J. Cent. South Univ. (2020) 27: 1012-1038

DOI: https://doi.org/10.1007/s11771-020-4349-3

Recent research development of energy-absorption structure and application for railway vehicles

GAO Guang-jun(高广军)^{1, 2, 3}, ZHUO Tian-yu(卓天宇)^{1, 2, 3}, GUAN Wei-yuan(关维元)^{1, 2, 3}

1. Key Laboratory of Traffic Safety on the Track of Ministry of Education, Central South University, Changsha 410075, China;

2. Joint International Research Laboratory of Key Technology for Rail Traffic Safety, Central South University, Changsha 410075, China;

3. National & Local Joint Engineering Research Center of Safety Technology for Rail Traffic Vehicle, Central South University, Changsha 410075, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: As the application of energy-absorption structure reaches an unprecedented scale in both academia and industry, a reflection upon the state-of-the-art developments in the crashworthiness design and structural optimization, becomes vital for successfully shaping the future energy-absorption structure. Physical impacting test and numerical simulation are the main methods to study the crashworthiness of railway vehicles at present. The end collision deformation area of the train can generally be divided into two kinds of structural design forms: integral absorbing structure design form and specific energy absorbing structure design form, and different energy-absorption structures introduced in this article can be equipped on different railway vehicles, so as to meet the balance of crashworthiness and economy. In pursuit of improving the capacity of energy dissipation in energy-absorption structures, studies are increasingly investigating multistage energy absorption systems, searching breakthrough when the energy dissipation capacity of the energy-absorption structure reaches its limit. In order to minimize injuries, a self-protective posture for occupants is also studied. Despite the abundance of energy-absorption structure research methods to-date, the problems of analysis and prediction during impact are still scarce, which is constituting one of many key challenges for the future.

Key words: railway vehicle; energy-absorption structure; crashworthiness

Cite this article as: GAO Guang-jun, ZHUO Tian-yu, GUAN Wei-yuan. Recent research development of energy-absorption structure and application for railway vehicles [J]. Journal of Central South University, 2020, 27(4): 1012-1038. DOI: https://doi.org/10.1007/s11771-020-4349-3.

1 Introduction

Safety is the eternal theme of transportation. According to Wikipedia, collisions account for 56% of world railway accidents, causing huge casualties and property damage. The crashworthy structure of railway vehicles can absorb the kinetic energy in collisions as much as possible, which significantly reduces the impact force and minimizes casualties [1]. Researches on energy-absorption structure improve the passive safety protection technology of railway vehicles, which has important practical significance for the train operation safety.

Since the 1990s, a number of projects including safe-trains, safe-trams, train-safety and safe-interiors have been funded by the European Union to modify the crashworthiness of current railway vehicles. EN 12663 [2], EN 15227 [3] and the technical specification for interoperability (TSI) [4] were issued based on the results of these projects. These standards and GM/RT 2100 [5] are the most important achievements of the research on railway vehicles in European countries. In the United States, the Federal Railroad Administration (FRA) has investigated crashworthiness strategies and carried out research on the safety of high-speed passenger trains beginning in 1989 [6-8]. The main achievement of the FRA full-scale impact tests was CEM, a design technique that enhances crashworthiness, which has been used in the rail industry [9]. Currently, one of the most important considerations of implementing LNG as a railroad industry-wide locomotive fuel in the United States is the crashworthiness of the locomotives and tenders [10].

With the worldwide emphasis on energy- absorption structure, as shown in Figure 1, energy- absorption structure has evolved greatly over the last three decades [11]. Demand is increasing for effective energy-absorption structure in an array of engineering fields such as railway vehicles, passenger cars, ships and aerospace. As a result, various kinds of energy absorbers with different structures such as circular tubes [12-15], polygon tubes [16-19], plate [20-22], honeycomb [23-25], foams [26-28] and multistage structures [29, 30] have been proposed recently (as shown in Figure 2).

A lot of work has been conducted on energy-absorption structures of rail trains, which has a wide range of applications. Thin-walled tube is one common kind of energy-absorption structure used in railway vehicles, which usually includes two main types: circular tube and polygon tube. The energy-absorbing mechanism of thin-walled tubes can be mainly achieved through the expansion, shrink, splitting and cutting of the tube body, and through different combinations of various energy- absorption structures, the energy-absorbing efficiency and space utilization. Aiming at the compression mode of the circular tube, ALEXANDER [31] proposed an approximate theoretical model, and analyzed the mechanical interactions and energy dissipation laws of the fold formation during the compression process of the simplified model, which pointed the direction for subsequent research. Subsequently, the buckling modes of round and square pipes under static and dynamic loads have been concluded [32], the factors affecting the tear energy absorption of tubes [33] and the energy-absorption characteristics of an elliptical tube [34] were all studied. Based on this, the energy absorption characteristics of a multi-cell thin-walled square tube have been researched [35], and SONG et al [36] found that the change of the wall shape made the impact force curve more stable by analyzing the deformation process of the thin-walled structure of quadrangular, hexagonal and octagonal. Using neural network and genetic algorithm [37], the crashworthiness of hollow tubes can be optimized, the maximization problem of objective functions also can be solved by multi- objective crashworthiness optimization [38]. For the energy-absorption structure of passenger coaches, GAO et al [39] analyzed the energy absorption responses of conventional tubes and tubes with diaphragms by means of finite element simulation. Based on it, TANASKOVIC et al [40] carried out quasi-static tests and dynamic tests to analyze the characteristics of modified tube absorbers for kinetic collision energy of passenger coaches.

