ENERGY ABSORBING MATERIALS AND STRUCTURES IN THE FUTURE DESIGN OF THE ROAD SAFETY EQUIPMENT

Transcription

1 ENERGY ABSORBING MATERIALS AND STRUCTURES IN THE FUTURE DESIGN OF THE ROAD SAFETY EQUIPMENT Dr.sc. Dražen Živković, Mechanical engineer The University of Split, Faculty for Electrical Engineering, Mechanical Engineering and Naval architecture Republic of Croatia ABSTRACT It is an obvious fact that road traffic participants will always make errors. These errors have generally resulted in traffic accidents. While the majority of automobile crashes involve vehicle-to-vehicle impacts, a number of traffic incidents involve vehicles impacting roadside equipment. Existing roadside equipment are designed to maximize performance and minimize cost. However, there are still many fatalities and serious injuries on the roads. Leading EU countries have recognized the need for passive safety improvement of roadside equipment as an important step to achieving reduction of serious occupant injuries. The improvement of existing roadside equipment and the future development of new structures, as well as the testing methods is of high importance for everyone who deals with road traffic problems. Devastating injury statistic of traffic road accidents with car roadside structure collision requires continuing research which should lead to improved roadside equipment. The automotive industry makes great effort to meet more and more rigid norms for the occupant passive safety. These efforts will be fully expressed only with parallel development of roadside equipment. On 23 rd of September 2007 the European Committee for Standardization (CEN) approved the new EN 167. Roadside equipment must be designed in the manner to meet the requirements of EN 167. The aim of this article is to point out that in the road traffic world there is a constant need for research in the field of roadside equipment, with the main purpose to improve of traffic passive safety. The improvements as, usage of energy absorbing materials and structures in the roadside equipment, will insure more favorable decelerations during a crash/accident resulting in less injury for the car occupants. This article contains some pictures that may cause uncomfortable feelings to someone. The author apologizes himself. KEYWORDS: roadside structures, crash energy absorbing material, passive safety, EN 167

2 1. INTRODUCTION Thousands of deaths are statistics, and the death of a person is drama. [1] The improvement of the road traffic safety is an important task for anyone who deals with automotive and road transport industry. In the last few decades a great efforts have been made in automotive industry to improve the vehicles safety devices as are: bumper systems, airbags, crash energy zones, etc. In the same time less attention has been paid to developments in the roadside equipment. Existing roadside equipment (as: fencing, lighting columns, support structures of vertical signs, W-beam guardrail systems, etc.) are designed according the principle of acceptable performance with minimum production cost. The analysis of accident problem involving single vehicle off road collisions, leads to conclusion that most of existing roadside equipment are dangerous structures. This time nobody died! Figure 1. The lighting column penetrated in to the car body [2] The impacts with lighting columns, vertical signs posts and the trees cause serious injuries, very often with the fatality. Usually the columns, various posts and the trees act against car body as the knife acts against cheese or butter. High kinetic energy and the narrow rigid structure of the road equipment, that stops the car in a fraction of a second, produce high deceleration of the car body and the occupants. This usually results in the overall damage to the car, often to the extent the car beyond recognition. The occupants have very little chance to stay alive. Similarly the great safety traffic problem is tree-lined streets. Along these mainly rural and city streets the lines of trees are planted, often less than 1 m far from the carriageway edges. No matter how much, these lines of trees make more comfortable felling in the city environment, in the same time they are dangerous objects alike lighting columns. Trees kill about 200 people a year in the UK [2].

3 Figure 2. The tree penetrated in to the car body [2], the tree market with flowers Sometimes the similar thing is with guide rails and protection barriers that are currently installed along the roads. The severity of the consequences depends on the car speed and the impact angle. Steel guide lines systems are relative safe for passenger cars, but most of these systems are not capable of containing the heavier trucks and modern road vehicles as SUV. On the other side the concrete barriers, capable of containing the heavier trucks, are less safe for smaller cars, causing serious injuries to the car occupants. Figure 3. The protection barrier entered in to the occupant compartment [3] This demand for more safe roadside support structures was also recognized and pointed out by the European Commission, who announced its decision to take concrete action on road infrastructure safety. In the year 2000 European Standard EN 167 Passive safety of support structures for road equipment - Requirements and tests methods was published. This European standard provided a methodology for crash testing a support structure and objective criteria for then deciding if the support structure was sufficiently passively safe. The crash testing procedures and criteria essentially followed the protocols developed for testing crash barriers. The editor of Designing safer roads A Handbook for Highway Engineers civil engineer David Milne, also one of the pioneers for passive road safety equipment, said: Passive Safety is the use of lighting columns, signposts, camera masts and other street furniture which doesn t kill you or severely injure you when you drive into it. You walk away after an accident. Mr. Milne also put the question: How do you know if an item of street furniture is passively safe?, and he gave the answer: It has been tested to a class in EN 167.

