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Composite Structures 78 (2007) 495–506

Integrated development of CFRP structures for a toplesshigh performance vehicle

Paolo Feraboli a,*, Attilio Masini b, Luigi Taraborrelli c, Andrea Pivetti c

a Department of Aeronautics and Astronautics, University of Washington, Box 352400, Seattle, WA 98195-2400, USAb Advanced Composites R&D Division, Automobili Lamborghini S.p.A, Sant’ Agata Bolognese, Italyc Analysis Department R&D Division, Automobili Lamborghini S.p.A, Sant’ Agata Bolognese, Italy

Available online 4 January 2006

Abstract

When modern saloon cars are re-engineered as convertibles (or roadsters) it is typical for them to lose 50% or more of the body’storsional rigidity. Consequently the vehicles rarely handle quite as crisply, nor do they ride as well as the coupes from which they derived.This paper highlights the fundamental contributions of advanced composites in achieving the desired value of handling of the MurcielagoRoadster without penalizing the overall weight of the vehicle. To compensate for the absence of the roof structure, the vehicle wasstrongly redesigned by introducing new structural members and reinforcing existing critical components. A new all-carbon/epoxy com-posite sub-frame, which spans the entire engine bay, is comprised of elliptical tubular members, and it is the first of its kind in a pro-duction vehicle. Engineering of the sub-frame, from preliminary design to manufacturing decisions, is the focus of this paper, and isachieved through a Building Block approach that sets the program within the Integrated Product Development (IPD) strategy thatthe Advanced Composites Division of Automobili Lamborghini S.p.A. employs for its technology demonstrators. The strategy consistsin a concurrent analytical and experimental development of the product, from the initial conceptual design and coupon testing, throughthe stages of element and subcomponent engineering, to final component manufacturing.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Polymer matrix composites; Selection of material processes; Destructive testing

1. Introduction

1.1. Background

The chassis of a car is the framework that provides themounting points for the suspensions, as well as the supportfor the steering mechanism, the power train, the fuel tankand the seats for the occupants, and every additional elec-trical cable and fluid line fundamental for the operation ofthe vehicle. While fulfilling these functions, it has to pro-vide sufficient rigidity for accurate handling, absorb crashenergy for the safety of the occupants, and resist opera-tional static and cyclic loads due to the interaction with

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doi:10.1016/j.compstruct.2005.11.011

* Corresponding author. Tel.: +1 206 543 2170; fax: +1 206 543 0217.E-mail address: [email protected] (P. Feraboli).

the road. Lastly, it needs to be light enough to increase fueleconomy and speed, and to reduce inertia, thereby increas-ing performance [1].

A chassis is typically designed for stiffness rather thanstrength. It has to be stiff enough not to influence thedynamical behavior of the vehicle, and to resist large defor-mations, which can deteriorate the mechanical propertiesof joints and components. The concept of torsional stiffnessis fundamental from the handling perspective, since itallows the lateral (i.e. cornering) loads to be distributedfrom front to rear, proportionally to the suspension rollstiffness. It is therefore an indication of the ability of thevehicle to closely follow the road surface. A common mea-sure of torsional stiffness is the twist produced under tor-sion [2], having units of [N m/deg] or [ft lb/deg], which isobtained by measuring the maximum deflection of the vehi-cle that occurs under a prescribed load applied at the front

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496 P. Feraboli et al. / Composite Structures 78 (2007) 495–506

axis, with the remaining one clamped. On the other hand,flexural stiffness, which is also an important indicator ofcrashworthiness and ride quality, is calculated by applyinga vertical load at specific locations along the longitudinalaxis and measuring the resulting bending deflections aboutthe rocker panels. The present work will focus only on thecontributions of CFRP (carbon fiber reinforced polymers)to the increased torsional stiffness of the Murcielago Road-ster (Fig. 1).

From the early days of automotive design, the appear-ance of the chassis has changed dramatically. Large-vol-ume passenger cars (not trucks or truck-based vehicles)are currently built with an integral or unitary (unibody)approach, whereby the classical box frame has disap-peared, and each body panel is a load-carrying member.On the other hand, low-volume, high-performance, super-luxury cars are built with either a complex spaceframeapproach or a monocoque design, while body panels arenot significantly stressed. A typical spaceframe is com-prised of transverse members, or bulkheads, which providethe majority of strength and stiffness, interconnected bylongitudinal and diagonal hollow tubular sections. Theintegral design of a monocoque chassis has instead theadvantage of providing the highest stiffness to weight ratio,particularly if built employing lightweight materials such asCFRP. However, while the fixed costs associated with theuse of CFRP materials are sensibly lower [3], the variablecosts remain higher due to the inherent cost of the rawmaterial and the highly labor-intensive process (hand lay-up, vacuum bag and autoclave cure), therefore the vehiclesemploying a composite monocoque approach for the entirepassenger compartment are usually very limited productionvehicles.