Figure 1 Number of publications of energy absorption based on the Scopus database [11]

Figure 2 Various kinds of energy absorbers with different structures

Since the early 1980s, researchers have conducted a lot of researches on the deformation mechanism of honeycomb materials because of their good impact resistance. GIBSON et al [41] theoretically analyzed the mechanical behavior of static stretching and compressive failure in view of the traditional rules of material constants (the elastic modulus E, the Poisson ratio v, the shear modulus G) of hexagonal honeycomb structures, and put forward a classic cellular material theory. The finite element method can be used to simulate the sub-yield behavior [42] and the structure surface compression behavior rules of a regular hexagonal honeycomb [43], both experimental results and finite element results are expressed by theoretical formulas, which overestimate the ultimate stress. In order to research the energy-absorbing characteristic of foams, REID et al [44] studied the uniaxial compression behavior of wood, and this theory has been extended and applied to the characterization of the impact resistance of honeycomb and foam structures by RUAN et al [45].

Recently, the foam-filled tube has attracted the interest of many scholars, the fillers include aluminum foam, polymer foam, and wood chips. It is found that filling can significantly increase energy absorption, and the deformation stability of thin-walled tubes is generally improved after filling. Further research found that the energy absorbed by the foam-filled tube is greater than the sum of the energy absorbed by the thin-walled tube and the filler. This enhanced energy absorption originates from the interaction between the tube wall and the filler. MASOUD et al [46] proposed a method that adding a small square tube in ordinary square tube to form double square tube, and used foam materials, such as metals, polymers, to fill thin-walled structures, thereby increasing the energy absorbed by the structure [47, 48]. YIN et al [49] optimized the way that cell aluminum foam filled multi-cellular tubes and used the response surface, radial basis, Krikin and support vector machines to construct the function formula of energy absorption and initial peak force.

With the continuous development of materials and structures, new energy-absorption structures and modes of energy dissipation applied to vehicles have been widely studied. In order to research the multistage structure, WANG et al [50] studied the energy absorption capacity and the stress of the platform under different velocity gradient structures. Using the extended finite element method, SINGH et al [51] simulated the functional gradient of cellular materials containing defects and reinforced particles, and explored the structural defects and reinforcing particles on the influence of stress concentration factor. Furthermore, in the study of plate energy-absorption structure, cutting model according to the cutting plate test data has been established [52, 53] and a simple calculation formula for energy absorption through data analysis, which includes cutting length, yield stress, cutting depth and cutting tool angle, was got [54]. It was found that the cutting force increases rapidly to a constant during the process of steady-state wedge cutting through steel plates [55-59]. To make better use of the energy-absorbing capacity of plate, DONG et al [60] proposed a new type of energy-absorption structure based on the splitting and bending behaviors of steel plates and analyzed the deformation mode of steel plates subjected to axial crushing through numerical simulations and experimental tests. In addition, GAO et al [30] proposed an active-passive integration energy absorber based on metal cutting deformation and investigated the energy-absorption performance of aluminum and steel tube cutting.

This review focuses on the energy-absorption structure for railway vehicles. Based on a broad review of the literature, the global state of recent research development of energy-absorption structure is summited comprehensively.Physical impacting test and numerical simulation are presented as the main approaches used for the analysis and design of energy-absorption structures. This article aims to compare the difference between various energy-absorption structures, it is necessary to choose the appropriate energy-absorption based on structural design forms, concentrating on the crashworthiness under collision. Furthermore, in order to expand the design ideas of energy- absorption structures for railway vehicles, this review dedicates a separate section (see Section 5) to energy-absorption structures applied in other fields. Based on the reviewed papers and the latest advancements in this field, the derived trends and projected developments provide a clear and challenging future for the energy-absorption structure of railway vehicles.

2 Research approaches of energy- absorption structure

Through analyzing the huge amounts of research data [6, 7] of FRA during 1990s [61-63], KIRKPATRICK et al [64-67] compared various experimental, analytical and computational approaches and evaluated rail vehicle crashworthiness. They summarized many approaches used in previous studies on vehicle crashworthiness, and explained their advantages and disadvantages. Because it is difficult to evaluate the behavior and deformation of the train collision exactly, physical impacting test and numerical simulation are often used to study the crashworthiness of railway vehicles at present.

The researches on energy-absorption structures and crashworthiness of railway vehicles mainly include four approaches: physical impacting test, theoretical calculation, finite element analysis and multibody dynamic analysis. The methodology, model and tool used for the analysis and design of energy-absorption structures are shown in Table 1. Through the use of these methodologies, models and tools, the energy-absorption structure can be effectively analyzed and designed.

