Stiffness of BIW ISSN 2395-1621 - IERJournal and Improve Torsional... · ISSN 2395-1621 Stiffness of BIW ... Figure 1- Process Flow ... attached to measure responses on BIW. Design

www.ierjournal.org International Engineering Research Journal (IERJ) Special Issue 2 Page 2093-2100, 2015, ISSN 2395-1621

© 2015, IERJ All Rights Reserved Page 1

ISSN 2395-1621

Evaluate and Improve Torsional

Stiffness of BIW

Using FEA and Validate Results with

Test

Sushant S. Dinde

[email protected]

Mechanical Department, Pune University

ABSTRACT

ARTICLE INFO

In a vehicle design, global body torsional stiffness is one of the important design

parameter which affects ride & handling performance of ground vehicles [2]. Also, in a

vehicle it’s important that front and rear suspension act in right way relative to each

other & this behavior of suspension systems are driven by body torsional stiffness.

Apart from vehicle dynamics performance, torsional stiffness plays an important role

for improving the structural capabilities of a vehicle body. Hence, it becomes important

to evaluate the torsional stiffness performance of a BIW (body in white) [3].

Current industrial methods for evaluating torsional stiffness are having several

downsides like high cost for test setups [2, 3], more time consumption for test setup

preparation which leads in delay of programs. Recent technique like finite element

analysis helps to evaluate car body torsional stiffness & also helps to improve torsional

stiffness if any shortfall of target by performing several iterations using FEA in shorter

time.The objective of this project is to evaluate the torsional Stiffness of Body-In-White

[BIW] in initial design phase using FEA methodology & validate FEA results with

Physical test [1, 2].

In the initial design phase, body doesn’t meet torsional stiffness target because of

immature design CAD data. Hence, this project also aims to improve torsional stiffness

by identifying susceptible areas in body & without adding significant weight in body,

using finite element analysis technique. However, it’s important to validate final FEA

results with Physical test to gain confidence about FEA technique.

Keywords— Torsional stiffness, BIW, CAE, Physical Testing

Article History

Received :18th

November

2015

Received in revised form :

19th

November 2015

Accepted : 21st

November ,

2015

Published online :

22nd

November 2015

I. INTRODUCTION

In a vehicle, Body in white (BIW) [3] is major constitute of

which main role is to hold & support the other subsystems

like powertrain, suspension system, closures systems like

door, bonnet, tailgate, exhaust system & so many. „Body in

White‟ is referring one of important term in design phase of

vehicle & mainly consist of all sheet metal parts welded

together without painting. In short, BIW refers to assembly

of sheet metal parts without any trim parts. This major

portion of vehicle is having significant role in vehicle ride &

handling performance [2] & so many other design

performances. This ride & handling performance is driven

by stiffness of body which can be described as a resistance

to deflection. Based on deflection of body, stiffness is

categorized in bending stiffness & torsional stiffness.

„Torsional stiffness‟ of body in white can be defined as

the resistance offered by body to twist because of

asymmetric load. It is essential to have a stiff body which

acts a better rigid structure for other subsystems. Stiffer

BIW improves design performance like it gives better

resistant to noise, vibration & also plays major role in

vehicle dynamics to act front & rear suspension in right way

[2]. Hence, it is important in designing of vehicle to

evaluate stiffness of BIW which undergoes for asymmetric

loading condition & causing torsion of body. It is must to



compare this torsional stiffness of BIW against generic

target which are decided based on benchmarking of other

vehicle‟s BIW & decided at the start of any program.

BIW is complicated structure & it become difficult to

evaluate torsional stiffness performance analytically.

Currently, in an automotive field, computer aided

engineering (CAE) methods are increasingly used to predict

design performance [1]. This paper includes the evaluation

of torsional stiffness of a body in white using Finite element

analysis (FEA) techniques & compares it against generic

target decided by benchmarking. Since CAD data of BIW

provided by designer is immature in its initial design phase

of program. Hence, this paper also covers improvement of

torsional stiffness by identifying weaknesses in structure if

any shortfall of target, by performing different iterations in

CAE. It aims to provide better design solution which is

efficient in terms of weight & also meet the torsional

stiffness performance.

