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CHAPTER 3

STATIC ANALYSIS OF CHASSIS

3.1 INTRODUCTION

The chassis forms the backbone of the dump truck and its chief

function is to safely carry the maximum load, whether the vehicle is in static

or dynamic condition. There are some advantages of off-road vehicles chassis

over other chassis. It suffers from some major disadvantages like heavy

weight, fatigue failure and welding cracks. With the help of simulation tools

like finite element method, the extent of the effects of chassis input conditions

can easily be estimated with minimum experimental cost. As a test study, a

3-D finite element model was developed to predict the complete static stresses

and fatigue life of the high strength structural chassis, determine the stresses

on the chassis.

Main objective is to determine the static loads defined in pay load

and vehicle weight acting in the form of reaction forces was adopted for static

load condition. Another main load case is designed to verify the absence of

any risk of fatigue cracks occurring under the combined effect of the main

forces encountered during running condition. These load cases consist of

different load scenarios subjecting chassis involving running of uneven track

with 12% gradient, curve negotiation, rolling and bouncing effect and track

twist. The load combinations are mainly three cases like bending, torsion and

combined bending and torsion.

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The truck chassis is the main system of the vehicle and it is

integrated with the main truck component systems such as the axles,

suspension, power train, cab and body. The truck chassis has been loaded by

static, dynamic and also cyclic loading. Static loading comes from the weight

of payload and net vehicle weights. Individual system weights are acting on

the CG points of the chassis. The movement of truck affects a dynamic

loading to the chassis. The vibration of engines and the roughness of roads

give a cyclic loading. The existing truck chassis design is normally designed

based on static analysis. The emphasis of this design is chiefly on the strength

of structure to support the loading placed upon it. However, the truck chassis

is loaded by complex type of loads, including static, dynamic and fatigue

aspects. It is estimated that fatigue is responsible for 85 to 90% of all

structural failures (or) crack propagation on the chassis. The knowledge of

dynamic and fatigue behavior of truck chassis in such environment is thus

important to determine the stress and deflection on the mounting point of the

components like engine, suspension, transmission. Many researchers have

carried out studies on truck chassis to investigate stress analysis of a truck

chassis with riveted joints using FEM. Numerical results showed that stresses

on the side member can be reduced locally increasing the side members

thickness. If the thickness change is not possible, then increasing the

connection plate length may be a good alternative method. Industrial

experience FE-based fatigue life predictions were better results. The objective

of this research work is mainly focus to determine the fatigue life and static

analysis on the heavy duty truck chassis. Localized system analysis

techniques have been applied on the critical areas in order to find the more

reliable, more accurate and faster ways of simulation.

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3.2 LINEAR STATIC ANALYSIS OF CHASSIS

Determine the stress distribution on the chassis for both bending

and torsion load. According to the equation of equilibrium condition, the

vertical tire reaction forces act upward direction and body weight with

payload act on downward direction.

Figure 3.1 Dump Truck

Dump truck is mainly used for coal application. The chassis carries

50~60 ton range of payload as shown in Figure 3.1.

Static analysis calculates the effects of steady load conditions on a

structure, while ignoring inertia and damping effects, such as those caused by

time varying loads. A static analysis, however, includes steady inertia loads

such as gravity and rotational velocity, and time-varying loads that can be

approximated as static equivalent loads such as the static equivalent tire

reaction loads and vehicle weight plus payload.

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3.2.1 Computation of Chassis Design

The chassis forms the main structure of the dump truck and its chief

function is to safely carry the maximum load wherever the operation

demands. Basically, it must absorb engine and axle torque and absorb the

shock loads over twisting, pounding and uneven roadbeds when the vehicle

moves along the road.

The chassis consists of a box - section design, incorporating two

rear castings in high stress areas, with deep penetrating and continuous wrap

around welding. Mild steel is used throughout and it provides flexibility,

durability, and resistance to impact loads even in cold climates. Both castings

have a large radius with internal reinforcing ribs to dissipate stress. Castings

move welds to lower stress areas and provide two to three times the strength

of equivalent-sized fabricated structures (Topac et al 2009). The frame

provides for ease of serviceability. The box section frame allows simple

access to power train components. This open design reduces overall removal

and installation time, lowering repair costs. Generally transmission access is

excellent under raised and pinned bodes.

