Design and Evaluation

of a Carbon Fibre Bus BodyScania CV AB

David Nordin

Mechanical Engineering, master's level

2018

Luleå University of Technology

Department of Engineering Sciences and Mathematics

1 Abstract

The automotive industry is in constant development and in recent years the environmental legisla-tions have been getting tougher. The need for lighter and stronger materials has increased accordingto these changes and composite materials such as carbon fibre reinforced polymers is showing po-tential of being a solution due to their high specific properties. This thesis is an investigation anddesign proposal for one way of making a carbon fibre bus body wall structure by the use of pultrudedbeam elements and a certain number of standardised node elements. This is done to increase thepossibility of mass production and possibly lower the manufacturing cost for a carbon fibre structure.

The methodology is based on a product development process where a market research as well asa literary study was conducted initially to see what work had been done in the area. Needs wereinvestigated and formulated to a product specification from which concepts was generated usingbrainstorming methods as well as discussions with bus design engineers at Scania.

A number of materials and manufacturing methods was analysed for the node elements and aftercomparing and scoring different concepts, a carbon fibre node element was chosen. Dimensioningcalculations were made based on standardised tests which simulates different driving scenarios. Theconcept was then designed in 3D-cad and the final weight of the concept was measured to 194 kg.

A comparison of the concept with a steel bus was made by the use of the life cycle analysis tool inCES Edupack 2017 which resulted in a difference of 47 tonnes carbon dioxide released for a dieseldriven light goods vehicle during the first six years of the lifetime. The overall results show thata carbon fibre bus body might be economically beneficial during the entire lifetime of a bus eventhough the purchase price is higher.

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2 Preface

This master’s thesis has been conducted at Scania CV AB in Sodertalje, Sweden for 20 weeks fromJanuary to June of 2018. The project is the final part of my M.Sc. in Mechanical Engineering atLulea University of Technology and corresponds to 30 credits. I would like to thank the departmentof YTMR, Materials Technology where I have done most of the work, for the support given throughout the entire project.

I would also like do direct a special thanks to:

• Magnus Burman, my supervisor at Scania, for giving me advice continuously during theproject and supporting with great expertise in composite materials.

• Patrik Gustafsson, my manager at YTMR for helping me solve problems and giving mesupport when ever needed.

• Dan Jonsson and Hannes Berg for giving me feedback on my work and giving me adviceon material related problems.

• Eric Falkgrim for giving me the opportunity to do this thesis as well as showing great interestin the progress of my work.

• Linus Ahrlig and Robert Sjodin for the information regarding bus technology and how tothink about the future of transportation.

• Karim Karim for answering questions about how buses are built at Scania.

• Rachid Younsi for informing about how calculations on bus body structures are made.

• Marcus Bjorling, my supervisor at LTU for giving me feedback on the methodology andguidelines for the project.

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3 Nomenclature

Symbol Variable Unita Mean radius [m]Iy Area moment of inertia [m4]t Material thickness [m]h Cross section height [m]b Cross section width [m]Kv Torsional cross section factor [m4]Amaterial Area of material in cross section [m2]δ Displacement [m]P Load [N]E Young’s modulus [Pa]β Coordinate of applied force -l Overlap length [m]M Applied moment [Nm]σmax Maximum tension [Pa]zmax Distance from center of beam to edge of beam [m]εmax Tension -Φ Angle of rotation radiansMv Torsional moment [Nm]τ Shear stress [Pa]A Overlap area [m2]ax Side acceleration [m/s2]ay Longitudinal acceleration [m/s2]∆Fuel Difference in amount of fuel [l]∆E Energy difference [J]EDiesel Amount of energy in one litre of diesel [J]

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Contents

1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

2 Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

3 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

4 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2 Aim of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.3 State of the Art Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

5 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.1 Cross Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.2 Dimensioning of Beam Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65.3 Manufacturing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5.3.1 Metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.3.2 Composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.4 Adhesive Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.1 Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2 Concept Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6.2.1 Need Finding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2.2 Product Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2.3 Concept Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136.2.4 Concept Scoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6.3 Detail Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136.3.1 Dimensioning of Beam and Node Elements . . . . . . . . . . . . . . . . . . . 146.3.2 Dimensioning of Adhesive Bond . . . . . . . . . . . . . . . . . . . . . . . . . . 156.3.3 Design of Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6.4 Concept Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167.1 Concept Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7.1.1 Need Finding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167.1.2 Product Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167.1.3 Concept Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187.1.4 Concept Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

7.2 Detail Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227.2.1 Manufacturing Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227.2.2 Dimensioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227.2.3 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237.2.4 Concept Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

8 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298.2 Structural Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298.3 Manufacturing Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298.4 Dimensioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298.5 Concept Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

10 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

A Gantt Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

B Node Cross Drawing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

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4 Introduction

Below follows an introduction to this thesis. First of all the background is presented together withthe aim, as well as a state of the art analysis.

4.1 Background

Tougher environmental legislations increases the demand for new and more sustainable transportsolutions [1]. Because of this, in recent years the electrification of the automotive industry has takenlarge technological steps with fully electric cars available for purchase, and most brands have theirown electric hybrid models. But one current problem with electric vehicles is the size and weight ofbatteries if a big range is required. It means carrying a lot of heavy batteries which compromisesthe load carrying capacity [2] and the range of the vehicle. This poses a problem for commercialvehicles when the amount of load is directly related to the amount of income. One way of increasingthe load carrying capacity and range is to make the body of the vehicle lighter. This is somethingthat increases value for the customer. For example, a lighter bus can take more people and increasethe efficiency of the transportation, both in term of emissions but also in added economical valuefor companies whom can make more money per trip.

It is however the inner-city transportation sector where it is considered to be most beneficial to moveto fully electric transportation. This would according to a study made by KTH in colaboration withStockholm Stad [3] make it possible to deliver goods to cities by night instead of in the day, mostlybecause electric vehicles are silent compared to conventional fossil fuel vehicles. In turn, this coulddecrease the delivery times by two thirds compared to regular deliveries during the day. Becauseof the lack of delivery trucks causing traffic jams, traffic would not be upheld and not contributeas much to the pollution. Also the way that buses in city areas are driven, with almost constantlystarting and stopping, means that the weight will have an even bigger influence on the energyconsumption and consequently the range of the vehicle.

The demand for a lower weight has forced companies to look for new and lighter materials, like lightmetal alloys and composite materials such as carbon or glass fibre reinforced polymers (CFRP andGFRP). Lightweight materials combined with optimized design, could reduce the weight by 50% incomparison with a traditional steel design [4]. According to [5], it is probable that the future ofthe automotive industry will not cope without the use of carbon fibre reinforced structures. Thisis shown in that more mass produced passenger cars have started to implement carbon fibre bodystructures to a greater extent. One example is BMW that have the electric city car i3 that has mostof the load carrying structure and body made in CFRP.

It is however not only environmental legislations that affects the need for material changes. In recentyears the safety demands on buses has increased with respect to roll over demands such as EconomicCommission for Europe Regulation 66, which puts pressure on the structure to be more rigid. Thecombined demands for rigidity and weight reduction makes a carbon fibre structure suitable becauseof its high specific strength.

4.2 Aim of the Thesis

The aim of this thesis is to develop a concept for how a modular bus body side structure could bemade in carbon fibre reinforced polymer for a manufacturing rate of 1000 buses per year. The mainarea of investigation is to utilise the principle of standardised beam elements connected by a certainnumber of standardised nodes. A product specification will be formulated based on needs obtainedby Scania and benchmarking data will be collected and inserted into the product specification froma regular bus used as reference. A number of concepts will then be generated on how to design thestructure and after concept selection, the chosen concept will be compared to a traditional, steelconstruction bus with regards to the metrics in the product specification. This is done to investigatethe potential of a full scale production in the future and possibly expand to other areas than citybuses.