2.1 Physical impacting test

The scale-model testing is considerably less expensive than full-scale testing, but the scale-model testing is probably not appropriate for collision responses in which gravitational effects are important [65, 68]. Therefore, the full-scale test is the most direct, reliable, and authoritative way to investigate the crashworthiness of trains. As shown in Figure 3, FRA [69-71] has conducted various full-scale train-to-train impact tests, and British Rail Research [72, 73] also carried out the crush tests concentrated on vehicle behavior under simulated overriding conditions. In addition, Figure 4 shows the impact test system at Central South University, which can test the crashworthiness of a 35 t car at 80 km/h [74] and provided a lot of research data for the improvement of the railway vehicle crashworthiness [75-77].

However, due to their expensive cost, complex procedures, and the inability to repeat the tests,full-scale train impact tests are not preferred. Therefore, the technology of the scaled model test has been widely used in various engineering fields. π theory can be used to investigate the scaled collision tests, but with the increasing speed of trains, the required impact speed of the scaled test became high accordingly [78]. Figure 5 shows the structure of scaled car body, which applied the above theory. Based on this, the scaled similitude rule for train collision [79] was proposed, which follows the principle of acceleration consistency. If the similarity ratios of mass and initial impact speed of cars, crushing force, and initial length of the energy-absorption structures of trains with different proportions meet the requirements of the scaled similitude rule, the impact accelerations of each are considered to be consistent. As shown in Figure 6, XU et al [80] studied the energy- absorption design of subway vehicles based on a one-seventh-scale model crash test to obtain the characteristics of collision energy absorption. LU et al [81] proposed a new method of force/stiffness equivalence-scaled modelling for highspeed trains, which can also be used for the scaled modelling of a prototype composed of thin-walled structures. An initial scaled model of a head car was established by scaling the energy absorbing part and undeforming part in a full-scale prototype.

Table 1 Methodology, model and tool used for analysis and design of energy-absorption structures

Figure 3 Train-to-train impact test conducted on March 23, 2006 in the US

Figure 4 Full-scale testing at Central South University [75]

Figure 5 Structure of scaled car body [78]

2.2 Numerical simulation

The numerical simulation approaches to study the crashworthy structure of trains mainly include theoretical calculation, finite element analysis and multibody dynamic analysis. The highest fidelity method of analysis for railway vehicle crashworthiness is application of 3D non-linear finite element analyses, which includes two different models: explicit time integration and implicit time integration.

Many popular simulation software using explicit time integration are LS-DYNA, RADIOSS, PAMCRASH and MSC-DYTRAN, and ANSYS and ABAQUS are commonly used implicit time integration code. The proprietary program OASYS DYNA3D can be used to undertake a theoretical non-linear finite element analysis for the collapsible structure [82] and provide more details about the performance of the collision. ABAQUS was also used to simulate the quasi-static crushing of the perforated square tubes [83] and generate the training and test sets for the ANNs to optimize the cylindrical aluminum tubes [84].

Compared with the finite element analysis, the multibody dynamic analysis has the advantages of simple model and less calculation time [85], however, the difficulty of model verification and inaccurate simulation results also restricts its application [86]. Generally, multibody dynamic analysis can be used to predict the gross motion and energy dissipation distribution of the train [87-89], and the performance of the train after a collision can also be analyzed to optimize the design of the energy-absorption structure, anti-climbing devices and anti-derailment devices [90-93].

With the development of computer technology and optimization algorithms, more and more researchers use optimization algorithms and numerical simulations to optimize the design of energy-absorbing structures. Both the finite element analysis and the multibody dynamic analysis can use optimization algorithms to optimize the simulation results, so that the energy-absorption structure has better crashworthiness. XIE et al [94] proposed an optimization process based on surrogate modal and used dynamic explicit finite element method to complicate this design of the energy-absorption structure. ZHANG et al [77, 95] proposed a hybrid solution methodology that combines NSGA-II, C-BW and GRA, formulated a 3D train-track coupling dynamics model of a high-speed train that contains eight vehicles in MADYMO multibody dynamic software, and obtained a better structure design for crashworthiness. These attempts have effectively improved the crashworthiness of trains and reduced casualties caused by collision accidents.

2.3 Comparison of typical models used for crashworthiness analysis

In order to clarify the application of the above typical models, Table 1 shows the main features of them. The advantages and disadvantages are self-explanatory in the table.

The choice of research approaches for energy- absorption structures will be adjusted based on the research objectives. Different crashworthiness requirements will determine different collision conditions, thus changing the methodologies and models used in the study.

Full-scale test obviously has the highest level of fidelity and certainty [65], hence it is most often used to verify numerical simulation and provide the crashworthiness indicators. Scaled test is often used in the iterative test design of crashworthy structures due to its inexpensive cost.

