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Title: Microstructural Modeling of Dynamic Intergranular and Transgranular Fracture Modes in Zircaloys

Abstract

In this time period, we have continued to focus on (i) refining the thermo-mechanical fracture model for zirconium (Zr) alloys subjected to large deformations and high temperatures that accounts for the cracking of ZrH and ZrH2 hydrides, (ii) formulating a framework to account intergranular fracture due to iodine diffusion and pit formation in grain-boundaries (GBs). Our future objectives are focused on extending to a combined population of ZrH and ZrH2 populations and understanding how thermo-mechanical behavior affects hydride reorientation and cracking. We will also refine the intergranular failure mechanisms for grain boundaries with pits.

Authors:
 [1];  [1];  [1]
  1. North Carolina State Univ., Raleigh, NC (United States)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1409268
Report Number(s):
ORNL/TM-2017/260
74851
DOE Contract Number:
AC05-00OR22725
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Mohammed, I., Zikry, M.A., and Ziaei, S. Microstructural Modeling of Dynamic Intergranular and Transgranular Fracture Modes in Zircaloys. United States: N. p., 2017. Web. doi:10.2172/1409268.
Mohammed, I., Zikry, M.A., & Ziaei, S. Microstructural Modeling of Dynamic Intergranular and Transgranular Fracture Modes in Zircaloys. United States. doi:10.2172/1409268.
Mohammed, I., Zikry, M.A., and Ziaei, S. Sat . "Microstructural Modeling of Dynamic Intergranular and Transgranular Fracture Modes in Zircaloys". United States. doi:10.2172/1409268. https://www.osti.gov/servlets/purl/1409268.
@article{osti_1409268,
title = {Microstructural Modeling of Dynamic Intergranular and Transgranular Fracture Modes in Zircaloys},
author = {Mohammed, I. and Zikry, M.A. and Ziaei, S.},
abstractNote = {In this time period, we have continued to focus on (i) refining the thermo-mechanical fracture model for zirconium (Zr) alloys subjected to large deformations and high temperatures that accounts for the cracking of ZrH and ZrH2 hydrides, (ii) formulating a framework to account intergranular fracture due to iodine diffusion and pit formation in grain-boundaries (GBs). Our future objectives are focused on extending to a combined population of ZrH and ZrH2 populations and understanding how thermo-mechanical behavior affects hydride reorientation and cracking. We will also refine the intergranular failure mechanisms for grain boundaries with pits.},
doi = {10.2172/1409268},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sat Apr 01 00:00:00 EDT 2017},
month = {Sat Apr 01 00:00:00 EDT 2017}
}

Technical Report:

