Διερεύνηση της μηχανικής συμπεριφοράς συγκολλητών διατομών αλουμινίου με υπολογιστικές μεθόδους

Abstract

The properties of aluminium alloys, low weight/strength ratio, their corrosion resistance, weldability and the easy shaping of aluminium into di erent contour shapes and sizes (extrusion process), enhance the use of aluminium in structural applications. The present dissertation concerns the investigation of some aspects of the behaviour of welded aluminium double T beams. The investigation was conducted by means of a shell nite element model, taking into account material nonlinearity. Geometrical nonlinearity is neglected. Aluminium alloys develop relatively medium strengths (up to 180 N=mm2), whereas heat treatment increases their strength substantially (up to 350 N=mm2). Heat treatments have no a ect in ductility (8-10%), Young Modulus (E=70000N=mm2) and other physical properties. As is well known, aluminium alloys exhibit a nonlinear stress-strain behaviour, characteristic for each aluminium alloy. In the literature, many stress-strain relationships are proposed. In the present work, the thorougly experimentally tested, Ramberg-Osgood law is used. The exponential # ## " law is given by the following expression: " = # E + 0:002 # f0:2!n (E.1) where E is the Young Modulus of elasticity (E=70000 N=mm2, f0:2 conventional yield limit corresponding to a permanent deformation 0f 0.2% and n hardening parameter characteristic for each alloy. Taking into account their varying hardening characteristics, and their mechanical behaviour in the heat a ected zone, three aluminium alloys were selected from table 3.2b of Eurocode 9, for the purposes of the numerical investigation: # 6063 T5, (f0:2=130 N=mm2, high hardening, ultimate elongation "t = 8%, and 50% reduction of strength in HAZ) # 6061 T6, (f0:2=240 N=mm2, low hardening, ultimate elongation "t = 8%, and 52% reduction of strength in HAZ) # 7020 T6,(f0:2=290 N=mm2, medium hardening, ultimate elongation "t = 10%, and 27% reduction of strength in HAZ) An important problem concerning the use of aluminium sections in structural applications is the lack in standard aluminium pro le series (like IPB, HEA, HEB in steel). Additionally, an unwanted but nertheless unavoidable side effect of the welding process in aluminium is the creation of the so-called Heat A ected Zone (HAZ) on the immediate vicinity of the weld due to changes in the microstructure of aluminium alloys. In the HAZ the 'conventional' yield strength f0:2 and ultimate strength fu of heat treated alloys are reduced up to 55%, whereas ductility and other mechanical characteristics remain unaffected. # ## # The most widely applied method for the estimation of the width of the heat a ected zone is the well-known 'one inch rule'. According to this approach, HAZ's width is extended one inch in all directions from the vicinity of the weld. Similar approaches are proposed in regulations (DIN4108). In Eurocode 9 the width of the HAZ is determined by the welding method and the weld size. In the present analysis, the more accurate and rational method, proposed by Richardson and Dwight and based on heat ow equations is used. Factors such as the size of the weld, the welding method, the thermal control of the welding procedure and uncertainities about the amount of the heat input (human factor) are herein considered. The alteration of the material behaviour due to the HAZ is introduced at a Gauss-point level in the model (the Castem2000 software was used for that). Although in the developed software the possibility of a transition zone inside HAZ was also considered, this was not implemented into the parent analysis due to a luck in necessary bibliographical data concerning the phenomenon. The results of the analysis show early failure of the welded beam due to the accumulation of shear deformation in the boundary between the HAZ and the una ected material in the areas of the co-existence of high bending and shear. This failure mechanism constitutes a decoupling of the cross section in three parts (web and two independent anges) and seems to take place far earlier than the initiation of any plastic ow in an extruded (i.e HAZ-free) cross section A low value of the Ramberg-Osgood exponent n is shown to have a benign e ect on the failure mechanism shown.