FINAL
L REPO
ORT:
Ligh
htweigh
ht Twist Beam
m Devellopmen
nt
Prepared by
LLIGHTTWEIGHT TWIS
ST BE
EAM
FINAL REPOR
RT
w
www.autossteel.org
Lightweight Twist Beam Development – Final Report
Acknowledgements
OEM Project Team:
Doug Howe
Ford Motor Company
Ranvir Singh Jalf
Chrysler Group LLC
William Pinch
General Motors Company
Cory Taulbert
General Motors Company
Steel Project Team:
David Anderson
Steel Market Development Institute
Jon Fleck
AK Steel Corporation
Tom Wormold
ArcelorMittal USA LLC
Dean Kanelos
Nucor Corporation
Srinivasan Laxman
Severstal North America
Jon Powers
Severstal North America
Paul McKune
ThyssenKrupp Steel USA, LLC
Bart DePompolo
United States Steel Corporation
Multimatic Engineering Team:
Nik Balaram
Tudor Boiangiu
Pardeep Dhillon
Eric Gillund
Bob Howell
Scott Keefer
Paul Saadetian
Murray White
2
Executive Summary
The objective of this project was to develop a lightweight steel proof-of-concept twist
beam design that achieves a 15 to 25% mass reduction with equivalent structural and
elasto-kinematic performance relative to the baseline design at a ≤ 10% cost premium. A
current production original equipment manufacturer (OEM) twist beam assembly was
used to establish the baseline for package, performance, mass and cost.
Computer-aided engineering (CAE) structural optimization methods were used to
determine the initial designs. Two designs were selected for further development and
one design was subsequently selected as the best-performing and lightest alternative
that met all typical performance criteria.
An iterative optimization strategy was used to minimize the mass of each design, while
meeting the specified strength, durability and elasto-kinematic requirements. The
manufacturing cost was estimated for the preferred design relative to the baseline
design for three production volumes.
The results of the study indicate that the preferred U-Beam Design based on 22MnB5
tubular construction with DP780 and SPFH540 sheet achieves a 30.0% mass reduction
relative to the baseline assembly, at a 12 to 15% premium in manufacturing cost. The
S-Beam Design based on 22MnB5 sheet, DP780 tube and HSLA550 materials was
predicted to have a 14.9% mass reduction relative to the baseline assembly.
All designs were deemed manufacturable based on expert manufacturing assessment
and relevant production application examples.
Lightweight Twist Beam Development – Final Report
Table of Contents
Executive Summary ..................................................................................................................... 3 Purpose .......................................................................................................................................... 9 Conclusions ................................................................................................................................... 9 Recommendations........................................................................................................................ 9 Baseline Design .......................................................................................................................... 11 Design Targets ............................................................................................................................ 12 Structural Performance .......................................................................................................... 13 Mass .......................................................................................................................................... 14 Package..................................................................................................................................... 16 Corrosion ................................................................................................................................. 16 Cost ........................................................................................................................................... 16 Development Process ................................................................................................................ 18 1. Concept Development ....................................................................................................... 18 2. Design Development .......................................................................................................... 19 3. Manufacturing and Corrosion .......................................................................................... 20 4. Cost Assessment ................................................................................................................. 20 Design – Package Effects ........................................................................................................... 21 Design Proposals ........................................................................................................................ 23 U-Beam Design ....................................................................................................................... 23 S-Beam Design ........................................................................................................................ 24 Performance ................................................................................................................................ 26 Materials................................................................................................................................... 26 Material Modeling Considerations ...................................................................................... 28 Material Selection ................................................................................................................... 29 Durability ................................................................................................................................. 32 Extreme Loads ......................................................................................................................... 34 Performance Summary .......................................................................................................... 37 Mass ............................................................................................................................................. 38 Elasto-Kinematic Performance ................................................................................................. 40 Manufacturing ............................................................................................................................ 44 U-Beam Design ....................................................................................................................... 44 S-Beam Design ........................................................................................................................ 44 Corrosion ..................................................................................................................................... 46 4
Lightweight Twist Beam Development – Final Report
