1-s2.0-0266353886900023-main

Composites Science and Technology 26 (1986) 251-281

The Potential for Composites in Structural Automotive Applications

P. Beardmore and C. F. Johnson

Ford Motor Company, Dearborn, Michigan 48121 (USA)

SUMMARY

The primary use of fiber-reinforced composites in automobiles, with the exception of a few specialized low volume vehicles, has been in semi- structural or decorative parts. Use of composites in primary structural areas of the vehicle, such as body structures, has been very limited to date. Such applications offer a tremendous opportunity for future expansion of composites in the automotive industry. In addition to materials cost, there are two over-riding criteria for significant application of FRP materials in automotive structures: (1)proof of structural functionality/durability; and (2) development of rapid, reproducible fabrication procedures to optimize manufacturing economics.

From a structural viewpoint, there are two major categories of material response which are critical to the application of composites to automobiles: fatigue (durability) and energy absorption. An abundance of evidence is accumulating relating the functional properties of these materials in simple structures. It is clear that the fundamental requirements of energy absorption and fatigue resistance are satisfied by composites and the main challenge is to translate these capabilities into complex structures with less well-defined load inputs. The less quantifiable, but equally important, functional requirement of ride quality (usually defined in terms of noise, vibration and ride harshness, NVH) also appears to be attainable through the utilization ofcomposites. Even though this factor has been historically related to vehicle stiffness, and composite materials are less stiff than steel, all the indications are that the effective stiffness of composite structures meet NVH requirements--the elimination o f joints through part integration plays a critical role in achieving such synergistic effects.

251 Composites Science and Technology 0266-3538/86/$03.50 Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

252 P. Beardmore, C. F. Johnson

Many of the properties of composite structures depend on the control o! fiber location and part integration which in turn are a direct function oj the fabrication process. Current high production rate fabrication processes such as compression molding of sheet molding compound (SMC) type materials go only part way to optimizing the properties and economics. Optimum automotive composite structures will probably require a combination of processes, some of which will need significant development, to realize the enormous potential for composites in the automotive industry. Full-scale structures may involve SMC type molding, thermoplastic stamping and the developing preform molding (HSRTM) process which has, perhaps, the greatest potential of all the processes for revolutionizing the use of composite structures. Techno- logical breakthroughs in fabrication technology do not appear to be necessary, the main requirement is the development of existing assorted techniques combined with a concerted effort by all aspects of the composite and automotive industry.

INTRODUCTION

The economic constraints in a mass production industry such as the automotive business are quite different from those of the aerospace or even the speciality vehicle business. This is particularly true in the potential application of high-performance composite materials which hitherto have primarily been developed and applied in a cost-intensive manner, both from a basic materials cost and from a fabrication viewpoint. Except for a few notable exceptions, virtually all uses of plastics and composites in high volume vehicles are restricted to decorative or semi-structural applications. Sheet molding compound (SMC) materials are the highest performance composites in general automotive use today: the most widely used SMC materials contain approximately 25 wt ~o of chopped glass fibers and cannot really be classified as high-performance composites. Typically, SMC materials are used for grill opening panels on many car lines and closure panels (hoods, decklids, doors) on a few select models. A characteristic molding time for SMC is of the order of 3-4 minutes, which is on the borderline of viability for automotive production rates.

The next major step for composites in the automotive business is extension of use into truly structural applications such as the primary body structure (Fig. 1) and to chassis/suspension systems. These are the

Composites in structural automotive applications 253

Fig. 1. Schematic of primary body structure.

areas which have to sustain all the major road load inputs and impact loads. In addition, these major structures must deliver an acceptable level of vehicle dynamics such that the passengers enjoy a comfortable ride. These functional requirements must be totally satisfied for any new material to find extensive application in these structures and it is no small challenge to structural composites to meet these criteria effectively. Of course, these criteria must also be satisfied in a cost-effective manner, and appropriate composite fabrication procedures must be applied or developed which satisfy the high production rates but still maintain control of fiber distribution so critical to successful usage of composites in structural applications.

The present paper is an attempt to summarize the current state of development of high-performance composites from an automotive viewpoint. In particular, the specific types of composites holding the most promise for structural applications and the viable and developable fabrication processes most likely to be used will be summarized. The state of knowledge of the particular properties of most relevance to automotive applications will be presented together with a scenario for the necessary extension of knowledge required before high confidence can be placed in the structural application of the materials.


