14
OPIMISATION OF SIDE STRURE D SIDE PADDING OF A PASSENR WITH T AID OF TATICAL EE SIMUIATION J. Hofmann AUDI NSU AUO UNION AG TRACT Side structure stif fness and side dding characteristics are optimised with the aid of a ical simulation el for the side-on vehicle-to- vehicle llision . The relation between the optim side padding characteristics and the side structure stiffness is determined on the sis of a defin cbination of test conditions . The results are formulated in terms of a design concept , a nd the effectiveness of this concept for occupant protection is investigated in crash tests . IDION cupant protection a side-on collision is influenced to a very great extent by o rameters: the stiffness of the side structure relative to the stiffness of the pacting y , a nd the defoation characteristics of the side padding. th rameters can be optimised in such a way as to give rmmm occupant loading, in which case the optim dding stiffness is a function of the stiffness of the side structure . The following contribution describes a theoretical optimisation of th rameters with the aid of a thtical simulation el. The results are foulated into terms of a design concept. A ified vehicle is constructed with a stiffened side structure and optimised padding in the פlvis imct area , and the vehicle is crash tested. The test results are compared with those for the corresnding crash test for the standard pruction vehicle. The nefit of these improvts is shown as a reduction in structural def oation d the loadings to which the occupants are subjected. TICAL INVESTITION The theoretical analysis provides information regarding the optim degrees of stiffness for the side structure and side padding, and their functional inter- relationship. The lysis is conducted using the s imulation el show n in Fig. 1 . Refer to / 1 / and /2/ for a re detailed description of the el. 256

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Page 1: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A PASSENGER CAR WITH THE AID OF MATHEMATICAL

AND EXPERIMENTAL SIMUIATION

J. Hofmann

AUDI NSU AUI'O UNION AG

ABSTRACT

Side structure stif fness and side padding characteristics are optimised with the aid of a rnathematical simulation model for the side-on vehicle-to­vehicle collision. The relation between the optimum side padding characteristics and the side structure stif fness is determined on the basis of a def ined combination of test conditions . The results are formulated in terms of a design concept , and the effectiveness of this concept for occupant protection is investigated in crash tests .

INTRODUCTION

Occupant protection in a side-on collision is inf luenced to a very great extent by two parameters: the stiffness of the side structure relative to the stiffness of the impacting body , and the deformation characteristics of the side padding. Both parameters can be optimised in such a way as to give rmmmum occupant loading, in which case the optimum padding stiffness is a function of the stiffness of the side structure.

The f ollowing contribution describes a theoretical optimisation of both parameters with the aid of a rnathernatical simulation model. The results are forrnulated into terms of a design concept. A modified vehicle is constructed with a stiffened side structure and optimised padding in the pelvis impact area , and the vehicle is crash tested. The test results are compared with those for the corresp:::>nding crash test for the standard production vehicle. The benefit of these improvements is shown as a reduction in structural def ormation and the loadings to which the occupants are subjected.

THEDRETICAL INVESTIGATION

The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side padding, and their functional inter­relationship. The analysis is conducted using the simulation model shown in Fig. 1 . Refer to / 1 / and /2/ for a rrore detailed description of the model.

256

Page 2: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

Fig. 1

m7

m5

m1 '% \- o, m4

c,

e- m3

Ma.thematical simulation model for the side on vehicle-to-vehicle collision

This model makes it i:ossible to program any desired side structure characteristics and variations in the thickness and energy absorption of the side padding. The input data for the rnodel were derived from a vehicle-to-vehicle side-on collision with standard production vehicles . The procedure of optimisation is based on a 50 kph collision of a 1 700 kg vehicle into the side structure of a stationary vehicle at an angle of 90° .

