An improvement on the end-of-life of High-speed rail rolling stocks considering CFRP composite material replacement

It is undeniably true that transportation systems positively impact human life but, the use of transportation systems also emits greenhouse gasses in the environment which leads to global warming. In this day and age, various technologies are being combined with transportation systems, aimed at reducing the global impact of greenhouse gas emissions. The high-speed rail (HSR), which is well-known for low CO 2 emissions, has continually been improved, especially its model. In this study, the material replacement method is taken to account for a feasibility study on the next model of the HSR. There are three types of composite materials that consist of glass filled with epoxy, glass filled with polyester and glass filled with nylon, replacing the six chief parts of HSR components. As a result, the total weight of rolling stock has significantly reduced to 24.61% on average so, it can decrease the amount of fuel in the HSR operation. Also, the feasibility of the recyclability (%R cyc ) and recovery rate (%R cov ) is critical in judging the suitability of the replacement. The research found that the replacement of CFRP (Carbon fiber reinforced polymer) generates a positive result on %R cyc value at 73.9% compared with 61.4% in the original model. Besides, the %R cov at 78.9% is higher than the 73.1% of the original model.


Introduction
With regards to the dramatic growth in railway industries around the world, rails have become one of the most crucial public transportation modes, directly creating a positive impact on human life. However, growth and demand are causing large scale issues in terms of its global impact. Many researchers have revealed that the increase of the world's temperature to 36.5 F is an effect of transportation emissions, which includes high-speed rails around the world [1,2].
The report from the UIC showed that transportation sector CO2 emissions, accounted for 24.7% of global emanation in 2015. In comparisons of CO2 emissions from transportation sectors, nearly three quarters came from the road (motor vehicles etc.); while 4.2% of the global transportation emissions was a result of rail sectors around the world [2]. Therefore, attempts to get passengers to change their mode of transport from cars to trains, has boosted the gradual growth of rail networks in many areas i.e., China, France and the USA.
The issue of most concern is the methodology needed to reduce the amount of CO2 released into the atmosphere from the transportation sector which directly relates to air quality, and which is a significant cause of severe diseases.
Heavy duty vehicles confirmedly have a greater effect on air quality compared to rail and passenger vehicles [3]. The high rates of CO2 emissions impact the ozone and PM2.5, which is a particle matter air pollution less than 2.5 in diameter, especially in big cities. Health effects from PM2.5 have manifested with numerous symptoms i.e., airway damage [4,5,6], cardiovascular impairment [7,8,9] and diabetes mellitus [10,11,12]. Therefore, the train as a means of transport has become a critical driver in promoting a cleaner environment.
Moreover, improvements in the railway industry can be found in various aspects, aiming to provide a positive impact and improvements in global railway operation efficiency; for example, many researchers have focused on combustion systems changing from diesel to electric operated trains [13,14,15]. Chan and et al.'s research (2013) mentioned that the substitution of electric systems on rail networks could reduce GHG emissions (including CO2) up to 27,000 tons per year or a decrease of 98% from the current operation on the rail network [16].
Some have pointed to the management of end-of-life rolling stock [17,18,19]. Regarding Kaewunruen and et al.'s (2019) findings, the end-of-life rolling stock of HSR contains 37 components and, the study described that recyclability rates (%Rcyc) and recoverability rates (%Rcov) were 61.4% and 73.9%, respectively [17]. The main reason for this was that the rolling stock model was composed of steel and mixed aluminium and steel. Consequently, waste management methods through end-oflife rolling stock, can reduce left over landfill waste which in turn leads to a decrease in CO2 emissions.
Lightweight materials have also been applied across transportation industries. In the USA, nearly one-third of pollution was emitted from transportation. Many researchers revealed that the reduction on the total weight (i.e. car body) could decrease fuel consumption; in other words, the amount of CO2 emissions would be degraded [20]. Nowadays, car industries have entirely adopted lightweight material replacement techniques in their model.
In this study, the material replacement technique is applied to the HSR model. The "original model" of the end-of-life HSR from Kaewunruen and et al.'s research (2019) will be compared with the "new model", which includes substitution with lightweight material on the HSR's body and other parts. The study aims to improve the performance of both Rcyc and Rcov rates which leads to a reduction in CO2 emissions which greatly impacts human health.

