Experimental Testing of Energy-Absorbing Structures Used to Enhance the Crashworthiness of the Vehicles

Abstract

Selected structures intended to absorb impact energy have been analysed in respect of their use in the rear underrun protective devices (RUPD) of motor trucks. The main purpose of the RUPD is to prevent a passenger car from running under the rear of a motor truck provided with such a device. From the point of view of the safety of the car occupants, it is important to take into account the components whose additional role would be to absorb a part of the impact energy so that the loads on the said occupants were minimised. This article presents experimental test results concerning selected energy-absorbing structures. Based on quasi-static strength tests, simplified material models were defined. As a result of experimental crash tests, the possible applications of selected energy absorbers to the RUPDs as their components accountable for the passive safety of passenger cars were indicated. Absorbers proposed in this paper can be considered effective energy-absorbing structures, e.g., in the case of the central impact of a medium-class car with a speed of about 40 km/h. They are relatively inexpensive in production and easily implementable to motor trucks, even taking into account some limitations related to the type-approval regulations on the European market.

1. Introduction

Crashworthiness refers to the vehicle’s properties that have an impact on its passive safety level. In general, the less severe the injury to vehicle occupants, the higher the crashworthiness of the vehicle. The designing of vehicles to achieve their best possible crashworthiness has been a challenge for manufacturers for many years, especially when the social and legal requirements are continually growing. The development of structures and materials capable of dissipating kinetic energy during impact or at intensive dynamic loads has been the subject of much research, in particular for the automotive and military industries [1,2,3]. In consequence, a number of solutions successfully used in passenger cars have been developed over the recent decades [4].

Since motor trucks are generally intended for commercial use, they are designed, first of all, for their load capacity and cargo space volume to be as big as possible. The questions of road traffic safety are in principle limited to active safety, i.e., braking and steering systems, stability of motion, etc. [5]. The issue of passive safety is predominantly focused on the driver’s space, i.e., the vehicle cabin, and the front, side, and rear underrun protective devices, because in the process of type approval in the European market, the cabin is subjected to strength tests for conformity with UN ECE Regulation No 29 [6,7], and the said underrun protective devices must meet analogous type approval regulations. The basic function of such devices is to ensure the safety of other participants in road traffic [8,9,10].

In 2021, rear-end collisions made up 12% of all road accidents in Poland, i.e., every second collision between vehicles in Poland was a collision of this kind [11].

Among such accidents, those where a passenger car hits the rear end of a motor truck are considered the most dangerous; therefore, it is extremely important to ensure the adequate crashworthiness of the motor truck’s end. Such a role in the truck is played by the rear underrun protective device (RUPD), whose purpose is to prevent a passenger car from running under the rear of the truck [12].

Given the fact that the market share of electric (BEV) or hybrid vehicles (HEV, PHEV) is constantly growing [13], the issue of road safety is related to another significant threat. In vehicles with internal combustion engines, passenger safety during a frontal collision is dominated by issues related to minimizing the loads and ensuring the adequate rigidity of the passenger space. The risk of fire or fuel explosion is limited only by protection against fuel leakage due to mechanical damage to the fuel tank or fuel system [14]. In BEV or hybrid vehicles, the problem of ensuring the safety of passengers is associated with minimizing the risk of fire too. However, apart from protecting batteries against mechanical damage, it is also important to minimize the shock loads to which the energy storage system (REESS) may be subjected. The battery packs used in electric and hybrid vehicles may be damaged due to significant loads, which may result in a vehicle fire. It is therefore important that the shock loads acting on a passenger car do not exceed 28 g [15,16]. In the event of an electric or hybrid passenger car hitting the rear end of a motor truck, the value of these loads also depends on the stiffness of the rear underrun protective device.