Cost Estimates ............................................................................................................................ 47 Assumptions ............................................................................................................................ 47 Material Costs ...................................................................................................................... 47 Design ................................................................................................................................... 47 Program ................................................................................................................................ 48 Variable Costs ...................................................................................................................... 48 Fixed Costs ........................................................................................................................... 48 Component Costs ................................................................................................................ 48 Cost Comparison .................................................................................................................... 48 References ................................................................................................................................... 51 Appendix 1: Kinematics and Compliance Plots .................................................................... 52 5
Lightweight Twist Beam Development – Final Report
List of Figures
Figure 1: Baseline OEM Twist Beam Assembly....................................................................... 9 Figure 2: OEM Baseline Design................................................................................................ 11 Figure 3: Twist Beam Design Targets...................................................................................... 13 Figure 4: Structural Performance Targets ............................................................................... 14 Figure 5: Baseline twist beam Assembly Mass Summary .................................................... 15 Figure 6: Package Volume and Design Environment ........................................................... 16 Figure 7: Development Process Diagram ............................................................................... 18 Figure 8: Volume Topology Optimization ............................................................................. 19 Figure 9: Volume Topology Optimization Using an Extruded Constraint ....................... 19 Figure 10: Candidate Design Concepts ................................................................................... 20 Figure 11: Twist Beam Example with Outboard-Driven Hub Fasteners ........................... 21 Figure 12: Damper Attachment for OEM Baseline and U-Beam Designs ......................... 22 Figure 13: U-Beam Design Concept ........................................................................................ 24 Figure 14: S-Beam Design Concept ......................................................................................... 25 Figure 15: Twist Beam Finite Element Models ...................................................................... 26 Figure 16: Engineering Stress-Strain Curve Comparison .................................................... 27 Figure 17: Material Fatigue Property Reductions – HAZ .................................................... 28 Figure 18: HAZ – Durability Implementation ....................................................................... 29 Figure 19: HAZ – Strength Implementation .......................................................................... 29 Figure 20: U-Beam Design Material Selection and Gage...................................................... 30 Figure 21: S-Beam Design Material Selection and Gage....................................................... 31 Figure 22: OEM Baseline Design Material Selection and Gage ........................................... 31 Figure 23: Predicted Durability Life Comparison ................................................................. 32 Figure 24: Predicted OEM Baseline Design Durability Life ................................................ 33 Figure 25: Predicted U-Beam Design Durability Life ........................................................... 33 Figure 26: Predicted S-Beam Design Durability Life ............................................................ 34 Figure 27: Predicted Extreme Load Permanent Set Comparison ........................................ 35 Figure 28: Predicted OEM Baseline Extreme Load Plastic Strain ....................................... 35 Figure 29: Predicted U-Beam Design Extreme Load Plastic Strain .................................... 36 Figure 30: Predicted S-Beam Design Extreme Load Plastic Strain ..................................... 36 Figure 31: Twist Beam Assembly Mass Comparison............................................................ 38 Figure 32: Twist Beam Structure Mass Comparison ............................................................. 39 Figure 33: K&C Results: Bounce – Bump Steer...................................................................... 41 6
Lightweight Twist Beam Development – Final Report
Figure 34: K&C Results: Bounce – Bump Camber ................................................................ 41 Figure 35: K&C Results: Roll - Roll Steer ................................................................................ 42 Figure 36: K&C Results: Longitudinal Braking – Toe Stiffness ........................................... 42 Figure 37: K&C Results: Lat Parallel – Toe Stiffness ............................................................. 43 Figure 38: K&C Results: Align Opposed – Toe Stiffness ...................................................... 43 Figure 39: Stamping Formability for 22MnB5 Main Beam Structure ................................. 45 Figure 40: Relative Cost Comparison ...................................................................................... 49 Figure 41: Relative Cost Comparison Plot.............................................................................. 50 7
Lightweight Twist Beam Development – Final Report
List of Tables
Table 1: Detail OEM Baseline Twist Beam Assembly Mass Summary .............................. 15 Table 2: Steel Sheet Material Properties .................................................................................. 27 Table 3: Performance Summary ............................................................................................... 37 Table 4: Detail Mass Summary................................................................................................. 39 Table 5: Assumed Material Costs ............................................................................................ 47 8
Lightweight Twist Beam Development – Final Report
Purpose
The objective of this project was to develop a lightweight steel proof-of-concept twist
beam design that achieves a 15 to 25% mass reduction with equivalent structural and
elasto-kinematic performance relative to the baseline design at a ≤ 10% cost premium. A
current production OEM twist beam assembly was used to establish the baseline for
package, performance, mass and cost.
Figure 1: Baseline OEM twist beam assembly
Conclusions
The results of the study support the following conclusions:

The U-Beam design is predicted to be 30.0% lighter than the OEM baseline design at
a 12 to 15% cost premium at production volumes of 30,000 to 250,000 vehicles per
year, respectively. The design is deemed production feasible based on expert
manufacturing assessments.

The S-Beam design is predicted to have the best strength performance at a 14.9%
mass reduction relative to the OEM baseline design. The design is deemed
production feasible based on expert manufacturing assessments.
Recommendations
The twist beam designs are driven by durability and strength requirements at the
component level and elasto-kinematic requirements at the vehicle level.
9
Lightweight Twist Beam Development – Final Report
Durability (Max Twist load case) and strength (Max Vertical load case) are the primary
design drivers for both designs.
CAE fatigue and strength modeling guidelines for the weld Heat Affected Zone (HAZ)
have been developed based on Steel Market Development Institute (SMDI)
recommendations. Currently, OEM best practices specify reduced material properties
for all MIG welds and adjacent material in the weld HAZ to account for the effect of
welding. Typically, the same reduced material properties are specified, regardless of the
grade of steel. Some studies have shown that reduction in fatigue performance of
advanced high-strength steel (AHSS) can be minimized by optimizing joint geometries
[1]. Further study and development of robust high-volume welding practices and other
advances in the area of sheet steel joining, especially with AHSS and ultra high-strength
steel (UHSS) are recommended. Welding practices that result in improved HAZ
properties could enable additional mass reduction by improving durability
performance and thus more fully exploiting the benefits of high-strength materials in
chassis components.
Additionally, with the aggressive gage reductions enabled by the use of AHSS and
UHSS, typical corrosion protection strategies may not be sufficient for these materials in
chassis applications. Additional studies of the corrosion performance of these materials
in welded assemblies, including pre- and post-assembly coatings, are recommended
with the goal of developing definitive corrosion treatment strategies for chassis
applications.
Finally, although variable gage materials with gage differences up to 1.0 mm were
considered and analytically assessed during the project, the final designs consist of
constant-thickness components which meet the performance requirements. Additional
studies of variable-gage components with smaller gage differences could be undertaken
to investigate further incremental mass savings.
10
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Baselin
ne Design
n
The baseeline design
n, as chosen
n by the SM
MDI team, iss depicted iin Figure 2.. The main
structure is compriised of a tub
bular transv
verse beam
m and tubular trailing aarms extend
ding
from thee bushings to the damper mountss. Upper an
nd lower reeinforcemen
nts are addeed at
the jointts between the transveerse and lon
ngitudinal m
members. T
The transveerse beam
features an inverted
d “V” in cro
oss section,, rotated fro
om verticall orientation
n. Each
damper mount is in
n single-sheear via a threaded sleeeve welded
d to the long
gitudinal tu
ube.
The spin
ndle mountts are cantillevered abo
ove the trailling arm tu
ubes and incclude
reinforceement platees, machineed for rear wheel
w
staticc alignmen
nt.
Figure 2: OE
EM baselin
ne design
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Lightweight Twist Beam Development – Final Report
Design Targets
The overall project design targets are illustrated in the schematic shown in Figure 3.
The objective was to develop a minimum mass design within the packaging constraints
that met the structural and elasto-kinematic performance targets. Corrosion
requirements are addressed by appropriate selection of material coatings, which
typically do not add significant mass, but can increase cost. Mass and cost are the
primary outputs of the study.
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Figu
ure 3: Twisst beam dessign targetss
Structurral Perform
mance
The speccific structu
ural perform
mance requ
uirements arre summarrized in the schematic
shown in
n Figure 4. The fundam
mental design requireements are sstrength, du
urability an
nd
elasto-kiinematic peerformancee (as demon
nstrated by subsystem
m-level Kinematic and
Complia
ance or K&C, perform
mance).
The strength requirrements incclude (5) qu
uasi-static eextreme loaad cases in w
which the tw
wist
beam may
m not exhiibit more th
han the allo
owable perm
manent set. The durab
bility
requirem
ments inclu
ude a total of
o (16) load cases that m
must be sattisfied.
Only thee load casess that drivee the design
ns will be diiscussed in this reportt. These include
the top (3)
( extreme load cases and the top
p (3) durab
bility load cases.
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Figure
e 4: Structu
ural perform
mance targeets
Mass
A high-llevel breakd
down of the OEM baseline twist beam assem
mbly mass is shown in
n
Figure 5.
5 The comp
plete assembly mass off 24.9 kg, in
ncluding bu
ushings, is u
used as the
overall basis
b
for comparison of
o the desig
gns with resspect to mass. A detailled compon
nent
mass breeakdown fo
or the twistt beam asseembly is pro
ovided in T
Table 1.
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Fig
gure 5: Base
eline twist beam assem
mbly masss summary
Table 1: Detail
D
OEM
M baseline twist beam
m assembly
y mass sum
mmary
O
OEM
Baseline Twiist Beam Asy
Mass per Asy
C
Component
kg
% of tottal
S
Seat-Spring
2.16
8.7%
P
Pipe-T/Arm
Bush
0.70
2.8%
B
Brkt-Spindle,
Rr Ax
xle
2.15
8.6%
B
Brkt-Damper,
Rr Ax
xle
0.41
1.7%
P
Plate-Spindle,
Rr Axle
A
0.98
3.9%
T
Trailing
Arm - Rr Axle
A
7.39
%
29.7%
B
Beam-Rear
Axle
7.26
29.2%
%
B
Brkt-Parking
Brakee Rr
0.16
0.7%
W
Welding-Rr
Axle
0.33
1.3%
R
Reinf-Beam
Upr
0.48
1.9%
R
Reinf-Beam
Lwr
0.44
1.8%
P
Patch-T/Arm
0.07
0.3%
P
Pipe-Insulator
0.16
0.7%
22.71
%
91.2%
2.19
8.8%
24.90
%
100.0%
T
Twist
Beam Asy lesss bushings
B
Bushings
C
Complete
Twist Bea
am Asy
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Package
e
The overall design environmeent and resu
ulting availlable packaage space is illustrated in
Figure 6.
6 The twist beam pack
kage space is
i defined b
by the fuel ttank, the sp
pare tire weell,
the rear floor pan, the
t tire env
velope, the required
r
cleearances to
o these com
mponents an
nd
the requ
uired twist beam
b
suspeension traveel.
Figure
F
6: Pa
ackage volu
ume and deesign envirronment
on
Corrosio
The projject target for
f corrosio
on performa
ance was baased on typ
pical OEM ccorrosion
requirem
ments. Thesse requirem
ments vary, but were asssumed to require a m
minimum
10-year life in a hig
ghly-corrosiive environ
nment.
Cost
Recogniizing the ag
ggressive weight
w
reducction targetts enabled b
by the use o
of AHSS an
nd
UHSS, th
he project cost
c
target was
w a ≤ 10%
% increase rrelative to tthe OEM baaseline desiign.
To assesss cost, the manufactur
m
ring cost was
w estimateed for the seelected U-B
Beam propo
osal
and com
mpared to th
he baseline design cost.
The projject costing
g assumptio
ons were:

Manufacturing cost for thee twist beam
m assembly
y including the structu
ure and
bush
hings;
16
Lightweight Twist Beam Development – Final Report

Production volumes of 30,000, 100,000 and 250,000 vehicles per year; and

Program life of six years.
17
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Develo
opment Process
An itera
ative optimiization strategy was used
u
to miniimize the m
mass of each
h design, w
while
meeting
g the specifiied structurral requirem
ments. A sch
hematic of the overalll developmeent
strategy
y is shown in
n Figure 7. The key eleements of tthe strategy
y are discusssed in the
followin
ng sections.
Figure
e 7: Develo
opment pro
ocess diagraam
ept Develo
opment
1. Conce
Initial deesign conceepts were developed
d
based
b
on sizze and shap
pe optimizaation of the
availablee design sp
pace shown in Figure 6.
6 Stiffness aand strengtth-based to
opology
optimiza
ation metho
ods were used to iden
ntify promissing concep
pts using th
he optistrucct
solver [2
2]. Without manufactu
uring constrraints, the o
optimizatio
on output w
was a truss
structure with a disstinct “U” shape
s
in pla
ain view. T
This result w
was interpreeted into a
s
in Fiigure 8, nam
med for its plan-view shape.
concept “U-Beam” design as shown
Various draw constraints werre also used
d to identify
y potential d
design conccepts. The
extruded
d constrain
nt that resullted in a seccond initial concept is shown in F
Figure 9. Th
his
concept was termed
d the “S-Beeam” since the
t cross-seection deveeloped into an “S” shape
as additional optim
mization wa
as performeed.
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Figure
e 8: Volume
e topology optimizatiion
V
top
pology optiimization u
using an exxtruded con
nstraint
Figure 9: Volume
gn Develop
pment
2. Desig
A total of
o two (2) ca
andidate deesign conceepts were id
dentified in
n the concep
pt developm
ment
stage forr further deevelopmentt. As indica
ated in Figu
ure 10, thesee were the U
U-Beam an
nd
19
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the S-Beeam concepts. Various optimization strategiees were utilized to meeet each loaad
requirem
ment, whilee minimizin
ng the overa
all mass. Sh
hape optim
mization wass used to
develop
p the compo
onent geom
metry. Numeerous addittional desig
gn iteration
ns were
conductted to fine tu
une the ma
aterial selection, thickn
ness and loccal geometrry to meet
strength
h, durability
y and elasto
o-kinematicc requiremeents. The ellasto-kinem
matic
requirem
ments were cascaded into
i
component twist b
beam stiffn
ness requireements
allowing
g for rapid early assessments in Abaqus
A
befo
ore creation
n of flex bo
odies and fu
ull
K&C asssessments via
v Adams.
The U-B
Beam design
n was later selected ass the preferrred alternattive due to its superio
or
structural, mass an
nd elasto-kin
nematic perrformance. The S-Beam
m design d
details are also
provided in this pa
aper.
Figure 10: Cand
didate desi gn conceptts
ufacturing and
a Corros
sion
3. Manu
The man
nufacturing
g feasibility
y of each design was asssessed at v
various stag
ges of the
develop
pment proceess. Additio
onal design
n developm
ment was con
nducted to meet
manufaccturing feassibility requ
uirements. Corrosion rrequiremen
nts includin
ng selection
n of
coatingss were conssidered as part
p of the manufactur
m
ring feasibillity assessm
ments.
4. Cost Assessme
A
nt
The fina
al step of the developm
ment processs was to esstimate the manufactu
uring cost fo
or
the seleccted U-Beam
m design an
nd the OEM
M baseline d
design. Pro
oduction costing
methodo
ologies werre applied to
t estimate the manufaacturing co
osts for the U
U-Beam
design, and
a the cossts were com
mpared to the
t OEM baaseline costt.
20
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Design
n – Packa
age Effectts
Two dessign changees with systtem-level efffects were made to th
he candidatte twist beaams
relative to the OEM
M baseline design:
d
nting strateegy will req
quire attach
hment of thee hub from the outboaard
First, thee hub moun
rather th
han inboard
d direction (similar to the Honda Fit or Ford
d Fiesta dessigns as sho
own
in Figure 11).
am example
e with outb
board-driveen hub fastteners
Figure 11: Twist bea
amper loweer attachmeent was mov
ved 40 mm
m outboard o
on both dessigns
Second, the rear da
to facilittate a much
h improved load path to
t the beam
m structure.. The movee maintaineed
the tire clearance
c
en
nvelope an
nd resulted in
i a dampeer motion raatio changee from 1.12 to
1.11. Thiis is illustra
ated in Figu
ure 12.
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Figure 12: Damper atttachment for
f OEM baaseline and
