leaf spring analysis

DESIGN AND ANALYSIS OF AUTOMOTIVE MULTI-LEAF SPRINGS USING

COMPOSITE MATERIALS

U. S. RAMAKANTH & K. SOWJANYA

Assistant Professors, SVP Engineering College, Andhra University, Vizag, Andhra Pradesh, India

ABSTRACT

Leaf springs are one of the oldest suspension components they are still frequently used, especially in commercial

vehicles. The automobile industries have shown interests in replacement of steel springs with composite leaf springs due to

high strength to weight ratio. This work is carried out on multi leaf springs having nine leaves used by a commercial

vehicle. A Finite element approach for analysis of a multi leaf springs using Ansys software is carried out. The model is

generated using solid works and imported in Ansys. The material of the leaf springs is 65Si7 (SUP9), composite leaf

springs and hybrid leaf springs. Fatigue analysis of leaf springs is carried out for steel leaf springs, and Static analysis for

steel leaf springs, composite leaf springs and hybrid leaf springs.

KEYWORDS: Leaf Springs, Steel Leaf Springs, Composite Leaf Springs, Hybrid Leaf Springs

INTRODUCTION

Increasing comptetion and innovations in automobile sector tends to modify the existing products or replacing old

products by new and advanced material products. Also to meet the needs of the natural resource conservation and energy

and economy, the automobile manufacturers have been attempting to reduce the weight of the vehicle in recent years [1]. A

suspension system of a vehicle is also an area where these innovations are carried out regularly. More efforts are taken in

order to increase the comfort of the user. Appropriate balance of comfort riding qualities and economy in the

manufacturing of leaf springs becomes an obvious necessity. Many past recorded data shows that steel leaf springs are

manufactured by EN45,EN45A,60Si7, EN47,and 50CrMoCV etc these materials are widely used for the manufacture of

the conventional multi leaf springs.

The introduction of the composite materials made it possible to reduce the weight of the leaf springs without any

reduction of load carrying capacity and stiffness. Studies were conducted on the application of the composite materials for

automobile suspension system (leaf springs) [2,3]. Compared to steel spring, the composite leaf spring is found to have

64.95% higher stiffness and 126.98% higher natural frequency than that of existing steel leaf springs [2].Multi leaf springs

used in the automotive vehicles normally consist of full length leaves and graduated length leaves. The specimen under this

research work consists of nine leaves. Finite element analysis using ANSYS software has been carried on conventional leaf

springs to determine the safe stress and pay loads and it is observed that inner side of eye sections [4].Composite mono leaf

springs were manufactured with integral eye and tested under static loading condition.

Also fatigue life prediction was done to ensure a reliable number of life cycles [5]. Leaf springs were modelled in

conventional way and simulated for the kinematic and dynamic comparatives [6]. An artificial genetics approach for the

design optimisation of the composite leaf springs were conducted [7]. Static testing and finite element analysis have been

conducted to obtain the characteristics of the spring. Load – deflection curves and strain measurement as a function of

loads for the three design tested have been plotted for the comparison with the FEA predicted values [8]. Fatigue strength

of shot peening leaf spring from laboratory samples of EN45 steel spring is calculated. A lot of research has been done to

International Journal of Mechanical

Production Engineering Research and

Development (IJMPERD)

ISSN 2249-6890

Vol. 3, Issue 1, Mar 2013, 155-162 © TJPRC Pvt. Ltd.

156 U. S. Ramakanth & K. Sowjanya

improve fatigue strength of material by creating compressive residual stress field in there surfaces through shot peening [9].

A double tapered beam for automotive suspension leaf springs has been designed and optimized [10].Demonstration of the

feasibility of using the program as a tool in establishing the initial design consideration s and in developing of preliminary

design was done [11]. Investigation of the fundamental properties of the dimensioning of the FRP leaf spring made from

GFRP to replace four steel springs [12]. The fatigue strength of 65Si7 spring steel has been evaluated experimentally as a

function of shot peening parameters for the application in automotive vehicles [13]. The main objective of this work is to

perform finite element analysis of multi leaf springs of three different combinations and to find the bending stresses, to

perform static analysis of the three combinations.

METHODS

Materials and Properties

• Material properties and Design parameters of steel leaf springs, composite leaf springs, hybrid leaf springs:

• Design parameters of the multi leaf spring used in this work are:

• Total span length (eye to eye): 1450mm

• Number of full length leaves: 02

• Length of full length leaves (L-1 and L-2): 1450 mm each

• Width of all leaves: 70mm

• Thickness of all leaves: 12mm

• Number of graduated length leaves: 07

• Length of graduated length leaves;

• (L-3, L-4, L-5, L-6, L-7, L-8 and L-9): 1320mm,

• 1140mm, 940mm, 800mm, 640mm, 464mm & 244mm respectively.