This article presents the results of experimental tests carried out on the selection of an energy absorber installed in the rear underrun protective device of a motor truck. In the second chapter, the most desirable features that should characterise the energy-absorbing structures are described. Relevant results reported in other articles are also pointed out. In the third chapter, based on quasi-static strength tests, simplified material models are defined. The samples analyzed in this work are characterized in detail. The characteristics of the tested materials obtained in static tests are presented. The fourth chapter describes the results of dynamic tests. The test results and the level of deformation of the analyzed samples are presented. The fifth chapter presents a detailed analysis of the results based on selected indicators. Finally, as a result of experimental crash tests, the possible applications of selected energy absorbers to the RUPDs as their components accountable for the passive safety of passenger cars have been indicated.

2. General Features of Energy-Absorbing Structures

The vehicles are so designed that they undergo only small elastic deformations under loads that occur during normal operation. Meanwhile, the structures that are to absorb energy must show high impact resistance; therefore, when the process of their deformation is designed, issues such as a change in their geometry, plastic hardening effects, as well as the issue of deformation rate should be taken into account. In automotive engineering, metal elements in the form of profiles (three-dimensional structures) with diversified shapes and multi-cell type structures (e.g., thin-walled structures with hollow cells of the honeycomb or metal foam type) are commonly used for the dissipation of energy (e.g., impact energy). In principle, energy absorbers are made of materials with plastic properties, such as low-carbon steels, aluminium alloys, composites, as well as metallic and non-metallic foams [4,17,18,19].

The general features that should characterise the energy-absorbing structures may be described as follows [4]:

• Irreversible energy conversion: The kinetic energy of a moving object (vehicle) should be converted into the work performed by the structure or material deformation forces resulting from the non-elastic deformation, in order to avoid the effect of the cumulation of elastic strain energy in the material.
• The force value being kept at a limited and constant level: For the energy absorbability to be as required, the peak values of the loads on the energy-absorbing structures or materials, resulting from the impact, should be below a predefined level (e.g., related to the biomechanical resistance of the human body). Moreover, the reaction force should remain constant for the deceleration growth rate to be limited.
• The energy losing path being as long as possible: The reaction force should remain constant, and its value must not exceed a predefined level. However, the work performed by the deformation force is equal to the product of this force and the deformation size; therefore, if a specific structure is to absorb a sufficient amount of energy, then an adequate deformation distance must be ensured.
• Stable and repeatable deformation process: The external dynamic loads to be borne by the energy-absorbing structures are diverse and depend on many factors, e.g., the place of application, direction, and value of the force impulse. Therefore, the energy-absorbing structures and materials should ensure a stable and repeatable deformation process insensitive to the variable factors mentioned above and, simultaneously, should ensure the energy absorbability as required.
• Low mass: The energy-absorbing structure or material should have the lowest possible mass and, simultaneously, the maximum capability of absorbing the kinetic energy, this being of critical importance, for example, in aircraft designing.
• Low production costs and easy installation: Economic issues should not be disregarded when new products are designed. Excessive manufacturing, installation, or maintenance costs may adversely affect the implementation of effective energy absorbers because the aspect of cost-effectiveness and budgetary limitations is a matter of considerable importance from the entrepreneur’s point of view.

The thin-walled tubes, such as honeycombs, are widely used as energy absorbers because they are relatively inexpensive and light. Over the years, much research work was conducted, including experimental tests, to determine the properties of such structures [19,20,21,22,23]. Much research work was also conducted to determine the characteristics of combinations of various multi-cell structures [18,24,25].

The structures based on foamed metal, e.g., aluminium foam, can also successfully function as energy absorbers because, as it is in the case of the honeycomb structures when they are deformed, the value of the crushing force remains at the same level. The foamed metals used in thin-walled tubes may improve the stability of the structure deformation process, thus enhancing the overall crashworthiness of the structure [26]. The results of the tests reported in [27,28,29,30,31] show that crashworthiness may be improved by using composite structures. The energy absorbability of such structures is higher than that of, for example, the hollow multi-cell ones.

