DEVELOPMENT AND PRODUCTION OF AN ADVANCED COMPISITE MONOCOQUE STRUCTURE FOR A SOLAR POWERED RACE VEHICLE Van Tricht Diederik*, Van Hooreweder Brecht, Ceulemans Guido GROUP T Leuven Engineering College in K.U.Leuven Association, Belgium KEYWORDS – monocoque structure, weight optimization, solar powered vehicle, lightweight design, carbon fiber prepreg ABSTRACT - The development of the chassis for a high performance solar powered vehicle is a challenging engineering problem. The structure needs to fit in the complex aerodynamic geometry. Moreover, strength, stiffness and durability requirements should be met and a minimum overall weight is desired to reduce the energy consumption of the vehicle. This paper describes the development and production of an advanced composite monocoque structure for the third Belgian solar powered vehicle: the Umicore Inspire. After a comprehensive concept and material study, finite element calculations were used to find the most optimal material distribution and layer orientation for a given set of loading conditions. To ensure consistency between the simulations and the physical product, state of the art production techniques were used. The result is a robust structure (4.3m x 1.8m x 1.2m) with a weight of only 48 kg.

1. INTRODUCTION The monocoque structure was developed for the solar powered race vehicle of the Belgian Umicore Solar Team. The team participated in the World Solar Challenge 2009 in Australia. This event is organized to stimulate and promote more environmentally responsible cars and industries. The Umicar Inspire (Figure 1) is designed and manufactured by 14 students of the Engineering College GroupT. The calculated average speed of the car was 105 km/h, in a race that would last 3000 kilometres throughout the Australian desert. Unfortunately, the team had a severe accident, and was cut out after 400 km, competing for the first place at that moment.

Figure 1: Umicar Inspire

To design a lightweight and reliable composite monocoque structure, free-size optimization is used to optimize the thickness for each ply in the laminate. Not only was the result a strong and stiff structure with a mass of only 48 kg, but also the frontal surface area decreased with 23%, leading to a lower aerodynamic resistance.

2. OBJECTIVE

In 2005 and 2007, the Umicore Solar Team built the first and second Belgium Solar Car, to participate in the World Solar Challenge. Both those cars used an aluminium tubular space frame, because of the reliability and the reparability in case of an accident. Furthermore, strength calculation can be accurate and straight forward because of the fact that bending moments are transmitted as tension and pressure loads, along the length of each tube (1). Since a low aerodynamic resistance of the body as well as a low overall mass of the car are one of the most important criteria for an economical energy consumption (Table1) ( 2), the team choose for a monocoque structure. This because composite monocoques have the potential to be very light and the fact that we can eliminate the aluminum tubes who enclose the driver in the cockpit.

Parameter Variation Speed increase

Incoming energy + 10% 4.0%

Aerodyn. resistance - 10% 3.1%

Roling friction - 10% 1.3%

Weight - 10% 0.9% Table 1: Most important parameters, influencing the speed When the driver is enclosed only with composite material, directly around his body, the frontal surface area (A) of the car will diminish (figure 2&3). This results in a lower aerodynamic resistance, leading to a lower energy consumption of the car. (F=1/2*ρ*v2*A*Cw)

Figure 2: ‘A’ of Umicar Infinity (2007) Figure 3: ‘A’ of Umicar Inspire (2009)

Figure 4: Comparison between different frontal surface area

The use of this composite structure enables us to create a high performant solar powered vehicle, by reducing the Cd*A and diminishing the weight. Nevertheless the numerous design variables complicate the calculations compared to a aluminum space frame (4). Furthermore, the lack of experience with monocoque structures makes the link between the theoretical solution and the production more difficult. Despite these difficulties, we took up the challenge with this innovative concept, convinced to store a profit.

3. CONSTRAINTS There are several aspects that can influence the design of the monocoque structure. Due to the high importance of a low aerodynamic resistance (table 1), the design space is quite complex. This shape is aerodynamically optimized and is the starting point for the further design. To comply with the technical regulations of the World Solar Challenge (3), there were a few rules to take into account. For example there was a specification for the roll bar, namely that the structure must be made of a material of sufficient tensile strength to protect the occupant from a force of 4 times the weight of the car itself. Also the seating angle must not exceed 27° from the vertical plane. To determine the position of each rib, all the components must be taken into account. For example the battery package, mounting points of the suspension, as well in the front as in the back, electronics, etc (figure 3).

