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。     

A theoretical analysis for the quasi-static axial crushing of top-hat and double-hat thin-walled sections

A theoretical analysis, using superfolding elements (SE), has been carried out to determine the mean crushing loads for the quasi-static axial loading of top-hat and double-hat, spot-welded mild steel sections. The theoretical predictions compare favourably with experimental results reported in a companion article and with the experimental and numerical predictions of other authors.     

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。     

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)     

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.     

Validation of constitutive models applicable to aluminium foams

An extensive experimental database has been established for the structural behaviour of aluminium foam and aluminium foam-based components (foam-filled extrusions). The database is divided into three levels, these are: (1) foam material calibration tests, (2) foam material validation tests and finally (3) structural interaction tests where the foam interacts with aluminium extrusions. This division makes it possible to validate constitutive models applicable to aluminium foam for a wide spectrum of loading configurations. Several existing material models for aluminium foam from the literature are discussed and compared. To illustrate the use of the database, four existing material models for foams in the explicit, non-linear finite element code LS-DYNA have been calibrated and evaluated against configurations in the database.     

内部加强帽形结构轴向压缩吸能特性研究

通过仿真和落锤试验研究了内部加强帽形结构动态压溃时的吸能特性。首先,从理论上分析内部加强对帽形结构吸能特性的影响;然后,根据仿真结果,对内部加强帽型结构的轴向吸能特性进行了分析,并与无内部加强帽形结构进行了对比;最后进行了试验验证。结果表明：通过改变加强结构,帽形结构的轴向压缩稳定性和吸能特性有了很大的提高,且失效现象得到改善,还有利于结构的轻量化。     

Static crushing of square aluminium extrusions with aluminium foam filler

An experimental investigation was carried out to study the behaviour of square aluminium extrusions filled with aluminium foam under quasi-static loading conditions. Based on the experimental work, simple relations between dimensionless numbers governing the influence of the foam on the characteristics of the crush problem were identified. Furthermore, a simplified set of equations applicable for design of foam-filled components was proposed.     

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.     

Optimum design for energy absorption of square aluminium columns with aluminium foam filler

The performance in axial compression of square aluminium columns with aluminium foam filler has been assessed based upon existing design formulas for average crush force, maximum force and effective crushing distance. Using an optimisation algorithm, the combination of (1) foam density, (2) column wall thickness, (3) column width, (4) column material strength and (5) total component length giving the component of minimum mass was determined for specific cases. It was found that optimum foam filled columns compared to the traditionally designed non-filled columns showed smaller cross section dimensions in addition to less weight. As a consequence, mass-, length- and volume reductions are possible by utilising foam filler.     

基于简化基本折叠单元法的蜂窝耐撞性优化设计

采用简化基本折叠单元法,即基于简化基本折叠单元的超折叠单元理论对三种常用蜂窝结构的轴向平均压缩应力进行理论求解。为验证理论解的准确性,采用基于LS-DYNA的非线性有限单元法对这三种常用蜂窝结构的轴向压缩进行仿真计算,仿真结果与理论结果吻合很好。基于轴向平均压缩应力理论解,进一步采用多目标粒子群算法分别对这三种预压缩常用蜂窝结构的耐撞性进行多目标优化,发现在所研究的三种常用蜂窝结构中,正六边形蜂窝结构的耐撞性最好。采用LS-DYNA对优化设计的蜂窝结构进行轴向压缩仿真,仿真结果与基于所给出理论公式的耐撞性优化结果相差不大,说明基于所给出理论公式的蜂窝结构耐撞性优化是可行且有效的。     

Dynamic indentation and penetration of aluminium foams

Failure of metal foams caused by dynamic indentation and penetration is very common in practice, such as light-weight structural sandwich panels, packing materials and energy absorbing devices. Rational application of these materials requires a sound understanding of deformation and energy absorption mechanisms of the aluminium foams as well as the effect of impact velocity. In this study, following experimental investigations into compression, tension, sharing and indentation of CYMAT aluminium foams of various densities, a finite element (FE) analysis using ABAQUS is conducted for dynamic indentation process of aluminium foams under a rigid, flat-headed indenter. Two methods of applying impact velocities are considered: the indenter is pushed into the foam at a constant velocity through the whole process or with an initial velocity which then decreases with indentation. Two energy dissipation mechanisms are considered: compression of the foam ahead of the indenter and fracture along the indenter edge. Effect of impact velocity is noted on the size of a localized deformation and the total energy absorbed. A plastic structural shock theory developed by previous researchers is applied to calculate the resistance force with indentation depth during indentation process and fair agreement is obtained between the analytical and numerical results.     