A research work that is particularly interesting from the point of view of the issue under consideration was presented in the publication [17]. The authors carried out a series of numerical tests representing the striking of a rigid impactor with a mass of 600 kg with a velocity of 20 m/s against a test specimen fastened to an un-deformable barrier. The specimens represented various configurations of absorbers consisting of five oblong chambers, which formed a cuboid with dimensions 60 × 60 × 200 mm. The chambers were filled, in various configurations, with a honeycomb-type structure or aluminium foam. It was found that the filled multi-cell structure can absorb even 70% more energy in comparison to the hollow one. The maximum value of the energy absorbed was about 8 kJ. It is not unimportant, however, that the value of the peak crushing force (PCF) increased by more than 20% and amounted to 88.8 N. Similar results were obtained for the structures filled with aluminium foam. An analysis carried out by the authors led to the conclusion that the best results were obtained for the structures filled in part only.

Results of numerical tests carried out for composite structures were also described in the publication [17], with the difference being that a multilayer structure with dimensions 80 × 80 × 240 mm was used in place of the structure consisting of oblong chambers. For the specimens with homogenous and multilayer structures, the values of the peak crushing force were close to each other and amounted to about 120 kN. The authors showed that if the foam density in the individual absorber layers is properly selected, then the value of the energy absorbed can be increased, with the value of the crushing force remaining unchanged at the same time.

The authors of the two publications mentioned above, however, did not express their opinion regarding questions concerning the manufacturing of complex energy absorber structures, while this undoubtedly can determine the possibilities related to the introduction of such structures to production and sale. When working on the desirable features of energy absorbers, it is also worth taking into account the questions related to the minimisation of the production costs.

Table 1. The most significant advantages and limitations of the structures described in the selected articles.

Material Description	Advantages	Limitations
FGF—graded foam filled thin-walled structure [17]	The SEA index value of the FGF specimen is greater than that of the uniform foam-filled specimen. The peak impact force of the FGF column is lower than that of the uniform foam column.	The finite element model of the FGF-filled column cannot be directly verified, as experimental data were not reported.
Honeycomb and foam interactive filled structures [18]	Filler material can improve the energy absorption of thin-walled structures. However, it may impose adverse effects for SEA index. For single material filled structures, filling the corner cells of the structure with honeycomb or foam is preferable to filling the central part of the structure. For mixed filling structures, filling the corner cells with honeycombs and central part with foams can improve the crashworthiness characteristics.	Filler material may impose adverse effects for the SEA index. Crashworthiness.Characteristics of the honeycomb-filled and foam-filled structures are dependent upon material properties and geometric dimensions.
Honeycombs (generally) [19]	Good energy absorption level. The overall large densification strain allows for energy absorption at an almost constant load over a greater strain interval.	Anisotropic material, resulting in large directional variations of the load response.
Foams (generally) [19]	Relatively high specific energy absorption.Most of the materials are inexpensive.Isotropic—their response is independent of the direction of the applied load.	The behaviour of the foam differs considerably under dynamic conditions.
Thin-walled square steel tubes [20,21]	The quasi-static theoretical predictions for the mean axial crushing loads meet with the corresponding dynamic test results.	Global bending and asymmetric deformation during the axial load. Anisotropic material—large directional variations of the load response.
Foam-filled double-cell and triple-cell columns [24]	Total crushing resistance by amounts equal to 140% and 180% of the direct resistance of the foam for double cell and triple cell, respectively. The foam-filled sections generally improved specific energy absorption values and weight-efficiency in energy absorption.	The finite element model cannot be directly verified, as experimental data have not been reported.
Square multi-cell columns [25]	The energy absorption efficiency of a single-cell column can be increased by 50% when the section is divided into smaller cells.	Anisotropic material, resulting in large directional variations of the load response.
Aluminium foam-filled conical tubes [26]	Foam-filled tubes absorb significantly more energy than empty tubes. The influence of foam filler is more visible for the mean load than for the peak load. The dynamic energy absorbed for a foam-filled conical tube can be improved by increasing the wall thickness or increasing the semiapical angle or increasing the density of the foam filler.	The finite element model cannot be directly verified, as experimental data have not been reported.
Box columns filled with aluminium honeycomb or foam [27]	Aluminium honeycomb filling is more weight efficient than aluminium foam filling. The superior strength-to-weight ratio of honeycomb compared to foam is due to its lower density. For a certain crushing strength, the density ratio between aluminium foam and honeycomb is on the order of greater than 3. The ratio of the strengthening effect to the specimen produced by alu-foam and alu-honeycomb filling is only on the order of two. This leads to the superiority of aluminium honeycomb in the specific energy absorption.	In the combination of compressive and bending issues, aluminum foam filling could give better crash behaviour than aluminum honeycomb filling.
Filled aluminium crash boxes [28]	The foam-filled tube that absorbs the same energy as an empty tube has more than 19% lower weight. The bending strength of the foam-filled tube is higher than the empty tube.	If the foam with a lower or higher density than the optimum one is selected, the specific energy absorption decreases.
Nomex® honeycomb filled thin-walled aluminium tubes [29]	Nomex® honeycomb might be an alternative to aluminium foam filler in thin-walled tubes as long as the tube crushing load is comparable to the honeycomb crushing load.	Anisotropic material, resulting in large directional variations of the load response.

