Generalized Mohr-Coulomb strain criterion for bulk metallic glasses under complex compressive loading.

The Mohr-Coulomb (M-C) stress criterion is widely applied to describe the pressure sensitivity of bulk metallic glasses (BMGs). However, this criterion is incapable of predicting the variation in fracture angles under different loading modes. Moreover, the M-C criterion cannot describe the plastic fracture of BMGs under compressive loading because the nominal stress of most BMGs remains unchanged after the materials yield. Based on these limitations, we propose a new generalized M-C strain criterion and apply it to analyze the fracture behaviors of two typical Zr-based BMG round bar specimens under complex compressive loading. In this case, the predicted initial yielding stress is in good agreement with the experimental results. The theoretical results can also describe the critical shear strain and fracture angle of BMGs that are associated with the deformation mode.

In-Plane Heterostructures Enable Internal Stress Assisted Strain Engineering in 2D Materials.

Conventional methods to induce strain in 2D materials can hardly catch up with the sharp increase in requirements to design specific strain forms, such as the pseudomagnetic field proposed in graphene, funnel effect of excitons in MoS2 , and also the inverse funnel effect reported in black phosphorus. Therefore, a long-standing challenge in 2D materials strain engineering is to find a feasible scheme that can be used to design given strain forms. In this article, combining the ability of experimentally synthetizing in-plane heterostructures and elegant Eshelby inclusion theory, the possibility of designing strain fields in 2D materials to manipulate physical properties, which is called internal stress assisted strain engineering, is theoretically demonstrated. Particularly, through changing the inclusion's size, the stress or strain gradient can be controlled precisely, which is never achieved. By taking advantage of it, the pseudomagnetic field as well as the funnel effect can be accurately designed, which opens an avenue to practical applications for strain engineering in 2D materials.

Graphene Foam: Uniaxial Tension Behavior and Fracture Mode Based on a Mesoscopic Model.

Because of the combined advantages of both porous materials and two-dimensional (2D) graphene sheets, superior mechanical properties of three-dimensional (3D) graphene foams have received much attention from material scientists and energy engineers. Here, a 2D mesoscopic graphene model (Modell. Simul. Mater. Sci. Eng. 2011, 19, 054003), was expanded into a 3D bonded graphene foam system by utilizing physical cross-links and van der Waals forces acting among different mesoscopic graphene flakes by considering the debonding behavior, to evaluate the uniaxial tension behavior and fracture mode based on in situ SEM tensile testing (Carbon 2015, 85, 299). We reasonably reproduced a multipeak stress-strain relationship including its obvious yielding plateau and a ductile fracture mode near 45° plane from the tensile direction including the corresponding fracture morphology. Then, a power scaling law of tensile elastic modulus with mass density and an anisotropic strain-dependent Poisson's ratio were both deduced. The mesoscopic physical mechanism of tensile deformation was clearly revealed through the local stress state and evolution of mesostructure. The fracture feature of bonded graphene foam and its thermodynamic state were directly navigated to the tearing pattern of mesoscopic graphene flakes. This study provides an effective way to understand the mesoscopic physical nature of 3D graphene foams, and hence it may contribute to the multiscale computations of micro/meso/macromechanical performances and optimal design of advanced graphene-foam-based materials.

Intrinsic Notch Effect Leads to Breakdown of Griffith Criterion in Graphene.

Due to lack of the third dimension in 3D bulk materials, the crack tip in graphene locates on several atoms implying that its fracture behavior can be closely associated with its lattice structure, i.e., the bond length and angle. As the bond length reflects the discrete nature of the atomic structure, theoretical discussion is focused on the concomitant size effect at the nanoscale with few or no reports about the influence of the bond angle. Through the comparisons between theoretical calculations and experimental data, here it is first demonstrated that the bond angle is essential for understanding the fracture behavior in graphene, serving as an intrinsic notch reducing the stress singularity near the crack tip (the intrinsic notch effect), leading to the breakdown of the Griffith criterion in graphene. The work provides a framework for the studying of the brittle fracture in 2D materials, which gives rise to the more reliable device design based on 2D materials. More importantly, the significance of the intrinsic notch effect is profound and far-reaching, paving the way to a more comprehensive and deep understanding of the mechanical properties in nano as well as nanostructured materials.

Fracture behaviors under pure shear loading in bulk metallic glasses.

Pure shear fracture test, as a special mechanical means, had been carried out extensively to obtain the critical information for traditional metallic crystalline materials and rocks, such as the intrinsic deformation behavior and fracture mechanism. However, for bulk metallic glasses (BMGs), the pure shear fracture behaviors have not been investigated systematically due to the lack of a suitable test method. Here, we specially introduce a unique antisymmetrical four-point bend shear test method to realize a uniform pure shear stress field and study the pure shear fracture behaviors of two kinds of BMGs, Zr-based and La-based BMGs. All kinds of fracture behaviors, the pure shear fracture strength, fracture angle and fracture surface morphology, are systematically analyzed and compared with those of the conventional compressive and tensile fracture. Our results indicate that both the Zr-based and La-based BMGs follow the same fracture mechanism under pure shear loading, which is significantly different from the situation of some previous research results. Our results might offer new enlightenment on the intrinsic deformation and fracture mechanism of BMGs and other amorphous materials.

PHOX2B mutation in a Taiwanese newborn with congenital central hypoventilation syndrome.

Congenital central hypoventilation syndrome (CCHS) is characterized by defective automatic regulation of breathing, mostly during sleep. The diagnostic criteria of CCHS include persistent sleep hypoventilation without primary cardiac, pulmonary disease or neuromuscular dysfunction, and no arousal response to hypoxemia and hypercapnia. Mutations in the PHOX2B gene have been indentified in 93-100% of patients with CCHS. We report a CCHS case with presentation of hypoventilation during sleep and Hirschsprung disease; moreover, a genetic study of the patient confirmed the PHOX2B gene mutation as polyanaline stretch.

Utilizing nasal-tragus length to estimate optimal endotracheal tube depth for neonates in Taiwan.

OBJECTIVE: To assess the application of the nasal-tragus length (NTL) to predict the proper endotracheal tube (ETT) depth; also, as relatively thinner size of Asian than Caucasian, the fitness of using the NTL to estimate the optimal ETT depth for neonates in Taiwan was examined. METHODS: The newborn infants who do need intubation orally were included. Those with midface dysmorphism, craniofacial anomalies, head trauma and/or facial injury in whom it was unable to measure NTL, were excluded. The data were collected after a satisfactory ETT tip position was confirmed on chest roentgenogram. Equations were established via the polynomial and the linear regression of the NTL and the actual ETT; simplified formulae as NTL+0.5 and NTL+1 were assumed accordingly. Paired t test was used to assess the coefficients. RESULTS: The 63 neonates, weighing 410 through 4,196 g and with gestation 21 through 41 weeks, were enrolled. No statistical difference was found between the actual ETT depth and the estimated ETT depth via the NTL+1 cm in neonates weighing ≤ 2,500 g (n=41, p=0.06), and also between the actual ETT depth and the estimated ETT depth via the NTL+0.5 cm in group weighing >2,500 g (n=22, p=0.171). CONCLUSIONS: Using the NTL to predict the optimal ETT depth with the formula, NTL plus 1 cm, was clinically practical for newborn infants in Taiwan weighing ≤ 2,500 g, and a modified formula, NTL plus 0.5 cm, was more suitable for neonates weighing >2,500 g.