In terms of in-service failures, cyclic fatigue is the most
prevalent form of fracture. Despite the wealth of
information on fatigue failures in traditional structural
materials such as (ductile) metals and alloys, far less is
understood about the susceptibility of the newer advanced
materials, such as (brittle) intermetallics, ceramics and
their composites. In this presentation, the mechanics and
mechanisms of fatigue damage and crack propagation are
examined with particular emphasis on the similarities and
differences between cyclic crack growth in ductile metallic
materials, and corresponding behavior in the more brittle
advanced materials. This is achieved by considering the
process of subcritical crack growth as a mutual competition
between intrinsic mechanisms of microstructural damage ahead
of the crack tip, which promote crack growth, and extrinsic
mechanisms of crack-tip shielding behind the tip, which
impede it. This approach is shown to be important for the
understanding of the structural fatigue properties of
advanced materials, such as monolithic and composite
ceramics, and a range of intermetallics (e.g., TiAl, MoSi2,
Nb3Al), as the mechanisms of fatigue in these brittle
materials are conceptually distinct from that associated
with the well known metal fatigue. Examples of the
application and life-prediction methodologies for such
materials in fatigue-critical situations will be given from
the aerospace and bioengineering industries.
[W17.002] Depinning with dynamic stress pulses
J. M. Schwarz, Daniel S. Fisher (Harvard University)
Quasistatic models of extended elastic objects --
interfaces, vortex lattices, crack fronts, etc. -- that are
driven through random medium by a uniform applied force F
show a second-order-like transition from a pinned to moving
phase as F is increased through a history-independent
F_c. The effects of dynamic stress transfer via elastic
waves, are investigated, in particular the effects of
additional transient stresses caused by the motion of
portions of the interface. Even though the stress will
eventually settle down to its quasistatic form, a
stress overshoot can cause extra portions of the interface
to depin, making the depinning transition hysteretic along
with a discontinuous jump in the average velocity as F
increased. Nevertheless, as the depinning transition is
approached from above, numerical studies suggest that the
critical behavior remains in the same universality class as
in the quasistatic case, at least for small stress
overshoots. These results on the depinning transition
suggest that second-order-like and first-order-like features
can coexist.
[W17.003] The effects of quantum-mechanics on fracture in silicon
D. W. Hess, N. Bernstein (Center for Computational Materials Science, Naval Research Laboratory, Washington, DC)
The transition between brittle and ductile fracture has long been studied using silicon as a model system. However, a microscopic understanding of the nature of fracture in this system has been difficult to obtain because of the failure of popular empirical potentials such as Stillinger Weber and EDIP to describe even the apparently simple brittle regime [1,2]. We have developed an atomistic multiscale method that couples an explicitly quantum-mechanical tight-binding model of bonding near the crack tip to a large empirical potential simulation that describes the rest of the mechanically loaded sample. Results of simulations using this method are in agreement with experimental observations of brittle fracture at the elastic energy threshold for crack propagation (the Griffith criterion). Using this method we have studied the material properties that control the nature of fracture. We discuss the applicability of fracture criteria based on energy balance and those based on stress balance to the different models and to the real material. We discuss the variations between the empirical-potential and tight-binding results.
[1] D. Holland and M. Marder, Advanced Materials Vol. 11, 793 (1999).
[2] F. F. Abraham, N. Bernstein, J. Q. Broughton, and D.
Hess, MRS Bulletin Vol. 25, 27 (2000).
[W17.004] Shear-transformation-zone theory of plastic deformation near a circular hole
Leonid Pechenik, Daniel Rabinowitz, James Langer (Physics Department, University of California, Santa Barbara)
We use the recently developed STZ theory of plastic
deformation [1] to investigate the transmission of high
stresses to the neighborhood of a defect in a solid
material. Here, we correct and extend previous findings on
the expansion of a circular hole in a large plate [2]. By
taking advantage of the simple geometry of the problem, we
are able to observe analytically features such as linear
viscoelastic flow at small stresses, strain hardening at
larger stresses, and a dynamic transition to viscoplasticity
at the yield stress. Our analysis predicts the formation of
a well-defined plastic zone around the hole for hard
materials, and a more diffuse plastic zone for soft
materials. The deviatoric stress always exceeds the yield
stress on the boundary of the hole, but its value also
depends on the softness of the material. We study the shape
stability of the hole and speculate that our results may be
related to the dynamics of fracture. [1] M.L. Falk and J.S.
