Bending energy absorption of extruded aluminum beams

ELSEVIER

JSAE Review 18 (1997) 385 392

alP'

Bending energy absorption of extruded aluminum beams Yukio Yoshida a, Xiang Wan a, Masayuki Takahashi a, Toshiyuki Hosokawa b aVehicle Evaluation and Ad~'anced Engineering Department, Kanto Auto Works Co., Ltd., 7-7l Funakoshi-cho, Yokosuka-shi, Kanagawa, 237 Japan bMetal Research Center Aluminum Technical Research Department, Furukawa Electric Co., Ltd., 560 Dotoh Oyama-Shi, Tochigi, 323 Japan Received 10 January 1997

Abstract

The bending collapse behavior of extruded aluminum and hybrid beams was studied theoretically and experimentally. Very good agreement was found between the theoretical results for extruded square section tubes with some partitions. For seam bending energy absorption, the hybrid beams achieved significant reduction of weight compared with traditional reinforced thin-walled steel beams. The strain rate dependence of different heat treated extruded aluminum was explained by the high speed tensile tests. Static and dynamic bending tests were carried out to investigate the influence of different materials, heat treatments and sections on bending energy absorption. @ Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V.

1. Introduction Weight reduction in vehicles is an important issue from the standpoint of energy conservation and improvement of the motion characteristics of automobiles, but this need seems to run contrary to the need for improved automobile safety. From the standpoint of compatibility of weight reduction of automobile bodies with their need to absorb energy for safety reasons, extruded aluminum beams are regarded as superior to the current monocoque construction, in which steel plates are used as the structural members of automobile bodies, with respect to degree of freedom in cross-sectional shape. In this paper, the collapse mechanism and energy absorption characteristics of extruded aluminum beams are observed both theoretically and experimentally, and the application of extruded aluminum beams to reinforcement members are examined by conducting a static bending test using a hybrid structural member constructed of aluminum and steel.

(2) Examination of the static bending-energy absorption characteristics of hybrid beams: (3) Examination of the strain-rate dependence of extruded aluminum beams; (4) Examination of the dynamic bending-energy absorption characteristics of extruded aluminum beams; and (5) Comparison between static and dynamic bendingenergy absorption characteristics.

3. Static bending test 3.1. Specimen The specimen materials were obtained by heat-treating 6000 extruded aluminum beams in three different ways. Four sectional shapes of a hollow square, a double square, a threefold square and a quartered square were used. Table 1 shows the mechanical properties of the specimens.

2. Examination procedure

3.2. Test method

The procedures for examination are as follows: (1) Examination of the static bending-energy absorption characteristics of extruded aluminum beams;

In examining side collisions of automobiles, a threepoint bending test was used, as shown in Fig. 1. Bulkheads in the extruded aluminum beams with the sectional

0389-4304/97,/$17.00 (', 1997 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved PII S 0 3 8 9 - 4 3 0 4 ( 9 7 ) 0 0 0 2 9 - 5

JSAE9735826

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1/2 Yoshida et al. / JSAE Review 18 (1997) 385 392

Table 1 Typical mechanicalproperties Material

Tensile strength (MPa)

Yield stress (MPa)

Elonganon (%)

AI A2 A3

133.23 243.55 310.65

75.160 145.48 296.77

14.8 12.3 5.80

Specimen ~"~_.""/~.~ Loadinghead

-

• ¢ 100 '-

i

4.2. Work made by bending and rolling

+

800 1000

t=.o

4

t=l. 5 Sect ion

~" Fig. 1. Loadingmethod.

