31-07-2012, 02:48 PM
Mono Composite Leaf Spring for Light Weight Vehicle – Design, End Joint Analysis and Testing
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INTRODUCTION∗
In order to conserve natural resources and economize
energy, weight reduction has been the main focus of
automobile manufacturer in the present scenario. Weight
reduction can be achieved primarily by the introduction of
better material, design optimization and better
manufacturing processes.
The suspension leaf spring is one of the potential items
for weight reduction in automobile as it accounts for ten to
twenty percent of the unsprung weight. This helps in
achieving the vehicle with improved riding qualities. It is
well known that springs, are designed to absorb and store
energy and then release it. Hence, the strain energy of the
material becomes a major factor in designing the springs.
DESIGNS AND FABRICATION OF
COMPOSITE MONO LEAF SPRING
Considering several types of vehicles that have leaf
springs and different loading on them, various kinds of
composite leaf spring have been developed. In multi-leaf
composite leaf spring, the interleaf spring friction plays a
spoil spot in damage tolerance. It has to be studied
carefully.
The following cross-sections of mono-leaf composite
leaf spring for manufacturing easiness are considered.
1. Constant thickness, constant width design.
2. Constant thickness, varying width design.
3. Varying width, varying thickness design.
In this paper, only a mono-leaf composite leaf spring
with varying width and varying thickness is designed and
manufactured. Computer algorithm using C-language has
been used for the design of constant cross-section leaf
spring. The results showed that a spring width decreases
hyperbolically and thickness increases linearly from the
spring eyes towards the axle seat. Fig. 2 shows flowchart
of computer algorithm for design of composite leaf spring.
The parameters of composite leaf spring are shown in
Table 2. The material properties of E-Glass/Epoxy [2] are
listed in Table 3.
Lay up Selection
The amount of elastic energy that can be stored by a
leaf spring varies directly with the square of maximum
allowable stress and inversely with the modulus of
elasticity both in the longitudinal direction. Composite
materials like the E-Glass/Epoxy in the direction of fibers
have good characteristics for storing strain energy. So, the
lay up is selected to be unidirectional along the
longitudinal direction of the spring. The unidirectional lay
up may weaken the spring at the mechanical joint area and
require strengthening the spring in this region.
Fabrication Process
The constant cross section design is selected due to its
capability for mass production, and to accommodate continuous
reinforcement of fibers and also it is quite suitable
for hand lay-up technique.
Many techniques can be suggested for the fabrication
of composite leaf spring from unidirectional GERP. Composite
leaf spring was fabricated using wet filamentwinding
technique [1]. In the present work, the hand lay-up
process was employed.
Testing of Steel and Composite
Mono Leaf Spring
The steel and composite leaf springs are tested by
using leaf spring test rig. The experimental set up is shown
in Figs. 4 (a), (b) and ©. The leaf springs are tested
following standard procedures recommended by SAE. The
spring to be tested is examined for any defects like cracks,
surface abnormalities, etc. The spring is loaded from zero
to the prescribed maximum deflection and back to zero.
The load is applied at the centre of spring; the vertical
deflection of the spring centre is recorded in the load
interval of 50 N.
RESULTS AND DISCUSSION
Experimental results from testing the leaf springs
under static loading containing the stresses and deflection
are listed in the Table 4. These results are also compared
with FEA in Table 4. Testing has been done for unidirectional
E-Glass/Epoxy mono composite leaf spring only.
Since the composite leaf spring is able to withstand the
static load, it is concluded that there is no objection from
strength point of view also, in the process of replacing the
conventional leaf spring by composite leaf spring. Since,
the composite spring is designed for same stiffness as that
of steel leaf spring, both the springs are considered to be
almost equal in vehicle stability. The major disadvantages
of composite leaf spring are chipping resistance. The
matrix material is likely to chip off when it is subjected to
a poor road environments (that is, if some stone hit the
composite leaf spring then it may produce chipping) which
may break some fibres in the lower portion of the spring.
This may result in a loss of capability to share flexural
stiffness. But this depends on the condition of the road.