4 2. EN 167 INFALLIBLE TOOL IN TRAFFIC ACCIDENT REDUCTION This European Standard specifies performance requirements and defines levels in passive safety terms intended to reduce the severity of injury to the occupants with the permanent road equipment support structures. Consideration is also given to other traffic and pedestrians. Three energy absorption types are considered and test methods for determining the level of performance under various conditions of impact are given. (EN 167 Scope). The EN 167 standard requires two successful crash tests using a specified instrumented car to determine whether a support structure is passively safe. All the tests use a light vehicle to verify that impact severity levels are satisfactorily attained and compatible with safety for occupants of a single vehicle. One of the crash tests refers to low speed test at 35 km/h. High speed test conducted at a selected speed of 50, 70 or 100 km/h is the other one. If the test results show that a structure passed the tests, the structure will be classified according energy absorption levels. The energy absorption levels are determined by following categories: NE non energy absorption, LE low energy absorption and HE high energy absorption. Energy absorption categories NE, LE and HE are based on impact speed and exit speed at impacts of 50, 70 or 100 km/h. If there is difference between impact and exit speeds, a part of kinetic energy has been absorbed and in practice all vehicles will loose some of their kinetic energy and therefore slow in an impact. Kinetic energy will be spend on to the vehicle deformation (which is dangerous if it goes too far) but preferably should also be lost by progressive deformation or degradation of the structure being hit. This kinetic energy loss will reduce the forces acting on the occupants and the vehicles body. Energy absorbing support structures slow the vehicle considerably and thus the risk of secondary accidents with structures, trees, pedestrians and other road users can be reduced. Non-energy absorbing support structures permit the vehicle to continue after the impact with a limited reduction in speed. Non-energy absorbing support structures may provide a lower primary injury risk than energy absorbing support structure [4]. Table 1 Energy absorbing categories [4] Impact speed, v i (km/h) Energy absorption category Exit speed, v e (km/h) HE v 0 0 v 5 0 v 50 e e e LE v e NE v e v e v e v e v e 3.1. Levels of occupant safety There are four levels of occupant safety. Levels 1, 2 and 3 provide increasing levels of safety in that order by reducing impact severity. Level 4 comprises very safe support structures (nonharmful products) classified by means of a simplified test at the class impact speed.

5 The occupant safety level is determined by means of the ASI and THIV values, related to the speed class and energy absorbing categories. Acceleration Severity Index (ASI) value is calculated from the triaxial vehicle accelerations (decelerations). Theoretical Head Impact Velocity (THIV) is the velocity that a hypothetical point mass occupant impacts the surface of a hypothetical occupant compartment. Both values are calculated in accordance with EN In table 2 shows the values of occupant safety level in relation to each speed class and energy absorption category. Table 2 Occupant safety [4] Energy absorption categories HE LE NE Occupant safety level Mandatory low speed impact test 35 km/h Maximum values ASI 0,6 THIV (km/h) 11 Speeds Speed class impact tests 50, 70 and 100 km/h Maximum values ASI 1,4 1,2 1,4 1,2 1,2 0,6 THIV (km/h) NE 4 No requirement No requirement Non-harmful equipment For a given combination of the energy absorption category and occupant safety level, the ASI and THIV values shall not exceed the maximum values specified in Table Figure 4. The scheme of energy absorption categories [5]

6 The classification consists from three parts: 1. Selected high speed test (i.e. 100, 70 or 50 km/h), 2. The energy absorption category which determines the loss of velocity during high speed impact, 3. Safety level of 1, 2 or 3. A classification for a passively safe structure according to the three parameters could be say 100 NE 3. This classification designation means: the structure has been successfully crash tested at 100 km/h (including the mandatory speed test at the 35 km/h), the vehicle retained most of its speed through the high speed test (vehicles exit speed would be more than 70 km/h), the structure achieved the highest safety level 3 when ASI and THIV are considered. The different occupant safety levels and the energy absorption categories enable national and local road authorities to specify the performance level of an item of road equipment support structures in terms of the effect on occupants of a vehicle impacting with the structure. The factors to be taken into consideration include: perceived injury accident risk and probable cost benefit; type of road and its geometrical layout; typical vehicle speeds at the location; presence of other structure, trees and pedestrians; presence of the vehicle restraint system. [4] 3. WHY DO OCCUPANTS GET HURT? The statistic data show that majority of people will experience at least once in their lives a car accident. The road traffic accidents are characterized with unintentional kinetic energy exchange. In a fraction of a second the car losses its speed, usually from crash speed to 0 km/h. This is especially outlined for single vehicle off road object collisions. From the elementary kinematic laws it is known that the deceleration increases when crash time decreases. High deceleration means the high inertial forces will act against human body. Figure 5. Linear acceleration at impact speed of 35 km/h-rigid structure [6]