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4.3 State of the Art Analysis

A current research project within VW Group Research is working with a concept for regular steelbuses to eliminate the need of welding in critical areas of the structure. The idea is to develop aconcept for how the structure can be divided by the use of node elements which makes it possibleto move the welded zones further from the joint and thus decrease the stress concentrations in thewelded areas. The node elements are thought to be manufactured in one piece which makes itpossible to make specific shapes to connect the hollow profiles in any desired direction. But to makesure that no intrusions of existing inventions were made, a thorough literary study was conductedby looking for patents and research within the same area. These patents are also relevant for thisthesis to investigate because of the similarities in the basic idea.

The patents are from several different companies from the automotive industry, including Daim-ler(Mercedes), Audi and BMW. Since BMW is currently one of the biggest car manufacturers atmass production of CFRP components in cars, and the patent (DE 10 2013 209 102 A1) is forconnecting a carbon fibre structure with a metal node, this was examined further. The node is men-tioned to preferably be manufactured by casting of aluminium and then fastened by either injectionbonding or plug-in adhesive connection. In the connection, a layer of glass fibre is added to serve asa protective surface against galvanic corrosion according to the patent description. Several patentsfrom Daimler Ag was also relevant to the investigation. The patent called ”Rohrrahmenkonstruktionfur einen Kraftwagenaufbau” (DE 102010033289 B4) considers knot elements made of either CFRPor cast aluminium in a tubular frame chassis with the tubes made of carbon fibre. The patentpoints out the benefits of strength and weight of nodes made from carbon fibre but the economicaladvantage of the aluminium.

Most of the previously mentioned patents and projects have all suggested to manufacture the nodeelements from casting of different kinds of metals. However, some smaller companies that currentlyonly produce in small volumes, suggest another approach. Companies like Divergent Technologiesand Bastion Cycles have both adopted the metal 3D-printing technology and prints its node elementsfrom light metal alloys like aluminium and titanium. In both cases standardised beam elements ofcarbon fibre are used to connect the nodes and build the desired shape. The Bastion Bicycle can beseen in Figure 1.

Figure 1: Bastion bike utilizing the node and beam element principle. [6]

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The company Divergent Technologies’ ”3D-printed supercar” is built mostly of 3D-printed alu-minium nodes and carbon fibre elements that connects them. The carbon fibre tube elements arestandardised parts manufactured by filament winding and on their homepage the following quote isfound[7]:

”Divergent provides a disruptive new approach to auto manufacturing that incorporates 3D printedjoints, which we call a NODETM, connecting carbon fiber structural materials that results in anindustrial strength chassis that can be assembled in a matter of minutes.”

Another market that has adopted the utilisation of carbon fibre reinforced beam elements withmetal joints, is the bike industry. Most carbon fibre bikes are built by nodes and beam elements andassembled using adhesive bonding. As previously mentioned the company called Bastion Cycles fromAustralia is utilizing this concept to be able to tailor each bike to specific customer needs and wishes.The modularity of the parts and flexibility of 3D-printing makes it possible to easily customize thegeometry of the bike by making longer beams and changing the angle of the connecting parts. Themetal parts are made of 3D-printed Titanium which makes the bike less stiff than a complete carbonstructure, which improves the ride comfort according to their website. Both the Divergent Bladeand Bastion bicycles are both low-production volume companies and in the case of the DivergentBlade, it is only prototypes.

When building buses at Scania today, the customisation of the bus according to customer needs areachieved by having a modular design. The bus consists of bigger standardized parts that can bejoint together with different elements to tailor each bus. The front of the bus, counting from thefront axle and forward is one standardised module with one or two variants only. The same principleis applied to the overhang in the back, counting from the rear axle and back. By having these asstandard, with the exception for the possibility to change door configuration, the part that changedepending on length is the middle part between the axles. There are standardised modules like thelength of a window that can be added between the front and end modules depending on the desiredlength and what type of door that is configured.

Current bus bodies are usually made from a steel or aluminium structure which is joint either bywelding or screw joints respectively. The two methods have their own benefits and shortcomings,the welded design is good for not having to add weight of screws and nuts but since the weldingis changing material properties, it can happen that the metal will get misaligned and with risk ofgetting built in tensions. Screw joints however had the advantage that it can be disassembled aswell as aligned as wanted since it is not a permanent fastening method and it does not affect thematerial properties. Mercedes has gone with another approach, similar to the VW Research projectby having node elements that makes the welded areas further away from the joint. This is shown ina collage in Figure 2

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Figure 2: Collage of the Mercedes Citaro node elements.

In this case the node is manufactured by pressed steel instead of casting but is still in one piece.

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5 Theory

The following section is a description of the theory and equations that are used in this thesis forcalculations when dimensioning the concept.

5.1 Cross Section

The following theory was used to compare different cross sections that the structure could consistof. Each type has different performance during different types of loading. To establish which crosssection performs best in terms of bending and torsion, calculations were made using equationsfrom [8]. The area moment of inertia Iy is used to establish performance during bending and theperformance during torsional loading is determined by the torsional stiffness cross section factor Kv.Three types of cross sections were evaluated; circular, rectangular and the special case of a rectanglethat is a square. The circular cross section has an area moment of inertia as described in equation(1)

Iycirc = πa3t (1)

and for a rectangular cross section as shown in equation (2).

Iyrec =1

6th3 +

1

2tbh2 (2)

Both equations follows the labelling according to Figure 3 where a is the mean radius, t is thethickness (for this comparison t = t1 = t2), h is the height and b is the width.

Figure 3: Circular and rectangular cross section.[8]

Further, the strength during torsional loading for a circular section can be calculated by usingequation (3)

Kcirc = 2πa3t (3)

and a rectangular cross section according to equation (4).

Krec =4(hb)2

2( b+ht )

(4)

To display the behaviour of the area moment of inertia for a rectangular cross section, it is plottedin Figure 4. The rectangular factor RectFact is a relationship between the height and the width

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where a value of 0.5 means that the width b is half the height h and the value 1 is for a square crosssection. This is for a cross section that has the same area and material thickness.

Figure 4: Comparison plot of Iy for the strong and weak side of a rectangular cross section dependantof the rectangular factor.

As seen in the graph the stronger side grows quicker than the weak side decreases, this is due toequation (2) having the height to the power of three and the width to the power of one. To determinewhich cross section area that has the best performance per weight for the same cross section areaand material thickness, equation (5) was used to calculate a number that could be compared relativeto each other.

Performance

Weight=

(Iyh + Iyb +Kv)/3

Amaterial(5)

Amaterial is the part of the cross section area that is material which means that the cross sectionwith the largest circumference is the one with largest mass.

5.2 Dimensioning of Beam Elements

Technical beam theory states that the displacement of a console beam, with a force P applied at adistance β from the edge, will be described according to equation (6) where E is Young’s modulus[8].

δ =Pl3

3EIβ3 (6)

The maximum tension will occur at the point where the beam is fastened and can be described asin equation (7) where M is the moment and zmax is the coordinate for the point furthest away fromcenter of the cross section.

σmax =M

Izmax (7)

From this equation, the maximum strain can be calculated from Hooke’s law in equation (8).

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εmax =σmax

E(8)

To calculate the maximum strain due to torsional loading of a console beam, the Saint-Venantstheory can be applied to cross sections with the exception for circular [8]. For other cross sectionssubjected to torsion, the angle of rotation is described as in equation (9) where Mv is the torsionalmoment applied, GK is the torsional stiffness and L is the length of the beam.

Θ =MvL

GK(9)

5.3 Manufacturing Methods

Below follows a description of different manufacturing methods that was considered for manufac-turing of the node elements. There are two main materials considered; metals and composites andwith different materials comes different possibilities for manufacturing. The node elements that areconsidered for manufacturing in this section are supposed to be hollow for saving weight and withthe ability to fasten hollow beam elements.

