14-06-2013, 02:49 PM
Analytical and Experimental Studies on Fatigue Life Prediction of Steel
and Composite Multi-leaf Spring for Light Passenger Vehicles Using
Life Data Analysis
Analytical and Experimental.pdf (Size: 225.72 KB / Downloads: 96)
ABSTRACT
This paper describes static and fatigue analysis of steel leaf spring and composite multi leaf spring made up of glass fibre
reinforced polymer using life data analysis. The dimensions of an existing conventional steel leaf spring of a light
commercial vehicle are taken and are verified by design calculations. Static analysis of 2-D model of conventional leaf
spring is also performed using ANSYS 7.1 and compared with experimental results. Same dimensions of conventional
leaf spring are used to fabricate a composite multi leaf spring using E-glass/Epoxy unidirectional laminates. The load
carrying capacity, stiffness and weight of composite leaf spring are compared with that of steel leaf spring analytically
and experimentally. The design constraints are stresses and deflections. Finite element analysis with full bump load on
3-D model of composite multi leaf spring is done using ANSYS 7.1 and the analytical results are compared with
experimental results. Fatigue life of steel leaf spring and composite leaf is also predicted. Compared to steel spring, the
composite leaf spring is found to have 67.35 % lesser stress, 64.95 % higher stiffness and 126.98 % higher natural
frequency than that of existing steel leaf spring. A weight reduction of 68.15 % is also achieved by using composite leaf
spring. It is also concluded that fatigue life of composite is more than that of conventional steel leaf spring.
Keywords: composite multi leaf spring, E-glass/Epoxy, static analysis and fatigue life, ride comfort.
INTRODUCTION
In the present scenario, weight reduction has been the
main focus of automobile manufactures. The suspension
leaf spring is one of the potential items for weight
reduction in automobiles as it accounts for ten to twenty
percent of the unsprung weight, which is considered to be
the mass not supported by the leaf spring. The introduction
of composite materials made it possible to reduce the
weight of the leaf spring without any reduction on the load
carrying capacity and stiffness. Studies were conducted on
the application of composite structures for automobile
suspension system [1, 2]. A double tapered beam for
automotive suspension leaf spring has been designed and
optimized [3]. Composite mono leaf spring has also been
analyzed and optimized [4].
The leaf spring should absorb the vertical vibrations
and impacts due to road irregularities by means of
variations in the spring deflection so that the potential
energy is stored in spring as strain energy and then
released slowly. So, increasing the energy storage
capability of a leaf spring ensures a more compliant
suspension system. According to the studies made [3, 4], a
material with maximum strength and minimum modulus of
elasticity in the longitudinal direction is the most suitable
material for a leaf spring. Fortunately, composites have
these characteristics [5].
Finite element analysis of steel leaf spring
A stress analysis is performed using finite element
method (FEM). All the calculations are done using the
version 7.1 of ANSYS [8]. The finite element model for
the leaf spring is two-dimensional [9]. A plane strain
solution is obtained because of the high ratio of width to
thickness of a leaf. The model is restrained to the right half
part only because the spring is symmetric. The contact
between leaves is emulated by interface elements. Nodes
are created based on the values of co-ordinates calculated
and each pair of coincident nodes is joined by the interface
elements that simulate the action between the neighboring
leaves. The element that is selected for this analysis is
SOLID 42 [9], which behaves as the spring, and the
interface elements CONTA174 and TARGE170 are used
to represent contact and sliding between adjacent surfaces
of leaves. An average coefficient of friction 0.03 is taken
between surfaces [10].
Design and finite element analysis of
composite leaf spring
The dimensions of the composite leaf spring are taken
as that of the conventional steel leaf spring. The number of
leaves is also the same for composite leaf spring. The
design parameters selected are as follows: each composite
leaf consists of 20 layers; thickness of a single layer is
0.275 mm; width of each leaf, fiber and resin is kept at
34 mm; thickness of each leaf, fiber and resin are 5.5 mm,
0.2 mm and 0.075 mm respectively. Since the properties of
the composite leaf spring vary with the directions of the
fiber, a 3-D model of the leaf spring (Fig. 1) is used for the
analysis in ANSYS 7.1. The loading conditions are
assumed to be static. The element chosen is SOLID46,
which is a layered version of the 8-node structural solid
element to model layered thick shells or solids [8]. The
element allows up to 250 different material layers. To
establish contact between the leaves, the interface elements
CONTACT174 and TARGET170 are chosen.
STATIC TESTING AND RESULTS
The individual leaves are fabricated using a filamentwinding
machine. A fiber volume fraction of 0.6 is used.
All individual leaves are assembled together using a center
bolt and four side clamps. Also metal spring eyes are fixed
at both the ends.
After the fabrication, the composite multi leaf spring
was tested with the help of an electro-hydraulic leaf spring
test rig. Steel leaf spring weighs 13.5 kg whereas
composite leaf spring weighs only 4.3 kg. For a light
passenger vehicle with a camber height of 175 mm, the
static load to flatten the leaf spring is theoretically
estimated to be 3250 N. Therefore a static vertical force of
3250 N is applied to determine the load-deflection curves
(Fig. 2).
PASSENGER RIDE COMFORT
The leaf spring of light passenger vehicles has to be
designed in such a way that its natural frequency is
maintained to avoid resonance condition with respect to
road frequency to provide good ride comfort. The road
irregularities usually have the maximum frequency of
12 Hz [3]. Therefore the leaf spring should be designed to
have a natural frequency, which is away from 12 Hz to
avoid the resonance (poor ride comfort zone). The first
natural frequency of the steel leaf spring is found to be
6.3 Hz whereas that of composite leaf spring is 14.3 Hz. It
is found that the first natural frequency of composite leaf
spring is nearly 1.2 times the maximum road frequency
and therefore resonance will not occur. Therefore it is
obvious that composite leaf spring provides improved ride
comfort.