Elsevier

Thin-Walled Structures

Volume 92, July 2015, Pages 48-54
Thin-Walled Structures

Plastic buckling of dented steel circular tubes under axial compression: An experimental study

https://doi.org/10.1016/j.tws.2015.02.018Get rights and content

Highlights

  • Dent׳s depth was the most important variable on the load capacity.

  • The closer the dent to the one end the less the ultimate capacity.

  • Axial stress flow lines deflected to the sides upon approaching the dent.

  • SG.1 and SG.2 recorded negative values whereas SG.3 showed positive.

  • The orientation of the dent seemed not to have a remarkable effect.

Abstract

This paper examines the effect of large local imperfections, known as dents, on the plastic buckling capacity of short steel tubes under axial compression. A total of 11 tests on such short columns were carried out. The specimens were indented through a separate process and the ultimate axial capacity was subsequently obtained through compression tests. Dent imperfections with various depths were introduced to different locations on the body of the specimens. Plastic buckling modes as well as the ultimate capacity of the specimens were thoroughly investigated. The adverse effect of such a local damage on the load carrying capacity was quantified for different values and types of imperfections.

Introduction

The very popular usage of CHS members as structural elements has stimulated many researchers to investigate the structural behavior of such elements. Many researches have focused on these elements under compression. Copious papers have also looked into the sensitivity of such structures to geometrical irregularities, also known as geometric imperfections. In some cases these elements are highly prone to physical contacts caused by collisions such as columns in car a parking, tubular members in offshore structures collided by supply boats and so forth.

A limited number of papers have, however, studied the dent imperfections in hollow sections. Most recently, Ghanbari Ghazijahani studied the buckling behavior of very thin cylindrical shells subject to axial stresses [1]. 27 specimens were tested in this study with different dents. It appeared that the capacity of such structures decreased due to the effect of the damaged areas. Buckling of dented short carbon steel cylindrical shells under axial loading was studied parametrically by Prabu et al. [2]. Angle, size and inclination of the local dent imperfection were investigated in this research. The effect of dent imperfections was examined by Ghanbari Ghazijahani et al. for slender shell specimens under pressure loading, which similarly led to the capacity reduction [3], [4], [5]. Bending capacity of dented CHS tubes was investigated through two respective studies by the authors [6], [7], and the failure modes as well as the ultimate capacity were thoroughly assessed for locally imperfect specimens. Indentation of ring-stiffened cylinders using wedge-shaped indenters was studied by Karroum et al. through both FE and an experimental research [8]. The failure location for all specimens was identified to be at a bulkhead of the specimens, where the plastic strain values were also evaluated.

Eyvazinejad Firouzsalari and Showkati studied steel dented tubes with D/t ratio of around 55 [9], [10], [11]. It was found that axial compression had a significant effect in some tests on the behavior of pipes against local lateral loading. Residual strength of dented structures was studied by Paik [12], which addressed the mechanics of dented members based on test observations and FE computations. An indentation process employing wedge-shaped indenters and a single spherical indenter was adopted to investigate tubular members [13]. The conducted tests were presented along with simplified analytical models which yielded closed-form expressions.

Despite the mentioned studies, relevant papers on local large imperfections seem insufficient to cover all aspects of structural behavior of shells with damaged geometries. To that end, this study aims to address the plastic buckling of steel CHS tubes under pure compression. The key aims of this study are to: (i) conduct the indentation by a proper V-shaped indenter, (ii) perform the main tests on the tubes under axial compression, (iii) evaluate the failure modes considering the presence of dent shape pre-deformation, (iv) discuss the load carrying capacity of the dented tubes considering different local imperfections, and (v) evaluate the results against previous data.

Section snippets

Specimens

All specimens were cold formed mild steel CHS tubes. The geometry of the specimens and the properties of the material used for fabrication of the tubes are given in Fig. 1 and Table 1. The material properties were obtained from tensile coupon tests and a stress strain curve obtained from this test is available in Ref. [14]. In Table 1, Table 2 Ltube and Dtube are the length and the diameter of the tubes. Additionally, de, Le and Wd are the depth of the dent, distance of the dent from the bottom

Observation of the test

The intact specimen, as expected, failed in elephant foot plastic buckling. The buckling bulge occurred approximately 15 mm from one end in a ring shape as an outward deformation. This end of the tubes deformed quite symmetrically.

Deformations for the dented specimens were initiated from the dented area such that the dents predominantly extended at the both ends towards the sides. This was further accompanied by the deepening of the dents. As deformations developed further, a U-shaped wave

Load carrying capacity of the dented specimens

Load bearing capacity of the present specimens is examined in this section. Fig. 12 presents the overall trend of the reduction in the capacity for locally imperfect specimens, which is plotted for different values of de (depth of the dent). Clearly the dent depth is the most important variable in determining the load capacity reduction. Table 3 lists the ultimate capacity of different specimens of TUD.2–11 with various dents as well as the capacity reduction in percentage. For the dent

Evaluations of the results against standards and other studies

A comparison of the results was drawn in this section on the basis of two parameters, viz. full-section strength, Psection, and slenderness parameter, λe. Note that these parameters along with non-dimensional buckling parameter, α, are used in different studies and mentioned in standards, in which Psection is obtained using the yield stress (fy=0.2% proof stress) [16], [17]. In Fig. 14, λe is plotted versus Psection. Slenderness and buckling parameters are defined asα=(E/fy)/(Dtube/t)λe=E250α

In

Summary and conclusion

Plastic buckling of dented CHS tubes under axial compression was investigated through an experimental study. In total 11 tests on short columns were carried out. Dent imperfections with various depths were introduced to different locations on the body of the specimens and ultimate axial strength was evaluated. The intact specimen TUD.1, as expected failed in elephant foot plastic buckling. The buckling bulge occurred approximately 15 mm from one end in a symmetric ring shape wave. Deformations

Acknowledgment

The efforts of UTAS workshop technicians in the School of Engineering and ICT in preparing the specimens and setting up the apparatus are greatly appreciated.

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