summarizes the most relevant test results of the energy-absorbing structures described in the selected articles. The significant novelty of the results presented by other authors was emphasized. Limitations of individual structures, which were demonstrated in the selected publications, are also presented. In consideration of the fact that the widely published results of research on energy absorbers were obtained for specimens with different dimensions and that those tests were carried out in diverse conditions, the authors of this contribution deemed it necessary to carry out their experimental tests where the future conditions of using the absorbers in vehicles would be represented. The test results described herein constitute a continuation of the research project, the results of which were given in [12,32]. In those publications, the assumptions adopted for the rear underrun protective device provided with an energy absorber were defined. In those assumptions, not only the obvious aspect of crashworthiness but also the economic questions and the possibility of, in the future, putting the device into production were taken into account.

3. Experimental Identification Tests

In the examination of the properties of energy-absorbing structures and materials, the quasi-static strength tests should be first taken into account. Since some structures are more sensitive to dynamic effects in comparison to others, the use of simple material models, such as the Cowper-Symonds material model, may be insufficient for their full parametrisation. Nevertheless, the use of simple models often simplifies the calculation process and facilitates the variant approach to the problem and to the analysis of the test results.

If the elastic strains are insignificant, then the ideal plastic material model, as shown in Figure 1a, may be considered. The ‘ideally plastic model’ means that the material does not show any tendency to plastic hardening, i.e., stiffness growth with increasing strain, or that such a tendency is inconsiderable. In other words, the material used in the ‘ideally plastic model’ is plastically deformed until the deformation is complete, and the value of the force applied remains at a constant level. If the plastic hardening is significant, then the elastoplastic and piecewise linear model of cold-work hardening (Figure 1b) or the elastoplastic non-linear (parabolic) model of cold-work hardening (Figure 1c) may be considered, depending on the actual behaviour of the material during experimental tests [4].

The materials and structures used for the absorption of kinetic energy usually undergo large plastic deformations. The plastic deformation is much bigger than the elastic one; therefore, the latter usually may be ignored in the analysis. The idealised material model is referred to as the ‘rigid-plastic model’.

To determine the physical properties of the materials considered, several experimental tests were carried out. At first, the specimens characterised in

Table 2. Description of the specimens subjected to quasi-static tests.