5.3.1 Metal

There are several different ways of shaping metal in to the desired shapes that would fulfil thedemands for the node elements, one of them being casting. Die casting in particular comes in twomain principles; cold-chamber die casting and hot-chamber die casting. Hot chamber die casting issometimes referred to as ”goose-neck” process due to the way that the metal is inserted into thedie via a goose-neck shaped leader. As seen in Figure 5 the hot-chamber die casting has the meltedmetal surrounding the intake supplying the die. When the cylinder is pushed upwards, it sucks thematerial into the cylinder and is then pushed out through the goose-neck when the plunger appliespressure.

Figure 5: Hot-chamber die casting. [9]

This method is however mostly used for metals with relatively low melting points such as lead,zinc and magnesium since this does not erode the surrounding components [10], [11]. For metalswith higher melting points such as Aluminium alloys or casting steels the cold-chamber die castingis preferred. Cold chamber die casting is similar to the hot chamber die casting in respect to themelted material being forced into the die by pressure and then cooled and ejected by separating thetwo halves of the die. However, the difference is that the material in this case is poured in separately

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into the cylinder, either manually or automated. This means that the material can be melted inanother part of the process and then brought to the machine as shown in Figure 6.

Figure 6: Cold-chamber die casting. [9]

Cold chamber die casting is also used in different variations, high pressure and low pressure casting.The high pressure die casting has the material injected into the die with a high pressure whichrequires the clamping force of the die to be high and leads to expensive tooling costs. The benefitis that since the material is injected into the die with high pressure, thin walls and smooth surfacescan be achieved as well as short cycle times. Because of the high tooling cost it is suitable for ahigher production volume to make it economically beneficial. Low pressure die casting utilises alower pressure for injecting the material into the die. As seen in Figure 7 the metal is inserted fromthe bottom of the die, counter gravity by inserting pressurised gas in the chamber.

Figure 7: Low pressure die casting. [10]

Since the pressure is low in the process, complex shapes can be achieved by the use of sand cores toget hollow structures which is not possible with high pressure die casting due to the high pressure.The quality of the material is ensured by keeping the pressure to the melted material as the cast iscooled down to eliminate air pockets in the die due to trapped air or shrinkage of the material as itcools. Limitations of this method is that the wall thickness is limited to about three millimetres aswell as having a slower cycle time.

Two other casting methods are investment casting and gravity die casting. Gravity die casting isanother die casting method and is mostly suitable for heavier and thicker structures. This is because

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of the only thing that is pushing the metal in the die is gravity which makes it difficult to ensurethe melt has penetrated thinner sections of the part. Investment casting is a completely differentmethod where the required shape is achieved from forming wax as the shape of the finished productand then investing this into a ceramic cover that functions as the mould. The wax is then burnt outin an oven and the mould can be filled with metal. This is commonly done by having the mouldplaced above the melt and then having vacuum pull the metal through the mould until it comes outon top.

5.3.2 Composite

For short fibre composite materials, a commonly used method of manufacturing is sheet mouldingcompound or SMC for short. In SMC the most commonly used fibre is glass fibres that are usuallychopped in length of 10-50 mm and randomly distributed on a carrier film. Before adding the fibre,some resin filler (usually polyester) is added to the film which holds the fibres in the composite.Then another layer of resin is added on top of the first sheet according to the same princible to coverthe SMC from contamination as well as separating the layers when rolled up during continuousproduction. Depending on the performance and cost requirements for the specific use, the amountof fibres can vary from around 25-50 wt%. After it has been rolled up it is aged or matured forusually about three to five days to let the resin thicken and let the material reach a stage where itis ready for forming and final stage of the production. The sheets are cut into pieces after whichthe carrier film is removed and the fibre and resin sheets are placed in a compression mould inwhich it is formed and cured under heat and pressure [12]. The mechanical properties of glass fibrereinforced SMC are not usually competitive with a continuous fibre reinforced composite. Howeverthe possibility of mass production are good and commonly used for larger covers without the demandfor high structural strength. [10]

For continuous fibres there are two major areas of manufacturing, one is by the use of so called resinpre impregnated sheets called pre-pregs and the other is the use of dry fibres and then injectingresin separately. The benefit of pre-preg is that the resin is already impregnated into the fibresheets which means that no resin needs to be handled separately which simplifies the layup of thematerial. This also gives the advantage of being able to control the fibre volume fraction as well asthe thickeness of the mateiral precisely. Pre-preg are often placed by hand in a mould in a certainstacking sequence depending on the application and mechanical properties required. The molds arethen bagged in special vacuum bags that they can suck vacuum in which applies a pressure andmakes sure that no air is trapped inside. The mould is then placed in an autoclave which cures theresin under pressure and heat to make sure it cures properly and that the resin fills the entire part.Usual resins are thermoset polymers such as epoxy and typical fibres are carbon or glass fibres. [10][13].

Thermoplastic composite moulding is a manufacturing method that utilises a thermoplastic polymerwhich unlike a thermoset polymer can be reshaped by reheating it. This is used when forming thecomposite by first heating a sheet of the composite and then quickly inserting it into the press whereit is shaped and cooled. The tool opens and the shaped composite part can be taken out. Theadvantage of this is that the possibility for automating the process is good and the volumes canbe high due to low cycle times. Because of the thermoplastic not needing a chemical process tocure, it is a clean process as well as good from a health perspective. The disadvantage of using athermoplastic over a thermoset is that a good surface quality is difficult to achieve and due to thehigher viscosity it can be difficult to impregnate the resin. [10] [13].

Wet Press Moulding is a manufacturing method that is good for mass production since it has lowcycle times and low tooling costs. Resin and hardener is added to a mat of fibres, preform or drymat and these are then inserted to a mould where it is formed and cured under pressure. Both longand short fibres are possible, but the disadvantage is the limited shape complexity that is achievablewithout having to add additional steps which might be costly. BMW has let the method grow biggersince its application to the 2015 BMW ”Carbon Core” 7-series where the method is used in massproduction of CFRP reinforcement in the body structure. [14], [10].

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Resin transfer moulding or RTM is a process conducted in several steps. The first step is to makethe fibre layup by unrolling the fabric of the roll and cutting it into the desired shapes. These cutfibres are stacked and fixed together in the right sequence to get the desired properties. They arethen formed in a preform to go from a 2D contour to a 3D shape and then inserted to the final diewhere the resin is infused and then cured. Depending on which method is used for injecting the resin,the methods are called differently. If the resin is infused in the middle of the structure and thendrawn to the edges by vacuum, it is traditional RTM. High pressure RTM or HP-RTM is when theresin is infused by having the die completely closed and then by having the resin injected by force,making sure that it is spread through the entire preform. HP-CRTM (high pressure compressionresin transfer moulding) is when the resin is infused in the die while it is not completely closedwhich lets the resin be infused quickly. The die is then closed which pushes the resin out throughthe entire part until it it flows out to the edges. This requires a hydraulic press and expensive steelor aluminium tooling which supports a larger volume production. [10]

Resin Film Infusion is another process that uses a dry mat of fibres that resin is infused in. In thiscase it is done by laying the fibre in a mould and then on top of that placing a semi-solid film ofresin that when it is heated melts and spreads through the fibre. Before heated up, the mould isplaced in an elastic bag from which the air is sucked out by vacuum. The curing usually is in anautoclave or under external pressure. [10]

A method commonly used when manufacturing hollow structures with such a shape that a continuousprocess such as pultrusion or filament winding is impossible, is called bladder moulding. The processnormally utilises pre-preg materials that are draped by hand around an inflatable bladder which isshaped according to the finished product. When the pre-preg tape is stacked according to preference,the bladder and carbon fibre is inserted into a mould which closes around the bladder. Pressure isthen inserted in the bladder which pushes the material against the mould until the part is cured.The curing is either from heating elements inside the mould or the entire mould is inserted into anautoclave (the most common). Since the material cost of pre-preg material is high and the use ofan autoclave to cure is time consuming, a method which uses a combination of bladder mouldingand RTM is more suitable for larger annual volumes. Since the method instead of pre-pregs is usingpreforms and then injecting the resin into the mould, the material costs decreases drastically. Thefibre preform is inserted into a die in which the resin is transferred into while the bladder keeps thepressure on the fibres from within, which gives a good surface quality on both sides of the part.The pressure applied by the bladder is also easily regulated by the amount of gas inserted into thebladder. However, this is a relatively new method and is not widely used within the industry. Butwith research being made on how to improve the method, it might show potential for larger scale ofmanufacturing in the future.