The implicit time integration is usually used in structural calculations and design, but the explicit time integration is suited for dynamic applications such as shock simulation, impact analysis and crashworthiness [65]. Because the researches on the crashworthiness of energy-absorption structures mostly consider the short-term large-scale deformation caused by dynamic shock on the structure, and more often use the explicit time integration method to perform simulation analysis. If the deformation caused by the collision can be negligible, the rigid-body dynamic model is a good way to study the entire group of train.

Figure 6 Test scenarios:

Table 2 Comparison of typical analysis models

3 Design strategies of energy-absorbing structure for railway vehicles

In order to develop North American passenger equipment regulations, TYRELL et al [96-100] collected the research data conducted by FRA and analyzed the conditions of railway vehicle accidents to study the crashworthiness strategy, and illustrated the flow diagram of crashworthiness research. As shown in Figure 7, design strategies of the railway equipment crashworthiness research can be described as the following steps [99]:

1) Define the collision scenarios of concern.

2a) Develop information on the features of existing designs that influence crashworthiness.

2b) Develop options for design modifications.

3) Determine the effectiveness of existing design and alternative design equipment.

4) Compare the crashworthiness of the alternative designs with the existing designs.

Through the above progress, the crashworthiness structure of railway vehicles can be researched. British Rail Research has progressed the development of crashworthiness in railway vehicles before 1990s, SCHOLES and LEWIS [72, 73] concluded these research results in 1993, and proposed the structural design philosophy to the production of improves specifications for future BR vehicles. They considered that the most effective approach to obtain the required crashworthiness is by absorbing the collision energy in a graduated and controlled manner with a progressively increasing force when the collapse progresses, and the anti-climbers can reduce injuries considerably. Following these principles can effectively improve the crashworthiness of the energy absorbing structure for railway vehicles.

In the design of the energy-absorption structure of the train, in order to ensure that the plastic deformation of the train body structure is limited to the preset collision deformation areas, the carrying capacity of regional passenger train body structure must be significantly higher than the deformable region on the end of the train [101]. The anti-creeping device is installed at the front part of the end of the train to prevent the train from climbing. The design of energy absorbing structure of train body collision and deformation is usually used in one or more combined energy-absorption structures such as honeycomb structure, pressure collapse energy absorption structure, cutting energy absorption structure, etc. Under the action of longitudinal impact force, energy-absorption structure installed in the end of the train take place axial plastic buckling, tearing, flip, bulging and cutting deformation mode, and the force displacement curve usually presents the oscillation waveform during deformation process with larger initial peak force. By using local weakening and changing the shape of cross section, induction mechanism is added in the design of energy-absorption structure. In the course of the collision, there is no initial peak force or the initial peak force is reduced to a reasonable level.

There are two kinds of structural design forms in the end collision deformation area of the train: 1) integral absorbing structure design form, the collision deformation energy-absorption structure on the end of the train are fully integrated with the chassis structure 2) specific energy-absorption structure design form, modular anti climbing energy absorbing component integrated by common energy-absorption element and anti-creeping device structure is assembled on the front end of the chassis structure through the mechanical connection of the bolt. Table 3 shows the comparison of the above two kinds of structure design form.

Figure 7 Flow diagram, crashworthiness research principal tasks and utilization of accident information [99]

The design of integral absorption structure needs to meet multiple conflicting requirements. It not only needs to bear the dynamic load during the impact, but also to meet the static load of the train during the normal operation. The train static strength design requires that the train body structure has a very strong stiffness, but the train body needs a certain plastic deformation capacity for collision safety requirements and secondary structure of train body has plastic deformation, and main structure has small elastic deformation during collision process, which is shown in Figure 8. We need take the static strength and impact safety of the train body into consideration when we have the design of integral absorption structure. By the finite element simulation and component test technology, repeated design of integral absorbing structure should be done to match energy-absorption area and passenger area structure strength. Under the condition of ensuring the static strength of the train body, the maximum limit of the train body is improved.

Table 3 Two kinds of structural design forms

Figure 8 Bearing energy-absorbing structure of crashworthy vehicle (Unit: mm) [102]

As shown in Figure 9, specific energy- absorption structure as a relatively independent module suffers little limit by the static strength of the train body. But it is necessary to ensure that the longitudinal stiffness of the energy-absorption structure is lower than the longitudinal stiffness of the structure of the train body, which causes the plastic deformation of the main structure in the course of the collision, and to absorb most of the impact energy. At the same time, the design of the specific energy-absorption structure has great flexibility and less constraint conditions, which can optimize the geometric parameters of the structure, so that it can have a good energy-absorption characteristic in the collision process. Specific energy-absorption structure installed on the chassis of the end of the train could be more easily assembled, disassembled and replaced after the collision occurred. Even if there is a serious accident, after the collapse of the energy-absorption structure, the damage degree of the train body structure is limited. The structure of the specific energy-absorption structure with anti-creeping function is usually installed at the end of the anti-climbing gear, which requires it to be able to withstand the vertical load of 150 kN to prevent the energy-absorption structure in the process of collision occurred eccentric. In some extreme collision scenarios, the specific energy- absorption structure must bear some bending moment, so as to ensure the orderly plastic deformation.

