Analysis of Traffic Crash Barriers - Case Study Example

Crash Barriers Introduction Traffic crash barriers are basically installed on the roadside to prevent motor vehicles from colliding with dangerous obstacles such as large storm drains, buildings, and boulders. These barriers are normally installed on the roadside or divided highways to prevent motor vehicles from entering the opposing carriageway traffic, and as a result head on collisions are minimized. These barriers can also be used to provide protection to facilities such as hospitals, fuel tanks, pedestrian zones, and school yards. In order to ensure that barriers installed are manufactured using the right kind of material, these products normally undergo extensive computerized simulation and full scale crash testing before being approved for general use. Simulation programs determine performance limits of these traffic barriers hence providing some reasonable level of protection to road users. Performances of crash barriers are composed of two main things and these are the containment level and the working width. Working width is described as the amount of deflection that occurs when barrier is impacted. In most cases Safety crash barriers are benchmarked against European Standards EN1317,a and crash barriers that are designed for normal containment are often tested with vehicle weighing approximately 1.5 tones where the vehicle would hit the barrier at a slant angle of 20 degrees when travelling at 70mph. High containment barriers are normally tested using heavy commercial vehicles that weight up to 38 tons while travelling at speed of 40mph and these vehicles would hit the barrier at an angle of 20 degrees. SECTION A (1)Functions and in-service requirements of crash barriers used on motorways In North America and Western Europe crash barriers are usually made of concrete due to their durability. Traffic crash barriers are generally categorized into two and this is mainly according to the function these barriers serve, and their level of deflection when a vehicle rums on to them. Functions Roadside barriers These barriers are mainly used to protect vehicle traffic from colliding with hazards. Mainly used in steep slopes that may cause rollover crashes. Median Barriers They are mainly used to prevent motor vehicles from crossing over a median and colliding head on with an incoming vehicle. These barriers are designed to be struck from either side. Bridge Barrier This is mainly designed to preventing the motor vehicle crashing to the side of the bridge and falling on to the railroad, river, or roadway. This particular barrier is normally a little bit higher compared to other barriers so that it can prevent cyclists, pedestrians, buses, and trucks from toppling over the barrier and falling on the other side of the structure. These barriers are normally made of reinforced concrete parapets, or multi-rail tubular steel. Work Zone Barriers These are mainly used in the protection of traffic from hazards in work environments. These barriers are unique in the sense that they can be relocated depending with road works conditions. These barriers may be either water filled, or temporary concrete. Barrier Stiffness Crash barriers are classified into how they deflect when hit by a motor vehicle and the mechanism that a particular crash barrier embraces in resisting impact forces. For instance, in United States of America crash barriers are classified according to AASHTO manual that assesses safety hardware standards. Flexible barriers This includes weak post corrugated rail and cable. They are referred to as flexible because they only deflect between 1.6-2.6 meters when hit by a light passenger vehicle or truck. Rigid barriers These are made of reinforced concrete and deflect negligible when hit by a vehicle. These barriers are designed with a shape that direct the vehicle to a parallel path, and this implies that rigid barriers protect traffic from hazards that are very close to the barrier. (2)Crash Barrier Material Properties Following very many years of testing it has been revealed that pre-galvanized steel (42 um) on both sides produces the best results. This is mainly because the material boasts of absence of lead not mentioning total cost of pre galvanization in comparison to post galvanization. Safety barriers are usually installed so that they can contain shock when the vehicle collides. Steel meets these requirements. Even the Acceleration Severity Index (ASI) that indicates the vehicle deceleration in crash test has even demonstrated the steel superiority when it comes to minimizing injuries in vehicle occupants. SECTION B (1) Justify a material performance index that can be used to assess suitable candidate material based on resistance to impact Crash barriers in European Union countries must fulfill EN 1317 standard in terms of the motor vehicles containment level. This barrier must sustain impact from different vehicle types ranging from buses, trucks, to passenger vehicles. In case the collision is from light passenger vehicle the barrier should come with the ability to absorb the kinetic energy from the restraint and vehicle deformation. (2) This particular