735 research outputs found
On the fatigue propagation of multiple cracks in friction stir weldments using linear and non-linear models under cyclic tensile loading
No full textIn this work, the propagation life of a friction stir-welded sample made of ductile materials is estimated by employing linear elastic fracture mechanics (LEFM) and small-scale yielding (SSY) conditions. The purpose is to demonstrate that by considering the SSY, the prediction of the propagation life of the welded sample can be improved when compared to the traditional LEFM. The process of friction stir welding (FSW) of an AA2024-T3 butt joint is then simulated by using the finite element method. Hence, a thermal analysis of the numerical model is performed, and the calculated temperature field is subsequently subjected to thermo-mechanical analysis. In the latter model, the two defects located in the most critical position, detected experimentally by performing fatigue tests on the same component, are introduced into the model by using the constrained crack faces technique. Furthermore, to enable the thermo-mechanical simulation of the FSW process, temperature-dependent non-linear material properties, material softening, and isotropic hardening are considered. Concerning fatigue crack growth analysis, three simulations of the fatigue crack propagation are performed by using three different propagation laws. The first is performed by considering linear elastic material properties and Vasudevan's law on fatigue crack propagation; the second is by employing non-linear material properties and Kujawski–Ellyin law; the third takes into account the non-linear material properties and UniGrow law. Thereafter, appropriate constraints and a remote fatigue load are applied to the specimen to allow residual stress redistribution and fatigue crack growth, respectively. The constraint effect is also evaluated by the calculation of the T-stress parameter. Finally, a comparison between the numerical and experimental results is presented; consequently, a better agreement with the case of the non-linear model under the SSY conditions is found
Crack closure in friction stir weldment using non‐linear model for fatigue crack propagation
No full textThe fatigue crack propagation in a friction stir-welded sample has been simulated herein by means of two 3-dimensional finite element method (FEM)-based analyses. Numerical simulations of the fatigue crack propagation have been carried out by assuming a residual stress field as a starting condition. Two initial cracks, observed in the real specimen, have been assessed experimentally by performing fatigue tests on the welded sample. Hence, the same cracks have been placed in the corresponding FE model, and then a remote load with boundary conditions has been applied on the welded specimen. The material behaviour of the welded joint has been modelled by means of the Ramberg-Osgood equation, while the non-linear Kujawski-Ellyin (KE) model has been adopted for the fatigue crack propagation under small-scale yielding (SSY) conditions. Owing to the compressive nature of the residual stress field that acts on a part of the cracked regions, the crack closure phenomenon has also been considered. Then, the original version of the KE law has been modified to fully include the closure effect in the analysis. Later, the crack closure effect has also been assessed in the simulation of fatigue propagation of three cracks. Finally, an investigation of the fracture process zone (FPZ) extension as well as the cyclic plastic zone (CPZ) and monotonic plastic zone (MPZ) extensions have been assessed
From test data to FE code: A straightforward strategy for modelling the structural bonding interface
No full textA straightforward methodology for modelling the cohesive zone (CZM) of an adhesively bonded joint is developed, by using a commercial finite element code and experimental outcomes from standard fracture tests, without defining a damage law explicitly. The in-house developed algorithm implements a linear interpolated cohesive relationship, obtained from literature data, and calculates the damage at each step increment. The algorithm is applicable both to dominant mode I or dominant mode II debonding simulations. The hypothesis of unloading stages occurrence is also considered employing an irreversible behaviour with elastic damaged reloading. A case study for validation is presented, implementing the algorithm in the commercial finite element method (FEM) software Abaqus®. Numerical simulation of dominant mode I fracture loading provides with satisfactory results
Multiple Crack Propagation in Friction Stir Welded Aluminium Joints
No full textThis paper is concerned with the simulation of crack propagation in friction stir welded butt joints, in order to assess the influence of process induced
microstructural alterations and residual stresses on the fatigue behaviour of the assembly. The approach employed is based on the coupled use of the finite element method and the dual boundary element method in order to take advantage of the main capabilities of the two methods. The distribution of the process induced residual stresses has been mapped by means of the contour method. Then, the computed residual stresses field has been superimposed, in a dual boundary element environment, to the stress field as a result of a remote fatigue traction load and the crack growth is simulated. A two-parameter crack growth law, based on the evaluation of two thresholds, for the material being analysed, is used for the crack propagation rate assessment. The stress intensity factors are evaluated using the Jintegral technique. Computational results have been compared with experimental data, provided from constant amplitude crack propagation tests on welded samples, showing the subdivision of the overall fatigue life in the two periods of crack initiation and crack propagation
