R. M. F. Paulo

Integrated design and numerical simulation of stiffened panels including friction stir welding effects

Stiffened panels are usually the basic structural building blocks of airplanes, vessels and other structures with high requirements of strength-to-weight ratio. They typically consist of a plate with equally spaced longitudinal stiffeners on one side, often with intermediate transverse stiffeners. Large aeronautical and naval parts are primarily designed based on their longitudinal compressive strength. The structural stability of such thin-walled structures, when subjected to compressive loads, is highly dependent on the buckling strength of the structure as a whole and of each structural member. In the present work, a number of modelling and numerical calculations, based on the Finite Element Method (FEM), is carried out in order to predict the ultimate load level when stiffened panels are subjected to compressive solicitations. The simulation models account not only for the elasto-plastic nonlinear behaviour, but also for the residual stresses, material properties modifications and geometrical distortions that arise from Friction Stir Welding (FSW) operations. To construct the model considering residual stresses, their distribution in FSW butt joints are obtained by means of a numerical-experimental procedure, namely the contour method, which allows for the evaluation of the normal residual stress distribution on a specimen section. FSW samples have been sectioned orthogonally to the welding line by wire electrical discharge machining (WEDM). Displacements of the relaxed surfaces are then recorded using a Coordinate Measuring Machine and processed in a MATLAB environment. Finally, the residual stress distribution is evaluated by means of an elastic FE model of the cut sample, using the measured and digitalized out-of-plane displacements as input nodal boundary conditions. With these considerations, the main goal of the present work will then be related to the evaluation of the effect of FSW operations, in the ultimate load of stiffened panels with complex cross-section shapes, by means of realist numerical simulation models.

Influence of friction stir welding effects on the compressive strength of aluminium alloy thin-walled structures

The main objective of the present work is to investigate the effect of the residual stresses originated by the friction stir welding (FSW) process in the compressive strength of aluminium alloy plates. The finite element method (FEM) is used to simulate the welding process and calculate the distribution of the residual stresses. The model is validated using a residual stress map obtained by means of the contour method from a friction stir welded AA2024-24 plate. The results from the welding simulation were then used to numerically assess the influence of the residual stresses on the collapse load of the plate.

Numerical Simulation of AA2024-T3 Friction Stir Welding (FSW) Process: Sensitivity Analyses and Influence of Decisions during Modelling Stages

The main objective of the present work is to assess the influence of several parameters relevant for Finite Element Analysis (FEA) in modelling Friction Stir Welding (FSW) processes on AA2024-T3 plates. Several tests were performed including variations on the type of shell elements, number of integration points across thickness direction and mesh refinement levels, aiming for good accuracy and low computational cost. On the one hand, several setups of the mechanical boundary conditions, modelling the clamping systems, were also tested, leading to the conclusion that the results, in terms of longitudinal residual stresses, are significantly affected by this factor. On the other hand, variations on the heat input distribution showed a reduced effect, or almost null, on the final results.

Numerical simulation of friction stir welding (FSW): Prediction of the heat affect zone using a softening model

In this work a numerical model is proposed to simulate Friction Stir Welding (FSW) process in AA2024-T3 plates. This model included a softening model that account for the temperature history and the hardness distribution on a welded plate can thus be predicted. The validation of the model was performed using experimental measurements of the hardness in the plate cross-section. There is an acceptable prediction of the material softening in the Heat Affected Zone (HAZ) using the adopted model.

A reliability study on the structural performance prediction of reinforced aluminium panels with respect to variations in input parameters of numerical simulations

Stiffened panels are composed of a base plate with stiffeners in one or more directions, leading to lightweight structures with high resistance. The structural design, in most cases, focuses mainly on the longitudinal compressive loads that the panels are subjected and can safely withstand. In the present work, a set of Finite Element Method Analyses (FEA) were carried out, using ABAQUS commercial simulation software, and compared with experimental data in order to infer about the sensitivity of the results to the initial geometrical imperfections (either in magnitude and shape). The developments in the present work aim to provide a range of models able to properly reproduce the experimental behaviour of aluminium stiffened panels subjected to compressive loads. It was shown that FEA using shell finite elements were able to obtain accurate predictions of the ultimate load, considering large deformation and elasto-plastic behaviours. The effect of using different shapes and magnitudes of the initial geometrical imperfections on the numerical simulation of the panels was also inferred and tested using previously obtained eigenvalue (EV) buckling modes.