Behaviour of Aluminium Extrusions Subjected to Axial Loading
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This thesis deals with the transition from progressive to global buckling of axially loaded thin-walled aluminium extrusions. The behaviour of the extrusions was studied experimentally and numerically using the finite element code LS-DYNA. Material tests were performed to provide stress-strain characteristics of the material to be used in the numerical simulations. The transition between progressive and global buckling of axially loaded aluminium extrusions in alloy AA6060 temper T6 was studied by quasi-static and dynamic tests. The primary variables in the tests were the local (b/h = 17.78 - 40) and global (L/b = 5 - 24) slenderness of the extruded members and the impact velocity. The critical global slenderness is defined as the slenderness where direct global buckling or a transition from progressive to global buckling occurs. In the quasi-static tests the critical global slenderness was found to be an increasing function of the local slenderness. In contrast, the critical global slenderness was a decreasing function of the local slenderness when the impact velocity was 20 m/s. The energy absorption was found to be very dependent on the collapse mode. Significantly more energy is absorbed in the progressive buckling mode than in the global bending mode. In the case of transition from progressive to global buckling, the energy absorption depends on the time of transition. The difference in energy absorption between the different deformation modes decreases for increasing impact velocity. This is due to inertia forces preventing the direct global buckling mode and the early transition from progressive to global buckling. In addition to experimental tests, numerical simulations using LS-DYNA were carried out. A numerical model was validated against the experimental tests. Good agreement between the progressive buckling pattern in the numerical simulations and experimental tests was found. The numerical simulations were capable of giving a relatively accurate prediction of the collapse mode found in the experimental tests. However, the numerical model underestimated the mean force level found in the experimental tests. Thus, additional simulations were performed to investigate the influence from some key parameter on the mean force level. Stabilization of the behaviour and increasing the critical global slenderness by use of a trigger has been investigated. The introduction of a heat affected zone at the impacted end caused the progressive buckling to start from this end. This had a positive effect on the critical buckling length. Finally the maximum and mean forces found in the experimental tests were compared to existing analytical expressions and a design code. A relatively good agreement was found. The collapse modes observed in the quasi-static experimental tests were compared to analytical models for predicting the response of axially loaded aluminium extrusions. The analytical models for predicting the global buckling gave relatively accurate results.