Figure 5: Components in the solar powered vehicle

To minimize the frontal surface area, the cockpit wall has to enclose the driver very tight. To guarantee this, the driver is scanned and a 3D image was created (figure 6). This image could then be used to design the shape of the cockpit. To create the 3D image, we used “ATOS”, a high-end 3D digitizer that is based on the principle of triangulation (4). Two cameras observe a projected pattern and 3D coordinates for each camera pixel are calculated with high precision, generating a polygon mesh of the object’s surface.

Figure 6: 3D image of the driver

4. METHODOLOGY

Materials Based on criteria as tensile strength, fatigue resistance, laminate density and cost, the carbon fiber HexPly M52 prepreg (Table 2) came out as the best suitable option for the material of the monocoque structure. This particular material is available as a cross ply in 0°/90° fabric, as well as in +45°/-45°, which improves the manufacturability. Mainly the low laminate density and good manufacturability were the decisive factors for this choice. HexPly M52 prepreg is an epoxy resin suitable for low pressure molding processes. Moreover, it can also be used for large components subjected to severe environmental conditions. This prepreg has a low resin density of 40% and gives a good surface finish, which is important for the aerodynamic resistance and the overall mass of the vehicle.

Laminate density 0.322 kg/ m2 Tensile strenght 751 MPa Tensile modulus 63.8 GPa

Table 2: Mechanical properties of HexPly M52 prepreg For the core material of the monocoque structure we used Rohacell 31, a closed-cell polymethacrylimide-(PMI-) rigid foam, that has very good mechanical and thermal properties and an outstanding strength-to-weight ratio. This core material was the most suitable solution, because of the very low density, high shear modulus (figure 7) and the fact the foam is deformable.

Density 32 kg/m3 Tensile strenght 0.979 MPa Mode of elasticity 35.3 MPa Shear modulus 12.8 MPa Shear strenght 0.393 MPa Heat distortion temperature 180°C

Table 3: Mechanical properties of Rohacell foam

Figure 7: Shear moduli of different foams

Because of the complex shape of the design space, the thermoformability of Rohacell is indispensable. The foam becomes deformable at temperatures between 165°C and 230°C, depending on the density and thickness of the sheet. Due to the low heat capacity of the foam, the sheets must be protected against cooling while they are moved from the heating plates to the mould. To ensure this, the sheets have to be covered with a cotton cloth or breather. After being placed in the mould, a uniform cooling is needed to avoid springback.

CAD The use of innovative analytical, numerical and experimental methods, the optimal aerodynamic shape for the body of the solar powered vehicle appeared to be the Morelli-profile with a 3-wheeler concept, placing 2 wheels in the front. To obtain the design space for this structure, Catia V5 is used to draw the aerodynamic shape and assemble all the components within this shape. The suspension, the battery package, the driver, etc. Based on the position of these components and taking the weight distribution into account, the suitable place for all the ribs was determined. These ribs, together with the shell, set up the design space for the FEM. Monocoque structure optimization Because we choose to work with composite materials, we have the advantage that the structural performance can be controlled precisely by choosing the appropriate ply thickness, ply orientation, stacking sequence, and so on. The fact that we can vary all these parameters gives us greater flexibility, but at the same time the optimization is becoming more and more complex. By using Altair Optistruct we have the ability to optimize the ply thickness, ply orientation and stacking sequence for composite structures. Free-size optimization handles the thickness of each ply as a design variable and optimizes the monocoque structure by determining the optimal thickness for each ply in the laminate in an iterative numerical design cycle. Starting from the design space, we create a mesh using 2D linear shell elements. Both quadrilateral and triangular elements with side lengths of 7mm were used to obtain a consistent and uniform mesh. The mesh contained 120000 elements within each node six degrees of freedom. To guarantee a high-quality mesh it was necessary to minimize the amount off skewed elements and verify warpage and aspect ratio of the elements by redistributing the node locations. The next step was to create groups that contain components with a similar laminate composition. For example ribs that are present on both sides of the structure, as well on the right-hand side as on the left-hand side. These components are marked with the same color as shown on figure 8.