4 Collision performance of energy- absorption structure for railway vehicles

4.1 Axial folding of thin-walled tubes

GAO et al [39] compared the energy- absorption response of square tubes with or without diaphragms under quasi-static axial impact loading, which is shown in Figures 10 and 11. Tubes with diaphragms can change their final deformation pattern, increase the number of lobes and improve energy absorption and the crashworthiness of tubes. The crushing behavior of tubes with diaphragms under quasi-static axial impact loading was studied experimentally, but there are differences between the experimental results and the results of numerical simulation of the ideal structure. The discrepancy is attributed to modelling perfect geometry, whereas imperfections are likely to exist in the actual test specimens.

To improve the crashworthiness of subway vehicles, a composite energy-absorption structure (EAS) is designed by coupling a thin-walled metal tube and aluminum honeycomb structures [104, 105]. XU et al [106-109] presents an investigation of a newly designed gradual energy-absorption structure subjected to impact loads using a test trolley for experimental and numerical simulations, as referenced in EN15227:2008. This variation resulted in the crushing force–displacement curve to be divided into four phases (I, II, III and IV), which was consistent with the four-stage structures of the new gradual energy-absorbing structure. Then they address the energy-absorption response and crashworthiness optimization of a gradual energy-absorption structure which is composed of nested thin-walled square tubes. WANG et al [110, 111] proposed a combined multi-cell thin-walled energy-absorption structure made from aluminum with octagonal and hexagonal cells, which will be used in high speed train. By comparing the average crushing force of different structures, it can be easily found that the combined five-cell thin-walled structure is more efficient in energy absorption. YAO et al [112] studied the collision performances under a vertical offset of 0-80 mm and a horizontal offset of 0-40 mm. Based on the concept of gradient material, the honeycomb strength in the structure is changed into gradient distribution. Figure 12 shows the above different types of thin-walled tubes, which are applied to the crashworthiness design of various trains.

Figure 9 Special energy-absorbing structure of crashworthy vehicle [103]

Figure 10 Experimental and numerical results:

Figure 11 Force–displacement curves of tubes with different thicknesses [39]

4.2 Circular tube energy-absorption structure

4.2.1 Expanding of circular tubes

The energy dissipation mechanism of expansion circular tube is widely studied for its stable maximal force and high energy-absorption efficiency. The expansion of circular tube dissipates collision energy by elastic–plastic bending of the tube and friction between the tube and the die [113]. Based on this theory, as shown in Figures 13 and 14, YAO et al [114] proposed the energy absorption and crashworthiness optimization of an expanding structure under axial loading, which has been applied to a railway vehicle coupling device.

To evaluate the crashworthiness of the expanding circular tubes energy absorber with cylindrical anti-clamber under eccentric loading, it was essential to predefine crashworthiness indicators. Three indicators are used [30]: energy absorption (EA), mean crushing force (MCF), peak crushing force (PCF).

Figure 12 Axial folding of thin-walled tubes:

Figure 13 Comparison of deformation process:

Energy absorption (EA) denotes the total amount of energy absorbed by the structure during structure crushing deformation. It can be expressed as:

                             (1)

Figure 14 Force-time curves of experiment and simulation [114]

where F(l) is the crushing force and l is the effective crushing displacement.

Meanwhile, peak crushing force (PCF) represents the maximum crushing force that occurs during deformation. Mean crushing force (MCF) for a given crushing displacement value l can be calculated as:

                              (2)

4.2.2 Shrinking of circular tubes

Similar to the deformation mechanism of expansion tube, the shrink of circular tube has already been employed as a forming method to shape the end of thin-walled tube [115-117]. However, the analysis of shrink circular tube as an energy absorber received few attentions. LI et al [118] examined the energy absorption behavior of shrink circular tube. As shown in Figures 15 and 16, the 6061-T6 aluminum alloy circular tubes were experimentally shrunken in radial direction by a cone bush under quasi-static loading, which demonstrated the feasibility of shrink circular tube as an energy absorber. It is found that shrink circular tube deforms as designed and exhibits good crashworthiness performance, the driving force in the stable deformation stage is almost constant. Also, shrink circular tube has a higher energy absorption efficiency when compared to expansion circular tube.

As shown in Figure 17, YAO et al [119] proposed a new straight-tapered shrink (STS) circular tube, which is planned to be applied on railway coupler for energy absorption and overload protection. The deformation modes of the STS tubes can be classified into two modes, namely,shrink mode (S-mode) and buckling mode (B-mode). The compressive material properties are more suitable for accurately modeling the STS tube. A typical shrinking tube test is shown in Figure 18. In addition, TANASKOVIC [120] designed a new structure to make circular tube deform by combining shrink and split modes. The new combined absorber had approximately 60% higher absorption power than the shrinking absorber by itself.

