Frédéric Lani

A multiscale parametric study of mode I fracture in metal-to-metal low-toughness adhesive joints

The failure of adhesively-bonded joints, consisting of metallic adherends and epoxy-based structural adhesive with a relatively low toughness ~200 J/m2, has been studied. The failure was via quasi-static mode I, steady-state crack propagation and has been modelled numerically. The model implements a ‘top-down approach’ to fracture using a dedicated steady-state, finite-element formulation. The damage mechanisms responsible for fracture are condensed onto a row of cohesive zone elements with zero thickness, and the responses of the bulk adhesive and of the adherends are represented by continuum elements spanning the full geometry of the joint. The material parameters employed in the model are first quantitatively identified for the particular epoxy adhesive of interest, and their validity is verified by comparison with experimental results. The model is then used to conduct a detailed study on the effects of (a) large variations in the geometrical configuration of the different types of specimens and (b) the adherend stiffness on the predicted value of the adhesive fracture energy, Ga. These numerical modelling results reveal that the adhesive fracture energy is a strong nonlinear function of the thickness of the adhesive layer, the other variables being of secondary importance in influencing the value of Ga providing the adhesive does not contribute significantly to the bending stiffness of the joint. These results which fully agree with experimental observations are explained in detail by identifying, and quantifying, the different sources of energy dissipation in the bulk adhesive contributing to the value of Ga. These sources are the locked-in elastic energy, crack tip plasticity, reverse plastic loading and plastic shear deformation at the adhesive/adherend interface. Further, the magnitudes of these sources of energy dissipation are correlated to the degree of constraint at the crack tip, which is quantified by considering the opening angle of the cohesive zone at the crack tip.

A substructured FE/XFE method for stress intensity factors computation in an industrial structure

The introduction of the eXtended Finite Element Method (X-FEM) into a commercial Finite Element (FE) software was achieved through a substructuring method. For fracture mechanics problems, the domain is decomposed into cracked and safe subdomains which are solved by the XFE-code and the FE-software, respectively. The interface problem is solved using a FETI solver. The new approach is compared with a classical FE-approach in the case of a planar crack in a compressor drum of a turbofan engine.

Damage detection in composite structures using autonomous wireless systems: simulation & validation

In the European FP6 project ADVICE, units that harvest energy from structural vibrations have been developed. These autonomous units are capable of wireless communication and are used as actuators of guided ultrasonic waves used to identify changes in structural behaviour. The growing use of composite structures in aeronautics brings new challenges to be able to predict and detect damage that may occur in these new materials. Part of the ADVICE project focused on studying the possibilities of damage detection in structures using Lamb waves. In order to do this, finite element simulations were performed and were compared with experimental data. This paper presents the finite element simulations performed to predict the behaviour of a structural health monitoring system. Using the technique and algorithms developed to quantify the amount of damage, we compare the numerical results to those that were obtained experimentally on a small representative composite structure. This allows evaluating how it is possible to design a monitoring system for composite structures and what needs to be addressed in the future.

A Coupled Mean Field / Gurson‐Tvergaard Micromechanical Model For Ductile Fracture In Multiphase Materials With Large Volume Fraction of Voids

In the framework of the European project PROHIPP (New design and manufacturing processes for high pressure fluid power product — NMP 2‐CT‐2004‐50546), CENAERO develops a library of constitutive models used to predict the mechanical response of a family of cast iron. The present contribution focuses on one particular microstructure, corresponding to a ferrite matrix containing spheroidal graphite and isolated inclusions of pearlite. An incremental mean field homogenisation scheme such as the one developed by Doghri and Ouaar is used. In the present application, the ferrite matrix is described by a Gurson type constitutive law (porous plasticity) while the pearlite inclusions are assumed to obey the classical isotropic J2 plasticity. The predictions of the micromechanical model are compared to the results of Finite Element simulations performed on three‐dimensional representative volume elements (RVEs).

Nonconformal mesh-based finite element strategy for 3D textile composites

A finite element procedure is developed for the computation of the thermoelastic properties of textile composites with complex and compact two- and three-dimensional woven reinforcement architectures. The purpose of the method is to provide estimates of the properties of the composite with minimum geometrical modeling effort. The software TexGen is used to model simplified representations of complex textiles. This results in severe yarn penetrations, which prevent conventional meshing. A non-conformal meshing strategy is adopted, where the mesh is refined at material interfaces. Penetrations are mitigated by using an original local correction of the material properties of the yarns to account for the true fiber content. The method is compared to more sophisticated textile modeling approaches and successfully assessed towards experimental data selected from the literature.