Due to the impressive electromechanical properties, ferroelectric materials are widely used in actuators, memory devices and other electronic devices. However, the instinct weak strength and brittleness of ferroelectrics make it easy to failure under external force and electric field. Furthermore, giant strain gradient will be generated near the crack tip with the action of external forces. As a consequence, flexoelectricity is expected to sharply affect the domain configuration and local electromechanical behavior. In this work, the time-dependent Ginzburg-Landau (TDGL) theory and phase field model are used to study the influence of flexoelectric effect on the domain structure and fracture toughness of PbTiO3 in the vicinity of the crack tip. The results show that flexoelectric effect reduces the fracture toughness near the crack tip and breaks the symmetry of the domain structure and fracture toughness. Fracture toughness of parallel to the polarization direction decreases more than that perpendicular to the polarization direction. Compared with anti-parallel to the polarization direction, the flexoelectric effect is more likely to weaken the fracture toughness of parallel to the polarization direction. As a conclusion, flexoelectric electric effect is vatal influence on the reliability design of ferroelectric materials and devices.
EXTENDED ABSTRACT
Themes: Microstructures and Fracture in Advanced Materials
EFFECT OF STRAIN RATE AND REFORMED AUSTENITE ON MECHANICAL PROPERTIES OF AISI 415 STAINLESS STEEL
Hydraulic turbine blades are exposed to cyclic loading which favors formation and propagation of fatigue cracks. Due to different in-service loading regimes, the crack tip is subjected to a range of strain rates. The present study proposes an experimental investigation of the mechanical properties of a 13%Cr-4%Ni martensitic stainless steel at strain rates (ε ̇) ranging from 4.7E-6s-1 to 6E-2s-1. The ε ̇ was chosen to simulate plastic deformation rate at the crack tip for load cycles frequency ranging from 0.3 to 35 Hz. Two heat treatments were applied to the alloy to obtain a martensitic microstructure containing 2% and 20% of reformed austenite (RA). For the sample containing 2% of RA, increasing ε ̇ resulted in a difference in yield strength (σ_y) and ultimate tensile strength (UTS) of 10% and 7%, respectively. As for the sample containing 20% of RA, an increase in the RA content had no significant effect on the σ_y strain rate sensitivity. On the other hand, it reduced the UTS strain rate sensitivity to 1%. These results indicate that σ_y is strain rate sensitive for both tested microstructure. Results also show that presence of RA increased 23% the uniform elongation as compared to microstructure containing
EXTENDED ABSTRACT
CHARACTERIZATION OF ICE ADHESION: MODES OF LOADING AND MICROSTRUCTURE
We present fracture mechanics-based approaches to characterize interfacial fracture parameters for the tensile and shear behavior of a typical ice/aluminum interface. An experimental framework employing single cantilever beam, direct shear, and push-out shear tests were developed to achieve near mode-I and near mode-II fracture conditions at the interface. Both analytical (beam bending and shear-lag analysis), and numerical (finite element analysis incorporating cohesive zone method) models were used to extract mode-I and II interfacial fracture parameters. The combined experimental and numerical results, as well as surveying published results for the direct shear and push-out shear tests, showed that mode-II interfacial strength and toughness could be significantly affected by the test method due to geometrically induced interfacial residual stress. As a result, the apparent toughness of the zero-angle push-out test could reach an order of magnitude higher than those derived from direct shear tests. Moreover, it was found that the interfacial ice adhesion is fracture mode insensitive and roughness insensitive for tensile and shear modes, for the observed modes of failures in this study.
EXTENDED ABSTRACT
FROM CONTINUUM TO QUANTUM MECHANICS STUDY ON THE FRACTURE OF NANOSCALE NOTCHED BRITTLE MATERIALS
The fracture of nanoscale notched brittle materials is investigated using the multi-scale analysis of cohesive zone modeling and first-principles calculations based on the notched nano-cantilever bending experiment. first-principles calculations are performed to investigate the inherent fracture properties of single-crystal silicon from atomic and electronic viewpoints. The fracture surface energy and critical bond length for the break of atomic bonds during the fracture are compared with the cohesive energy and failure length parameter, which indicates that the consumed energy is an effective linkage to quantify the fracture of brittle materials at different scales.