• The different parameters related to steel leaf springs are tabulated in the table1 below:

Table 1: Material Properties

Parameter Value

Material selected-steel 65Si7

Young’s modulus, E 2.1*105 N/mm2

Poisson’s Ratio 0.266

BHN 400-425

Ultimate Tensile Strength 460 Mpa

Tensile strength Yield 250 Mpa

Spring stiffness 221.5 N/mm2

Mass 58.758 kg

Normal static loading 35000N

Density 0.00000785Kg/mm3

Behaviour Isotropic

Material properties of E glass/ Epoxy (GFRP): The different parameters related to GFRP leaf springs are tabulated

in the table below:

Design and Analysis of Automotive Multi-Leaf Springs Using Composite Materials 157

Table 2: Material Properties

Parameter Value

Tensile modulus along X direction 34000 Mpa

Tensile modulus along Y direction 6530 Mpa

Tensile modulus along Z direction 6530 Mpa `

Tensile strength 900 Mpa

Compressive strength 450 Mpa

Shear modulus along XY direction 2433 Mpa

Shear modulus along YZ direction 1698 Mpa

Shear modulus along ZX direction 2433 Mpa

Poisons ratio along XY direction 0.217

Poisons ratio along YZ direction 0.366

Poisons ratio along ZX direction 0.217

Density of the material 2.6*103 kg/mm

3

Behaviour orthotropic

Material properties of Hybrid leaf springs: Hybrid leaf springs are just alternate layers of steel 65Si7 and Epoxy

GFRP.

Leaf springs are made out of flat plates. The advantage of leaf spring over helical spring is that the ends of the

spring may be guided along a definite path as it deflects to act as a structural member in addition to the energy absorbing

device. Thus leaf spring may carry lateral loads, brake torque, driving torque, in addition to the shocks. The ability to

absorb and store more amount of energy ensures the comfortable operation of a suspension system. However, the problem

of heavy weight of spring is still persistent.

This can be remedied by introducing composite material, in place of steel in the conventional leaf spring.

Research has indicated that the results of E-glass/Epoxy were found to have good characteristics for storing strain energy

[2], It is also proved through the further stages of this research. So a virtual model of the three combinations of the leaf

springs are modelled using Solid works 12 and static analysis is done by importing these models into Ansys 12 software

package.

Modelling

For the above given specification of the leaf spring, the static analysis is performed using ANSYS to find the

stresses and deflection. After modelling of the leaf spring with given specifications it is subjected to analysis. The Analysis

involves the following discretization called meshing, boundary conditions and loading. A virtual model of each leaf is

modelled separately, and then it is assembled together using solid works 12 (figure 1).

This model is then imported into Ansys 12 for conducting static and fatigue analysis. Same model is used for the

static analysis with three different materials steel 65Si7, epoxy GFRP, and hybrid leaf springs. These materials are selected

in the Ansys software by inserting the appropriate material properties and the analysis of the corresponding material is

done.

Meshing of the Leaf Spring Model

Meshing involves division of the entire of model into small pieces called elements. This is done by meshing. It is

convenient to select the free mesh because the leaf spring has sharp curves, so that shape of the object will not alter. To

mesh the leaf spring the element type must be decided first. Here, the element type is solid 72. Fine mesh is created with

16916 nodes and 2212 elements. After meshing is done the contacts and targets must be defined in between individual

leafs of the leaf spring (figure 2). The material properties of the given leaf spring are given in table 1 and table 2.


Applying Boundary Conditions

The leaf spring is mounted on the axle of the automobile; the frame of the vehicle is connected to the ends of the

leaf spring. The ends of the leaf spring are formed in the shape of an eye. The front eye of the leaf spring is coupled

directly with a pin to the frame so that the eye can rotate freely about the pin but no translation is occurred.

The rear eye of the spring is connected to the shackle which is a flexible link; the other end of the shackle is

connected to the frame of the vehicle. The nodes of rear eye of the leaf spring are constrained in all translational degrees of

freedom; Figure 3 shows the boundary conditions of the leaf spring. The load is applied along Fy direction as shown in

Figure 3. Same procedure is conducted for composite leaf springs and hybrid leaf springs.

Figure 1: Model of Leaf Springs Using Solid Works Figure 2: Meshed Model Using Ansys

Static Analysis

As the finite element analysis of multi leaf spring is performed using ANSYS-12 workbench. The multi leaf

spring for conventional steel showing deflection and bending stress under load is shown in figure-4 and Figue-5.From

Figure 4, it is obvious that maximum stress developed is at inner side of the eye sections i.e. the red colour indicates

maximum stress, because the constraints applied at the interior of the eyes. Since eyes are subjected to maximum stress,

care must be taken in eye design and fabrication and material selection.

