Introduction. The increasing competition and intensification of production in the engineering industry make it necessary to introduce new materials with desired properties, as well as to search for new unconventional applications of previously known technologies of metal forming, e.g., extrusion. One of such innovations is an attempt to apply the elements of extrusion technology in the design of devices absorbing the impact energy. When designing this type of solutions, the detailed information on the behavior of a metallic material applied in the range of high-speed deformations up to 5000 s −1 is necessary. There are mathematical models pertmitting description of the behavior of material subjected to deformation with a very high speed. However, noteworthy is that these models require the determination of a number of constants and coefficients for each material [1–3]. The alternative solution is to use the data on the material behavior in the range of high-speed deformations obtained by using the advanced research equipment. The most commonly used method for determining material properties at high strain rates is the split-Hopkinson pressure test [4–9]. There is also a method with use of a rotary hammer, which allows stretching and bending of the specimens with a linear velocity ranged from 5 to 40 m/s, which is equal to strain rate ranged from 102 to 104 s −1 [10]. The critical issue of the research on dynamic deformation, also conducted on flywheel machines, is the course and interpretation of the achieved performance graph. There are many doubts as to the correctness of the analysis of the results, especially at the starting phase of deformation. Whether the observed varied performance graphs are caused by the specific design and work of the dynamic deformation of the research device, the structure of the gripper/handle, the specimen shape and the ability to identify and tune-off/filter the interferences [11, 12].

In this paper, the test results obtained with an application of the Hopkinson bar and numerical calculation results of backward extrusion process of 1050A aluminum grade at a speed of 10 m/s are presented. The elements of extrusion technology have been used in the process of designing of the impact energy-absorbing device.

Currently used technology of the controlled acquisition and absorption of impact energy consists mainly of two types: crash technology and technology of springs. The crash technology, innovative and patented by Axtone company, is based upon a permanent plastic deformation of a metal filament sheared from the outer surface of the bumper body (including peeling and folding technology). Upon the collision, the outer surface of the bumper is machined by the cutting blades. The level of absorbed energy depends on the size and the cross section of the cutting tape, which makes possible a rather wide range of control parameters of the device, depending on the needs of the particular application. The crash bumpers, currently produced in accordance with the applicable standards, guarantee a controlled acquisition of large amounts of energy emerging as a result of a collision, in case of emergency situations. Energy absorption capacity of a bumper often reaches 1300 kJ. For comparison, the currently used standard bumper can absorb about 70 kJ [13]. The key aspect of all safety systems is their reliability. This applies to all bumpers, whose efficiency must be confirmed by a number of full-scale tests. Crash technology is based on a controlled takeover and absorption of a very high energy under the conditions of the accident taking place at high speeds. To ensure the maximum flexibility of the proposed solutions, the individual solutions, in accordance with the diverse needs of customers, are currently being designed based on the patented cutting technology. In view of the increasing competition, there is a search for new solutions in this field, which can expand the range of products offered and introduce a better performance of these products. This concerns the quantity of energy absorption, stability or strength during operation. The devices for absorbing impact energy must meet certain requirements, such as the amount of energy absorbed, a stable force level during an operation, a certain value of force, etc. When analyzing the requirements for such devices, it is noteworthy that, in the case of a test application of extrusion technology, the condition of a constant force can be met by the backward extrusion technology.

A very important issue is the choice of the charge material, which, while deforming, allows one to absorb the stored energy. Due to the low propensity to strengthening and a low weight, the aluminum alloys have been taken into account (Table 1), from which the available 1050A aluminum alloy has been chosen.

Table 1 Properties of the Selected Low Strain Hardening Materials [14]
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Material Test. In order to obtain data on the material behavior for a wide range of strain rates, the static compression tests and dynamic tests using the Hopkinson bar have been performed. Static compression tests of aluminum alloy have been carried out on the Instron 8862 testing machine. The test specimens in the form of cylinders of dimensions D = 10 mm and L = 7.5 mm have been prepared by turning (Fig. 1).

Fig. 1
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1050A aluminum alloy specimens before the compression tests.

