1,721,044 research outputs found
Mode I fracture toughness of fibre-reinforced concrete by means of a modified version of the two-parameter model
No full textAbstractThe present paper proposes a method to calculate Mode I plane-strain fracture toughness of concrete, by taking into account the possible crack deflection (kinked crack), even in the case of a far-field Mode I loading. As a matter of fact, during fracture extension, cracks may deflect as a result of microstructural inhomogeneities inside the material. Concrete is an inhomogeneous mixture due to aggregates embedded in the cementitious matrix, but additional inhomogeneities may be represented by fibres. Firstly, a two-parameter fracture model based on Mode I analytical expressions of the linear elastic fracture mechanics is employed. Then, in order to take into account the possible crack deflection as a result of the above inhomogeneities, a modified version of such a model is here discussed. Three-point bending tests on both plain concrete specimens and concrete specimens reinforced with micro-synthetic polypropylene fibrillated fibres are experimentally performed, and the modified model is applied
Mechanics of interface debonding in fiber-reinforced materials
No full textThe evaluation of damage in multiphase materials plays a crucial role in their safety assessment under service mechanical actions. In this context, the quantification of the damage associated to fibre–matrix detachment is one of the most important aspects to be carried out for short fibre-reinforced materials. In the present article, the problem of progressive fibre–matrix debonding is examined and a mechanics interpretation of such a phenomenon is developed by relating the shear-lag and the fracture mechanics approach in order to determine the fibre–matrix interface characteristics. A multiscale approach is employed: at macroscopic level, composites with dilute dispersed fibres, arranged in a undirectional or in random orientation, are analysed through a homogenization approach, whereas the problem of axisymmetric debond growth in short fibres is examined at microscopic level. Moreover, a ‘structured’ linear elastic interface framework model for crack propagation analysis is applied by defining a microscopic truss structure, enabling to relate each other the classical shear strength approach and the fracture mechanics approach. Finally, a fibre pull-out test and some simple fibre-reinforced structural components are examined. This new proposed point of view on the debonding phenomenon allows a deep understanding of the mechanics of the fibre–matrix interface and enables to characterize such an interface layer that has a relevant role in mechanics design of composites materials
Micromechanical crack growth-based fatigue damage in fibrous composites
Full text linkA partially debonded fibre can be analysed as a 3-D mixed Mode fracture case, for which the fibre–matrix
detachment growth – leading to a progressive loss of the composite’s bearing capacity – can be assessed
through classical fatigue crack propagation laws. In the present research, the above mentioned case is
firstly examined from the fracture mechanics theoretical point of view, and the effects of the stress field
in the matrix material on the Stress Intensity Factors – SIFs – (associated to the crack representing the
fibre–matrix detachment) are taken into account. Fatigue effects on the matrix material are accounted
for by means of a mechanical damage, quantified through a Wöhler-based approach. A damage scalar
parameter aimed at measuring the debonding severity during fatigue process is also introduced.
Finally, some numerical simulations are performed, and the obtained results are compared with
experimental data found in the literature
Tension failure assessment at lug hole edges
No full textThe lug is the critical component of lug joints, and one of its failure mode is the tension failure at hole edges. In the present paper, the fatigue crack growth simulation of a semi-elliptical crack located in the lug net cross-section, at one of the two hole edges, is carried out by employing a theoretical model based on the Paris law. The lug sizes, loading conditions and crack configurations are assumed in accordance with those related to some experimental tests available in the literature, performed on lugs of PolyMethylMethAcrylate under pulsating tension
Fatigue Crack Propagation Simulating Fibre Debonding in Cyclically Loaded Composites
No full textAbstractA partially debonded fibre can be analyzed as a 3-D mixed Mode fracture problem for which the fibre-matrix detachment growth – leading to a progressive loss of the composite's bearing capacity – can be assessed through classical fatigue crack propagation laws. In the present study, the above mentioned problem is firstly examined from the theoretical point of view, and the effects of the stress field in the matrix material on the SIFs (associated to the crack representing fibre-matrix detachment) are taken into account. Suitable fatigue crack propagation laws for mixed mode SIFs are employed in order to quantify the crack growth rate corresponding to the fibre-matrix debonding growth rate, while the matrix material undergoes a mechanical damage quantified through a Wöhler-based approach to fatigue. A damage scalar parameter aimed at measuring the debonding severity during fatigue process is also introduced. Finally, some numerical simulations are performed, and the obtained results are compared with results found in the literature
Simplified analysis of fracture behaviour of a Francis hydraulic turbine runner blade