Material Symbol View of the Specimen	Description	Height × Width × Length (m)	Mass (kg)
HC1.71	Honeycomb structure made of aluminium 3003 sheets, sheet thickness 0.076 mm, cell dimensions 6.4 mm, mass density 82.6 kg/m3, crushing strength 1.71 MPa, expansion process manufacturing.	0.12 × 0.12 × 0.15	0.18
HC1.71x2	Honeycomb structure made of aluminium 3003 sheets, sheet thickness 0.076 mm, cell dimensions 6.4 mm, mass density 82.6 kg/m3, crushing strength 1.71 MPa, expansion process manufacturing.	0.12 × 0.24 × 0.15	0.34
HC1.71_BOX	Honeycomb structure made of aluminium 3003 sheets, sheet thickness 0.076 mm, cell dimensions 6.4 mm, mass density 82.6 kg/m3, crushing strength 1.71 MPa, expansion process manufacturing, additionally reinforced with steel sheet walls.	0.125 × 0.125 × 0.15	0.78
HC1.71x2_BOX	Honeycomb structure made of aluminium 3003 sheets, sheet thickness 0.076 mm, cell dimensions 6.4 mm, mass density 82.6 kg/m3, crushing strength 1.71 MPa, expansion process manufacturing, additionally reinforced with steel sheet walls.	0.125 × 0.245 × 0.15	1.56
CCAF	Aluminium closed-cell foam, mass density 260 kg/m3, crushing strength 3 MPa.	0.12 × 0.12 × 0.15	0.56
CS_01	Ceramic structure made of silicon carbide and aluminium oxide.	Diameter × length [m] 0.1 × 0.04	0.18

were subjected to quasi-static strength tests.

Using a test rig at Łukasiewicz Research Network – Automotive Industry Institute (Łukasiewicz – PIMOT), compression strength tests were carried out (Figure 2), and the values of the force applied and of the actuator piston’s displacement were recorded. During the tests, strength characteristics (Figure 3) were determined for the materials taken into consideration, and some specimens were selected on these grounds for further research work.

The strength characteristics of the specimens tested, approximated according to the models shown in Figure 1, are presented in

Table 3. Strength characteristics of the specimens having been tested.

Material Symbol and Model Adopted	Characteristic Curve	Description
HC1.71—Ideally plastic model		The material was plastically deformed until the deformation was complete; the value of the force applied remained constant.
HC1.71x2—Ideally plastic model		The material was plastically deformed until the deformation was complete; the value of the force applied remained constant.
HC1.71_BOX—Elastoplastic model		The value of the force applied changed with the deformation. The linearization of the force vs. the deformation curve without taking into account the outermost values, however, makes it possible to identify a model close to the model of softening by compression.
HC1.71_BOXx2—Elastoplastic model		The value of the force applied changed with the deformation. The linearization of the force vs. deformation curve without taking into account the outermost values, however, makes it possible to identify a model close to the cold-work softening one.
CCAF—Ideally plastic model for a limited deformation range		The ideally plastic model could be applied but for a limited deformation range. The material was plastically deformed, and the value of the force applied remained constant but only for the first half of the specimen length. Then, the model turned into the cold-work hardening one.
CS_01—Strongly non-linear model		No adequate idealised material model could be found.

.

Summary of the Analysis of the Quasi-Static Test Results

When the curves shown in Figure 3 and Table 2 were analysed, it was noticed that, apart from specimen CS_01, the materials selected underwent predominantly plastic deformation. When the yield point was exceeded, the aluminium honeycomb structures were deformed plastically until the deformation was complete, while the values of the forces applied remained constant or slightly varied. In the case of specimen CCAF, the force vs. deformation curve was observed to turn from a linear part into a parabola. To assess the specimens examined, a statement may also be made that only some of them have all the features required from the materials that are to absorb kinetic energy (

Table 4. Assessment of the specimens tested in respect of the desirable features of energy-absorbing materials.

Material Symbol	Irreversible Energy Conversion	The Force Value Is Limited and Constant	Stable Deformation Process	Low Mass (in Relation to the Specimen Volume)
HC1.71	Yes	Yes	Yes	Yes
HC1.71x2	Yes	Yes	Yes	Yes
HC1.71_BOX	Yes	Yes	Yes	Yes
HC1.71_BOXx2	Yes	Yes	Yes	No
CCAF	Yes	Yes *	Yes	No
CS_01	Yes	No	No	Yes

* within a limited deformation range.

).

