Composite side-door impact beams for passenger cars

ELSEVIER

Composite Structures Vol. 38, No. 1-4, pp. 229-239, 1997 0 1997 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0263-8223/97/$1J.O0 + 0.00

PII:SO263-8223(97)00058-S

Composite side-door impact beams for passenger cars

Seong Sik Cheon: Dai Gil Lee” & Kwang Seop Jeongb aDepartment of Mechanical Engineering, Korea Advanced Institute of Science and Technology Taejon 305-701, Korea

bDepatient of Textile Engineering, Yeungnam University, Kyongsan 712-749, Korea

The fuel efficiency and emission gas regulation of passenger cars are two important issues nowadays. The best way to increase fuel efficiency without sacrificing safety is to employ fibre-reinforced composite materials in the body of cars because fibre-reinforced composite materials have higher specific strengths than those of steel.

In this study, the side-door impact beam for passenger cars was developed using glass-fibre-reinforced composite materials as metals usually have a lower capacity of impact absorption energy at low temperature compared with that of glass-fibre-reinforced composite materials. Static tests were carried out to determine the optimum fibre stacking sequences and cross-sectional thickness for the composite impact beams taking consideration of the weight saving ratio compared to the high strength steel.

Dynamic tests were carried out at several different temperatures using the pneumatic impact tester, which was developed to investigate the dynamic characteristics of impact beams at a speed of 30 mph. Also, finite- element analyses were performed using ABAQUS, a commercial software to compare the simulated characteristics of the impact beams with the experimental results.

From the comparison, it was found that the results from the finite- element analvses showed eood agreement with the exnerimental results, although several assumptioks were’ mad e in the finite-element analyses. 6 1997 Elsevier Science Ltd.

INTRODUCTION

Fibre-reinforced composite materials have been used in aircraft and space vehicles as they have high specific strength (strength/density) and high specific stiffness (stiffness/density) [ 11. They also have high damping [2] and impact characteristics [3]. As the price of composites has fallen, they are now widely used for sport goods, leisure supplies, machine tools and in the structure of automobiles [4,5]. Reports from the United States and Canada predicted that plastics and composites would be widely applied to body panels, bumper systems, flexible compo- nents, trims, driveshaft and transparent parts of cars [6-81. Also, rotors manufactured using RTM (resin transfer moulding) for air compres- sors or superchargers of cars have been used to substitute for metal rotors which are difficult to

machine [9]. Composites have also been used to substitute flexspline materials in harmonic drives or traction drives [lo, 111. In industrial robots, in particular, stiffness is very important and an increase in the robot stiffness leads to an increase in the body weight, which reduces the payload of a robot. Therefore, composites were tried in the body of SCARA-type [12] or anthropomorphic robots [ 131. As mentioned above, substitutions using composites for exist- ing metal structures have been widely tried and successfully achieved in several cases.

The weight of cars has been continuously reduced to increase the fuel efficiency, which sacrifices the safety of cars. The best way to increase the fuel efficiency of cars without sacrificing safety is to employ fibre-reinforced composite materials in the body of cars because the fibre-reinforced composite materials have a

229

230 S. S. Cheon, D. G. Lee, K. S. Jeong

higher specific strength compared to that of metals. Because glass-fibre-reinforced and aramid-fibre-reinforced composites have high impact energy absorption characteristics, a car body made of these composite materials would bring about an increase in fuel mileage and a reduction in human body injuries when acci- dents occur.

In this study side-door impact beams, which require a large impact energy absorption capability, were manufactured using glass-fibre-epoxy composites. Design parameters such as fibre stacking sequence and cross-sectional thickness were selected and determined in order to manufacture composite impact beams. Moreover, various cross-sectional shapes of composite impact beams were designed to prevent local collapse as composites have a tendency to buckled at low level of external load owing to local collapse by concen- tric loads. Moulds for composite impact beams were fabricated and the prototype composite impact beams were manufactured. Three-point static bending tests were performed for the steel and composite impact beams.