OPI'IMISATION OF SIDE STRDCrURE STIFFNESS

The safety principle of the rigid passenger cell cannot be realised in practice for the side-on collision as it can for the frontal collision. The side stif fness values of modern cars are lower in relation to the stif fness of the front structure (by a factor of 1 /2 - 1 /6 ) , so the occupant sitting on the side of the collision is struck by the side structure penetrating into the cell and accelerated towards the opi:osite side of the vehicle. The acceleration imparted to the vehicle is therefore less relevant for the occupant , and the decisive factor is the large amount of relative motion between the side structure and the vehicle. By stiffening the side structure this relative rrovement is reduced, at the same time increasing vehicle acceleration . Theoretically the optimum stif fness is the value at which the internal deformation of the cell just equals the initial distance between the occupant and the side padding. The velocity of impact between the occupant and the side padding is then at a minimum; the occupant is subjected to the lowest acceleration values /1 / . Insufficient side stiffness leads to a high internal impact velocity, while excessive side stiffness leads to a high level of vehicle acceleration .

257

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calculation was based on the Audi 4000. The front and side structure characteristics employed in the computation model are shown in Fig. 2 .

z � Q) u L 0

...

Fig. 2

600

500

400

300

200

1 00

0 ' 1 0 ,2 0 , 3

side structure ; AUDI 4000, 1 9 8 1 basel ine

0 ,4 0 ,5

defl ect ion [m)

0 ,6 0 ,7 0 , 8 0 , 9

Deformation characteristics of front and side structures

The front structure characteristics correspond to those of the Audi 4000 , extrapolated for a vehicle mass of 1 700 kg. The side stiffness was now gradually increased , maintaining the same characteristic configuration , and the effect on the relevant parameters was calculated . The result is shown in Figs . 3 and 4 . Side penetration is naturally reduced as side stiffness is increased , whilst at the sarne time the ex:tent of front deformation of the colliding vehicle increases . The clearance between the side padding and the occupant was 0 . 1 1 5 m in this calculation . In order to limit side intrusion to this arrount it is necessary to increase side stiffness by a factor of 5 in relation to the standard vehicle. The calculated acceleration values for the durrany (Fig. 4 ) do indeed reach minimum levels with stiffness increased by a factor of 5 , so this does correspond to the minimum occupant loading.

258

Page 4: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

Fig. 3

Fig. 4

259

c: 0

0 . 4 +----+-�-+---+----f----1-----i _ _j__

0 , 2

0 . 1 f r o n t d e f o r m a t i o n

s t i f f n e s s of s l de s t r u c t u r e C/c 0

i !

_J " C0" =- b a s e l l n ,..

8

Deformation of front and side structure versus side stif fness

130

llO

110

100

"' 90

80

70

60

50

40 30

20

IO OplfNl sidc stiHness

stiffness of side structure C/c0 ( C0 -: baseline )

Durrmy acceleration versus side structure stiffness ; mass of the striking vehicle: 1 700 kg; no additional interior padding

Page 5: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

It is therefore p:>ssible to establish an optimum side structure stiffness for a given set of test conditions , but this stiffness is so extremely high that a practical design is not p:>ssible. In order to exceed the FMVSS 208 protection criteria it will also be necessary to improve the side restraint system, i . e . the intemal padding , because of the limited design freedom in stiffening the side structure .

OPI'IMISING SIDE PADDING STIFFNESS

The side padding is defined by 2 parameters : the padding thickness and the ability to absorb energy. The required padding for maximum occupant protection can thus be defined as follows: the padding should be as thick as p:>ssible subject to considerations of vehicle shape , interior width and comfort requirements . In the pelvis area it would be p:>ssible to provide padding with a thickness of 0 . 05 - 0 . 1 0 m. additional padding in the shoulder impact area is however problematic because the interior width measured at this p:>int is a comfort requirement. The padding thickness will therefore ultimately be detennined by the priority given to occupant protection in side impact.