In the theoretical model shown in Fig. 3, energy absorption by sectional collapse can be divided into internal bending work Wb and internal rolling work Wr. Fig. 4(a) shows the bending of an ideal plate. The workload made by this bending can be expressed by the following equation: W b :

25

Mp'L'(o,

(1)

A3

"..nn II

[]

[]

i

°0

collapse will occur at the center of the member, as shown in Fig. 3. In order to theoretically examine the energy absorption of this sectional collapse, a geometric model of a non-elongation deformation constituted by polyhedrons, as shown in Fig. 3, was proposed based on observation of actual sectional collapse patterns. In this collapse pattern, dimensional relation can be determined by means of geometry with the bending angle of rotation as an independent variable. The workloads of bending and rolling phenomena are then calculated.

100 0 Deformation

100 0

i

i

i

i

lOO

where Mp = t2/4O-s is a full sectional plasticity moment per unit width, is the angle of rotation, L is the plate width, and as is yield stress. In Fig. 3, assuming that an action of bending rate 0 --* bending rate 1/r -~ bending rate 0 is generated at AC and AF as a sectional collapse progresses, a rolling phenomenon will occur. This rolling phenomenon is the same as the phenomenon in which a plate as shown in Fig. 4(b) is rolled and extended by

[mm]

Fig. 2. Staticenergyabsorption of extruded aluminum. ~

~

Web

shapes of a double square and a threefold square were oriented in parallel to the load.

Web~

~,~)M

lange

3.3. Energy absorption characteristics

Y F Figure 2 shows the results of static bending tests of extruded aluminum beams heat-treated in three different ways. The A3 material shows the highest value for the maximum load but this value decreases rapidly after the maximum load is reached. The A2 material shows higher energy absorption with the sectional shape of a quartered square than with the other three shapes.

El_

if

dD

"X

Fig. 3. Theoreticalmodel.

4. Bending energy absorption .-''; 4.1. Sectional collapse pattern

"W';Y/

"X~

When a moment is given to an extruded aluminum beam with the sectional shape of a square by three-point bending and a certain load is exceeded, a sectional

L ~ .. t =thickness (a) bending

L

(b) rolling

Fig. 4. Idealbendingand rollingdeformation.

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Y. Yoshida et al. / JSAE Review 18 (1997) 385--392

25

a roller without friction. The workload can be calculated by the following equation:

2.0mm6N01-T6

20 Wr = (2Mp/r)'Ar,

(2)

where, r is the bend radius, Ar is the passage area of the rolling line, and mp is the same as above.

Z

15 ~

Y

J

5

4.3. Calculation of the Ioa~displacement curve A bending load-displacement curve can be calculated in three parts, as shown in Fig. 5. That is, it is divided into 1. elastic region, 2. elastoplastic region, 3. sectional collapse region. In the elastic region, calculations are made by the beam theory of material strength, and the maximum load is calculated using the full plasticity moment. For the load-displacement curve following sectional collapse, first the sum of bending and rolling work of the theoretical model is derived from Eq. (3): M(O) = [ W (O + AO) - W (O)]/AO

(3)

and the sum is differentiated by the angle of rotation, and the bending moment is calculated. Then, load and displacement can be obtained from Eq. (4):

I

20

0

I

I

I

40 60 8() Deformation (mm)

I

]00

120

Fig. 6. Comparison of the experimentalresults and the theoretical model.

SPC270

1.0

SPC270

Extrudedalumintan

1.0

Spot welded

Rivet

P(O) = 4. M(O)/L,

(4)

U(O) = (L/2). tan (0).

\Reinforce material 4.4. Comparison between the experimental and theoretical values Figure 6 shows a comparison between the experimental and theoretical values for the bending of extruded aluminum beams. The bulkheads oriented in parallel to a load on the extruded aluminum beams contribute to energy absorption, and energy absorption is highest with the sectional shape of a threefold square, followed by a double square, and then a hollow square design. This result is consistent with the assumption derived from the theoretical model. In addition, the experimental values are almost the same as the theoretical values, so they can

Full ~plasticity ©

I@

~

Q

)

@@Elasticregion Elastoplastic region Beamcollapse

Deformation Fig. 5. Theoreticalloadand deformationcurve.

