1,913 research outputs found
Evolving discontinuities and cohesive fracture
Multi-scale methods provide a new paradigm in many branches of sciences, including applied mechanics. However, at lower scales continuum mechanics can become less applicable, and more phenomena enter which involve discon- tinuities. The two main approaches to the modelling of discontinuities are briefly reviewed, followed by an in-depth discussion of cohesive models for fracture. In this discussion emphasis is put on a novel approach to incorporate triaxi- ality into cohesive-zone models, which enables for instance the modelling of crazing in polymers, or of splitting cracks in shear-critical concrete beams. This is followed by a discussion on the representation of cohesive crack models in a continuum format, where phase-field models seem promising
Homogenization of cohesive fracture in masonry structures
We derive a homogenized mechanical model of a masonry-type structure
constituted by a periodic assemblage of blocks with interposed mortar joints.
The energy functionals in the model under investigation consist in (i) a linear
elastic contribution within the blocks, (ii) a Barenblatt's cohesive
contribution at contact surfaces between blocks and (iii) a suitable unilateral
condition on the strain across contact surfaces, and are governed by a small
parameter representing the typical ratio between the length of the blocks and
the dimension of the structure. Using the terminology of Gamma-convergence and
within the functional setting supplied by the functions of bounded deformation,
we analyze the asymptotic behavior of such energy functionals when the
parameter tends to zero, and derive a simple homogenization formula for the
limit energy. Furthermore, we highlight the main mathematical and mechanical
properties of the homogenized energy, including its non-standard growth
conditions under tension or compression. The key point in the limit process is
the definition of macroscopic tensile and compressive stresses, which are
determined by the unilateral conditions on contact surfaces and the geometry of
the blocks
Phase field approximation of cohesive fracture models
We obtain a cohesive fracture model as a -limit of scalar damage
models in which the elastic coefficient is computed from the damage variable
through a function of the form , with diverging for close to the value describing undamaged
material. The resulting fracture energy can be determined by solving a
one-dimensional vectorial optimal profile problem. It is linear in the opening
at small values of and has a finite limit as . If the
function is allowed to depend on the index , for specific choices we
recover in the limit Dugdale's and Griffith's fracture models, and models with
surface energy density having a power-law growth at small openings
A phase-field model for cohesive fracture
In this paper, a phase-field model for cohesive fracture is developed. After casting the cohesive zone approach in an energetic framework, which is suitable for incorporation in phase-field approaches, the phase-field approach to brittle fracture is recapitulated. The approximation to the Dirac function is discussed with particular emphasis on the Dirichlet boundary conditions that arise in the phase-field approximation. The accuracy of the discretisation of the phase field, including the sensitivity to the parameter that balances the field and the boundary contributions, is assessed at the hand of a simple example. The relation to gradient-enhanced damage models is highlighted, and some comments on the similarities and the differences between phase-field approaches to fracture and gradient-damage models are made. A phase-field representation for cohesive fracture is elaborated, starting from the aforementioned energetic framework. The strong as well as the weak formats are presented, the latter being the starting point for the ensuing finite element discretisation, which involves three fields: the displacement field, an auxiliary field that represents the jump in the displacement across the crack, and the phase field. Compared to phase-field approaches for brittle fracture, the modelling of the jump of the displacement across the crack is a complication, and the current work provides evidence that an additional constraint has to be provided in the sense that the auxiliary field must be constant in the direction orthogonal to the crack. The sensitivity of the results with respect to the numerical parameter needed to enforce this constraint is investigated, as well as how the results depend on the orders of the discretisation of the three fields. Finally, examples are given that demonstrate grid insensitivity for adhesive and for cohesive failure, the latter example being somewhat limited because only straight crack propagation is considered
Numerical modelling of sandstone uniaxial compression test using a mix-mode cohesive fracture model
A mix-mode cohesive fracture model considering tension, compression and shear
material behaviour is presented, which has wide applications to geotechnical
problems. The model considers both elastic and inelastic displacements.