Figure 15 Deformation of shrink circular tube [118]

Figure 16 Force-displacement curves of experiment and simulation [118]

Figure 17 Straight-tapered shrink tube [119]

Figure 18 Shrinking–splitting tube [102]

4.2.3 Splitting of circular tubes

Splitting of thin-walled structure is an efficient energy dissipation mechanism, which is used to dissipate collision energy in many structures such as automobiles and trains. The splitting of thin-walled structure was first introduced by STRONGE et al [121], who pressed the square metal tube against a die and then the square tube was torn to absorb energy. Recently, axial splitting of thin-walled aluminum tubes under the blast load by a special cutter has been investigated [122], and some theoretical relations were derived to predict the instantaneous axial force of the circular metal tubes during the splitting process under the axial compression [123]. The principle of shrinking and splitting the circular cross section of tubes can be used to absorb the kinetic collision energy [124]. Inspired by this, as shown in Figures 19 and 20. LI et al [125, 126] proposed a new structure which combines expansion and splitting of circular metal tube and designed the special metal circular tube and die. Using this system of deformation, the total energy absorption is obtained by the three mechanisms: elastic–plastic bending of the tube, splitting of the tube wall and friction between the tube and the die.

4.2.4 Cutting of metal circular tubes

Employing the principle of metal cutting, the cutter and the metal tube make relative motion during the collision and chips are produced by the contact extrusion of the cutter and the metal surfaced. Through the investigation by PENG et al [127], the energy absorption and its efficiency of the metal tube under the cutting deformation mode mainly lie in the shape and size of the chip, and its cutting force is relatively stable. The results are shown in Figure 21. The deformation process of the aluminum tube is divided into three stages, which includes metal plastic deformation and fracturing, chip formation and crimp deformation. Relying on these, GUAN [128] proposed a method to characterize and optimize the crushing performance of a new cutting aluminum tube absorber for railway vehicles. With the aim of improving passive safety protection in subway vehicles, WANG et al [129] investigated a cutting energy-absorption structure, and passed the simulation verification of a FE coupled thermal–structural model using the explicit FE software LS-DYNA, which is shown in Figure 22. However, it must be noted that their study does not consider manufacturing tolerances, material-property tolerances, the cutting-edge eroding manner, vibration, or any of the other attributes that are critical to an overall design.

Figure 19 Deformation of splitting circular tube [125]

Figure 20 Force-displacement curves of experiment and simulation [125]

Figure 21 Force-time curves of experiment and simulation [127]

4.2.5 Capped cylindrical tubes

Thin-walled cylindrical tubes are the most common elements used as energy-absorption structures, and the presence of the porous materials in thin-walled tubes improves crush stability and collapse mode [130]. Therefore, the research about capped cylindrical tubes was launched to be used in the nose of the reentry sounding rockets as an energy absorbing device. Through the simulation and test, GHAMARIAN [130-132] considered that capped cylindrical tubes are convenient shock absorber for structures which are sensitive to deceleration level.

Figure 22 cutting-type energy-absorbing structure [129]

As shown in Figures 23 and 24, KUMAR [133-136] preformed finite element simulation and quasi-static test on the capped cylindrical tubes and studied the crashworthiness of capped cylindrical tubes under axial crush and lateral compression. Compared with the non-capped cylindrical tubes, the initial peak crushing force of shallow-spherical and hemispherical capped cylindrical tubes is reduced by 20%-40% significantly [135]. Under quasi-static lateral compression, capped cylindrical tubular structure, especially the shallow-spherical capped cylindrical tube, is strongly validated to considerably increase the EAC and SEA [136].

4.3 Metal plate energy-absorption structure

4.3.1 Bending-straightening of metal plate

To explore diversified energy absorption methods, a new energy-absorption structure was proposed based on the plastic forming mechanisms of stamping. The U-shaped plates, as shown in Figures 25 and 26, are subjected to stable and repeated plastic bending in the gaps of the structure [137]. Frictional energy dissipation lags are behind the energy dissipation of plastic deformation, whilst the frictional energy dissipation rises more rapidly. Therefore, the plastic forming of stamping can be used as a new energy dissipation strategy. Through the use and combination of it, YU et al [138] presented a novel energy absorber that can generate a larger deformation stroke than its free length, which is shown in Figure 27. The effective crushing distance rate (ECDR) of this structure can exceed 1. During collision, the impact kinetic energy is dissipated by the elastic-plastic deformation of a steel plate and aluminum honeycomb.

Figure 23 Fabricated tube specimens [133] and drop mass impact testing setup [134]

Figure 24 Numerically predicted crushing history of shallow-spherical capped cylindrical tube:

Figure 25 Deformation of U-shaped thin plate energy absorber (Unit: mm) [137]

Figure 26 Force-displacement curves of experiment [137]

Figure 27 Bending-straightening energy absorber [138]

4.3.2 Splitting–bending steel plate

Based on the splitting and bending of a steel plate, a new type of energy absorption structure is presented in the reference [60, 139], which is shown in Figures 28 and 29. Using this type of absorber, energy absorption occurs through the splitting of the steel plate, elastic–plastic bending and friction between the steel plate and the die. Tests show that the deformation is stable and the cutting line is smooth for each steel plate; the force increases gradually from zero to the steady-state force with increasing displacement.