In order to have good confidence about CAE work which

gives better design solutions, it is essential to have physical

testing [3]. Body in white with best design solution

developed in CAE to meet its torsional stiffness target, is

being tested to check torsional stiffness performance. CAE

helps to save cost & time of physical test by doing testing

only on best designs delivered using FEA methodology.

How Torsion Load Causes Twisting of BIW:

Normally, when vehicle is running on road & when it come

across some pothole or bumps, one of vehicle wheel get

upward & other downward which applies asymmetric load at

front and at rear shock tower. This asymmetric loading causes

the twisting of BIW components & causes BIW torsion. This

vertical asymmetric load case gives a combination of bending

and torsion on the vehicle [1]. The pure torsion load cannot be

experienced on the road, since the weight of the vehicle is

always acting in downward direction so that there cannot be a

negative wheel reaction [1].This opposite loading at shock

tower (where suspension system gets attached to body)

produces twisting of body along length at some angle which

is called as angle of twist.

II. METHODOLOGY

Below figure refers to methodology used during this project.

Figure 1- Process Flow

This project is basically includes four major steps as a

described below-

1. Finite Element Modeling & Modal Analysis:

This is the most time consuming & important part of this

project work. To start with FE modeling of different BIW

parts, all sheet metal parts are modeled as 2D elements

(TRIA & QUARD elements) whereas casting & solid parts

are modeled as 3D elements (TETRA elements). Modeling

of casting parts with tetra elements becomes easy & less

time consuming because of complicated geometry.

Appropriate connections are made based on provided data

by design engineer like spot welds, adhesives etc. BIW

model which considered for this project work is Fixed Metal

roof BIW. Element sized used for modeling the BIW parts is

10 mm & all the parts are need to mesh as per standard

modeling practices & can meet the element quality criteria.

There are some modeling checks which needs to do after

complete modeling as per below-

The following items must be verified for the full model:

No duplicated grids or elements.

No unintentional „flying‟ parts. No free end for

rigid elements.

No loop for rigid elements.

No unmatched local coordinate system for spring

elements.

No missing/duplicate parts.

The total weight of FE model should match the design

weight.

Once the meshed model is ready then it‟s important to

check the structural integrity of model for any missing

connection or any free component. This can be possible by

running the free free modal analysis which will give first six

rigid body modes if all parts are properly connected. However,

modal animations need to check in details for any missing

spot welds by scaling the animation view. Below checks

needs to do in order to make sure the model is perfectly

connected & is ready to use for further analysis otherwise it

may give the wrong results.

It is recommended to run a free-free normal mode

analysis to check the model connections. The following are

the items that should be verified in BIW FE model:

The mass & C.G are reasonable.

There should be six rigid modes for body, with their natural

frequencies less than 0.1 Hz.

The six rigid body modes should represent both the three

translational modes and three rotational modes in x, y and z

directions, individually or any linear combination of them.

The frequencies of flexible modes are reasonable. No „flying‟

parts.

In consultation with Test engineer, the FE model of

torsional stiffness rig modeled & confirmed to use for

further analysis work.

2. Run Static Stiffness Analysis &Calculate Torsional

Stiffness:

After confirmation of structural integrity of FE model, job

need to run for static stiffness analysis which will gives the

output as a displacement after applying a static load which

will twist BIW. With this FE output data, need to do some

calculations to evaluate torsional stiffness & also need to

compare with generic torsional stiffness target of BIW.

3. Perform Iterations to Improve Torsional Stiffness & Find

Best Design Solution:

Since, CAD data provided by design engineer is of initial



design phase & it‟s pretty impossible to meet design target in

first place & hence some design iterations need to perform in

CAE which can meet torsional stiffness target with providing

best design solution in terms of weight & saving of cost.

4. Physical Test & Validation of Results:

With the best design solution find in CAE, physical testing

done & validated the CAE vs. Test results which his has

saved a cost & time of test setup.

FEA Tools Used:

There are different FEA tools available in market & having

different capabilities.

Torsional stiffness is a static load case & MSc Nastran is well

known for static analysis. MSc Nastran (version 2013) is used

as a solver while hyper mesh 12.0 is used as a preprocessor &

hyper view 12.0 as a post processor.

III. CAE ANALYSIS

To start with this project, we first required perfect FE

model of BIW modelled in hyper mesh. Along with BIW FE

model, we have modelled the test rig model in FE. This test

rig has modelled in FE in consultation with Test engineer.