5.4 Adhesive Bonding

The advantages of an adhesive bond compared to a welded or a bolted joint is that an adhesivejoint does not compromise the properties and geometry of the material. Welding two metal piecesrequires high temperatures and will cause the material to be heated to a degree that changes themechanical properties. If this is then subjected to heavy loading it might cause the joint to fail dueto weaker strength. An adhesive bond on the other hand usually do not require high temperaturesfor curing and makes it suitable if the material is sensitive to temperature change. The fact thatthe load is spread over the entire bond makes the stress distributed over a bigger area and thusdecreases the stress concentrations in the material which might lead to problems with fatigue life.Another benefit is that the sometimes flexible joint will consume some vibrations and serve as somedamping in a structure. Further, the adhesive also can serve as a separator between surfaces inmulti-material structures that might otherwise have a problem with compatibility such as galvaniccorrosion or conductivity. It can also make it possible to join complex shapes such as sandwichstructures or several different parts at the same time which might be impossible or difficult to dowith a mechanical connection.

The downsides of the use of adhesive joints is that for optimal use, it is important that parts

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are designed with adhesive joining in mind. If not, the stresses might be unfavourable such as highpeeling stresses that might cause the joint to fail. Another limitation of adhesives is the temperatureand chemical resistance. Most adhesives contain their strength up to a temperature of about 70-220◦C when the adhesive will start to deform plastically and if it is placed in an environment whereit might be subjected to solvents it can break depending on the type of adhesive. When massproducing parts with bonding through adhesion, there are two main problems to solve; curing timeand quality control. If the curing time of the adhesive is not long enough means that the strengthof the joint might not be enough to hold the parts by them selves after application. Often somesupporting structure is required, or sometimes even supportive welding might be necessary to holdthe structure together during the curing time. With high demands on efficiency in a productionline seconds can be important and then the time to cure might be compromised. Application inthe right way and in the right amount is also something that has proven difficult, especially inmass production. The method used is often to put extra adhesive on the surfaces until it comesout between the material and then visibly check that it has been applied sufficiently. This causes awaste of material and sometimes also require after-treatment to eliminate the redundant glue beforepainting.

As mentioned above, when disigning an adhesive joint there are a number of different parameters toconsider; joint geometry, adhesive selection, mechanical properties of adhesive and adherent, stress inthe joint and the manufacturing conditions [?]. The design and geometry of the components desiredto be joint will affect what type of stresses that will act on the adhesive. An adhesive joint is bestat handling compressive, shear and tension stresses and performs worst with peel stress. Keepingthis in mind when designing a joint could mean the difference between a joint working or failing. Ifknowing the type of loading before designing the part, this can be optimised. When dimensioning asingle lap joint shown in Figure 8, the overlap l is of interest to determine.

Figure 8: Double lap joint.

To establish the dimensions of the joint, loaded in the same way as Figure 8, the shear stress τ inthe adhesive is determined by the applied load P and the overlap area A. To calculate the shearstress of a single overlap, if the adherents (the materials joint) are considered rigid, equation 10 isused.

τ =P

A(10)

Where overlapping area A is l multiplied by the width of the joint.

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6 Method

The methodology of this thesis is mostly based on the product development process described in[15]. The project was initiated by conducting a literary study to establish the state of the art withinthe subject of building structures with modular joints connected by standardised elements. Theneeds were identified, a product specification was made and then concepts could be generated andthen evaluated according to the product specification.

6.1 Planning

When the problem was formulated, a project plan was made in the form of a Gantt chart to clearlyvisualise the steps that had to be made in order to complete the project in time. The project isdivided in four main gates in which a number of deliverables must be completed before moving onto the next. Each gate has its own corresponding colour as seen in the project plan in Appendix A.

6.2 Concept Development

The concept development phase follows the main principles described in the theory based on [15].The work is initiated by establishing the needs from which a product specification is formulated.Then the concepts can be generated using creative methods, based on the criteria from the productspecification. The concept development is the major part of this project since the final result is atan early concept stage and not a finished product designed in close detail.

6.2.1 Need Finding

In order to make a product specification, the underlying needs has to be investigated [15]. The basicfoundation of the needs are obtained directly from statements from Scania and then interpreted intoneeds. The needs also comes from a template that was supplied by Scania with vehicle propertiesthat a new product might affect, such as Fuel Economy, Safety and Prestige. For this concept,several of these categories are applicable and helpful in making sure that no needs are overlooked.Needs are collected in the form of a table with corresponding number and importance to each metric.The importances are determined by specifications from Scania and discussed to archive the wantedpriority for the solution.

6.2.2 Product Specification

The needs transforms into measurable metrics according to the methodology described in [15].Several needs can be covered with the same metric, and several metrics might be needed to describea single need. Each metric has a corresponding unit that can be measured to determine how theconcept performs. The metric also has an importance which indicates how valuable each metricis for fulfilling the product purpose. To determine the importance some metrics could be directlyrelated from the needs, but metrics covered by more than one need with different importance hadto be re-evaluated.

To get an idea of how competitors perform in the areas considered in the product specification, abenchmarking is conducted. This data is also used to identify the desired place for the producton the market in respect to the competition. The chosen product to compare with is a regular buscalled ”CN UB 4x2 with long rear overhang”. Information was obtained directly by Scania as well asthe use of CES EduPack 2017 to obtain material data which was then inserted in the benchmarkingtable. These numbers also gives an idea for the marginal and ideal values for the final specification.The marginal value is the value that the solution must be better than to compete in the desiredmarket segment and the ideal value is the best possible result that can be hoped for.

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6.2.3 Concept Generation

By discussions with bus-body builders and other Scania employees with experience in the field of busdesign, valuable information was obtained on how to design a bus structure and what to think aboutwhen doing so. A number of different concepts was generated on how the side could be designed byusing mainly brainstorming to get several different ideas. The concepts were mainly visualised insketches with equal detail level across all concepts to be able to make an equal comparison betweenthem. The global geometry was however basically the same for all concepts and the difference liedin how the structure was divided in different sections.

Since the scope of this project has been to investigate the node element structure, this was theconcept that was defined and investigated further. The global geometry sketches made it possibleto define the node elements needed for building the two sides of the bus body. The next step was todefine which type of cross section to use since this would affect the shape and characteristics of theentire concept. First of all the type of cross section was limited to a closed cross section because ofthe higher torsional stiffness compared to open cross sections such as C- beams or I beams. This isbecause of the loading being complex due to different driving scenarios and can not be idealised tostrictly bending. Three main types of cross sections were evaluated; circular, rectangular and square.To determine their characteristics and to have a logical ground of which to base the decision, thearea moment of inertia as well as the torsional stiffness cross section factor Kv was used to determinethe performance in bending and torsion respectively. Since it is important to optimize the weight ofthe structure, the performance to weight ratio was used as the determinative factor. To see whichcross section that has the highest ratio, the same cross section area and wall thickness were assumedfor all variants which means that the cross section with the largest circumference is the one withthe largest mass. The performance was determined as the mean value between the bending stiffnessin two directions and torsional stiffness. The results were analysed and additional needs as how tointeract with surrounding structure was considered as well as performance.