Methodology
Aiming to reduce rolling stock body mass, the feasibility of substitution of materials used in the original model, with lightweight materials, must be considered as follows. First, the selection of materials for replacement in the original model, which mostly consists of aluminium and steel; determining which rolling stock parts need to be replaced with light-weight material should be thoroughly considered. Second, the calculation of component mass, after replacing the old materials, is addressed by analyzing the change in total mass of the rolling stock. Lastly, the impact of end-oflife rolling stock must be considered; since the change in body weight leads to leftover landfill waste CO2 emissions.

Material replacement technique
Based on Kaewunruen and et al.'s (2019) research, there are some parts of HSR rolling stock that can be replaced by lightweight materials, i.e. composite material, Advanced High Strength Steel (AHSS) and Magnesium. An obvious benefit of the lightweight material in the vehicle's body was revealed by the US government showing that a "10% reduction in vehicle weight can lead to an 8% improvement in fuel economy" [17]. This refers to the decrease in fuel demand for powering vehicle, which in turn degrades CO2 emissions. Nonetheless, the main body and other parts of the HSR's rolling stock are made from aluminium and steel so, the substitution of those parts must be of a lighter weight than aluminium.
Nowadays, composite materials are applied to some car body components in order to reduce total rolling stock weight and GHG emissions [17,21,22]. The material contains remarkable properties, i.e., high stiffness, high strength, low density and is suitable for reforming in to multi-shaped components [17].
Composite materials are made from more than one component and, a well-known composite material is reinforced plastic [23,24] which is currently being used in various industries. Reinforced plastic was applied to some parts of the HSR and passenger trains, mostly in the car body.
In this study, three types of composite materials were chosen to replace some parts of the HSR, including glass-filled epoxy (35%), glass-filled polyester (35%) and glass-filled nylon (35%).
Moreover, reinforced plastic contains higher values of energy recovery rates compared to other materials, thus making it a suitable replacement material for the rolling stock model [25][26][27][28][29].
The main reason of selecting the three above-mentioned composite materials is the current market availability, especially for glass-filled epoxy [30,31]. Moreover, many industries have made attempts to lower the cost of composite materials. This study therefore provides case studies on replacement composite materials in HSR rolling stock.

Calculation of components' mass
The primary purpose of replacing the material is to reduce the total mass of the train which in turn leads to a decrease in CO2 emissions. The "original model" has a total weight of 265,000 kg, as this model contains steel and aluminium in more than 50% of all components. However, the replacement of composite material effects the body weight. Thus, the weight of the "improved model" of the HSR rolling stock, can be calculated using Eq. 1. The perspective on replacing composite materials focuses on some rolling stock parts based on the feasibility study. There are six main parts including the roof, door, car body/tumblehome, brake control unit, break rheostat/dynamic brake and bogie transom. Composite material replacement is divided into three cases studies, as shown in Table 1. As a result, the total weight of the HSR rolling stock is changed, reducing CO2 emissions due to higher Rcyc and Rcov rates compared to aluminium or steel.  The key finding in Table 1  respectively. The low-density property of composite materials significantly impacts the total weight of the HSR, and so, positively contributing to a light-weight car body. Therefore, the replacement with composite materials leads to a reduction in the amount of fuel consumed during the operation.

