The Role of Hydrogen on the Behavior of Intergranular Cracks in Bicrystalline α-Fe Nanowires

Hydrogen embrittlement (HE) has been extensively studied in bulk materials. However, little is known about the role of H on the plastic deformation and fracture mechanisms of nanoscale materials such as nanowires. In this study, molecular dynamics simulations are employed to study the influence of H segregation on the behavior of intergranular cracks in bicrystalline α-Fe nanowires. The results demonstrate that segregated H atoms have weak embrittling effects on the predicted ductile cracks along the GBs, but favor the cleavage process of intergranular cracks in the theoretically brittle directions. Furthermore, it is revealed that cyclic loading can promote the H accumulation into the GB region ahead of the crack tip and overcome crack trapping, thus inducing a ductile-to-brittle transformation. This information will deepen our understanding on the experimentally-observed H-assisted brittle cleavage failure and have implications for designing new nanocrystalline materials with high resistance to HE.

Hierarchical levels of organization of the Brazil nut mesocarp

Aiming to understand Nature´s strategies that inspire new composite materials, the hierarchical levels of organization of the Brazil nut (Bertholletia excelsa) mesocarp were investigated. Optical microscopy, scanning electron microscopy (SEM), microtomography (MicroCT) and small-angle X-ray scattering (SAXS) were used to deeply describe the cellular and fibrillary levels of organization. The mesocarp is the middle layer of the fruit which has developed several strategies to avoid its opening and protect its seed. Fibers have a different orientation in the three layers of the mesocarp, what reduces the anisotropy of the structure. Sclereids cells with thick cell walls fill the spaces between the fibers resembling a foam-filled structural composite. The mesocarp has several tubular channels and fractured surfaces which may work as sites for crack trapping and increase toughness. The thick and lignified cell wall of sclereids and fibers and the weak interface between cells can promote a longer and tortuous intercellular crack path. Additionally, fibers with high strength and stiffness due to microfibrils oriented along the main cell axis (µ = 0° to 17°) were identified in the innermost layer of the mesocarp. Such an understanding of each hierarchical level can inspire the development of new cellular composites with improved mechanical behavior

Multiple Toughening Mechanisms of Laminated Ti-TiBw/Ti Composites Fabricated by Diffusion Welding

Laminated Ti-TiBw/Ti composites behave a moderate loading capacity and high fracture ductility with non-castropic failure stage under three-point bending test. Fracture characteristics of laminated composites reveal many extrinsic toughening behaviors, such as, interfacial delamination, bucking and crack deflection, and the interfacial delamination is attributed to the weak bonding strength. Many strengthening and toughening mechanisms are presented in the TiBw/Ti composite layer, such as de-bonding effects of TiB whiskers, multi-fracture of TiB whiskers, crack trapping effect and crack bridging effect.

Microfibres Containing Laminates Prepared by EPD: Microstructure and Fracture Behaviour

Recently it was possible to prepare tailored laminates with perfect and strong interface of layers with precise thickness management. Tailoring of ceramic laminates to obtain optimal mechanical properties with enhanced fracture resistance is possible when predictions based on numerical calculations are employed. Extraordinary mechanical properties were achieved via high internal stresses development during material processing. The aim of this investigation can be seen in two directions. The enhanced crack free green bodies through incorporating small volume fraction of micro-fibres to the powders were prepared. Additionally, control of the crack propagation by incorporated directionally oriented micro-fibres both in the volume and in individual layers. In this contribution both alumina and zirconia micro-fibres were used to help eliminate drying defects in the green body stage before sintering. The co-deposition of ceramic micro-fibres and powder led to the preparation of microstructures having unique orthogonal fracture properties. Developed laminate with thin layers created by zirconia micro-fibres in the alumina matrix seems to be the most promising. This type of material exhibited potential of the crack trapping and deflection even when very small amount of micro-fibres was used.

Micro-Textured Surfaces With Parallel Wall-Like Structures: ‘Modulation’ of Adhesion Properties With the Direction of the Applied External Moment

The aim of the present work is to investigate the adhesive properties of a biomimetic micro-structured surface with strongly direction-dependent adhesion properties. The system is constituted by parallel elastic wall-like structures topped with a thin film. The micro-walls are assumed in perfect contact with a rigid substrate and the adhesive interaction is modeled by considering full adhesion. In a previous work of the authors, it is shown that this geometry, when loaded with an external moment acting perpendicularly to the walls direction, enhances the adhesive properties with respect to a simple flat surface as a result of its crack trapping behavior. In the present paper we investigate what happens when the crack propagates with a generic angle with respect to the walls, in order to determine the variation of the critical conditions for detachment with the direction of the applied moment. Results show that the crack trapping can occur only when the crack propagates perpendicularly to the walls. In all the other cases, the system compliance linearly increases with the crack length. As a result, the energy release rate at the crack tip is constant during the crack propagation and the crack trapping phenomenon cannot occur.

Biomimetic surfaces with controlled direction-dependent adhesion

We propose a novel design of a biomimetic micro-structured surface, which exhibits controlled strongly direction-dependent adhesion properties. The micro-system consists of parallel elastic wall-like structures covered by a thin layer. Numerical calculations have been carried out to study the adhesive properties of the proposed system and to provide design criteria with the aim of obtaining optimized geometries. A numerically equivalent version of the double cantilever beam fracture experiment is, then, simulated by means of finite element analysis to investigate the anisotropic adhesion of the structure. We find that, because of inherent crack trapping properties of these types of structures, the wall-like geometry allows us to strongly enhance adhesion when the detachment direction is perpendicular to the walls. On the other hand, when the detachment occurs parallel to the walls, the system shows low adhesion. This controlled direction-dependent adhesive property of the proposed structure solves one of the key problems of biomimetic adhesive surfaces, which usually show very strong adhesion, even larger than biological systems, but are not suitable for object manipulation and locomotion, as detachment always occurs at high loads and cannot be controlled.

Barnacles resist removal by crack trapping

We study the mechanics of pull-off of a barnacle adhering to a thin elastic layer which is bonded to a rigid substrate. We address the case of barnacles having acorn shell geometry and hard, calcarious base plates. Pull-off is initiated by the propagation of an interface edge crack between the base plate and the layer. We compute the energy release rate of this crack as it grows along the interface using a finite element method. We also develop an approximate analytical model to interpret our numerical results and to give a closed-form expression for the energy release rate. Our result shows that the resistance of barnacles to interfacial failure arises from a crack-trapping mechanism.