HISTORICAL PERSPECTIVE

Composite body structures have been used in a variety of speciality vehicles for the past three decades, Lotus cars, which began production in 1956, being a particularly well-known example. The composite material used was invariably glass fibers in, typically, a polyester resin. A variety of production methods have been used but perhaps the only common thread is that all the processes were slow, primarily because a very low production rate was required (typically, a maximum of 5000 per annum). The other common factor among these vehicles was the general use of some type of steel backbone or chassis which absorbed most of the road loads and crash impact energy. Thus, while the FRP body can be considered structural, the major structural loads were not imposed on the FRP materials.

High FRP content vehicles in existence today were initially designed to use FRP materials. Consequently, there is no direct comparison available between an FRP vehicle and an identical steel vehicle to relate baseline characteristics. Perhaps the best comparison is the prototype Graphite LTD built by Ford to afford a direct comparison between a production steel vehicle and a 'high-tech' FR P vehicle, l Although graphite fibers were used and the vehicle was fabricated by hand lay-up procedures, several interesting features were evaluated.

The graphite fiber composite (CFRP) car is shown in Fig. 2, and an

Fig. 2. Photograph of the Ford CFRP vehicle.

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TABLE I Major Weight Savings in Carbon Fiber Reinforced Plastics

Component Weight ( lb ) Reduction (/b)

Steel CFRP

Body-in-white 423-0 160.0 253.0 Front End 95.0 30.0 65.0 Frame 283.0 206-0 77.0 Wheels~5) 91-7 49-0 42.7 Hood 49-0 17.2 32.3 Decklid 42.8 14.3 28.9 Doors (4) 141-0 55.5 85.5 Bumpers (2) 123-0 44.0 79.0 Drivesha ft 21.1 14-9 6.2 Total Vehicle 3 750 2 504 1 246

exploded schematic showing the composite parts is shown in Fig. 3. The weight savings for the various structures are given in Table 1. While these savings (of the order of 55-65 ~) might be considered optimal because of the use of graphite ifibers,I other cost-effective fibers can achieve a major portion of these weight savings (see below). The CFRP vehicle weighed 2500 lb, compared with a steel production vehicle of 3750 lb, but vehicle evaluation tests indicated no perceptible difference between the vehicles. Ride quality and vehicle dynamics were judged at least equal to top quality production steel cars. Thus, on a direct comparison basis, a vehicle with an entire FRP structure proved to be at least equivalent to a steel vehicle, at a weight level only 67 ~/0 of the steel vehicle.

The CFRP car clearly showed that high-cost fibers (graphite) and high- cost fabrication techniques (hand lay-up) can yield a perfectly acceptable vehicle. The real challenge is to translate that performance into realistic economics by the use of cost-effective fibers, resins and fabrication procedures.

The existing speciality cars do not provide the answer to how extensively lower-cost composites will go in replacing steel in automotive structures because steel is used as the major load-carrying structure. The governing design guidelines for composites need to be further developed to ascertain, for example, how to design with low-cost composite materials, and to ascertain allowables for stiffness in situations where major integration of parts in composites eliminates a multiplicity of


joints. The following sections will summarize the extant knowledge in composites potential for automotive structures.

FRP MATERIALS

By far the most comprehensive property data have been developed on aerospace type composites, in particular graphite fiber reinforced epoxies. Extensive data banks are available on these materials and it would be very convenient to be able to build from this data base for less esoteric type applications such as automobile structures. Graphite fibers are the favored choice in aerospace because of the superb combination of stiffness, strength and fatigue resistance exhibited by these fibers. Unfortunately for the cost-conscious mass-production industries, these properties are only attained at significant expense. Typical graphite fibers cost in the region of $25 per lb. There are intensive research efforts devoted to reducing these costs by utilizing a pitch-based precursor but the most optimistic predictions are in the region of $8 per lb which still keeps these fibers in the realm of very restricted potential for consumer- oriented industries.

The fiber with by far the greatest potential for automobile structural applications is E-glass fiber based on the optimal combination of cost and performance. Likewise the resin systems likely to dominate are polyester and vinyl-ester resins based primarily on a cost/processability trade-off against epoxy resins. Higher performance resins will only find specialized application (in much the same way as graphite fibers) even though the ultimate properties may be somewhat superior.

The form of the glass fiber used will be very application-specific, and both chopped and continuous glass fibers will find extensive use. It might be expected that most structural applications involving significant load inputs will utilize a combination of both chopped and continuous glass fibers with the particular proportions of each depending on the component or structure. Since all the fabrication processes expected to play a significant role in automotive production are capable of handling mixtures of continuous and chopped glass (see below), this requirement should not represent any restriction. One potential development which is likely if glass fiber composites come to occupy a significant portion of the structural content of an automobile is the tailoring of glass fiber sizings and corresponding speciality resin development. The size of the industry (each pound of composite per vehicle translates to approximately 10


million pounds per year in North America) dictates that it would be economic to have fiber and resin production tailored exclusively for the automotive market. The advantage of such an approach is that these specific developments will lead to incremental improvements in specific properties which in turn will lead to increased applicability and increased cost effectiveness for the materials. For example, a 10% increase in fatigue performance by sizing improvements should lead to a directly proportional increase in composite utilization in the appropriate components and structures.

