FEM simulation of a crack propagation in a round bar under combined tension and torsion fatigue loading

An edge crack propagation in a steel bar of circular cross-section undergoing multiaxial fatigue loads is simulated by Finite Element Method (FEM). The variation of crack growth behaviour is studied under axial and combined in phase axial+torsional fatigue loading. Results show that the cyclic Mode III loading superimposed on the cyclic Mode I leads to a fatigue life reduction. Numerical calculations are performed using the FEM software ZENCRACK to determine the crack path and fatigue life. The FEM numerical predictions have been compared against corresponding experimental and numerical data, available from literature, getting satisfactory consistencyN/

Crack propagation calculations in aircraft engines 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 thermomechanical fatigue, creep and crack propagation can be quite important. The combination of stresses due to centrifugal loads 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 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 crack growth calculation 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 allows to also consider the spectrum effects and the creep effects: both conditions relieve residual stresses that the crack 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

Stress Analysis of an Endosseus Dental Implant by BEM and FEM

In this work the Boundary Element Method (BEM) and the Finite Element Method (FEM) have been used for an elastic-static analysis of both a Branemark dental implant and a generic conic threaded implant, modelled either in the complete mandible or in a mandibular segment, under axial and lateral loading conditions. Two different hypotheses are considered with reference to degree of osteo-integration between the implant and the mandibular bone: perfect and partial osteointegration. The BEM analysis takes advantage of the submodelling technique, applied on the region surrounding the implant. Such region is extracted from the overall mandible and the boundary conditions for such submodel are obtained from the stress analysis realised on the complete mandible. The obtained results provide the localisation of the most stressed areas at the bone-implant interface and at the mandibular canal (containing the alveolar nerve) which represent the most critical areas during mastication. This methodology, enriched with the tools necessary for the numerical mandible reconstruction, is useful to realise sensitivity analysis of the stress field against a variation of the localisation, inclination and typology of the considered implant, in order to assess the optimal implant conditions for each patient under treatment. Due to the high flexibility in the pre- and post-processing phase and accuracy in reproducing superficial stress gradients, BEM is more efficient than FEM in facing this kind of problem, especially when a linear elastic constitutive material law is adopted

ANALISI BEM DELLE SOLLECITAZIONI A CARICO DEL NERVO TRIGEMINO CAUSATE DA IMPIANTI DENTALI MULTIPLI

In this work an elastic-static analysis is developed in order to assess how the masticatory loads, applied on a system of multiple independent implants in a mandible, are transferred to the trigeminal nerve. The simulation is based on the Boundary Element Method (BEM) and consider a perfect osteo-integration between implant and bone (continuity condition for traction and displacement at the implant-bone interface). The analysis considers the mandible segment containing two adjacent implants at different distances, each one loaded with a vertical load. Both the cortical and spongy bone are modeled as transversally isotropic. A non linear contact analysis is needed in order to properly model the nerve-canal interactions. The results obtained show the pressure on the trigeminal nerve against a varying distance between implants. Such information is useful for a correct surgical planning

FEM-DBEM procedure for crack analysis in baffle module of Wendelstein 7-X

“Wendelstein 7-X” is the world’s largest nuclear fusion experiment of “stellarator type” in which a hydrogen plasma is confined in a magnet field, generated with external superconducting coils, allowing the plasma to be heated up to fusion temperature. The water-cooled Plasma Facing Components (PFC) protect the Plasma Vessel (PV) against radiative and convective heat from the plasma. After the manufacturing process of the heat shields and baffles, several cracks have been found in the braze and in the cooling pipes. Due to heat loads occurring during each Operational Phase (OP), thermal-stresses are generated in the heat sinks, brazes and cooling pipes, that encourage cyclic crack-growth and, eventually, the water leak through the pipes. The aim of this study is to predict the operational limits of the baffles and heat shields under cyclic heat loads, by using a numerical model based on a FEM-DBEM approach, in order to provide an assessment on the risks of premature failure for segments assembled in the PV

A computational strategy for damage-tolerant design of hollow shafts under mixed-mode loading condition.

Three‐dimensional numerical analyses, using the finite element method (FEM), have been adopted to simulate fatigue crack propagation in a hollow cylindrical specimen, under pure axial or combined axial‐torsion loading conditions. Specimens, made of Al alloys B95AT and D16T, have been experimentally tested under pure axial load and combined in‐phase constant amplitude axial and torsional loadings. The stress intensity factors (SIFs) have been calculated, according to the J‐integral approach, along the front of a part through crack, initiated in correspondence of the outer surface of a hollow cylindrical specimen. The crack path is evaluated by using the maximum energy release rate (MERR) criterion, whereas the Paris law is used to calculate crack growth rates. A numerical and experimental comparison of the results is presented, showing a good agreement in terms of crack growth rates and paths.N/

Integrated system for monitoring the initiation and propagation of fatigue cracks

Damage tolerant design requires accurate prediction of fatigue crack growth under service conditions and typically this is accomplished with the aid of numerical codes and experimental tests on simple specimens. In this work an in house made integrated system for automated fatigue tests is presented: such system is aimed to provide crack initiation detection, propagation monitoring and remote test machine control. The initiation is automatically devised by the variation of a strain gauge signal. The strain gauge is placed on the specimen close to the most probable initiation site and bonded in such a way to measure the strain in that specific direction which enhances its sensitivity to crack length variations. The crack length is correlated to the measured strain by means of a previous calibration based on a numerical simulation with the Boundary Element method (BEM). The automatic crack length assessment is provided in a twofold manner, with different level of accuracy: the strain gauge measurement is directly converted in the crack length, via an in house made Labview software and the aforementioned calibration; such measurement is used to control the movement of a micrometric motion system that keeps the camera aligned to the crack tip in order to grab focused images of the advancing crack, that, post-processed by an image analysis software, will provide a more accurate crack length assessment. The remote machine control, by a personal computer, is realized by means of a Labview software and a GPIB (General Purpose Interface Bus) hardware. The whole system is applied to assess the crack retardation for a specimen undergoing a fatigue load spectrum

A FEM based methodology to simulate multiple crack propagation in friction stir welds

In this work a numerical procedure, based on a finite element approach, is proposed to simulate multiple three-dimensional crack propagation in a welded structure. Cracks are introduced in a friction stir welded AA2024-T3 butt joint, affected by a process-induced residual stress scenario. The residual stress field was inferred by a thermo-mechanical FEM simulation of the process, considering temperature dependent elastic-plastic material properties, material softening and isotropic hardening. Afterwards, cracks introduced in the selected location of FEM computational domain allow stress redistribution and fatigue crack growth. The proposed approach has been validated by comparison with numerical outcomes provided by a consolidated FEM-DBEM procedure, available in literature. The discussed procedures are substantially equivalent in terms of SIFs evaluation along the crack front at the cracks insertion, as well as with respect to crack sizes measured in three different points for each propagation step. This FEMbased approach simulates the fatigue crack propagation by considering accurately the residual stress field generated by plastic deformations imposed on a structural component and has general validity