The suspension system is designed to dissipate haul road and

loading impacts for extended frame life. Four, independent, variable -

rebound, suspension cylinders absorb shocks. Rear cylinders allow axle

oscillation and absorb both the bending and twisting stresses. Front cylinders

which are mounted on the frame, serve as steering kingpins, thus providing

excellent maneuverability and reduced maintenance.

3.2.2 CAD Model of Chassis

A 3D model of main frame structure is modeled using Pro-E

software. It consists of bumper which is at the front side of the truck. The

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front rail structure is located next to the bumper on both LH and RH side.

Horse collar, which is the portion at the middle of the chassis, carries engine

and suspension system. Rear torque tube structure which is used to join left

and right side of the rail structure is also used to withstand the load while the

vehicle is in torsion and cornering cases. The body pad is fitted on the chassis

to carry payload. Finally, rear rail structure is at the end of the chassis mainly

used for mounting rear axle and suspension of the chassis. Totally 143 parts

are assembled as per the fabricated structure.

chassis is a main member of the vehicle, it is not a choice to use

fewer members in frame. The second way to decrease the cost of the frame is

using thinner sections for the profiles. There should be a limit for the

thickness. If the limit is exceeded, the truck chassis is no longer robust or

reliable. Initial CAD part of the truck chassis was modeled in Pro-E as shown

in Figure 3.2. Boundary conditions are applied on the CAD model.

Figure 3.2 Main frame chassis pro-E model

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3.2.3 Selection of Plates and Casting Materials

In the truck chassis, rail structures, horse collar consists of plates and rear tail or suspension mounting point consists of casting. Rail structure is a welded high strength structured steel box format. There are two side rail structure and two cross way torque tubes. The side rails and the cross torque tube are closely constructed welded by the upper and lower cover plates and the inner and outer side plates, with horizontal backup plates in the box. The upper and lower cover plates have thickness of 24 mm. The thickness of the inside and outside plates is 12 mm and the horizontal backup plates of 6 mm. The rear axle castings and plates are welding complete chassis. A high strength electrode (E 70) used for weld the complete chassis. In order to withstand heavy load and to prevent fatigue failure of the chassis, high strength structural steel (IS 1030) material as per the ASTM A 148 Gr.90-60 is used for plates and ASTM A148 Gr.150-125 is used for castings and high strength steel plates are used to fabricate chassis.

In the chassis manufacturing technology, especially steel and its alloys are used for the material of the frame geometry. For the frame models a wide variety of materials, composite materials and different kind of alloys can be used. In the present study the manufacturer doesnt want to change the type of material and the material property of the structural steel is given in Table 3.1.

Table 3.1 Material properties of the Truck Chassis

Material Properties

Young Modulus 200GPa

Poisson Ratio 0.33

Density 7827.08kg/m3

Symmetry Linear isotropic

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In the finite element analysis of the truck chassis, the linear

isotropic material model of high strength structural steel was used.

3.3 OVER ALL DIMENSIONS OF THE CHASSIS

All over length : 9090 mm

All over width : 4440 mm

All over height : 4570 mm

Wheel base : 4060 mm

Wheel tread : 3480 mm

Table 3.2 Weight of the Vehicle

Total Weight of the Vehicle And Payload

kg N

Pay load 54500 534645

NVW 43500 426735

Maximum GVW 98000 961380

3.3.1 Calculation of Bending and Torsion Case Load

Consider a simply supported beam

Figure 3.3 Calculations of Forces

GVW

Front Rear

2/3 WB 1/3 WBWB

F1V + F2V F3V + F4V

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FFT = F1V + F2V (3.1)

FRT = F3V + F4V (3.2)

where, WB = Wheel Base

FFT = Force at Front Tire

FRT = Force at Rear Tire

According to the equation of equilibrium condition:

H = 0, V = 0, M = 0

3.3.1.1 Simplified Free Body sketch

Figure 3.3a Simplified sketch

where, FFT = 4060 mm

2/3 WB = 2707 mm

1/3 WB = 1353 mm

Taking moment at front (F) i.e MF = 0

FRT 4060 = 961380 x2707

961380 N

F R

2/3 WB 1/3 WBWB

FRT

FRT = 640999 N

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Taking moment at rear(R) i.e MR = 0

FFT 4060 = 961380 x1353

3.3.1.2 Bending load acting on the frame

Front suspension load at each bolt: FFT = 320381/24

Rear suspension load at each bolt: FRT = 640999/4

3.3.1.3 Torsion Load acting on the frame

Load distribution of the tire for torsion load conditions

Right ramp loading

F1v = 0 N

FFT = F1v + F2v

FFT = F2v = 320381 N

Taking moment at a rear left tire point i.e. (M FFT = 0)

F4v RT = (GVW x RT/2) + {RT+ (FT-RT)/2 F2v}

Taking moment at a rear right tire point i.e (M F4v = 0)

FFT = 320381 N

FFT = 13349 N

FRT = 160250 N

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Left Ramp Loading

F2v = 0 N

F3v RT = (GVW x RT/2)- (F2v )x( FT-RT)/2 +RT

F RT = F3V + F4V

GVW = 961380 N

RT =2920 mm

FT = 3480 mm

F4V 2920 = (961380 x2920/2) +(320381) 2920 +(3480-2920)/2

F4V 2920 =1403414800 +1025219200

160250 = F3V + 831724 F3V = 160250 - 831724

3.4 PICTORIAL REPRESENTATION OF LOAD

Different types of loads act on the tire (FBD) as shown in

Figure 3.4. All forces like reaction force, traction force, accelerating force and

cornering force which represent by different colors and also acts on the tire.

F2v = 320381 N

F4v = 831724 N

F3v = -671474 N

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Figure 3.4 Pictorial Representation of Load Cases

3.4.1 Load Calculation of the Chassis

Calculation of different loads were calculated like bending and

torsion with 1G and 2G level, torsion left ramping and torsion right ramping

with 2G level loads acting on the chassis.

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Table 3.3 Load Calculation of the Chassis

MGVM = 98.000

kg Maximum Gross Machines Mass (i.e mass of vehicle itself plus payload)

MGVW = 961.380

N Maximum Gross Vehicle Weight (i.e weight of vehicle itself plus payload)

MGVW1/3 = 320.460

N 1/3 of the MGVW is distributed across the front axle of the vehicle

MGVW2/3 = 640.920

N 2/3 of the MGVW is distributed across the rear axle of the vehicle

VF = 160.230 N 1 g vertical reaction force at one (1) Front tire patch

Vg = 320.460 N 1g vertical reaction force at one (1) Rear tire patch

1g F1V= 1VF = 160,230

N F2V = 1 VF = 160,230

N F3T = 1 VR = 320,460

N F4V = 1 VR =320,460

N

2g F1V = 2 vF = 320,460

N F2V = 2 VF = 320,460

N F3T = 2 VR = 640,920

N F4V = 2 VR =640,920

N

Left Ramp

F1V = 0 = 0 N F2V = 2 VF = 320,460

N F3T = 0.5 VR= 160,230

N F4V = 1.5 VR= 480,690

N

Right Ramp

F1V = 2 VF =320,460

N F2V = 0 = 0 N F3T = 1.5 VR= 480,690

N F4V = 0.5 VR= 160,230

N

3.5 MESHING OF CHASSIS FE MODEL

The IGS file format of the model from Pro-E has been imported

into HYPER MESH software for geometry cleanup, followed by 2D and 3D

meshing. The complete meshed model is shown in Figure 3.5.

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3D element - tetra

2D element - trias

Total number of nodes = 81679

Total number of elements = 230104

Figure 3.5 Meshed model

The model is cleaned by removing surface penetration,

interference, edge cleanup failure surfaces and failure edges etc.

3.5.1 2D Meshing

The 2D mesh can be carried out to the entire chassis structure using

trias mesh to get accurate results. Front rail structure and horse collar portion

has been meshed both quad and trias (mixed mesh). The purpose of 2D mesh

has been considered for creating 3D four nodded tetra mesh. 3D mesh is a

mesh, which forms a matrix method to find out the displacement and stress

distribution of the chassis. Quality check like aspect ratio warpage, Jacobian,

skew, max and min angle has been done for the entire chassis.