The energy absorption capability on the other hand can be optimised with the goal of. minimum occupant loading . The optimum characteristics are achieved when the whole thickness of the padding is utilized to absorb the movement of the occupant relative to the side structure , with a constant force which is as low as p:>ssible. This requirement of maximum energy absorption with a minimum force level results in a rectangular characteristic curve . By using the padding thickness the defonnation force can be controlled in such a way that it is maintained below the force threshold applicable for the given occupant protection criteria .

The influence of the padding characteristics shown in Fig. 5 on the occupant loadings was now calculated with the computation model. The maximum deformation capability of the padding and the force limit were varied .

1 ,ß i

1 1 ,6 ' "' 1

"" 1 ,4 � 0

: 1 ,l J " t 1 ,0 -... 1 g- 0,8 ...

� 0,6 "1 11

0 ' 1

force 1 1 m i t 1 0 kll maxtml11 deformation 0 , 1 2 m

force l lmt t 4 kll

1 0,2

0 ,06 m

deformation of side padding (mJ

Fig. 5 Padding characteristics with different force limitations and def ormation lengths

260

Page 6: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

The result of the ealeulation is shown in Fig. 6 with the example of the ehest aeeeleration . With inereasing padding stiffness and deformation eapability the loading on the dummy deereases towards a rrilnirnmn. At the rninirnmn level the total deformation capability of the padding is j ust utilized to absorb the motion of the oceupant relative to the side strueture . If the padding is too soft the rigidity of the eompressed padding after eomplete deformation has a negative effeet ; if the padding is too hard the available energy absorption eapability is not utilized fully. Fig. 6 shows that ehest aeeeleration remains below the eritieal value by using ideal padding eharaeteristies and a deform­ation distanee of 0 . 06 m.

Fig. 6

100

90

80

70

_ 1 1m i val e , i to FHVSS Z08 50 1

lO -1

1 / 1 0 J / / /

/

/

, ,

/

/

0 +--.,--.--.-�--.--·-+---'-�--+---.--�-.-0 Z 6 8 10 lZ forco li•it (kill

14 16 18

Influenee of foree lirnitation and deformation eapability of the side padding on the dummy ehest aeeeleration

The optirnmn limit of the padding foree is of eourse a funktion of the side strueture stiffness (Fig. 7 ) .

26 1

Page 7: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

26 24 22 20 1 8 1 6

;;:' 1 4 �

1 2 „ V ... 0

,._ i O 8 6

Fig. 7

0,0 0 ,02

curve o f optima 1 energy absorption i n the s i de padding of a baseline car ( c0 )

dunvny

above cri t i ca l va 1 ues

loadings below crit i ca l va 1 ues

�t-- r- �t-- c0 side s t i ffness of

r--1"-"- "'r-. r-r--, I"- r- 1 ,5 x Co basel ��� l · r--. / r-r- r-f-..j t---r---....._ !'!"'- L! 2,0 x c0

) r-r-r-�� n 3 ,0 x C0 increasing ·-·-r-side stiffness ,.__ 4,0 x c0

0 ,04 0,06 0,08 o , 1 n maximum deformat1on capabi l i ty o f padding [m]

Optimum padding def onnation f orce as a f unktion of padding deformation ca:p:lbility and side structure stiffness

The f igure shows the optimum padding defonnation force as a function of the padding deformation capability and the side structure stiffness. This diagrarn provides the first basis for devising and developing a new side padding. When the stiffness of the side structure is known , the required minimum def ormation length and the energy absorption of the padding necessary to remain under the protection criteria can be determined . The diagrarn illustrates two significant p'.)ints :

CD With the stiffness of the standard side structure the ideal padding requires a defo:rrnation length of 0 . 06 m in order to ensure occupant protection

If side stiffness is increased by a factor of 4 , the required protection is p'.)Ssible without additional side padding

Since a padding defo:rrnation length of 0 . 06 m with a rectangular characteristic curve , using a realistic padding design taking into account the length when fully compressed and non-ideal characteristics , would require an unacceptable padding thickness of > 0 . 1 2 m, and since a four-fold increase in the stiffness of the side structure is not practicable, it is therefore necessary to employ a combination of side structure stif f ening and interior padding in order to comply with the occupant protection and comfort requirements .