Figure 8: Different components in the monocoque structure

In the Altair software we used the HyperLaminate module for calculations with composite materials. HyperLaminate gives us two types of view in which we have to fill in certain parameters. The first view is the material view, this enables us to enter all the relevant material properties such as shear modulus, tensile strength and material density. This is needed for the carbon fiber prepreg material, as well for the properties of the Rohacell foam that is used as core material. In the ply layup view all the possible ply orientations and the stacking sequence of each laminate has to be filled in. To make sure that the manufacturability didn’t became too complex, the ply orientation and stacking sequence was set on 0°,+45°,-45° and 90°. Once the design space is meshed, all the forces that are acting on several nodes of the mesh can be applied. The magnitude of the applied forces varies from 300 N for the battery package to up to 5000 N when the vehicle is breaking and turning. These forces are grouped in 12 different loadcases, as represented in the figure below. The monocoque structure has to meet the requirements for strength and stiffness in every load case.

Loadcase Max. magnitude Lift vehicle left 1500 N Lift vehicle right 1500 N lift vehicle back 1000 N rol of the vehicle 2000 N weight driver 1000 N breaking 5644 N bump 6745 N acceleration 5298 N driver mounting vehicle 1000 N weight solar panel 300 N belt 1000 N turning 4562 N

Table 4: Different loadcases After generating the mesh and applying the forces, the constraints need to be defined. The monocoque structure is not clamped, so the feature ‘inertia relief’ is used (6). With this feature unconstrained structures can be simulated in a static analysis. Typical applications of inertia relief include a satellite in space, an aircraft in flight and a vehicle on a track. When applying this type of constraint, the inertia of the structure is used to resist the applied forces, assuming that the structure is in a state of static equilibrium even though it is not constrained. Now stresses and deformations can be calculated in every element of the mesh. To run the optimization, an objective must be set, namely to minimize the weight of the design space. Other parameters , such as optimization constraints, need to be defined for the optimization model. One of the constraints is setting the maximum limit for the Von Mises Stresses in each ply. Taking the maximal shear stress of the Rohacell core material into account, this limit is set to 150 MPa. The other optimization constraint is defined by a maximal displacement between the mounting points of the front suspension and the mounting points of the back suspension. This

maximal displacement is determined by a torsion stiffness of 1500 Nm/deg between these mounting points. The optimization process will now calculate the most optimal thickness for each ply, without exceeding any of the optimization constraints. This process took ten hours processing time on a workstation with quadcore processor (2 x 3.6GHz) and extended ram memory (8GB).

5. PRODUCTION Once the monocoque structure is numerically optimized, it is not directly producible. First each layer has to be analysed for its optimal thickness. During the optimization process, the thickness of the carbon fibre layer always varied with 0.2mm. Due to the thickness of one layer of the HexPly M52 Prepreg, which is 0.2mm, it is easy to determine to exact amount of layers that is needed. Starting from a positive model in polyurethane foam, the body of the vehicle is created. The surface of this model has to be very smooth, due to the high aerodynamic performance that is needed from this body. To ensure this, a milling machine with a accuracy of 0.01mm is used and afterwards the model is grinded and polished. After that, several layers of carbon fibre are hand laminated on top of this positive model, with a negative mould as result. Carbon fibre is used as mould material because of the thermal expansion coefficient that needs to be the same as the material used for the body itself. This to avoid deformation of the product, leading to a deviation from the original shape. The negative mould also has to be very smooth, because every roughness or irregularity can lead to an increasing aerodynamic resistance. We used a vacuum installation to lower the possibility of air getting trapped between 2 layers. If this happens, there will be an expansion of the air bubble, leading to a bulge on the surface of the mould. Once the negative mould is finished the defined number of prepreg layers are laid out, as well as the Rohacell foam for the core material. On the places where the radii of the fillets are too large , the foam needs to be thermoformed, by heating it to 195°C for 8 minutes. When the correct amount of prepreg layers are laid, a peel ply is placed on top of the laminate. This fabric will absorb the excess amount of resin and create a rough surface on the inside of the body. This can be useful when we place the ribs on top of this shell, ensuring a good bound between the separate layers of the body and the rib. On top of this peel ply, a release film is used, which is a perforated foil separating the underlying laminate with the breather foil. This prevents the laminate to bound with the breather foil and ensures an easy removal of this foil. The breather foil enables the vacuum to spread throughout the entire mould, guaranteeing a uniform cured result. The laminate has to cure in an oven for 4 hours at a temperature of 110°C, under an applied pressure of 0.1 Bar.