Although the static energy absorption of composite beams was low, because composites usually have low failure strains compared with those of metals, the dynamic impact energy absorption of composites are high due to fibre pull-out, matrix cracking and delamination.

Because the dynamic energy absorption of side-door impact beams is more important than the static energy absorption, a pneumatic impact tester, whose impact velocity was 30 mph, was developed to investigate the dynamic behaviour of steel and composite impact beams. With the developed impact tester, the dynamic energy absorption of steel and composite impact beams was investigated at several low temperatures, as well as room temperature.

Also, numerical analyses using ABAQUS/ Standard, a commercial finite-element analysis package developed by H.K.S. Inc. (Hibbitt, Karlsson and Sorensen Inc.), were performed to compare the numerical results with the experimental ones.

Side impact beam

Fig. 1. Shape and mounting configuration of the side-door impact beams.

DESIGN PARAMETERS FOR COMPOSITE IMPACT BEAMS

Figure 1 shows the shape and mounting configuration of the side-door impact beams. The lengths of the front-door impact beam and the rear-door impact beams for compact passenger cars were 803 mm and 507 mm, respectively.

To manufacture the composite impact beams, design parameters such as the type of composite materials, stacking sequences [14], shape and the thickness of cross-sections should be determined. Table 1 shows the mechanical properties of high strength steel and glass-fibre-epoxy composites [3]. Consulting the Charpy impact energy absorption in Table 1, the glass-fibre- epoxy composite was selected for the impact beam material. Also, compositions of high strength steel (AISI 4340) for steel impact beams are shown in Table 2 [15]. The stacking sequences of the beam were determined using the results of the three-point static bending tests.

Table 1. Properties of high strength steel and glass-fibre- epoxy composites

High strength steel

(*Is!3?40)

Glass-fibre- epov

composites

Charpy impact (kJ/m’) [3]

Density (kg/m3) Ex (GPa)

E, (GPa) X7 2 GPa) (GPa)

214 622

7870 1980 210 43.5

210 0.3 2”j 80.8 1.5 1.0 5

Table 2. Compositions of AISI 4340 steel [ 151

C Mn P S Si Ni Cr MO

AISI 4340 0.38-0.43 0.60-0.80 0.035 0.040 0.15 1.65-2.00 0.70-0.90 0.20-0.30

Composite side-door impact beams for cars 231

Five different stacking sequences, such as

P%T, [k 15’lnT, [OJ90°1nT, [OJ9001nT and beams. The stacking sequences of [ + 15&. and

P2°~9nir showed similar behaviour; however, [01c/9040]T, for the glass-fibre-epoxy composite the latter was desirable because of the ease of impact beams were selected. The mass of each of the 507 mm impact beams was 0.25 kg and

cutting and handling the prepreg. Finally, an additional 0” layer was included to enhance the

the cross-sectional shapes of each of the impact bending stiffness, beams was circular. The outside and inside

resulting in the stacking sequence [0,“/90’],,.

diameters of the composite impact beam were Composites have a tendency to buckle by 31.8 mm and 26.3 mm, respectively. The longi- concentrated loads which give rise to local col- tudinal or axial direction was designated to be lapse, consequently fracture would occur at 0”. The three-point static bending tests were relatively low external loads. Therefore, various carried out using Instron 4206 to determine the cross-sectional shapes, such as shown in Fig. 3, optimum stacking sequences for the composite for prohibiting local collapse were devised. impact beams. The jig span and diameter of the The impact beam (a) in Fig. 3 has a hollow loading cylinder for the three-point static bend- circular cross-section, (b) has a regular square ing tests were 250 mm and 25.4 mm, cross-section, (c) is a composite wrapped onto a respectively. Figure 2 shows the results of the low carbon circular steel tube, (d) is a compo- three-point static bending tests. site wrapped onto a low carbon regular square

