To mitigate the degree of damage to passengers caused by automobile collisions, a friction damper was built and used in experimental tests to test its effectiveness in impact energy attenuation. The study revealed that energy absorption capacity of a bumper can be improved with the addition of a friction damper. The results revealed that the addition of the friction damper to an automobile bumper to give a bumper-damper system could attenuate about 32.5 % more energy than with the bumper alone. It can be concluded that the effectiveness of automobile bumpers to withstand impact of vehicles by absorbing the kinetic energy from the impact can be improved with the use of a passive friction damper. That is, a passive friction damper system could be used to attenuate more road vehicle impact energy in collisions.
Road traffic crashes and their effects are quite complicated and expensive. Costs attributable to this include social costs, medical costs, loss of production, human costs, material costs, settlement costs and traffic jam costs. Road traffic accident costs Ghana US$ 165 million annually, which is about 1.6% of its Gross Domestic Product (GDP) [
Over the past 20 to 30 years, Bumper design concepts have changed drastically. EEA [
An automobile bumper is the front-most or rear-most part, specially designed to allow the car to sustain low speed impact without damage to the vehicle’s safety systems. That is, they are not capable of reducing injury to vehicle occupants in high speed impacts [
For passenger cars in USA, the law specifies 10 bumper tests, including pendulum tests and crashes into a fixed flat barrier. This is in line with the bumper standards that stipulate the impact resistance of vehicles in low speed front and rear collisions. The purpose of the Bumper standard for Passenger Motor Vehicles other than Multipurpose Passenger Vehicles was to reduce physical damage to the front and rear ends of a passenger motor vehicle from low speed collisions [
Bumpers could be designed to absorb more energy than they usually do with some modification of the design and possibly with the use of additional energy absorption devices. This work seeks to apply a passive control system which is an uncontrolled damper that requires no input power to operate. Passive control systems attenuate or absorb vibrations automatically without the need of an electrical control system. They are simple and generally low in cost, but are unable to adapt to changing needs after installation. The passive control system was selected for this work because of its stability, simplicity and low cost in its application. Passive systems include base isolation systems, viscoelastic dampers, bracing systems and friction dampers [
King et al. [
By using stiffness of a vehicle and its components, the kinetic energy loss in deformation on vehicles can be estimated. In an investigation, Vangi [
Different impact attenuation measures have been used in the railway industry, for example, to mitigate the effect of high forces on the sleeper. In one of such examples, the force content is filtered and attenuated by the softening medium, rail pad installed between sleeper and rail [
Global road traffic fatalities by sex and age (Source: [3] )
An experiment was performed using an impact test machine to investigate whether the impact attenuation capacity of a bumper could be enhanced with the addition of a friction damper. A model of friction damper was designed using springs with a definite stiffness. Special fixtures were made to adapt the impact test machine to the test conditions. Fixtures were fastened to the hammer and machine to give a good surface for the impact. The impact fixture was made in the shape of an L, with webs to strengthen the welded joints. This was clamped to the impact machine as shown in the schematic diagram in
During the experiment, the bumper specimen and the bumper-damper combination, where applicable, were arranged together and the hammer of the impact machine allowed swinging freely to impact on it. The hammer of the impact test machine was raised to specific heights and allowed to fall under gravity to hit the bumper specimen in the experimental setup. During the experiments four different heights were used to give four impact forces.
The angle at which the hammer swings from rest,
Impact test machine with impact (R) and hammer (O) fixtures
Schematic of a simplified pendulum hammer of an impact test machine
Destructive impact tests were performed on pieces of the bumper specimen. Specimen from two bumpers B and C, from two different cars were used in the analysis. 4 specimens of bumper B and 8 specimens of bumper C were used. The average length of the specimen was 35 cm. The specimen, and where applicable both specimen and damper, were put together, placed on the impact fixture and the hammer allowed to swing freely to impact on it/them.
The friction damper was made with springs of stiffness 44 kN/m. For bumper B, the four bumpers were tested without a friction damper; but for bumper C, four specimens were tested without a damper, and four tested with a damper. The impact forces during the tests were computed and used in the analysis. The results obtained for the two sets of tests are tabulated and presented in
Consider a hammer swings through an angle, θ before impact, as indicated in
Then, the sum of moments at point A is given by
This implies
Hence
But
Then
But ω1 = 0, since the hammer swings from rest.