No full textEach welded connection between blades and band or crown of a Francis hydraulic turbine runner can be considered as a T-joint subjected to pure bending induced by the water action. A semi-elliptical crack is assumed to exist at the surface of one of the aforementioned welded connections. The actual geometry of the T-joint can be simplified, that is, only the cracked plate (representing the blade) under a given stress distribution acting on the defect faces is examined. A numerical procedure already proposed by the authors to compute the stress-intensity factor (SIF) along the crack front is here applied by introducing some
changes to simplify such computations. The obtained values of SIF are compared with some results available in the literature
A frequency-domain HCF criterion for biaxial random loading
No full textA frequency-domain critical plane criterion is here proposed to estimate fatigue life of smooth
metallic structural components under multiaxial random loading. The procedure consists of the
following three steps: (a) definition of the critical plane; (b) PSD evaluation of an equivalent normal
stress; (c) computation of the fatigue life. A new formulation to define the critical plane is adopted in
order to improve the criterion as far as the lifetime estimation is concerned. The criterion is
numerically validated through comparison of the obtained results with experimental fatigue data,
available in the literature, for structural steel subjected to non-proportional bending and torsion
Effect of fibre arrangement on the multiaxial fatigue of fibrous composites: a micromechanical computational model
Full text linkStructural components made of fibre-reinforced materials are frequently used in engineering
applications. Fibre-reinforced composites are multiphase materials, and complex mechanical phenomena take
place at limit conditions but also during normal service situations, especially under fatigue loading, causing a
progressive deterioration and damage. Under repeated loading, the degradation mainly occurs in the matrix
material and at the fibre-matrix interface, and such a degradation has to be quantified for design structural
assessment purposes. To this end, damage mechanics and fracture mechanics theories can be suitably applied
to examine such a problem. Damage concepts can be applied to the matrix mechanical characteristics and, by
adopting a 3-D mixed mode fracture description of the fibre-matrix detachment, fatigue fracture mechanics
concepts can be used to determine the progressive fibre debonding responsible for the loss of load bearing
capacity of the reinforcing phase.
In the present paper, a micromechanical model is used to evaluate the unixial or multiaxial fatigue behaviour of
structures with equi-oriented or randomly distributed fibres. The spatial fibre arrangement is taken into account
through a statistical description of their orientation angles for which a Gaussian-like distribution is assumed,
whereas the mechanical effect of the fibres on the composite is accounted for by a homogenization approach
aimed at obtaining the macroscopic elastic constants of the material. The composite material behaves as an
isotropic one for randomly distributed fibres, while it is transversally isotropic for unidirectional fibres. The
fibre arrangement in the structural component influences the fatigue life with respect to the biaxiality ratio for
multiaxial constant amplitude fatigue loading. One representative parametric example is discussed
QUASI BRITTLE MATRIX COMPOSITE MATERIALS: A COMPUTATIONAL APPROACH BASED ON DISCONTINUOUS-LIKE FE AND FRACTURE MECHANICS DEBONDING SIMULATION
No full textIn the present paper, the damaging mechanisms observed in fibre reinforced composite materials are simulated. In particular, quasi brittle matrix composites are examined: such cases typically involve the matrix damage associated to the creation and propagation of cracks, whose opening is mitigated by the presence of the reinforcing fibre phase. On the other hand, the effectiveness of fibres on the composite’s load bearing capacity is also heavily affected by the debonding phenomenon. Both degrading mechanisms are considered in the proposed mechanical model: the matrix cracking is accounted for by adopting a discontinuous-like FE approach which allows us to consider cracks depending on the matrix stress field, whereas the fibre-matrix detachment is simulated through a crack growth model based on the critical interface fracture energy; the remote critical stress causing debonding (i.e. crack advancing) can be obtained from the interface SIF. The main mechanical aspects of the developed model are described and, through some numerical examples compared with existing literature data, the two damaging aspects are shown to properly describe the complex phenomena arising in such multiphase materials
Spectral fatigue life estimation for non-proportional multiaxial random loading
Full text linkA frequency-domain High-Cycle Fatigue (HCF) criterion based on the critical plane approach is here proposed
to estimate fatigue life of smooth metallic structural components under multiaxial random loading.
The procedure consists of the following three steps: (a) definition of the critical plane; (b) PSD
evaluation of an equivalent normal stress; and (c) computation of the fatigue life. A new formulation
to define the critical plane is adopted in order to improve the lifetime estimation. The criterion is validated
through comparison of the obtained numerical results with experimental fatigue data available
in the literature for structural steel specimens subjected to a combination of random non-proportional
bending and torsion
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