To produce the ribs of the monocoque structure, a plate with a limited amount if layers is cured in the oven. Next, the ribs are precisely mild out off this plate and are placed into the body, using highly accurate measuring systems, with an accuracy of 0.2mm.

Once the ribs are correctly positioned, the remaining number of layers are placed over the ribs with their corresponding fibre orientation. To complete the finishing of the vehicle, the body is filled, grinded, painted and polished, to maximise the smoothness of the surface.

6. RESULT When the optimization process is finished, it is possible to display the stresses in every layer, as well as the displacements. It is clear that a very stiff structure is created between the torsion-box and the back of the chair. This structure contains the mounting points for the front and back suspension. The ribs next to the driver contain for example 11 layers of carbon fiber, on each side of the core material. In some areas of the cockpit, 14 layers are used, without the core material, leading to a strong massive side with a thickness of 2.8mm. This cockpit also ensures the safety of the driver, who is now enclosed by a very strong and stiff structure. This is proven during the World Solar Challenge, when the vehicle went off the highway and crashed into a tree at a speed of 110km/h, without the driver being injured.

Figure 9: Composite monocoque structure of the Umicar Inspire

The composite monocoque structure (4.3m x 1.8m x 1.2m) has a total weight of only 48 kg. Furthermore, the structure has a torsion stiffness of 1500 Nm/deg between the mounting points of the front and back suspension.

7. CONCLUSION

By using free-size optimization, it is possible to determine the optimal thickness for each ply in the monocoque structure, leading to a very strong and lightweight solution. To ensure a light, reliable and producible structure, several parameters have to be taken into account. Not only the mesh quality and optimization constraints are important, but also manufacturability is a very significant aspect to keep in mind when setting the parameters for the optimization. The major advantage of this composite structure appeared to be the smaller frontal surface area of the vehicle. Compared to the previous Belgian solar powered vehicle, containing a tubular aluminum space frame, a reduction of 23% of the frontal surface area is achieved, resulting in a much lower aerodynamic resistance. Using only composite materials, it is also possible to create a lighter structure, with a lower rolling friction as result. The theoretical diminution in weight appeared to be less than expected, mainly because of extra weight added during the production. In spite of this, there was a profit of almost 6 kg compared with the aluminum space frame and composite body used in the vehicle in 2007.

Particularly on the area of weight reduction there is a chance to store a profit for the next solar car. Using the experience and knowledge that is gained during the design, development and production of this vehicle, the composite monocoque structure for the next car can be lighter and more efficient. REFERENCES (1) Van Hooreweder, B., Faid, S. (2008). Development of a lightweight tubular space

frame of a solar powered vehicle using 2D topology optimization. In : Proceedings of the Fisita 2008 World Automotive Congress Fisita 2008 World Automotive Congress. München, Germany, Sep 14-19, 2008 (pp. F2008-SC-0-48).

(2) Douglas R. Carroll, “The Winning Solar Car: A Design Guide for Solar Race Car Teams”, SAE International, 2003.

(3) World Solar Challenge, Regulations for the 2009 World Solar Challenge, Part 2-Technical, 16 pg, 1 September 2008.

(4) GOM, Optimal measuring techniques, ATOS (5) Potter, Kevin. “An introduction to composite products: design, development and

manufacture”. Chapman and Hall, London, 1997. (5) Hyperworks 8.0 user manual, “Inertia Relief Analysis”, Altair Engineering, 2006.