While the thermoset matrix composites discussed above will probably constitute the bulk of the structural applications, thermoplastic-based composites formed by a stamping process may well have a significant, but comparatively minor, role to play. Most of the thermoplastic matrices in commercial use today tend to concentrate on polypropylene or Nylon as the base resin. The reason is simply the economic fact that these materials tend to be the most inexpensive of the engineering thermoplastics and are easily processed. Both of these materials are somewhat deficient in either heat resistance and/or environmental sensitivity relative to vehicle requirements for high-performance structures. Other thermoplastic matrices for stampable glass-fiber-reinforced composites are under development, and materials such as polyethylene terephthalate (PET) hold significant promise for the future.

PERFORMANCE CRITERIA

From a structural viewpoint, there are two major categories of material response which are critical to the application of composites to automobiles. These are fatigue (durability) and energy absorption. In addition, there is another critical vehicle requirement, namely ride quality, which is usually defined in terms of noise, vibration and ride harshness (NVH) and generally perceived as directly related to vehicle stiffness. Obviously, material characteristics play a significant role in this category of vehicle response. These three areas are summarized separately below.

Fatigue

The specific fatigue resistance of glass-fiber-reinforced composites (GRP) is a sensitive function of the precise constitution of the material but there


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are characteristic trends developing to generate a reasonably clear picture of the sensitivity to cyclic stresses. For unidirectional GRP materials, the fatigue behavior can be characterized as illustrated in Fig. 4. A fairly well- defined fatigue limit is exhibited by these materials and as a guiding principle this limit can be estimated as approximately 35-40 % of the ultimate strength. Thus, for a 55 vol. % continuous glass-fiber-reinforced epoxy material such as might be used in a leaf spring, the ultimate strength would be approximately 180000 psi (1.24GPa) with a corresponding fatigue limit of approximately 63 000 psi (434 M Pa). A chopped glass fibre composite, by constrast, would have a fatigue limit closer to 25 % of the ultimate strength and .would exhibit much greater scatter in properties (Fig. 5). It is also important to realize that the different failure

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modes in composites (in comparison to metals) can result in different design criteria for these materials depending on the functionality involved. For example, a decrease in modulus can occur under cyclic stressing long before physical cracking and strength deterioration occur. If stiffness is a critical part of the component function, the loss in stiffness under the cyclic road loads could result in loss of the stiffness-controlled function (usually NVH) with no accompanying danger of any loss in mechanical function. This type of phenomenon does not occur in steel.

As a guiding principle, it follows from the above that, wherever possible, automotive structures should be designed such that continuous fibers take the primary stresses and chopped fibers should be present to generate some degree of orthotropic behavior. It is critical to minimize the stress levels, particularly fatigue, which have to be borne by the chopped fibers.

There is clear, emerging evidence from both fundamental research data and field experience with composite components that glass-fiber- reinforced composites can be designed to withstand the rigorous fatigue loads experienced under vehicle operating conditions. The success of composite leaf springs and SMC componentry attests to the capability of composites to withstand service environments.

The composite leaf spring represents an interesting example to typify the kinds of stresses, both ultimate and fatigue, which can be successfully tolerated by continuous glass-fiber-reinforced composites. The examples are derived from the design methodology for leaf springs published by Robertson. 2 For a typical composite containing 55% by volume of continuous glass fibers, a maximum allowable cyclic fiber stress of 120 000 psi (827 MPa) can be used. This corresponds to a composite stress of approximately 66 000 psi (455 MPa), a value slightly above the fatigue limit stress because the cyclic requirements of a spring in the field are less stringent than the conditions represented by the fatigue limit.

An additional, less obvious, potential problem in the leaf spring is the development of inter- and intra-laminar shear stresses, both monotonic and cyclic. These stresses are shown schematically in Fig. 6. Typical interlaminar shear stresses for failure in glass-fiber-reinforced epoxy are approximately 7000 psi (48 MPa). Evidence from a variety of composite materials 3,4 clearly indicates that these materials are not very sensitive to cyclic interlaminar shear stresses (Fig. 7) and a conservative design value for resisting these cyclic stresses is 60 % of the interlaminar shear failure stress. Thus, a simple failure test to evaluate the shear failure


TENSILE (BEND) TLS FAILURE FAI LURE

Fig. 6. Schematic illustration of interlaminar shear stresses.

strength will yield a design value for cyclic shear stresses. The corresponding leaf spring design must ensure that interlaminar shear stresses remain below this value.