After a cross section had been chosen according to the above named methodology, the node ele-ments were investigated further. Concepts with different materials and manufacturing methods wasgenerated. The process started by mapping out all possible materials that the node element couldbe manufactured by, first by category such as metal, plastic or composite. These were then brokendown further into subgroups in each category such as different metals or subgroups of composites.This process was repeated until specific materials were specified in enough detail to be able to get in-formation on each material in areas such as material cost, mechanical properties and carbon dioxidefootprint. However, this requires investigating different manufacturing methods since it has a greataffect on the material properties and cost. Information was retrieved primarily from the softwareCES Edupack 2017 as well as meetings with cost engineers at Scania. The different manufacturingmethods considered can be seen in 5.3 in the Theory section.

6.2.4 Concept Scoring

When the amount of materials was narrowed down to four different types, a comparison by therelative performance was made. A ferritic stainless steel was chosen as the reference material and itsperformance in each category was divided by the data for the other materials in the same category.This was done in order to eliminate the factor of estimation when determining the score for eachmaterial. The data was inserted into the scoring matrix where the relevant categories from theproduct specification was weighted according to their importance, from one to five. The relevantmetrics that could be compared were; cost per unit, CO2 footprint per unit and the square root ofYoung’s modulus divided by the density from the theory described by Ashby for bending of a beamoptimized by weight. [16].

6.3 Detail Design

The methods used for establishing the possible dimensions of the concept is presented below. Bothcalculations of adhesive overlap and material thickness is described as well as the load cases from

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which they are based upon.

6.3.1 Dimensioning of Beam and Node Elements

The next step when the material for the node elements was chosen was to make some dimensioningcalculations to be able to in the end get the total weight of the structure. At this stage, with thelevel of detail on the concept and in line with the scope of the thesis, it was considered sufficient tomake some initial calculations based on basic strength of material load cases. The calculations wereconducted according to the theory described in section 5.2 and performed in Matlab.

Three different load cases were considered; driving in a tight corner, stopping suddenly and a verticalacceleration to simulate driving in a bump. These driving conditions give stresses to the bodystructure due to the weight of the roof and objects that are mounted on top of it such as aircondition units. The weight and size of an air condition unit was given by Scania to about 250kg and three meters long. According to a Scania standard load case, the bus should handle anacceleration of 0.5g on the side (simulating driving in a corner), de-acceleration of the vehicle (0.6g)and a resulting vertical acceleration of the bus of 0.8g. For a certain height of the cross section, thethickness that would fulfil the criteria was calculated for all three cases. There is also a torsionaltest which simulates the bus driving of an edge with one wheel making the whole bus turn, but sincethis is considered too complex to solve in a representative way with simply hand calculations, it wasnot used for dimensioning.

The outer dimensions, as the height and width of the beam were chosen to have the optimum rela-tionship to fulfil the dimensioning criteria in both directions according to the load cases describedabove. The relationship between the longitudinal loading and the transverse loading gave the re-lationship as shown in equation 11 where ax is the side acceleration and ay is the longitudinalacceleration.

axay

= 0.8333 (11)

Since the area moment of inertia grows to the power of three and the width linearly, the optimumrelationship could be calculated as shown in equation 12.

IybIyh

=axay

(12)

Solving this equation with the height of the cross section of 1 m gives what the width will be tomatch the ratio of the test. This gives rise to a polynomial equation as displayed in equation 13.

b3 + 3hb2 − 2.499h2b− 0.00833 = 0 (13)

The three roots were analysed and the rectangular factor was set according to the result. A testwas also done to verify that the ratio was correct by calculating the required material thickness forboth accelerations and making sure the calculated thickness was the same in both cases.

The dimensioning parameters such as maximum strain and displacement was controlling the thick-ness of the beam, while parameters such as outer dimensions and material properties are inputs tothe calculation. The maximum strain allowed for the material is set to 0.3 % and the maximumallowed displacement 12 mm. The height of the cross section can then be varied to calculate thethickness which also needs to be possible for manufacturing by methods such as pultrusion. Fromthis, the required thickness was calculated from the dimensioning criteria. Also the height and widthcould not be too big because of intruding the space for the passengers in the bus. After applyingthe load, the results was analysed and the size of the beams was changed to make sure that neitherthe strain nor the displacement was outside of the requirements.

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6.3.2 Dimensioning of Adhesive Bond

The vertical load case was used to calculate the needed overlap between the beam and the nodeelement for the adhesive bonding. Equation (10) from theory described in section 5.4 was used withthe maximum shear stress that the adhesive can take as the dimensioning factor. Equation (10) canbe rewritten for the overlap length l as shown in equation (14) for the entire area covered by thebeam when it is plugged onto the node element.

l =P

(2h+ 2b)τmax(14)

The force applied P is mass times 1.8g in acceleration according to standard test. The mass is 250kg since the worst case scenario would be right below the air condition unit.

6.3.3 Design of Concept

To be able to evaluate as well as visualise the concept, it was modelled using a 3D-CAD softwarecalled Siemens NX. The parts were designed according to the specifications calculated when dimen-sioning the concept and when all parts were finished, the two sides of the bus were assembled. Thesandwich structures was added to the assembly and then the total weight of the structure could becalculated by measuring the volume for parts with the same material and then multiplying themwith each density respectively.

6.4 Concept Evaluation

When the weight was established, a number of data with regards to the performance of the conceptcould be calculated. Based on statistics from the Sveriges Bussforetag covenant from March 2018[17], a bus is driven most kilometres within the first six years of its lifetime and on average about62000 km/year. This results in 170 km/day if it is calculated that a bus is used every day of theyear. From the use of CES Edupack 2017’s ”Eco Audit” feature, a life-cycle analysis was performedfor the first six years of the bus life when the most kilometres are driven. From the life cycle analysis,different materials can be compared by specifying weight, manufacturing method, how the materialis used as well as how it is recycled during the end of the lifetime. From the Eco Audit results theenergy consumption during a life-cycle could be determined for both the carbon fibre node elementsconcept but as well as a regular steel bus (converted to 8m in length). The energy could then beconverted into carbon dioxide release which also was compared for both cases.

The Eco Audit was made with the weight of the node concept, specified with the manufacturingmethod for the node elements (Resin Transfer Moulding) and set as used in a mobile application.Further, the type of vehicle was specified to a diesel driven light goods vehicle which gives an energyconsumption of 1.4 MJ/tonne.km. The difference in energy consumption during the ”Use” phasewas then used to calculate the difference in diesel consumed, ∆Fuel by the use of equation (15).

∆Fuel =∆E

EDiesel(15)

Where EDiesel is the energy that is carried by one litre of Diesel. The total cost difference wasthen calculated by multiplying the amount of diesel calculated by (15) with the price of one litre ofdiesel.

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7 Results

The results from this thesis can be seen in the section below, starting from the concept developmentto the final specifications of the chosen concept.

7.1 Concept Development

The concept development phase follows the main principles described in the theory of Ulrich andEppinger[15] with the end result being a chosen concept that can be developed further.

7.1.1 Need Finding

The interpreted needs are collected in Table 1 with a corresponding number, description as well asan importance between one and five where five is the most important.

Table 1: Need finding table.

NeedNo.

Interpreted Need Importance

1 Low weight 52 Low material cost 43 Low manufacturing cost 44 Short assembly time 35 Able to handle standardised tests for rigidity 56 Function during the bus lifetime 57 Carry loads from surrounding structure 58 Compatible with the bus chassis and surrounding walls 59 Dimensions within prototype design space 310 Capable of regular production volume 411 Producible in several dimensions 412 Ergonomic for passengers 313 Aesthetically pleasing 214 Sustainability 5

7.1.2 Product Specification

The list of metrics can be seen in Table 2 with the metrics with corresponding need numbers,importance and units.

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Table 2: List of metrics.

MetricNo.

NeedNo.

