Enhanced FEM-DBEM approach for fatigue crack-growth simulation

2016 - 2017To comply with fatigue life requirements, it is often necessary to carry out fracture mechanics assessments of structural components undergoing cyclic loadings. Fatigue growth analyses of cracks is one of the most important aspects of the structural integrity prediction for components (bars, wires, bolts, shafts, etc.) in presence of initial or accumulated in‐service damage. Stresses and strains due to mechanical as well as thermal, electromagnetical, etc., loading conditions are typical for the components of engineering structures. The problem of residual fatigue life prediction of such type of structural elements is complex, and a closed form solution is usually not available because the applied loads not rarely lead to mixed-mode conditions. Frequently, engineering structures are modelled by using the Finite Element Method (FEM) due to the availability of many well‐known commercial packages, a widespread use of the method and its well-known flexibility when dealing with complex structures. However, modelling crack-growth with FEM involves complex remeshing processes as the crack propagates, especially when mixed‐mode conditions occur. Hence, extended FEMs (XFEMs) and meshless methods have been widely and successfully applied to crack propagation analyses in the last years. These techniques allow a mesh‐independent crack representation, and remeshing is not even required to model the crack growth. The drawbacks of such mesh independency consist of high complexity of the finite elements, of material law formulation and solver algorithm. On the other hand, the Dual Boundary Element Method (DBEM) both simplifies the meshing processes and accurately characterizes the singular stress fields at the crack tips (linear assumption must be verified). Furthermore, it can be easily used in combination with FEM and, such a combination between DBEM and FEM, allows to simulate fracture problems leveraging on the high accuracy of DBEM when working on fracture, and on the versatility of FEM when working on complex structural problems... [edited by Author]XVI n.s. (XXX ciclo

fatigue crack growth in a compressor stage of a turbofan engine by fem dbem approach

Abstract In this work, the fatigue crack-growth process in a rotating disk of an aircraft gas turbine engine has been simulated. The considered crack nucleated in the attachment between a blade and the disk of a compressor stage, both made of a two-phase titanium alloy. The fatigue crack-growth process of such crack has been simulated by means of two codes, ABAQUS and BEASY, based on Finite Element Method (FEM) and Dual Boundary Element Method (DBEM) respectively. In particular, a variant of the submodelling technique, based on the superposition principle, has been used for coupling the two codes in order to exploit simultaneously their peculiar strength points. The FEM code has been used to compute the global stress field whereas the DBEM code has been used to calculate the fracture parameters, useful to predict the crack-growth evolution. The J-integral method and the Minimum Strain Energy Density Criterion (MSED) have been used for calculating K values and predicting crack kinking respectively. In this work, the FEM-DBEM crack path is compared with both the path obtained by a full-scale experimental test and the path predicted via a full FEM approach: having in an initial stage considered the only centrifugal load, with no allowance e.g. for the fluid pressure on the blades and for the blade dynamic behaviour, some discrepancies are found between numerical and experimental results. The computational advantages of the proposed submodelling approach are highlighted, in addition to a preliminary fatigue assessment provided for the considered compressor disk (further analyses are under development)

Multi-axial fatigue numerical crack propagation in cruciform specimens

Abstract Two cracks, initiated from the opposite tips of a 45° inclined central notch, were considered in cruciform specimens made of Ti6246. A static load was applied along an arm of the cruciform specimen together with an alternating load (R=-1) applied along the other arm. Experimental results were available from literature allowing a validation of the numerical procedure adopted to calculate the crack propagation paths and crack growth rates monitored during such tests. In particular, numerical evaluations by means of the Dual Boundary Element Method (DBEM) were performed, using the Minimum Strain Energy Density (MSED) criterion for the crack path assessment and J-integral approach for SIFs evaluations. Allowance for non-linear contact with friction was provided for those load cases in which contact between the crack faces occurred. Crack growth rates were predicted by using the Walker law, previously calibrated using the set of data coming from the first tested specimen. A good agreement between experimental and numerical crack paths was obtained. It was found that the cracks propagate without appreciable kinking, on the initial notch plane, for static loads lower than 10-15% of dynamic load amplitude, whereas the cracks develop perpendicular to the static load direction when the latter exceeds 25% of the dynamic load amplitude. Both propagation paths and crack growth rates were provided as a function of the static to dynamic load ratio

Design for NVH: topology optimization of an engine bracket support

Noise Vibration and Harshness (NVH) issues are proven to be the main drivers for customer dissatisfaction in the latest years. This work relies on the framework of Design For X (DFX), specifically, Design for NVH. Main goal of this work was to perform a Topology Optimization (TO) of an engine bracket based on its vibrational behavior, in order to reduce the vibrations transmitted from the engine to the chassis and, consequently, improving the comfort for passengers. In particular, the target function was defined with the aim of increasing the first natural frequency of the bracket, whereas the bracket mass reduction was considered as a constraint function for the TO process. The vibrational characterization of the bracket was based on Frequency Response Function (FRF) analyses which, conducted via FEM (Finite Element Method), allowed to identify the resonant frequencies of the different bracket configurations built up during the TO. The FEM models included the cylinder head, with the related engine bracket support under optimization; the latter is connected to the bracket on which the simulation load was applied. The TO turned out to be effective in lowering the mass of engine bracket support of nearly 20% and, at the same time, increasing the first natural frequency of nearly 10%, this latter result was sufficient to guarantee an improvement of the comfort for passengers