A similar approach to the one outlined above for the leaf spring must be followed for all parts of the structure which undergo fatigue loading.

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While the data for all combinations of composite materials are not yet available, a sufficiently large data base is available such that conservative estimates can be generated and lead to reliable designs. It cannot be overemphasized, however, that the mechanical properties of composites (much more than isotropic materials) are very sensitive to the fabrication process. It is imperative that properties be related to the relevant manufacturing technique to prevent misuse of baseline data.

Energy absorption

The stress-strain curves for all high-performance composites are essentially linear in nature (Fig. 8). This is in contrast to most metals which exhibit a high degree of plasticity (Fig. 9) and is much more akin to the behavior of brittle materials such as ceramics. The point to be drawn from comparison of the stress-strain behavior is that materials which are essentially elastic to failure (composites, ceramics) might be considered to have no capacity for energy absorption since no plastic deformation energy is available to satisfy such requirements. However, ceramics have been used for decades as armored protection against high- velocity projectiles. Such energy absorption is achieved by spreading the localized impact energy into a high-volume cone of fractured ceramic material as shown schematically in Fig. 10. A large amount of fracture

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high performance composites, steel and aluminum.


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Fig. 10. Schematic of energy dissipation mechanism in ceramics.

surface area is created by fragmentation of the solid ceramic, and the impact energy is converted into surface energy resulting in successful protection and very efficient energy dissipation. Clearly, elastic (so-called 'brittle') materials can be very effective energy absorbers but the mechanism is fracture surface energy rather than plastic deformation.

The analogy given above leads directly to the conclusion that, in a similar manner, high-performance composites may well be able to absorb energy by a controlled disintegration (fracture) process. Clear evidence is emerging from laboratory test data on the axial collapse of composite tubes that efficient energy absorption of the type needed for vehicle structures can be achieved in these materials.

A comparison between the energy absorption mechanisms in metals and composites is shown in Fig. 11. Glass fiber and graphite fiber reinforced composites behave as indicated in Fig. 1 l(b). By contrast, composites utilizing fibers consisting of highly-oriented long-chain polymers (e.g. Kevlar) collapse in a metal-like fashion utilizing plastic deformation as the energy absorbing mechanism. The fragmentation/ fracture mechanism typical of glass-fiber composites can be very weight effective and Table 2 illustrates this point by giving typical relative values

TABLE 2 Energy Absorption (Typical Properties)

Mater&l Relative energy absorption (per unit weight)

High-performance composites 100 Commercial composites 60-75 Mild steel 40


Fig. 11. Photographs illustrating the different collapse mechanisms in (a) metal and (b) composite tubes for energy absorption.


for different materials. It is particularly significant that while high- performance, highly-oriented composites generate the maximum energy absorption, commercial type composites yield specific energy numbers considerably superior to those of metals. Thornton and coworkers have accumulated extensive data on energy absorption of composites s,6,7 and have generated formulae for estimating energy absorption for composites:

E~ = Pm/Asp

where E~ is specific energy absorption; Pm is desired average crush load; A s is cross section area of tube; p is density.

Typical values for E~ are 60 J g- 1 for 0/90 graphite-epoxy, 40 J g - 1 for 0/90 glass-epoxy, and 30 J g- 1 for a variety of SMC type materials. The values of E~ correspond to axial collapse of circular tubes--these values must be changed by a multiplication factor (0.8 for square tubes, 0.5 for rectangular tubes) to take account of geometrical effects on structural rigidity. The above equations can be used to estimate the capabilities of tubes or structures to absorb impacts.

In a manner directly analogous to metal structures, it is imperative to trigger the energy absorption process to guarantee successful generation of the required process and to eliminate the initial load spike which otherwise occurs. This is illustrated in Fig. 12, which shows identical composite tubes (a) without a trigger and (b) with a trigger. In the absence

Fig. 12. Photographs illustrating the effect of a trigger mechanism. (a) Local'shear-out' in absence of trigger and (b) progresswe collapse in a triggered tube.