From Fig. 2 it was found that [O’],= angle steel tube, (e) is centre-part enhanced, (f) is a could not effectively sustain external load regular square cross-section strengthened by a because of its low strength under hoop stress. In rib and (g) is an I-type cross-section composite the case of [0160/904”]T, the 0” fibre could not impact beam. Two different thicknesses for the withstand the hoop stress after the 90” fibre impact beams were used to make the weights of yielded. Therefore, it was found that the 90” the impact beams 50% and 70% weight ratios fibre should be uniformly placed through the with respect to the high strength steel impact entire cross-section of the composite impact beams whose outer and inner diameters were

10

9

8

I I I I I I I 1

0 5 10 15 20 25 30 35 40

Displacement (mm)

Fig. 2. Load vs displacement diagram of composite impact beams with respect to stacking sequences.


I

6) O-9

W (4 W (0

Fig. 3. Various cross-sectional shapes for the composite impact beams.

31.8 and, 27.4 mm, respectively. The mass and length of the high strength steel impact beam were 0.825 kg and 507 mm, respectively.

MANUFACTURE OF THE COMPOSITE IMPACT BEAMS

Moulds for the various cross-sectional shapes were designed and manufactured. The UGN 150 type uni-directional glass-fibre-epoxy prepreg fabricated by Sun Kyung Industry (Suwon, Korea) was used for the manufacture of composite impact beams by the autoclave-vacuum bag degassing method.

Figure 4 shows the manufacturing sequence for the circular composite impact beam. First, a non-porous Teflon sheet was wrapped on a steel mandrel as shown in Fig. 4(a). Then, the prepreg was cut to the appropriate size and

(a) 09

(4 Fig. 4. Schematic diagram of the manufacturing sequence

for the circular composite impact beams.

angle and was stacked on the Teflon-sheet- wrapped mandrel, as shown in Fig. 4(b). After removing the mandrel, as shown in Fig. 4(c), the rolled prepreg was placed inside the bottom mould, as shown in Fig. 4(d). After assembling the upper and lower moulds, with bolts as shown in Fig. 4(e), the prepreg inside mould was cured in an autoclave after bagging the whole assembly with a vacuum bag, as shown in Fig. 4(f).

Figure 5 is a photograph of the manufactured circular cross-sectional composite impact beam which was adhesively bonded to the mounting brackets. After painting, the yellow-green col- our of the glass-fibre-epoxy composite was changed to black.

The regular square composite impact beams were manufactured using the mould with a regular square cross-sectional shape. The manufacturing methods for the regular square cross-section composite beam strengthened by the rib, as well as the I-type cross-section composite impact beam, are shown in Fig. 6.

The regular square cross-section composite impact beam strengthened by the rib was manufactured by co-curing [16-181 a prepreg layer inserted between the interface and two prepreg layers both placed on the top and the bottom surfaces of the two rectangular composites already manufactured. A similar manufacturing method was used for the I-type cross-sectional composite impact beams.

Figure 7 shows the cure cycle used for the manufacture of the composite impact beams.

As shown in Fig. 7, the 30 min dwelling stage at 80°C was employed to promote consolidation between the plies of prepreg. Then the inside temperature of the autoclave was increased to 120°C to cure the prepreg. During the entire cure cycle a vacuum state was maintained inside the vacuum bag, while a 0.6 MPa air pressure was applied outside the vacuum bag. A photograph of the cross-sections of the manufactured composite impact beams of the types shown in Fig. 3(a), (b), (f) and (g) are shown in Fig. 8. The cross-sections of the impact beams were painted to enhance their visability.

STATIC BENDING TEST AND NUMERICAL ANALYSIS

The jigs and loading cylinders were prepared based on the FMVSS (Federal Motor Vehicle


Rg. 5. Photograph of the circular cross-sectional composite impact beam bonded to the mounting brackets.

Fig. 6. Schematic diagram of the manufacturing methods for the regular square cross-section impact beam strengthened by the rib, as well as the I-type cross-section

composite impact beams.

