1.1 Background
A transportation system that carries large number of passengers or amounts of freight must be reliable and safe. In the early times of the railway industry, conventional materials like wood and steel were used for rail vehicle construction that could meet the structural requirements achieved by the technology of that era. However, while providing the necessary durability, these materials were disadvantageous in terms of high energy consumption, demanding traction-braking systems, and damage/wear of wheel/rail because of their heavy weight. When compared with a lighter vehicle, a heavier vehicle will consume more energy/fuel, consequently increasing costs in operation. Increased energy/fuel consumption also implies a likelihood of higher CO2 emissions at some point in the energy supply chain (Robinson et al. 2012). In addition, wheel or rail damage caused by heavy mass can result in line closures, leading to huge losses for companies. Therefore, rail vehicles incorporating light-weight materials such as composites can be capable of boosting the efficiency of the system in many different aspects.
The past decades have seen the rapid development of composite materials in many areas like aviation/aerospace, offshore-marine applications, automotive, defence, and wind energy. It is particularly important for the aerospace sector to use lightweight materials since the main challenge is against gravity in the first place. As a result, specific knowledge is obviously more advanced for this sector. Although not used as widely as the other sectors, railway vehicles have included parts made of composites for many years, for example, vehicle doors, seats, tables, and inner panels (Batchelor & Wilson 1984). However, using composite materials for main structural components in rail vehicles remains a challenge.
One of the main obstacles in the implementation of lightweight composite materials in the railway sector is mainly the lack of structural standards and certification procedures in view of their specific applications. Existing structural standards are only applicable for metallic materials. However, metals are isotropic materials, so they have the same mechanical properties in all directions, while this is not the case for composites materials where the mechanical properties are tailored to meet specific loading requirements. Key load paths can be secured through aligning the reinforcing fibres along the loading direction while unnecessary weight can be reduced in other directions, creating “lighter” designs compared to metals. This allows designers to create weight optimised designs. Consequently, it is necessary for new standards
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applicable to composite materials for railway applications to be put in place, if the potential benefits of the composite materials are to be realised. To address this issue, the European Commission funded project – REFRESCO – investigated the applicability of lightweight materials, with the aim to create a regulatory framework for the use of new structural materials such as laminated and sandwich composites.
The REFRESCO project had dedicated activities which analysed the compatibility of lightweight materials with respect to specific requirements such as structural requirements (static and fatigue loading), crashworthiness (energy absorption), fire-noise-electromagnetic compatibility, joint and manufacturing, prognostic and health management (damage/failure), and lastly reparability and maintainability. Before these activities could take place, a benchmarking study was carried out on material solutions applicable to the railway sector, and a gap analysis study for existing rail standards in order to identify the short comings. The results of the study revealed that most of the railway standards are applicable when the composites are introduced in carbody shells. The exceptions were EN12663 – Structural requirements of railway vehicle bodies, EN15227 – Crashworthiness requirements of railway vehicle bodies, EN50121 – Electromagnetic compatibility, and EN15273 – Gauges, which need adaptation or modification. Among these standards, EN12663 defines the admissible proof loads a rail vehicle carbody can withstand in various loading conditions. This is the most relevant standard to this thesis although importantly, it lacks information on evaluation criteria for object impact.
Impacts from small objects are especially important for high-speed train (HST) lines, where the aerodynamic forces generated by either an individual train or two passing trains coupled with mechanical/natural influences become strong enough to make objects airborne. Airborne objects can contact with infrastructure or rolling stock and cause damage. The closest solid object around the track – a piece of ballast – is influenced primarily, hence this phenomenon is called “flying ballast damage” in most cases. However, sometimes ice piled up under the rail vehicle or around the bogie could fall and results in hard ice or ballast projection as well (also known as snow/ice falling). Consequently, ballast flight damage is a concerning phenomenon for HST operation and is classified as a highly probable phenomenon (50% chance) (REFRESCO WP7-D7.2 2014).
