Gabriele Cricri

Three-Dimensional BEM and FEM Submodelling in a Cracked FML Full Scale Aeronautic Panel

This paper concerns the numerical characterization of the fatigue strength of a flat stiffened panel, designed as a fiber metal laminate (FML) and made of Aluminum alloy and Fiber Glass FRP. The panel is full scale and was tested (in a previous work) under fatigue biaxial loads, applied by means of a multi-axial fatigue machine: an initial through the thickness notch was created in the panel and the aforementioned biaxial fatigue load applied, causing a crack initiation and propagation in the Aluminum layers. Moreover, (still in a previous work), the fatigue test was simulated by the Dual Boundary Element Method (DBEM) in a bidimensional approach. Now, in order to validate the assumptions made in the aforementioned DBEM approach and concerning the delamination area size and the fiber integrity during crack propagation, three-dimensional BEM and FEM submodelling analyses are realized. Due to the lack of experimental data on the delamination area size (normally increasing as the crack propagates), such area is calculated by iterative three-dimensional BEM or FEM analyses, considering the inter-laminar stresses and a delamination criterion. Such three-dimensional analyses, but in particular the FEM proposed model, can also provide insights into the fiber rupture problem. These DBEM-BEM or DBEM-FEM approaches aims at providing a general purpose evaluation tool for a better understanding of the fatigue resistance of FML panels, providing a deeper insight into the role of fiber stiffness and of delamination extension on the stress intensity factors.

Thermo-mechanical crack propagation in aircraft engine vane by coupled FEM-DBEM approach

New generation jet engines are subject to severe reduced fuel consumption requirements. This usually leads to thin components in which damage issues such as thermo-mechanical fatigue, creep and crack propagation can be quite important. The combination of mechanical and thermal stresses usually leads to mixed-mode loading. Consequently, a suitable crack propagation tool must be able to predict mixed-mode crack propagation of arbitrarily curved cracks in three-dimensional space. To tackle this problem a procedure has been developed based on a combined FEM (finite element method) - DBEM (dual boundary element method) approach. Starting from a three-dimensional FEM mesh for the uncracked structure a subdomain is identified, in which crack initiation and propagation are simulated by DBEM. Such a subdomain is extracted from the FEM domain and imported, together with its boundary conditions (calculated by a previous thermal-stress FEM analysis), in a DBEM environment, where a linear elastic multiple crack growth analysis is performed. Once the crack propagation direction is determined a new crack increment can be calculated and, for the new crack front, the procedure can be repeated until failure. The proposed procedure also allows the consideration of the spectrum effects and creep effects: both conditions determine residual stresses that the crack will encounters during its propagation. The procedure has been tested on a gas turbine vane, getting sound results, and can be made fully automatic, thanks to in house made routines needed to facilitate the data exchange between the two adopted codes.

Multiple Crack Propagation with Dual Boundary Element Method in Stiffened and Reinforced Full Scale Aeronautic Panels

In this work, the performance of a new methodology, based on the Dual Boundary Element Method (DBEM) and applied to reinforced cracked aeronautic panels, is assessed. Such procedure is mainly based on two-dimensional stress analyses, whereas the three-dimensional modelling, always implemented in conjunction with the sub-modelling approach, is limited to those situations in which the so-called secondary bending effects cannot be neglected. The connection between the different layers (patches and main panel) is realised by rivets: a peculiar original arrangement of the rivet configuration in the two-dimensional DBEM model allows to take into account the real in-plane panel stiffness and the transversal rivet stiffness, even with a two dimensional approach. Different in plane loading configurations are considered, depending on the presence of a biaxial or uniaxial remote load. The nonlinear hole/rivet contact, is simulated by gap elements when needed. The most stressed skin holes are highlighted, and the effect of through the thickness cracks, initiated from the aforementioned holes, is analysed in terms of stress redistribution, SIF evaluation and crack propagation. The two-dimensional approximation for such kind of problems is generally not detrimental to the accuracy level, due the low thickness of involved panels, and is particularly efficient for studying varying reinforcement configurations, where reduced run times and a lean pre-processing phase are prerequisites.The accuracy of the proposed approach is assessed by comparison with Finite Element Method (FEM) results and experimental tests available in literature.This approach aims at providing a general purpose prediction tool useful to improve the understanding of the fatigue resistance of aeronautic panels.KEYWORDSDBEM, full scale aeronautic panel, 2D/3D crack growth, MSD, doubler-skin assembly, damage tolerance