Metric Unit Importance

1 1 Weight kg 52 2 Material cost SEK/kg 43 3 Manufacturing cost SEK 44 4 Assembly time s 35 5,7 Maximum loading MPa 56 6 Fatigue load MPa 57 8 Producible at Scania Binary 28 9 Within design space Binary 39 10 Production volume Buses/year 410 11 Modular Binary 411 12 Vibration comfort m/s2 312 12 Acoustics dB 313 13 Aesthetically pleasing Subj. 214 14 Recycleability % 415 14 CO2 footprint kg 4

Below follows a list with an explanation to each metric to give a better understanding of its meaning.

1. Weight: The total weight of the structure.

2. Material cost: The material cost for the structure.

3. Manufacturing cost: The cost related to the choice of manufacturing method; requiredtooling etc.

4. Assembly time: The assembly time on production line.

5. Maximum loading: Maximum allowed tension from load according to standard.

6. Fatigue load: Maximum amount of tension the structure can handle with respect to fatigue.

7. Producible at Scania: Capable of being produced at Scania.

8. Within design space: Is the structure within the maximum outer dimensions. Length,width, height.

9. Production volume: The number of buses capable of being produced. The aim is 1000/year.

10. Modular: Possibility of making different configurations with standardized parts.

11. Vibration comfort: Vibrations that the structure might give rise to.

12. Acoustics: Noise inside the bus that the structure might give rise to.

13. Aesthetically pleasing: To what extent visible parts are aesthetically pleasing.

14. Recyclability: The amount of material that can be reused after lifetime.

15. CO2 – footprint: The amount of carbon dioxide that has been released during entire lifecycle.

The product specification with inserted benchmarking data can be seen in Table 3.

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Table 3: Benchmarking table.

MetricNo.

NeedNo.

Metric UnitCN UB 4x2

(8m)1 1 Weight kg 7552 2 Material cost SEK/kg 53 3 Manufacturing cost SEK -4 4 Assembly time s -5 5,7 Maximum loading MPa 2506 6 Fatigue load MPa 1357 8 Producible at Scania Binary Yes8 9 Within design space Binary Yes9 10 Production volume Buses/year 30010 11 Modular Binary Yes11 12 Vibration comfort m/s2 -12 12 Acoustics dB -13 13 Aesthetically pleasing Subj. No14 14 Renewability % 10015 14 CO2 footprint kg 57000

After the product specification had been updated it was as shown in the final specification.

Table 4: Final product specification.

MetricNo.

NeedNo.

Metric Unit Imp.Marginal

ValueIdealvalues

1 1 Weight kg 5 1000 1502 2 Material cost SEK/kg 4 350 1003 3 Manufacturing cost SEK 4 100000 100004 4 Assembly time s 3 - -5 5,7 Maximum loading MPa 5 200 6006 6 Fatigue load MPa 5 135 2507 8 Producible at Scania Binary 2 Yes Yes8 9 Within design space Binary 3 Yes Yes9 10 Production volume Buses/year 4 1000 100010 11 Modular Binary 4 Yes Yes11 12 Vibration comfort m/s2 3 - -12 12 Acoustics dB 3 - -13 13 Aesthetically pleasing Subj. 2 Yes Yes14 14 Renewability % 4 0.1 10015 14 CO2 footprint kg 4 400000 100000

7.1.3 Concept Generation

Three main types of concepts was thought of; fully integrated- non modular structure, partlyintegrated- modular structure and a node-element structure with complete modularity. The firstconcept is a fully integrated structure which means that the whole side of the bus would be manu-factured in one single piece. This can be seen in Figure 9 where also the basic structure is shown.One complete structure makes it non modular and if customers would like to specify different com-binations of doors or vehicle length, all combinations would have to be manufactured from differenttools or moulds.

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Figure 9: Fully integrated concept displaying the global geometry of the structure.

As seen in the figure above, the structure has one main door and two wheel arches. On the side ofthe doors and on top of the wheels, the structure is meant to be dressed with windows to make thebus bright and give an open impression. The axle distance is 6 meter, the total length is 8 metersand with the door placed in the center of the bus, between the axles, makes getting in and out of thebus easy since it is where the floor is the lowest. Between the doors there is one centre pillar whichmakes the opening divided into two halves instead of one big door. The sections between the doorsand the wheels are strengthened with diagonal beams that prevents the structure from collapsingduring linear loading. The opposite side of the structure is shown in Figure 10 and as seen in thesketch it has the same type of structure as beside the doors with the diagonal beams.

Figure 10: The opposite side structure.

Inspired of the way that current buses are designed, with the same structure from behind the frontwheel and the same structure from in front of the real wheel and back for most buses, the nextconcept was designed as shown in Figure 11.

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Figure 11: Partly integrated concept displaying the global geometry of the structure.

The sides of the structure highlighted with thicker lines are parts that are made from one singlepiece of CFRP. This gives the opportunity of making bigger integrated sections but still keep themodularity in between the sections if a longer bus is specified. The modularity is thought to bedone by also making ”window frames” in sections that can be mounted together between the wheelsto extend the length of the bus. By dividing the structure in these sections, the larger sections canbe manufactured in one single tool to make all variations, plus the manufacturing of the extensionframes.

The third concept is the one that is the main subject of investigation in this thesis, which is the nodeand beam element structure. The structure is thought to be built up of the node elements shown inFigure 12. As seen in the picture, a total of 20 joints in four different versions are needed in orderto archive the desired shape for the door-side of the bus with the majority of the joints being theT-shape connection.

Figure 12: Node elements that are meant to connect the structure.

On the opposite side, the nodes has a bit more complex shapes due to having more attachments foreach node. The nodes has the configuration as shown in Figure 13

Figure 13: Node elements that are meant to connect the structure.

This gives a total of 39 node elements in seven different variations for one 8 m long bus. Afterdiscussions with supervisors at Scania and further investigations, the idea of having a sandwichstructure instead of the diagonal beams came up. The sandwich would provide stiffness as wellas insulation from the environment and would remove the need for additional covers beneath thepart of the wall that is windows. In Figure 14 the node elements for the structure with integratedsandwich structure is shown with a total of three different types to make the entire bus.

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Figure 14: Node elements with sandwich structure.

This redesign would enable the annual volume to increase from in average 5600 nodes per year toan average of almost 10000 nodes per year due to the decreased number of different nodes per bus.This is a considerable increase in annual volume which reduces the manufacturing costs per nodeand also gives the nodes less complexity.

7.1.4 Concept Selection

The results from the cross section screening is shown in Table 5. The bending performance for therectangular cross sections were calculated in both directions with the weak side in parenthesis.

Table 5: Cross section screening results.

Cross Section Weight Iy Iy/Wt Kv Kv/Wt Mean Perf. Tot Perf./WtCircular 0.89 0.76 0.85 1.01 1.14 0.87 0.97Square 1 1 1 1 1 1 1

Rect. 0.7 1.071.80

(0.61)1.13 0.94 0.88 1.09 1.02

Rect. 0.6 1.142.41

(0.50)1.28 0.88 0.77 1.21 1.06

Rect. 0.5 1.263.50

(0.40)1.55 0.80 0.63 1.46 1.16

As seen in the table, the rectangular cross section is the one with the highest score with regardsto performance during bending but gets the lowest score with regards to torsional stiffness, wherethe circular cross section gets the highest score. Over all, and the most important aspect is theperformance per weight ratio, where the rectangular cross section gets the highest score. However,other factors that this type of performance test does not take into account, such as how easilywindows and surrounding structures can be attached, would also benefit a rectangular or squarecross section since there are flat surfaces to attach to or align with.

The node element material selection data that are used in the scoring table are shown in Table 6.

Table 6: Material data.