Fig. 7. Formationof the hybrid beam.

be examined theoretically. The theoretical calculation of energy absorption of the quartered square section will be discussed later because of its specificity.

5. Energy absorption of the hybrid beam 5.1. Formation of the hybrid beam The application of extruded aluminum beams to actual automobile-body structures involves factors of problems such as production facilities, member coupling, and costs. The traditional reinforcement method of sheet-metal box sectional members in automobile-body structures has been achieved through the use of steel reinforcement materials. In order to reduce the weight of such structural members and improve energy absorption, reinforcement through the use of extruded aluminum material rather than steel reinforcement material was attempted, as shown in Fig. 7. Table 2 shows the mechanical properties

388

E Yoshida et al. JSAE Review 18 (1997) 385-392

Table 2 Typical mechanical properties Material SPC270 SPH440 A3

25

Tensile strength (MPa}

Yield s t r e s s (MPa)

Elongation (%)

301.80 446.90 288.10

180.30 323.40 282.20

31.30 23.00 6.70

(8) Hybrid beam .......... Theory ExDeriment

20

(7) High tension steel R/F (6) Steel R/F Steel

I5

5 I

25 57% .._~

20

n:J

~ is R/Fweight

ratio

6NO1-T6 Partition t=l.5nam

100% ~¢/

15

I

I

I

20 40 60 Deformation (mm)

I

80

Fig. 9. Comparison of the theoretical model and the experiment results.

SPH440 t=l. Omm

10 Table 3 Failure load of specimens

5 0

0

2;

4b 6; 80 10; Deformation (mm)

120

Specimen Section number

l[

Fig. 8. Energy absorption of the hybrid beam.

of specimen beams. The extruded aluminum reinforcement member was fixed to a single-hat steel member with rivets for partial reinforcement.

(2)

~

(3)

~

(4)

~

Material SPC270 sPc270 R/F SPC270 SPC270 R/F SPH440 SPC270 R/F A3

CalculatedExperimental max. load max.load 668.97

409.66

1115.18

1056.11

1429.13

1166.36

2104.30

2274.41

5.2. Energy absorption of the hybrid beam A comparison between the hybrid beam and steel reinforcement beam in terms of bending energy absorption is shown in Fig. 8. When steel reinforcement material was placed on the single-hat sectional beam, energy absorption was considerably increased. The hybrid beam reinforced by extruded aluminum exhibited the highest degree of energy absorption, and, moreover, weight of the extruded aluminum reinforcement beam is 57% of that of the steel reinforcement beam. 5.3. Comparison between the experimental and theoretical values Figure 9 shows a comparison between the experimental and theoretical values for each beam. The experimental values are consistent with theoretical values for soft steel (SPC270) reinforcement beams and hybrid beams, though there is a significant difference between the experimental and theoretical values at m a x i m u m load for soft steel (SPC270) and high-tension steel (SPH440). In particular, the m a x i m u m load for the high-tension steel reinforcement beams is virtually the same as that of the soft steel reinforcement beam and significantly different

from the theoretical value, despite the use of a hightension steel plate. It is believed that this is caused by elastic buckling. 5.4. Failure load of specimens A comparison between the experimental values of failure load and full plasticity load for each beam is shown in Table 3.

6. High-speed tensile test 6.1. Test method and test piece It is necessary to clarify the strain-rate dependence of the extruded aluminum beam to understand its dynamic energy absorption characteristics. The strain-rate dependence of aluminum alloy in the high-strain rate range has been examined mainly through the use of the compression or tensile type of the Hopkinson polar method [1,2]. In this test, the influence of the treatment conditions on the strain-rate dependence of extruded