3.5.2 Quality Check

The most important aspect of meshing is to check the quality

parameters of elements like Merge 2 or more elements to get a single element.

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To improve the quality of selected elements and to satisfy the quality criteria

Quality index Set criteria for all standard quality checks simultaneously

evaluate element quality Combine all checks into a single value called

composite quality index value Edit nodes and elements interactively or by

automatically maximizing element quality.

3.5.3 Tetra Meshing

Hyper Mesh provides two methods of generating a tetrahedral

element mesh. The volume tetra mesh works directly with surface or solid

geometry to automatically generate a tetrahedral mesh without further

interaction with the user. Even with complex geometry, this method can often

generate a high quality tetra mesh both quickly and easily. The standard tetra

mesh requires a surface mesh of trias or quad elements as input, which

provides us with a number of options to control the resulting tetrahedral mesh.

This offers a great deal of control over the tetrahedral mesh, and provides the

means to generate a tetrahedral mesh for even the most complex models. The

tetra mesh panel allows filling an enclosed volume with first or second order

tetrahedral elements. A chassis critical region is considered castings with

plate welded areas. Complete welded areas were bounded by a solid, shell and

spider elements.

3.6 LOAD CASES

It is necessary to use a detailed model of the structure for analyzing

of the vehicle chassis. It becomes even more necessary, when the center of

gravity of the vehicle is towards left or right of the central plane of the

vehicle. A lot of practical works has been done before finalizing the boundary

conditions and load cases.

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The finite element model shows the possibilities of fulfilling

requirements related to sufficient model description as well as efficiency. The

finite element model has been tested with an experimental result available for

some of the cases.

The same finite element model can be used for the fatigue analysis

of the chassis, if the adequate data are available on the repetitions of the loads

creating stresses on the chassis above the endurance limit of the chassis

material. The same model can also be used to analyze the lateral vibrations

responses.

It is impossible to cover all the conditions analyzing of the vehicle

on road conditions; however the above-mentioned boundary conditions can be

used as the starting point for analyzing stresses in the chassis.

3.6.1 Computation of Bending Load Case

Assumption: Liner static analysis is carried out for the chassis

structure of dump truck at zero inclination. Calculated reaction forces applied

at the front and rear suspension mounting points vertically upward direction.

Body weight and payload will act at the top of the body pad on the chassis. as

shown in Figure 3.6 free body diagram below.

3.6.2 Free Body Diagram of Bending Load

The load is assumed to be a point load acting on the CG of the

body. Details of loading are shown in Figure 3.6 the magnitude of force on

the CG point of the chassis 54500 N has been acting.

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Figure 3.6 Free body diagrams for bending load

3.6.3 Boundary conditions of bending load

Front axle tire reaction force is applied on the chassis front

suspension mounting point on both LH and RH sides. Rear axle tire reaction

force is acting on the rear suspension mounting point and body with pay load

weight in terms of force acting on the top of the chassis is shown in

Figure 3.7.

Figure 3.7 Bending Load on the chassis

Force

Force

Force

Force

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3.7 SOLVER AND OUTPUT OF BENDING LOAD

The location of bending load total displacement occurs at the front

bumper portion of the chassis. The numerical value of displacement is 7.02

mm as shown in Figure 3.8.

Figure 3.8 Displacement plot (Bending)

3.7.1 Torsion Load Analysis of Chassis

Assumption: Liner static analysis is carried out for the main frame

structure of dump truck at zero inclination. Torsion loading consists of, left

and right torsional racking conditions. Calculated reaction forces applied at

the left front (LF) and right rear (RR) suspension mounting points vertically

upward direction. Body weight and payload will be constrained at the top of

the body pad on the chassis. This is called torsional left racking condition.

shown in Figure 3.9.

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3.7.2 Free Body Diagram of Torsion Load

The load is assumed to be a point load acting on the CG of the

body. Details of loading as shown in Figure 3.9, the magnitude of force on the

CG point of the chassis 54500 N.