Test engineer has explained actual working of test rig &

then modelled in FE. This discussion helps to understand

how BIW is held on rig & where the accelerometers

attached to measure responses on BIW.

Design engineer has provided the CAD data of BIW

which contains around 120 components, mostly sheet metal

part & few casting parts. This all components are meshed in

hyper mesh (fig 2). Sheet metal parts are represented as 2D

elements (CTRIA & CQUARD) while casting parts are

represented as 3D elements (CTETRA). All spot welds are

modelled as REB3-HEXA- REB3 connecting to different

panels. Spot welds which are connecting to two panels are

modelled as 2T while spot welds connecting to three panels

are modelled as 3T. Structural adhesives are modelled as

RBE3-HEXA REB3. Normally, in manufacturing adhesives

are spread along the flanges of panel which doesn‟t allow

move panels & to have perfect spot welding.

On a similar way of test, response points, loading &

constrained are modelled in FE rig (fig 3).

Figure 2- FE Model of BIW

Figure 3- FE model of Test Rig representing load & constraints

The response points on front side of BIW are near to

outboard bolt of circular plate as shown in fig.4 whereas on

rear side its points on outboard longit. The position of

response point locations are provided by test engineer. As a

shown in figure 5, circular plates are bolted to front shock

tower of BIW whereas L shaped plates are bolted to rear

shock tower.

Figure 4 – Response points on Front shock tower & RR longit



Figure 5– FE model of circular plate connecting to front shock tower

In FE model of test rig, BIW is fixed at sub frame by

arresting dof 123456 of rod to avoid the lateral movement of

BIW when applied a lateral load. Also, rear shock tower is

constraint in order to avoid the movement of BIW in

longitudinal direction.

Lateral load applied in such a way that it produces the

twisting of BIW & then can measure the displacement at

response point at front shock tower &rr longit area.

Torsional Stiffness Calculation:

Because of application of lateral load at front &

constrained at rr end of BIW, vertical deflections are

measured at front shock tower &rr longit. From this

deflection values, angle of twist measured in a BIW. Torque

required to produce the unit angular displacement gives the

torsional stiffness value. Detailed calculation formulas are

as below-

Kt = Torque Applied (T) / Average angle of twist of BIW (Q)

Where,

Kt = Torsional stiffness in N-mm/deg

T = Torque applied in N-mm

Q = Average angle of twist of BIW in deg

Torque applied T = Lateral load applied * Perpendicular

distance

Q =Average twist at front (Qf)- Average twist at rear(Qr)

Figure 6– Angle of twist at front

Z1, Z2- Vertical displacement at front shock tower

Q1, Q2- Angle of twist

Y1, Y2- Distance b/w front shock towers

Average angle of twist at front

Qf= ½[tan-1(Z1/Y1) +tan-1(Z2/Y2)]

Similarly, average angle of twist is calculated at rear end (Qr)

& then average angle of twist of BIW.

Baseline torsional stiffness is calculated using stated

formula earlier. Below image shows displacement values at

front shock tower & at rear longit points.

Figure 7– Front & RR Displacement of Baseline Model

Generic target for torsional stiffness is 22 KN-mm/deg&

baseline torsional stiffness is shortfall of generic target &

hence need to improve to meet target.

Iterations to Improve Torsional Stiffness:

Different iterations are performed in CAE to improve

torsional stiffness of a baseline model to meet target value.

These iterations are performed in area where BIW looks

weak & weak area in BIW is identified by studying torsion

animation & identifying the maximum strain energy area in

BIW. Stiffness is increased reducing the strain energy

concentration in weak areas. Figure 8 shows the maximum

strain energy in a baseline BIW at rr quarter panel &rr longit

area.

Figure 8– Maximum Strain Energy area in a baseline BIW

After identifying weak zone in baseline BIW model,

different iterations performed to reduce strain energy

concentration at rr longit area. Since, there is no support in

lateral direction at rr longit as encircled in fig. 8, it makes rr

longit weak & bends in lateral direction. Hence, iterations

are carried to make rr longit section more stiff as a described

below.

Iteration -1

Iteration 1 is with addition of triangular steel plate of 2.5

mm on LH & RH side connecting to outboard rr longit &

top face of inboard rr longit as shown in fig 9. Intial idea is



just to check whether it really gives an improvement or not.