4.4 Telescopic type collision energy absorption device for rail vehicle

Passive safety technology has become one of the critical technologies of railway vehicles, and multistage energy absorption systems have become the main form of crash structural design of railway vehicles. As shown in Figures 30 and 31, GAO et al [29, 30] proposed a design method of an active–passive integration energy absorber on the basis of a multistage energy absorption system, and the new type of absorber is applicable to various types of railway vehicles. The absorber consists of a crush tube, anti-climber gear, die, support tool, guide tube, and reversible actuator. Before collision, the crush tube of the absorber is extended due to the effect of the reversible actuator, which breaks through the coupler restrictions on the longitudinal dimension of the absorber and significantly increases the deformation stroke of the absorber. During collision, the kinetic energies are dissipated by the cutting of the circular aluminum or steel tube, shear sliding, and friction between the circular tube and the die.

Figure 28 Deformation of splitting-bending steel plate [139]

Figure 29 Force-displacement curves of experiment and simulation [139]

4.5 Summit of energy-absorption structure for railway vehicles

According to the different features of the above energy-absorbing structure, different energy- absorption structures can be equipped on different railway vehicles, so as to meet the balance of crashworthiness and economy. Table 4 illustrates the summit of energy-absorption structure for railway vehicle, which compares the difference between various energy-absorption structures.

Figure 30 Active–passive integration energy absorber [30]

Circular tubes are the main energy-absorption structure used to improve the crashworthiness of railway vehicle, which include: expansion circular tube, shrink circular tube, splitting circular tube, cutting circular tube and capped cylindrical tube. Circular tube has many different forms of energy absorption and its deformation form can be predicted easily, so it is most widely used. Expansion circular tube and shrink circular tube have some similarities, but the energy absorption efficiency of shrink circular tube is relatively higher. Capped cylindrical tube is sensitive to deceleration level and has less initial peak crushing force, which is suited to be convenient shock absorber for structures.

Figure 31 Comparison of the physical compression of interface of head car of traditional subway vehicles and active–passive safety subway vehicles at different impact velocities [29]

Table 4 Summit of energy-absorption structure for railway vehicle

Table 5 Summit of energy-absorption structure for railway vehicle

Metal plate also plays an important role in energy- absorption structure and can be combined with other energy absorbing structures, such as honeycomb aluminum, to increase the energy dissipation of the entire energy-absorption structure. The effective crushing distance rate of bending- straightening metal plate can exceed 1, and the efficiencies of stamping forces for splitting– bending metal plate are greater than 80%.

Telescopic type collision energy absorption device for rail vehicle is a new design. It can break through the coupler restrictions on the longitudinal dimension of the absorber, which is aimed at enhancing the deformation stroke of the absorber. This may be a new breakthrough when the energy dissipation capacity of the energy-absorption structure reaches its limit.

5 Energy-absorption structure applied in other fields

5.1 Injury analysis of occupant protection in train collisions

By considering that collision force hurls occupants against interior parts of the compartment, scholars conducted a lot of studies on interior structure of the trains [140]. Simulation results [141] suggested that standing passengers could drastically minimize the risk of head and thorax injuries by squatting on the floor at the time of collision. As the vehicle decelerates, occupants are thrown forward in a secondary collision, striking the seat or table in front of them, depending on the seating configuration [142]. This is likely to result in injuries to the chest, head, face, and lower leg. In order to minimize injuries, YANG et al [143] proposed a self-protective posture with hands laced behind head and body curled up for occupants. The curled-up posture with hands laced behind head can effectively reduce injury degrees of head and neck of occupants in collision, which is shown in Figure 32. To fit this posture, the seats can be designed to turn to the optimal curling angle automatically, so as to assist occupants reaching the optimal protective state.

5.2 Energy-absorbing structure for passenger cars

Accident database information shows that 40% to 50% of car occupants experience injuries that are serious (AIS3+) and in many cases fatal [144]. National Highway Traffic Safety Administration (NHTSA) recorded the first car fatal accident occurred in 1889 in New York City [145], since then the researches about crashworthiness of the passenger cars has been carried out. The research on the crashworthiness of passenger cars is mainly focused on the front-end structure [146, 147] and bumper beam [148] of the automobile, and there are also optimized designs for rear under-ride protection device of the vehicle [149].

According to the research of WITTERMAN [150], occupant injuries and vehicle damage are mainly due to rapid deceleration then rapid acceleration of automobiles in the collision. The crashworthiness is the ability of body structure including progressive crush zones to absorb part of the crash kinetic energy, and the accuracy, speed, robustness and development time should be considered to optimize the crashworthiness of the car [151]. HAMZA [152] used the equivalent mechanism to predict the aggregated behaviors of structural members during crush and design the crashworthiness of vehicles. The proposed approach consists of two main phase which is exploration of the good crash mode and identical the design to the desired crash mode.