Inelastic displacement comprises fracture and plastic displacements. The norm
of inelastic displacement is used to control the fracture behaviour. Meantime,
a failure function describing the fracture strength is proposed. Using the
internal programming FISH, the cohesive fracture model is programmed into a
hybrid distinct element algorithm as encoded in Universal Distinct Element Code
(UDEC). The model is verified through uniaxial tension and direct shear tests.
The developed model is then applied to model the behaviour of a uniaxial
compression test on Gosford sandstone. The modelling results indicate that the
proposed cohesive fracture model is capable of simulating combined failure
behaviour applicable to rock
Phase-field approximation for a class of cohesive fracture energies with an activation threshold
We study the -limit of Ambrosio-Tortorelli-type functionals
, whose dependence on the symmetrised gradient is
different in and in , for a
-elliptic symmetric operator , in terms of the
prefactor depending on the phase-field variable . This is intermediate
between an approximation for the Griffith brittle fracture energy and the one
for a cohesive energy by Focardi and Iurlano. In particular we prove that
functions with bounded -variation are
Phase-field models for brittle and cohesive fracture
In this paper we first recapitulate some basic notions of brittle and cohesive fracture models, as well as the phase-field approximation to fracture. Next, a critical assessment is made of the sensitivity of the phase-field approach to brittle fracture, in particular the degradation function, and the use of monolithic versus partitioned solution schemes. The last part of the paper makes extensions to a recently developed phase-field model for cohesive fracture, in particular for propagating cracks. Using some simple examples the current state of the cohesive phase-field model is shown
Porous LSCF/Dense 3YSZ Interface Fracture Toughness Measured by Single Cantilever Beam Wedge Test
Sandwich specimens were prepared by firing a thin inter-layer of porous
La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) to bond a thin tetragonal yttria-stabilised
zirconia (YSZ) beam to a thick YSZ substrate. Fracture of the joint was
evaluated by introducing a wedge between the two YSZ adherands so that the
stored energy in the thin YSZ cantilever beam drives a stable crack in the
adhesive bond and allows the critical energy release rate for crack extension
(fracture toughness) to be measured. The crack path in most specimens showed a
mixture of adhesive failure (at the YSZ-LSCF interface) and cohesive failure
(within the LSCF). It was found that the extent of adhesive fracture increased
with firing temperature and decreased with LSCF layer thickness. The adhesive
failures were mainly at the interface between the LSCF and the thin YSZ beam
and FEM modelling revealed that this is due to asymmetric stresses in the LSCF.
Within the firing temperature range of 1000-1150C, the bonding fracture
toughness appears to have a strong dependence on firing temperature. However,
the intrinsic adhesive fracture toughness of the LSCF/YSZ interface was
estimated to be 11 Jm2 and was not firing temperature dependent within the
temperature range investigated.Comment: 13 figures, 1 table, journal paper publishe
Effect of airborne particle abrasion on microtensile bond strength of total-etch adhesives to human dentin
Aim of this study was to investigate a specific airborne particle abrasion pretreatment on dentin and its effects on microtensile bond strengths of four commercial total-etch adhesives. Midcoronal occlusal dentin of extracted human molars was used. Teeth were randomly assigned to 4 groups according to the adhesive system used: OptiBond FL (FL), OptiBond Solo Plus (SO), Prime & Bond (PB), and Riva Bond LC (RB). Specimens from each group were further divided into two subgroups: control specimens were treated with adhesive procedures; abraded specimens were pretreated with airborne particle abrasion using 50 mu m Al2O3 before adhesion. After bonding procedures, composite crowns were incrementally built up. Specimens were sectioned perpendicular to adhesive interface to producemultiple beams, which were tested under tension until failure. Data were statistically analysed. Failure mode analysis was performed. Overall comparison showed significant increase in bond strength (p < 0.001) between abraded and no-abraded specimens, independently of brand. Intrabrand comparison showed statistical increase when abraded specimens were tested compared to no-abraded ones, with the exception of PB that did not show such difference. Distribution of failure mode was relatively uniform among all subgroups. Surface treatment by airborne particle abrasion with Al2O3 particles can increase the bond strength of total-etch adhesive
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