With addition of triangular plate, it shows potential to

improve torsionl stiffness by 0.46%.

Figure 9– Addition of Triangular steel plate 2.5 mm

Iteration -2

Iteration 2 is with a addition of three flange steel

reinforcment of 2.5 mm LH & RH side connecting to

outboard rr longit,top face of inboard rr longit & with two

extra spot ( as shown in encircled area) on inner face of

inboard rr longit as shown in fig 10.With addition of three

face steel reinforcement, improvement of 0.92% in torsionl

stiffness.

Figure 10– Three flange Reinforcement steel plate 2.5 mm

Iteration -3

Iteration 3 is addition of lateral bottom face in iteration 2 &

with two extra spots on lateral bottom face as shown in

encircled area fig 11.With addition of lateral bottom face in

iteration 2, improvement of 1.75% in torsionl stiffness.

Figure 11– Addition of Lateral bottom face with two spot welds

Iteration -4

Iteration 4 is with a change in thickness of reinforcemnt

added in iteration 3. Thickness changed from 2.5 mm to 3.5

mm .With change in thickness from 2.5mm to 3.5 mm,

improvement of 2.05% in torsionl stiffness of baseline

model as shown in fig 12. Improvement 0.3% is because of

only change in thickness of reinforcement.

Figure 12– Steel Reinforcement of 3.5 mm

Iteration -5

Iteration 5 is with thermoplastic baffles which is injection

molded as shown in fig 13. As shown in below image,

designed ribbing parttern provids good lateral support &

hence get maximum improvement of 3.25% in torsional

stiffness.

Normally, thermoplastic baffles are placed in a sheet

metal parts along with some fixing to hold it as shown in fig

14 & also with some adhesive material. During the painting

and anti-corrosion a process, the BIW is subjected to bake-

oven temperatures and the baffle adhesive material expands

and cures inside the cavity.

The thermoplastic materials are injection moldable and have

an advantage in that they can be manufactured into complex,

three-dimensional shapes.

One of the important benefits of using baffles is that they

require no major upfront capital expenditure for in plant

material handling or installation equipment.

Figure 13– Thermoplastic Baffle with best ribbing pattern

0.46%

Improvement



Figure 14– Thermoplastic Baffle hold in Sheet metal

Design Effectiveness-

To choose best design out of five iterations, design

effectiveness is calculated by ratio of change in torsional

stiffness to mass penalty to improve that torsional stiffness.

Design effectiveness & respective mass penalties to improve

that torsional stiffness is summarized in below table.

TABLE 1– Design Iterations Summary

BIW models

Torsional

Stiffness

(KN-m/deg)

Change in

Torsional

Stiffness

(KN-m/deg)

Mass

Penalty

(Kg)

Design

Effectiveness

(KN-m/deg/Kg)

Baseline 21.4 - - -

Iteration 1 21.5 0.1 0.18 0.6

Iteration 2 21.6 0.2 0.23 0.9

Iteration 3 21.78 0.38 0.38 1.0

Iteration 4 21.85 0.45 0.5 0.9

Iteration 5 22.12 0.72 0.12 6.0

From design effectiveness chart as below in fig 15, it‟s

pretty clear that adding the baffles would increase more

torsional stiffness with minimal mass penalty than other

iterations.

Hence, by discussing with design engineer the feasibility

of manufacturing & considering mass saving, iteration 5 is

selected as best design solution & decided to go for

physical testing based on best CAE design solution i.e.

with addition of baffles on each side (LH & RH side) .

Figure 15– Design Effectiveness Chart

IV. PHYSICAL TESTING

The BIW model with baffles added in RR longit area is

being used for testing. BIW is bolted on test rig at front strut

using circular plates. These circular plates are as shown in

fig 16 while L shaped brackets are used to bolt BIW on rear

strut as shown in fig 17.

Figure 16 – Circular plates connecting to front strut of BIW

Figure 17 – L Brackets connecting to rear strut of BIW

Procedure-

Linear displacement traducers are used to measure

the displacement near to outboard bolt position for front

shock whereas at outboard position of rear longit area as

shown in fig 18.After mounting the BIW on test rig,

pneumatic actuators are used to apply a torque to twist

BIW as shown in fig 19. The torque is applied in steps of

600 Nm in both clockwise & anticlockwise directions.

Force traducers are used to measure test force. Traducers

reading at front & rear are noted in both clockwise &

anticlockwise direction. A data acquisition system with

signal conditioning should be interfaced with a computer

to facilitate measurement reading and data storage.

