International Journal of Engineering Trends and Technology (IJETT) – Volume 34 Number 4- April 2016
Determination of Light-Weighting Potential of
Orthogonally Ribbed Polyamide-Steel
Polymer-Metal Hybrid Material in Vehicle APillar Design
Rindai P. Mahoso#1, Sagar S. Parihar*2
#1 #2
Department of Mechanical Engineering, ITM University, Gwalior, India
Abstract - The application of polymer-metal hybrid
materials in the automotive industry is aimed at
advancing vehicle light-weighting whilst reaping
performance benefits at the same time. In the
information available on the public domain, it
indicates that the application hybrid materials has
been restricted to non-safety critical components of
vehicle bodies. Therefore this paper seeks to show
the feasibility of application of a polyamide-steel
hybrid material in a safety critical component, the
A-pillar going on to show the benefits of application
of the technology as far as weight saving is
concerned. This paper does so through the
implementation of multi-objective geometric
algorithm optimisation procedure on a prototypical
material setup for A-pillar design and shows that
weight saving of up to 29.7% is possible with
maintenance of acceptable performance in
structural stiffness.
Keywords — A-pillar, rollover accident, vehicle
light-weighting, Mass optimisation
I. INTRODUCTION
In modern day automotive design, operator and
passenger safety are key considerations in the design
process and they are major influences in structural
design and material selection. As such safety devices
and technologies, both active and passive are always
evolving to ensure the safety of all aboard [1]. This
evolution has also been greatly influenced by the
industry’s light-weighting drive where designers and
engineers are seeking to increase motor vehicle’s
fuel economy and yet increasing safety and integrity
of their vehicles in all aspects.[2] As is known
vehicle occupant safety is at the greatest risk in the
event of an accident and accidents occur in many
different modes including rollover accidents, frontal
impact, side impact and rear impact accidents and
the key to protecting occupants in these encounters
lies in ensuring the integrity of passenger
compartments or safety cells so that other active
safety devices such as seatbelts and airbags can
perform their designed tasks effectively.[3] One of
the accidents modes of gravest concern in modern
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day car design are rollover accidents, which are
arguably the most fatal mode of accident due to their
occurrence-fatality ratio.[4] It is for this reason that
vehicle roof integrity is important as it is the best
way to ensure passenger safety space. It has also
been shown that the A-pillar is a crucial member in
providing the roof integrity during rollover as top of
the A-pillar is the usual point of impact thus this
member has been shown to bear the greatest load in
roof collapse. [5] In the light-weighting revolution,
steel has stood its ground in the manufacture of
automotive bodies, especially the safety critical parts
like the A-pillar. This has been largely due to the
material’s availability and the advent of advanced
steels which include dual phase steels and other
multiphase steels which are easy to manipulate in
terms of required physical traits for different uses on
the motor vehicle. However other technologies have
also gained considerable favour amongst researchers,
case in point Polymer Metal Hybrid (PMH) material
technology which offers the possibility of improving
overall part performance, reducing part weight and
also reducing part complexity.
The technology has been used in several parts in
motor vehicle Body-In-White (BIW) parts including
front ends and roof headers by large manufacturer
like Audi [6] but it has not yet been applied to safety
critical components of the vehicle BIW structure. It
is this researchers aim to design an A-pillar using the
PMH technology and assess its performance.
In the use of PMH materials different
technologies are taken and used together to produce
a composite material with the performance
characteristics of the materials involved with the
added advantage of light-weighting. [7] In this paper
Direct-Adhesion Polymer-Metal-Hybrid Technology
is used and the required level of adhesion strength is
attained through direct adhesion where the
infiltration and of the thermoplastic melt into the
micron-size asperities on the metal substrate surface
is facilitated through injection moulding. The
general production principle is that a formed metal
part is placed in the mould as the insert and an
appropriate polymer is injected around it. [8] This
method relies upon metal forming and plastics
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International Journal of Engineering Trends and Technology (IJETT) – Volume 34 Number 4- April 2016
injection moulding which are two very economical
methods of series production, thus boosting the
possibility of offering solutions for structural
automotive parts that are light, cheap, reproducible
with functional manufacturing integration and
functionally competitive in a single process step.
