Plastic collapse and energy absorption of empty circular aluminum tube under transverse quasi-static loading

This paper presents an experimental investigation on plastic collapse and energy absorption of empty circular aluminum tubes under quasi-static transverse loading. Tubular structures being a critical demand as material saving, high energy absorption and good strength characteristics were of major concerns due to its wall thinness, and so, its various diameter-to-thickness (D/t) ratios and span lengths. Studies found that empty circular Al-tube structure subjected to transverse standard three-point bending loading undergone three plastic deformation phases, starting with crumpling phase, crumpling and buckling phase, and lastly the structural collapse. The results found that energy absorption of empty aluminum tubes for a constant D/t ratio decreases as span length. On the contrary, the energy absorption of empty aluminum tubes for a given constant span length increases with the increase in D/t ratio.     

泡沫填充薄壁结构的抗弯性分析及多目标优化设计*

泡沫铝填充薄壁结构具有轻质、较大承载能力以及高效吸能特性,越来越多地应用于各种工程结构。提出一种新颖的轴向梯度泡沫填充薄壁结构,采用试验与数值分析的方法,系统地分析空管、均匀泡沫填充及梯度泡沫填充薄壁圆管在弯曲工况下的力学响应及能量吸收特性。研究发现,泡沫填充薄壁结构比空管具有更好的抗弯性能。与均匀泡沫填充结构相比,梯度泡沫不仅使得填充薄壁结构的变形模式从单褶皱模式变为多褶皱模式,截面扁化量和抗弯刚度损失显著减小,而且有效地提高了填充结构的承载力及吸能特性。为了进一步探索填充结构的最优耐撞性,结合Kriging近似技术与粒子群数值优化方法,对均匀泡沫和功能梯度填充泡沫薄壁结构进行多目标优化设计,得到了泡沫填充薄壁结构耐撞性的最佳参数匹配设计,并有效提高了结构的抗弯性能,为泡沫填充薄壁结构抗弯性设计提供了参考依据。     

泡沫铝填充帽型结构轴向冲击吸能特性的试验研究

利用冲击试验系统,通过试验方法研究了泡沫铝填充帽型结构在轴向冲击工况下的吸能特性。首先进行了泡沫铝、空心帽型结构以及泡沫铝填充帽型结构的轴向冲击试验;然后根据试验结果,对泡沫铝填充帽型结构轴向冲击工况下的吸能特性进行了分析,评估了填充泡沫铝以及应变率对帽型结构吸能特性的影响。试验结果表明, 与空心结构相比,填充泡沫铝之后帽型结构的轴向压缩稳定性和吸能特性有明显的改善;由于材料对应变率敏感, 与准静态压缩相比,结构的吸能特性有一定的提高。     

Experimental studies on the axial crash behavior of aluminum foam-filled hat sections

Drop hammer tests were carried out to study the axial crash behavior of aluminum foam-filled hat sections. First, the axial crash tests of the empty hat sections, aluminum foam and the aluminum foam-filled hat sections were carried out; then, based upon the test results, the axial crash behavior of the aluminum foam-filled hat sections were analyzed. It was found that aluminum foam filling can increase the energy absorption capacities of the hat sections. Compared with the non-filled structures, aluminum foam-filled structures were much more stable and needed less mass to absorb the specified energy. __________ Translated from Chinese Journal of Mechanical Engineering, 2006, 42(4) (in Chinese)     

Axial crushing behavior of the intermittent tack-welded cylindrical tubes

The objective of this paper is to evaluate the effect of intermittent weldment of cylindrical tubes on the energy absorbing behavior under axial crushing. This paper describes the test results for cylindrical empty and foam-filled tubes and discussions of the improvement of energy absorbing efficiency by the sequential rupture of intermittent weldment. The weldment rupture of a cylindrical foam-filled tube reduces the peak values of crush load and increases the valley values, while the mean crush load is maintained at a similar level as in the fully welded tube. The weldment rupture of a cylindrical foam-filled tube improves the energy-absorbing efficiency by reducing the crush load amplitude without a loss of total energy absorption.     