Test rig circular plates bolted at front

strut



Figure 18 – Front & Rear traducers on BIW

Figure 19 – Pneumatic system to apply torque

Below table shows readings of BIW torsion test.

Table 2– BIW torsion test readings

CAE Vs Test Correlation-

Torsional stiffness measured using CAE methodology for a

best design solution obtained & torsional stiffnes calculated

using test is compared.. Figure 20 shows CAE vs Test

tosional stiffness values & comparison of torsional stiffness

result & physical test result.A correlation of 96% was

established with the physical testing results and CAE as

shown in fig 20.

Figure 20 – Comparison of Test Vs CAE results

V. CONCLUSION

In this paper, BIW torsional stiffness is evaluated using

CAE & target is achieved with minimum mass penalty.

Results have been tested with best design solution obtained

in CAE & show good correlation of CAE vs. TEST results.

A correlation of 96% is established with the physical test

and CAE for best design solution.

In conclusion, this paper helps to understand

importance of CAE technique to evaluate torsional stiffness.

CAE techniques also help to meet target of torsional

stiffness of BIW by performing different iterations in quick

time which ultimately helps to save time & cost of test setup.

ACKNOWLEDGEMENT

The author would like to thank project guide Prof. P.A.

Narwade, PG coordinator Prof. R.Navthar& HOD Prof. P.A.

Deokule for their valuable guidance, support and help from

time to time during work. Also, author would like to thanks

TATA Technologies, Ltd, Pune for giving an opportunity to

work on this project.



DEFINATION/ABBREVIATIONS

CAE - Computer Aided Engineering

CAD - Computer Aided Designing

DOF- Degree of freedom

FEA- Finite Element Analysis

Kt = Torsional stiffness in N-mm/deg

T = Torque applied in N-mm

Q = Average angle of twist of BIW in deg

Qf= Average angle of twist at front in deg

Qr= Average angle of twist at front in deg

Z1, Z2- Vertical displacement at front shock tower

Y1, Y2- Distance b/w front shock towers

REFRENCES

[1] Ramachandran, R., Dehariya, N., Kumar, G., Agarwal,

H. et al.,

"Methodology to Measure BIW Torsional Stiffness and

Study to Identify

and Optimize Critical Panels," SAE Technical Paper 2015-

26-0224,

2015,doi: 10.4271/2015-26-0224.

[2] Hari Krishnan M, N Sreeraj, C Bhaskar, G Nagaraju

and R Mugundaram “Establishing Correlation between

Torsional and Lateral Stiffness Parameters of BIW and

Vehicle Handling Performance” SAE Technical Paper

2011-01-0089, 2011, doi: 10.4271/2011-01-0089

[3] Karan R. Khanse and Shekhar P Pathak, “Test Set-Up

of BIW (Body in White) Stiffness Measurements”, SAE

Technical Paper 2013-01-1439, 2013, doi:

10.4271/2013-01-1439

[4] Steven Tebby1, Ebrahim Esmailzadeh2 and Ahmad

Barari3 “Methods to Determine Torsion Stiffness in an

Automotive Chassis” doi:

10.3722/cadaps.2011.PACE.67-75

[5] J. Helsen 1,L. Cremers 2,P. Mas 3, P. Sas 1,” Global

static and dynamic car body stiffness based on a single

experimental modal analysis test”

K.U.Leuven, Department of Mechanical Engineering,

Celestijnenlaan

300 B, B-3001, Heverlee, Belgium.

[6] MSC Nastran 2012.2 Installation and Operations

Guide.

[7] http://www.adhesivesmag.com/articles/polymers-

elastomers-and-two-component-foams-seal-reduce-

noise-vibration-and-harshness-reduce-vehicle-weight-

and-provide-easy-application

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Stiffness of BIW ISSN 2395-1621 - IERJournal and Improve Torsional... · ISSN 2395-1621 Stiffness of BIW ... Figure 1- Process Flow ... attached to measure responses on BIW. Design