Elsevier

Thin-Walled Structures

Volume 59, October 2012, Pages 87-96
Thin-Walled Structures

Effects of buckling initiators on mechanical behavior of thin-walled square tubes subjected to oblique loading

https://doi.org/10.1016/j.tws.2012.03.002Get rights and content

Abstract

Thin-walled structures usually collapse in Eulerian buckling mode under oblique loads. Energy absorption capacity and crush force efficiency of the structure in this type of collapse are low. Collapse initiators are used to improve these properties. In this research, effect of collapse initiators on energy absorption characteristics of square tubes under oblique quasi-static loads is investigated both experimentally and numerically. Initiators are in the form of cuttings on the tube corners. Results show that collapse initiators in most of the specimens change deformation mode from general buckling to progressive buckling and decrease considerably the peak load; therefore increase crush force efficiency. Furthermore, effect of location and number of initiators is studied. There is good agreement between the numerical results and data from experiments.

Highlights

► Buckling initiators increase the crush force efficiency of the thin-walled tubes. ► Energy absorption for the highest initiator at the opposite of the first contact side is greater. ► The crush force efficiency of specimens whose initiators located at top end is greater. ► CFE of specimens with highest initiator at first contact side is greater for higher loading angles.

Introduction

Aluminum alloys are used widely in automotive industries due to their light weights, so that if they are used instead of steel causes about 25% reduction of weight and this in turn decreases fuel consumption and increases cleanliness of the surrounding [1]. Energy absorbers are designed in order to prevent or reduce the impact induced damages of main structures. They absorb energy by plastic deformation and this capability increases with the number of folds occur in collapse. There are many publications about pure bending and axial loading of thin-walled tubes and about the effects of various collapse initiators on the energy absorption and initial peak load of them.

Gupta and Gupta [2] investigated axial loading of annealed steel and aluminum tubes with different dimensions and various holes with different diameters, numbers and locations and showed that the holes change the collapse mode and prevent from general buckling of the tubes. Abramowicz and Jones [3] studied transmission of collapse process from general buckling to progressive buckling. They showed that increasing the width/thickness ratio increases the critical value of length/width ratio. Cheng et al. [4] used geometrical discontinuities as collapse initiators in square tubes and concluded that these initiators can reduce the first peak load between 5.2% and 18.7% and increase the crash force efficiency between 54.5% and 95.8%. Daneshi and Hosseinpour [5], [6] conducted quasi-static tests on tubes with peripheral grooves and compared the results with their proposed analytical method and concluded that it is possible to control the energy absorption characteristics by means of grooves distances. Zhang and coworkers [7], [8] studied the effect of buckling initiators on the first peak load of axially loaded thin-walled tubes. These initiators consist of a pre-hit column along the axis of the tube and several pulling strips around the top of the tube. They reported 30% decrease of the peak load at the optimum state but this was associated with decrease in energy absorption capacity. Adachi et al. [9] studied experimentally and numerically the effect of ribs on cylindrical tubes subjected to axial impact load. Their results showed that tube deformations depend on the ribs sections and distances; furthermore the tubes absorbed more energy with less deformations at the presence of ribs. Alavi Nia et al. [10] studied the effect of cracks on energy absorption characteristics of square and cylindrical tubes. They made these cracks in different situations and various angles and showed that the first peak load decreases in crack angles from 0 to 45°and increases for angles between 45°and 60°.

The previously mentioned studies were based on axial loading. Some other studies have considered pure bending [11], [12], [13]. In some cases the loads are a combination of these two cases and automotive industries need absorbers with capability of bearing loads in different angles. Recently, some researchers have studied oblique loading on energy absorbers; for example oblique loading of frusta [14], [15], [16], [17]. Ahmad et al. [16] investigated behavior of foam-filled frusta subjected to oblique load and showed that this specimen had better performance than empty frusta; besides these structures had good capabilities both in axial and oblique loading. Nagel et al. [17] compared the energy absorption of square and frusta tubes subjected to oblique loading and showed that frusta tubes had better response. Reyes et al. [1], [18], [19] conducted great researches in oblique loading field and studied experimentally and numerically quasi-static and dynamic oblique loading of square empty and foam-filled aluminum tubes. They investigated the effect of some parameters like the loading angle, thickness, length and temperature behavior of the tube on energy absorption capacity. Han and Park [20] studied numerically energy absorption characteristics of structures in oblique loading and showed that there is a critical angle of loading for transmission from axial to bending collapse. Heung et al. [21] tested prismatic specimens subjected to combination of compressive and bending loads and proposed a theoretical method for analyzing these tubes.

In this paper, quasi-static oblique loading of aluminum thin-walled tubes with buckling initiators is studied both experimentally and numerically. Buckling initiators, which are made in the form of cuttings at the specimen corners when inserted at angles 7°, 14° and 21° and the effects of load angle and initiators locations on crashworthiness characteristics are studied.

Section snippets

Samples specifications

Aluminum square tubes with a length of 150 mm and edge and thickness of 35 and 1.5 mm were used in this research. Fifteen types of these specimens were prepared so that three types were without any buckling initiator and reminder of them had initiators at different locations of the tube. Three samples of each type were tested; therefore total number of the samples was forty-five. Initiators were made in the form of cuttings at the corners of the samples. In this report, an initiator means two

Quasi-static tests results

Load-displacement curves of the tested specimens are shown in Fig. 5(a)–(c), belong to 7°, 14°and 21° loading angles, respectively. Experimental data are listed also in Table 2.

Peak load

Peak load, Pmax refers to initial maximum load during loading after which the first folding of the tube occurs. This is an important parameter in optimum design of energy absorbers and attempts are made to reduce its value with respect to residual energy absorption capacity. Fig. 6(b) shows Pmax for each type of specimen

Simulation

Oblique loading of square tubes with buckling initiator is analyzed using explicit software LS-DYNA. The finite element model of the tube constructed with shell elements and a rigid plate inserted the load; the load velocity and stroke are selected as in experiments. MAT-RIGID-TITLE and MAT-PIECWIES-LINEAR-PLASTICITY-TITLE are used to define material behavior of the plate and the tube, respectively. CONTACT-AUTOMATIC-SURFACE-TO-SURFACE and CONTACT-AUTOMATIC-SINGLE- SURFACE defined the contact

Conclusions

There are many published reports about characteristics of energy absorbing systems subjected to axial and bending loading. In some of these reports, effects of collapse initiators are studied. Some researches dealt with oblique loading of tubes but we did not find any report about initiator effect on improving energy absorption characteristics of tubes under oblique loading. In this research, the effects of buckling initiators in the cutting form on the mechanical behavior especially changing

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