7 During the car traffic accident consequently any one or combination of the following may occur: the occupants experience high deceleration forces causing serious injuries, particularly to internal organs (especially brain); the object breaks into the vehicle body leading to direct physical contact between the victim and object; the occupant may be ejected from the vehicle leading to secondary impact with the ground or other objects; unrestrained occupants may be thrown around inside the vehicle suffering secondary impact with the vehicle, intruding objects or other occupants. The experience of high deceleration first occurs and negatively acts on human body (neck and brain). The neck will be violently pushed either forward or backwards at a range that is past the normal range of motion of the neck. Whiplash occurs. This is characterized as high speed velocity change (high speed car accident) and is called Isotropic Stress. Whereas diffuse axonal injury involves the deforming or stretching of the brain tissue, resulting in tearing, isotropic stress causes damage through a pulse or pressure wave that moves through the brain at extraordinarily high speeds. The damage is caused by a sudden change in the density of the inside of an individual brain cell. The instant compression causes damage to the internal structures of the brain cells. [7] Figure 6. Neck injury during whiplash [8] An understanding of what type of crashes occur and how occupants get hurt in such crashes is vital for person who checks road designs for future operational safety. In order to reduce the consequences of such collisions it is necessary to increase over several seconds the collision time. In that case the deceleration and inertial forces will decrease, getting the chance to the occupants to walk away after an accident. The road equipment that acts in described manner it is called forgiving equipment.

8 4. THE FREQUENCY OF ACCIDENTS AND THE NUMBER OF FATALITIES As someone like to see some statistic data for making up own opinion about the problem, it will be presented some Croatian traffic accident statistic. This statistic data are taken from the Croatian Ministry of the Interior Bulletin of the Road Safety, and they refer to year The consideration of road safety problem needs to determine the types of accidents that mostly happen, and also the kind of road that these accidents have the highest frequency. Figure 7. The types of traffic accidents year 2010 [1] Figure 8. The number of fatalities - types of traffic accidents year 2010 [1] Figure 9. The traffic accidents by the road category year 2010 [1]

9 Figure 10. The number of fatalities by the road category year 2010 [1] The analysis of the above charts leads to the conclusion that the traffic accident vehicle run-off the carriageway is very significant type of the accident. Also analyzing the accidents and the number of fatalities by road category it is obvious that most accidents with fatalities happen in the urban and rural areas. Speaking about single vehicle collisions with various road equipment and objects, it is important to analyze the statistic data about the objects being hit in mentioned accidents. On the figure 10 the statistic data of UK (year 2005) is presented. Figure 11. The objects most commonly hit [9] The three graphs in Figure 11 reveal which objects are most commonly struck, by road type and crash severity. In the first case for urban roads we see that lamp posts are the most commonly struck object, followed by trees and then road signs. Trees accounted for 54 deaths on urban roads in In the second example for rural roads we observe that trees are most frequently hit, followed by crash barrier and then road signs. On rural roads trees accounted for 184 deaths in Finally, in the third graph, for motorways, we see that crash barrier is the most commonly struck object. Crash barrier, also known as safety fence, accounted for 29 deaths on motorways in 2005, and 67 deaths in total on all roads. [9] According to the statistic data the most dangerous road objects are lamp posts and trees in urban and rural areas.

10 5. THE ROAD SAFETY EQUIPMENT Towards the EN 167 there are three possible energy absorption categories. Depending of the road category, the speed limit and the accident frequency, the road designers chose adequate road equipment. So, for the motorways and trunk roads, where the driving speeds are over 70 km/h, the adequate equipment is that from NE absorption category. For trunk roads also the LE absorption equipment may be accepted. The preferred equipment for urban and rural areas has to be in HE absorption category. In urban (built-up) areas the speed limits are usually 50 km/h, and reasonable expected extreme driving speeds are not higher than 60 km/h. In the built-up areas there is another problem of possible secondary impacts. These impacts may occur with other cars and object, but what is more dangerous also with pedestrians. In this case the road equipment has to stop the vehicle absorbing the kinetic energy without possibility for bad injury of the occupants. There are a few manufactures in EU who design and produce the safety road equipment. Mainly this equipment refers to NE and LE absorption categories (Lattix NE, WIMED ProLife LE). Future development needs to pay more attention to HE absorption equipment that will be used in built-up areas. Also the future research in this field has to include safety equipment which can protect the occupants from trees along the carriageways. The manufacturer from Belgium Safety Product had developed and tested their ZIP pole according to EN 167. They are ranked in the high energy absorbing class of impact with a vehicle and they guarantee the highest occupant safety level. The reason that ZIP pole reaches the highest safety for occupants and the highest absorption of energy lies in its innovative design and construction. The cross section of a ZIP pole is nine angles and in the longitudinal direction is conical. It is made of steel plate. The main differentiator of the pole is that it is constructed of pliable yet strong steel and is not welded but riveted. This construction enables the column to be strong enough for functional use, this means that it can support the lamp or other equipment, but at the same time, in case of a vehicle impact, it starts flattening gradually, so that it changes shape from almost round into a ribbon which means that the crumpling of a column overtakes the impact force of a vehicle. [10] Figure 12. ZIP pole construction detail and function principle [10]

