167 research outputs found
Decaying into the Hidden Sector
The existence of light hidden sectors is an exciting possibility that may be
tested in the near future. If DM is allowed to decay into such a hidden sector
through GUT suppressed operators, it can accommodate the recent cosmic ray
observations without over-producing antiprotons or interfering with the
attractive features of the thermal WIMP. Models of this kind are simple to
construct, generic and evade all astrophysical bounds. We provide tools for
constructing such models and present several distinct examples. The light
hidden spectrum and DM couplings can be probed in the near future, by measuring
astrophysical photon and neutrino fluxes. These indirect signatures are
complimentary to the direct production signals, such as lepton jets, predicted
by these models.Comment: 40 pages, 5 figure
Failure analysis of a cracked plate based on endochronic plastic theory coupled with damage
An anisotropic model of damage mechanics for ductile fracture incorporating the endochronic theory of plasticity is presented in order to take into account material deterioration during plastic deformation. An alternative form of endochronic (internal time) theory which is actually an elasto-plastic damage theory with isotropic-nonlinear kinematic hardening is developed for ease of numerical computation. Based on this new damage model, a finite element algorithm is formulated and then employed to characterize the fracture of thin aluminum plate containing a center crack. A new criterion termed as Y R -Criterion is proposed to define both the crack initiation angle and load. Experiments have been conducted to verify the validity of the proposed damage model and it is found that the theoretical crack initiation loads correspond closely with the measured values.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42776/1/10704_2004_Article_BF00034511.pd
Numerical investigation of 3-D constraint effects on brittle fracture in SE(B) and C(T) specimens
This investigation employs 3-D nonlinear finite element analyses to conduct an extensive parametric evaluation of crack front stress triaxiality for deep notch SE(B) and C(T) specimens and shallow notch SE(B) specimens, with and without side grooves. Crack front conditions are characterized in terms of J-Q trajectories and the constraint scaling model for cleavage fracture toughness proposed previously by Dodds and Anderson. The 3-D computational results imply that a significantly less strict size/deformation limit, relative to the limits indicated by previous plane-strain computations, is needed to maintain small-scale yielding conditions at fracture by a stress- controlled, cleavage mechanism in deep notch SE(B) and C(T) specimens. Additional new results made available from the 3-D analyses also include revised {eta}-plastic factors for use in experimental studies to convert measured work quantities to thickness average and maximum (local) J-values over the crack front
Influences of non-singular stresses on plane-stress near-tip fields for pressure-sensitive materials and applications to transformation toughened ceramics
In this paper, we investigate the effects of the non-singular stress ( T stress) on the mode I near-tip fields for elastic perfectly plastic pressure-sensitive materials under plane-stress and small-scale yielding conditions. The T stress is the normal stress parallel to the crack faces. The yield criterion for pressure-sensitive materials is described by a linear combination of the effective stress and the hydrostatic stress. Plastic dilatancy is introduced by the normality flow rule. The results of our finite element computations based on a two-parameter boundary layer formulation show that the total angular span of the plastic sectors of the near-tip fields increases with increasing T stress for materials with moderately large pressure sensitivity. The T stress also has significant effects on the sizes and shapes of the plastic zones. The height of the plastic zone increases substantially as the T stress increases, especially for materials with large pressure sensitivity. When the plastic strains are considered to be finite as for transformation toughened ceramics, the results of our finite element computations indicate that the phase transformation zones for strong transformation ceramics with large pressure sensitivity can be approximated by those for elastic-plastic materials with no limit on plastic strains. When the T stress and the stress intensity factor K are prescribed in the two-parameter boundary layer formulation to simulate the crack-tip constraint condition for a single-edge notch bend specimen of zirconia ceramics, our finite element computation shows a spear shape of the phase transformation zone which agrees well with the corresponding experimental observation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42782/1/10704_2004_Article_BF00018779.pd
Continuum and Micromechanics Treatment of Constraint in Fracture
Two complementary methodologies are described to quantify the effects of crack-tip stress triaxiality
(constraint) on the macroscopic measures of elastic-plastic fracture toughness, J and CTOD. In the continuum
mechanics methodology, two parameters, J and Q, suffice to characterize the full range of near-tip
environments at the onset of fracture. J sets the size scale of the zone of high stresses and large deformations
while Q scales the near-tip stress level relative to a high triaxiality reference stress state. The material's
fracture resistance is characterized by a toughness locus, Jc(Q), which defines the sequence of J-Q values
at fracture determined by experiment from high constraint conditions (Q=O) to low constraint conditions
(Q < 0). A micromechanics methodology is described which predicts the toughness locus using crack-tip
stress fields and critical J-values from a few fracture toughness tests. A robust micromechanics model
for cleavage fracture has evolved from the observations of a strong, spatial self-similarity of crack-tip principal
stresses under increased loading and across different fracture specimens. The micromechanics model
employs the volume of material bounded within principal stress contours at fracture to correlate Jc values
for different specimens and loading modes. This report explores the fundamental concepts of the J-Q description
of crack-tip fields, the fracture toughness locus and micromechanics approaches to predict the
variability of macroscopic fracture toughness with constraint under elastic-plastic conditions. Computational
results are presented for a surface cracked plate containing a 6: 1 semi-elliptical, a=t/4 flaw subjected
to remote uniaxial and biaxial tension. Crack-tip stress fields consistent with the J-Q theory are demonstrated
to exist at each location along the crack front. The micromechanics model employs the J-Q description
of crack-front stresses to interpret fracture toughness values measured on laboratory specimens for
fracture assessment of the surface cracked plate. The computational results suggest only a minor effect of
the biaxial loading on the crack tip stress fields and, consequently, on the propensity for fracture relative
to the uniaxial loading.U.S. Nuclear Regulatory Commission. Office of Nuclear Regulatory Research. Division of Engineering.Contract No. N61533-90-K-0059Contract No. N00167-92-K-003
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