E Yoshida et al. / JSAE Review 18 (1997) 385-392

aluminum beams was examined using a high-speed tensile dynamic tester, which is relatively easy to use. The tension speed was set in eight stages, from 0.01 mm/s to 15 m/s, in consideration of vehicle collision speeds. A high-speed tensile specimen is shown in Fig. 10. 6.2. Strain-rate dependence of extruded aluminum beams Figure 11 shows the strain-rate dependence of the yield stress of extruded aluminum beams with various heat treatments. As shown in Fig. 11, strain-rate dependence of material deformation resistance rarely occurs in regions in which the strain rate is approximately 102/S o r less. However, in the regions above 102/s, yield stress tends to increase along with the strain rate. The A2 beam shows a remarkable increase in yield stress in the high-speed strain region compared to that of the other beams. It is believed that the strain rate dependence of deformation resistance in metal material, especially face-centered cubic metal, is generated as the rate-determining mechanism of motion shift moves from the thermal active process on the lowspeed side to viscous resistance control on the high-speed side [3]. In addition, aluminum alloy has mechanical properties that can be improved by heat treatment, and changes in micro organinization caused by heat treatment are believed to affect the strain-rate dependence of extruded aluminum beams.

389

7. Dynamic bending test 7.1. Test method A dynamic three-point bending test was conducted to examine the dynamic energy absorption characteristics of extruded aluminum beams. Figure 12 shows the dynamic testing equipment. A truck was operated at 15 km/h, and an extruded aluminum beam was made to collide with a push tool on which a load cell was mounted. Displacement of the beam was measured using a laser displacement guage. 7.2. Energy absorption characteristics Figure 13 shows the results of the dynamic bendingenergy absorption characteristic test of extruded aluminum beams. There are no significant differences between the dynamic and static test results, and the A2 beam with the quartered-square section exhibited greater energy

Laser deformation measure

15tgn/hr

Body I r-,

~men

r--,

t)

[__

~

Damper

I

thickness

t=2mm

" 56 "'Fig. 10. High-speed tensile specimen.

Fig. 12. Dynamic bending test equipment. 500

AI-Mg-Si alloy

25

400

A1 I[

300

A3

finE]

q3 200

,,t - ~ f [

9-,

,,.}.;:,<...--

100 A1 0

I

10-4

I

10-2

I

I

I

10o

I

102

I

104

Strain rate Fig. 11. Strain-rate dependence of extruded aluminum.

0

JO0 0 Deformat ion

100 0

100

(mm)

Fig. 13. Dynamic energy absorption of extruded aluminum.

Y. Yoshida et al. / JSAE Review 18 (1997) 385-392

390

Fig. 14. Dynamic beam collapse pattern following the test.

25

Lfl A1

A2

elongat ion

A3

Dynamic .

.

.

.

.

Static

m

i

" ~ " section

" [] " s e c t i o n

Fig. 16. The theoretical model.

o

100 0 100 0 Deformation (mm)

100

Fig. 15. Comparison of dynamic and static bending.

absorption than any of the other members, as with the static test result. Figure 14 shows the sectional collapse of an extruded aluminum beam following the dynamic bending test. The collapse pattern of the static test is almost identical to that of the dynamic test, and a crack was generated on the tensile side of the A3 threefold and quartered-square sections and the A1 quartered-square section, which is considered to cause a significant decrease in energy absorption. Thus, it is necessary to consider not only crosssectional shape but also mechanical properties to obtain larger energy absorption.

8. Comparison between the static and dynamic test results Figure 15 shows a comparison between the static and dynamic bending-energy absorption characteristics of typical extruded aluminum beams. The static and

dynamic test results are almost identical for the A1 and A3 beams, but the shape of the static and dynamic load-displacement curves are different, which is considered to be caused by the effect of strain-rate dependence.