While modern Passenger cars exhibit average torsionalstiffness values of 4000–9000 N m/deg, high performance

Fig. 1. Three-quarter view and cutaway of the 2005 Murcielago Roadster highlCFRP/tubular steel chassis.

vehicles need to exhibit much higher values, in the range15,000–30,000 N m/deg. In this context, the original Lam-borghini Murcielago Coupe exhibits a total of21,000 N m/deg for the chassis-body assembly while theLamborghini Gallardo Coupe achieves 28,000 N m/degthanks to its aluminum monococque construction and itssmaller dimensions. Many are the factors that influencethe measurement of torsional stiffness during the test,which is sometimes conducted on the body-in-white(BIW), and sometimes on the complete vehicle. The pres-ence of the windshield, side windows and non-BIW bodypanels (such as the doors) influences the measurement ofthe torsional stiffness. It is therefore important to use thesenumbers for comparison purposes only if a thoroughunderstanding of the test conditions is available.

A great disadvantage associated with convertible vehi-cles is the loss in torsional and bending stiffness due tothe absence of the roof structure, which completes the pas-senger compartment and forms a closed section, and theassociated loss in vehicle performance and crash resistance.While these considerations apply to every topless car, theyare particularly evident in the case of high performancevehicles, as in the case of the 2005 Murcielago Roadster.In the earlier stages of development, full-scale torsionaltests indicated a loss in rigidity exceeding 50%, whichgreatly reflected on the handling characteristics, noise andvibration, and crash performance. A structural re-designprocess is therefore undertaken, and it involves the intro-duction of new members as well as the reinforcing of exist-ing ones. The new all-carbon/epoxy composite sub-frame,which spans the entire engine bay compartment, is com-prised of elliptical tubular members, and its engineering,from preliminary design to manufacturing solutions, is herediscussed in detail because it is the first of its kind in a pro-duction vehicle. Other critical composite components,

ights the all CFRP body panels, which are adhesively bonded to the hybrid

P. Feraboli et al. / Composite Structures 78 (2007) 495–506 497

already in use on the Murcielago Coupe [4], are stiffenedwith selective or global reinforcements, as in the case ofthe doorsill structural elements, the rocker panels and thefloor pans, which together also contribute to increasingthe vehicle rigidity and crash resistance. The structural tun-nel is also stiffened with selective unidirectional plies, andwas extended from the rear to the front firewall in orderto transfer the kinetic energy to the massive rear structurein the event of a crash [5,6].

1.2. Approach

The Integrated Product Development (IPD) strategy,sometimes referred to as Concurrent Engineering, consistsin the development of a new product by means of a multi-functional approach, and is antithetic to the traditionalform of sequential functional development. The IPD phi-losophy involves a simultaneous integration of the productmanufacturing, R&D activities, and business strategy. Anearly involvement in the product development of market-ing, program management, manufacturing, material sup-ply, testing, processing, and quality control provides amulti-functional perspective and facilitates the paralleldesign of product and process, reducing design iterationsand production problems. Through IPD, the design ofthe manufacturing process is then integrated with thedesign of the product in order to optimize the performance,availability and cost of the product. It becomes thereforenecessary for the design engineer to fully understand exist-ing and planned process capabilities and constraints. Anal-ogously, the manufacturing engineer needs to be activelyinvolved in the early decision-making process, so that pastmistakes are not repeated.

To implement an IPD strategy is fundamental to inte-grate Computer Aided Design, Manufacturing and Engi-neering (CAD/CAM/CAE) tools, working with acommon digital model of the product, which facilitatesthe definition, analysis, and refinement of product and pro-cess design data. These tools facilitate process design anddefinition, and translate into a reduction in lead-time toproduction. When intelligently and cost effectively applied,can lead to a streamlined development process and projectorganization. Analysis and simulation tools such as FiniteElement Analysis (FEA) and Virtual Manufacturing orTesting can be used to develop and refine both productand process design inexpensively. These tools should beused early in the development process to reduce the num-ber of time-consuming design/build/test iterations formock-ups and prototypes.