Figure 32 Dynamic response posture of dummy 6 in collision at speed of 48 km/h [143]:

Through researching the crashworthiness of the passenger cars, the proportion of energy reversibly absorbed by the bumper beam should be confined and the high kinetic energy should be preferably dissipated by plastic deformation [153], which is shown in Figure 33. In addition, rear under-ride protection devices [154, 155] for trucks also contributed to the safety of occupants by preventing car-to-truck rear-end accidents.

5.3 Energy-absorbing structure for ships

Lloyd’s register report placed collision and grounding as highest [156, 157] and the crashworthiness of ship structure against these accidental loads were addressed by MINORSKY [158] initially. As shown in Figure 34, PRABOWO et al [159-161] analyzed the crashworthiness of the ship’s bottom for two different ship-grounding scenarios by the finite element method, and provided detailed design parameter recommendations to assist the design of the hull energy-absorbing structure.

Figure 33 GMT bumper beam of Samand [153]

Ship-to-ship collision also play an important role in the ship collision accidents, the collision energy absorption capability depends on the thickness of structural parts, such as the outer and inner shells and side stringers in Figure 35 [162]. During the collision, the damage and energy dissipation switches between ships [163] and it is different between rebounding and fully stuck [164].

5.4 Energy-absorbing structure for aerospace

The research about energy-absorption structure for aerospace is mainly focused on the anti-crash of aircraft. As of 2003, NASA Langley has conducted 41 full-scale crash tests on general aircraft and 11 full-scale crash tests on helicopters [165]. PIFKO et al [166] used a combination of DYCAST and NASTRAN to study the crashworthiness of the helicopter fuselage. MCCARTHY et al [167] used PAM-CRASH software to carry out a numerical study on the crashworthiness of the fuselage abdomen and compared it with the test results. As shown in Figure 36, ZHOU et al [168-170] design a new helicopter subfloor intersection element structure by replacing the skin and floor of the subfloor structure with folded core sandwich structure.

Figure 34 Double bottom structures of ship [159]

Figure 35 Cargo tank geometry model for SSS (a) and DSS (b) bulk carrier vessels [162]

Figure 36 Subfloor structure of helicopter [168] (a) and folded core sandwich structure [169] (b)

6 Conclusions and outlook

This article presents a review of the crashworthiness of railway vehicles from the design strategy and collision performance of energy- absorption structure. Aspects deemed important for future work on the design and optimization in energy-absorption structure, are as follows:

1) The reasonable matching technology of the strength and stiffness of the end structure of train and the optimization design of vehicle bearing energy absorbing structure should be a focus of future research.

2) How to capture the impact force and the plastic and elastic stress wave properly in a very short time is always the problem of impact test of high-speed train. The dynamic response characteristics of materials and structures which have higher impact safety velocity needed to be analyzed.

3) The prediction of deformation is still not accurate enough. For example, the prediction of the number of hoop folds in diamond mode still lacks an accurate model. In addition, the theoretical research on inclined loading conditions and composite tubes is not enough, which need further study.

4) Gradient thin-walled structures can use asymmetric geometry or uneven material distribution to optimize the performance of energy- absorption structure, but how to build a deformation model of uneven-thin-thickness tubes and how to explain the energy-absorption behavior of gradient thin-walled tubes is a topic worthy of increased research in the academic community.

5) The deceleration level of energy-absorption structure can be considered as a factor of structural design to improve the occupant protection, and the energy absorber inside the vehicle should also be researched to reduce the damage caused by the secondary collision of passengers.

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(Edited by HE Yun-bin)

中文导读

铁路车辆吸能结构的研究进展及应用

摘要：随着吸能结构在学术界和工业界的应用达到前所未有的规模，关于列车耐撞性设计和结构优化最新进展的思考对于成功塑造未来的吸能结构就显得至关重要。实车冲击试验和数值仿真是目前研究铁路车辆耐撞性的主要方法。列车的端部碰撞变形区域一般可采用两种结构设计形式：承载式吸能结构设计形式和专有吸能结构设计形式，本文介绍的不同吸能结构可以适用于不同的铁路车辆，从而达到耐撞性和经济性之间的平衡。为了提升吸能结构能量耗散的能力，对于多级吸能系统的研究层出不穷，力求在现有吸能结构能量耗散能力达到极限时寻求突破。为了减少冲击伤害，乘员的自我保护姿势也被广泛研究。迄今为止开展了大量的吸能结构的研究，但针对吸能结构冲击过程的分析与预测所进行的研究较少，这也是未来吸能结构研究的诸多关键挑战之一。

关键词：铁路车辆；吸能结构；耐撞性

Foundation item: Project(2018YFB1201701-08) supported by the National Key R&D Program of China; Project(ZLXD2017002) supported by the Strategic Leading Science and Technology Project of Central South University, China; Project(2019zzts145) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2019-12-12; Accepted date: 2020-03-26

Corresponding author: GUAN Wei-yuan, PhD; Tel: +86-731-82655294; E-mail: wy_guan@csu.edu.cn; ORCID: 0000-0002-8518-3509