chapter is going to determine the material that is best to design the crash barrier using the values that were state n materials requirements the software application CES is going to be used to provide list of the best materials that can be used. In order to achieve this all the values are entered in the limit function in order to remove materials which are less suitable for producing crash barriers. With some of the values that have previously been mentioned, some of the most suitable materials have been found to be: Galvanized Steel Aluminium Medium carbon steel Titanium Alloy The next step is to select the most appropriate material and to produce a graph that compares Cost vs. PI. The graph that is established is going to be produced below. From the graph it is revealed that the material with the best Performance Index is Titanium alloy followed closely by the Low Alloy steel. This has made the Low Alloy steel a little bit cost effective when building roads because public money is well spent. SECTION C Manufacturing Method In this chapter we are going to discuss some of the available Crash barrier manufacturing methods. This is going to be done by exploring some features that Crash barriers need to possess together with the economics involved. Manufacturing Requirements From the requirements that were discussed in the above chapters the following features of possible manufacturing methods were identified. The manufacturing method should produce some 1000 crash barriers The mass range should be between 10-15 KG Labour intensity – Low Production rate – minimum o f30 pieces per hour Immediately after these features were analyzed, the manufacturing methods should integrate into CES four possible manufacturing methods, but two could be rejected on the spot because they were only meant to produce products made from plastics. The two methods involved; injection molding thermosets, and injection molding Thermoplastics. The two selected processes were Forging: whereby the metals are compressed between two dies by using gradual or impact to form the crash barriers. Forging provides excellent finishing and also provides high strength materials compared to when the product is made using a different method. Because forging requires very many tools and equipments, its cost tends to be high and therefore require very many pieces to be produced so that the entire can be economical. Pressing and Sintering – In this particular process the metal is in powder form and it is pressed tightly by very high forces in moulds. Immediately the pressing is not complete, the finished product is normally not very strong and this implies that the crash barrier should be given some additional heat treatment so that it gets stronger. Because the metal is on powder form, most raw materials are always useful and not wasted and this also contributes to the process efficiencies. However tools and equipments required in this process are extremely expensive hence involves very high startup costs whenever producing components. By carefully evaluating each of the manufacturing process it becomes easier to make decisions on the best method to use in producing crash barriers. In our case we are going to select pressing and Sintering as the method to manufacture the crash barriers because it is relatively cheaper compared to forging. This production methodology also enables high production rates and higher number because it is automated with low labour density hence economically viable. Pressing and Sintering also produces components that have better tolerances in comparison to forging. SECTION D Microstructure and defects Low alloy comprises of iron and carbon. There are also other elements that can be combined with Low alloy steel to improve on its properties even though they only take less than 5% of the entire volume. Some of the commonly used additives include; Vanadium, Nickel, Molybdenum, Managenese, and Chromium. However, the only defect that is likely to arise with Pressing and Sintering process when using low alloy steel is there is a likelihood of Isotropic properties. This implies that this particular property is going to be uniform throughout the Crash barrier. However, common defects associated with Pressing and Sintering is Lamination cracking. This is a situation whereby air gets trapped inside the powder during the pressing stage and the sides of the components may show some signs of cracking when air tries to escape out. This defect is mainly caused when powder is being compressed at a very fast rate. References Ashby, M.F. (2005) “Materials Selection in Mechanical Design”, 3rd edition, Butterworth-Heinemann, Oxford, UK CES Eco (2005) “The Cambridge Engineering Selector”, Granta Design, Cambridge (www.grantadesign.com) Farag, M.M. (1989) "Selection of materials and manufacturing processes for engineering design", Prentice-Hall, Englewood Cliffs, NJ, USA. Johnson, K.W., Langdon, P. and Ashby, M.F., (2001) “Grouping Materials and Processes for the Designer – and Application of Cluster Analysis”, to appear in Materials and Design. Ulrich, K.T. and Eppinger, S.D. (1995) “Product design and development”, McGraw Hill, New York, USA. Read More