Unit CFRP GFRP Aluminium SteelE [GPa] 61 22 71 203ρ [kg/m3] 1500 1830 2710 7830Material Cost [SEK/kg] 160 27 18.5 7.2CO2 [kg/kg] 17 4 12 4

The CFRP is an epoxy resin infused, woven biaxial layup composite with a fibre volume fraction of60 %. The chosen aluminium is a casting aluminium (Al 356,0), the steel is a stainless steel (EN1.4003) and the GFRP is a polyester resin with woven fabric, biaxial layup and 65 % fibre volumefraction. The results from the material scoring process can be seen in Table 7.

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Table 7: Material selection scoring results.

CFRP GFRP Aluminimum SteelCategory Importance Grade Score Grade Score Grade Score Grade ScoreCO2 footp. 4 0.673 2.693 0.805 3.220 0.580 2.321 1 2√E/ρ 5 2.862 14.308 1.409 7.043 1.741 8.704 1 5

Cost 4 0.129 0.515 0.38 1.50 0.716 2.86 1 4

Sum 17.52 11.77 13.89 13Continue? YES NO NO NO

As seen in the table, the carbon fibre epoxy composite is the material which gets the highest scorewith regards to the importance according to the product specification. Since the material gets thehighest score, and wins by a margin of 20 % compared to the aluminium on second place, it waschosen as the material for the node elements. The glass fibre composite gets the lowest score inthe comparison due to its mechanical properties and relatively high price. The material cost inthis comparison is limited to raw material and the CO2 footprint is for the CO2 released duringmanufacturing of the material. Other factors such as material compatibility with regards to galvaniccorrosion and thermal expansion coefficients are also important to consider. The thermal expansioncoefficient difference of two different materials could also lead to challenges when designing for largetemperature ranges. Since the carbon fibre beam elements would have similar properties as thenodes, the problems would be kept to a minimum.

7.2 Detail Design

The results from the material selection showed that a carbon fibre node element would be mostsuitable according to the priorities of this concept. The section Detail Design is where the specifica-tions of the chosen concept is set which in this case means that the dimensions and manufacturingmethod will be specified. Also, calculations of the overlap of the adhesive bond will be examined aswell as the type of sandwich structure that will be placed in the lower part of the structure.

7.2.1 Manufacturing Method

The manufacturing method of the node and beam elements had to be chosen in order to specifythe final specifications of the concept. The beam elements was early chosen to be manufacturedby pultrusion because of the competitive price point and good surface finish on both the inner andouter side of the beam since it is pulled trough a die shaped as the cross section. The beams arethen cut continuously in the desired lengths of the finished product. Other methods such as fibrewinding and filament winding would also be possible but does not come with the same tolerancesand surface finish of the pultruded beams.

The node element could be manufactured in several different ways but due to its complex, hollowshape combined with a high production rate makes the process choice narrower. An extensiveinvestigation of manufacturing methods was conducted and other markets such as hockey stickmanufacturing and BMW’s body panels was looked into. The result is that the most suitable wayof making the node elements in one piece is the use of bladder assisted resin transfer moulding(BARTM). The process is suitable for manufacturing of structural hollow parts with continuousfibre reinforcements. Other processes such as short fibre composite methods such as spray-up orinfusion does not give good enough structural performance to be an option. For a bladder assistedmanufacturing, it is perfect for making the part in one section since the possibility to apply pressurefrom inside the mould on the entire part at once.

7.2.2 Dimensioning

The outer dimensions, as the height and width of the beam were chosen to be about 100 mm inheight to have a starting point for the calculation. Since the cross section comparison showed that

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a rectangular cross section is the most efficient with regards to performance per weight, this waschosen. The rectangular factor was calculated to be 0.885 from equation (13) which gave threepossible roots. Since only one of them was positive (0.885) this was set as the rectangular factor.The height of the cross section was set to 0.08 m which multiplied by the rectangular factor gave awidth of 0.0708 which was simplified to 0.07 m. These inputs gave a calculated thickness of 1.5 mmwhich multiplied with a safety factor of 2 is set to 3 mm for the whole structure.

The results for dimensioning of the adhesive overlap between the nodes and the beam elementswas calculated to about 2 mm depending on which adhesive used, which in this case was an epoxyadhesive with a shear strength of 10 MPa. With a safety factor of 10, the overlap was set to 20 mm tomake sure that the adhesive will cover unexpected loads as well as make the beam and node elementhave a sufficient overlap which provides stability during the curing of the adhesive and minimise theneed for additional supporting structures during assembly.

7.2.3 Design

The final design for both sides of the structure was designed according to the dimensions calculatedpreviously and can be seen in Figure 15 for the door side with the dark red panels being thesandwich panels as described before. A similar panel is also placed on top of the door for extrasupport surrounding the door.

Figure 15: Render of the door side of the bus.

This is also displayed better in Figure 16 which is the side facing in to the bus. As seen in therender, an additional beam has been added above the door compared to the initial sketches. Thiswas recommended as a strengthening measure surrounding the door.

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Figure 16: Render of the back of the panels on top of the door.

Figure 17 shows a render of the structure for the no-door side with panels running along the entirelength of the wall.

Figure 17: Render of the no-door side of the bus.

The design of the node elements can be seen in Figure 18.

(a) T-node. (b) Cross-node. (c) L-node.

Figure 18: Renders of the three node elements.

All three variants have a similar design theory with the same distance from the center of the nodeto the connection of the beam of 120 mm. This means that the connection is moved away from thepoints where the stresses are the highest and the material is not compromised in the critical areas of

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the structure. For the L-node, this however means that the length from the vertical side on the leftto the connection on the right side is 160 mm. Drawings with the main measurements can be seen inthe drawings in Appendix B. The total number of node elements needed is 40 with the distributionof eight cross-nodes, eight L-nodes and 24 T-nodes.

The sandwich structure described previously is shown in more detail in Figure 19. The outer partsare made from a GFRP composite with a filler material of a polymer foam, for example PVC or PET.The glass fibre composite shell gives the structure the strength and the foam is there to separatethe layers and give the panel stability while maintaining the low weight.

Figure 19: Exploded view of the sandwich structure.

The back panel of the sandwich wraps round the filler material and is then joint with the flat frontpanel which is the one facing out from the bus. Both the inner and outer panel is 2 mm thick andfastened to the surrounding structure by adhesion. The back of the panel has the same measurementsas the slots beneath the windows as seen in Figure 20.

Figure 20: Sandwich panels mounted.

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The flange surrounding the filler is pushed against the surrounding beam elements when mounted.The flange is 40 mm wide all way round which makes it cover half of the width of the beam. Thismeans that the next panel or window on either side gets the same fastening area and the panelsform an almost seamless transition between each other.

An exploded view of the no-door side can be seen in Figure 21 which displays the node and beamelements and how they are mounted together.

Figure 21: Exploded view of the no-door side.

A summary of all beam elements that are needed for both sides of the bus can be seen in Table 8.

Table 8: Specification of beam elements.

Quantity 3 9 4 14 8 11 12 1l [mm] 70 590 750 940 1040 1250 1260 1770

When summarized, the total quantity adds up to 62 beam elements in eight lengths with a combinedlength of 60.64 m. The total mass for both sides of the bus with the sandwich panels and nodeelements included, adds up to 194 kg.

7.2.4 Concept Evaluation

The results from the life-cycle analysis can be seen in Figure 22 where the dark green pillar is thedata for the carbon fibre concept and the light green is for the steel bus. The y-axis shows theenergy that is consumed for the different phases of the life cycle which are placed along the x-axis.

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Figure 22: Energy comparison for life-cycle analysis.

As seen in the chart, even though the carbon fibre is a more energy consuming material to produce,the fact that a lot less material is needed makes the total energy consumption lower than for thesteel. The biggest difference is found in ”Use” phase where the difference is almost a factor of eight,with the carbon fibre consuming 16 GJ and the steel bus consuming 130 GJ per year. This differencein energy can then be translated into litres of fuel, which when multiplied by the cost for one litreof fuel gave a difference of about 39000 SEK/year.