3D non-linear MSD crack growth simulation for a riveted aeronautic reinforcement by DBEM
No full textEffects of temperature in novel crimp-free based composite materials for aerospace applications: An experimental assessment
No full textThis work shows the effects produced on mechanical behaviour by cold and elevated temperatures in a novel Non-crimped Fabric (NCF) composite for aerospace use. A mechanical characterization of the new composite was performed through several experimental tests. Composite plaques with different ply orientation, number of plies and thicknesses were produced through the Resin Transfer Moulding (RTM) process. Coupons required for mechanical characterisation of the composite were obtained from each of these plates using a well-designed plate cutting plane. The experimental tests were performed in a certified laboratory with electromechanical machines and according to ASTM standards. To characterize the mechanical behaviour of the composite material at elevated and cold temperatures, experimental tests were carried out in a controlled temperature environment and the results were compared with those obtained from previous tests at room temperature. This comparison was necessary to understand if the new composite can be used as a structural component for aerospace. Finally, an analysis of the variation coefficient was carried out, on the basis of the statistical average of the parameters calculated with the experimental tests, to evaluate the reproducibility of the tests in different laboratories
A unified approach to simulate the creep-fatigue crack growth in P91 steel at elevated temperature under SSY and SSC conditions
No full textThe Finite Element (FE) model of a Compact Tension C(T) specimen, made of P91 steel with different values of loading conditions and holding times, has been chosen for simulating the creep-fatigue crack propagation in high temperature (625 degrees C). Two FE-based commercial software have been used considering both the Small-Scale Yielding (SSY) and the Small-Scale Creep conditions (SSC) so that Low Cycle Fatigue (LCF) properties and the C(t) integral have been used to perform the numerical simulations of creep-fatigue crack propagation. Hence, the elastic-plastic material behaviour of P91 steel has been modelled by means of the Ramberg-Osgood equation while the creep behaviour has been modelled with the Norton's model. To calculate numerically the crack growth rates for the creep-fatigue crack propagation, a modified version of the UniGrow model has been adopted also considering the creep-fatigue interaction. Finally, numerical and the experimental results available in the literature have been compared with each other. This work presents a general methodology for the simulation of the creep-fatigue phenomenon. The method can also be applied to structural components with complex geometry and challenging load conditions
Non-linear models for assessing the fatigue crack behaviour under cyclic biaxial loading in a cruciform specimen
No full textThis work addresses FEM-based numerical simulations of fatigue crack propagation in a cruciform specimen under biaxial loading and small scale yielding (SSY) conditions. Three non-linear models are used as fatigue crack propagation laws. The cruciform specimens are made of aluminium alloy D16T and modelled with non-linear material properties. In the experimental tests, starting from a surface flow mechanically created in the centre of the cruciform specimen, biaxial loading conditions are applied to create various pre-cracked configurations that correspond to different values of initial crack aspect ratios. In the numerical simulations, the material behaviour is modelled with a low hardening bilinear law. Each simulation is carried out with one of the established non-linear laws. At the end of the fatigue crack propagation simulations, several crack aspect ratios are obtained. Finally, these aspect ratios are compared with the corresponding experimental tests available in the literature
A comparison between numerical and approximate methods for rapid calculation of NSIFs
No full textTo more rapidly predicting the integrity of components weakened by sharp notches, notch stress intensity factors (NSIFs) are usually evaluated. Many methods have been proposed evaluating NSIFs, ranging from the stress gradient based formulation to methods evaluating strain energy density averaged over a control region. Here, we compare different numerical and approximate methods applied to a square plate with a 45° tilted crack, positioned at the center of the plate. Hence, approximate methods need to FE solutions, obtained from models discretized with fine mesh or, alternatively, with coarse mesh, having to consider the mean value of the local Strain Energy Density (SED) to calculate NSIFs. Furthermore, 2D and 3D numerical simulations have been carried out to investigate the solutions provided by these analytical methods. For this aim, two software based on the Finite Element Method (FEM) and on the Dual Boundary Element Method (DBEM), respectively, have been adopted to solve the proposed study case of the square plate with a 45° tilted crack. These methods can supply accurate predictions of SIFs by means of J-integral calculation. Then, the NSIFs have been calculated at the intersection between the crack front and the free surface of the plate and, to assess the thickness effect on the provided solutions, they have also been calculated at half the thickness along the crack front and for increasing thickness values. Finally, the NSIFs obtained from the analytical and numerical methods have been compared each other to evaluate the level of agreement
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