150 I.0

s 3 loo 2 1 5 z 0.5 2

; g i? 50 b;

F k

0 I 2 3 4 5 6

Time (hour)

Fig. 7. Cure cycle for the glass-fibre-epoxy composites.

Safety Standards) 214 regulation which regu- lates the static properties of the side-doors of passenger cars. Figure 9 shows the jig for the three-point bending test of the impact beams.

As shown in Fig. 9, the jig span was set at 470 mm (18.5 in.), and two 25.4 mm (1 in.) diameter cylinders were used to support the impact beam. The load was given by an Instron 4206 through a 304.8 mm (12 in.) diameter half cylinder at the midpoint of the impact beam. The stacking sequence of all the composites was [03°/900]nT. Prototype composite impact beams were manufactured and tested with a weight ratio of 50% and 70% with respect to steel impact beams. From the experiments, it was found that the load-carrying capacity of the high strength steel impact beams was 27.3 kN, while the load-carrying capacity of the circular composite impact beams was 16.2 kN. The composite impact beam was collapsed locally by the concentrated load which was prominent at the circular cross-section. The regular square cross-section composite impact beams were able to resist a 25.3 kN external load because the contact area of the regular cross-section was larger than the circular cross-section. The impact beam manufactured by wrapping composite prepreg onto the circular low carbon steel yielded at low external load of 12.5 kN, which has not only low load-carrying capacity but also low weight saving effect due to the embedded steel. The composite impact beam which had an


Fig. 8. Photograph of the cross-sections of the composite impact beams.

enhanced centre part also yielded at a relatively low external load of 17 kN. The centre-part size of the impact beam could not be increased beyond a certain limit because of the limitation of the mounting space. Therefore, the composite impact beam whose centre part was enhanced was shown to be irrelevant. The regular square cross-section composite impact beams strengthened by the rib could resist an external load of 27.1 kN, which is similar to that of the impact beam made of high strength steel. The I-type cross-section composite impact beams yielded at 25 kN, which was similar to

e? 12 in

I- 470mm -4 Fig. 9. Jig for the three-point static bending test.

the impact beams with the regular section. Figure 10 shows the static capacity of the impact beams.

square cross- load-carrying

The load-carrying capacities of the impact beams were numerically analysed [19] using ABAQUS/Standard. The employed element both for the high strength steel and for the glass-fibre-epoxy composites was C3D8R (solid three-dimensional, eight nodes, reduced inte- gration, hour-glass control) to prevent both shear locking and hour-glass mode. Only a quarter of the cross-section of the impact beam was modelled to include the contact phenome- non between the impact beam and the loading cylinder. The loading cylinder was assumed to

I * 3 4

Fig. 10. Static load-carrying capacity of the composite impact beams of [0,“/90”],, stacking sequence. 1, Circular cross-section; 2, regular square cross-section; 3, regular square cross-section strengthened by a rib; and 4, I-type

cross-section.


3

L.- ‘1.. \

DISPLACEYEHT IIICATIOW FACTO

;hsr,nr TIlL COIPLETCD IN %A.

MAWS VERSIOIP, 5.4-l L 09.41.11

ST&P 1 IRCRCIERT 10

(a) (b)

Fig. 11. Deformed shape of the steel impact beam.

Displacement (mm)

- Experiment

... Analysis

25 50 15 100 125 1

Displacement (mm)

-Experiment

.--- .-. Analysis

125

Displacement (mm)

___ Experiment

Analysis

0-r 0 25 50 15 100 125 1

Displacement (mm)

50

Fig. 12. Comparison between the static test results and the finite-element analyses. (a) Circular cross-section, (b) regular square cross-section; (c) regular square cross-section strengthened by a rib; and (d) I-type cross-section.


be rigid. The friction force between the contact surfaces was assumed to be negligible. The orthotropic composite properties as shown in Table 1 were used taking into consideration the stacking sequences. The deformed shape of the steel impact beams from finite-element analyses is shown in Fig. 11 as a representative sample.