As it can be seen from the table above, most of the specimens selected were in conformity with the requirements adopted for the materials to be used as kinetic energy absorbers.

4. Crash Tests of Selected Structures

The analysis of the results of the quasi-static strength tests showed that the specimens denoted as CH1.71, CH1.71_BOX, CH1.71x2, CH1.71_BOXx2, and CCAF have properties that are desirable from the energy absorption point of view. In further analyses, specimen CS_01 was given up because of its strong non-linear force vs. deformation curve. To determine the dynamic energy-absorption characteristics of the specimens selected, crash tests were carried out at Łukasiewicz – PIMOT using a test rig presented in Figure 4. For the tests, a sled with a mass of 325 kg was prepared, onto which the specimen under testing was mounted. The test sled was accelerated to a pre-set speed so that the specimen hit against an un-deformable barrier. During the test (i.e., the impact), the acceleration of the sled was measured. Additionally, the course of every impact was filmed by means of a high-speed camera (1000 frames per second, with resolution 2048 × 1024 pixels). The test results are presented in Figure 5 and Figure 6.

Apart from the range of the maximum specimen deformation values (over 0.12 m), the resultant accelerations did not exceed 28 g (g = 9.81 m/s²). An exception is the CCAF material, for which, though an acceleration of 26 g was recorded at a deformation of 0.1 m, the acceleration value reached as much as 42 g at a deformation of 0.12 m.

Based on the analysis of the results of the crash tests carried out, it may be stated that the force values determined to coincide, to a certain extent, with the values recorded in the quasi-static tests. Except for the peak values recorded in the tests of both kinds for the maximum specimen deformations, the materials underwent plastic deformation until the deformation was complete, while the values of the force applied remained constant or slightly varied.

When comparing the deformation behaviour of the specimens, the main differences were found (

Table 5. Deformation behaviour of the specimens.

HC1.71x2		HC1.71_BOX		HC1.71_BOXx2		CCAF
	0 s		0 s		0 s		0 s
	0 m		0 m		0 m		0 m
	0.003 s		0.003 s		0.003 s		0.003 s
	0.026 m		0.026 m		0.024 m		0.025 m
	0.006 s		0.006 s		0.006 s		0.006 s
	0.05 m		0.05 m		0.046 m		0.049 m
	0.009 s		0.009 s		0.009 s		0.009 s
	0.073 m		0.072 m		0.065 m		0.071 m
	0.012 s		0.012 s		0.012 s		0.012 s
	0.094 m		0.094 m		0.082 m		0.09 m
	max: 0.02 s		max: 0.023 s		max: 0.024 s		max: 0.022 s
	0.142 m		0.147 m		0.126 m		0.125 m

).

The deformation process of the HC1.71x2 specimen was stable. The deformation occurred by the progressive collapse of the cell walls of the structure directly at the front plate of the test sled. During the compression process, the deformed area destabilised the adjacent areas of the cell walls, propagating the deformation mode until the end of the structure length. The CCAFx2 sample deformation model is similar to the HC1.71x2 model. Both absorbers were homogeneous structures. The two boxes with honeycomb cores were deformed in an unstable way. The influence of the steel walls on the deformation process of the aluminium core was noticeable. Already in the initial phase, the deformation of the side walls occurred in a random alignment. The walls of the boxes collapsed, then curled and overlapped in an unpredictable way. The results of the analysis of the absorber’s deformation process are consistent with the results of strength tests, especially quasi-static ones.

5. Analysis of the Test Results Obtained

In most cases, the crashworthiness is assessed on the grounds of the force vs. deformation curve, which shows how the force applied changes with the specimen deformation. Such a graph makes it possible to read the value of the maximum deformation force as well as the maximum value of the reduction in the corresponding specimen’s dimension (‘specimen shortening’). For the analysis to be more precise, some indices that would enable comparisons between the test results obtained must be determined:

−

The EA index is defined as the total deformation energy defined during the plastic deformation:

$$EA={\int }_0^{x}F\left(x\right)dx$$

(1)

where F(x) is the instantaneous crushing force and x is the specimen deformation (shortening).