EXTENDED ABSTRACT
MICROMECHANICAL MODELING OF THE FRACTURE PROCESS IN ADVANCED METAL SANDWICH PLATES USING FFT-BASED HOMOGENIZATION
The fracture behavior of the complex core material of Hybrix sandwich plates was investigated by micromechanical modeling using FFT-based homogenization. A method for generating virtual Representative Volume Elements (RVEs) based on experimental observations was developed and the homogenization results were compared to experiments in peel mode I. The applicability of micromechanical simulations to the optimization of fracture properties of the Hybrix core is discussed.
EXTENDED ABSTRACT
GROWTH AND COALESCENCE OF MULTIPLE CRACKS – EXPERIMENTS AND FRACTURE MECHANICS BASED MODEL
Short crack growth tests are carried out on the coarse-grained nickel-based cast alloy Iconel 100 (IN100) and two microstructures of the austenitic stainless steel AISI 347 using the replica technique. IN100 is tested under TMF and AISI347 isothermally. For both materials, several cracks are found which grow together to form the final main crack. Atypically, the final main crack length does not develop exponentially. To describe the damage evolution of the final main crack, a model is developed based on inelastic fracture mechanics, which includes the different crack driving forces along the crack front, and applied to the test results.
EXTENDED ABSTRACT
NUMERICAL MODELING OF SPALLING PHENOMENON ON ALUMINA BY DISCRETE ELEMENT METHOD.
The numerical Discrete Element Method (DEM) approach has already proven its legitimacy to represent the behaviour of brittle or quasi-brittle materials such as ceramics at quasi-static regime. The present study investigates the DEM approach in reproducing the dynamic behaviour of an AL23 ceramic under dynamic spalling tests. Elastic microscopic parameters of the DEM model are calibrated using quasi-static uniaxial tensile tests in order to match the macroscopic elastic behaviour of an AL23 ceramic. The DEM model is then used to simulate the stress waves propagation, interactions and fracture mechanisms generated during spalling damage tests. Rear face velocity profiles have been measured and compared to the numerical results. The strain-rate sensitivity of the spalling stress of AL23 ceramic has been observed experimentally. The anisotropic DFH (Denoual-Forquin-Hild) damage model is implemented in DEM to take into account the strain rate sensitivity. Several methods to manage anisotropy in DEM are tested.
EXTENDED ABSTRACT
CHARACTERIZATION OF THE DAMAGE TOLERANCE OF NANODESIGNED COATINGS BASED ON HIGH ENTROPY ALLOYS
Coatings are primarily designed to offer excellent wear and corrosion resistance. However, these properties are adjusted at the expense of the damage tolerance of the materials applied. By introducing the concept of high entropy alloys new property combinations are expected. In this contribution coatings composed of purely refractory HEA nitride as well as coatings containing non-refractory elements such as Al or Si will be presented. The focus is on a comparison of different experimental strategies to evaluate the damage tolerance of these coatings.
EXTENDED ABSTRACT
THE JUMPING DIELECTRIC BREAKDOWN BEHAVIOR INDUCED BY CRACK PROPAGATION IN FERROELECTRIC MATERIALS: A PHASE FIELD STUDY [Keynote]
Ferroelectric materials often experience fracture and dielectric breakdown under large mechanical and/or electrical loadings. These two failure behaviours can affect each other and may lead to new phenomena. In the present study, a new type of jumping dielectric breakdown are predicted during the crack propagation in ferroelectric materials by using a phase field model with multiple order parameters. The driving force for the jumping dielectric breakdown induced by crack propagation is analysed based on the concept of configurational force. It is found that the jumping dielectric breakdown is attributed to the competition between the crack propagation and the breakdown initiation at the moving crack tip. The breakdown pattern is related to the polarization domain structure and thus greatly influenced by the applied electric field and the crack boundary conditions. Further, the jumping breakdown associated with possible mechanical degradation on the material can change the driving and resistant forces for the crack propagation. The present work not only suggests a novel dielectric breakdown behaviour during crack propagation but also provides new insights into the coupling failure of ferroelectric materials.
EXTENDED ABSTRACT