DBEM and FEM Analysis of an Extrusion Press Fatigue Failure

This paper presents an application of the Dual Boundary Element Method (DBEM) to the simulation of a fatigue crack propagation affecting the main cylinder of an extrusion press for aluminum sections. The crack initiates at the inner surface of the cylinder and propagates through the thickness causing a leakage of the pressurized oil and consequent production stop. The fatigue load is induced by the pressure variation inside the cylinder as needed to push each section through the extrusion hole. The aim of the simulation is to assess the most probable initial crack dimensions that, after the recorded in service fatigue cycles, lead to the final crack scenario. This was requested in order to assess if there was a rogue detectable flaw introduced by the manufacturing process. For validation purposes, the DBEM numerical results in terms of Stress Intensity Factors (SIFs) in the initial cracked configuration are compared with corresponding Finite Element Method (FEM) results. DBEM SIFs are calculated by both J-integral and Crack Opening Displacement (COD) approaches, whereas for FEM SIFs only COD is used.

Assessment of Crack Growth from a Cold Worked Hole by Coupled FEM-DBEM Approach

The main objective of the present work is the study of the effect of residual stresses, induced by a cold working split sleeve process, on the fatigue life of a holed specimen. The crack propagation is simulated by a two-parameters crack growth model, based on the usage of two threshold material parameters (ΔKth and Kmax,th) and on the allowance for residual stresses, introduced on the crack faces by material plastic deformations. The coupled usage of Finite Element Method (FEM) and Dual Boundary Element Method (DBEM) is proposed to simulate the crack propagation, in order to take advantage of the main capabilities of the two methods. The procedure is validated by comparison with experimental results (crack growth rates and crack path) available from literature, in order to assess its capability to predict the crack growth retardation phenomena.

Micro- and macro-failure models of heterogeneous media with micro-structure

Abstract In this paper, the effectiveness of homogenization techniques for media with micro-structure subject to large deformations has been studied by comparing their micro- and macro-failure mechanisms. The material has been studied by considering its representative volume element (RVE) which entails all the geometric and constitutive information of the micro-structure. First, the formulation of the elastostatic problem governing the non-linear (large deformation and non-linear elastic) behavior of the structure of the RVE is presented. The RVE is subject to loading paths that produce uniform macroscopic strains . In this way it has been possible to use an homogenization procedure in order to simulate the overall behavior of the material, i.e. its constitutive tensor, at each point of the equilibrium path. Then, a macro failure surface has been defined as the locus of the points, in the macroscopic stretches space, corresponding to the loss of positivity of the macroscopic fourth order constitutive tensor in terms of the Biot stress [Encyclopedia of Physics, vol. 3(3), Springer-Verlag, Berlin]. Further, a micro-failure surface is defined as the locus of the points, in the overall stretches space, corresponding to the first critical point detected along the equilibrium path which can be characterized by an eigenmode compatible with the boundary conditions. Finally, a representative volume element, schematized by plane rods with strongly non-linear elastic constitutive behavior, is considered and the corresponding micro- and macro-failure surfaces are obtained in order to validate the proposed methodology.

A consistent use of the Gurson-Tvergaard-Needleman damage model for the R-curve calculation

The scope of the present work is to point out a consistent simulation procedure for the quasi-static fracture processes, starting from the micro-structural characteristics of the material. To this aim, a local nineparameters Gurson-Tvergaard-Needleman (GTN) damage law has been used. The damage parameters depend on the micro-structural characteristics and must be calculated, measured or opportunely tuned. This can be done, as proposed by the author, by using an opportunely tuned GTN model for the representative volume element simulations, in order to enrich the original damage model by considering also the defect size distribution. Once determined all the material parameters, an MT fracture test has been simulated by a FE code, to calculate the R-curve in an aeronautical Al-based alloy. The simulation procedure produced results in a very good agreement with the experimental data.

Modelling the mechanical behaviour of metal powder during Die compaction process

In this work, powder compaction process was investigated by using a numerical material model, which involves Mohr-Coulomb theory and an elliptical surface plasticity model. An effective algorithm was developed and implemented in the ANSYS finite element (FEM) code by using the subroutine USERMAT. Some simulations were performed to validate the proposed metal powder material model. The interaction between metal powder and die walls was considered by means of contact elements. In addition to the analysis of metal powder behaviour during compaction, the actions transmitted to die were also investigated, by considering different friction coefficients. This information is particularly useful for a correct die design.