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Corresponing author Mir Irfan Ul Haq*1 E-mail address: haqmechanical@gmail.com

Doi: http://dx.doi.org/10.11127/ijammc2016.09.04 Copyright@GRIET Publications. All rights reserved.

Advanced Materials Manufacturing & Characterization Vol 6 Issue 2 (2016)

Advanced Materials Manufacturing & Characterization

journal home page: www.ijammc-griet.com

Role of Computer Aided Engineering in Mitigation of Automotive Chassis

Failures

Mir Irfan Ul Haq*1, Sanjeev Anand2, Yatheshth Anand1, Tawqeer Nasir Taq3

1Department of Mechanical Engineering, SMVD University, J&K, India

2Department of Energy Management, SMVD University, J&K, India 3 Department of Aerospace Engineering, Indian Institute of Technology Bombay, Powai Mumbai, India

Abstract With the advent of high speed computers and analysis softwares,

Computer Aided Engineering (CAE) is playing a vital role in the new

product development processes. In an attempt to shorten the new product development life time and to withstand the tremendous

competitive market scenario vis a vis quality, global automobile industry

uses various physical and virtual testing methods. The various virtual tests being carried out for an automobile chassis structure so as to

analyze it statically as well as dynamically due to various loads generated

because of rough road conditions, engine excitation, sudden acceleration, deceleration; cornering, gross weight and net weight. The

tests include the vertical bending test, lateral bending test, 3G bending

test, the torsional test, modal analysis, accelerated durability test, tow hook test and engine and transmission mount test. The paper reviews

the details of these tests being widely used by automotive industry that

usually includes developing a CAD model of the chassis and then subjecting it to different loads depending on the type of test, virtually.

Acceptance criteria for all the parameters obtained in the tests, viz.

torsional stiffness, bending stiffness, yield strength and modal frequency etc. are deduced from data collected via experimenting and reverse

engineering.

Keywords: Computer Aided Engineering, Virtual Testing Methods,

Chassis

1. Introduction

For a pro active design organization failure does not refer to

the breaking of the machine element but every deviation from

the desired behaviour is termed so. The durability of any

automotive structure depend on the structural strength it offers

and fatigue resistance. The structural strength is the ability of

the automotive component to maintain function peak loads are

applied to it and the fatigue resistance is the capacity to maintain

function when repetitive cyclic loading is applied to the system.

The structural durability is a function of three main parameters

the loads to which the structure is subjected, the material used

in the component, the geometrical features chosen by the

designer, manufacturing process through which the component

is produced, and the environmental conditions in which the

component is working[1].To mitigate the failures after the

design has reached a particular stage in a design development

cycle various virtual tests being carried out for an automobile

chassis structure so as to analyze it statically as well as

dynamically due to various loads generated because of rough

road conditions, engine excitation, sudden acceleration,

deceleration; cornering, gross weight and net weight. The tests

include the vertical bending test, lateral bending test, 3G bending

test, the torsional test, modal analysis, accelerated durability

test, tow hook test and engine and transmission mount test.

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1.1 Computer Aided Engineering

The use of computer softwares to aid engineering processes

like designing, analysis, manufacturing, planning and

optimization has become the backbone of every product design

process. Computer Aided Design (CAD) can be defined as the use

of computer systems to assist in the creation, modification,

analysis, or optimization of a design; Computer aided

manufacturing (CAM) may be defined as the use of computer

systems to plan, manage, and control the operations of a

manufacturing plant through either direct or indirect interface

with the plant’s production resources [2]. With tremendous

competitive market scenario and the market trend for frugal

engineering the use of computers has further become

indispensable for all the product development organizations in

general and automobile manufacturers in particular to deliver

the products in time and maintain the quality standards[3][4].

Further the cost effectiveness of the computer analysis has made

computer aided analysis an important process in any product

development cycle. In concept generation during the design

process, using reverse engineering as a tool for benchmarking,

ensuring accuracy, precision and quality of the designed

products, computers are playing a vital role in the modern

automobile industry [5]. The use of various materials which are

light in weight and exhibit better structural and other

engineering properties has become an engineering practice.