Therefore
. Experimental results for the two sets of tests.
Impact Load without a Friction Element | Impact Load with a Friction Element | |||
---|---|---|---|---|
Impact Load (N) | Deformation of Bumper B (mm) | Deformation of Bumper C (mm) | Impact Load (N) | Deformation of Bumper C (mm) |
3662.80 | 2.00 | 7.00 | 3552.26 | 4.00 |
5856.50 | 3.00 | 13.00 | 5788.10 | 8.00 |
7491.70 | 11.50 | 20.50 | 7438.27 | 9.00 |
9122.80 | 20.00 | 40.00 | 9078.95 | 27.00 |
Therefore,
Also,
Hence the impact force F from the pendulum is
Hence Equation (1) gives the impact force, F, from the pendulum swinging through the angle
Therefore the maximum impact force, F, from the pendulum is when
4. Discussion of Results
Using the following values for the machine, for a hammer of mass of 20 kg;
Using the Equation (1), different angles,
These results were used to determine the energy absorbed in the impact by plotting a graph of Impact force against deformation and finding the area under the curve. The details are given in the next section.
The strain energy absorbed can be calculated by plotting a graph of impact force versus deformation and using integration method to find the area under the curve. The impact forces and their respective deformations were plotted and curves fitted to the plots. MATLABTM was used to find the best fitting curves and their equations. It was observed that the curves had a general equation of the form:
. Angle of pendelum hammer and corresponding impact forces.
Impact Load without a Friction Element | Impact Load with a Friction Element | ||||
---|---|---|---|---|---|
. Angle of pendelum hammer and corresponding impact forces. | . Angle of pendelum hammer and corresponding impact forces. | Impact Load (N) | . Angle of pendelum hammer and corresponding impact forces. | . Angle of pendelum hammer and corresponding impact forces. | Impact Load (N) |
57 | 89 | 3662.80 | 57 | 82 | 3552.26 |
36 | 89 | 5856.50 | 36 | 82 | 5788.10 |
19 | 89 | 7491.70 | 19 | 82 | 7438.27 |
0 | 89 | 9122.80 | 0 | 82 | 9078.95 |
where y is the displacement and x is the impact force ´ 10−5; and A, B and C are constants. That is Displacement = A(Impact force ´ 10−5) + B(ln(Impact force ´ 10−5)) + C. Equations of the curves are also given in
Plots of the curves of the experimental results are presented in
The results of bumper C with a damper give an exponential curve shown in
The amount of energy absorbed in each case (with or without a damper) for a deformation of 2.5 cm was calculated. The strain energy absorbed as a result of the deformation of the bumper is given by the areas under the respective curves. From
The areas were calculated using integration of the equations of the respective curves.
From the calculations, energy absorbed by the bumper without a friction damper was 119.42 J, and that absorbed by the bumper C with friction damper was 158.22 J. This implies that the bumper-damper system absorbed 38.8 J more energy than that without the friction damper. This represent 32.5% more energy for the one with the friction damper than the one without the friction damper.
Results for bumper B without a friction element
. Curve-fitted equations of the deformation for different impact forces for the different specimen.
Bumper Specimen | . Curve-fitted equations of the deformation for different impact forces for the different specimen. |
---|---|
B-without a Friction Element | . Curve-fitted equations of the deformation for different impact forces for the different specimen. |
C-without a Friction Element | . Curve-fitted equations of the deformation for different impact forces for the different specimen. |
C-with a Friction Element | . Curve-fitted equations of the deformation for different impact forces for the different specimen. |
Results for bumper C with (b) and without (a) a Dam- per
A friction damper was built and tested with a bumper to check for its effectiveness to attenuate impact energy. The study showed that energy absorption capacity of a bumper can be improved with the addition of a friction damper. The experimental results revealed that the addition of the friction damper to a bumper to give a bumper- damper system could attenuate about 32.5% more energy than with the bumper alone. It can be concluded that the effectiveness of automobile bumpers to withstand impact of vehicles by absorbing the kinetic energy from the impact can be improved with the use of a friction damper.
The method used for the experiment was a destructive one therefore each specimen could be used only once. Since material manufacturing methods cannot guarantee that the material strictly had the same properties, deviation of material property in the same bumper could also have affected the experimental results. This could also have influenced the experimental results.