The material must have good ductility, resilience and toughness to avoid sudden fracture. Thus factor of safety

must be increased near the eye. The same procedure is carried out for composite leaf springs, hybrid leaf springs, by

changing the material properties of the corresponding materials.


Figure 3: Boundary Conditions

Figure 4: Bending Stress (Von-Misses Stress) Figure 5: Total Deflection

Fatigue Analysis

The finite element model from earlier stage was applied with the SAE transmission loading to simulate the actual

road loading condition. In this simulation, the Goodman approach, Soderberg’s theory, Gerber’s theory and mean stress

curves has been used. The material that has been used for the simulation of the spring is 65Si7(SUP 9)steel, which is

normally used for the spring fabrication. Ultimate Tensile Strength of the material is 460 Mpa. The results obtained using

ANSYS 12 software. . The results are compared and the theory which gives the lowest value of life and the highest value is

chosen to be best in the analysis of steel leaf springs Loading data is chosen as history data of SAE transmission and is

applied.

Life data analysis: This is carried out by applying a load of 1000N and the analysis is carried out by the above

mentioned four approaches. Goodman’s and Gerber’s approach: Using the Gerber’s theory the life data obtained is similar

to the Goodman’s theory.

Mean stress theory approach: Using mean stress curves it is observed that there are less red coloured region (fig

11) compared to the Goodman’s and Gerber’s hence this theory is not preferred for the life data analysis of the leaf springs.


Soderberg’s approach: In this approach it is seen that red coloured region is greater (figure 13). It shows less life

compared to other approaches, hence it is most preferred in the analysis of the leaf springs.


RESULTS AND DISCUSSIONS

Static Analysis

A graph is plotted as shown in Figure 6 between Load versus Von-mises stress with Load on the X-Axis and Von-

mises stress on the Y-Axis. It is seen that from the graph that when load increases the bending stress increases linearly. So

load-stress graph gives the straight line relationship. Also it is seen that hybrid leaf springs has properties between the steel

leaf springs and composite leaf springs.

Table 3: Stress Analysis Using FEA Software

Load

(N)

Stress

65Si7 Steel

Leaf Springs

(Mpa)

GFRP Leaf

Springs

(Mpa)

Hybrid Leaf

Springs

(Mpa)

1000 9.242 8.401 8.812

2000 18.484 16.802 17.624

3000 27.726 25.203 26.436

4000 36.968 33.604 35.248

5000 46.21 42.005 44.06

Fatigue Analysis

Fatigue analysis is conducted with four approaches namely Goodman’s approach (fig8), Gerber’s approach, mean

stress approach (fig11) and Soderberg’s approach (fig 13), by applying a load of 1000N, with a loading condition from

history data – SAE Transmission. From the above said approaches it is seen that Soderberg’s approach shows a maximum

value of life 1.171*107 cycles which is represented in blue colour in the life data figure and the least value of life is shown

in red colour. And from the Soderberg’s approach we find that the red coloured region is greater it shows less life

compared to other approaches, hence it is most preferred in the analysis so the designer can increase the safety of the leaf

springs.

CONCLUSIONS

• Under the same static load conditions the stresses in leaf springs are found with great difference. Stresses in

composite leaf springs is found out to be less as compared to the conventional steel leaf springs, also a new

combination of steel and composite leaf springs (hybrid leaf springs) are given the same static loading and is

found to have values of stresses in between that of steel and composite leaf springs.

• Conventional 65Si7 (SUP9) leaf springs were found to weight about 58.757kgs, while the composite leaf springs

weighed only 19.461kgs, and the hybrid leaf springs weighed 41.14 kgs for the same specifications.

• The cost of the GFRP composite is very high when compared to conventional steel leaf springs, while the cost of

hybrid leaf springs may be lesser when compared to GFRP composite leaf springs.

• The fatigue analysis of the steel leaf springs are carried with four approaches, Soderberg’s approach is found out

to give better results for the analysis of life data for leaf springs.

REFERENCES

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2. Senthil Kumar, Sabapathy Vijayarangan,” Analytical and experimental studies on fatigue life prediction of steel

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material procedings of japan US conference, Tokyo 1981:pp 529-538

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5. Shiva Shankar and Vijayarangan,”Mono composite leaf springs for light weight vehicle design, End joint, analysis

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Structure 79 (2001) 1121-1129

8. Hou J.P Cherrualt J Y,”evolution of the eye-end design of a composite leaf spring for heavy axle loads,”

composite structure, 28,351-358(2006)

9. M.L.Agarwal, V.P.Agarwal,”A stress approach model fpr prediction of fatigue life by shot peening of EN45 steel

spring”,International Journal of Fatigue 28 (2006) 1845-1853.

10. Yu.W.J, Kinm.H.C,”Double Tapered FRP Beam from Automotive suspension leaf springs”, Composite structures

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