Lightweight materials like Alumunium, Magnesium, etc. are

extensively being used for automotive applications including

chassis. The use of aluminium metal matrix composites for

chassis components has also gained much interest. The use of

such materials and their prior testing before actual use can be

done by the use of computer aided engineering[6-8].Apart from

the computer aided body styling, emission control, numerous

packages are used to foresee the static and dynamic behaviours

of the raw designs without actual physical testing.

1.2 Modern Product Development Process

The advent of Computer Aided Engineering (CAE) has led to

shortening of the product development cycles as it has obviated

or lessened the extensive use of physical testing to determine the

quality of designs. The new product development cycle is based

on feedback and performance evaluation of the design concepts

at various stages. The performance including structural

durability, aesthetics, manufacturability and other parameters

are checked virtually before actual manufacturing starts. Apart

from the time saving it also leads to economic benefits as the

material and energy utilized in the physical testing and

consequent design iterations is also saved [9]. The modern

product development process also involves the active

involvement of the end user throughout the design process. The

modern product development cycle also involves the concept of

concurrent engineering in which the various “Design for X”

strategies are followed, wherein X may be environment,

assembly, manufacturability, cost, disassembly, recyclability, etc

[10][11]. Moreover the global concern for sustainability and

greener energy options has led to development of structures

with light weight which poses a pressure to the automobile

designers to develop structures which are light in weight. The

use of Computer Aided Engineering has helped a lot to develop

structures which are lighter and are more durable by fast design

iterations and freedom to alter the designs frequently without

any material inputs[12][13].

1.3 Chassis/Frame Design

There is much confusion over the meaning of the word

chassis as discussed by Aird in

the book “The Race Car Chassis” [14]. The conventional

automobiles possess a unit on which the remainder of the units

are built unlike the recent frameless or monocoque vehicles. The

purpose of car chassis is to maintain the shape of the vehicle and

to support the various loads applied to it. An automobile chassis

may be shaped into a number of different types which include

ladder chassis, backbone chassis, monocoque chassis and

tubular space frame chassis Ladder chassis is considered to be

one of the oldest forms of automobile chassis that is still been

used by most of the SUVs till today. It also resembles the shape

of a ladder having two longitudinal rails inter linked by several

lateral and cross braces. The lateral and cross members provide

rigidity to the structure [15]. The most

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common chassis design is referred to as the “Ladder Frame

” due to its resemblance to a conventional lean-to ladder

consisting of long members and cross members [16]. A narrow

front portion, upswept at the rear and the front to accommodate

the movement of the axles due to springing, to ensure a smaller

turning radius and a lower chassis height; bumper brackets,

body mounting brackets, and engine and transmission mounting

brackets are part of the chassis design. Assessments of the

performance of a vehicle structure are related to its strength and

stiffness. A design aim is to achieve sufficient levels of these with

as little mass as possible. An automobile chassis is usually made

of a light metal or composite plastic that provides strength

needed for supporting vehicle components and load into it.

The fundamental principle of a chassis design states that the

chassis is to be designed to achieve the torsional rigidity and

light weight in order to achieve good handling performance.

Generally, the effect of the torsional rigidity on space-frame is

different to the monocoque due to their construction format, but

the structure is used to approximate the same results as the

difficult to twist monocoque chassis. A space-frame chassis lies

somewhere between the ladder chassis and the monocoque, it is

constructed from an arrangement of small, simple members

which make up a larger frame. A space-frame is analogous to a

truss style bridge which is made up of small (generally straight)

members in a triangular pattern which are always in pure

compression or tension. By having members in pure

compression or tension (i.e. they do not experience bending

forces) they do not have to be oversized to support bending

loads [17]. The most common types of chassis structures used

in a chassis are channel section, box section and tubular section.

To ensure the desired performance designers manipulate

the geometry, thickness, material composition and even the

position of the cross members and the brackets with respect to

the long rails. The cross sections used also affect the

performance, the frequently used ones include the channel

section, box section, hat section or an I- section. The various

loads which a chassis has to withstand may be a vertical load due

to the weight of the vehicle and the occupants resulting in

vertical bending of the chassis, a vertical load when one of the

four wheels comes across a bump resulting in torsional bending

of the frame, forces as a result of road camber, side wind,

cornering force while taking a turn, resulting in lateral bending

of side members, force due to one wheel being obstructed and

other wheels moving forward resulting in a typical distortion in

the automotive chassis resulting in a parallelogram shape,

torque due to engine at the time of engaging the clutch and due

to braking action, impact load at the time of a crash due to an

accident, forces due to the engine vibrations whose frequency

may match with the natural frequency of the chassis resulting in

resonance[18].