This gives this technology its economic advantage
over other methods being researched for BIW design.
[8] [9] Direct adhesion under optimal process
conditions an adhesion strength as high as 40MPa
which has been shown by several researchers as
adequate for use in BIW uses. [6] [10] [11].
This paper therefore seeks to show the potential
weight saving that can be achieved by using
polyamide-steel polymer metal hybrid material for a
safety critical component like the A-pillar. It
achieves this by explaining the optimisation problem
itself with its major parameters and constraints and
goes on to describe the simulation setup to test this
which was followed by an explanation and
discussion of the results.
II. OPTIMISATION PROBLEM
One of the major draws of applying the PMH
technology to the motor vehicle parts is that of lightweighting and in this paper the application of a
polyamide-steel PMH material to BIW members is
to be assessed for its merits in the weight saving
criterion. The BIW member of note in this paper is
the A-pillar. Its main functions on a vehicle are to
provide structural support to the roof at the foremost
end, to form a support structure for the windshield
and to form part of the closure for the doorframe of
the front doors. However in this paper we seek the
optimal material combination design and the extent
to which the material can offer weight saving. For
the purposes of this assessment we employ the use
of an Erlanger-Trager beam which has bounding box
dimensions approximate to those of a mid-size sedan
A-pillar as a prototype member. The control member
will be a prototypical all steel pillar sharing the same
bounding box dimensions of 100mm depth and
width and 800mm length.
The main objectives in this paper’s optimisation
problem are the minimisation of mass and the
maximisation of stiffness. The parameters that we
are to focus on to achieve our best stiffness and mass
for the A-pillar member are the geometric
parameters of steel insert thickness and polyamide
rib thickness. The average sheet thickness used in
mid-range sedan A-pillars is 1mm and a material
availability of high strength steel sheets is there to a
gauge of 0.55mm and the polymer rib thickness was
initially set at the maximum advised for reinforced
injection moulded wall and rib thickness at 5mm but
reduced to 3mm due to the maximum weight
restriction. The minimised output parameter will be
the maximum deflection under a nominal axial
compressive load of 30kN as this will give us a
direct indication of the stiffness performance of the
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prototype member which we seek to maximise. The
Erlanger-Trager beam polymer reinforcement design
parameters are in line with the guidelines given by a
few polymer part design handbooks and the metal
insert design parameters are all within the limits of
material supply and general current manufacturer
general practise. [12] [13] [14]
The above stated optimisation problem, the
maximisation of stiffness:
k =AE/L
It is clear that by maximising the elastic modulus
of our hybrid material, we will also maximise our
part stiffness. Therefore, using the rule of mixtures,
we can estimate our material elastic modulus using
volume fractions of the constituent materials. After
expressing the volume fractions in terms of metal
insert thickness (m), ts, and polyamide rib thickness
(m), tp, we can seek the values of these variables that:
Max f(Ec) = [Es(0.24ts-1.6ts2)+Ep(0.3663tp-4.158tp2)]
/ (0.24ts-1.6ts2+0.3663tp-4.158tp2 )
Subject to
0.0005≤ts≤0.001
0.0015≤tp≤0.003
Where Es and Ep are elastic moduli for Dual
Phase 980 HSS and 43% Glass Fibre reinforced
Polyamide respectively.
For the second objective to minimise mass, we
can use density volume relationships to ts and tp to
give us the following optimisation problem:
Min f(M) = ρs(0.24ts-1.6ts2)+ρp(0.3663tp - 4.158tp2)
Subject to
0.0005 ≤ ts ≤ 0.001
0.0015 ≤ tp ≤ 0.003
As can be noticed that these objectives do not
have an obvious optimal point, however trade-offs
need to be done to produce an optimal point. Hence
for this complicated optimisation we will employ the
use of ANSYS tool DesignXplorer.
An overall objective function for the optimisation
problem for the system can be written as:
f(X) = α1f1(Ec) + α2f2(M)
Where α1 and α2 are weighting constants.