As shown in this paper, a correct approach to IPD willlead to continuous improvements and re-engineering of thedesign process. Even though a particular process may yieldalready satisfactory results, it is likely that it could be fur-ther improved, from a quality and efficiency standpoint. Itis the key to the success of a project involving advancedcomposites, which are often seen as an expensive optionalrather than a viable alternative, to consider cost projections

the fundamental part of decision-making to proactivelymanage product and life cycle costs.

When composites are to be used in structural compo-nents, a design development program is generally initiatedduring which the performance of the structure is assessedprior to use, and it generally consists of a complex mix oftesting and analysis. Testing alone can be prohibitivelyexpensive because of the number of specimens needed toverify every geometry, loading, environment, and failuremode. Analysis techniques alone are usually not sufficientlyreliable to adequately predict results. By combining testingand analysis, the cost of the overall effort is reduced andreliability is increased. This approach is conducted at var-ious levels of structural complexity, often beginning atthe coupon level, progressing through structural elementsand details, and terminating at the to sub-component andfull-scale component level. This substantiation process,using both testing and analysis in a program of increasinglycomplex levels, has become known as the ‘‘Building Block’’approach, and is usually collocated with supporting tech-nologies and design considerations within an IPD strategy[7]. The major purpose of employing this approach is toreduce program cost and risk while meeting all technical,regulatory, and customer requirements.

2. Results and discussion

2.1. CFRP structural components

Among the CFRP structural elements that already finduse in the Murcielago Coupe, the transmission tunnel(Fig. 2) is a monolithic laminate of total thickness of0.157 in. (4 mm), exclusively comprised of [±45] 2 · 2 twillplies, which exhibit high torsional stiffness [4,5], and itextends from the rear firewall halfway through the passen-ger compartment. To compensate for the lower torsionalrigidity, the structural tunnel of the Roadster is reinforcedintroducing 11 unidirectional plies, each 0.006 in. (0.15 mmthick), oriented longitudinally and at 45�. The thicker lam-inate (0.223 in. or 5.7 mm) increases the bending and tor-sional stiffness. Furthermore, its length is also extendedto reach the front firewall, thereby further increasing theoverall bending rigidity of the vehicle and providing a morefavorable load path.

The existing CFRP sandwich floor panels (Fig. 3), whichhave to exhibit high torsional as well as bending stiffness,are comprised of two [0/90] and one [±45] fabric plies forface sheets, and of an aramid honeycomb core of0.196 in. (5 mm) thick throughout the entire length. Thecore is terminated 1.57 in. (40 mm) away from the edgeof the component, and the face sheets are joint togetherto form a peripheral border. The 6-ply symmetric and bal-anced laminate thus obtained is 0.197 in. (5 mm) thick, andis used for the hybrid joining [4] of the composite panels tothe tubular steel spaceframe, since previous experience hasshown that the transition from sandwich to monolithiclaminate has beneficial effects on the joining procedure.

Fig. 2. Extended structural transmission tunnel.

Fig. 3. Floor pan.

Fig. 4. Front wheel housing reinforcement (in light tone), and sheet steelpassenger compartment (in dark tone).


It was initially noted in the unstiffened Roadster’s offsetfrontal crash tests, that the passenger compartment had atendency to buckle in the front doorsill area, due to theintrusion of the wheel/brake/suspension assembly, whichin turn prevented the post-crash opening of the doors.The portion of the front firewall corresponding to thewheel housing (Fig. 4), originally comprised of solely sixfabric plies, was also reinforced with 8 unidirectional (tape)plies, equally distributed in the [+45] and [�45] orienta-tions, for a total of 0.157 in. (4 mm). Similarly to the floorpans, an aluminum honeycomb core 0.197 in. (5 mm) thickis also used away from the edges, and the previous laminateis split in two face sheets, each 0.078 in. (2 mm) thick.

In the effort of replenishing the loss of mass and struc-tural integrity due to the absence of the upper structure,new doorsill stiffeners are added to provide increased bend-ing stiffness and crash resistance (Fig. 5). Similarly to thefloor pans and wheel housing reinforcements, the periphe-ral portion of the laminate (Fig. 6) differs from the remain-

Fig. 5. Internal doorsill stiffening members.