Plastic collapse and energy absorption of circular filled tubes under quasi-static loads by computational analysis

This study presents the finite element analysis of plastic collapse and energy absorption of polyurethane-filled aluminium circular tubes under quasi-static transverse loading. Increasing focuses were given to impact damage of structures where energy absorbed during impact could be controlled to avoid total structure collapse of energy absorbers and devices designed to dissipate energy. ABAQUS finite element analysis application was utilized for modelling and simulating the polyurethane-filled aluminium tubes, different set of diameter-to-thickness ratios and span lengths, subjected to transverse three-point-bending load. Different sets of polyurethane-filled aluminium tubes subjected to the transverse loading were modelled and simulated. The failure modes and mechanisms of filled tubes and its capabilities as energy absorbers to further improve and strengthening of empty tube were also identified. The results showed that plastic deformation response was affected by the geometric constraints and parameters of the specimens. The diameter-to-thickness ratio and span lengths had shown to play crucial role in optimizing the PU-filled tube as energy absorber.     

Parametric study and numerical analysis of empty and foam-filled thin-walled tubes under static and dynamic loadings

In this paper the crushing behavior of thin-walled tubes under static and dynamic loading is investigated. First, a finite element (FE) model for empty thin-walled tube was constructed and validated by available experimental and numerical data. The comparison between the FE results and the existing numerical solutions as well as the available experimental results showed good agreements. Next, a model for the foam was adopted and implemented in an in-house FE code. The implemented isotropic foam model was then used to simulate the behavior of foam-filled tubes under both static and dynamic loadings. Good agreement was observed between the results from the model with those obtained by analytical relations and experimental test data. The validated FE model was then used to conduct a series of parametric studies on foam-filled tapered tubes under static and dynamic loadings. The parametric studies were carried out to determine the effect of different parameters such as the number of oblique sides, foam density and boundary conditions on crushing behavior of rectangular tubes. The characteristic included deformed shapes, load–displacement, fold length and specific energy absorptions.     

Axial crushing of foam-filled tapered sheet metal tubes

Following earlier work on the axial crushing of foam-filled sheet metal tubes of square and rectangular cross-section and empty tapered tubes the behaviour of foam-filled tapered tubes is considered. Theoretical estimates of the variation in the mean crushing loads for both quasi-static and dynamic loading conditions are provided and compared with experimental data.     

Axial crushing of wood-filled square metal tubes

The behaviour of thin tubes made of sheet metal and not so thin extruded tubes filled with wood and subjected to axial crushing is studied. Experiments show that the mode of elastic buckling is changed by the presence of the wood filler. The plastic crushing of thin tubes resulted in Euler-type buckling, while a considerable enhancement in the load carrying capacity and energy absorption was seen in the case of thicker walled tubes which were examined both under quasi-static and dynamic loading conditions. A new idealized deformation mechanism for the progressive crushing of the wood-filled tube is suggested and analysed. The results obtained for the mean loads agree reasonably with experimental observations. An alternative method employed to predict the mean crushing load of empty tubes and using a thickness that provides an equivalent stiffness to that of a filled tube is also seen to produce reasonable agreement with the experiment.     

Energy absorption of aluminum foam filled braided stainless steel tubes under quasi-static tensile loading conditions

A theoretical model to predict the energy absorption capabilities of aluminum foam filled braided stainless steel tubes under tensile loading conditions has been developed and is presented. Experimental testing was completed on braided tubes, with a nominal diameter of 64.5 mm and woven from 304 stainless steel wires with a diameter of 0.51 mm, filled with rectangular prisms of closed cell aluminum foam with densities ranging from 248 to 373 kg/m³. Based upon observations from experimental testing and applying a unit cell concept to the braided tube, a theoretical model which incorporates two stages of deformation was developed. Within the first stage of deformation, which occurs prior to tow lockup of the braided tube, energy absorption is primarily due to compression of the aluminum foam core. After tow lockup has occurred the energy absorption behavior of the assembly is a sole result of the deformation of the braided tube. Comparisons between the energy absorption predictions of the analytical model and experimental observations were found to be in good agreement for assembly lengths of approximately 400 mm. For the tensile loading conditions and geometry of aluminum foam filled braided tubes considered in this research energy absorption ranged from approximately 5.2 to 7.9 kJ with corresponding tube elongations of 400 mm.     

