Effects of Initial Geometrical Imperfection on Square Tube Collapse

Random geometric imperfections are natural in structures. The initial imperfections used to be ignored in structure strength analysis and thus geometric-perfect models were used in most case of numerical simulation. However, collapse of axially compressed square tubes is not such a case. LS-DYNA is used to simulate the effects initial geometrical imperfection has on square tube collapse. This study proves that dynamic progressive buckling of square box columns is sensitive to initial geometrical imperfections. The simulation results show that ideal square tubes tend to buckle in extensional mode, though is not likely to happen in experimental studies. Previous theoretical analysis suggests, that from the view of energy absorption extensional mode is a dynamic procedure of higher energy absorption characteristics than that of each of symmetric and asymmetric mode of square tube in case of high c/h. This phenomenon suggests that extensional mode is an unstable equilibrium that will easily change to another equilibrium – symmetric mode. In a real world, geometrical imperfection renders extensional mode almost unachievable for hollow square tubes. Three kinds of imperfections: deflection of wall, thickness deviation and length of section side unequal were discussed in this paper. The amplitude of imperfection was compared with the geometry tolerance. Numerical simulations are then performed using LS-DYNA. Compared with the experimental datum, deflection of wall is the main reason for the predominance of symmetric mode of axially impacted hollow square tubes. Several characteristic values with regard to the amplitude of wall deflection are discussed in particular. It is found that when the λcr , the initial impact force peak amplitude of deflection is less than a certain critical value value and the critical buckling load are almost the same and unchanged at a determined impact velocity. When deflection exceeds the critical value, buckling take place in elastic area and critical buckling force drops quickly. Energy absorbed before buckling also quickly drops to near zero when deflection is considerably large.