Fig. 6. The body-in-white structure of the 2005 Murcielago Roadster. The new and modified CFRP members are shown in red, the existing floor pans arein beige; the rear sub-frame is dark gray.


ing portion of the laminate. The stacking sequence at theperiphery is comprised of six [0/90] 5-harness satin plies,each one interleaved by two unidirectional plies orientedin the axial direction, for a total nominal component thick-ness of 0.157 in. (4 mm). However, away from the freeedge, the laminate thickness is increased to 0.343 in.(8.7 mm) and the majority of the lay-up is comprised of[0�] tape. In the event of a crash, these members aredesigned to transfer, together with the transmission tunnel,the kinetic energy from the frontal crush members throughthe passenger compartment, toward the rear bulkhead,where the majority of the vehicle’s mass is contained (the12-cylinder engine/power train assembly constitutes a largeportion of the curb weight of the vehicle). These inner stiff-ening members are also adhesively joined to the tubularsteel chassis by means of methacrylate adhesive, and aresupposed to contribute in resisting a large portion of theload coming from the wheel assembly and prevent it fromintruding in the compartment.

2.2. Development of the rear sub-frame

2.2.1. Initial design

The single, most complex application that contributesthe majority of the torsional stiffness increase (�50%) inthe upgraded Roadster (Fig. 6) is the rear tubular sub-frame, which embraces the entire engine bay. Fundamentaldesign requirements are to increase torsional rigidity, resistthe cyclic loads coming from the rear suspension attach-ments, be of easy removal in order to allow for accessingthe engine, and fit within the existing geometric envelope.

The first solution adopted by the Technical Develop-ment is to assemble a tubular welded steel sub-frame, com-prised of the same members as the main spaceframe. Themetallic frame is sized according to the loads measuredduring previous road operation of the vehicle, and consistsof circular cross-section steel rods, 0.98 in. (25 mm) diame-ter and 0.059 in. (1.5 mm) thick, weighing approximately32.8 lb (14.9 kg).

Fig. 8. Picture of the sub-frame, showing the prototype and theproduction versions of the sub-frame. The latter features transparentCFRP struts and titanium-painted, integral CFRP joints.


Further development suggested that employingadvanced composites would not only reduce the weightof the component, but if properly engineered would alsoallow for a more versatile design and a more estheticallyappealing product.

During the early stages of the development, the CFRPsub-frame is developed within the R&D division by handlay-up over an aluminum mandrel of prepreg tape, followedby vacuum-bagging for autoclave cure. The sub-frame iscomprised of 17 separate cross-members of lengths between3.93 and 13.78 in. (100–350 mm), having elliptical cross-sec-tion, and is mechanically fastened to the steel spaceframe.Unlike the metallic frame, which is available only in circularcross-section, CFRP allows the engineers to exploit the ben-efits of an elliptical cross-section to achieve the highestmoment of inertia while minimizing the vertical dimension(shell height), and remain within the 1 in. (25.4 mm) enve-lope. After extraction from the tool, each 3.05 ft (1 m) longtube was cut to the specified length.

The individual struts are then assembled into complexthree-dimensional joints, referred to as knees (Figs. 7 and8), which are also made of the same CFRP material. Thesejoints are separately fabricated, as two half-shells that arepartially cured in individual monolithic epoxy femalemolds, then co-cured with a layer of film epoxy adhesive.Each strut is then bonded to its knee housing via a singlelap joint, comprised of an internal tapered sleeve(pseudo-conical) of length 1.18 in. (30 mm). Thanks tothe symmetry of the elliptical geometry the joint actuallybehaves as a balanced joint, thereby compensating forcomplex out-of-plane stress that might develop. The clear-ance between the outer member and the internal sleeve is0.008 in. (0.2 mm) on each side at the joint, tapering to0.019 in. (0.5 mm). A paste epoxy adhesive is employedfor this application because of its more forgiving nature,which allows for compensating inevitable manufacturinginaccuracies.

The final nominal thickness of the CFRP element is0.134 in. (3.4 mm), with major and minor axes 1.53 and

Fig. 7. In its production version, the three integral sub-components are connemechanical fasteners.

1.00 in. (39.0 and 25.5 mm) long respectively, for a totalweight of 15.1 lb (6.85 kg). The class-A finish cross-mem-bers are varnished, by applying an additional layer of

cted by means of universal joints (circle) and fixed to the chassis through


transparent epoxy resin, in order to expose the ‘‘checker-board’’ pattern of the plain weave, while the knees aresprayed with an opaque polyurethane titanium paint. Amore detailed discussion regarding the surface finish andthe general rules adopted for the manufacturing of class-A body panels can be found in [4].

