Contents lists available at ScienceDirect Thin–Walled Structures journal homepage: www.elsevier.com/locate/tws Full length article Experimental and numerical investigation of dynamic plastic behavior of tube with different thickness distribution under axial impact R. Rajabiehfard a , A. Darvizeh b, ⁎ , M. Alitavoli a , H. Sadeghi a , N. Noorzadeh c , E. Maghdouri c a Department of Mechanical Engineering, University of Guilan, P.O. Box 3756, Rasht, Iran b Department of Mechanical Engineering, Faculty of Engineering, Islamic Azad University, Anzali Branch, Bandar Anzali, Iran c Department of Mechanical Engineering, Ahrar Institute of Technology & Higher Education, Rasht, Iran ARTICLE INFO Keywords: Tube Different thickness distribution Axial impact Energy absorption Progressive buckling ABSTRACT In this study, the dynamic behavior of uniform thickness, stepped thickness and functionally graded thickness tubes under axial impact are investigated experimentally and numerically. A striking mass has been used to impact tubes. Experimental tests are performed by using gas gun and numerical results are obtained by FE simulation. The effect of thickness distribution on shortening, energy absorption, axial force and buckling shape of tubes are investigated. It is found that a change in thickness distribution of tube can convert the buckling shape from buckling with mild folds to progressive buckling and vice versa. In addition, it is found that stepped thickness tube can be an approximate of functionally graded thickness tube which in this case almost, their behavior is identical. This study reveals that stepped thickness and functionally graded thickness tubes in comparison with uniform thickness tube absorb the same energy with more shortening and less peak load or less mean load; thus, they are better energy absorption specimens. With comparing experimental and numerical results, it is found that there is a good agreement between them. 1. Introduction Tubes have been becoming increasingly popular in different in- dustries due to their economic, lightness and efficiency. Because of their high energy absorption capacity, long crushing distance and high ratio of energy absorption into weight, one of their most important applications is in the energy absorption systems. Thus, investigation on dynamic behavior of tubes is significant. This paper aims to study the dynamic behavior of uniform thickness, stepped thickness and func- tionally graded thickness tubes under axial impact. The dynamic behavior of uniform thickness tubes was investigated experimentally [1], numerically [2–5] and theoretically [6–8] by different researchers. Florence and Goodier [1] experimentally inves- tigated the plastic buckling of tubes subjected to axial impact. In this paper, results of shortening, impact duration, half-wave number and buckling shape of tubes were reported. Finite element analysis was used to investigate the buckling of tubes subjected to axial impact [2–5]. By studying the effects of material properties, shell geometry, boundary and loading conditions on energy absorption and buckling shape of tubes by Karagiozova and Jones [2], it was found that a large proportion of the initial kinetic energy is absorbed through a shortening mechanism in steel tubes, whereas aluminum tubes absorb the energy with folding mechanism. Karagiozova and Jones [3] explored the influence of approximation of strain hardening modulus on the type of buckling shape (plastic or progressive buckling) and also the effect of axial inertia on initiation and development of buckling shape of tubes under axial impact. In the dynamic plastic buckling "the whole length of a tube wrinkles before the large radial displacements develop [3]", and in the dynamic progressive buckling "the folds in a tube form consecutively [3]". Energy absorption characteristics of tubes under axial loading were studied using nonlinear finite element software LS-DYNA by Tai et al. [4]. This study revealed that with assuming of equal cross sectional for two different materials, the energy absorption of tube made of high strength steel is higher than the energy absorption of tube made of mild steel. With investigation the effect of different parameters such as material properties, geometry and impact velocity on the peak load of the tube which was subjected to axial impact, an approximate equation for estimation of peak load at a relatively low velocity (v < 40 (m/s) ) 0 was proposed by Chen and Ushijima [5]. Lepik [6] studied the buckling improvement of elastic-plastic tubes under axial impact with Galerkin method. Axial impact of elastic- plastic tubes with linear strain hardening material was studied using a discrete model by Karagiozova and Jones [7]. It was found that inertia http://dx.doi.org/10.1016/j.tws.2016.08.017 Received 25 February 2016; Received in revised form 23 August 2016; Accepted 24 August 2016 ⁎ Corresponding author. E-mail addresses: adarvizeh@guilan.ac.ir, adarvizeh.guilan@gmail.com (A. Darvizeh). Thin--Walled Structures 109 (2016) 174–184 0263-8231/ © 2016 Published by Elsevier Ltd. crossmark