This work aims to carry out a hybrid experimental and numerical investigation on the fracture properties of the zirconium cladding tube coated with Cr2AlC, which belongs to the group of MAX phase materials. A macroscopic failure criterion is finally developed based on the experimental and numerical simulation results, thus contributing to the design of the accident-tolerant fuel system (ATFs) in nuclear power plants. A series of in-situ bending tests involving various sample geometries covering a wide range of stress states are carried out under a quasi-static condition. Oxidized samples and samples aged in hot water under high pressure are also involved to consider the aging and oxidation effect on material failure. The modified Bai-Wierzbicki (MBW) damage model and the analytical Yoon2014 model are coupled in the simulation so the damage and strength differential effect can be considered in modeling material failure. By transferring boundary conditions between the micro- and macroscopic model as a weak macro-micro coupling, homogenization is achieved so that a micromechanical sub-model can also be developed and the micromechanical simulation and macroscopic simulation can be cross-scale bridged.
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Themes: Microstructures and Fracture in Advanced Materials
ADVANCES IN NECKING-ASSISTED CONTROLLED FRAGMENTATION BY COMPOSITE COLD DRAWING [Keynote]
Fracture of materials has been regarded as the major danger to structures and is to be avoided in design, manufacture and maintenance. However, the application of classical cold drawing technique to advanced composites consisting of brittle semiconductor/glass/2D materials and ductile polymers prone to necking enables controlled fragmentation of the target component, resulting in structured patterns in micro- down to nano- scales. The controlled fragmentation can thus be taken advantage of to produce microstructures in large scale. Mechanism of controlled fragmentation and key parameters for tuning fragment size are revealed through theoretical modeling, experiment and finite element analysis. Effects of the addition of a sacrificial layer/capping layer on fragment size to improve capability of the cold drawing technique will also be discussed.
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FATIGUE DAMAGE MODELLING OF ALUMINIUM ALLOY POLYCRYSTALS CONTAINING INTERMETALLIC PHASES
The objective of this work is to model fatigue damage of the aluminium alloy AA2139 at the microscopic scale. It combines an experimental campaign and numerical simulations for a complete modelling of the alloy. Special attention is given to the reproduction of the alloy grain morphology and crystallography. Moreover, intermetallic phases are preferred sites for fatigue crack initiation in this alloy. Therefore, a method for taking into account the alloy microstructural complexity including the presence of intermetallic phases is presented. Finally, a fatigue damage model using a fatigue indicator parameter (FIP) is considered for the introduction of a crack and its propagation in simulations.
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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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