III. SIMULATION SETUP
The aim of the work in this paper is to minimize
the part’s mass yet maximising its structural
performance. Therefore the prototypical A-pillar
members are modelled in Autodesk Inventor and
thereafter the optimisation building blocks are set up
in ANSYS Workbench. The first step is to select the
most favourable reinforcement design to be used in
the prototypical PMH A-pillar comparing the
performance to the control all-steel A-pillar. Then
the chosen design member is set up as the geometry
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International Journal of Engineering Trends and Technology (IJETT) – Volume 34 Number 4- April 2016
for a non-linear axial loading simulation which will
be our benchmark for the stiffness of different
parameter settings. At this point a Central Composite
Design type Design of Experiments (DOE) is run to
assess parameter correlation and sensitivities.
Thereafter the sensitivities guide us as to which
parameters are more effective and important to our
desired goal of maximising stiffness. The desired
parameters can then be plotted on the Kriging
Response surface. This then gives us an appreciation
of how our parameters vary with each other and
finally we use the Multi-Objective Genetic
Algorithm (MOGA) optimisation to find candidate
design points that give us the best trade-offs which
are closest to our desired goals.
The polyamide used for the simulation is a 43
percent Nylon 6/6 and the steel used is 600MPa dual
phase steel for the control beam and 980MPa dual
phase steel for the PMH beam. These steels are
common automotive steels and the 600MPa steel is
the steel usually used in A-pillar construction by
automotive constructors. The ANSYS material
models are shown below:
TABLE I: 43% GLASS FIBRE REINFORCED NYLON 6/6
MATERIAL MODEL
Property
Value
Density
1480 kg/m3
Compressive Yield Strength
2.6e8Pa
Tensile Ultimate Strength
2.25e8Pa
Table III: DUAL PHASE 980 Y700 VERY HIGH STRENGTH
STEEL
Property
Value
Density
7850 kg/m3
Tensile Yield Strength
7e8Pa
Compressive Yield Strength
9.1e8Pa
Tensile Ultimate Strength
9.8e8Pa
Isotropic elasticity
Young’s Modulus
2.1e11Pa
Poisson’s Ratio
0.3
Bulk Modulus
1.75e11Pa
Shear Modulus
8.0769e10Pa
For modelling the contact between the steel and
polyamide, the contact debonding tool in ANSYS
will be used which uses Cohesive Zone Modelling
(CZM) material. This cohesive zone material is used
in modelling the contact elements between contact
surfaces applying the bilinear behaviour model. The
metal sheets in the control member are modelled
with spot weld elements.
Isotropic elasticity
Young’s Modulus
1.4e10Pa
Poisson’s Ratio
0.35
Bulk Modulus
1.5556e10Pa
Shear Modulus
5.1852e9Pa
Fig. 1 Prototypical 3 component all-steel member
Table II: DUAL PHASE 600 VERY HIGH STRENGTH STEEL
Property
Value
Density
7850 kg/m3
Tensile Yield Strength
3.3e8Pa
Compressive Yield Strength
4.29e8Pa
Tensile Ultimate Strength
5.8e8Pa
Isotropic elasticity
Young’s Modulus
2e11Pa
Poisson’s Ratio
0.3
Bulk Modulus
1.75e11Pa
Shear Modulus
8.0769e10Pa
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Fig. 2 Prototypical Erlanger-Trager type PMH member
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International Journal of Engineering Trends and Technology (IJETT) – Volume 34 Number 4- April 2016
Table IV: STIFFNESS TEST RESULTS USED FOR REINFORCEMENT CONCEPT DESIGN
Reinforcement type
Mass (kg)
All steel pillar
3.46
Axial
buckling
14154
Longitudinal
3.35
Orthogonal
Diagonal
loading
Reinforcement side
buckling
3533.8
Wall side buckling
16613
1348.5
1233.4
3.44
45586
1862.4
3180.9
3.40
24675
1096.8
1957.9
1415.3
The Erlanger-Trager composite members are
designed initially using the upper design limits
permitted for injection moulded polyamide parts and
this results in the all steel part being almost equal to
the PMH member in mass. The all-steel prototype
member was roughly designed with the 3 sheet
configuration in most A-pillars, where there is an
outer member, inner member and a middle
reinforcement member spot welded together.
