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This paper deals with the simulation of multiple crack propagation in friction stir welded butt joints and the aim is to assess the influence of process induced residual stresses on the fatigue behavior of the assembly. The distribution of the process induced residual stresses is mapped by means of the contour method; then, the computed residual stress field is superimposed, in the DBEM environment, to the stress field due to a remote fatigue traction load and the crack growth is simulated. A two-parameters crack growth law is used for the crack propagation rate assessment. The Stress Intensity Factors are evaluated by the J-integral technique. Computational results have been compared with experimental data, provided by 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.

In this paper a numerical–experimental procedure useful to numerically assess the fatigue performance of friction stir welded aluminium joints is presented. The impact of manufacturing residual stresses on crack propagation in the joint driven by a remote fatigue load can be predicted. The proposed sequential procedure starts with the experimental residual stress assessment by the contour method and proceeds through the multiple crack growth simulation by the Dual Boundary Element Method. In the frame of Linear Elastic Fracture Mechanics, the superposition principle is invoked to provide the mathematical foundation supporting the proposed modelling strategy. In order to validate the proposed procedure, simple specimens are fatigue tested, obtaining multiple crack propagation scenarios that are monitored, in order to compare experimental and predicted crack growth rates.

The fatigue behavior of a riveted lap joint is analyzed with the Dual Boundary Element Method (DBEM). A Multiple Site Damage (MSD) scenario is obtained from the simultaneous initiation, from the most loaded holes, and propagation of different cracks. The analysis is bidimensional, with no allowance for secondary bending effects that are judged negligible, due to the reduced thicknesses of the involved plates. The lap joint considered has three rivet rows and undergoes a uniaxial fatigue load. When the cracks are short in comparison to plate thickness and hole diameters the allowance for nonlinear pin-rivet contact condition is provided. Crack faces are meshed using “discontinuous” quadratic elements and Stress Intensity Factors (SIFs) are calculated by the J-integral approach. The crack growth rate is calculated by the well-known Paris’ law, getting a satisfactory correlation between numerical and experimental findings (the latter available from literature).

This 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 J-integral 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.

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.

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 (ΔKthand 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.