## 1 Introduction  
Energy-absorbing structures are encountered in many real-life situations, which involves an exchange of compressive loads between two or more objects. Critical equipment or occupants should be protected from the resulting energy transfer using dedicated components that absorb the crush energy. Composite materials have been extensively used in recent years for energy absorption applications because of their lightweight, corrosion resistance and high energy absorption characteristics [1]. When designed accordingly, composite materials crush under the compressive load and therefore absorb a significant amount of energy transfer that stems from compressive loads. For example, in the automotive and aerospace industry, thin-walled tubes with different types of filler are commonly used as passive energy-absorbing sacrificial structures [2, 3]. Composite tubes for energy absorption applications are positioned to bear the load in the uniaxial direction, however, in some applications such as helicopter skid landing gears, lateral loading is necessary in addition to bending loads [4]. The design of passive energy-absorbing structures, such as tubes, aims to dissipate a fixed amount of energy by crushing at an invariable load over an increasing stroke (crushing distance) without exhausting the crushing stroke. In the case of consuming all stroke, i.e., when the tube \"bottoms out\", high peak loads emerge [5]. The high peak loads and violent fluctuations in loads increase the risk of whiplash [6]. To prevent this undesirable consequence and increase energy absorption capacity, auxetic materials have been considered [7]. Thanks to their negative Poisson's ratio, auxetic materials are useful in energy absorption since they accumulate more material under the crush region as opposed to conventional materials. Numerous studies have been conducted to understand and identify the crushing behavior of composite materials, and especially their failure mechanisms, which heavily depend upon the fiber and matrix properties [8; 9; 10]. A significant number of studies have also appeared in the literature about the effect of the stacking sequence [11; 12; 13; 14]. [PERSON] et al. [1] suggested that the overall energy absorption increases when fibers parallel to the loading direction were laterally supported, such as in the case of the [0/90/0/90] s layup. Besides unidirectional composites, the energy absorption and damage resistance capabilities of 3D woven composites under crushing load after multiple impacts [15] and the effect of different architectures of 3D carbon fiber woven composites were investigated [16]. Several design aspects are also critical in the crush resistance of composites. Tubes with chamfered, inward folding or outward-splaying crush-caps, or the combination of the two (chamfered-end and crush-cap) were investigated as designs with potential embedded failure triggering mechanisms. The objective of the trigger mechanism is to create a local region to elevate stress at one tip of the tube for failure initiation and disseminate it to the rest of the tube. As a result, the stable progressive crushing is promoted while an instant catastrophic failure is avoided. Geometric aspects, such as the tube thickness (t) and the outer diameter (D), have been mentioned as important parameters related to the energy absorption and deformation modes of carbon fiber/PEEK composite tubes [17; 18; 19]. [PERSON] [20] showed the presence of a nonlinear variation of the energy absorption versus D/t ratio. [PERSON] and [PERSON] [21] observed that higher specific energy absorption (SEA) could be obtained using 2-3 mm thickness tubes. [PERSON] et al. [22] also demonstrated that one can prevent the global buckling and the catastrophic failure of axially crushed composite circular cross-section tubes by maintaining the D/t ratio between 15 and 40. The crush tube length is another critical parameter, which should be determined by considering the crash speed.