11 Figure 13 presents the crash test into conventional rigid pole and ZIP pole. While in the first case the car stop in the fraction of a second, in the case of ZIP pole the car was gradual slowing down increasing the stopping time. Figure 13. Crash into conventional pole almost immediate stop (green car) and crash into the ZIP pole gradual slow down (red car) [10] This ZIP pole can probably be more efficient (protect the occupants at higher speed more than 60 km/h) if the profile will be filled with the additional structure that has good energy absorbing characteristic. This non-flammable structure may be aluminum alloy foam. This idea, considered at Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, is in the Programme of virtual testing by computer modeling. There are also a few ideas concerning the problem of occupant protection in the vehicle-tree collisions. One of them is to put the protecting cushion around the tree. To make the sight more comfortable it is possible to cover the cushion by green bushes. The bushes also act as energy absorption objects. This protection might be especially effective for motorists. Figure 14. Tree line protected with energy absorbing cushions and covered by green bushes [11] 6. CONCLUSION It may be said that, in spite of intensive development of the car passive safety systems, it is impossible to avoid the driver errors that cause the traffic accidents. All modern philosophy defines the traffic safety problem as health problem (health loss). Health, in this context, can be defined as a state where crashes or collisions do not lead to health loss. If a vehicle hits some object from the road equipment with damage to the vehicle and road equipment but with no personal injuries, it is not a safety problem it is only a cost. From this point of view, the

12 human body's tolerance for biomechanical forces is the starting point for the design of a safe traffic system. So, the traffic systems have to be designed and constructed in the manner where this human tolerance isn't exceeded. The infallible tool EN 167 is a big step foreword in improving the passive safety of road equipment. All the traffic accidents analysis leads to the conclusion that it is very important to research and improve existing roadside equipment, and also to develop new design with new materials structure. The design has to be developed in the direction of predictable structure deformation, and the structural material must have as better as possible kinetic energy absorption property. One of the promising structural kinetic energy absorbing material is non flammable aluminum alloy foams. LITERATURE [1] BILTEN O SIGURNOSTI CESTOVNOG PROMETA (Bulliten of the Road Safety), Ministry of the Interior, Republic of Croatia, ISSN , Zagreb [2] MILNE, D., A history of the use of passively safe street furniture to EN 167 in the UK, 2 nd International Conference MTSM 2011, ISSN , 29 th -30 th September 2011, SPLIT, Republic of Croatia, [3] HEGLUND, K., 10 years with EN 167 and Zero vision, 2 nd International Conference MTSM 2011, ISSN , 29 th -30 th September 2011, SPLIT, Republic of Croatia, [4] European Standard EN 167, Passive safety of support structures for road equipment Requrements, classification and test methods, November 2007, Belgium, Brussels, [5] Dabczynski, Z., Passive safety products safe lifes Real facts from WIMED experience in Poland, 2 nd International Conference MTSM 2011, ISSN , 29 th -30 th September 2011, SPLIT, Republic of Croatia, [6] Thomson, R., Naing, C., Klootwijk, C., Garcia, J., Grosse, U. and RISER Consortium, Roadside Infractructure for Safer European Roads, Chalmers University of Technology, February Sweden, [7] [8] [9] Proctor, S., Single vehicle collisions and the role of Road Safety Audit, Designing safer roadsides A Handbook for Highway Engineers, ISBN , UK, 2008, [10] Prislan, D., ZIP poles for public lighting with regard to EN 167 passive safety of support structures for road equipment, 2 nd International Conference MTSM 2011, ISSN , 29 th -30 th September 2011, SPLIT, Republic of Croatia, [11] Živković, D., Energy absorbing structures for the roadside safety equipment, International Conference MATRIB 2008, June 2008, Vela Luka, Island of Korčula, Republic of Croatia