From Design to Manufacture of a Carbon Fiber Monocoque for a Three-Wheeler Vehicle Prototype

materials

Article From Design to Manufacture of a Carbon Fiber Monocoque for a Three-Wheeler Vehicle Prototype

Alessandro Messana , Lorenzo Sisca, Alessandro Ferraris , Andrea Giancarlo Airale , Henrique de Carvalho Pinheiro, Pietro Sanﬁlippo and Massimiliana Carello *

Department of Mechanical and Aerospace Engineering, Politecnico di Torino, 10129 Turin, Italy; [email protected] (A.M.); [email protected] (L.S.); [email protected] (A.F.); [email protected] (A.G.A.); [email protected] (H.d.C.P.); pietro.sanﬁ[email protected] (P.S.) * Correspondence: [email protected]; Tel.: +39-011-0906946

Received: 18 December 2018; Accepted: 16 January 2019; Published: 22 January 2019

Abstract: This paper describes the design process of the monocoque for IDRAkronos, a three-wheeler hydrogen prototype focused on fuel efficiency, made to compete at the Shell Eco-Marathon event. The vehicle takes advantage of the lightweight and high mechanical performance of carbon fiber to achieve minimal mass and optimized fuel consumption. Based on previous experiences and background knowledge, the authors describe their work toward a design that integrates aerodynamic performance, style, structural resistance and stiffness. A portrayal of the objectives, load cases, simulations and production process—that lead to a final vehicle winner of the Design Award and 1st place general at the 2016 competition—is presented and discussed.

Keywords: carbon fiber; structural analysis; monocoque structure; lightweight design; low consumption vehicle; three-wheeler vehicle

1. Introduction The importance of lightweight design and high mechanical performance in contemporary automotive development is undisputed. Certainly, one of the main related goals is fuel efficiency. In order to achieve a good compromise between mass, mechanical properties, aerodynamic performance and style, the use of carbon fiber stands out. Its application allows creative shapes to couple with elevated engineering performance, permitting high design freedom. The main issues of carbon fiber technology are the cost of production and the recyclability; therefore, its major applications still lie in the racing field and niche markets. This paper presents the design and manufacturing of a three-wheeler prototype for the Shell Eco-marathon competition [1], which awards the vehicle that obtains the lowest fuel consumption measured in terms of km, with a liter equivalent fuel (calculated by Shell, using chemical formulations). Starting from experience in other vehicles designs [2–4] at the Politecnico di Torino and also by other teams [5], the new vehicle—IDRAkronos—has been developed, using a carbon fiber monocoque to reach minimal mass, while maintaining structural resistance. The prototype has three wheels; two front steering wheels (covered by the body to reduce aerodynamic drag) and one rear wheel powered by a brushed electric motor, supplied by a hydrogen fuel cell. The upper limit of 40 kg has been respected, including by all the sub-systems; steering, breaks, wheels, cockpit, electric wiring, controls, fuel cell, the electric motor, and transmission. During the thorough technical inspection that takes place before the competition [1], one of the most challenging tests for the vehicle body was the application of a 70 kg load on the highest point of the vehicle (roll bar). The structure of the vehicle should not deform permanently, nor show ruptures

Materials 2019, 12, 332; doi:10.3390/ma12030332 www.mdpi.com/journal/materials Materials 2019, 12, 332 2 of 11 during the test. To withstand this load, a good technological solution is the carbon fiber composite, which has excellent performance in terms of low density and mechanical properties [6,7], allowing us to attain minimum mass and high resistance, but also to make an aerodynamically optimized shape. Apart from the technical regulation constrains and some geometrical fixed features (for instance, the vehicle track) the design followed was conducted in order of CAD (Computer Aided Design), FEM (Finite Element Method) analysis, CFD (Computational Fluid Dynamics) analysis and some iteration between structural resistance and aerodynamic performance. Once the final shape was achieved, a definitive FEM model was developed, concurrently with the ply book for the manufacturing process. A typical manufacturing technology used to produce carbon fiber components for automotive applications is the use of pre-pregs, which are fabric composite materials already littered with resin. The pre-pregs are shaped manually in molds and then polymerized in an autoclave, using a vacuum bag around the mold. Unidirectional and multi directional carbon fiber fabrics were used, with a specific ply lay-up to balance the stress due to pressure and temperature variation, which occurs during the curing cycle in the autoclave. By doing so, it is possible to obtain components with elevated uniformity and performance.