In conclusion, the traditional hand lay-up over malemandrel/vacuum bag, autoclave process leads only to mar-ginal results with regard to surface finish. It also provesexcessively time consuming both in the lay-up and bondingoperations, thereby increasing the cost of the final compo-nent to over seven times the cost of the baseline steel frame.

Further development work is therefore performed bytube rolling, a technique used extensively for the manufac-turing of golf shafts, where the individual plies are rolledover an aluminum mandrel, and subsequently wrappedwith an external thermo-retractable material (cello-wrap)before being cured in the autoclave.

A disadvantage related to this processing technique isthat the component requires extensive sanding to removethe imprint of the wrapping material before applicationof the transparent layer. Such operation in turn affectsthe integrity of the fibers and results in another labor-inten-sive process. A further disadvantage of this process for thepresent elliptical geometry concerns the cello-wrap itself,which exerts more pressure around the corners than alongits major axis. The result is a component of variable thick-ness (0.016 in. or ±0.4 mm), which in turn affects the bond-line clearance at the location of the tapered joint (Fig. 9,left). While the results obtained by this method are moresatisfactory from an aesthetical standpoint, manufactur-ing/assembly costs still exceed the baseline metallic sub-frame by over 400%, and concerns arise on the high vari-ability of component quality.

Fig. 9. Cross-section comparison of the tubular specimens obtained by tube rothe female mold.

In order to provide a competitive product, further re-engineering of the process is required. A third and finalsolution involves a technique used successfully for the man-ufacturing of complex shape commercial applications, suchas high performance bike frames. It consists in an externalaluminum tool, wherein the plies are arranged, and in aninternally pressurized Nylon bladder, which substitutesthe traditional vacuum-bag. The component can be curedin a regular oven, rather than in an autoclave, with notice-able cost reduction. Also, with this process it is possible toachieve the required class-A surface finish [4] since the exte-rior of the component faces the tool-side. This techniqueallows for a uniform pressure distribution, which in turnyields a constant wall thickness through the length of thecomponent (Fig. 9, right). Furthermore, with this processit is possible to manufacture of non-open ended parts, suchas the terminating sub-frame members, by leaving theinternal bladder inside the component, while with a tradi-tional vacuum bagging of mandrels the same operationwould have to be performed in a separate curing cycle,and likely with less success.

The entire sub-frame is now manufactured as two single,complex, large components, and two small individual rods,including the complex CFRP three-dimensional joints.These components are connected by means of universaljoints, which are incorporated in the component duringthe lamination process (Fig. 7), in the same fashion as othermetallic inserts [4]. The costly and time-consuming practiceof adhesively bonding each tubular member to a separatelyis then avoided. Furthermore, the structural integrity of thecomponent is, from a designer perspective, increased due tothe continuity of the one-piece, single-cure construction.However, while previous techniques resulted in a more for-giving product, whereby eventual manufacturing inaccura-

lling/wrapping over male mandrel (left) and by hand lay-up/internal bag in


cies could be compensated for at the time of sub-compo-nent assembly, in the case of this final solution, the fabrica-tion and assembly operations need to be more carefullyperformed. The larger frame sub-components are manufac-tured in a single two stage curing cycle, whereby the firstpartial cure involves only the bottom plies, up to thedesired wall thickness, and the second stage involves thefull cure of the completed component. In order to avoidundesired discontinuities, the plies continuously overlapeach other. Ply pick-ups and drops are designed to occurprogressively along the periphery of the component, at adistance greater than 0.157 in. (4 mm) from each other.With this last manufacturing process, the cost of the finalcomponent is thereby contained to about twice that ofthe original welded steel one, for an overall weight whichis noticeably less than half the metallic counterpart.

2.2.2. Test results

In the early stages of development, when both tube-roll-ing and bladder-molding manufacturing methods are beingconsidered, a first stacking sequence is evaluated formechanical properties. The stacking sequence, balancedand symmetric, is comprised of two [0/90] and one [±45]plain weave plies, followed by seven [0] unidirectional plies,and by other three fabric plies as the above ones. Benefitingof the geometrical symmetry of the tube, a second lay-up isthen made unsymmetric by removing two of the bottomfabric plies, and substituting them with two [0], one[+45], and one [�45] tape plies. While a detailed numericalanalysis could yield an optimum lay-up (fiber orientationand wall thickness) for each member of the sub-frame,manufacturing considerations and time restraints favorthe use of one lay-up for every tubular member. For theelastic properties of the unidirectional tape and plain weavelaminae see Table 1.