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Compression tests have been carried out with controlling of the displacement motion of the grips at a rate of 5 mm/min. For an accurate measurement of specimen contraction/shortening, the Instron extensometer attached directly to the compression pads has been used. The static compression test procedure is illustrated by Fig. 2.

Fig. 2
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Static compression tests of 1050A aluminum alloy.

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In the static compression tests, the relations between stresses and strains have been determined. Overall figures obtained in tests of static compression of the aluminum alloy are presented in Fig. 3. The graph shows the engineering stresses and strains. The shapes of specimens after the tests are also shown in Fig. 3.

Fig. 3
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Collective graph of 1050A aluminum alloy static compression test and shapes of specimens after the static compression tests.

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Compression tests at high strain rate have been carried out using a specialized test bench with a system of elastic rods (modified Hopkinson bar or the Kolsky bar). A striker bar of specified length produces an adequate loading duration. The impact of the striker bar triggers the automatic data registration from the strain gauges located on both the incident and transmission bars. Stress, strain, and strain rate are then calculated from the data collected via the strain gauges. A general schematic representation of the equipment is shown in Fig. 4. For the signal amplification from the measuring rods (electroresistant extensometer) and the signal acquisition from the bench (the signals from the laser barriers for measuring the projectile velocity) the fast-process amplifier LTT500 by Tasler (LTT Labortechnik GmbH Tasler, Germany), the measuring card NI USB-6366 by National Instruments and the originally developed specialized software have been applied. In the described bench, the amplifier and the A/D measuring card have a bandwidth equal to 1 MHz. During the test the signals have been sampled at the time interval of 0.5 μs.

Fig. 4
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A perspective view of the SHPB apparatus for testing materials at high strain rates (SHPB = split-Hopkinson pressure bar).

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A typical specimen inserted between measuring bars is depicted in Fig. 5, while samples of waveforms of the measuring rods are shown in Figs. 6 and 7.

Fig. 5
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Specimen between the measuring bars.

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Fig. 6
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Exemplary signals from the measuring bars.

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Fig. 7
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Signals from measuring bars processed for calculation of stresses and strains.

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On the basis of the registered signals from the measuring rods, the calculations of strains, stresses and strain rates in a specimen have been performed according to the following relationships for the elastic rods:

$$ \upvarepsilon (t)=-2\frac{C_0}{L}{\displaystyle \underset{0}{\overset{t}{\int }}{\upvarepsilon}_R(t)dt,} $$
(1)
$$ \upsigma (t)=\frac{E{S}_{p0}}{S_{pr}}{\upvarepsilon}_T(t), $$
(2)
$$ \overset{\cdot }{\upvarepsilon}(t)=-2\frac{C_0}{L}{\upvarepsilon}_R(t), $$
(3)

where C 0 is wave propagation velocity, L is length of the specimen, E is the Young modulus of measuring bars, S p0 and S pr are cross-sectional areas of bar and specimen, respectively, ε R (t) and ε T (t) are signals in time for the reflected wave and the wave in transmitted bar, respectively.

The calculated values of ε(t) and σ(t) were used to construct plots of the stresses and strains for the given strain rate.

The overall graphs obtained in the compression tests at high strain rates are shown in Figs. 8–10. Figure 11 shows some examples of specimens after the dynamic tests.

Fig. 8
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Collective graph of 1050A aluminum alloy (compression test at high strain rates) D10/L5.

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Fig. 9
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Collective graph of 1050A aluminum alloy (compression test at high strain rates) D10/L3.

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Fig. 10
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Collective graph of 1050A aluminum (compression test at high strain rates) D10/L1.

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Fig. 11
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Exemplary specimens of 1050A aluminum after dynamic compression test at high strain rates: (a) D = 10 mm, L = 5 mm; (b) D = 10 mm, L = 1 mm.

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Comparing the relationship between stresses and strains for different strain rates, it can be seen that the tested 1050A aluminum alloy shows a small sensitivity to the strain rate. The maximum stress at a strain of 0.3 for the specimen type D10/L5 is equal to 180 MPa, for the specimen type D10/L3 at the same deformation is 220 MPa, and for the specimen type D10/L1 is 270 MPa.

The data obtained from the static compression tests and from the tests with an application of the Hopkinson bar have been applied for simulation of the material behavior in the backward extrusion process at high speed.