2. Materials and Methods

Materials2.1. Geometry 2018, 11 Design, x FOR PEER REVIEW 3 of 11 a rearA opening significant which change allows in IDRAkronos’the air inside layoutthe rear (Figure compartment1)—with to respect flow outside to the prototypes, avoiding the designed spoon effect.previously The refrigeration [2–4]—is the of location the fuel ofcell the was rear one wheel of the behind main objectives the firewall,; in particular and the fuel, the celltemperature with the profilehydrogen was tank estimated at the endon its of duty the car. cycle This during solution the grantsrace, at not different only a times considerably of the day. stiffer The rear temperature assembly gapand abetween shorter thewheelbase, two solutions but also justifie higherd the design presence freedom of thefor the NACA external ductshape in orde ofr the to vehicle’s improve tail. the powertrain’sMoreover, this efficiency. position enhances the ventilation of the fuel cell, increasing the powertrain’s efficiency.

Figure 1. IDRAkronos’ layout layout..

The position of the driver was set such that the steering bar passed below the knees, while the wheels were leveled with the hip. The choice of the vehicle’s height was the result of a compromise, where on the one hand, a low height would reduce the frontal area (and therefore the aerodynamic drag) and avoid rollover, but a more elevated floor would prevent the downforce caused by ground effects, thus reducing the rolling resistance. The engine was directly mounted on the chassis in order to minimize relative motion between the pinion and the gear mounted on theFigure wheel. 2. IDRAkronos’ This way, a external better coupling shape. efficiency was achieved during cornering, where the components typically tend to move apart. TheMoreover, body of IDRAkronos IDRAkronos had is subdivided three particularly into the well-developed following parts characteristics: (Figure 3): 1.• TheCovered front frontend wheels that reduced turbulence generated by the spinning motion. In this case, the • Thewheel central arch volumepart had to include the envelope of the steering wheels, increasing the frontal area • Theof the rear car, end which was compensated for, by a significant reduction in the drag coefficient. • The top cover The front end, rear end, and the cover did not add a relevant contribution to the stiffness of the vehicle, and the central part had the purpose of withstanding all static and dynamic loads that acted on the vehicle.

Top cover Rear end Central part

Front end

Figure 3. Subdivisions of the IDRAkronos’ body.

Materials 2018, 11, x FOR PEER REVIEW 3 of 11 a rear opening which allows the air inside the rear compartment to flow outside, avoiding the spoon Materials 20182019, 1211, 332x FOR PEER REVIEW 3 of 11 effect. The refrigeration of the fuel cell was one of the main objectives; in particular, the temperature profilea rear opening was estimated which allowson its duty the aircycle inside during the therear race, compartment at different to times flow outsideof the day., avoiding The temperature the spoon 2. Reduced wheelbase of the vehicle helps decreased the frontal area of the car and also the volume gapeffect. between The refrigeration the two solutions of the fuel justifie cell wasd the one presence of the main of the objectives NACA ; ductin particular in order, the to improvetemperature the of the wheel arches, improving the aerodynamic resistance (a lower wheelbase means a lower powertrain’sprofile was estimated efficiency. on its duty cycle during the race, at different times of the day. The temperature steering angle is needed to run the same curve). Another advantage of the reduced wheelbase is gap between the two solutions justified the presence of the NACA duct in order to improve the that the length of the structural part of the car was shortened, allowing a reduction of the bending powertrain’s efficiency. stresses on the monocoque, and thus helping to achieve a lighter vehicle. 3. Increased length of the body allowed us to close the tail with a smaller angle, delaying the transition from laminar to turbulent ﬂow as far back as possible, reducing the dimensions of the wake, and therefore the drag coefﬁcient.