Figure 12 shows the results of the finite-element analysis where they showed a relatively

Electromagnet

TUP -

Impact beam Accelerometer

(b) Fig. 13. Dynamic impact tester accelerated by a pneumatic

cylinder. (a) Schematic diagram. (b) Photograph.

good agreement with experimental results. The low- and high-level lines represent 50% and 70% weight ratios, respectively, with respect to the steel. Also, the solid and the dashed lines represent experiment and analysis, respectively.

From the experiments and analyses, it was found that the composite impact beam of the regular square cross-section strengthened by the rib had a comparable static strength compared to the high strength steel impact beam. How- ever, the composite impact beam had a low energy absorption capability because composites usually do not have plastic regions after yield- ing. However, as impact beams undergo dynamic loads in a car crash, the dynamic impact energy absorption capability [20] of impact beams is more important than the static energy absorption capability. In addition, the new FMVSS 214 regulation, revised in 1993, introduced these dynamic tests between cars.

DEVELOPMENT OF AN IMPACT TESTER

An impact tester, whose impact velocity was increased by a pneumatic cylinder, was developed to investigate the dynamic characteristics of impact beams, as shown in Fig. 13.

The 25 mm nose radius impact tup of the dynamic impact tester is accelerated when the electromagnet which holds the piston in the pneumatic cylinder is switched off. When the air pressure in the cylinder was 0.5 MPa, the velocity of the 13 kg impact tup was greater than 30 mph. During the impact process, veloci- ties of the impact tup before and after impact were measured with four photo-sensors. The upper two photo-sensors were used to measure the time difference through 50 mm of movement, while the lower two photo-sensors were used to measure the time difference through 100 mm of movement. Because the lower two photo-sensors were infrared emitted-retrore- flector-type, the interval between them was set to 100 mm considering the space for the retro- reflective mirror mounting, whose diameter was 85 mm. On the other hand, as the upper two photo-sensors were optical-fibre-type without mirrors, their interval wns set at 50 mm.

The acceleration of the impact tup was also measured with an accelerometer attached at the impact tup. The signals from the photo-sensors and the accelerometer were processed by an IBM 486 computer through an A/D converter.


IO

8-

From senscsr 2, 9.2 ms

4

-2 -

-4

-6 - From sensor 4. 46.2 ms

-8 - From sensor 3. 34.7 ms

-10 I 1 1 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.h

Time (set)

Fig. 14. Signal from the photo-interrupt sensors.

The mass of the impact tup was adjusted in the range of lo-15 kg to vary the impact magni- tude. Also, the impact velocity was adjusted in the range of 1-25 m/s by varying the pressure inside the cylinder. The measured signals from the accelerometer were low-pass filtered with a 100 kHz cut-off frequency.

DYNAMIC IMPACT TESTS

The high strength steel circular impact beam whose outer diameter and inner diameter were 30 and 27.4 mm, respectively, was impact tested. The high strength steel, which was heat-treated through its thickness, had an ultimate tensile strength of 1.5 GPa. The mass and length of the high strength circular impact beam were 0.50 kg and 507 mm, respectively. The simply supported jig span of 360 mm was used during impact tests. When the tup mass and the cylinder pressure were 13 kg and 0.5 MPa, respectively, the velocity of the tup was 13.1 m/s (29 mph, 47 km/ h), which was close to the standard velocity of 30 mph for the side crash test of FMVSS 214. In the tests, about 1100 J of dynamic energy was given to each specimens.

In dynamic tests, because the Fomposite impact beams of 0.25 kg (50% weight ratio) showed a sufficient dynamic energy absorption capability, only 0.25 kg composite impact beams were tested. Figure 14 shows the time differ- ences checked by the four photo-interrupt- sensors during dynamic testing of the high strength steel impact beams at 25°C.

