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From:
Rui Miguel Ferreira Paulo, “Modelling of friction stir welding processes and their influence on the structural behaviour of aluminium stiffened panels”, Ph.D dissertation, Dept. of Mechanical Engineering, University of Aveiro, 2015
ABSTRACT: Aluminium stiffened panels are the basic elements of many structures with requirements of high strength/weight ratio. They are used in a wide variety of applications such as airplane wings and fuselage, ships and off-shore structures. Friction Stir Welding (FSW) process has been used in the manufacturing of such structures as an alternative of other joining techniques, such as riveting or fusion welding processes, with many advantages. The understanding of the effects of FSW processes in the structural behaviour of stiffened panels is a relevant research area, useful to avoid conservative design choices often motivated by an attempt to compensate for structural analysis uncertainties.
The present work focus on the numerical simulation of FSW processes, aiming to predict its effects, and also on the numerical simulation of the longitudinal compression of aluminium stiffened panels, including these effects. The numerical model for FSW was firstly developed to simulate single plates’ joining and subsequently validated using data from experiments, also performed in the scope of this work. The validated model was afterwards adapted to simulate FSW operations on stiffened panels. Sensitivity analyses performed showed an insignificant variation in the results coming from models using distinct heat input distributions related to the geometry of the toll. On the contrary, both models showed significant sensitivity to the variations of the mechanical boundary conditions that simulate the clamping system, although the single plate model was seen to be more affected by this modelling parameter.
The simulations regarding the longitudinal compression of the panels included the study of the influence of each one of the welding effects (distortion, residual stresses and material softening) on the structural behaviour. The numerical modelling of these welding effects was performed adopting two different procedures: using the results from FSW numerical analyses; and using a simplified methodology based on the literature. Regarding the compression of the panels, other modelling details were also tested and compared, such as: the numerical solving methodology; the plastic behaviour definitions of the material and the welding effects on the transversal edges. All the mentioned welding effects were able to affect the panel behaviour and, namely, the collapse load. Finally, a complementary study on initial geometrical imperfection was performed using distinct deformed shapes obtained from eigenvalue analyses and applied with different magnitudes. The variation of the collapse load with the increase of the imperfection magnitude revealed to be dependent on the shape of that imperfection.
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