The software used to design the surfaces is Alias (2015 Version) by Autodesk, San Rafael, CA, USA. The design of IDRAkronos (Figure2) was influenced by profile 4415 of NACA (National Advisory Committee for Aeronautics) airfoil, with dimensions according to their characteristics and the compatibility with the proportions of the prototype. The NACA duct was designed to work with a rear opening which allows the air inside the rear compartment to flow outside, avoiding the spoon effect. The refrigeration of the fuel cell was one of the main objectives; in particular, the temperature profile was estimated on its duty cycleFigure during 1. IDRAkronos’ the race, at layout different. times of the day. The temperature gap between the two solutions justified the presence of the NACA duct in order to improve the powertrain’s efficiency. Figure 1. IDRAkronos’ layout.

Figure 2. IDRAkronos’ external shape.

The body of IDRAkronos is subdivided into the following parts (Figure 3): Figure 2. IDRAkronos’ external shape shape.. • The front end • The centralbody of part IDRAkronos is subdivided into the following parts (Figure3 3):): • The rear end • The front end • The top cover • The central part The front end, rear end, and the cover did not add a relevant contribution to the stiffness of the • The rear end vehicle, and the central part had the purpose of withstanding all static and dynamic loads that acted • The top cover on theThe vehicle. front end, rear end, and the cover did not add a relevant contribution to the stiffness of the vehicle, and the central part had the purpose of withstanding all static and dynamic loads that acted on the vehicle. Top cover Rear end Central part Top cover Rear end Front end Central part

Front end

FigureFigure 3. 3. SubdivisionSubdivisionss of of the the IDRAkronos’ IDRAkronos’ b body.ody.

Figure 3. Subdivisions of the IDRAkronos’ body.

Materials 2019, 12, 332 4 of 11

The front end, rear end, and the cover did not add a relevant contribution to the stiffness of the vehicle, and the central part had the purpose of withstanding all static and dynamic loads that acted on the vehicle. With IDRAkronos, the objective was to have a closed cross section in order to signiﬁcantly improve the moment of inertia of the monocoque, improving the stiffness due to the geometry factor, and therefore allowing us to use less material to achieve the same displacements.

2.2. Materials The monocoque of IDRAkronos is made of 0.25 mm thick sheets of 2 × 2 twill T300 carbon ﬁber/epoxy pre-preg, layered in a sandwich structure within the Nomex honeycomb. The mechanical properties of these materials are reported in Table1. Carbon ﬁber based composites show a high strength-to-volume ratio, which make them ideal for applications that need both lightness and high strength. Preferred orientations used for pre-preg sheets were 0◦, 45◦, −45◦, and 90◦, while avoiding 30◦ and 60◦ for simplicity during the manufacturing process.

Table 1. Mechanical properties of carbon ﬁber laminate and Nomex honeycomb.

Property T300/Epoxy Composite Nomex Honeycomb E—Young’s Modulus (GPa) 57 0.9 G—Shear Modulus (GPa) 3 0.3 σt—Tensile Strength (MPa) 570 - Materialsσ 2018c—Compressive, 11, x FOR PEER Strength REVIEW (MPa) 530 80 5 of 11 ν—Poisson’s Ratio 0.05 0.4 $—Specific Mass (kg/dm3) 1.4 0.06 confirmed the above requirements, outputting the forces acting on the wheels right before rollover (Table 2). The sandwich structure is frequently used in the motorsport and aerospace industry due to its ability to combine the best properties ofTable both materials.2. Rollover forces Nomex. honeycomb is a core material made up of aramid polyamide fibers, which have low specific mass and high torsional resistance, impact Tire Fz [N] Fy [N] toughness and vibration damping. The possibility of using a core material with very low density External front 650 350 allowed us to employ a higher thickness core (ranging from 6 mm to 13 mm in this application, as Internal front 0 0 shown in Figure 10) and, importantly, to increase the moment of inertia of the section without adding Rear 350 150 too much mass.

2.3. FEMIn r Modelegards to the other loads, the mass of the head was applied in a rectangular shape onto the firewall, since the head was placed above the fire extinguisher-holding structure. The loads on the backLinear and hips static were analyses applied of on IDRAkronos an area with were a width made roughly using the equal Hypermesh to the shoulder and Hyperview width of the software driver, (v.14)and the and loads using caused the Optistruct by the driver’s solver (bothfeet were provided applied by on Altair, the front Troy, end MI, USA).of the vehicle. Figure4 Constraintsshows the , CADsimilar design to those of the on body the load and itsstep smaller were placed, subdivisions, which withassured different that the sizes driver and curvaturewas inside used the vehicle. to better analyzeFurthermore, and control the f FEMirewall mesh had quality. the function In the of pre-processing a roll bar, which interface, had to the be geometrycapable of clean-upwithstanding and meshinga static load process of 700 was N, carefully as the regulations carried out, state as depictedd. in Figure5.