Crashworthiness of aluminum extrusions subjected to oblique loading: experiments and numerical analyses

Oblique loading was studied through quasi-static experiments and numerical simulations. The behavior of square aluminum columns in alloy AA6060 subjected to quasi-static oblique loading was investigated experimentally for three different load angles. The square columns were clamped at one end and oblique load conditions were realized by applying a force with different angles to the centerline of the column. These tests were used to validate a numerical model. Numerical studies of oblique impact were carried out using the validated model, and the mean crush load was investigated through factorial analysis with parameters as load angle, thickness, length, and heat treatment of the alloy and impact velocity.     

Optimization of functionally graded foam-filled conical tubes under axial impact loading

Metallic foams as a filler in thin-walled structures can improve their crashworthiness characteristics. In this article, nonlinear parametric finite element simulations of FGF foam-filled conical tube are developed and the effect of various design parameters such as density grading, number of grading layers and the total mass of FGF tube on resulting mode shapes, specific energy absorption and initial peak load is investigated. Multi design optimization (MDO) technique and the geometrical average method, both are based on FE model are applied to maximize the specific energy absorption and minimize the impact peak force by estimating the best wall thickness and gradient exponential parameter “m” that controls the variation of foam density. The results obtained from the optimizations indicated that functionally graded foam material, with graded density, is a suitable candidate for enhancing the crashworthiness characteristics of the structure compared to uniform density foam.     

Analytical and experimental investigations into the controlled energy absorption characteristics of thick-walled tubes with circumferential grooves

In this paper, the energy absorption characteristics of grooved circular tubes are investigated under quasi-static loading condition. For experiments, thick-walled tubes with circumferential grooves are prepared. The grooves divide the thick-walled tube into several shorter thin-walled portions. Specimens are subjected to axial crushing load to observe the effect of distribution of circular grooves on the deformation mechanism and energy absorption capacity. Geometrical parameters of the specimens are designed utilizing the Taguchi method to cover a reasonably wide range of groove length-to-wall thickness ratios. An analytical approach based on the concept of energy dissipation through the plastic hinges is applied. Taking the effect of strain hardening into account, the obtained analytical results are in good agreement with the experimental ones. The agreement between analytical and experimental results may indicate the validity of the proposed analytical approach. Desirable mechanism of deformation observed justifies the pre-forming method for obtaining favorable energy absorption characteristics.     

On functionally-graded crashworthy shape of conical structures for multiple load cases

Many studies on energy absorbers have been focused on tapered tubes because they have significant advantages in crashworthiness and provide a desired constant load-deflection response. However, few studies have been reported on tapered tubes with nonlinearlyvariable diameters along the longitudinal direction. This paper presents thin-walled Functionally graded tapered tube (FGTT) with a diameter varying nonlinearly subject to axial (0°) and oblique (10°, 20°, 30°) impacts. To explore the advantages of FGTT, conventional Straight/Conical circular tube (SCT/CCT) with the same mass are compared; and FGTTs with a gradient exponent n > 1 are found to be preferable to others in terms of energy absorption capacity under small impact angles. Then, crashworthiness analyses of different crushing distances are conducted and it is found that under a large impact angle (e.g. 20°, 30°), FGTT with a short crushing distance (e.g. 40 mm) have a higher mean crashing force than long crushing distance (e.g. 120 mm), especially for n > 1. In addition, the effect of geometric parameters, such as the gradient exponent n and diameter range ΔD between top (incident) and bottom (distal) diameters of FGTTs, are also studied and it is found that the FGTT with ΔD = 40 mm and n > 1 exhibits better crashworthiness than the others under small impact angles (0°, 10°). This paper demonstrates that such FGTT structures have a certain potential to be an energy absorber.

     

Experimental and numerical crashworthiness investigation of combined circular and square sections

Quasi-static experimental and nonlinear finite element analyses are performed to compare the energy absorption and initial peak load of combined circular and square sections with those of regular circular and square sections. The combined circular and square sections have higher energy absorption and lower initial peak load. These tubes can be widely used to ensure passenger safety in automotive and aerospace landing structures. The predicted numerical crushing load and fold pattern are in good agreement with experimental results. The specific energy absorption capability of the combined tube is significantly higher than that of the square tube and is close to that of the circular tube. The initial peak load of combined tube is significantly lower than that of the circular tube and somewhat lower than that of the square tube. Changing the section dimensions and their length results in higher energy absorption of the combined section than that of circular and square sections. Moreover, the initial peak load in the combined section is lower than that of the circular and square sections in all cases.     

