CONCEPTION AND CONSTRUCTION OF AN IMPACT MACHINE (100KJ)
FOR FRONTAL AND SKEW SHOCKS
Etienne Pecquet1, Jérôme Tchuindjang2, Serge Cescotto1
Department ArGEnCo – Solids, Structures and Fluids Mechanics – University of Liège
2
Department A&M – Metallurgy and Materials Science – University of Liège
1&2
Bât. B52/3 – 1, Chemin des Chevreuils – B-4000 Liège
[email protected], [email protected], [email protected]
1
ABSTRACT
Often, to characterize materials and products in dynamic or impact loading, especially for road safety products and more
generally in logistics, two types of tests exist on opposite scales: tests on small specimen and with relatively low energy
absorption or complete crash tests that are very expensive. Here, the goal is the development of a machine between these two
situations. So, this device will allow best product developments and money saving by reducing failed tests in real scales.
The global concept consists in using of the height of the laboratory to create the kinetic energy: a big ramp guides a mass from
the top to the basement of the laboratory where the tested product is impacted.
A big particularity of this equipment is the availability of frontal or skew shocks. This is especially interesting for the road safety
applications.
Introduction
The conception and the construction of the impact machine are financed by the Communauté Française de Belgique.
This is performed in a fundamental research that deals with the conception, the elaboration and the characterization of metallic
macro-foams. Contrary to micro-foams which are made for example from liquid metals injecting gazes producing voids
measuring about one millimeter, macro-foams are obtained by assembly of metal pieces by different methods (welding, gluing,
bolding, packaging, …) letting big voids, from about one to ten centimeters.
The South of Belgium has historically lots of activities in steel industry. Because of the current difficult socio economical
context, new activities are searched for metallic products in this region, that’s the reason concentrating on metallic materials.
The research project also includes ecological considerations like energy consumption reduction. This induces the study of the
direct re-use of metallic materials without return to the electrical furnace. Some of these materials can be used soft drink or
food cans (without or with more of less damage due to the collecting process), wires, chips … It has also been chosen to study
the expanded metal material. This one is currently used for several protections but seldom in structural applications where
however it could be very efficient.
Another imposition is to create, in comparison to micro-foams, cheaper products that can find applications in civil engineering
and in road safety.
The ultimate goal of the research is the improvement of the safety of humans and goods. This includes several domains like
safety in transports, protections against terrorism or seismic actions and protection of goods especially during transports.
These domains of applications require lots of products able to dissipate energy in case of shocks, impacts or explosions. So, a
major idea of the research is the development of metallic products dissipating energy by plastic deformations.
Lots of products can be developed. Here are listed some examples of products: bumpers and safety barriers (new models or
for example improvement of very stiff concrete ones) for road safety, vehicles elements increasing crash performances,
products warranting safety in nuclear material transportation, products damping parachuted elements, products for resistance
of buildings to explosions and seismic actions, other buildings elements like decks or walls.
As it can be seen, lots of applications for metallic macro-foams require dynamic characterization of developed products. Our
laboratories have only small equipments to make dynamic experiments. That’s the reason explaining the design and the
construction of the machine described in this paper.
Performances of the equipment
As already mentioned, a big part of the applications studied in this research concerns the road safety. On the one hand, tests
can be made on very small specimen in classical dynamic presses. On the other hand, real complete crash tests can be
performed but are really very expensive. So, the present equipment is situated between both extreme experimental conditions.
It will allow best developments of products and money saving by reducing failed tests of accreditation in real situations.
The global concept of the machine consists in using the height of the laboratory to create the kinetic energy. So, a big ramp is
constructed on which masses move from the top to the basement of the laboratory. Figure 1 gives a schematic view of the
ramp.
A big advantage with this equipment is obtaining horizontal shocks, like very often observed in road crashes. In comparison,
with vertical shocks, the disadvantage is the influence of the impact mass on the tested elements even after the crash
situation. So, in this case, it is impossible to observe real displacements and deformations of the different parts after the crash.
Figure 1. Schematic view of the ramp in the laboratory
The maximum impact load of this equipment is about 700kg. This value is not far from the weight of a small car. To obtain this
mass, metallic plates are added on the impact wagon, but the machine can work only with the weight of the wagon (no added
mass) and then, the minimum impact load is about 80kg.
Because of the 15m in height between the top and the basement of the laboratory, the maximum theoretical impact speed is
about 60km/h, as explained by equation 1. The initial height of the impact mass can also be freely chosen.
Combining the maximum mass and speed, the maximum available energy is calculated by equation 2.
V = 2⋅g⋅H max = 17,1 m/s ≈ 60 km/h
(1)
E = m ⋅ g ⋅ H max = 100 kJ
(2)
These performances should be reduced a little bit by friction between the wagon and the ramp. Normally, these ones will
remain small because of the use of ball bearings only.
The impact mass is a simple wagon only put on the ramp. Many shapes and materials can be mounted on this wagon.
Because of geometrical constraints, the maximum front area of the wagon is about 1,6m in width and 1m in height.