Shafts subject to combined bending and torsion are tra-ditionally designed with an outer stack of [0�] plies, whichprovide the highest axial stiffness, and a stack of [±45�]plies, known as the torque core, which supplies torsionalrigidity. In the present application the use of the outer fab-ric plies provide not only a pleasant esthetical effect, butalso a more damage resistant design. Experience shows thatthe [0�] plies on the surface are prone to delamination ini-tiation and crack propagation, which are unacceptable fora component exposed to the sight of the customer.

The advantage of a CFRP spaceframe construction isthat it takes full advantage of the high axial (compressive)

Table 1Rear sub-frame elastic and material properties

Description E1 Msi (MPa) E2

Fabric prepreg (supplier) 8.70 8.7Unidirectional prepreg (supplier) 19.34 1.1Laminate theory 11.94 4.3Tubular component—wrapping 13.79 –Tubular component—bladder 13.08 –

properties of the carbon fibers. However, torsional andbending stresses also manifest during vehicle road opera-tion, and the tubular struts need to be engineered to exhibitresist them. Two tests are employed to characterize theresponse of the tubular struts manufactured with the twodifferent techniques for comparative purposes [5,6].

For the bending test, the specimens are machined to alength of 9.84 in. (250 mm), and clamped at one end to pro-vide a cantilever beam setup. The clamp is manufactured toaccommodate the complex geometry of the tubular mem-ber, then rigidly fixed to an immovable plate. The otherend is also enclosed in a similar fixture, but instead of beingfixed to a plate it is connected to the load cell mounted onthe movable cross-head of the test machine. From therecorded load–displacement curves, failure is determinedto be ultimate load, or the maximum load sustained bythe structure before total loss in stiffness. In the case ofthe struts obtained by wrapping, failure is sudden, and isnot anticipated by earlier drops in the load curve [6]. Forthese specimens failure is also catastrophic, being associ-ated to a very large drop in stiffness, with load pickingup at an average value 60% lower than the failure load.In the case of the specimens obtained with the internalbladder, ultimate failure is preceded by at least one smallerdrop in the load curve, and at ultimate failure load dropsonly an average of 25% of the maximum value. Failurealways occurs in the vicinity of the clamped end of thespecimen.

For the torsion test, a similar setup is used, however, theload is applied with a prescribed eccentricity from the axisof the specimen. To minimize the bending contributiongiven by the cantilever beam setup, the tubular memberis supported immediately outside the loading fixture. Theload is then applied, resulting in a state of nearly constanttorsion, and a twist angle of 12.8� is imposed for all speci-mens. Failure is again defined as the peak or ultimate loadrecorded in the load–displacement curve. For both wrap-ping and bladder specimens ultimate failure is precededby a series of smaller drops [6], associated with the initia-tion of internal damage by means of cracking, splittingand delamination.

The results plotted in Fig. 10 are the average and stan-dard deviation values of three specimens per test configura-tion. The effective structural stiffness is the slope of theload–deflection curve and shall not be mistaken for thematerial stiffness or modulus of the material. For the bend-ing test it has dimensions of N/mm, while for the torsion

Msi (MPa) G12 Msi (MPa) m12 Msi (MPa)

0 0.57 0.310 0.87 0.296 1.84 0.49

– –– –

Fig. 10. Flexural and torsional test results, showing effective stiffness andfailure load per unit weight, used to compare the performance ofspecimens fabricated by wrapping and by internal bladder.


test it has units of N m/deg. Due to the slight differences inspecimen length and thickness, the results have been nor-malized by the weight of each component for both struc-tural stiffness and failure load.

2.2.3. Post-mortem inspection

Failed specimens are cross-sectioned with a diamond tipdisk saw, then polished for micrographic inspection. Animmediate observation concerns the non-uniform wallthickness distribution that characterizes the specimensobtained by wrapping (Fig. 9, left). The radii exhibit a25% lower wall thickness than the sides and, as a result,the resin content in the wall is higher. This can be verifiedupon inspection under the microscope (Fig. 11, left), which

shows evident resin-rich layers between the plies, and evenwithin individual unidirectional tape plies in the areasaround the midplane, where the pressure exerted by theautoclave is lower. In the case of the specimens manufac-tured by bladder molding, the wall thickness is uniformthroughout the entire length of the specimen (Fig. 9, right).Further micrographic inspection (Fig. 11, right) alsoreveals a much more compacted laminate, with individualplies equally distributed through the thickness, and withmuch smaller interlaminar resin layers. However, in certainarea, such as in proximity of the joints where a change incross-section occurs, some fiber distortion can be observed,but it is contained within acceptable limits. Another obser-vation that can be made by inspecting the cross-sectionaway from the failure zone is that, for both specimens,the 12 K tow used for the unidirectional tape plies, origi-nally selected because of the lower cost than finer tows, istoo thick for this application. A finer 6 K tow is then imple-mented in the production version.