Figure 4. Central part CAD design (left) and subdivision to enhance mesh quality (right). Figure 4. Central part CAD design (left) and subdivision to enhance mesh quality (right).

Figure 5. FEM shell elements mesh definition (left) and pre-processing geometry cleanup (right).

3. Results

3.1. FEM Results The design phase was composed of three principal steps; free-size-optimization, discrete-size- optimization, and shuffle-optimization. As the monocoque of IDRApegasus (the predecessor of IDRAkronos) never reached more than 18 mm in thickness, the maximum thickness of the laminate was set to 20 mm during optimization. Total displacement response to the load steps “driver inside, stand, and exit” was considered to be 1 mm. For most parts, two plies of carbon fiber for each orientation were more than enough, and the minimum thickness for each orientation was, in fact, set to 0.5 mm, corresponding to two plies, in order to keep the laminate balanced and guarantee a certain degree of isotropy.

Materials 2018, 11, x FOR PEER REVIEW 5 of 11

confirmed the above requirements, outputting the forces acting on the wheels right before rollover (Table 2).

Table 2. Rollover forces.

Tire Fz [N] Fy [N] External front 650 350 Internal front 0 0 Rear 350 150

In regards to the other loads, the mass of the head was applied in a rectangular shape onto the firewall, since the head was placed above the fire extinguisher-holding structure. The loads on the back and hips were applied on an area with a width roughly equal to the shoulder width of the driver, and the loads caused by the driver’s feet were applied on the front end of the vehicle. Constraints, similar to those on the load step were placed, which assured that the driver was inside the vehicle. Furthermore, the firewall had the function of a roll bar, which had to be capable of withstanding a static load of 700 N, as the regulations stated.

Materials 2019Figure, 12, 332 4. Central part CAD design (left) and subdivision to enhance mesh quality (right). 5 of 11

Figure 5. FEM shell elements mesh definition (left) and pre-processing geometry cleanup (right). Figure 5. FEM shell elements mesh definition (left) and pre-processing geometry cleanup (right). Two-dimensional elements have been used for the mesh, since one dimension is negligible with 3. Results respect to the others for the panels, under the hypothesis that the distribution of stresses along the axis orthogonal to the element plane is irrelevant. Although stiffer than quadrilateral elements, 3.1. FEM Results triangular elements were necessary to follow the fairly complex geometry. The quality of an element was determinedThe design phase by minimum was composed length, of skew three angle, principal Jacobian, steps; aspectfree-size ratio,-optimization, and warpage. discrete In-s fact,ize- reducingoptimization the size, and of s thehuffle elements-optimization. increased As their the number, monocoque so results of IDRApegasus were more accurate (the predecessor with a longer of computationalIDRAkronos) never time. reached more than 18 mm in thickness, the maximum thickness of the laminate was MATset to 820 was mm used during in Hypermesh optimization to. defineTotal d linearisplacement temperature-independent response to the load orthotropic steps “driver materials inside, forstand two-dimensional, and exit” was elements. considered The to PCOMPP be 1 mm. card For was most used parts in combination, two plies with of carbon the STACK fiber andfor each PLY cardsorientation to create were composite more than properties enough, throughand the minimum the Ply-based thickness composite for each definition. orientation was, in fact, set to 0.5Composite mm, corresponding materials to are two highly plies, anisotropic in order to andkeep behave the laminate differently balanced based and on guarantee their orientation. a certain Thedegree 0◦ direction of isotropy. was oriented along the x-axis of the vehicle. The track on which the race was held was analyzed and, due to the speed profile expected during the race, a target on the limit for rollover conditions was set, assuming infinite adherence from the tires. From the Shell Eco-marathon regulations [1], the vehicle was required to be able to run a 10 m radius curve at a speed of up to 27 km/h. Multibody dynamic simulations made with Adams View confirmed the above requirements, outputting the forces acting on the wheels right before rollover (Table2).