Figure 3.9 Free body diagrams for torsion load

Figure 3.10 Torsion Left Ramp Load

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Left right front (RF) and left rear (RR) suspension mounting points

act vertically downward direction. Body weight and payload will be

constrained at the top of the body pad on the chassis. This is called torsional

right racking conditions as shown in Figure 3.10.

3.7.3 Boundary conditions of torsion load

Diagonally vertical upward and vertical downward directional loads

acts on the chassis as shown in Figure 3.11.

Figure 3.11 Left and Right Ramp Load on the chassis

3.8 SOLVER AND OUTPUT OF TORSION LOAD

The location of torsion load total displacement occurred at the front

bumper portions of both LH and RH side of the chassis. The numerical value

of displacement is 9.07 mm as shown in Figure 3.12.

Force

Force

Force

Force

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Figure 3.12 Displacement plot (Torsion)

3.8.1 Stress Histories at Specified Location of Chassis

The results from the FE-analyses are maximum displacement and

von misses stress in the form of contour plots. It can be noticed that the color

bar to the right in the figure, which indicate the stress level in the structure,

have different maximum values and interval steps of the scale for the

maximum bending and torsion load cases.

3.8.2 Von-misses Stress at Maximum

Bending and torsion load cases have been evaluated and the results

indicate that the vertical load case is the most critical area at the rear rail

structure of the chassis. The location of Maximum Von-misses stress

distribution occurred at the rear rail structure of the chassis, which is in

contact with the body, as shown in Figure 3.13. The stress magnitude is

noticed at critical point.

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Figure 3.13 Von-misses stresses Torsion

Vertical maximum load case gives the highest stress at the position

where the side member changes the cross section height. Two of the major

cross members take much of the load from the vehicle and reaches stresses up

to 304 MPa in the rear rail structure. Some of the minor reaches stresses up to

approx 222.6 MPa, but many of them are low stressed. We also see that the

rear part of the side member is less stressed, near the horse collar and the rear

rail structure exhibits more stress on the chassis.

Figure 3.13 Von-misses stress shows maximum stress on the both

the sides of the chassis. A Figure 3.14 maximum principle stress shows the

bottom rail structure of the chassis, it can be noted that the stress value is with

in the standard and recommended limit.

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Figure 3.14 Maximum Principal Stress (Combined bending and torsion)

3.9 RESULTS AND DISCUSSION

In this work, modeled chassis used in a heavy vehicle using Pro-E.

A structural analysis was done on the chassis using Hyper Mesh and ANSYS.

Analysis is done using three materials high strength structural steel has been

used. We have done bending and torsion load analysis using FE method.

In the usage of 16mm to 25 mm varying thickness sectioned chassis

is reliable at the same time heavy and expensive. For increasing the variable

thickness of the box type chassis profile, structural thickness optimizations

were performed for 12mm, 16mm, 24mm and 25mm. The total displacements

for the frame thickness of 7.02mm, In the static analyses of 50 ton load

carrying automotive chassis, thickness of 12mm (original model), 16mm and

24mm box type profile of chassis about 16t linear distributed load condition

were examined.

In finite element analysis the stress values can be more than these

values. The reason is the manufacturer requirements as a linear, elastic and

static analysis. In the static analysis, stress value is gets bigger without

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encountering any resistance. In addition the static torsion tests are processed

in a single vertical direction and axis. Reality loads can be affected different

angles. This situation makes the maximum. In the chassis analyses max. Von-

misses stresses for 12mm 24 mm is: 304 MPa and 16mm 25mm

222.6 MPa.

For further studies localized system analysis of loading conditions

can be evaluated as a more realistic determination in the next chapter.

3.10 CONCLUSION

From the study following results can be drawn:

The analyses are processed in the static and structural

conditions.

Used 6mm to 25mm varying thickness plate box type chassis.

It is heavy and expensive, more over stress value were standard

and recommended limit. 7.02 mm displacement results are

better.

Thickness of a 24mm truck chassis section profiles can transport

a more load with less stress.

The manufacturer gains much from their expensive chassis, by

the optimum analysis of thicknesses for the truck chassis.