262

Page 8: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

EXPERIMENI'AL INVESTIGATION

An experimental study was carried out to put these theoretically derived results into actual structures . r-bdifications to stiffen the side structure were applied to a standard Audi 4000 , and special side padding with optimised energy absorption capability was installed in the pelvis irnpact area.

STRUCI'URAL M:>DIFICATIONS TO INCREASE STIFFNESS

The basic approach is to create a kind of safety frame by increasing the bending stiffness of the B pillar, strengthening the roof frame with a cross member and adding a floor cross mernber ; the resulting structure is able to withstand high transverse loadings and is also a feasible design. The concept was optirnised in three steps of iteration with crash testing in each case .

The main structural modifications at the third stage of optimisation were :

- Metal gauge was increased and support plates used to stiffen the sill

- Bending stif fness of the B pillar was increased by using thicker gauge meta-1 and an additional reinforcement plate

- A f loor cross member between the B pillar and tunnel was employed as a new part. The keying of the B pillar into the sill was improved to enable large forces to be transmitted into the floor cross member

- A strong roof cross member with an irnproved joint at the B pillar was installed in the roof area

- The door beam was repositioned downwards and outwards at impact height

- The front seats were strengthened in the transverse direction by design modif ications

The weight increase associated with these combined modifications is about 1 2 kg per vehicle.

DEVEIDPMENT OF PADDING IN THE PELVIS IMPACI' AREA

It is only feasible to add affective side padding ( ">,. 0 . 06 m) in the pelvis impact area. At shoulder impact height additional padding causes problems since it restricts interior width, an essential comfort requirement.

One possibility is to design the door sheet itself to be deformable in the shoulder contact area so it absorbs energy on impact with the shoulder. But this is also subject to limitations because it would reduce side stiffness and the bracing effect of the door in a frontal collision.

263

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But with the HSRI side impact durrmy used for this study , which is very soft in the ehest and shoulder area, the ehest acceleration is in any case less critical than the pelvis acceleration . The deformability of the HSRI durnny has the same :positive effect as special padding in the shoulder impact area for a front impact durrmy .

Padding was now developed for the pelvis impact area in conjunction with this study . The padding consists of a 0 . 6 mm thick aluminium sheet outer skin with a plastic coating, filled with an energy-absorbing :polyurethane hard foam. The metal gauge and the density of the foam filler were optimised so that the resulting deformation characteristics were as close as :possible to the theoretically derived ideal characteristic curve (Fig. 8 ) .

Fig. 8

30

28

l6 -� 24 +---t-----t-- ;dedl paddrng charactcristic

20 --":::.l:Ctl.Lp.a.d.dinq characteristic

18 � 1 6 -1---r--+----t „ u L

14 .!=! lt

1 0

0,01 0,02 0,03 0,04 0,05 dcflection fm)

0,06 0,07 0,08

Real padding characteristic in comparison with the calculated ideal padding characteristic

The stif f ening ef fect 9f the mcx1ifications to the side structure was estimated at a factor of 1 . 5 in relation to the standard version . The data for the ideal padding characteristics were then taken f rorn the diagram shown in Fig. 7 . Results closey approximated the ideal rectangular characteristic curve . The weight of the padding for the pelvis area is 0 . 45 kg.

Alternative solutions were also examined , for example energy-absorbing designs using sheet metal with a honeycomb construction . However , the resulting characteristics were inferior and the structure was heavier than the version with the foam filler, although the sheet metal version has the advantage of a shorter fully compressed length than with foam padding.