Table 2. Rollover forces.

Tire Fz [N] Fy [N] External front 650 350 Internal front 0 0 Rear 350 150

In regards to the other loads, the mass of the head was applied in a rectangular shape onto the firewall, since the head was placed above the fire extinguisher-holding structure. The loads on the back and hips were applied on an area with a width roughly equal to the shoulder width of the driver, and the loads caused by the driver’s feet were applied on the front end of the vehicle. Constraints, similar to those on the load step were placed, which assured that the driver was inside the vehicle. Furthermore, the firewall had the function of a roll bar, which had to be capable of withstanding a static load of 700 N, as the regulations stated.

3. Results

3.1. FEM Results The design phase was composed of three principal steps; free-size-optimization, discrete-size-optimization, and shufﬂe-optimization. As the monocoque of IDRApegasus (the predecessor of IDRAkronos) never reached more than 18 mm in thickness, the maximum thickness of the laminate was set to 20 mm during optimization. Total displacement response to the load steps Materials 2019, 12, 332 6 of 11

“driverMaterials inside, 2018, 11 stand,, x FOR PEER andexit” REVIEW was considered to be 1 mm. For most parts, two plies of carbon fiber6 of for 11 each orientation were more than enough, and the minimum thickness for each orientation was, in fact, set toT 0.5o improve mm, corresponding correlation with to two the plies, real application in order to keepand reduce the laminate to bare balanced minimum and the guarantee amount of a certainmaterial degree used ofto isotropy.obtain the preset target in terms of stiffness, the contribution of the side windows was Tocomputed improve inside correlation the FEM with model. the realThis applicationis an essential and step reduce of the to design bare minimumand optimization the amount process, of materialsince an inaccurate used to obtain description the preset of the target general in terms behavior of stiffness, of the structure the contribution could lead of to the the side exaggerat windowsion wasof the computed thickness inside (and thetherefore FEM model. weight) This of the is an body. essential step of the design and optimization process, sinceFor an inaccurateoptical reasons, description 3 mm of thick the generalscratch- behaviorresistant ofantireflection the structure-certified could lead panels to the made exaggeration by Lexan ofwere the used thickness, resulting (and thereforein a 1.5 kg weight) mass addition of the body. to the vehicle. The lateral windows’ contribution to the bendingFor opticalstiffness reasons, of the monocoque 3 mm thick scratch-resistantwas very important, antireflection-certified as depicted by Figure panels 6, which made show by Lexans the wereeffect used, on the resulting roll-over in aconditions 1.5 kg mass, and addition Figure to 7,the which vehicle. shows The the lateral effect windows’ on the displacement contribution due to the to bendingthe driver stiffness’s hold ofdu thering monocoque the exit from was the very vehicle. important, It is clear, as depictedafter these by results, Figure 6that, which the inclusion shows the of effectthe glass on the contributed roll-over conditions, majorly toand the Figure structural7, which performanc shows thee, effect allowing on the displacement a proper setup due for to the the driver’soptimization hold duringand demonstrating the exit from that the these vehicle. elements It is clear, should after be these carefully results, designed that the and inclusion installed. of the glassThe contributed same approach majorly has to the been structural used for performance, the design of allowingthe front aend proper and setupthe tail for of thethe optimizationvehicle, with andlower demonstrating loads. that these elements should be carefully designed and installed.

Figure 6. Rollover displacements without (left) and with (right) windows. Figure 6. Rollover displacements without (left) and with (right) windows.

Figure 7. Exit displacements without (left) and with (right) windows. Figure 7. Exit displacements without (left) and with (right) windows. The same approach has been used for the design of the front end and the tail of the vehicle, with Figure 7. Exit displacements without (left) and with (right) windows. lower loads. GivenGiven strictstrict requirements requirements from from the the displacement displacement point point of of view, view, stresses stresses remained remain veryed very low, low, of the of orderthe order of few of MPa,few MPa, as shown as shown in Figure in F8igure. Nonetheless, 8. Nonetheless, for the for sake the of sake completeness, of completeness, a ply failure a ply failure check wascheck computed was computed using Tsai using Wu Tsai theory Wu for theory composite for composite materials. materials As expected,. As expected, ply failure ply never failure occurred never inoccurred any load in step, any load even step, in the even most in severe the most tests severe of the tests monocoque. of the monocoque.