In the basement, the testing zone is a large metallic horizontal floor measuring 6 x 6,5m. On it, eight metallic slabs, each one
with a mass of three tons, are placed as we want to maintain the specimen to crush in place. Because of the important
modularity of the eight reaction masses on this floor, another great interest of this machine is the availability to perform frontal
but also skew shocks.
So, for example, in the road safety domain, many products can be tested: bumpers in frontal or skew shocks, safety barriers –
these ones can be as well in metallic material as in concrete and why not in concrete but with metallic macro-foam added to
increase performances – ends of safety barriers, all types of shock absorbers,…
Especially concerning safety barriers in metal, as well the cable effect in the barrier as the rebound of the impacting mass after
the shock can be examined.
Always in the road safety domain, large size parts of cars can be tested to determine crash resistance. It is important to
mention than the tested products can be mounted as well on the floor in the basement as on the wagon circulating on the
ramp. It depends on geometries of tested products and crash surfaces (for example, modeling of walls, trees …). Finally, if
masses are correctly distributed to model the mass of the vehicle, as well the vehicle as the tree can move. Only the relative
speed between both pieces before the shock is significant.
Detailed description of the different parts of the equipment
The global view of the ramp is given in figure 1. The structure of this ramp is very stiff to prevent too large deformations or
vibrations during tests. It consists in a HEB300 profile on which a 400 x 20mm steel plate has been bolded. This plate was
necessary to dispose of very good plane surfaces. Figure 2 gives a view, from the basement of the laboratory, of the end and
the curved parts of the ramp.
Figure 2. End and curved parts of the ramp
The wagon containing the impact mass should be as free as possible, especially for skew shocks, to be able to reproduce the
right trajectory after impact. So, it cannot be fixed in three directions on the ramp, it is only posed on it. This induces that
another structure – also a wagon – can maintain the mass on the ramp from the top to the basement.
Masses are lifted to the top of the ramp by a winch. This one, located at the top of the laboratory, can stop at any height to give
the wanted speed to the impact mass. So, to start a test, a third minor wagon – containing principally a jack to disconnect the
two first wagons from the cable of the winch – is necessary. Here below, the two first wagons are described.
Both wagons are made from slabs measuring 400 x 400 x 80mm. In these blocks, voids are machined to include wheels,
fixations between both wagons … Figure 3 shows a view of the underside of both wagons. At the top of the figure is
represented the first wagon on which the impact mass will be fixed. At the bottom, the second wagon, more complex because
it guides by lots of ball bearings along the ramp.
Figure 3. Underside view of wagons
In figure 3, the first wagon is in a 4 wheels configuration (in brown on the picture), but it can also be mounted with 3 wheels
placing only one wheel at the center back side. Both wagons are attached together by two hooks (in red on picture).
As the first one, the second wagon can be put down on the ramp in a 4 or 3 wheels configuration. In addition, this wagon
disposes of two additional ball bearings mounted on springs that come on the underside of the ramp. They press the wagon on
the ramp to prevent any takeoff. Finally, there are still three other ball bearings – two fixed on one side and the third one
mounted on springs on the other side – to ensure the lateral trajectory.
As it can also been seen on figure 3, the second wagon that drives both wagons along the ramp has big lateral parts. The
reason of these parts is explained by figure 4. A major goal of the machine is to have an impact mass free of movement from
the beginning of the shock to the stop of the mass. For that, it is necessary to remove fixations between the two wagons at the
end of the ramp, before the impact zone.
Figure 4. Disconnecting zone between the ramp and the impact floor
Just before the impact floor and just after the curved part of the ramp, we have the disconnecting zone.
As it can be seen in figure 4, there are two cams on the ramp (in red on pictures) that will lift both hooks. Once this operation
ended, there remains no link between both wagons. Just after that, the second wagon comes in contacts with both hydraulics
dampers (in black on picture). These ones have been designed and constructed in function of the mass and the maximum
speed of the second wagon to be able to stop it. The maximum displacement of the dampers is about 20cm only.
Figure 5. Upper side view of the impact zone
After this disconnecting zone, only the impact mass is moving, comes on the impact floor and crushes the tested specimen
installed against the big reaction mass made of eight elements of three tons each ! This is illustrated in figure 5 by an upper
side view showing the end of the ramp (in blue), the disconnecting zone, the impact floor (in green) and the reaction masses
(in dark grey).
The location of reaction masses in figure 5 is only one example corresponding to a frontal shock for short tested structure. The
eight masses can be freely located on whole the impact floor.
In dynamic tests on so big structures, it is not easy to measure what happens only with forces, displacements or accelerations
measurements. In addition, tests are generally not in one direction (one degree of freedom) but can use the six degrees of
freedom in space. For these reasons, the major measurement system will be high-speed cameras.
Conclusions
A dynamic testing machine has been presented. It uses gravity to create the necessary kinetic energy. The impact direction is
horizontal. Frontal as well as skew shocks can be simulated. The key numbers representing the maximum performances are
700kg and 60km/h. These sizes allow best developments of products especially for road safety before validation tests that are
very expensive in real scales.
Measurements are principally made by high speed cameras to be able measuring displacements and deformations in three
dimensions.