/ / Fig. 13. Schematic of (a) the conventional bevel trigger and (b) the tulip trigger.

of a trigger, local shear-out can occur (Fig. 12(a)) with little energy absorption. In contrast, a trigger initiates progressive collapse with maximum energy absorption (Fig. 12(b)). Various trigger mechanisms have been proposed and tested 6'~ but the most favored types always involve a bevel at the initiation site (Fig. 13(a)). Effectively, the bevel acts as a stress concentrator and initiates the fracture without a major structural load spike. The propagating cracks travel into the uniform sections and a pseudo-square wave is generated (Fig. 14). Recently, a

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different type of generic trigger has been developed for composites by Thornton, 8 the so-called tulip trigger (Fig. 13(b)). The advantage of this particular trigger is that it appears to be capable of creating additional cracks during the fracture process which results in additional energy absorption particularly in the lower performance composites. Fig. 14 illustrates the increased energy absorption capability for several composites.S The tulip trigger acts in a similar manner to the bevel trigger in that the elevated points act as stress concentrators (Fig. 15) but the fact that all laminae are concurrently exposed to the first load input evidently leads to more uniform distribution of the load between all lamina and a greater number ofinterlaminar cracks (and more associated surface energy) is generated. The larger number of cracks is maintained as fracture propagates along the tube or structure. An additional feature of the tulip trigger is the more gradual rate of load build-up to the plateau which presumably can be tailored by varying the tulip angle.

Fig. 15. Section through composite tubes after initiation of collapse with a bevel trigger and a tulip.


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0.028 in wall.

The load-displacement plateau is, in practice, composed of a series of load undulations (Fig. 16). The variations about the mean level tend to be very large for energy absorption by plastic deformation (as in metals and Kevlar-reinforced composites), whereas the fluctuations are typically significantly smaller for glass and graphite fiber composites in which fracture is the controlling energy mechanism. This results in closer-to- ideal square-wave ibehavior for composites.

Virtually all the energy absorption data generated to date have been developed for axial collapse of relatively simple structures, usually tubes. It is still an open question concerning the ability to generate the same effective fracture mechanisms as the dominant energy mode in complex structures. In addition, it is well known from observations on metal vehicles that bending collapse normally plays a significant part in the collapse of the vehicle structure and it is consequently of considerable importance to evaluate energy mechanisms in bending failure. Just as in metals, meager data are available on energy absorption characteristics in


bending. There is no reason to believe that the relative energy values of metals to composites in bending should change significantly from the ratios in axial collapse except that bending failure (fracture) in composites may tend to occur on a more localized basis than plastic bending in metals. If, indeed, this does occur, then the ratio could change in favor of metals.

Stiffness and damping

Glass-fiber-reinforced composites are inherently less stiffthan steel. Some typical values for various types of composite are listed in Table 3. There are two offsetting factors to compensate for these material limitations. First, an increase in wall thickness can be used partially to offset the lesser material stiffness. Also, local areas can be thickened as required to optimize properties. Since the composite has a density approximately one-third that of steel, a significant increase in thickness can be achieved while maintaining an appreciable weight reduction. The second, and perhaps the major, compensating factor is the additional stiffness attained in composites by virtue of part integration. The integration leads directly to the elimination of joints, which results in significant synergism in stiffness. It is becoming increasingly evident that this synergism is such that structures of acceptable stiffness and considerably reduced weight are feasible in glass-fiber-reinforced composites. As a rule of thumb, a glass FRP structure with significant part integration relative to the steel structure being replaced should be designed for a'nominal stiffness level of 50-60 % of the steel structure. Such a design procedure should lead to adequate stiffness and to typical weight reductions between 30 and 50 %.

TABLE 3 Typical Stiffness of Composites

Material Modulus (106psi)

Unidirectional CFRP Unidirectional GRP Unidirectional Kevlar reinforced plastic XMC (high-performance molding compound) SMC-R50 SMC-25

20 6

11 4-5 2.3 1.3


The stiffness requirement for vehicles is normally dictated by NVH or vehicle dynamic characteristics. The historical axiom in the vehicle engineers' design principles is 'the stiffer the better'. However, there are some intangible factors which enter the overall NVH picture, in particular the damping factor. Obviously, it is an oversimplification to assume stiffness alone primarily dictates NVH, although it unquestionably dominates certain categories of NVH. Equally obviously, damping effects can play a significant role in many categories of NVH, and the fact that the damping of composite materials considerably exceeds that of metals must be relevant to the overall NVH scenario. Most people involved with composite component/structure prototype development express the opinion that some aspect of NVH (usually noise or vibration) is improved but little quantitative data are available to document the degree of improvement. There seems, however, to be little doubt that this increased damping capacity can only be a positive asset. It remains to be seen whether or not the additional damping can be of sufficient benefit as to become a cost-effective asset in that it allows beneficial iterative modifications to the vehicle.

