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An elementary approach to safety
01/09/2010 | Channel:
Rolling Stock, Health & Safety
The structural integrity of railway vehicles – and hence passenger safety – has been much improved in recent years. GRAEME ANDERSON discusses a project in which finite element analysis has been used to test new design ideas quickly, effectively and at little relative expense
Rail accidents are thankfully very rare. According to the Rail Safety and Standards Board (RSSB), last year there were just 39 ‘potentially higher risk’ accidents and 2009 was the fourth year out of the last five that there were no passenger or workforce fatalities in train accidents. In comparison, the latest DfT statistics reveal that there were 2538 deaths on UK roads in 2008. Yet, when serious accidents do occur on the railways, they often tend to involve trains travelling at high speed and therefore can result in many casualties. Accidents such as those at Potter’s Bar or Clapham Junction for example, will remain in the public consciousness for many years to come.
Maintaining the structural integrity of a carriage during an accident in order to protect its occupants is therefore a major consideration for train operating companies. To this end, many manufacturers are investing significant effort in designing and testing rolling stock that can be proven to withstand heavy impacts. In fact, this is a heavily regulated area. Euronorm (EN) standards have been set for assessing any form of passenger train, and in the UK British Standard EN 15227 defines the level of assessment required of railway vehicle bodies in order to demonstrate their crashworthiness. The British Standard applies to new designs of locomotives and passenger-carrying rolling stock, and its requirements apply to the vehicle body and associated parts that may be used to absorb energy in a collision. The intention of the standard therefore is to preserve the structural integrity of the vehicle in order to protect its occupants.
To assess the compliance of a new railway vehicle design against the requirements of BS EN 15227, rolling stock manufacturer Hyundai-Rotem recently appointed engineers at Frazer-Nash Consultancy to undertake a thorough analysis of the impact performance of a new aluminium bodied electric multiple unit (EMU) design currently in development for use as a commuter train in South Korea. Although the design is initially intended for use outside Europe, this work may be instrumental in demonstrating the train’s safety in order to help satisfy the demands of markets beyond South Korea, possibly including the UK.
Whereas the design stages of many of the trains currently in service would have depended on expensive prototyping and physical tests, designs can now be analysed far more effectively and cheaply before reaching a need for any manufacture. With rapid increases in the amount of software and hardware power now available for undertaking computer-based analysis, such analysis can also incorporate a greater level of complexity and produce comprehensive data that can allow the designs to be modified and fine tuned in order to improve predicted performance long before any physical testing is performed.
The engineering team constructed a complex computer model that was used to analyse the proposed train design under a range of impact conditions. Using the geometry of the train together with appropriate manufacturing data, an accurate 3D model of the train was constructed that included full details of all of the key structural components, including all relevant section thicknesses and strain-rate dependence of material properties. The new EMU has been designed to be manufactured primarily from aluminium, so using their knowledge of the properties of this and the other materials used, the team developed a finite element analysis (FEA) computer model (see box). The FEA model was suitable for analysing a number of collision scenarios (as given within the EN standards) in order to understand the structural performance of each of the carriages and to identify critical components that control the overall vehicle response to various impact conditions.
One of the significant causes of injury in any rail crash, however, is not necessarily the actual deformation of the passenger compartment in the initial impact, but instead the rapid deceleration of the passenger compartment. Such deceleration can lead to such violent movement of passengers that they risk potentially experience serious injury through impact with the internal furniture or the train structure itself. For this reason, much of the focus of engineers and designers in recent years has been on understanding the mechanisms that control the deceleration of the passenger compartments so that, in the unlikely event of an accident occurring, the risk of injury can be limited.
Therefore, of particular importance to the team was use of the computer model to develop a detailed understanding of how kinetic energy within the train would be dissipated in the event of an accident. With a focus on this aspect of structural performance, a series of theoretical impact scenarios were examined, with each of these scenarios involving an impact between the front of the train and an obstacle. For example, crash scenarios examined during the work included ‘head-on’ collisions with an identical train and impact with a deformable obstacle intended to represent a road tanker at a level crossing. The data generated by analysing these scenarios enabled the team to understand the performance of the various structural components used to manufacture the train and the complex interaction between these components under crash conditions. In turn, an understanding was built up of the potential risk to both the driver and passengers, and design changes suggested if necessary.
One of the key considerations to ensure that passengers are protected in such accidents is increasing the level of energy absorption around the front of the train, thereby reducing the level of deceleration that occurs in the passenger compartments. As a general rule, when deceleration occurs at a lower level but over a longer period of time, the number of injuries experienced by passengers decreases. For this reason, crashworthiness standards tend to demand a relatively low average level of deceleration in new trains. At the same time, however, when focusing on a frontal impact for an EMU, the driver’s compartment needs to be protected and must retain sufficient room for the driver following the crash; it is not sufficient to allow the entire driver’s cab to be used as a crush zone to absorb the kinetic energy of the train and reduce deceleration. The challenge is to find a balance between protecting the train driver as well as rail passengers. This balance depends on providing a reasonable level of ‘survivable space’ for the driver, whilst allowing enough deformation to reduce deceleration and in turn reduce passenger injuries.
The analysis work performed using the finite element model developed in this project was validated against a series of small-scale component trials along with some larger crash tests performed on prototypes of the energy-absorbing components from the cab and intermediate ends of the carriages. Data obtained from the trials was successfully compared to the results from equivalent analyses performed using the finite element model in order to confirm that the model was an accurate representation of the train and was responding in the appropriate manner.
As a result of this work, the train manufacturer has been able to demonstrate that the new EMU design would be capable of meeting the relevant crashworthiness requirements of BS EN 15227. This is a successful outcome in itself, however the most beneficial achievement in taking this analytical approach is in reaching a sound conclusion at just a fraction of the potential cost that would have been involved in constructing and physically testing a full prototype of the entire train. By using an assessment approach based on testing and small-scale trials, examining potential modifications or improvements to the design can require expensive reconstruction or redevelopment of the prototype followed by a repeat of the crash test. By using a computer model of the train, such changes can be made far quicker and many more alternatives can be examined with ease. FEA therefore makes it possible to test new design ideas quickly, effectively and at little relative expense. As a tool for the assessment of vehicle crashworthiness then, FEA is here to stay, and through its increased use will help create many more of the trains that will run on our railways in the years to come.
What is FEA?
Finite Element Analysis has been used since the 1940s as a means of developing new products or examining refinements to existing products. Its major benefit is in providing engineers with numerical data about the performance and likely behaviour of a structure or system without the need for extensive physical tests.
In summary, the analysis technique involves dividing the structure under review into very small segments or ‘elements’. A mesh can be created in either 2D or 3D in order to represent the geometry of the structure under assessment. This mesh is then further defined with the properties of the materials it represents so that the performance of each element can be analysed under the conditions that represent the scenarios for which the analyst requires information. Often the mesh is particularly dense (contains a high number of small elements) around areas expected to be under high stress, perhaps based on experience, knowledge of previous failures or of the materials used.
Most finite element packages include a ready-made element ‘library’ so that different types of systems can be readily modelled, and many allow the analysis of a the behaviour of large number materials ranging from the classic elastic response of metals, to more complex biological materials, and everything in between. The engineer developing the model can apply various load conditions to the analysis, such as heat, pressure or displacement, in order to examine the overall response of the whole system to a large range of physical conditions.
A package known as LS-DYNA was used in the work described in this article.
Graeme Anderson is principal consultant at Frazer-Nash Consultancy