordinarily damage-tolerant architected materials. i.e. meta-crystals, necessitating in-depth fundamental studies to reveal the underlying mechanisms responsible for the strengthening in meta-crystals. Such understanding will enable greater confidence to control not only strength, but also spatial local deformation. In this study, the mechanisms underlying shear band activities were investigated and discussed to provide a solid basis for predicting and controlling the local deformation behaviour in meta-crystals. The boundary strengthening in polycrystal-like meta-crystals was found to relate to the interaction between shear bands and polygrain-like boundaries. More importantly, the boundary type and coherency were found to be influential as they govern the transmission of shear bands across meta-grains boundaries. The obtained insights in this study provide crucial knowledge in developing high strength architected materials with great capacity in controlling and programming the mechanical strength and damage path.

The role of side-branching in microstructure development in laser powder-bed fusion.

In-depth understanding of microstructure development is required to fabricate high quality products by additive manufacturing (for example, 3D printing). Here we report the governing role of side-branching in the microstructure development of alloys by laser powder bed fusion. We show that perturbations on the sides of cells (or dendrites) facilitate crystals to change growth direction by side-branching along orthogonal directions in response to changes in local heat flux. While the continuous epitaxial growth is responsible for slender columnar grains confined to the centreline of melt pools, side-branching frequently happening on the sides of melt pools enables crystals to follow drastic changes in thermal gradient across adjacent melt pools, resulting in substantial broadening of grains. The variation of scan pattern can interrupt the vertical columnar microstructure, but promotes both in-layer and out-of-layer side-branching, in particular resulting in the helical growth of microstructure in a chessboard strategy with 67° rotation between layers.

Forming limit prediction using a self-consistent crystal plasticity framework: a case study for body-centered cubic materials.

A rate-dependent self-consistent crystal plasticity model was incorporated with the Marciniak-Kuczynski model in order to study the effects of anisotropy on the forming limits of BCC materials. The computational speed of the model was improved by a factor of 24 when running the simulations for several strain paths in parallel. This speed-up enabled a comprehensive investigation of the forming limits of various BCC textures, such as γ, σ, α, η and ϵ fibers and a uniform (random) texture. These simulations demonstrate that the crystallographic texture has significant (both positive and negative) effects on the resulting forming limit diagrams. For example, the γ fiber texture, which is often sought through thermo-mechanical processing due to a high r-value, had the highest forming limit in the balanced biaxial strain path but the lowest forming limit under the plane strain path among the textures under consideration. A systematic investigation based on the results produced by the current model, referred to as 'VPSC-FLD', suggests that the r-value does not serve as a good measure of forming limit strain. However, model predictions show a degree of correlation between the r-value and the forming limit stress.

Thermally-activated constitutive model including dislocation interactions, aging and recovery for strain path dependence of solid solution strengthened alloys: Application to AA5754-O.

A thermally-activated constitutive model is developed based on dislocation interactions, crystallographic orientations and microstructural evolution to describe the elasto-plastic stress-strain behavior during multi-axial loading. The aim is to contribute to the quantification of complex strain path response in solid solution strengthened alloys. In detail, dislocation/dislocation interactions are incorporated in the model to quantify latent and kinematic hardening phenomena during loading path changes. Dislocation density-based constitutive relations are included to account for dislocation features such as dislocation forests, walls and channels. Moreover specifically, dislocation/solute atom interactions are also considered in order to account for both dynamic and static strain aging as well as static recovery. The model is validated against multiple multi-axial data sets for AA5754-O with changes of loading path and various degrees of pre-strain and time intervals between tests.