Multi-axial fatigue numerical crack propagation in cruciform specimens

Two cracks, initiated from the opposite tips of a central notch inclined by 45°, were considered in cruciform specimens made of Ti6246. A static load was applied to a cruciform arm while a cyclic load was applied along the other arm. Fatigue propagation of cracked specimens was performed by means of Dual Boundary Element Method (DBEM) and Finite Element Method (FEM) codes. For crack path assessment, the Minimum Strain Energy Density (MSED) and the Maximum Tensile Stress (MTS) criteria were adopted in DBEM and FEM approaches, respectively. Moreover, the J and M integrals’ formulations were used to evaluate the SIFs along the crack fronts for DBEM and FEM codes, respectively. Crack-growth rates were predicted by using a Walker law, calibrated on mode I fracture experimental data. A good agreement between numerical and experimental crack paths was obtained

fem dbem approach to simulate crack propagation in a turbine vane segment undergoing a fatigue load spectrum

Abstract In this work a thermo-mechanical fatigue application, related to a crack propagation in an aircraft turbine vane undergoing a complex load spectrum, is simulated. A computationally efficient FEM-DBEM submodelling approach, whose implementation leverages on the principle of linear superposition, is adopted. When tackling a crack propagation problem with a FEM-DBEM combined approach, the global analysis is generally worked out by FEM whereas the fracture problem is solved in a DBEM environment. In particular, a DBEM submodel is extracted from the global uncracked FEM model and, generally, is loaded on the boundaries with temperatures and either displacements or tractions; then the crack propagation is simulated by repeated thermal-stress DBEM analyses. Differently from that, the proposed equivalent approach solves the crack propagation problem by adopting a simpler pure stress DBEM analyses in which the boundary conditions, in terms of tractions, are just needed on the DBEM crack faces. Such tractions are evaluated by the FEM global analysis along a virtual surface traced by the advancing crack (the FEM model is uncracked). Such an approach provides accuracy enhancement and computational advantages

characterization of equivalent acoustic sources to reproduce the acoustic field generated by engines on an aircraft fuselage

Abstract This work presents a general procedure to characterize equivalent acoustic sources to reproduce the sound pressure field generated by the engines on an aircraft fuselage. The procedure would allow to set up ground experimental tests on aircraft components, by means of distributed loudspeakers, to obtain their vibro-acoustic performances as if they were tested in flight conditions. A FEM model of an aircraft fuselage mock-up was built up, comprising the structure, the internal acoustic cavities and the external air. The sound pressure field generated by the engines was considered as the reference solution, whereas an equivalent sound field, produced by distributed monopole sources surrounding the structure, was obtained by leveraging on the proposed Multi-Disciplinary Optimization (MDO) procedure. The MDO procedure was based on the mutual interaction between the commercial codes Siemens NX, for the CAE/FEM simulations, and Noesis Optimus, for the optimization framework

A Novel Optimization Framework to Replicate the Vibro-Acoustics Response of an Aircraft Fuselage

In this work, a novel optimization framework, based on a Multi-Disciplinary Optimization (MDO) procedure, applied to the vibro-acoustic Finite Element Method (FEM) model of an aircraft fuselage mock-up, is proposed. The MDO procedure, based on an Efficient Global Optimization (EGO)-like approach, is implemented to characterize acoustic sources that replicate the sound pressure field generated by the engines on the fuselage. A realistic sound pressure field, evaluated by aeroacoustic simulations, was considered as the reference acoustic load, whereas two equivalent sound fields, displayed by two different arrays of microphones and generated by the same configuration of monopoles, were calculated by the proposed vibro-acoustic FEM-MDO procedure. The proposed FEM-MDO framework enables to set up ground experimental tests on aircraft components, useful to replicate their vibro-acoustic performances as if tested in flight. More in general, such a procedure can also be used as a reference tool to design simplified tests starting from more complex ones

Experimental/Numerical Acoustic Assessment of Aircraft Seat Headrests Based on Electrospun Mats

The work proposes a methodology for the assessment of the performances of Passive Noise Control (PNC) for passenger aircraft headrests with the aim of enhancing acoustic comfort. Two PNC improvements of headrests were designed to reduce the Sound Pressure Level (SPL) at the passengers’ ears in an aircraft cabin during flight; the first was based on the optimization of the headrest shape, whereas the second consisted of partially or fully covering the headrest surface with a new highly sound-absorbing nanofibrous textile. An experimental validation campaign was conducted in a semi-anechoic chamber. A dummy headrest was assembled in different configurations of shape and materials to assess the acoustic performances associated to each set up. In parallel, simulations based on the Boundary Element Method (BEM) were performed for each configuration and an acceptable correlation between experimental and numerical results was obtained. Based on these findings, general guidelines were proposed for the acoustical design of advanced headrests