−

The SEA index is defined as the ratio of EA to the mass of the specimen under test:

$$SEA=EA/m$$

(2)

where m is the mass of the specimen under test.

−

The MCF index is defined as the mean crushing force for the known deformation length:

$$MCF=\frac{1}{x}{\int }_0^{x}F\left(x\right)dx$$

(3)

−

The PCF index (peak crushing force) is defined as the maximum value of the crushing force recorded during the test.

The results obtained are shown in

Table 6. Results of dynamic strength tests.

Material Symbol	Energy Absorbed (J)	PCF (kN)	SEA (kJ/kg)	MCF (kJ/m)	Deceleration (m/s2)
					Total	Working Length
HC1.71x2	13,683	504	40.2	96	599	283
HC1.71box	13,745	524	17.6	96	1439	214
HC1.71box_x2	13,643	210	8.8	99	1496	150
CCAF	12,478	260	11.1	88	864	414

and in the graphs that follow (Figure 7, Figure 8 and Figure 9).

Figure 7 shows the results of determining the SEA values. The highest one was obtained for specimen HC1.71x2, for which the SEA index was somewhat higher than 40 (kJ/kg), i.e., its value was more than twice as high as those obtained for the other specimens. In turn, the lowest value was obtained for specimen HC1.71box_x2, which was made of the same material as that of specimen HC1.71x2, with the difference being that this one was additionally enclosed with a steel sheet.

The SEA index is strongly related to the value of absorbed energy and the mass of the sample. The density of the honeycomb is three times lower than that of aluminium foam. This makes the honeycomb structures much lighter. The analysed structures absorbed a comparable amount of energy during crash tests; hence, the highest SEA values were obtained for the lightest specimen, HC1.71x2 (the structure without reinforcements). The results for the MCF index (Figure 8) for the honeycomb-type specimens (HC1.71…) were close to each other and generally reached close to 100 kJ/m.

Somewhat lower values (below 90 kJ/m) were recorded for specimen CCAF. This is because of the fact that various values of the kinetic energy effectively absorbed were obtained for the deformation values being close to each other. When the values of this index were juxtaposed with the maximum accelerations achieved, it was noticed that much higher acceleration values were recorded for the honeycomb-type specimens with sheet-metal enclosures.

An analysis of the results obtained for the PCF index (Figure 9) showed that the highest values of the force (blue bars) were recorded for specimens HC1.71x2 and HC1.71box. The relatively high value of this index is, obviously, unfavourable in terms of crashworthiness because it has an impact on the value of the loads exerted on the vehicle during a collision. It should be noted, however, that these results are not fully reliable because they were determined for the maximum deformation values. The PCF values that were determined for the forces developed when the deformation was within its ‘working range’, i.e., without taking into account the maximum deformations, are represented by the grey bars in Figure 9.

It was found that the PCF values thus determined were reduced, especially for the honeycomb-type specimens. Moreover, the differences between individual specimens decreased, too, and did not exceed 70 kN.

6. Discussion

The materials and structures, whose primary purpose is to absorb the impact kinetic energy, have been the subject of research for many years. In spite of that, it is still reasonable to carry out further research work aimed at the implementation of such solutions in different devices and machines, especially where their use may have a positive influence on people’s safety. The authors of this publication plan, within the project they are carrying out, to make use of the research results obtained hitherto to the implementation of energy absorbers in the RUPDs, i.e., in the devices where the minimisation of hazards to participants in the road traffic by the use of energy-absorbing materials has not been universally implemented until now, although it is the subject of numerous research works [8].

The main conclusions that can be drawn from the research results presented herein may be formulated as follows:

−

Employing honeycomb structures as energy absorbers in the RUPDs entails the necessity of using enclosures because the structures must be permanently mounted in the devices they are provided for. The analysis of the test results presented shows that the use of a metal enclosure causes the absorber to deform non-linearly; however, the peak load values do not markedly differ from those obtained for the other specimens.

