Numerical simulation of energy release rate for interface crack initiation due to thermal stress in environmental barrier coatings for Silicon Carbide (SIC) fiber reinforced in SIC matrix composite

1. Introduction In order to apply silicon carbide (SiC) fiber reinforced SiC matrix (SiC/SiC) composites as high-pressure turbine materials, environmental barrier coatings (EBC) is essential. EBC consists of several materials and thermal stress occurs by the difference in thermal property of EBC layers and SiC/SiC substrate during the fabrication process and usage environment. If energy release rate (ERR) exceeds interface fracture toughness, the interface crack can be initiated (Griffith theory). For structure design to maintain the property of EBC, it is necessary to theoretically predict ERR for interface crack while fracture toughness is obtained in experiments. This study is to perform numerical simulation of ERR for interface crack initiation due to thermal stress in EBC. 2. Theoretical equation for predicting ERR for interface crack in multi-layered structure In 1990’s, Suo and Hutchinson revealed that ERR for interface crack initiation in single-layered structure (isotropic elastic material, biaxial stress state) is written by strain energy of the layer multiplied by a dimensionless constant factor. To predict ERR for interface crack initiation in a multi-layered structure, we regard the coating layers above the objective interface as one layer and the other layers below the interface as a substrate. Then, ERR (G) is expressed by Here, Z′ is a dimensionless factor, σi, Ei, νi and hi are thermal stress, Young’s modulus, Poisson’s ratio and thickness of the coating layer above the objective interface, respectively. Note that Z′ is dependent not only on elastic properties of the components but also on thicknesses of the coating layers and substrate because ERR should be governed by the ‘effective’ mismatch between the layers above and below the objective interface. In order to examine the dependence of Z′ EBC layer thicknesses, we calculate ERR (GF), which is released strain energy per crack propagation area, by using thermal stress finite element method (FEM) analysis to be compared with ΠT. Please click Additional Files below to see the full abstract

Dislocation nucleation in a thin Cu film from molecular dynamics simulations: Instability activation by thermal fluctuations

To elucidate the mechanism responsible for structural instability at the atomic level, atomistic modeling simulation of tension in a Cu thin film containing a notch was performed using an embedded-atom method potential and dislocation nucleation was observed. Mechanical stability during tension was analyzed by solving the eigenvalue problem of the Hessian matrix taking into account all the degrees of freedom of the atoms in the system. Since an eigenvalue designates the curvature of the potential energy landscape in the direction of the corresponding eigenvector, which indicates a deformation mode, the system is unstable under vanishing temperature at the critical strain (εc) when any eigenvalue is zero or negative. At a strain smaller than εc where all the eigenvalues are positive, atomic fluctuations due to finite temperature may cause structural instability. We found that the path of activated instability (dislocation emission from the notch) could be written with a linear combination of the eigenvectors having small eigenvalues obtained under a corresponding external strain at zero temperature. The energy landscape has a much lower hill along the mixed-mode path than along any single-mode paths. In a molecular dynamics simulation under finite temperature, components of deformation modes having small eigenvalues fluctuate at low frequency, which dominate the activation of instability

Ab initio study of stress-induced domain switching in PbTiO3

We investigated the atomistic and electronic structure of the 90° domain wall in PbTiO3 and the fundamental mechanism of domain switching induced by shear stress using first-principles density functional theory calculations within the local density approximation. Under strain-free condition, the magnitude of polarization at the center of the domain wall decreased by 20% from that of the bulk, and the direction rotated within the transition region of 1.3 nm. Under strain, the applied shear deformation concentrated near the 90° domain wall, and the domain wall began to migrate in a direction perpendicular to itself after the stress reached the critical magnitude of 152 MPa. The migration direction was governed by the shearing direction. During stress-induced domain switching, a Pb-O covalent bond at the center of the domain wall broke, and concurrently, another bond on the neighboring Pb-O site was formed with a large movement of the Pb atom. Thus, reconstruction of the Pb-O bond was associated with the domain switching

Coarse-Grained Molecular Dynamics Simulation of Polycarbonate Deformation: Dependence of Mechanical Performance by the Effect of Spatial Distribution and Topological Constraints

Polycarbonate is an engineering plastic used in a wide range of applications due to its excellent mechanical properties, which are closely related to its molecular structure. We performed coarse-grained molecular dynamics (CGMD) calculations to investigate the effects of topological constraints and spatial distribution on the mechanical performance of a certain range of molecular weights. The topological constraints and spatial distribution are quantified as the number of entanglements per molecule (Ne) and the radius of gyration (Rg), respectively. We successfully modeled molecular structures with a systematic variation of Ne and Rg by controlling two simulation parameters: the temperature profile and Kuhn segment length, respectively. We investigated the effect of Ne and Rg on stress–strain curves in uniaxial tension with fixed transverse strain. The result shows that the structure with a higher radius of gyration or number of entanglements has a higher maximum stress (σm), which is mainly due to a firmly formed entanglement network. Such a configuration minimizes the critical strain (εc). The constitutive relationships between the mechanical properties (σm and εc) and the initial molecular structure parameters (Ne and Rg) are suggested

Nonsingular Stress Distribution of Edge Dislocations near Zero-Traction Boundary

Among many types of defects present in crystalline materials, dislocations are the most influential in determining the deformation process and various physical properties of the materials. However, the mathematical description of the elastic field generated around dislocations is challenging because of various theoretical difficulties, such as physically irrelevant singularities near the dislocation-core and nontrivial modulation in the spatial distribution near the material interface. As a theoretical solution to this problem, in the present study, we develop an explicit formulation for the nonsingular stress field generated by an edge dislocation near the zero-traction surface of an elastic medium. The obtained stress field is free from nonphysical divergence near the dislocation-core, as compared to classical solutions. Because of the nonsingular property, our results allow the accurate estimation of the effect of the zero-traction surface on the near-surface stress distribution, as well as its dependence on the orientation of the Burgers vector. Finally, the degree of surface-induced modulation in the stress field is evaluated using the concept of the L2-norm for function spaces and the comparison with the stress field in an infinitely large system without any surface

10 SCIENTIFIC HIGHLIGHT OF THE MONTH Ideal strength of nano-structured components

The ideal (theoretical) strength was originally defined as the stress or strain at which perfect crystal lattice became mechanically unstable with respect to arbitrary homogeneous infinitesimal deformation. This has been intensely investigated because the ultimate strength without defects is a fundamental mechanical characteristic of materials. In the analyses, the instability criteria have been studied on the basis of elastic constants. Recent developments in computational technology make it possible to analyze the ideal strength on the basis of quantum mechanics. On the other hand, it is well known that the mechanical strength of components is dependent not only on (1) material (atom species), but also on (2) loading condition and (3) structure. Because most studies on the strength in terms of atomic mechanics have focused on the factor (1) (materials), analysis has mainly been conducted on simple crystal consisting of perfect lattices (e.g. fcc and bcc) under simple loading conditions (e.g. tension), though some have explored the properties of bulk materials with defects (e.g. vacancy and grain boundary). Small atomic components (nano-structured components) such as nano-films, nano-wires (tubes) and nano-dots (clusters) possess their own beautiful