Figure 9. Free-size-optimization of honeycomb thickness: Top (left) and bottom (right) view.

Materials 2018, 11, x FOR PEER REVIEW 7 of 11 Materials 2019, 12, 332 7 of 11 Materials 2018, 11, x FOR PEER REVIEW 7 of 11

Figure 8. Stress distribution during the exit maneuver.

Many analyses, such as Figurethe free 8.- sStressize-o ptimization distribution during (Figure the 9), exit were maneuver.maneuver carried. out in order to define the best shapes and thicknesses of the honeycomb inside the body. This was extremely important in Many analyses,analyses, such as the free-size-optimizationfree-size-optimization (Figure9 9),), werewere carried carried out out inin order order to to deﬁne define order to achieve the goal of minimum mass (and ultimately lower consumption) for the vehicle, while thethe bestbest shapesshapes andand thicknessesthicknesses of thethe honeycombhoneycomb inside the body. This waswas extremelyextremely importantimportant inin maintaining mechanical performance. Bearing in mind that the limits on the thicknesses available order toto achieve the goal of minimum mass (and ultimately lower consumption) forfor the vehicle,vehicle, while were only 13 mm , 6 mm, and 3 mm, and the shapes that could be obtained due to different curvatures maintaining mechanical performance. Bearing Bearing in in mind that the limitslimits on thethe thicknessesthicknesses availableavailable of the monocoque, the final lamination ply book is reported in Figure 10. werewere onlyonly 1313 mm,mm, 66Figure mm,mm, and 7. Exit 3 mm, displacements and the shapes without that (left could ) and bewith obtained (right) windows. due to different curvatures of thethe monocoque,monocoque, thethe ﬁnalfinal laminationlamination plyply bookbook isis reportedreported inin FigureFigure 1010..

Figure 9. Free-size-optimization of honeycomb thickness: Top (left) and bottom (right) view. Figure 9. Free-size-optimization of honeycomb thickness: Top (left) and bottom (right) view. FigureFigure 9.9. FreeFree-size-optimization-size-optimization ofof hhooneycombneycomb thickness:thickness: TopTop ((leftleft)) andand bottombottom ((rightright)) viewview..

Figure 10. Final laminate (left) and stacking sequence (right). Figure 10. Final laminate (left) and s tacking sequence (right). 3.2. Production Process 3.2. Production Process Figure 10. Final laminate (left) and stacking sequence (right). The ﬁnal properties of the components in composite materials, other than the properties of the carbon3.2.The Production final ﬁber properties and Process matrix, of depend the components strongly on in thecomposite production materials, process. other For thisthan reason, the properties it was not of enough the carbon fiber and matrix, depend strongly on the production process. For this reason, it was not to chooseThe final the rightproperties type of the materials, components it was in also composite of great materials, importance other to than evaluate the properties the appropriate of the enough to choose the right type of materials, it was also of great importance to evaluate the carbon fiber and matrix, depend strongly on the production process. For this reason, it was not

enough to choose the right type of materials, it was also of great importance to evaluate the

Materials 2019, 12, 332 8 of 11 Materials 2018, 11, x FOR PEER REVIEW 8 of 11 productionappropriate process production in order process to guarantee in order to good guarantee quality good of the quality final product. of the final Inter-laminar product. Inter cohesion,-laminar for example,cohesion, wasfor example, strictly dependent was strictly on thedependent absence ofon air the bubbles absence absorbed of air bubbles during theabsorbed manufacturing during the of themanufacturing laminate. of the laminate. The most suitable process to realize the components of the prototype was the use of the autoclave vacuum bag, which require requiredd a phase of composite lamination in a mold and a phase of thermal consolidation both in vacuum and under pressure. Blocks of epoxy resin (Figure 11)11) RAKURAKU-TOOL-TOOL WB0691 (Rampf Company, Company, Gräfenberg Gräfenberg,, BY, BY, Germany)Germany) were used to reduce the costs of metal. Resin ma malele molds were used to realize carbon fiber fiber female molds, which carried out the firstfirst carboncarbon fiberfiber lamination.lamination. Figure Figure 12,12, on the left, shows the closure of the female molds in the release film, film, and Figure 12,12, on the right, shows the closure of the breather filmfilm andand vacuumvacuum bag bag before before the the autoclave autoclave cycle. cycle. The The autoclave autoclave cycle cycle used used lasted lasted 3 h,3 athours, 130 ◦ atC and130 °C 5.5 and bar. 5.5 bar.

