# Enrique GracianiUniversidad de Sevilla | US

Enrique Graciani

## About

53

Publications

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## Publications

Publications (53)

Experimental off-axis tensile tests aimed at characterising the in-plane shear behaviour of biaxial non-crimp fabric (NCF) composite laminates obtained from different directions within the same panel (warp and weft) reveal noteworthy variations. The in-plane shear modulus, shear strength, and shear strain at failure are significantly higher in the...

A mesoscopic scale 3D finite element model of its representative unit cell is used to study the progressive damage of a [0,90]n non-crimp fabric laminate under compressive loading. The tows of the unit cell have been generated with a straight finite element mesh, and the out-of-plane fibre crimp has been incorporated into the model by defining the...

A unified methodology to solve problems of frictionless unilateral contact as well as adhesive contact between linear elastic solids is presented. This methodology is based on energetic principles and is casted to a minimization problem of the total potential energy. Appropriate boundary integral forms of the energy are defined and the quadratic pr...

Composite laminates are materials where different scales are required for the understanding of the damage and for the calculation of structures made of these materials. The appearance of ultra thin plies of composites has opened the possibility of delaying the onset of damage, which at a first stage appears at a micromechanical level, debondings in...

Highly-curved laminates are prone to fail by delamination when they are loaded under a bending moment which tries to flatten the laminate. This failure, commonly called unfolding failure, has been traditionally associated with the tensile interlaminar stresses which appear in the laminate, due to its high curvature. However, some specimens failing...

A Galerkin boundary integral formulation for 3D axisymmetric Stokes flow is presented. The singular integrals are evaluated by splitting the complicated Green’s function kernels into a singular term that can be integrated analytically, plus a term for which Gauss quadrature provides sufficient accuracy. As in a previous axisymmetric Laplace impleme...

A novel semi-analytic model is developed to determine interlaminar stresses in laminates including highly curved parts, for obtaining a rapid, accurate and reliable design tool for evaluating the reserve factor of a certain aeronautic composite component prone to unfolding failure. The model assumes plane conditions and is based, firstly, in an app...

A numerical study of the onset and growth of debonds at fibre-matrix interfaces, for a fibre bundle under far field biaxial transverse loads, is presented. The behaviour of the fibre-matrix interface cracks was studied by means of a Linear Elastic-Brittle Interface Model (LEBIM). The simplified, but representative, model of an actual unidirectional...

The controversy about the symmetry of the debond onset at the fibre-matrix interface in single-fibre specimens under transverse tension is studied here applying the coupled stress and energy criterion of the finite fracture mechanics. This criterion enables to compare the two different post-failure configurations found in the literature studying th...

A theoretical model is developed to predict debond onset in spherical-particle-reinforced composites subjected to uniaxial tension. The coupled stress and incremental energy criterion of the finite fracture mechanics is applied employing an axisymmetric boundary element method code, which is highly accurate in the analysis of interface cracks in pr...

The problem of crack initiation at the interface of a spherical inclusion embedded in an infinite matrix subjected to a remote tension is studied by the means of the coupled criterion of the finite fracture mechanics. This allows us to predict the critical tension necessary to initiate the crack at the interface as a function of the elastic propert...

The ultrasonic compaction might be a suitable technique to obtain well compacted composite laminates in order to implement them in out-of-autoclave automated curing processes. This procedure generates an ultrasonic vibration over the laminate that makes the resin heat and allows the composite plies to be compacted. The numerical study of this heat...

Recent advances in out-of-autoclave manufacturing processes for composite materials may require the compaction of the pre-preg plies just after placing them. Ultrasonic compactors are a good solution for this task. The quality of the compaction will depend on the frequency and amplitude of the ultrasonic vibration, which produces a viscous heating...

Relations between different solutions of an interface crack in a neighborhood of the crack tip given by the open model, frictionless and frictional contact models of interface cracks are analyzed numerically for a penny-shaped interface crack subjected to remote tension. A new analytic expression for the size of the near-tip contact zone in presenc...

The instability behaviour associated to a crack onset and propagation along the fibre-matrix interface in a composite system under remote transverse loads is studied by a means of the linear elastic - (perfectly) brittle interface model (LEBIM). LEBIM has proved to model adequately the behaviour of this interface represented by a continuous distrib...

A mesoscopic scale 3D finite element (FE) model of the representative unit cell of a [0, 90]n non-crimp fabric laminate is used to study the progressive damage under compressive loading. The tows of the unit cell have been generated with a straight FE mesh, the waviness of the fibres having been taken into account with the definition of the mechani...

Typical analysis of composite laminates do not consider interlaminar stresses, being considered as negligible in most cases. These stresses are more significant in the case of a curved beam, where Lekhnitskii's equations are typically used for their calculation. However, Lekhnitskii's equations are only exact in the case of an homogeneous anisotrop...

Unfolding failure in composites is one of the most complicated to deal with since it does not involve failure of fibers, taking then place at a low nominal load level. The accurate prediction of tresses becomes a key factor in designing structural parts susceptible of suffering this kind of failure. Stresses responsible for unfolding failure in a c...