The specimens tested to failure under flexural loadingexhibit a compressive fiber fracture surface in the proximityof the clamp, oriented in the hoop direction, from which abuckling wrinkle proceeds longitudinally on the surfacefabric ply. After cross-sectioning, both specimen typesreveals the presence of a slightly off-axis failure surfaceconsisting of fiber tensile breakage for the (0/90) and(±45�) fabric plies, and of matrix cracking due to trans-verse shearing (Fig. 12). It is suspected that fiber breakageinitiates in the outer-most bottom (0/90) fabric ply, slowlyproceeds through the other two fabric plies, but when itreaches the first [0�] ply it changes failure mechanism.The crack front quickly proceeds on a slant along theweakest transverse plane through all seven plies, towardthe radii, until it is slowed down by the outer fabric plies.Specimens subjected to torsional loads fail in a very differ-ent way. Failure likely initiates around the radii, in theform of matrix delamination at the interface of the [0] plystack with the inner fabric plies. It then proceeds outwardsthrough the wall, and vertically along the sides until it dis-sipates toward the middle of the laminate. Damage mani-fest in a series of ‘‘concentric’’, or radially parallel,delamination planes, interconnected by a series of matrixcracks (Fig. 13). It can be seen that the majority of thedamage is contained within the central plies of unidirec-tional tape. Typical load–displacement curves can be foundin [6].

After the mechanical characterization of the wrappingand bladder molding specimens, it is concluded that forthis application, the practice of grouping multiple plieswith the same orientation should be avoided, as sinceit provides very little resistance to crack propagation,particularly in the case of resin rich areas. Similarly, itwas confirmed that adjacent plies should not be stackedwith a high fiber orientation mismatch, as the [0i/ ± 45j]ply-interface is a weak interlaminar plane. A portionof the angle plies is moved toward the surface, andis interspersed among the axial plies, and the initial

Fig. 11. Micrographic photograph showing the degree of compaction and uniformity of the laminate for the specimens obtained by wrapping (left) and bybladder molding (right).

Fig. 12. Micrographic picture of cross-section of a specimen failed in flexure, showing the radial fracture front.


[(0/90)2/(±45)/07/(±45)/(0/90)2]T stacking sequence is thenreplaced by [(0/90)2/02/(±45)/02/(±45)/02/(±45)/0/90]T,which is the more damage-resistant laminate selected forfinal production.

Furthermore, while test results for the specimensobtained by wrapping technique are higher in absolutevalue, other considerations favor the adoption of the inter-nal bladder process. Among the principal factors leading tothe selection of the latter are lower cost and faster produc-tion rate, but also more forgiving and less catastrophic fail-ure mechanisms.

2.2.4. Full-scale testing

There are two methods employed at Lamborghini forassessing the performance of a chassis, namely static tor-sional testing and modal analysis. The static torsional testis performed on a gantry where the rear axle and suspen-sion arms are rigidly clamped, then a load is applied witha lever hinged in the center of the front axle and the result-ing rotation and deflections are measured. A numericalmodel is realized by means of commercial finite elementcodes, and a virtual test is performed that closely repro-duces the experimental results obtained by the full-scale

Fig. 13. Micrographic and photographic pictures of the cross-section of a specimen failed in torsion, showing the ‘‘concentric’’ delamination planes at themidplane of the specimen.


torsional rigidity testing. Once the numerical model is cal-ibrated, other virtual tests can be performed without hav-ing the need to constantly verify the results with costlyand time-consuming experiments.