FABRICATION OF COMPOSITE MATERIALS

The successful application of structural composites to automotive structures is more dependent on the ability to use rapid, economic fabrication processes than on any other single factor. The fabrication processes must also be capable of close control of composite properties to achieve lightweight, efficient structures. Currently, the only commercial process which comes close to satisfying these requirements is compression molding of sheet molding compounds (SMC) or some variant on the process. There are, however, several developing processes which hold distinct potential for the future in that these techniques combine high rates of production, precise fiber control and high degrees of part integration. The overall philosophy behind composites fabrication for automobiles is summarized in Fig. 17. The requirements for precise fiber control (high performance), rapid production rates and high complexity demand that automotive processes be in the region of developing processes shown schematically in Fig. 18. The evolving processes are thermoplastic stamping and high speed resin transfer molding~ a variation on 'preform molding' (e.g. resin transfer molding, structural


Fig. 17.

FIBER RANAGElqEITT I ! { PROPERTY OPTIMIZATION & UNIFO~ITY

J MINIMUM WEIGHT DESIGN

PART INTEGRATION' " MINIMUM COST

Overriding requirements of fiber control and part integration for optimizing composite economics.

resin injection molding). Each of these processes is summarized separately below with some comments relative to its merits and potential disadvantages.

Compression molding

A schematic illustration of the sheet molding compound (SMC) process, depicting both the fabrication of the SMC material and the subsequent compression molding into a component is shown in Fig. 19. This technology is widely used in the automobile industry for the fabrication of grill opening panels on virtually all car lines, and for some exterior panels on selected vehicles. Tailgates (Fig. 20), and hoods (Fig. 21) are examples

SIZE COMPLEXITY PERFORMANCE

REGION OF DEVELOPING PROCESSES

(HmH ~ RESIN TeANSF~)

Fig. 18. NUMBER OF PARTS

Schematic of the relationship between performance and fabrication for composites and the area of required development.


Fig. 19.

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Schematic illustration of S MC material preparation and component fabrication.

on cars while the entire cab on some heavy trucks (Fig. 22) is constructed in this manner. The process consists of placing sheets of leathery textured SMC (1-2 inch chopped glass fibers in chemically thickened thermoset resin) into a heated mold (typically at 150 C) and closing the mold under pressures of 1000psi (7 MPa) for about 2 minutes to cure the material. Approximately 80 % of the mold surface is covered by the SMC charge and the material flows to fill the remaining mold cavity as the mold closes.

The above description of the SMC process delineates material primarily used for semi-structural applications rather than high-load bearing segments of the structure which must satisfy severe durability and

Fig. 20. Typical SMC production tailgate from a Bronco II.


Fig. 21. Typical SMC Econoline hood.

energy absorption requirements. To sustain the more stringent structural demands, it is normally necessary to incorporate appreciable amounts of continuous fibers in predesignated locations and orientations. The same basic SMC operation can be utilized to incorporate such material modifications either by formulating the material to include the continuous fibers along with the chopped fibers or by using separate charge patterns of two different types of material. The complexity of shape and degree of flow possible are governed by the amount and location of the continuous glass material. Careful charge pattern

Fig. 22. Typical SMC truck cab.


development is necessary for com~nents of complex geometry. A typical example of a prototype rear floor pan fabricated by this technique 9 is shown in Fig. 23.

The limitations on the usefulness of compression molding of SMC type materials in truly structural applications have yet to be established. Provided continuous fiber is strategically incorporated, these materials promise to be capable of providing high structural integrity and this may well prove to be the pioneering fabrication procedure in high-toad bearing applications. The advanced state of commercialization of this process relative to other evolving techniques will also provide a lead time for compression molding to branch into higher performance parts.

Although compression molding of SMC type materials is an

Fig. 23. Prototype Escort composite rear floor pan.


economically viable, high-production-rate process in current usage, there are some disadvantages inherent in the process which, in the longer term, will limit applications and tend to favor the developing processes. For example, the degree of flow required to optimize the mechanical properties gives rise to a spread in mechanical properties as a result of imprecise control of fiber location and orientation. Typically, variations in mechanical properties by a factor of two throughout the component are not unusual, based on an initial charge pattern coverage of approximately 70 %. Such uncertainty in properties introduces reliability issues and conservative design allowables yielding a heavier-than- necessary component or structure. Currently, extensive research efforts are underway to develop SMC type materials which allow 100 % charge pattern coverage and which attain high, uniform mechanical properties with minimal flow. These materials can also be molded at lower pressures on smaller capacity presses. Materials developments such as these may well make the newer breed of SMCs much more applicable to highly loaded stuctures than has ever been envisaged up to the present.

