Following the continuous miniaturization of the microelectromechanical systems (MEMS), a size-dependent phenomenon of flexoelectricity starts to play an essential role at the micro- and nanoscale. Direct flexoelectricity is an electromechanical coupling of strain gradients, which are inversely proportional to the length scale, and electric field. Due to the application of strain gradients, the centrosymmetry of the unit cells is broken, allowing a wider choice of dielectrics to be used in applications. In the proposed research, the nonlinear ferroelectric material behavior is further enhanced with strain gradients and applied to fracture problems with naturally occurring gradients of electromechanical fields near the crack tip. Or in another way, it is the incorporation of the remanent strains and polarization into the flexoelectric formulation. Our solution demonstrates the strong influence of the gradients on the ferroelectric domain switching behavior, leading to modified electromechanical fields close to the crack tip compared to the well-known ferroelectric problems.
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Themes: Microstructures and Fracture in Advanced Materials
CRACK TIP TRANSFORMATION ZONE MORPHOLOGY IN SMA MATERIALS WITH TRANSFORMATION SOFTENING [Keynote]
The pseudoelastic effect due to martensitic transformation in polycrystalline shape memory alloys is simulated with a phenomenological constitutive model based on a kinematic hardening framework with a gradient enhancement to regularize moving austenite-to-martensite boundaries that arise in softening materials. This constitutive modeling framework introduces a length scale within the theory which has yet to be deteremined experimentally. Calculations are presented for the evolution of the transformation zone around a stationary crack tip. The calculations uncover the interplay between the length scale associated with the size of the transformation zone around the crack tip and the material length scale inherent to the consistutive model. The calcualtions show that localized “fingers” or “needles” of deformation emenate from the transformation zone at a specific level of the applied stress intensity, which provide a comparison to experimental observations that then can be used to quantify the sie of the material length scale.
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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
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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.
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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.
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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.
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TOUGHNESS AND FATIGUE CRACK GROWTH MECHANISMS OF WC-CO CERAMIC-METAL COMPOSITES: A COMPARATIVE STUDY USING CONTROLLED SMALL INDENTATION FLAWS AND LONG THROUGH-THICKNESS CRACKS
Crack growth mechanics and mechanisms under both monotonic and cyclic loading are investigated in a WC-Co cemented carbide by using small and long cracks, both of them artificially introduced by indentation and cyclic compression of unnotched and notched specimens respectively. Agreement and discrepancies on fracture toughness and fatigue crack growth behavior are discussed and rationalized, on the basis of the similitude evidenced in toughening and fatigue degradation mechanisms for both crack types, by taking into account the residual stress field arising after indentation in the unnotched specimens.
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HIGH QUALITY GROWTH AND ADHESION ENERGY MEASUREMENT OF BILAYER GRAPHENE ON SAPPHIRE
One bottleneck in integrating graphene in the next generation of microelectronics devices is the efficient and effective transfer of graphene from its growth substrate to the substrate that is targeted for device fabrication. Dry transfer offers the potential for a relatively fast manufacturing process with minimal contamination of and damage to graphene. The paper describes the development of a chemical vapor deposition process for growing graphene on sapphire rather metal. It also demonstrates that graphene can be dry transferred to another substrate via a polymer carrier by exploiting rate and mode-mix dependent interfacial fracture.
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