The full-scale vehicle is modeled by mean of commercialcodes, such as Hypermesh� for the pre- and post-processingand Optistruct� or Nastran� for the actual processing. The3D geometry, created with software such as CATIA� or Pro-Engineer�, is imported in the finite element pre-processor,where the surfaces of the body-in-white are meshed with atotal of 369,047 elements, of which the majority are four-node quadrilateral (67%) and three-node triangular (25%)shell elements. Material properties for the composite areassigned by specifying the actual lay-up, since the shell ele-ment allows the use of layered material. Boundary condi-tions are applied so to accurately represent the real test,and the resulting displacements are compared with the onesmeasured during the experimental test at specified locationsalong the axis of the vehicle. The sole presence of the sub-frame greatly increased the stiffness of the vehicle, as shownqualitatively in the contour plots of Fig. 14, around 50% of

the original Roadster stiffness. There is no significant differ-ence observed in the results by replacing the steel sub-framewith the advanced composite one. However, to fully capturethe details of the behavior of a composite sub-frame it wouldbe necessary to develop and labor-intensive and computa-tionally expensive three-dimensional model of each jointand component. Out-of-plane stresses cannot be properlyaccounted for in the present model, which considers theglobal sub-frame and the laminate as a perfect continuum.

Since the composite is designed to exhibit the sameresponse to an applied load, the lower elastic propertiesof the multi-directional tape/fabric CFRP laminate are bal-anced by a greater component thickness. While the individ-ual influence of the extended tunnel, the doorsill stiffeners,and the other CFRP structural components has not beenevaluated, the contribution of these components has beenestimated to further increase the rigidity of the unstiffenedRoadster by another 30%, making it nearly as performingas the Murcielago.

Before the final component is installed on a roadand track test bed vehicle, and verified to handle pas by

Fig. 14. Deformation contours obtained by virtual testing of the BIW(body-in-white), before and after the introduction of the rear sub-frame,show a 50% improvement alone in torsional stiffness.


specification during actual driving conditions. TheMurcielago Roadster, like its roofed predecessor, has alsobeen designed to meet US Federal Motor Vehicle SafetyStandards (FMVSS) and European New Car AssessmentProgram (Euro-NCAP) regulations, and exceeded thesafety requirements. However, crashworthiness consider-ations deserve to be treated in a separate publication.

3. Conclusions

Advanced carbon fiber composites constituted animportant portion of the Murcielago structure, but pro-vided necessary for the development of the new MurcielagoRoadster. Traditionally, the additional structural compo-nents required to compensate for the loss in torsional andbending stiffness of the roofless vehicle tend to penalize

the weight of the vehicle. Advanced composite materialsallowed for the development of several reinforcing mem-bers that were introduced below the belt line of the vehicleto increase its dynamic and crash performance. Amongthese, the all-composite tubular sub-frame, first of its kind,whose engineering process from preliminary design to finalmanufacturing has been discussed, gave the most signifi-cant contribution. An extensive Integrated Product Devel-opment effort was undertaken to meet the requirementsimposed by the original metallic sub-frame, particularlywith regard to manufacturing costs, and the simultaneousdesign/manufacturing/testing process leading to compo-nent production provided a clear example of BuildingBlock approach. The final composite design achievedgreater operational and handling performance for an over-all weight saving in excess of 60%, while keeping theincreased manufacturing and material costs within twiceof the metallic benchmark.

Acknowledgements

The authors would also like to thank the TechnicalDevelopment of Automobili Lamborghini S.p.A. (Sant’A-gata Bolognese, Italy) for the dedication shown during thedevelopment of this wonderful product, in particular Dr.Ing. Ceccarani (Technical Director), Mr. Reggiani, Mr.Bonfatti and Mr. Gho’. They also would like to acknowl-edge the support of Dr. Galli and Dr. Scaramelli of HumanResources.

References

[1] Campbell C. The sports car: its design and performance. BentleyEditor, 1955.

[2] Saccone M. Composite design procedures for racing cars at DallaraAutomobili. In: 3rd SPE automotive composites conference, Troy, MI,September 2003.

[3] Jambor A, Beyer M. New cars–new materials. Mater Des1997;18(4):203–9.

[4] Feraboli P, Masini A. Development of carbon/epoxy structuralcomponents for a high performance vehicle. Compos Part B: Eng2004;35(4):323–30.

[5] Masini A, Taraborrelli L, Pivetti A, Feraboli P. Development ofcarbon/epoxy structural components for a topless high performancevehicle. In: ASC/ASTM 19th joint technical conference, Atlanta, GA,October 2004.

[6] Engineering Considerations on the CFRP subframe of the MurcielagoRoadster. In: SAMPE technical conference, Seattle, WA, November2005.

[7] Building block approach for composite structures. MIL-HDBK-17(Rev. F) vol. 3, 2002 [chapter 4].

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