Langer, Phys. Rev. E 57, 7192 (1998). [2] J.S. Langer and
A.E. Lobkovsky, Phys. Rev. E 60, 6978 (1999)
[W17.005] Fracture Mechanisms in ternary carbide Ti3SiC2
Peter Finkel (Drexel University), Michel Barsoum (Drexel University , PA), Tamer El-Raghy (Drexel University)
New class of Ti-based ternary carbides was shown to deform
plastically by a combination of delamination of individual
grains, shear and shear kink band formation. We report here
on the study of the acoustic emission produced during room
temperature deformation of polycrystalline, fully dense,
bulk samples of Ti3SiC2. The acoustic emission was monitored
during monotonic and cyclic compression tests. It was found
that acoustic emission activity is strongly related to the
strain rate and grain size. An attempt has been made to
quantify the acoustic emission and correlate it to the
various damage and fracture mechanisms. The acoustic
emission was shown to be able to continiously monitor the
evolution of the damage.
[W17.006] Analysis of Dislocation Emission during Microvoid Growth in Ductile Metals
James Belak, Robert E. Rudd (Lawrence Livermore National Laboratory)
Fracture in ductile metals occurs through the nucleation and growth of microscopic voids. This talk focuses on the initial stage when dislocations are first emitted from the void surface. The model system consists of a spherical void in an otherwise perfect crystal under triaxial tension. The stress field is calculated using continuum techniques, both finite element and analytic forms due to Eshelby, and compared with large-scale molecular dynamics (MD) simulation. The stress field is used to derive a criterion for dislocation nucleation on the glide planes intersecting the void surface. The critical resolved shear stress and the unstable stacking fault energy for the strain at the surface are used to compare to the critical stress for void growth in the MD simulations.
Acknowledgement: This work was performed under the auspices of the US Dept. of Energy at the University of California/Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48.
[1] J. Belak, "On the nucleation and growth of voids at high
strain-rates," J. Comp.-Aided Mater. Design 5, 193 (1998).
[W17.007] Atomistic Characterization of Dislocation Activity During Void Growth in Dynamic Fracture
Robert E. Rudd, James Belak (Lawrence Livermore National Laboratory)
We use multiscale simulations targeted at the atomistic level to characterize the dislocation activity in the plastic zone surrounding a microvoid system in dynamic fracture. The system is loaded in a state of triaxial tension at a strain rate of approximately 10^9. This talk focuses on void growth in single crystals, in which one or more voids have been seeded in a perfect crystal (e.g. via separation from an inclusion). The voids grow anisotropically under tension [1], causing the material about them to deform plastically. Novel techniques are used to monitor the nucleation and propagation of dislocations on the fly during the evolution of millions of molecular dynamics atoms. We present a complete analysis of the plasticity, including nucleation of partial dislocations about the void surface, the formation and evolution of pairs of partials bound by stacking fault ribbons, their development into well-faceted interstitial loops and their propagation out away in order to accommodate the growth of the void.
Acknowledgement: This work was performed under the auspices of the US Dept. of Energy at the University of California/Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48.
[1] J. Belak, J. Comp.-Aided Mater. Design 5, 193 (1998).
[W17.008] Fracture simulation in vitreous silica
J.M.D. Lane, Michael P Marder (University of Texas at Austin)
I will discuss the use of molecular dynamics simulation as a
tool to investigate the fracture properties of vitreous
silica. Silica was modelled using the Fueston-Garofalini
interatomic potential. Vitreous silica was formed through a
melt-quench technique from the crystalline base state. The
resulting glass was tested against experimental data with
relatively good quantitative agreement. The goal of this
research is to investigate the fracture and mechanical
properties of vitreous silica and compare these properties
with the results of discrete analysis for crystals.
[W17.009] Role of Elastic and Inelastic Deformation on the Tensile Strengths of Ceramics Under Plane Shock Wave Propagation
Dattatraya Dandekar (U. S. Army Research Laboratory, APG, MD 21005)
A material model for predicting the failure of brittle materials under applied stresses remains elusive. Investigations of the tensile strength (spall strength) of ceramics under plane shock wave propagation tend to both simplify the problem and provide valuable guidance for the development of a material model for this purpose. Simplification comes from the fact that the strains developed are one-dimensional, while the stresses are triaxial. Magnitudes of the triaxial stresses are calculable from knowledge of the elastic constants of the material, provided the spallation of the material involves elastic deformation globally while undergoing inelastic deformation locally. The spallation of ceramics tends to satisfy the above conditions pertaining to their shock-induced deformations. This talk summarizes the results of experiments performed to determine the spall strengths of single crystal and polycrystalline Al2O3, and polycrystalline TiB2 and SiC, in terms of their shock induced-response and possible interpretation. This talk also points out limitations of these experiments that are likely to influence the assumptions necessary to develop a material model for brittle materials like ceramics.