264

Page 10: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

TEST RFSULTS

An Audi 4000 with the combined modif ications as described was crash tested with a representative USA saloon (Chevrolet IMPAIA) with a mass of 1700 kg. The test results are summarized in Table 1 and compared against the corresp:mding results from the standard car crash test.

baseline vehicle modified vehi c l e

struck vehi c l e AUDI 4000 baseline AUDI 4000 modified

s t r i k i ng v e h i c l e Chevro let IMPALA Chevrolet IMPALA mass of striking vehicle 1700 kg 1 700 kg

mass of struck vehicle 1270 kg 1283 kg

impact speed 50 km/h 50 km/h

dummy s truck s i de HSRI HSRI dummy oppos i te s i de HYBRID I I HYBRID I I

max. outside deforma tion 0 , 461 m 0,345 m

max. intrusion 0,382 m 0,258 m

struck opposite struck oppos i te side s i de side side

�---- 4 - -----·-1 HIC 81 1 1 68 857 1 1 0

'O amax

220 ,0 g ! 2 2 , 0 g 1 70 ,0 g 3 2 , 0 g "' 1 00 , 0 g 1 1 QJ

33ms

22 ,0 g 1 0 3 ,0 g 1 31 ,5 g .s:;

"' 1 c:n SI 422 143 370 1 74 c 'O ...

amax

64 , 5 g 58,7 g 64 , 7 g 6 0 , 5 g "' "' ;?. QJ

.s:; 61 ,5 g 42 , 2 g 54,6 g 4 9 ' 1 9 >, u a 3ms � 1 :> 1 'O

S I 1364 1 31 7 601 233 "' 1 > a

max 1 3 0 , 8 g 1 7 6 , 6 g 81 ,5 g 5 3 , 3 9 Qj 1 0. a

3ms 1 1 8 , 5 g 6 4 , 7 g 79,0 g 48,3 g

Table 1 Sumnary of test results

The stif fening rrodif ications to the structure led to a reduction in the maximum amount of intrusion from 0 . 382 to 0 . 258 m. The structural deformation is illustrated in Fig. 9. It can be seen that there is an even reduction in deformation between the A and C pillars . Maximum deformation still occurs in the area around the B pillar. The reduction is greater at the sill/lower door level than in the centre of the door because the most effective stiffening measures were applied to the floor area.

As Table 1 shows , the loading values on the durmny in the pelvic area were considerably reduced , and are below the critical range. In terms of ehest loading the overall benefit of the concept is smaller because no additional padding was employed in the impact area and the acceleration level is in any case lower than in the pelvic area. The acceleration-time curves are shown in Fig. 1 0 .

265

Page 11: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

Fig. 9

/ 1 . .

1 5

2

34

4 -1- ' 1 \ \

\ 1 5 1 7 -

<5 27

34 --,- 36 -0 100 200 300 400 500 0 100 200 301

0 400 500 outs 1de dcfonnat 1on fi11111

;f�}, ' 0 100 200 300 400 500

outside deformation (nr.iJ

�__,� � . -+- - -t · _ _ .,... __ t-... . ............ / .. . -i- - '1-.. ""' / � ' / ' 1 / ' ,.,r

r' ....

43 13 14 15 16 1 7 18 19 20 21 22

F-t r--'� _..._, . ........, . ..- - --V '

"-.., 1 / I (

--+---1f---+---<-�--·-l- -+--l----+----44 23 24 25 26 27 28 29 30 31

measurCnM'nt point

500

400

300

200

100

0

500

400

300

200

1 00

! " -� � .e „ " „ " "' .., " 0

Structural deforrnations of the crashed vehicles baseline vehicle modif ied vehicle

266

Page 12: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

" 0 � „ ... "' 3

V „ .., "' „

c:

"' c. ...