d U-Beam d
designs
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Lightweight Twist Beam Development – Final Report
Design Proposals
The final U-Beam and S-Beam design proposals are shown in Figure 13 and Figure 14,
respectively.
U-Beam Design
The U-Beam design utilizes UHSS and AHSS to enable aggressive gage and mass
reductions.
The U-Beam design features hot-formed tubular transverse and swept longitudinal
members, all from 22MnB5 material with a constant 2.5 mm thickness. The transverse
member has a closed inverted “U” cross section to provide the desired shear center
location for roll steer performance. The roll steer can be tuned if required with this
design by adding a rear-view sweep to the beam. The final design presented in this
report has been tuned to achieve the OEM baseline roll steer with an unswept design
for simplicity.
The transverse member also features a fixed material gage but with a 20% increase in
OD near vehicle centerline. This adds stiffness via section enlargement with minimum
added mass. This increase in section is achieved either through the ACCRA®
hot-forming process or by a purchased variable-diameter tube.
The normally circular cross section of the longitudinal members is formed to a
rectangular cross section at the hub mounts, facilitating integration of the hub
attachment features without additional parts.
A unique feature of this design is the addition of structural “bulkheads” to locally
stabilize the beam assembly in the critical transition area from the lateral beam to the
longitudinal arms.
Trailing arms containing the bushings are simple inverted “U” profile stampings from
DP780 material.
All components are MIG-welded to form the assembly.
23
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Fig
gure 13: U-B
Beam desig
gn concept
S-Beam Design
The S-Beeam design
n features a hot-stampeed main beeam and an associated hot-stamped
lower reeinforcemen
nt, all from 22MnB5 material.
m
Th
he stamped beam desig
gn providees the
“S” crosss section deerived from
m optimizattion. The ov
verall shapee of the beaam is also a “U”
in the pllan view, reeflecting thee optimizattion results consistentlly observed
d during
develop
pment.
The S-Beeam also in
ncludes bulk
kheads to stabilize thee lateral-to-llongitudinaal beam
transitio
on area.
aining the bushings
b
arre closed-seection tubullar compon
nents from
Trailing arms conta
®
DP780 material,
m
alsso designed
d to be com
mpatible witth the ACCR
RA hot-forrming proccess.
All comp
ponents aree MIG-weld
ded to form
m the assem
mbly.
24
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Fig
gure 14: S-B
Beam desig
gn concept
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Performance
Finite element (FE) analysis methods
m
were used to p
predict the structural p
performancce of
each dessign. The FE
E model forr the two neew design cconcepts iss shown in
Figure 15.
1 As menttioned prev
viously, an iterative
i
op
ptimization strategy w
was used to
minimiz
ze the mass of each design while meeting th
he specified structural requiremen
nts.
Optistru
uct [2], Abaqus / Stand
dard [3] and
d nCode DeesignLife [44] softwaree products w
were
used to optimize an
nd assess th
he structura
al performaance of desiigns. The fiinal design
ural perform
mance are d
discussed in
n the follow
wing section
ns.
materiall selections and structu
Figure 15
5: Twist Beam Finite E
Element M
Models
Materia
als
For the U-Beam
U
and S-Beam designs,
d
thee material g
grade selecttion was priimarily
influencced by the durability
d
and
a extremee load casess. Addition
nal material selection
criteria included
i
fo
ormability, weldability
w
y, availabili ty and costt. A table su
ummarizing
g the
Auto/Stteel Partnerrship team recommend
ded sheet m
materials is provided iin Table 2.
ves for the sheet
s
and fo
orged mateerials utilizeed in this sttudy
Engineeering stress--strain curv
are comp
pared in Figure 16. Th
he yield and
d ultimate ttensile stren
ngths are in
ndicated forr
each ma
aterial. Fatig
gue propertties were ob
btained fro
om [5].
26
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Table
e 2: Steel sh
heet materiial propertiies
Fig
gure 16: En
ngineering stress-straiin curve comparison
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Materia
al Modeling
g Considera
ations
Materiall processing
g considera
ations were taken into account in
n the finite eelement
modelin
ng. Specifica
ally, the effects of weld
ding-inducced material property rreduction in
the Heatt Affected Zone
Z
(HAZ
Z) were inclu
uded in thee durability
y and streng
gth load casses
per the SMDI
S
team
m’s recommeendations as
a follows:

For the dura
ability load cases, mateerial fatiguee property reductionss of 20% in tthe
weld
w
HAZ were
w
applieed for all hig
gh-strength
h, advanced
d high-stren
ngth and
ultra-high
u
sttrength steeel grades [6
6]. To achiev
ve the redu
uced properrties, the K’’
parameter
p
of
o the cyclic stress-straiin amplitud
de curve waas scaled by
y 0.8. The sstrain
liife curve wa
as not mod
dified. This is
i shown in
n Figure 17..

For strength
h load casess, material strength
s
red
ductions of 20% in thee HAZ zonee
were
w
applied
d for UHSS
S grades with ultimatee strengths g
greater than
n 800MPa. [6].
Shell welds were assign
ned properrties corresp
ponding to the lower-sstrength
material
m
of the
t two join
ning compo
onents. The shell weld thickness w
was
determined
d
by a weigh
hted averag
ge between the two com
mponent th
hicknesses.
An exam
mple of the durability modeling
m
of
o the HAZ can be seen
n in Figure 18. An
examplee of the streength modeeling of the HAZ is illu
ustrated in Figure 19.
Fig
gure 17: Ma
aterial fatig
gue propertty reductio
ons – HAZ
28
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figure 18
8: HAZ – Durability
D
iimplementtation
Figure 19:
1 HAZ – Strength
S
im
mplementaation
al Selection
n
Materia
The matterials weree selected fo
or each desiign based o
on meeting all of the sttrength and
d
durability requirem
ments, form
mability considerationss and SMDII’s recommendations. The
29
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
resulting
g material selections
s
and
a gage are illustrated
d in Figure 20 and Fig
gure 21. Thee
materialls for the OEM baselin
ne design arre specified
d in Figure 222. The OEM
M baseline
materiall grades aree labeled geenerically to
o maintain OEM mateerial specificcation
confiden
ntiality.
Based on
n existing corrosion
c
gu
uidelines, a nickel-plaating coating
g process iss
recomm
mended for the
t U-Beam
m in consideeration of itts componeents at <2.0m
mm gage (ssee
Figure 20).
2 An apprropriate E-ccoat finish is also reco
ommended for both th
he U-Beam aand
the S-Beeam (see Fig
gure 21) dessigns.
Fig
gure 20: U--Beam design materiaal selection and gage
30
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figure 21: S-Beam desig
gn materiaal selection and gage
Figurre 22: OEM baseline design
d
mateerial selectiion and gag
ge
31
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Durabiliity
The pred
dicted dura
ability life performanc
p
e is comparred in Figu
ure 23 for all designs. T
The
minimum life values are show
wn for the worst
w
case ((3) load casees. For each
h design, th
he
ase loading results from
m the Max Twist even
nt. The asso
ociated life ccontour plo
ots
worst ca
are show
wn in Figurre 24 for thee OEM baseeline design
n, Figure 255 for the U-B
Beam desig
gn,
and Figu
ure 26 for th
he S-Beam design. In all
a cases, th
he limiting llocation of each design
n is
the transsition area near
n
the latteral-to-lon
ngitudinal trransition off the main b
beam structture.
In all casses the area
a of low lifee occurs in the
t parent m
material ratther than in
n a weld.
Note tha
at in Figuree 24, the OE
EM baselinee predicted design life is 0.18, sign
nificantly leess
than thee 1.0 life min
nimum targ
get. When this
t
result w
was first ob
bserved, inv
vestigationss
into poteential causees began. Multimatic
M
performed
p
coupon tessting from p
production
OEM ba
aseline twist beam parts to measu
ure as-form
med materiall propertiess and comp
pare
them to the publish
hed values for the tran
nsverse bea m materiall. While theese materiall
propertiies roughly doubled th
he predicted
d life, the reesults weree still well b
below the 1.0
life targeet. After furrther consu
ultation with
h the OEM
M, it was dettermined th
hat internal
OEM prredictions closely boun
nded the 0.1
18 life resullt, and conssequently, tthe OEM
agreed to
t use 0.18 lives
l
as the target for the
t max twiist load casse.
Figure 23: Predicted
d durability
y life compaarison
32
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figu
ure 24: Pred
dicted OEM
M baseline design durrability lifee
Figure
F
25: Predicted
P
U-Beam
U
dessign durabiility life
33
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figure
F
26: Predicted S--Beam Desiign Durabiility Life
e Loads
Extreme
The pred
dicted perm
manent set performanc
p
ce is compaared in Figu
ure 27 for alll designs. T
The
permaneent set valu
ues are show
wn for the worst
w
case (3) load casses. In each
h case, the w
worst
case load
ding is from
m the maxim
mum verticcal event co
ondition. Th
he U-Beam and S-Beam
m
both meeet the 1.0 mm
m maximu
um set requ
uirement ass measured
d at the wheeel center, w
with
the S-Beeam exhibitiing the bestt overall strrength perfformance.
The asso
ociated plasstic strain contours forr the maxim
mum verticaal event aree shown in
Figure 28
2 for the OEM baselin
ne design, Figure
F
29 fo
or the U-Beaam design aand Figure 30
for the S-Beam
S
desiign. The pla
astic strain contours aare contoureed to 1.00%
% for
visualiza
ation.
34
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figure
e 27: Prediccted extrem
me load perrmanent sett comparison
Figure
e 28: Predicted OEM baseline
b
exttreme load
d plastic strain
35
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figure 29: Predictted U-Beam
m design exxtreme load
d plastic strrain
Figure 30: Predictted S-Beam
m design exxtreme load
d plastic strrain
36
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Perform
mance Sum
mmary
The rela
ative structu
ural perform
mance of ea
ach design iis summariized in Table 3, wheree the
relative performance is defineed as the acttual perform
mance norm
malized by
y the indicatted
target va
alue. To meeet the requ
uired level of
o durabilitty performaance, the relative valuee
must be ≥ 1.0, whilee the relativ
ve value forr permanen
nt set due to
o extreme lloads must be
≤ 1.0. Th
he primary and
a second
dary design
n drivers forr each desig
gn are iden
ntified in thee
table. Alll of the dessigns are prrimarily fattigue limiteed by the m
max twist loaad case. A
seconda
ary design driver
d
for th
he OEM basseline and U
U-Beam deesigns is thee max verticcal
event strrength load
d case. Finally, both th
he U-Beam aand the S-B
Beam design
ns are furth
her
limited by
b the max cornering event dura
ability case.
Table 3: Perfformance ssummary
37
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Mass
The fina
al twist beam
m assembly
y mass resu
ults, includiing bushing
gs, are compared in Fiigure
31. The results
r
indiicate that th
he mass of the
t U-Beam
m design is 330.0% less tthan the OE
EM
baselinee assembly mass,
m
whilee the S-Beam
m design iss 14.5% ligh
hter than th
he OEM
baselinee.
The fina
al twist beam
m welded structure
s
mass
m
results are compaared in Figu
ure 32. The
results in
ndicate tha
at the mass of the U-Beeam structu
ure is 32.8%
% less than tthe OEM
baselinee mass, while the S-Bea
am structurre is 15.9% llighter than
n the OEM baseline.
The deta
ail component masses are summa
arized in Taable 4.
Figure
F
31: Twist
T
beam
m assembly
y mass com
mparison
38
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figure 32: Twist
T
beam
m structuree mass comp
parison
Table
T
4: Dettail mass su
ummary
39
Lightweight Twist Beam Development – Final Report
Elasto-Kinematic Performance
A key aspect of the development project was to closely match the OEM baseline elastokinematic behavior. Unlike many other suspension components, twist beam axles are
designed to exhibit significantly compliant behavior. This behavior is typically
measured by industry-standard kinematics and Compliance (K&C) testing.
An Adams [7] model of the OEM baseline twist beam was created so that the K&C
response of the proposed designs could be compared to the baseline design. First, flex
bodies for the baseline twist beam were generated based on mesh created for structural
analysis. Next, the Adams model was refined with the following information provided
by the OEM:

Spring rate / Spring preload at Design position;

Jounce / rebound bumper stiffness;

Bushing stiffnesses;

Tire stiffness; and

Tire unloaded radius.
Once the initial Adams model was complete, the response of Adams K&C simulations
were correlated with physical K&C test results. To further improve the correlation,
bushings from a current production OEM baseline twist beam were obtained, the
stiffnesses were measured as-installed in the beam, and the measured stifnesses were
incorporated into the Adams model.
A subset of the K&C plots for the selected U-Beam design and OEM baseline twist beam
are presented here as Figure 33-Figure 38. The U-Beam closely matched the OEM
baseline K&C characteristics, thereby providing confidence that the on-road vehicle ride
and handling behavior of the two beam designs would be very similar.
A full set of K&C plots is included in this report as Appendix 1: Kinematics and
Compliance Plots.
40
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figure 33
3: K&C ressults: bouncce – bump steer
Figure 34:: K&C resu
ults: bouncee – bump camber
41
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figurre 35: K&C results: rolll – roll steer
Figurre 36: K&C results: lon
ngitudinal braking – toe stiffness
42
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figure 37: K&C resullts: lat paraallel – toe sttiffness
Fiigure 38: K&C resultss: align opp
posed – toe stiffness
43
Lightweight Twist Beam Development – Final Report
Manufacturing
Each design was assessed to ensure manufacturing feasibility. Assessment included a
combination of expert engineering and manufacturing experience, including input from
partner companies on hot forming processes. Additional design development was
conducted in some cases to improve manufacturing feasibility.
U-Beam Design
The main manufacturing considerations for the U-Beam design were the feasibility of
forming the 22MnB5 transverse tubular member and the swept longitudinal members
that join the transverse member and provide the spindle mounting structure.
The 20% increased length of line near the centerline of the transverse tube is achieved
either through the Multimatic / Linde+Wiemann ACCRA® hot-forming process or by a
purchased variable-diameter tube. Both options were assessed by internal and external
manufacturing experts to confirm feasibility.
The “U” cross-section of the transverse beam is also achieved via the ACCRA® hotforming process. The swept longitudinal members include a 90° bend which was
judged feasible by manufacturing experts. The bulkheads are sub-assembled to the
transverse member during the beam assembly. A formed shoulder on the bulkheads is
recommended to provide location and self-centering on the transverse tube.
The remaining stampings are judged feasible based on the observation that the
individual twist beam components are simple stamped and / or blanked components
for which the trim lines can readily be developed and without major draw or other
geometric limitations.
The overall beam assembly was evaluated for weld access and judged feasible. Weld
length for this design is 5980mm.
S-Beam Design
The main manufacturing considerations for the S-Beam design were the feasibility of
the 22MnB5 main beam stamping and feasibility of MIG welding the assembly in
production.
To evaluate the stamping feasibility of the main beam, a hot-stamping supplier partner
was consulted to evaluate the design. Expert review revealed no concerns. In addition,
a one-step forming simulation was conducted to assess formability. Based on the results
of the forming simulation shown in Figure 39, the main stamping is judged feasible.
44
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
The rem
maining stam
mpings are judged feasible based
d on the obsservation th
hat the
individu
ual twist beeam compon
nents are siimple stam
mped and / or blanked
d componen
nts
for whicch the trim lines can reeadily be deeveloped an
nd withoutt major draw
w or other
geometrric limitatio
ons.
a
was evaluateed for weld
d access and
d judged feaasible. Weld
d
The overall beam assembly
or this design is 8510m
mm.
length fo
9: Stampin
ng formabillity for 22M
MnB5 main
n beam stru
ucture
Figure 39
45
Lightweight Twist Beam Development – Final Report
Corrosion
To meet OEM corrosion requirements, corrosion protection is generally applied to
components based on material gage. The sheet steel material gage limit is OEM specific,
and is assumed to be 2.0 mm for the purpose of this study.
Typical requirements on a component basis are as follows:

Gage > 2.0 mm: E-coat finish required.