Collapse of pressurized elastoplastic tubular members under lateral loads

The present work examines the collapse of tubular members subjected to lateral (transverse) quasi-static loading in the presence of uniform pressure. In particular, it investigates pressure effects on the ultimate lateral load of tubes and on their energy absorption capacity. External pressure is mainly considered, whereas internal pressure effects are also discussed. Tubes are modeled with shell finite elements, accounting for geometric and material nonlinearities. Relatively thick steel and aluminum tubes (D/t50), which exhibit significant inelastic deformations, are considered. Two-dimensional cases are examined first, where lateral loading is imposed by either two rigid plates or by two opposite radial loads. Three-dimensional cases are also analyzed, where the load is applied either through a pair of opposite wedge-shaped indenters or a single spherical indenter. The results are presented in terms of load–deflection curves for different levels of pressure, and indicate that the presence of pressure has significant effects on tube response. Deformed shapes of tubes are depicted and discussed, and comparison with test data from non-pressurized pipes is conducted. Finally, simplified analytical models are presented for two-dimensional and three-dimensional load configurations, which yield closed-form expressions, compare fairly well with the finite element results and illustrate some important features of tube response in an elegant manner.     

Torsional crushing of foam-filled thin-walled square columns

Torsional crushing behavior of foam-filled thin-walled square columns were investigated analytically, numerically and experimentally. The lower and upper bounds on the torsional resistance of foam-filled columns were established analytically. Numerical simulations were carried out and showed that the presence of the filler changes the torsional collapse mechanism and gives rise to higher order sectional collapse modes, which results in a higher torsional resistance. Torsional experiments were performed and results were compared to the analytical and numerical solutions with reasonably good agreement. It was found that bonding of the foam to the walls changes the deformation mode by spreading deformation over the whole length. The corresponding torsional resistance is also larger for the first 40° of rotation. It is concluded that fitting prismatic members with the aluminum foam of a density ranging from 0.14 to 0.28 g/cm³ can double the energy absorption of a given member.     

泡沫铝填充帽型结构轴向压缩吸能特性的试验研究

通过试验方法研究了泡沫铝填充帽型结构准静态压溃时的吸能特性。首先,进行了泡沫铝空心帽型结构以及泡沫铝填充帽型结构的轴向压缩试验;然后,根据试验结果,对泡沫铝填充帽型结构的轴向吸能特性进行了分析,并与空心帽型结构进行了比较。结果表明,填充泡沫铝之后,帽型结构的轴向压缩稳定性和吸能特性有了很大的提高。在吸收的能量一定时,泡沫铝填充能够减少吸能结构所需要的质量。     

Investigation of deformation and collapse mechanism for magnesium tube in axial crushing test

This paper demonstrates the quasi-static axial compression and high-speed axial compression tests of extruded magnesium alloy circular tubes for evaluating the crash and fracture behavior of mg parts. To capture the buckling and fracture behavior of Mg tube during the axial compression tests, FE simulation adopts different types of flow curves depending on the deformation mode such as tension and compression with LS-DYNA software. The Mg tube undergoes compressive plastic strain prior to buckling while according to the model based on Hill yield criterion only bulging along the radial direction is predicted. Due to the tension-compression asymmetry of Mg alloys, diameter of Mg tube expands largely at the initial plastic strain before having bulging or folding while only a bulging mode typical for materials with cubic crystal structure can be predicted. Simulation results such as punch load and deformation mode are compared with experimental results in the axial crushing test with AZ61 alloy.     

Energy absorption characteristics of composite tubes with different fibers and matrix under axial quasi-static and impact crushing conditions

The collapse characteristics and energy absorption capability of composite tubes with different fibers and matrix were studied in present article under axial quasi-static and impact crushing conditions. The sensitivity to fibers, matrix and loading conditions were thoroughly discussed for the crushing modes and energy absorption capability. Experimental results showed specimens with different matrix and fibers exhibit three typical types of crushing modes. Specimens G803/3234 and G827/3234 had better energy absorption performance than the specimens with 5224 matrix. Impact loading condition led to lower energy absorption capability as compared to quasi-static loading condition. Moreover, impact loading condition also caused the crushing mode transition from splaying mode to fragmentation mode for G803/3234 and G827/3234.