One other potential disadvantage of compression molding, which may turn out to be the real limitation in structural usage, is the degree of part integration attainable. The basic strategy in composite applications is integration of as many individual (steel) pieces as possible to minimize fabrication and assembly costs (which offsets increased material costs) and minimize joints (which increases 'effective' stiffness). Compression molding requires fairly high molding pressures (about 1000 psi (7 MPa)) and thus limits potential structures in areal size and complexity (particularly in 3D geometries requiring foam cores). Consequently, while compression molding is likely to play a key role in the development of composites in structural automotive applications in the next idecade, ultimately the process is unlikely to provide optimum structural efficiency and weight. It should be pointed out that this statement will only prove true if the alternative, more optimal processes undergo the developments required because currently compression molding is the only commercial structural composite process capable of satisfying the economic constraints of a mass production industry.

High-speed resin transfer molding

Fabrication processes allowing precise fiber control with rapid processability would overcome many of the deficiencies outlined above. The use

276 P. Beardmore,. C. F. Johnson

of some kind of preform of oriented glass fibers preplaced in the mold cavity followed by the introduction of a resin with no resultant fiber movement would satisfy these requirements. The basic concepts required for this process are practised fairly widely today in the boat building and speciality car business. Without exception the glass preform is hand constructed and the resin injection and cure times are of the order of tens of minutes at the fastest. Major time contractions and automation of all phases of the process are necessary to generate automotive production rates. However, the basic ingredients of precise fiber control and highly- integrated complex part geometries (including, for example, box sections) are an integral part of this process and offer large potential cost benefits.

There are two basic elements to the high-speed resin transfer molding (IffSRTM) process which must be developed. The assemblage of the glass preform must be developed such that it can be placed in the mold as a single piece. In addition, the introduction of the resin into the mold must be rapid and the cure cycle must be equally fast to generate a mold closed- mold open cycle time of a few minutes. A schematic illustration of the process is shown in Fig. 24.

There are two processes currently in use which may have the potential to offer rapid resin injection and cure times. Resin transfer molding,

~ ~ / ~ PRECU T MATERIAL

DRY GLASS F I B E R ~

EPOXY SHAPING DIE ~- ~ J

Fig. 24. Schematic of' high-speed resin transfer molding (HSRTM).


currently in wide usage at slow rates, could be accelerated dramatically by the use of low viscosity resins, multi-port injection sites, computer- controlled feedback injection controls and sophisticated heated steel tools. There do not appear to be any significant technological barriers to these kinds of developments, but it will require a strong financial commitment to prove out such a system. A schematic illustration of the process is shown in Fig. 25, which also illustrates a variant on the process usually termed squeeze molding.

The second technique which promises rapid injection and cure cycles is reaction injection molding. Once the dry glass preform is in the mold, the resin can be introduced by any appropriate procedure and reaction injection would be ideal provided the resultant resin has adequate mechanical properties. The fluidity of the reactants would obviously be ideal for rapid introduction into the mold. Typical reaction times for these systems are around 30 s, giving a potential mold closed-mold open cycle time of the order of 1 min. A schematic illustration of the process is shown in Fig. 26.

Full 3D geometries, including box sections, are attainable by preform molding. In addition, only low-pressure presses are necessary. The high degree of part integration maximizes 'effective' stiffness and minimizes assembly. In principle, major portions of vehicles could be molded in one piece--for example, Lotus car body structures consist of only two pieces (albeit molded very slowly) with one circumferential bond. If similar sized complex pieces could be molded in minutes, a viable production technique would result.

(a)

z Lr. ;T (b)

,

Fig. 25. Schematic of (a) squeeze molding and (b) resin transfer molding.


~ ~ PRECUT MATERIAL DRY G L A ~ ~ G L A S $ PREFORM

Fig. 26. Schematic of HSRTM using reaction injection molding.

Thermoplastic stamping

The process of thermoplastic stamping is attractive to the automotive industry because of the rapid cycle time and the potential utilization of some existing stamping equipment. Thermoplastic stamping at its current level of development achieves cycle times of one minute for large components. Figure 27 presents a schematic of the process. Typically a sheet of premanufactured thermoplastic and reinforcement is preheated above the melting point of the matrix material and then rapidly transferred to the mold. The mold is quickly closed until the point where the material is contacted and then the closing rate is slowed. The material is formed to shape and flows to fill the mold cavity much the same as compression molded SMC. The material is cooled in the mold for a short period of time and then the mold opens and the component is removed.