Gage < 2.0 mm: Hot dipped galvanized coating + E-coat finish required.
The specific type of galvanized coating is also OEM specific. Examples of coating
specifications include Hot Dip G60 / G60 (GI) or Hot Dip Galvanneal A-40 (GA).
For a complex welded assembly such as a twist beam, if components are included
below 2.0mm gage, the manufacturing recommendation from this study is a zinc-nickel
plating process after assembly followed by E-coat. This is a current production process
and eliminates manufacturing concerns by ensuring common welding processes for all
components.
The project also included consulting with the Auto/Steel Partnership Lightweight
Chassis Corrosion Project Team on corrosion countermeasures for the beam designs.
This team plans to conduct a build and test program to gather data on the corrosion
performance of AHSS and UHSS with various treatments. Possible approaches include:

Powder coating weld areas;

Mild alloying of materials via added copper or chrome; and

Post-formed coatings, especially for hot stamped boron steels.
46
Lightweight Twist Beam Development – Final Report
Cost Estimates
Production costs were estimated for the OEM baseline and selected U-Beam designs
based on the SMDI-provided project assumptions. All costs are reported relative to the
functionally equivalent OEM baseline design for comparison purposes. Costing was
completed using Multimatic’s proprietary production cost estimation methodology.
Assumptions
The following assumptions were used to estimate the cost of the twist beam structure
for the OEM baseline and the U-beam designs.
Material Costs
Sheet steel material costs were based on published data for the period of June 2013 to
July 2013, with the exception of DP780 and Mn22B5 costs, which are based on data from
material suppliers. The costs are summarized in Table 5.
Table 5: Assumed material costs*
Material Type
HSLA 550 CR, 1.5mm
HSLA 550 HR, 2.6mm
HSLA 300 HR, 2mm
SPH590 HR, 1.5mm
HSLA 550 HR, 3mm
DP780, 1.8mm
Mn22B5, 2.5mm
$CDN/lb
$0.4969
$0.5050
$0.4642
$0.5244
$0.4948
$0.6284
$0.6553
$US/kg
$1.0952
$1.1130
$1.0231
$1.1558
$1.0905
$1.3850
$1.4443
Design
*

OEM baseline design:
Content included the welded assembly and the bushings (Figure 1). E-Coat finish
included.

U-Beam design:
Content included the welded assembly and the bushings. Nickel plate and ECoat finish included.
CRU Index for steel costs, June 12, 2013 - July 10, 2013, except for DP780 and Mn22B5
47
Lightweight Twist Beam Development – Final Report
Program

Production volumes of 30,000, 100,000 and 250,000 vehicles per year were
evaluated.

Program life: six years
Variable Costs
The following were considered in estimating the variable costs:

Material blank size;

Material type and coating;

Plating and / or E-coating;

Purchased components (bushings, etc.);

Machining labor and burden;

Variable overhead;

Capital equipment; and

Selling, General & Administrative Expense (SG&A).
Fixed Costs
The following were considered in estimating the fixed costs:

Tooling (machining, stamping, welding, etc.); and

Fixed overhead.
Component Costs
The total component costs were calculated from the sum of the variable and fixed costs,
with the fixed costs calculated on an amortized basis. For comparison, only the total
costs for each design are reported as costs relative to the baseline design cost.
Cost Comparison
Component costs were estimated as a function of production volume based on the
above-mentioned assumptions. The relative cost results are summarized in a bar chart
in Figure 40, where the cost basis for the comparison is the cost of the baseline at the
indicated volume. The relative cost results are again plotted in the graph of Figure 41.
However, in this case, the cost basis is the cost of the baseline design at the highest
production volume (250,000 vehicles / year). The relative percent cost difference
between the U-Beam design and the baseline is also indicated in Figure 41.
The cost results indicate that the U-Beam design has a 12% higher cost compared to the
OEM baseline at the lowest production volume and a 15% higher cost at the higher
production volumes. Further, the results in Figure 41 indicate the relative change in the
baseline cost as a function of the production volume.
48
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figu
ure 40: Rela
ative cost ccomparison
n
49
Lig
ghtweight Twist
T
Beam
m Developm
ment – Fin
nal Report
Figure
e 41: Relativ
ve cost com
mparison plot
50
Lightweight Twist Beam Development – Final Report
References
1. Anderson, D., Sang, J, “Effects of Material Properties and Weld
Geometry on Fatigue Performance of DP780 and Mild Steel
GMAW Lap Joints,” Great Designs In Steel 2009,
www.autosteel.org.
2. Altair HyperWorks 11 Optistruct User's Guide; 1990-2012 Altair
Engineering, Inc.
3. Abaqus Analysis User's Manual v6.11; Dassault Systèmes Simulia
Corp, 2011
4. nCode DesignLife 8.0 User's Manual; HBM United Kingdom
Limited, 2012
5. Material Fatigue Properties, OEM proprietary document, April 23,
2013
6. Lightweight Twist Beam Team Recommendation, Southfield, MI,
March 22, 2013.
7. MD ADAMS 2010 Installation and Operations Guide,
MSC.Software, 2010
51
Lightweiight Twist Bea
am Developme
ent – Final Re
eport
A
Appendix 1:: Kinematics
s and Compliance Plots
s
52
Lightweiight Twist Bea
am Developme
ent – Final Re
eport
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am Developme
ent – Final Re
eport
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eport
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eport
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eport
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