Static Loads Source of the Load

Vertical Bending load Weight of the Vehicle

components, occupants,

Braking, Acceleration

Lateral Bending Load Turning and winds

Torsional Load Driving on a uneven load

Combined Bending and

Torsion

Resultant forces

1.4 Virtual Validation

In an attempt to shorten the product development cycles and

to maintain the quality standards the use of computer softwares

has considerably increased in the design circles. Different

analysis softwares are used in the automotive industry to

virtually analyze the designs by taking into account the loads to

which they are subjected to in the practical use. The CAE

portfolio covers strength & life estimation of components, Noise

Vibration and Harshness, performance prediction, crash, ride &

handling, advanced CFD and optimization. It is a unique

approach to accurately predict the real-world behavior of

vehicle durability and fatigue life. An important aspect of

designing any product is validation. Virtual design process (VDP)

is an alternative to hardware prototyping in which analysis of

designs can be done without manufacturing physical samples. In

recent years, VDP have been generated either for animation or

filming applications [19,20].

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1.5 Modal Analysis

Due to the interaction of inertial and elastic properties of a

material, resonant vibrations are caused within a structure.

Resonance is often a cause of, or at least a contributing factor to

many of the vibration and noise related problems that occur in

structures and operating machinery. The global vibration

characteristics of a chassis are directly related to both its

stiffness and mass distribution [21]. The frequencies associated

with the global bending and torsional vibration modes are

usually used as benchmarks for the structural analysis of the

chassis. The Finite Element Method (FEM) based modal analysis

has become a widespread method for finding the modes of a

machine or structure. In a nutshell, it could be said that modal

analysis is a process whereby a system is described by a

structure in terms of its natural characteristics, which are the

frequency, damping and mode shapes --its dynamic properties

[22]. Modes are inherent properties of a structure and are

determined by the material properties (mass, damping and

stiffness) and boundary conditions. If the material properties or

the boundary conditions of the structure change so do the modes

[23].

2. Discussion

In a typical product design cycle a 3D CAD model of the

chassis is prepared and then the model is tested virtually for

various mechanical tests. The various tests may be performed to

compute the maximum forces, which the structure can

withstand. Data for the loading is collected from actual tests

carried out on roads of various geographical locations, weather

conditions and road conditions by using measurement

instruments like strain gauges, etc. Different automobile

manufacturers have their own data and hence different loads,

constraints and acceptance criteria [24]. However depending on

the loading conditions and the constraints derived from actual

road data usually the following tests are carried out virtually

irrespective of the manufacturer.

2.1 Vertical Bending Test

This test involves subjecting the chassis to loads vertically at

two or four points and the displacement at the point of

application of load is calculated by the analysis software. The

chassis in constrained at all the body mount locations in all the

three directions. The stiffness of the chassis as a whole is

calculated by using the general formula for stiffness as given in

equation 1, which is finally compared to the acceptance criteria

based on past experience or actual tests or values got from

reverse engineering. Design iterations are carried out to reach

the acceptance criteria for the bending stiffness.

Equation-1 is given as: K = F/x ... (1) where K stands for

stiffness, F for force, and x for deflection.

Figure 1: Vertical Bending Test in a ladder type automobile

chassis

Description: The figure shows a chassis subjected to carry out

vertical bending test with triangles representing the constraints.

2.2 Lateral Bending Test

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In this test the chassis is loaded laterally by loads depending on

the test data and constrained in the vertical direction. Deflection

at the respective points is calculated by the software after

analysis. The general equation for calculating stiffness (Equation

1) gives the stiffness which is compared with the acceptance

criteria. Design iterations are carried out to reach the acceptance

criteria for the bending stiffness.