Although EN12663 does not provide an assessment for this phenomenon, to the best of author’s knowledge, there are other various national standards that identify specific object impact requirements for carbodies, for example, British railway group standard GM/RT2100, European standard EN 15152 - Front windscreens for train cabs, French national standards
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F07-101 and NF-F15-818, and UIC (Union Internationale des Chemins de fer - International Union of Railways) standard for windscreens. These safety standards identify the potential threats against different parts of carbodies such as front ends, roof areas, windscreens, and body side windows. Even though the aforementioned standards provide assessment and guidelines to deal with railway specific impact threats, the problem still remains as there is no harmony between most of these methods and the requirements are case specific. Therefore, a more flexible and unifying assessment method is needed.
Furthermore, a convenient assessment method on impact risks against carbody shells should be established if composite materials are to be considered as main structural elements. This necessity arises as a result of the rather sensitive nature of these materials to impacts, and the complex response of laminated composites to impact type loading, e.g. delamination – more commonly known as barely visible impact damage (BVID) – which is difficult to detect although it is an important type of failure which leads to unstable structural integrity.
1.2 Purpose and objectives
This thesis proposes a methodology for the assessment of flying object impact against composite rail vehicle carbodies, and evaluates it by performing a comparison with various national and international impact resistance requirements in order to determine whether the proposed concept can be applied. One important aspect of this work is that while existing standards are case specific, the research carried out here is innovative in that it proposes and creates a harmonised test. Harmonisation here means to draw the considered standards to a common ground, by proposing a method that produces similar outcomes as each of the individual standard does. That being said, the essence of this methodology is to remove the necessity of high-velocity impact experiments which are used to measure penetration resistance of composite materials when they are used in railway vehicles. These experiments are expensive therefore removing the need for high velocity experiments would result in significant cost savings during a composite rail vehicle’s development stage.
The proposed method leads to the following research questions:
To what extent is the new methodology a reliable representation of the assessment of an object strike?
Which parameter is the most reliable for assessment?
What are the challenges of the proposed method?
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As a result of these questions, the objectives of the evaluation are:
Determination of important impact parameters,
To analyse the extent of visible and invisible material failure,
To provide relevant numerical modelling guides,
To harmonize various impact related standards into one test,
To create a simpler method for assessing flying object strike for composite materials in railway vehicles.
1.3 Scope of the thesis
This thesis addresses small object impact risks on rail vehicles which is an issue that needs particular attention in HST operation. An experimental testing method is presented and respective numerical models are validated in static and dynamic loadings. Quasi-static punch tests (QSPT) were carried out with E-glass fibre reinforced laminates and foam core sandwich materials with such laminates, and the material data obtained by these experiments were used in finite element (FE) numerical models to simulate intermediate-to-high velocity impact loading scenarios. Strain rate effects were considered as well. The motivation of preferring E-glass fibre reinforced laminates and foam core sandwich materials is based on the fact that they are cost effective and suitable for rail vehicle applications. Taking into account the vehicle operation environment, conditions of specific threats, and the specifications of impact risk requirement standards, this thesis focuses mainly on the impact of small objects travelling at high velocities. It is beyond the scope of this thesis to examine large object – low velocity impacts which can be distinguished by global structural response.
1.4 Contribution
The significant and pioneering aspect of this research is in the attempt to create a single harmonised method which can be used for various different railway intrinsic small object impact resistance requirements. Another important contribution of this thesis to the railway industry is to present a well-documented source of railway specific impact risks and relevant analysis methods. The design and qualification of composite structures for specific applications is costly and time consuming. Given the fact that there is an increasing demand for the use of composites in the rail industry and that they are proven candidates for main structural elements in rail vehicles, it is believed that this research will prove to be a useful reference especially in
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the design and qualification process, and in particular for the rapid and cost effective approach on assessing flying object strikes.
1.5 Thesis structure
This thesis is composed of seven chapters. Chapter One gives the background and the purpose of the research, while Chapter Two reviews the existing composite applications in railway vehicles, and the previous research on impact response of composites while including the basic definitions and characteristics of composite materials. Chapter Three identifies the impact risks to rail vehicle carbodies, and the requirements from various national and international standards perspective. Chapter Four provides the explanation and presentation of the proposed methodology of this thesis which will be evaluated through relevant railway standards in Chapter Six. Chapter Five will investigate the experimental and numerical work that has been implemented in this research. Lastly, Chapter Seven will finalize the thesis with general conclusions and the recommendations for future research.
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