FEM Modeling. In the calculations of the impact energy absorption device, the SimufactForming software has been used [15]. The geometry of the model has been set as axisymmetric. It has been assumed that the deformable aluminum alloy charge closed in a rigid container gets into a contact with a rigid punch. Application of the feedstock in the form of a sleeve ensures that the total deformable material remains in the container (Fig. 12c).

Fig. 12
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Geometrical parameters of the impact energy-absorbing device: (a) punch; (b) ingot; (c) general view (e – punch, f – ingot, g – container).

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Motion of the punch has been forced by a single stroke with a suitably chosen large initial mass (20 ton) of the linear speed of 10 m/s. The dimensions of the aluminum alloy charge, in all cases, have been established, and in the case of the punch three different variants of outer diameter (d 1 = 100 mm, d 2 = 102 mm, and d 3 = 104 mm) have been applied (Fig. 12). In numerical calculations, elements of Quad 10 type of 0.5 mm size were used. A detailed description of the FEM model is given in Table 2.

Table 2 Selected Features of the Applied FEM Model
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Based on the calculations, the graphs of the force–punch displacement relations for three cases of the punch geometry have been constructed. Depending on the dimensions, three different values of the maximum force have been obtained (Fig. 13).

Fig. 13
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The relationship between force and displacement for the examined geometric variants of punch (1050A aluminum).

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The varied size of the resulting force depending on the geometry of the applied punch is related to the time, in which the impact energy is absorbed, and the time of deceleration in the tool from 10 to 0 m/s (Figs. 14 and 15). Exemplary results of FEM calculations (effective plastic strain and effective stress in ingot) are shown in Fig. 16.

Fig. 14
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Dependence between time and punch velocity for various geometric punch configurations (1050A aluminum).

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Fig. 15
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Dependence between time and absorbed energy for various geometric punch configurations (1050A aluminum).

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Fig. 16
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Exemplary results of FEM calculations: distribution of the effective plastic strain (a) and effective stress (b) in 102-70-500 ingot.

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The relations obtained with an application of three different geometric configurations of the punch make it possible to assess the extent, to which the device is able to absorb the necessary amount of energy by the controlled deformation of the 1050A aluminum. This is due to the time required to slow down the motion of the punch. The calculation results show that the use of an external diameter of the punch of 104 mm can meet the requirement to absorb 1 MJ of energy in the shortest possible time. In the transport industry, there is a need for devices absorbing different preset amounts of the impact energy, which vary with the particular vehicle weight. That is why an ability to change parameters of operation of such a device is very important and it can be achieved by variation of the punch geometry. At the next phase, it is planned to test the prototype of the designed device.

Conclusions. In this paper, the results of experimental research performed with an application of the Hopkinson bar and the numerical calculations of backward extrusion process of 1050A aluminum at a speed of 10 m/s have been presented. The extrusion technology have been used in designing of the impact energy-absorbing device. The analysis of results obtained made it possible to draw the following conclusions:

1. The research performed confirmed a possibility to use the extrusion technology in the design of the impact energy-absorbing devices.

2. On the basis of calculations it can be stated that the process of backward 1050A aluminum extrusion can be used in the construction of the impact energy-absorbing devices, while detailed results of the calculations should be experimentally verified.

3. Calculations have shown that the use of geometrical parameters of punch 100-70-500 makes it possible to completely absorb 1 MJ of the impact energy during the time of 0.072 s. An increase in the punch cross-sectional area results in reducing the time required to absorb the impact energy and increases the force.

4. The solution satisfies the industrial requirements of the constant force, while the ability to absorb a certain amount of energy turned out to be higher than that of the existing solutions, which can be used, in particular, in rail vehicles with a high weight. Application of the feedstock in the form of a sleeve allows one to keep the total deformable material in the container, in contrast to the existing crash bumper solutions.

5. A significant advantage of this solution is a possibility of continuous adjustment of the force, as well as of the amount of energy absorbed, depending on the weight of the vehicle, by a suitable choice of the device geometry, which it is advantageous for the industrial production. The developed impact energy-absorbing device may widen the scope of manufacturers of this type of equipment on the market.