<: � � ,;: :=. "' ::: � "' -B "' t

Fi g. 1 0

'6 7

"""''-../ ........... _ _ .....--====::... ___ --

�I� I ·W 1 �UD! 1000 base l i ne 1 1 1 1 1 1 HIC 3 1 1

llO J 1 1 a1:1ax l_O '1 1 l 1 � a{ 3„5) liJO 9

1 ' 1 1 1 r.uo 1 4000 ;,1od i f i ed

1 HIC 857

1 / amax 170 n

160 1, d (3''1S ) = 103 0 V

"" 1 -,---,-, -.-.- ---1--1 ·---. -,-- -r lO 40 bO 80 100

l frie

40

3U

1 2 0

2 0 40 60 80 100

time

Dumny accelerations versus time �- baseline vehicle --- modif ied vehicle

IZO 140

(111s)

...._ _

AUDI 4000 basel ine

s 1 422

a111,1 x 64 ,5 g

a!JasJ 6 1 ,5 9

A11n1 •noo n10di ficd

SI 370

amax 64 ,7 9

a(Jms) = 54 ,6 9

,---,-- 1 120 140

(ms)

1 0(1 l&O zoo

1- 1 160 180 200

Page 13: OPI'IMISATION OF SIDE STRUCTURE AND SIDE PADDING OF A ... · The theoretical analysis provides information regarding the optimum degrees of stiffness for the side structure and side

Because of its soft shoulder characteristics the HSRI dl.IlTITly used here gives rise to a new problem, narnely the hard impact of the head against the B pillar. In all earlier side-on tests with the HYBRID II durmny the head loading did not prove critical. But with the HSRI dumny the resulting head acceleration is a long way above the critical level , although the HIC value is admittedly still acceptable.

The head impact on the B pillar at a high impact velocity generates an extreme acceleration peak. However, the 3 ms value defined as the criterion for occupant protection is considerably lower because of the low energy value of the peak, and could p::issibly be easily reduced to a p::>int below the critical range with a small am:::iunt of padding on the B pillar (o. 5 - 1 . 0 cm) .

SUMMARY

The paper describes a theoretical procedure for optimising the side structure stiffness and the side padding of a vehicle. Both parameters were optimised for a given set of test conditions and the functional relationship between the stif fness of the side structure and ideal padding characteristics was detennined. The theoretical f indings were put into practice by nodifying a standard Audi 4000 for crash testing. The nodifications incurred a weight penalty of about 1 3 kg per vehicle , and were very beneficial in the pelvis impact area , gave a slight benefit in the ehest impact area, and no benefit in the head impact area. The head loading can only be significantly reduced by direct padding applied in the head impact area. The study has shown that occupant protection in a side-on collision can only be guaranteed by applying very extensive nodifications to the vehicle structure and interior. A realistic concept is only p::issible by combining nodifications to the side structure and the side padding.

Ref erences

/ 1 / Hofmann, J. Rechnerische Untersuchung zur optimalen Seitenstruktur von Personenkraftwagen Dissertation , Technische Universität Berlin , 1 980

/2/ Hofmann , J. ; Appel, H. Mathematical Simulation of Side Impact -- A Contribution to the Problem of Rigid/Defonuable Barriers VIII . ESV-Conference , Wolfsburg, October 1 980

268

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Illustrations

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 1 0

Tab. 1

269

.Mathematical simulation rrodel for the side collision

Deformation characteristics of front and side structure

Deformation of front and side structure versus side structure stiffness ; rrass of the striking vehicle: 1 700 kg

Dummy acceleration versus side structure stiffness ; rrass of the striking vehicle: 1 700 kg; no additional interior padding

Padding characteristics with different force limitations and def ormation lengths

Influence of force limitation and deforrration capability of the side padding on the dunmy ehest acceleration

Optimum padding deforrnation f orce as a function of the padding deformation capability and the side structure stif fness

Real padding characteristic in comparison with the calculated ideal padding characteristic

Structural defonnations of the crashed vehicles �- baseline vehicle --- rrodif ied vehicle

Durrmy accelerations versus time �- baseline vehicle --- modified vehicle

Summary of test results