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  • From the formation of microscopic cracks in the fuel pipe liner of the space shuttle to the safety of roadway bridges, the fracture of materials has enormous implications throughout our society. The ability to assess and design safe engineering structures requires a detailed knowledge of this failure process. The fracture process depends on both the loading history and the detailed microscopic structure (microstructure) of the material. Weak points, such as inclusions and grain boundary junctions, are the locations from which microscopic fractures (voids and cracks) originate. Once nucleated, these fractures quickly link together to form a macroscopic crack. Despite thismore » qualitative understanding, little is known about voids nucleation, plastic deformation in the surrounding material, and the mechanisms of linking. Central to Stockpile Stewardship is an understanding of shock loading of materials. During the passage of a shock wave, the material is compressed at a very high rate. This compression produces a high density of dislocation defects and other changes to the microstructure that are poorly understood. When the shock wave reflects from a free surface, the compression is rapidly released and extreme tension is produced inside the material. If this tension exceeds the internal rupture strength, microscopic fractures form and link up to create a spallation scab--a thin scab that separates from the bulk of the material. In this project, we use the LLNL gas gun facility to produce a planar stress pulse with controlled duration and amplitude. The sample is carefully captured in soft foam while measuring the free surface velocity profile. The amount of change in the surface velocity during release is related to the spallation strength. We study light metals (Al, V, Ti, Cu) with known initial microstructure: single crystal, polycrystalline, and single crystal with engineered inclusions. Light metals enable direct measurement of the three dimensional distribution of damage using X-Ray tomography. After the tomography experiment is complete, the samples are sliced and analyzed using 2D metallography.« less
  • Dynamic fracture is a material failure process at high strain-rates. Here, we limit our discussion to spallation fracture during shock wave loading. When a compressive shock wave reflects from a free surface, internal states of tension are created. If this tension exceeds the rupture strength of the material, it fails by nucleating and growing microscopic voids in ductile metals and cracks in brittle solids. This effect, known as spallation, was reported by Hopkinson in 1872 [1]. Rinehart and Pierson [2] give an historical introduction to spallation and other aspects of high strain-rate deformation. This phenomenology of the nucleation and growthmore » of microscopic voids is common to all fracture in ductile metals, including dynamic fracture. The importance of pulse duration was not fully appreciated until the 1960's through the work of Butcher and colleagues [3, 4], leading to the concept of cumulative damage. This concept of damage accumulation was put on a strong experimental foundation by Barbee et al. [5], who performed gas gun recovery experiments and tediously measured the size and distribution of microscopic flaws using 2D microscopy. The resulting continuum material model of dynamic fracture is known as the SRI-NAG model [6, 7], for Nucleation And Growth. In a critical assessment of dynamic fracture, Meyers and Aimone [8] reviewed the abundant continuum spallation models and found the model of nucleation, growth, and coelescense of voids to qualitatively reproduce the observed extent of damage from spall recovery experiments. As in the work of Barbee et al. [5], Meyers and Aimone do not find any voids smaller than about 1 {micro}m, suggesting that the voids either nucleated at that scale or when they are nucleated they rapidly grow to the micron size scale. The continuum models are phenomenological in nature with parameters fit to experiment. The advent of the Advanced Strategic Computing program has enabled direct numerical simulation of the nucleation and growth of microscopic voids. However, there exists no experimental data to both guide the modeling to the essential physical phenomena and to validate meso-scale modeling of dynamic fracture in ductile metals. The goal of this project is to obtain this new experimental data for dynamic fracture in ductile metals by characterizing and analyzing the microscopic processes using modern experimental facilities not previously available. In particular, we measured with increased precision the damage produced during spallation fracture in ductile metals. Dynamic gas-gun recovery experiments were used to generate incipient spallation damage while concurrently measuring the free surface velocity wave profile. This wave profile has been used extensively to characterize spallation fracture. The concurrent measurement enables us to correlate the observed incipient damage with features in the wave profile, such as the velocity pull back. The incipient spallation damage was characterized using both advanced Synchrotron-based 3D X-Ray tomography and traditional 2D microscopy. A direct correlation was made between the traditional 2D microscopy and the 3D distribution of voids on the same sample. 2D microscopy, including optical, EBS/OIM, and TEM, was used to quantify, for the first time, the plastically deformed zone in the metal surrounding an incipiently grown void. Nanoindentation hardness experiments were used to quantify the increased strength in the plastically deformed zone from the greatly increased dislocation density.« less
  • Microstructural fracture processes in A533B steel were studied in small standard tensile test specimens, and a computational model of these processes was constructed to allow calculation of various measures of macroscopic fracture toughness. Based on data from small smooth-bar tensile tests, computations were made of J/sub Ic/, the critical crack-opening angle, and the critical crack-opening displacement for a hypothetical center-cracked-panel large enough to fulfill the conditions for J-controlled growth. The calculated values of these parameters only slightly exceeded values typically measured. It is concluded that a viable route exists to obtain macroscopic fracture toughness measures from posttest metallographic examination ofmore » small tensile specimens. The resulting computational model can also be used to compute the conditions for failure under conditions of large-scale yielding, in the absence of macroscopic cracks, and to study expected variations in toughness caused by variations in inclusion concentrations.« less
  • This paper illustrates the use of advanced constitutive models in ABAQUS/Explicit together with highly focused finite element meshes to simulate the propagation of a fracture in a ductile medium. A double edge-cracked specimen under far field dynamic tensile loading is analyzed, and shows both rectilinear motion or unstable oscillatory motion of the crack depending on the material property constraints. Results are also presented for a simulation of ASTM`s standard fracture test E399. Comparisons of ABAQUS/Explicit results with experiments or other analytical/numerical results are made.
  • This work is part of an investigation with the long-range objective of predicting the size distribution function and velocity dispersion of shattered pellet fragments after a large cryogenic pellet impacts a solid surface at high velocity. The study is vitally important for the shattered pellet injection (SPI) technique, one of the leading technologies being implemented at ORNL for the mitigation of disruption damage on current tokamaks and ITER. The report contains three parts that are somewhat interwoven. In Part I we formulated a self-similar model for the expansion dynamics and velocity dispersion of the debris cloud following pellet impact againstmore » a thick (rigid) target plate. Also presented in Part I is an analytical fracture model that predicts the nominal or mean size of the fragments in the debris cloud and agrees well with known SPI data. The aim of Part II is to gain an understanding of the pellet fracturing process when a pellet is shattered inside a miter tube with a sharp bend. Because miter tubes have a thin stainless steel (SS) wall a permanent deformation (dishing) of the wall is produced at the site of the impact. A review of the literature indicates that most projectile impact on thin plates are those for which the target is deformed and the projectile is perfectly rigid. Such impacts result in “projectile embedding” where the projectile speed is reduced to zero during the interaction so that all the kinetic energy (KE) of the projectile goes into the energy stored in plastic deformation. Much of the literature deals with perforation of the target. The problem here is quite different; the softer pellet easily undergoes complete material failure causing only a small transfer of KE to stored energy of wall deformation. For the real miter tube, we derived a strain energy function for the wall deflection using a non-linear (plastic) stress-strain relation for 304 SS. Using a dishing profile identical to the linear Kirchkoff-Love profile (for lack of a rigorously derived profile) we derived the strain energy associated with the deflection and applied a virtual work principle to find a relationship between the impact (load) pressure to the measured wall deflection depth. The inferred impact pressure was in good agreement with the expected pressure for oblique cryogenic pellet impacts where the pellet shear stress causing cleavage fracture is well above the yield stress for pure shear. The section is concluded with additional discussion on how this wall deformation data lends further support to the analytical fracture model presented in Part I. In Part III we present three different size distribution models. A summary, with a few brief suggestions for a follow on study, is provided at the end of this report.« less