Materials 2018, 11, xFigure FOR PEER 11. MaleREVIEW resin molds; front and rear end (left),), and central part (rightright).). 9 of 11

For the second lamination, honeycomb sheets were glued in the central part of the body, only where designed by FEM (Figure 13, left). The composite layers were laminated to realize the internal shell (Figure 13, right). In order to avoid the collapse of Nomex, the second curing cycle was done at a maximum of 1.5 bar. After the body was released from the mold (Figure 14), some trimming was done on the tunnel, the windows, and the wheel mounting zone. To obtain a perfectly vertical laminate needed to mount the wheel, six of 22 plies of carbon fibers were sacrificed in the milling machine. Before starting the trim, the body was properly positioned and the symmetry plane was traced. This was crucial, so as to have the faces, on which the wheel brackets had to be mounted, perfectly parallel to the vehicle’s z–x plane, with an offset of ±210 mm along the y- axis, and therefore to minimize the error on the final wheelFigure alignment. 12. First lamination: Finally, Releaseall Release the holesfilm ﬁlm ( left leftwere)) and and drilled, vacuum the bag metal ( right inserts) on the screwed female central in, and mold mold. the. hooks affixed on the monocoque. ForThe themanufacture second lamination, of the firewall honeycomb was relatively sheets weresimple, glued with in only the centralone curing part cycle of the of body, 12 mm only of wherehoneycomb designed sheets by FEMbetween (Figure carbon 13, left).fiber Theplies. composite To properly layers glue were the laminated firewall on to realizethe monocoque, the internal a shellguide (Figure was built 13, with right). four In ordercentering to avoid pins theplaced collapse on the of main Nomex, body the to second guarantee curing the cycle perfect was placement done at a maximumof the firewall of 1.5 at bar.exactly 15° from the vertical plane. An error at this stage could have jeopardized the position of the rear wheel, both in relative misalignment and in absolute positioning, leading to a permanent, unwanted pitch or roll angle of the whole vehicle. The front end was laminated with two plies, while four plies were placed in the frame in contact with the main body, to increase the stiffness and the handling. The rear end was laminated with two plies, and after curing, the only operation made on the rear end was the cutting of a hole used to glue the NACA duct, which was 3D printed previously. Figure 13. Second lamination in the central part: Nomex honeycomb (left) and composite layers (right).

Figure 14. Final central part.

Figure 15. Full vehicle body assembled.

4. Discussion Mass reduction is a key factor in the design of a prototype vehicle aimed at a low consumption competition, such as the Shell Eco-marathon. In this paper, the structural design of IDRAkronos prototype's body, from the concept phase (Figure 3) to the manufacturing process (Figure 15) was

Materials 2018, 11, x FOR PEER REVIEW 9 of 11

Materials 2019, 12, 332 9 of 11 Figure 12. First lamination: Release film (left) and vacuum bag (right) on the female central mold.

Figure 12. First lamination: Release film (left) and vacuum bag (right) on the female central mold.

FigureFigure 13. 13.Second Second lamination lamination in thein the central central part: part Nomex: Nomex honeycomb honeycomb (left ) ( andleft) composite and composite layers layers (right ). (right). After the body was released from the mold (Figure 14), some trimming was done on the tunnel, the windows, and the wheel mounting zone. To obtain a perfectly vertical laminate needed to mount the wheel, six of 22 plies of carbon fibers were sacrificed in the milling machine. Before starting the trim, the body was properly positioned and the symmetry plane was traced. This was crucial, so as to have the faces, on which the wheel brackets had to be mounted, perfectly parallel to the vehicle’s z–x plane, with an offset of ±210 mm along the y- axis, and therefore to minimize the error on the final wheelFigure alignment. 13. Second Finally, lamination all the holes in the were central drilled, part: the Nomex metal honeycomb inserts screwed (left) and in, andcomposite the hooks layers affixed on the(right monocoque.).