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.     

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.     

Static and dynamic axial crushing of foam-filled sheet metal tubes

Experimental results are provided for the quasi-static and dynamic axial crushing of thin-walled square and rectangular tubes manufactured from sheet metal. The tubes were tested both empty and filled with polyurethane foam of various densities. Both the stability and the energy absorbing characteristics of the tubes are described and discussed. Simple theoretical models are proposed to explain and quantify the interaction between the foam and the sheet metal tubes.     

Deformation and energy absorption of aluminum foam-filled tubes subjected to oblique loading

Research to quantify the energy absorption of empty and foam-filled tubes under oblique loading with different loading angles and geometry parameters was carried out. Tests on circular tubes made of aluminum alloy AA6063 under quasi-static axial or oblique loading were performed. The collapse behavior of empty, foam-filled single and double tubes was investigated at loading angles of 0°, 5°, 10° and 15° with respect to the longitudinal direction of the tube. The tubes were fixed at both ends and oblique load was realized by applying a load at the upper end of a pair of specimens. When the foam-filled tubular structures subjected to oblique quasi-static loading, some new deformation modes, such as spiral folding mode, irregular extensional folding mode and irregular axi-symmetric or diamond deformation mode, were identified and ascribed to the bending of tubes and shearing of foam filler, as well as the interaction between the tubes and the foam. The energy absorption characteristics of empty and foam-filled single and double tube structures with respect to the load angle and wall thickness are determined. It is found that the energy-absorbing effectiveness factors of the circular tube structures with aluminum foam core are significant higher than those of the empty tubes and the energy absorption capacity of the foam-filled double tubes is better than that of the empty and foam-filled single tubes.     

Influence of spot-weld failure on crushing of thin-walled structural sections

Structural effectiveness differences have been observed in a recent study on the progressive axial collapse of thin-walled structural sections when made from different classes of steels (mild steel, interstitial-free rephosphorized high-strength steel and high-strength low-alloyed steel). A higher effectiveness was observed for spot-welded top-hat sections made from a mild steel than for similar sections made from a high-strength steel. For square sections, the structural effectiveness was not affected by the steel classes. It is anticipated that this observation applies not only for spot-welded top-hat and square sections, but for other joined and unjoined thin-walled structures as well.

The part and full failure of spot-welds, during the axial collapse of the thin-walled structural sections, is one possible explanation for the above inconsistency. This is investigated experimentally in this article using peel tests on spot-weld samples under quasi-static and dynamic conditions. Despite having a lower material strength, the mild steel spot-weld samples exhibited a higher peak force and similar energy absorption during failure when compared with a high-strength steel, both under quasi-static and dynamic loadings.

The potential contribution to the mean crushing force during progressive axial collapse is estimated from the experimental results and comparisons are made with deformed thin-walled structural sections from a recent experimental study. Possible implications for the determination of the mean crushing force from analytical and numerical models are identified and discussed.     

Classification of the axial collapse of cylindrical tubes under quasi-static loading

The results of an experimental investigation of the axial crushing modes and energy absorption properties of quasi-statically compressed aluminium alloy tubes are presented. In particular, the influence of tube length on these properties is discussed and quantified and a classification chart presented. This chart together with other experimental data, enables a designer to predict the energy absorbing properties of a given tube as well as its mode of crushing.     

Axial crushing of square tubes

The mechanics of the large deformation of a square tube under axial load is discussed. Theoretical results are substantiated by experimental results. The elastic buckling load is predicted by assuming the tube to be comprised of four plates simply supported at their edges. The highest load the tube can sustain is predicted by allowing for the development of plasticity near the corners. The mean crushing load is predicted from an incremental plasticity analysis which allows for travelling plastic hinges. Comparison with circular tube behaviour is considered and an attempt to explain some of the peculiarities is made.