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Title: Atomistic simulations of brittle crack growth.

Abstract

Ceramic materials such as lead zirconium titanates (PZT), low temperature co-fired ceramics and silica glasses are used in several of Sandia's mission critical components. Brittle fracture, either during machining and processing or after many years in service, remains a serious reliability and cost issue. Despite its technological importance, brittle fracture remains poorly understand, especially the onset and propagation of sub-critical cracks. However, some insights into the onset of fracture can be gleaned from the atomic scale structure of the amorphous material. In silica for example, it is well known [1] that the Si-O-Si bonds are relatively weak and, in angle distribution functions determined from scattering experiments, the bonds exhibit a wide spread around a peak at 150. By contrast the O-Si-O bonds are strong with a narrow peak in the distribution around the 109 dictated by the SiO{sub 4} tetrahedron. In addition, slow energy release in silica, as deduced from dissolution experiments, depends on the distribution of 3-fold and higher rings in the amorphous structure. The purpose of this four month LDRD project was to investigate the atomic structure of silica in the bulk and in the vicinity of a crack tip using molecular dynamics simulations. Changes in the amorphousmore » structure in the neighborhood of an atomically sharp tip may provide important clues as to the initiation sites and the stress intensity required to propagate a sub-critical crack.« less

Authors:
Publication Date:
Research Org.:
Sandia National Laboratories
Sponsoring Org.:
USDOE
OSTI Identifier:
908078
Report Number(s):
SAND2007-0901
TRN: US200722%%187
DOE Contract Number:
AC04-94AL85000
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; CERAMICS; CRACK PROPAGATION; FRACTURES; MACHINING; PZT; SILICA; GLASS; BRITTLENESS; MOLECULAR DYNAMICS METHOD; ELECTRONIC STRUCTURE; Ceramic materials.; Lead zirconate titanate (PZT); Ceramic materials-Fracture.; Fracture (Mechanics)

Citation Formats

Hoyt, Jeffrey John. Atomistic simulations of brittle crack growth.. United States: N. p., 2007. Web. doi:10.2172/908078.
Hoyt, Jeffrey John. Atomistic simulations of brittle crack growth.. United States. doi:10.2172/908078.
Hoyt, Jeffrey John. Sun . "Atomistic simulations of brittle crack growth.". United States. doi:10.2172/908078. https://www.osti.gov/servlets/purl/908078.
@article{osti_908078,
title = {Atomistic simulations of brittle crack growth.},
author = {Hoyt, Jeffrey John},
abstractNote = {Ceramic materials such as lead zirconium titanates (PZT), low temperature co-fired ceramics and silica glasses are used in several of Sandia's mission critical components. Brittle fracture, either during machining and processing or after many years in service, remains a serious reliability and cost issue. Despite its technological importance, brittle fracture remains poorly understand, especially the onset and propagation of sub-critical cracks. However, some insights into the onset of fracture can be gleaned from the atomic scale structure of the amorphous material. In silica for example, it is well known [1] that the Si-O-Si bonds are relatively weak and, in angle distribution functions determined from scattering experiments, the bonds exhibit a wide spread around a peak at 150. By contrast the O-Si-O bonds are strong with a narrow peak in the distribution around the 109 dictated by the SiO{sub 4} tetrahedron. In addition, slow energy release in silica, as deduced from dissolution experiments, depends on the distribution of 3-fold and higher rings in the amorphous structure. The purpose of this four month LDRD project was to investigate the atomic structure of silica in the bulk and in the vicinity of a crack tip using molecular dynamics simulations. Changes in the amorphous structure in the neighborhood of an atomically sharp tip may provide important clues as to the initiation sites and the stress intensity required to propagate a sub-critical crack.},
doi = {10.2172/908078},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sun Apr 01 00:00:00 EDT 2007},
month = {Sun Apr 01 00:00:00 EDT 2007}
}

Technical Report:

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  • A set of numerical analyses of crack growth was preformed to elucidate the influence of microcracking on the fracture behavior of microcracking brittle solids and composites. The random nucleation, orientation and size effects of discrete nucleating microcracks and resulting interactions are fully accounted for in a hybrid finite element model. The results obtained from the finite element analysis are compared with the continuum description of the microcracking. Although continuum description can provide a reasonable estimation of shielding, it fails to resolve the details of micromechanism of toughening resulting from microcracking, since not every shielding event during the course of crackmore » extension corresponds to an increase in the R-curve. Moreover, as seen in the composite cases, the local events leading to toughening behavior may not be associated with the microcracking even in the presence of a large population of microcracks.« less
  • In this project we developed t he atomistic models needed to predict how graphene grows when carbon is deposited on metal and semiconductor surfaces. We first calculated energies of many carbon configurations using first principles electronic structure calculations and then used these energies to construct an empirical bond order potentials that enable s comprehensive molecular dynamics simulation of growth. We validated our approach by comparing our predictions to experiments of graphene growth on Ir, Cu and Ge. The robustness of ou r understanding of graphene growth will enable high quality graphene to be grown on novel substrates which will expandmore » the number of potential types of graphene electronic devices.« less
  • A vibrating-beam method was used to determine the elastic modulus of graphite rods. The frequency and apparent modulus were determined as a function of compression end loading. Following fracture of the rod, the frequency and apparent modulus were decreased. At a compression and loading of about 0.83 MPa (120 psi), crack closure was sufficient for the fractured rod to behave similarly in vibration to the unfractured rod. Thus, the fractured material behaves in a bimodular fashion and crack closure can be achieved to enable unimpeded stress transfer across the fracture surface during vibration.
  • The critical stress intensity factors for propagating cracks, K/sub ID/, was experimentally found to decrease with increasing crack velocity. K/sub ID/ was measured using rapidly wedged double cantilevered beam specimens. The crack length versus time was continuously recorded using electro-potential techniques. The specimens were fractured at temperatures well below the brittle to ductile transition temperature. The crack velocity in these specimens varies continuously but is typically in the range of 100 to 200 m/s. The analysis of fully dynamic crack propagation in double torsional beam specimens was solved in closed analytic form. The solutions for rotational rate loading and formore » constant applied torque, predict constant crack velocity. The crack velocity is given by the applied rate of rotation, the magnitude of the applied torque and specifics about the beam. The maximum crack velocity is the torsional wave speed. The use of the analysis is to deduce K/sub ID/ during crack propagation without measuring the crack velocity directly. The measurements of static, dynamic loading and propagating crack, stress intensity factors establishes that rate is an important variable in specifying the stress intensity factor for fracture. (auth)« less