The impact with the end-of-life rolling stock
Regarding end-of-life rolling stock CO2 emissions, many researchers have measured its performance by greenhouse gas emission rates, through recycling rates (%Rcyc) and recovery rates (%Rcov) [17,25,33].
The recycling process of end-of-life rolling stock aims at turning waste parts into useable material and to decrease environmental impact. The full process includes three stages; pre-treatment, dismantling and shredding. First, the pre-treatment stage removes all liquid, toxicities, fuel and gases.
The process aims at preventing accidents during the transfer of rolling stock parts to the recycling process.  × 100% ] where;

Results
The The reason for this is the Rcyc and Rcov of CFRP is 95% [34]; whereas, the value of aluminium represents a Rcyc of 14% and Rcov of 33% [25,26].
As illustrated in Table 2 Figure. 1. The significant increase in both rates, greatly reduces both landfill waste and CO2 emissions in the long term.

The parametric study of time effect on MRF and ERF values
The technology revolution in manufacturing has increased recyclability production. Since the industrial era, various materials have been applied to abandoned products to support human needs.
However, un-recyclable products have significantly impacted the environment which has pushed the idea of recyclability and the recycling of end-of-life products across industries forward.
Aluminium and steel materials are mostly used in sectors such as transportation and construction, as well as in other products. Products that are made from those materials and which are leftover or unused have increased landfill waste, significantly damaging the environment. Nevertheless, the improvement of recycling technologies and regulations to limit CO2 emissions have provoked an increasing number of recycling products. As shown in the example of aluminum production in Figure. 2, the percentage of recycling components in aluminium production has risen after 1980 [35].
In the transportation sector, mixed aluminium with steel and pure steel make up the majority of components in HSR rolling stock which consists of 51.55% and 36.52%, respectively. The recycling rate of aluminium production relates to its respective industry. In 2007, for instance, the rate was approximately 90% in the transportation sector and 85% in the construction sector, but 60% in the beverage can production industry [36][37][38][39][40][41]. Fig. 2 The percentage of recycling component in aluminium production adapted from [35].
With regards to the end-of-life HSR rolling stock, recycling rates have continuously increased due the advancement of technologies [35,37,40]. The recycling rate of mixed aluminium and steel was only at 90% of the recycling rate in 2006 and it increased by 1% the following year. At present, the recycling rate sits at 94%, which is almost higher than any other materials. In terms of steel recycling, the trend in recycling rates has also increased from 88% in 2012 to 90% in 2019. The obvious increase of recycling rates on mixed aluminium with steel and pure steel are impacted by recycling technologies.
It directly generates a positive impact across end-of-life rolling stock, reducing the amount of waste leftover and the global environmental impact.

Discussion
The HSR system consistently provides a service that positively impacts human life; however, the dramatic growth of the HSR has created higher atmospheric CO2 emissions. Thus, the notion of reducing CO2 emissions has been addressed in various aspects, from changing operation systems to electric trains and even material replacement.
Within this study, the CFPR replacement material was developed based on the "original model" in As a result, the overall %Rcyc and %Rcov is dramatically improved by 73.1% and 78.9%, respectively.
The increase in both values refers to the reduction of CO2 emissions from landfill.
Lastly, the parametric study showed that the improvement in technology directly impacts the endof-life rolling stock's left over waste. In the original model, there is 88,201 kg left over residues but, in the predictive model in 2039, there will be only 13,094.98 kg left over waste. This means that almost all components can be recycled during the manufacturing processes. However, the research highly recommended following the material replacement method using CFRP material (improved model) for following two reasons. First, the "improved model" produces a slightly different amount of left-over waste. Furthermore, while the improved model is currently ready to implement, the predictive model in 2039 is requires future and developing technologies to support the recycling process.

Conclusion
The improved model of the HSR rolling stock, using replacement CFRP materials and techniques shows a decrease in CO2 emissions. This study confirms that the decrease in CO2 emissions in the atmosphere is a results of the reduction in fuel used for operating lightweight rolling stock.
Additionally, CFRP material is ideal for recycling and recovering processes, which in turn decreases end-of-life rolling stock waste. Therefore, using CFRP materials to replace components in the new HSR model is feasible and desirable.

Acknowledgement
The first author gratefully acknowledges the Royal Thai Government for the PhD scholarship at the

Data Availability
The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.