Figure 14. Final central part.

Figure 14. Final central part.part.

The manufacture of the firewallFigure was 15. Full relatively vehicle simple,body assembled with only. one curing cycle of 12 mm of honeycomb sheets between carbon fiber plies. To properly glue the firewall on the monocoque, a guide was4. Discussion built with four centering pins placed on the main body to guarantee the perfect placement of the firewall at exactly 15◦ from the vertical plane. An error at this stage could have jeopardized the Mass reduction is a key factor in the design of a prototype vehicle aimed at a low consumption position of the rear wheel, both in relative misalignment and in absolute positioning, leading to a competition, such as the Shell Eco-marathon. In this paper, the structural design of IDRAkronos permanent, unwanted pitch or roll angle of the whole vehicle. prototype's body, from the concept phase (Figure 3) to the manufacturing process (Figure 15) was The front end was laminated with two plies, while four plies were placed in the frame in contact with the main body, to increase the stiffness and the handling. The rear end was laminated with two plies, and after curing, the only operationFigure 15. madeFull vehicle on the body rear assembled end was. the cutting of a hole used to glue the NACA duct, which was 3D printed previously. 4. Discussion 4. Discussion Mass reduction is a key factor in the design of a prototype vehicle aimed at a low consumption Mass reduction is a key factor in the design of a prototype vehicle aimed at a low consumption competition, such as the Shell Eco-marathon. In this paper, the structural design of IDRAkronos competition, such as the Shell Eco-marathon. In this paper, the structural design of IDRAkronos prototype's body, from the concept phase (Figure 3) to the manufacturing process (Figure 15) was prototype’s body, from the concept phase (Figure3) to the manufacturing process (Figure 15) was illustrated. Trying to minimize the mass of the vehicle, the authors highlighted the main steps of Materials 2018, 11, x FOR PEER REVIEW 9 of 11

Figure 12. First lamination: Release film (left) and vacuum bag (right) on the female central mold.

Figure 13. Second lamination in the central part: Nomex honeycomb (left) and composite layers (right).

Materials 2019, 12, 332 10 of 11 the CAD design that creates the general shape, the aerodynamic requirements, and ﬁnally the FEM simulations and optimization process. In this way, the ply book of the composite was deﬁned before fabricating the body using the pre-preg,Figure hand 14. lay-up, Final central and autoclave part. production processes.

Figure 15.15. Full vehicle body assembled assembled..

4. DiscussionThe design of the carbon ﬁber monocoque delivered great results in terms of mass and stiffness. The technical solutions adopted were proved to be successful, and each detail was important in Mass reduction is a key factor in the design of a prototype vehicle aimed at a low consumption achieving the desired mechanical performance of the structure, and therefore the general objectives for competition, such as the Shell Eco-marathon. In this paper, the structural design of IDRAkronos the whole vehicle. The main body had a mass of 7.5 kg, a tail mass of 1.2 kg, while the front-end mass prototype's body, from the concept phase (Figure 3) to the manufacturing process (Figure 15) was was 2.5 kg. This lightweight design contributed to obtaining a total vehicle mass of just 39 kg.

The design methodology and insights provided by this paper may contribute to the development of similar low consumption vehicles, creating a guideline for the application of carbon ﬁber composite materials in monocoque structures [8–10], or any other component with similar complexity and variety of constrains and goals.

Author Contributions: Conceptualization, P.S. and M.C.; methodology, A.M. and L.S.; software, A.M., L.S. and P.S.; validation, A.F., A.G.A.; formal analysis, A.F., A.G.A., H.d.C.P.; investigation, L.S.; data curation, L.S. and A.M.; writing—original draft preparation, M.C. and L.S.; writing—review and editing, M.C. and H.d.C.P.; supervision, M.C.; project administration, M.C.; funding acquisition, M.C. Funding: This research was funded by Politecnico di Torino—“Commissione contributi e progettualità studentesca” and all the sponsor and partner of Team H2politO (www.polito.it/h2polito). Acknowledgments: The authors would like to thanks: Altair Italia s.r.l. for the software licenses and all the students of Team H2politO. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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