IV. RESULTS
Initially different designs of reinforcement were
tested to find the most appropriate ribbing design
for the Erlanger-Trager beam. When the
different ribbing methods were designed and
tested through linear buckling simulations, the
concepts compared as shown in
Table IV.
The first step in the design optimisation process
was to run a central composite design type design of
experiments (DOE). This method came up with a set
of points to determine the parameter relevance. The
sensitivity graph and linear correlation matrix
derived from the DOE are shown below:
Figure 4 Parameter sensitivity chart
As can be determined from the linear correlation
matrix from the ANSYS DesignXplorer tool shows
very good linear correlation between the chosen
parameters. Showing a very strong inverse
relationship between the geometry mass and the total
deformation of the member. With the reinforcement
thickness having a stronger correlation with the total
deformation. The sensitivity graph also shows that
the rib thickness has a greater effect on the output
parameters, which shows the importance of the
polymer reinforcement to the performance of the
design. This also shows that there is greater freedom
to play with the insert material gauge than it is to
tweak the reinforcement thickness as far as
maximising rigidity is concerned.
After the DOE, a response surface is then plotted
to show a more complete set of results through
extrapolation of the DOE points.
Fig. 3 Parameter Linear correlation matrix
Fig. 5 Optimisation response surface
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International Journal of Engineering Trends and Technology (IJETT) – Volume 34 Number 4- April 2016
The Kriging response surface obtained from the
ANSYS DesignXplorer minimization of the total
deformation which we desire correlates with
maximisation of both the insert thickness and the rib
thickness. This works the in the direct opposite way
with the objective of mass reduction. Therefore
trade-offs would have to be made. This can be
shown in by the distribution of the feasible points
which are marked by the blue shade on the surface.
In light of this the multi-objective optimisation
was run and to yield the candidate point distribution
shown below:
of PMH technology in the automotive industry can
hold.
REFERENCES
[1]
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[3]
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[4]
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[5]
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M. Grujicic, V. Sellappan, M. A. Omar, N. Seyr, M.
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Fig. 6 Candidate spread from optimisation
In the candidate point spread, the green lines are
the feasible ones. The results of the MOGA
optimisation therefore suggest that the advised rib
thickness range from 2.2507mm to 2.6278mm and
the metal insert thickness range from 0.50325mm to
0.5675. These ranges yield a deflection of our
polyamide-steel prototype member that ranges from
5.4518mm to 7.213mm. The best candidate yielded
by the MOGA optimisation gave us the metal insert
thickness of 0.55735mm and polyamide rib
thickness of 2.6278mm and this yielded a maximum
deformation under the 30kN load of 5.4518mm at a
part mass of 2.4293mm which represents a 29.7%
weight saving.
A linear buckling simulation was then performed
on the optimised design and it yielded at a buckling
load of 35387N which indicates a reduced
performance from the non-optimal ribbing but very
much competitive compared to the steel prototype
pillar.
V. CONCLUSIONS
The ANSYS DesignXplorer optimisation clearly
shows how well the PMH technology can achieve
reasonable levels of weight-saving whilst at the
same time maintaining performance of the member.
This therefore shows that there is a future for
application of this PMH technology in the safety
critical members of vehicle BIW. The maintenance
of performance within acceptable levels whilst
offering a 29.7 % weight saving is a huge promise
towards the potential benefits that further application
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[10] M. Grujicic, V. Sellappan, G. Arakere, N. Seyr and M.
Erdmann, "Computational feasibility analysis of directadhesion polymer-to-metal hybrid technology for loadbearing body-in-white structural components," Journal of
Materials Processing Technology , vol. 195, pp. 282-298,
2008.
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Metals Via Injection Molding for Macro-Composite
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[12] Modern Plastics, Modern plastics handbook, C. A. Harper,
Ed., Lutherville: McGraw-Hill, 1999.
[13] GE Plastics, GE Engineering Thermoplastics Design Guide,
GE Plastics.
[14] C. Maier , Design Guides for Plastics, Tangram Technology
Ltd., 2009.
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