From the same life cycle analysis the difference in CO2 footprint can be gathered as well. Here, thedifference is a total of 10400 kg for the carbon fibre concept and 57000 kg for the steel bus. This,even though the steel is calculated to be recycled in the end of its life and the CFRP to end up aslandfill.

The calculations for the total price of the carbon fibre concept can be seen in Table 9. The fillermaterial for the sandwich is calculated from a PVC-foam with a density of 80 kg/m3. The data forthe glass fibre panels and carbon fibre structure is gathered from [18] which describes manufacturingcosts for different processes and materials.

Table 9: Cost calculation results.

Part Weight [kg] Cost [SEK/kg] Total CostCarbon Structure 96 630 60500Glass Fibre Panel 64 400 25500Filler Material 34 135 4730

Adding up the material cost for the two sides including sandwich panels, results in a price of 90730.A price for the steel of 10 SEK/kg [10] gives a total price for the steel bus of 15200 SEK. This meansthat the difference for the material cost of the finished products is 75300 SEK. Dividing the initialhigher cost with the annual savings in fuel, gives a ratio of 1.9 which indicates that after abouttwo years, the initial higher cost for the carbon fibre structure will have been saved from the loverenergy consumption. If the weight difference instead is considered as additional batteries that a

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vehicle could carry, this would result in 1326 kg of batteries. Multiplying with the energy density ofan state of the art automotive battery’s energy density of 150 Wh/kg, results in an additional 200kWh of energy.

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8 Discussion

Below follows additional discussions regarding the results of this thesis.

8.1 Methodology

The methodology for this thesis has been based on the product development process described byUlrich and Eppinger because of the project being a product development project and the over allmethodology was found suitable. However, the method had to be adapted since all steps of theprocess was not suitable for this specific project at the current level of detail. Since basically allof the parts that the wall structure would be fitted to does not exist, the parts of the process thatinvestigates how the concept will interact or affect other parts of the bus have not been considered.Other processes for concept development could also have been used, but since the general processof Ulrich and Eppinger was considered suitable the need for changing to another process was notconsidered necessary.

8.2 Structural Layout

The over all geometry and layout of this concept is just one of the possibilities to manufacture withthe use of node elements. The specification of the current structure is based on measurements givenby Scania for what might be considered as a possible design for a future vehicle. The design wouldbe suitable for placing batteries in the floor where passengers could stand and then having a sittingarea on top of the wheel arches where the electric motors could be placed. The wide doors wouldmake it easy to get in and out and since the placement is central it is easy to reach for passengersfrom both sides of the bus.

The measurements can however easily be changed to the measurements from a current bus andperhaps this would have been better for comparing the concept in a more correct way, with thesame measurements. However, this design process has pointed towards the benefit of simplifying thestructure and only designing with perpendicular angles. This has shown to decrease the number ofnode elements as well as keeping the possibility of building any desired length.

8.3 Manufacturing Method

The node elements in this case was chosen to be manufactured from BARTM because of the hollowshape of the node elements. This does however not have to be the case if another joining method isused. Instead of plugging the beams on the overlap of the joint, the node elements could be madefrom two pieces and then assembled by clamping them over the beam elements at the same timethat the beams are assembled. Then the use of another manufacturing method would be possible.This would mean that the two halves could be pressed separately by the use of high/low pressureRTM for example. The problem with not having the node element in one complete section is thatthe joining of the two sides would create a weakness compared to a uniform material. The twoparts would also most certainly have to be placed on top of the beam elements from each side, thuscreating a difference in cross sectional thickness which could lead to problems when mounting thesandwich panels or windows against it.

8.4 Dimensioning

As described in the method, the dimensioning of the concept is based on simplified calculations dueto the level of detail in the concept. The mass of the roof for for instance was set to 800 kg includingweight of the air condition unit and weight of the actual roof panel. Since no actual roof exists theweight is an estimate and based on some rough calculations. It could probably be reduced if designedand optimised with the use of a lightweight sandwich panel. In the end, the calculated thicknessof the beam was multiplied with a safety factor because of the lack of knowledge of the real loadsthat might occur in the structure. By adding the safety factor and thus increasing the weight of the

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concept, makes the final comparison with the steel bus less beneficial since the structure might beheavier than it needs to be.

The same principle was used when dimensioning the adhesive overlap. A calculated overlap of 2 mmwas considered too narrow to be implementable so the overlap was instead increased significantly.The overlap of 20 mm corresponds to a load carrying capacity of 60 kN for one connection underthe assumption that the loading is strictly shear and that the average stress is applied through outthe entire overlap.

8.5 Concept Evaluation

The evaluation of the concept developed in this thesis was mostly done by making a life-cycle analysis.This shows some of the most important factors of the product specification such as CO2 footprintand energy consumption which converted into fuel, can be seen as part of the running cost. Thetool used (CES Edupack 2017) does not however have a life cycle analysis for heavy electric vehiclesand such a life-cycle analysis would probably look different. However, the energy consumption forthe vehicle should still be the same since the energy that is consumed when driving is dependenton the weight of the vehicle. The difference would be in where the economical benefits of a lighterbody is. For an electric vehicle, the saved weight could mean extra batteries which is described inthe results. This would in turn enable the bus to go longer without recharging and would be ableto carry more people during a shift due to the time saved by not having to stop for recharging asoften. Depending on the specific use and configuration of the vehicle, the lower weight might alsomean that more passengers can be transported at the same time instead of adding extra batteries.This would also increase the income for the company owning the buses.

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9 Conclusion

Based on the results of this thesis, a conclusion can be made that a carbon fibre bus body can beeconomically beneficial for the customer compared to a traditional steel bus by decreasing the weightby more than 1300 kg. The results also shows that due to the saved weight, the CO2 footprint canbe decreased during the entire life-cycle. This is however under the premises that the entire designapproach of the vehicle is for light weight structure with the most heavy components placed low inthe vehicle. By standardising parts, a total number of three different nodes could be used to buildan entire bus body. The use of perpendicular angles simplifies the design of the nodes which enablesthe annual volumes for each node to increase and thus enabling more sophisticated manufacturingmethods. The added assembly time compared to a fully integrated structure is considerable, but thechoice is dependent on where on the market the product is aimed at. If a completely customisableproduct can be considered beneficial, the node element concept have a great advantage.

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10 Future Work

To take this concept further, some additional work needs to be done. This section describes possiblefuture work that might be interesting or even necessary for this concept if it is to be implementedin a future vehicle.

To get a better idea of how to design and optimise the structure, a more thorough dimensioninganalysis has to be conducted. A FEM model of an entire bus might be necessary to determinestresses in specific parts. This might lead to a redesign of the node elements with larger radii orlonger adhesive overlap. A proper calculation model could also suggest how to optimise the fibrelayup to better tailor the behaviour of parts which are subjected to a certain type of loading. Forinstance, this could be economically beneficial if material could be taken away but would also meanmore variation which might lead to higher manufacturing costs. All of this however means thatsurrounding structures such as windows, roof and front/rear end of the vehicle would have to betaken into account. Also, the design aspect is important to consider since the outer design of thevehicle is controlling the dimensions for the underlying structure.

A proper study of the costs involved with an implementation would have to be conducted in orderto fully investigate the potential of the concept. Price information could be gathered from manufac-turers depending on wanted annual volumes which would make the raw material prices. Additionalproduction simulations and assembly times would need to be investigated to establish cycle timesand hand labour costs. Conclusions can then be made properly by how the node element conceptstands compared to a concept with fewer parts manufactured in bigger sections.

Since this concept has the opportunity of being configured in as many ways as possible, other marketsthan city buses could be of interest to investigate. To start with the inter-city and long distancecoaches would be a fist step which would also be able to implement the node element structure.Other markets such as trucks could also be a possibility if parts of the cab could be built in carbonfibre instead of steel.

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Appendix

A Gantt Chart

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B Node Cross Drawing

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