−

For the load conditions adopted for the tests, the length of the absorbers should be increased to reduce the rapid growth in the loads in the final phase of specimen deformation.

−

The achieved values of the energy absorbed were adequate to the assumptions described in publication [8], as in the case of a central impact of a medium-class car with a speed of about 40 km/h, the absorber is capable of absorbing 14% of the kinetic energy of the impacting vehicle.

−

When the acceleration values are analysed, they should be recognised as not critical (for the deformation considered as taking place within the ‘working range’ of the absorber); hence, the use of the absorbers presented herein enhances the crashworthiness of the RUPD and actually helps to improve the road traffic safety.

−

Among the tested samples, the highest total deformation energy defined during the plastic deformation was recorded for the HC1.71box and HC1.71x2 specimens. Differences in this range for all specimens did not exceed 9%.

−

The highest PCF index (peak crushing force) was determined for the HC1.71box. Slightly lower (by 4%) values were recorded for HC1.71x2, while those for the HC1.71box_x2 and CCF were lower by 60% and 50%, respectively.

−

The most favourable SEA index was obtained by the HC1.71x2 specimen, which was nearly 4.5 times higher than that for HC1.71box_x2.

−

The MCF index, defined as the average crushing force for a known deformation length, obtained similar values for the materials HC1.71… HC1.71box_x2. The difference between them did not exceed 3%. This parameter determined for CCF differed from the highest one by 11%.

−

Raw aluminium honeycomb structures are more weight-efficient than aluminium foams. The superior strength-to-weight ratio of the honeycomb compared to the foam is due to its lower density. For a certain crushing strength, the density ratio between aluminium foam and honeycomb is on the order of greater than 3. The ratio of the strengthening effect to the specimen produced by foam and honeycomb is only on the order of two. This leads to the superiority of aluminium honeycomb in the specific energy absorption. Unfortunately, the application of the absorbers proposed to motor trucks involves some limitations related to the current type-approval regulations [8,33]. In particular, the absorber dimensions are limited not only by the vehicle structure but also, indirectly, by legal constraints. The use of absorbers whose deformation distance would be too long (although this would be favourable from the point of view of energy absorption efficiency) may cause the deformation of the RUPD as a whole to exceed the limits specified in relevant normative documents, and this might result in the nonconformity of the rear underrun protective device with the legal regulations in force.

7. Conclusions

The energy-absorbing structures described in this paper are widely used in the construction and aircraft industry, but in the automotive industry, the use of those materials is very rare. Taking in to account the need to improve road safety, actions are still being taken to investigate new solutions in the field of passive safety. These solutions concern modifications both in terms of the structure of the car body itself, as well as additional elements that can dissipate the energy of a possible collision. It is particularly important in situations where the vehicles involved in the collision have significantly different body structures and curb weights, e.g., a passenger car and a truck. The absorbers proposed meet the basic requirements for energy-absorbing structures; however, unlike the structures presented in numerous publications (e.g., graded foam filled thin-walled structure, foam-filled double-cell or triple-cell column, aluminium foam-filled conical tube), they are relatively inexpensive in production and easily implementable because they do not require the use of complicated manufacturing processes (limited number of production steps due to the limited number of components) nor materials that are hard to reach. Given the above features, the proposed structures might be recognised as capable of finding successful application in the automotive industry as elements accountable for the passive safety of motor vehicles, especially commercial vehicles.

Appropriately used materials might significantly reduce the load values, which is also associated with health effects for the driver and passengers. Therefore, future research work is planned, especially in a manner of numerical simulations of the impact of a passenger car with the rear end of a motor truck. To optimise the crash-worthiness of a rear underrun protective device, regarding energy-absorption, it is desired to continue research with modified structures. It should be considered to use composites, the graded density of a filling, or graded wall thickness [17,18]. Moreover, new designs of energy-absorbers might be considered, e.g., composite sandwich structures, as described in [34].