Thermoplastic stamping is currently used in automobiles to form low- cost semi-structural components such as bumper backup beams, seats, and load floors. Commercially available materials range from wood-filled polypropylene, and short-glass-filled polypropylene which have low physical properties, to continuous, random, glass-reinforced materials based on polypropylene or PET which offer somewhat higher physical properties. Other materials based on oriented reinforcements and such


(a)

(b)

Fig. 27. Schematic of thermoplastic stamping. (a) Heated blank loaded into mold. (b) Mold closing, compressing material to fill cavity.

resins as polyether ether ketone (PEEK) and polyphenylene sulphide (PPS) are in use in the aerospace industry. These materials are expensive and limited in their conformability to complex shapes.

Higher levels of strength and stiffness must be developed in low-cost stampable materials before they can be used in truly structural automotive applications. Attempts have been made to improve the properties of stampable materials through the use of separate preimpregnated unidirectional reinforcement tapes. These materials are added to the heated charge at critical locations to improve locally the strength and stiffness. Use of these materials adds to the cost of the material and slightly increases cycle times. Although effective for simple configurations, location of the oriented reinforcement and reproduci- bility of location are problems in complex parts. To be cost-effective, these types of reinforcements will ultimately have to be part of the premanufactured sheet or robotically applied. Current research is in progress in the area of stampable sheet materials with oriented reinforcement in critical areas. For application to automotive structures, these materials will have to retain the geometric flexibility (i.e. ability to form complex shapes with the reinforcement in the correct location) in molding exhibited by today's commercial materials.

The question of part integration is the main open issue in the expanded use of this process. The high pressures (1000-3000psi (7-21MPa)) required limit, the size of components which can be manufactured on conventional presses. Thermoplastic stamping is also limited in capability to incorporate complex three-dimensional cores required for optimum part integration. If very large structures are economically favorable then


thermoplastic stamping will gain only the small components where geometry is relatively simple and part integration is limited due to physical part constraints, such as door, hood, and deck lid inner panels. If very large scale integration proves too expensive, then thermoplastic stamping will exhibit increased penetration. Continuing long-range research in the area of low-pressure systems, and incorporation of foam cores in stampings, could significantly alter this outlook in the long term.

CONCLUSION

The extension of use of composites to automotive structures will require an expanded knowledge of the design parameters for these materials together with major innovations in fabrication technologies. There is abundant laboratory evidence and limited vehicle evidence which strongly indicates that glass-fiber-reinforced composites are fully capable of meeting the functional requirements of the most highly loaded automotive structures. There are, however, sufficient unanswered questions (e.g. about long-term environmental effects) to ensure that applications will be developed slowly until adequate confidence is generated. Nevertheless, it seems inevitable that the functional questions will be answered; it only remains to be seen how soon. Perhaps the more imperative requirement is the fabrication advancement that appears to be a prerequisite to widespread use of composites in automotive structures. High-volume, less stringent performance componentry can be satisfied by variations on compression molding techniques. It is the high-volume, high-performance regime which needs the development, and HSRTM molding appears to be a 'sleeping fabrication giant' capable of developing into just such a process. All the elements for attacking the rapid, high- performance problem are scattered around the somewhat fragmented composites industry. It will require the appropriate combination of fiber manufacturers, resin technologists, fabrication specialists and industrial end-users to generate the necessary developments. Such developments may well be more dependent on commitment of all the parties concerned rather than requirements for any technological breakthrough.

REFERENCES

1. P. Beardmore, J. J. Harwood and E. J. Horton, Int. ConJ~ on Composite Materials, Paris, August 1980.


2. R. E. Robertson, Design of Composite Leaf Springs, to be published. 3. B. Pipes, ASTM STP546, 1974, pp. 419-32. 4. P. Beardmore, Fatigue 84: Proc. Second Int. Conf. on Fatigue and Fatigue

Thresholds, Birmingham, UK, September 1984, Vol. 2, pp. 1091-102. 5. P. H. Thornton, J. Composite Materials, 13 (1979), pp. 247-62. 6. P. H. Thornton and P. J. Edwards, J. Composite Materials, 16 (1982),

pp. 521-45. 7. P. H. Thornton, J. J. Harwood and P. Beardmore, Composites Science and

Technology, 24 (1985), pp. 275-98. 8. P. H. Thornton, Proc. Fifth Int. Conf. on Composite Materials, San Diego,

August 1985, to be published. 9. C. F. Johnson and N. G. Chavka, An Escort rear floor pan, Proc. 40th Society

of the Plastics Industry Annual Tech. Conf., Atlanta, January 1985.

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