9. Discussion of sectional-collapse energy absorption 9.1. Sectional collapse pattern and theoretical model According to the above results, the A2 quarteredsquare section beam shows the largest energy absorption. Here, bending energy absorption of the quartered-square section beam will be discussed. Figure 16 shows the condition of the hollow-square and quartered-square section beams following collapse. It is known from the observation of the collapse pattern that the rolling length of the quartered-square section beam (Lf2) is half that of the hollow-square section beam. Assuming that the rolling portion shows non-elongation deformation, a large elongation will occur on the tensile side of the beam at the time of sectional collapse as

Y. Yoshida et al. / JSAE Review 18 (1997) 3 8 5 - 3 9 2

391

15

a yd= a ys+K ~

V:15km/hr

1 rolling

0~

1110 i5 ~m d

rolling

angle of rotation

" [] " section

7J o

Web

i

!

| /

A2

~ / bending /rolling I

I

20 0

5

10

15

' 20

Flange

J

" []"

section

I

I

[

40 60 80 100 d e f o r m a t ± o n (mm)

I

120

tensile

angle of rotation

'~[]" section

Fig. 18. Comparison of the theoretical models and the experimental values.

Fig. 17. Comparison of energy absorption.

9.4. Discussion of beam strain-rate dependence shown in Fig. 16. By calculating the work made by this elongation and bending and rolling work, energy absorption at the time of collapse of the quartered-square section can be examined theoretically.

The maximum load is calculated by the following equation using the full-section plasticity moment in consideration of strain-rate dependence:

9.2. Comparison of energy absorption

Mp = a,d" 2

f

h/2

z(r/)r/dr/,

(5)

0

Figure 17 shows a comparison between the degree of energy absorption of the hollow-square and quarteredsquare sections following sectional collapse. It is clear that in the hollow-square section beam, energy absorption following sectional collapse is significantly affected by rolling work. In the case of the quartered-square section beam, tensile work increases along with an increase in the angle of bending rotation rather than bending and rolling work, and the energy absorption is extremely large. Since this tensile work will increase as long as the material elongation continues, apparent load would not fall if no crack was generated. Thus, when a material with sufficient elongation is used, extruded aluminum beams with bulkheads in the load and vertical directions show significant bending-energy absorption.

is the full-section plasticity moment, a~.d is dynamic yield stress, and z(h) is a sectional shape function. When the dynamic maximum load in Fig. 18 is calculated by the full-section plasticity moment, the dynamic yield stress is found to be approximately 200 Mpa. When this value is considered together with the case of the A2 beam in Fig. 11, the strain rate of the extruded aluminum beam is approximately 3 x 10z. In this strainrate region, the strain-rate dependence of the yield stress of the A2 beam is larger than that of the other beams. Thus, it can be presumed that the shape of the static and dynamic load-displacement curves of the A2 extruded aluminum beam in Fig. 15 changed as a result of the strain rate of the beam entering the above region.

9.3. Comparison between the test values and theoretical values

10. Conclusions

Figure 18 shows a comparison between the test values and theoretical values of the dynamic bending load-displacement curves of the A2 quartered-square section beam. For the load-displacement curve following sectional collapse, the total work load of tensile work and bending and rolling work is calculated, differentiated by the angle of rotation, and the bending moment is calculated. Next, load and displacement are calculated. As shown in Fig. 11, the theoretical values are nearly the same as the test values.

where, Mp

The following conclusions can be drawn from the results of this examination: 1. Extruded aluminum beams with bulkheads are more advantageous than steel in terms of bending energy absorption; 2. Hybrid beams of aluminum and steel are superior to the other beams in terms of weight reduction for energy absorption; 3. Heat-treatment conditions affect the strain-rate dependence of the yield stress of extruded aluminum beams;

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Y. Yoshida et al. / JSAE Review 18 (1997) 385-392

4. When materials are chosen properly, a quarteredsquare section beam features the largest bending-energy absorption; and 5. By fully analyzing tensile work in theoretical calculation, energy absorption following sectional collapse can be improved.

r

References [1] Sakino, K., Introduction (A), Vol. 58, No. 553, p. 1703 (1992-9). [3] Mukai, T., Introduction (A), Vol. 59, No. 566, p. 2350 (1993 I0). [3] Ferguson, W.G. et al., J. Appl. Phys. 38, p. 1863 (1967).

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