2.3 Torsional Bending Test

To simulate the couple which acts on a chassis when the

vehicle comes across a bump, Torsional Bending Test is

performed. The test involves loading a chassis with a couple at

two locations as shown in the Figure 2 and the angle through

which the chassis is twisted is calculated by the analysis

software and by simple calculations the torsional stiffness of a

chassis is calculated. The number and the cross section of cross

members has a considerable impact on the torsional stiffness.

Suitable number of design iterations are carried out to reach the

acceptance criteria for the torsional stiffness [25].

Figure 2: Torsional Bending Test in a ladder type chassis

Description: The figure shows a chassis subjected to torsional

bending test with triangles representing the constraints.

2.4 3G Bending Test

The whole chassis model is subjected to a load thrice its

weight (mg) to check for bending strength, hence the name 3G

Bending, and the chassis is analysed for stresses virtually. As per

the rudimentary laws of mechanics of materials, viz. theories of

failure, if the stresses are less than the yield strength of the

material used in the particular part in the chassis, the design is

considered to be safe otherwise design iterations by changing

the thickness, geometry, material are carried out[26-28].

2.5 Modal Analysis with mass of fuel tank

To ensure that the design doesn’t fail due to resonance modal

analysis of the automobile chassis is carried out [29]. In this test

the mass of the fuel tank is modelled by using lumped mass and

then the modal analysis is carried out. The various modes, viz.

vertical bending mode, lateral bending mode, matchbox mode

being the frequent modes encountered are properly analysed. A

slight variation from the desired outcomes is corrected by design

iterations.

2.6 Modal Analysis without mass of fuel tank

The modal analysis of the chassis without lumping the mass of

the fuel tank is carried out and various modes derived therein

are studied.

2.7 Accelerated Durability Test

To ensure that the chassis withstands the forces due to springing

action of the suspension system due to cornering, braking and

acceleration are modelled and applied at the sleeve of the

suspension system, accelerated durability test is carried out. The

stresses after analysis are compared with the yield stresses for

the material. To reach the acceptance criteria design iterations

are carried out. Veloso et al. Have carried out a study on failure

and stress analysis of an automobile chassis wherein the

reaserchers observed that failure occurred at bumpers fixation

points of the vehicle suspension during durability tests.

Moreover cracks were observed that further led to the fracture

of the component [30]. Shoval et al. have developed a robot for

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guidance of automatic durability test vehicles wherein the gear,

speed and acceleration can be automatically changed and

monitored to match the global test protocols. This research

contribution has made the durability testing easier which

otherwise was very difficult for the drivers to perform [31].

2.8 Tow Hook Test

The tow hook is an attachment for either towing a trailer vehicle

or being towed by another vehicle. A tow hook being an

attachment to a chassis is analysed for various loading

conditions and magnitude. The chassis used in this test is a

trimmed chassis, a model of the chassis with a half cut section of

the chassis constrained at the section in all the three directions.

The thickness of the welding in case of tow hook test is of

primary importance. An optimized welding thickness and shape

helps to lower the stresses. Petracconi et al. performed studies

on fatigue crack nucleation in a rear tow hook of an automobile

using a computational methodology. Moreover, the fatigue

analysis was also performed using local material response,

measured during experimental tests. The researchers

performed various tests to simulate the actual conditions that

are prevalent in an automobile while towing [32]. Palma et al.

also carried out a similar study which was concerned with the

Fatigue behavior analysis of a rear tow hook pin of an

automobile. Moreover, the methodology that was developed is

based on experimentation [33].

Engine and Transmission Mount Test

Lumped mass of engine along with the transmission or the 3D

model of engine and the transmission is mounted on the chassis

and the engine and transmission mount brackets are subjected

to all the engine excitation forces. The purpose of analysis and

optimization for an engine mounting system is to find the

optimum design parameters such as angle of each principal

elastic axis of every mounting component, location of elastic

center and stiffness in various directions of each mounting

component, so as to maximize the vibration isolation effect [34-

37].

3. Conclusion

This paper discusses the role of computer aided designing in the

modern automobile industry and it provides a detailed

discussion on the various virtual tests being carried out in

automotive industry on an automotive chassis for the mitigation

of failures in an attempt to shorten the product development

cycles. Moreover the new product development cycle in vogue in

the automobile industry has also been discussed in this paper. It

also presents the various steps being followed to virtually

analyse the chassis before actual fabrication.

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