From Fig. 14, the measured initial velocity of the impact tup was 13.1 m/s (Vi = 50 mm/ 3.819 ms), and the measured velocity after impact on the high strength steel impact beam was 8.75 m/s (Vf = 100 mm/11.43 ms). There- fore, the energy absorption rate, which is defined as 1 - Vf/VF, was 55%. The glass-fibre- epoxy composite impact beam, which had a 0.25 kg mass, was found to absorb 53% of the given impact energy at a room temperature of 25°C. Therefore, it was found that a 50% weight saving could be obtained when the composite impact beams were used instead of steel ones, based on the dynamic energy absorption capability. Moreover, composite impact beams showed very similar dynamic energy absorption capabilities regardless of their cross-sectional shapes, although the static strengths were much more dependent on the cross-sectional shapes

Table 3. Dynamic energy absorption rate of four sections of composite impact beams

Cross-sectional shape Circular type Regular square Regular square +rib (0.25 kg) (0.25 kg) (0.25 kg)

Energy absorption rate 53.2% 52.9% 52.5%

(O.&kg)

53.3%

238 S. S. Cheon, D. 6. Lee, K. S. Jeong

100 -- ---

I High strength steel, AlS14340 (0.50 kg)

90 I Circular section composite (0 25 kg)

80 1

IO I 0 - 1~ ~~~ -or I -T--~ .~_._ r ___.. _~ ,

~1 ~~~~~ -

-60 -SO -40 -30 -20 -;0 0 10 20

Temperature (“C)

Fig. 15. Dynamic energy absorption results with respect to temperature.

on the composite impact beams, as shown in Table 3. Therefore, it was decided that in this study only circular composite impact beams were to be investigated.

In order to investigate the temperature dependence of the impact beams, the impact tests were performed at several low temperatures. Figure 15 shows the impact energy absorption of the impact beams with respect to environmental temperatures. The low environmental temperature was established by placing the impact beams inside a box containing dry ice, and the outside temperature of the impact beam was measured by a touch-probe-type ther- mometer.

From Fig. 15, it was revealed that the energy absorption of the steel impact beams went down as the environmental temperature dropped; however, the composite impact beam had an almost constant energy absorption capability. As it was expected that the slope of energy absorption rate was steep between 0 and - 10°C an impact test was performed at -5°C. From the test it was estimated that the nil ductility temperature [21] of the steel impact beam might exist between -5 and - 10°C.

CONCLUSIONS

In this study composite side-door impact beams were designed and manufactured. From the three-point static bending tests it was found that the composite impact beams with a circular

cross-section had a tendency to be buckled by relatively low concentrated loads, which gave rise to local collapse and fracture. However, the regular square cross-section composite impact beams, especially strengthened by the rib, could resist external loads comparable with that of the high strength steel impact beams. The glass- fibre-epoxy composite impact beam has a 30% weight reduction compared to the high strength steel impact beam based on the static bending tests.

A pneumatic impact tester was developed to investigate the dynamic characteristics of the impact beams at several different environmental temperatures. From the dynamic tests it was found that the composite impact beam had bet- ter impact energy absorption capability than the high strength steel impact beams. The cross- sectional shape of the impact beams had little influence on the impact energy absorption capability. The impact energy absorption capability of the high strength steel impact beams dropped abruptly at environmental temperatures below - 10°C from which it was concluded that the nil ductility temperature of the high strength steel impact beam might exist at a temperature around - 10°C. The glass-fibre-epoxy composite impact beam has a 50% of weight reduction compared to the high strength steel impact beam based on the dynamic tests.

From the experiments it was concluded that the composite impact beams not only reduce the weight of the impact beams by more than 50% but also had a constant impact energy


absorption capability with respect to environmental temperature variation.

ACKNOWLEDGEMENTS

This work was supported by KOSEF (Korea Science and Engineering Foundation) under grant No. 95-02-00-14. Their support is grate- fully acknowledged.

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Composite side-door impact beams for passenger cars