The crack onset and propagation at the fibre-matrix interface in a composite
under tensile/compressive remote biaxial transverse loads is studied by a new
linear elastic - (perfectly) brittle interface model. In this model the
interface is represented by a continuous distribution of springs which
simulates the presence of a thin elastic layer. The...

A new simple and general technique for potential gradient recovery along the boundary in two-dimensional BEM is introduced and studied numerically. This technique is based on generalized Galerkin projections, and its basic formulation corresponds to a least-squares method. In preliminary numerical tests for Dirichlet and Neumann problems on smooth...

Detailed characterization of linear elastic stress states at corners and crack tips requires knowledge of the stress singularity orders, the characteristic angular functions and the generalized stress intensity factors (GSIF). Typically a high accuracy is found in the literature for the evaluation of the stress singularity orders and characteristic...

The onset and growth of transverse and delamination cracks in [0/90] symmetric laminates under traction loads in the fibre direction of the external laminas is studied. The problem is solved by means of the Boundary Element Method (BEM). Crack propagation is modeled by two different approaches. First, using the Virtual Crack Closure Technique (VCCT...

Single fiber fragmentation test is extensively employed to characterize the fiber-matrix interface in composites. A novel technique for evaluating fracture toughness and friction coefficient at the fiber-matrix interface in an epoxy sample containing a single glass fiber is proposed. Using experimental measurements of the average fragment and debon...

The behavior of the fiber–matrix interface under transverse tension is studied by means of a new linear elastic–brittle interface model. Similar models, also called weak or imperfect interface models, are frequently applied to describe the behavior of adhesively bonded joints. The interface is modeled by a continuous distribution of linear-elastic...

The problem of a crack in a thin adhesive layer is considered. The adherents may have orthotropic elastic behavior which allows composite laminates to be modeled. In the present work a linear elastic-brittle constitutive law of the thin adhesive layer, called weak interface model, is adopted, allowing an easy modeling of crack propagation along it....

The global elastic solution for the problem of a pressurized penny-shaped crack at the interface of two dissimilar half spaces has been numerically obtained employing the boundary element method (BEM). Using the Williams’ open model (for the whole range of feasible bi-material combinations), the comparison of the global BEM solution with an existin...

A numerical analysis, using the Boundary Element Method, of the stress state within the specimen in the single fibre fragmentation test is presented first. Thermal residual stresses and fibre–matrix interfacial friction along the debonding crack faces have been considered in the study. Special attention has been paid to the axial stresses along the...

A numerical study of the fundamental problem of a pressurized penny-shaped crack at the interface of two dissimilar half spaces is carried out allowing for the possibility of frictionless contact between crack faces. A new, highly accurate axi-symmetric formulation of the boundary element method (BEM) for the solution of elastic contact problems is...

Non-crimp fabric (NCF) composites are constituted by differently oriented layers of ideally unidirectional fibre tows. However, a slight waviness always appears in the tows. The applicability of the general laminate theory (GLT) to obtain the stiffness properties of NCF laminates is elucidated in this paper. For this purpose, a 3D finite element (F...

A general expression for estimating the location of the first interpenetration point in the open model of interface cracks
that, for a fixed reference system, can be directly applied to all material combinations is presented.

The boundary integral equation for the axisymmetric Laplace equation is solved by employing modified Galerkin weight functions. The alternative weights smooth out the singularity of the Green's function at the symmetry axis, and restore symmetry to the formulation. As a consequence, special treatment of the axis equations is avoided, and a symmetri...

A boundary element formulation of the axi-symmetric elastic problem, including contact conditions for multi-domain problems, is presented in this paper and applied to the analysis of interface cracks. A weak formulation of the equilibrium and compatibility equations has been developed to allow non-conforming discretizations to be used in the contac...

Single fiber fragmentation test is widely used for characterization of the fiber-matrix interfacial strength in composite materials. The aim of the test is to obtain a stable growth of a crack along the fiber-matrix interface in a single fiber sample. Qualitative values of interfacial strength are usually obtained with the use of semi-analytical so...

This paper deals with recovery of potential gradient (∇u) in the resolution of the two-dimensional Laplace equation by boundary element method. The emphasis is placed on ∇u-recovery near and on the boundary, where error behaviour and convergence rate are studied in detail. Conventional hypersingular
(HS) and strongly singular (SS) boundary integral...

A new approach for computation of potential gradient at and near boundary is introduced. A strongly singular boundary integral representation of potential gradient, whose integral density is the potential gradient, is derived and analysed. Applying the concept of the osculating circle, a local smoothing procedure which computes a continuous approxi...

A new approach for evaluation of the potential gradient in the 2D boundary element method is presented. The two key elements of the procedure developed are, firstly, a new Strongly Singular Boundary Integral Representation of Potential Gradient in complex variable formulation (equivalent to the Cauchy integral representation of analytic functions),...