**3. Composite Leaf Spring (CLS) (Senthilkumar & Vijayarangan,2007)**

Applicability of CLS in automobiles is evaluated by considering the types of vehicles and different loading on them. Theoretical details of composite mono-leaf spring are reported (Ryan,1985; Richrad et al., 1990). In some designs, width is fixed and in each section the thickness is varied hyperbolically so that thickness is minimum at two edges and is maximum in the middle (Nickel,1986). Another design, in which width and thickness are fixed from eyes to middle of spring and towards the axle seat width decreases hyperbolically and thickness increases linearly, has been presented (Yu & Kim, 1988). In this design, curvature of spring and fiber misalignment in the width and thickness direction are neglected. A double tapered CLS has been designed and tested with optimizing its size for minimum weight(Rajendran & Vijayarangan,2002). A composite mono-leaf spring has also been designed and optimized with joint design(Mahmood & Davood,2003). The mono-leaf spring is not easily replaceable on its catastrophic failure. Hence, in this work, a composite multi leaf spring is designed and tested for its load carrying capacity, stiffness and fatigue life prediction using a more realistic situation.

#### **3.1 Material selection**

Material selected should be capable of storing more strain energy in leaf spring. Specific elastic strain energy can be written as:

Design, Manufacturing and Testing

achieved by using CLS (4.3 kg) in place of SLS (13.5 kg).

Table 4. Design parameters of composite leaf spring.

Fig. 1. Electro-hydraulic leaf spring test rig.

of Polymer Composite Multi-Leaf Spring for Light Passenger Automobiles - A Review 63

clamps. Also metal spring eyes are fixed at both the ends. CLS is tested with an electrohydraulic leaf spring test rig (Fig. 1). Four CLSs were manufactured and tested. The spring, which provided the lowest stiffness and highest stress values, has been considered for comparative purpose because it satisfies the fail-safe condition. The reason for the stiffness and stress variations may be due to variation in volume fraction obtained in the fabrication process or due to lack of complete curing. A reasonably good weight reduction (68.15%) is

> Thickness of each leaf, mm 5.5 Width of the each leaf ,mm 34 Thickness of the fiber, mm 0.2 Width of the fiber, mm 34 Thickness of the resin, mm 0.075 Width of the resin, mm 34 Thickness of single layer, mm 0.275 Number of layers 20

For a light passenger vehicle with a camber height of 175 mm, 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).The load is gradually increased to obtain the deflection of steel spring first untill it becomes comepletely flat. Then, for similar deflection in composite leaf spring, the loads are measured for composite leaf spring. From Fig.2, it is understood that the deflection increases linearly as load increases in both steel and composite leaf springs. For a full bump load of 3250 N, composite leaf spring deflects to 94 mm only while steel leaf spring deflects 175 mm. The FEM results of longitudinal stress and deflection of CLS are shown in Figs.3&4. During full bump load test, experimental stress measurement (Fig. 5) is carried out to verify the results of FEM analysis (Figs 3 & 4). Fig.3 shows the variation of stress in CLS along the length of the spring. Fig.4 shows the deflection of CLS at various points along the length. It is found that CLS developes the maximum stress of about 215 MPa and it deflectes about 60 mm. E-glass/epoxy composite

**Parameters Values** 

Load indicator

Load cell Leaf spring

$$\mathbf{S} = \frac{1}{2} \frac{\sigma\_t^2}{\rho \mathbf{E}} \tag{1}$$

where t is allowable stress, E is modulus of elasticity and is density. Based on specific strain energy of steel spring and some composite materials(Yu&Kim,1988), E-glass/epoxy is selected as spring material having the mechanical properties given in Table 3. This material is assumed to be linearly elastic and orthotropic.


Table 3. Mechanical properties of E-glass/epoxy(Springer & Kollar,2003).

#### **3.2 Layup selection**

Stored elastic strain energy in a leaf spring varies directly with the square of maximum allowable stress and inversely with the modulus of elasticity both in the longitudinal and transverse directions according to Eq. (1). Composite materials like E-glass/epoxy in the direction of fibers have good characteristics for storing strain energy. So, lay up is selected to be unidirectional along the longitudinal direction of spring. This also helped in fabricating process using filament-winding machine.

#### **3.3 Design and finite element analysis of composite leaf spring**

With the extensive use of laminated composite materials in almost all engineering fields, the optimal design of laminated composites has been an extensive subject of research in recent years. The dimensions of the composite leaf spring are taken as that of the conventional steel leaf spring. Each leaf of the composite leaf spring consists of 20 plies of thickness 0.275 mm each. The number of leaves is also the same for composite leaf spring. The design parameters selected are listed in Table 4.

A 3-D model of the leaf spring is used for the analysis in ANSYS 7.1, since the properties of the composite leaf spring vary with the directions of the ber. 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. The element allows up to 250 different material layers. To establish contact between the leaves, the interface elements CONTACT174 and TARGET170 are chosen.

Individual leaves are fabricated using a filament-winding 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. CLS is tested with an electrohydraulic leaf spring test rig (Fig. 1). Four CLSs were manufactured and tested. The spring, which provided the lowest stiffness and highest stress values, has been considered for comparative purpose because it satisfies the fail-safe condition. The reason for the stiffness and stress variations may be due to variation in volume fraction obtained in the fabrication process or due to lack of complete curing. A reasonably good weight reduction (68.15%) is achieved by using CLS (4.3 kg) in place of SLS (13.5 kg).


Table 4. Design parameters of composite leaf spring.

62 Materials Science and Technology

<sup>2</sup> 1 2 *t E* 

**Parameters Values** 

where t is allowable stress, E is modulus of elasticity and is density. Based on specific strain energy of steel spring and some composite materials(Yu&Kim,1988), E-glass/epoxy is selected as spring material having the mechanical properties given in Table 3. This material

> Modulus of elasticity, GPa E11, 38.6 Modulus of elasticity, GPa E22, 8.27 Modulus of shear, GPa G12, 4.14 Poisson ratio, vxy, 0.26 Tensile strength, MPa σt11, 1062 Tensile strength, MPa σt22, 31 Compressive strength, MPa σc11, 610 Compressive strength, MPa σc22, 118 Shear strength, MPa τ12, 71

Stored elastic strain energy in a leaf spring varies directly with the square of maximum allowable stress and inversely with the modulus of elasticity both in the longitudinal and transverse directions according to Eq. (1). Composite materials like E-glass/epoxy in the direction of fibers have good characteristics for storing strain energy. So, lay up is selected to be unidirectional along the longitudinal direction of spring. This also helped in fabricating

With the extensive use of laminated composite materials in almost all engineering fields, the optimal design of laminated composites has been an extensive subject of research in recent years. The dimensions of the composite leaf spring are taken as that of the conventional steel leaf spring. Each leaf of the composite leaf spring consists of 20 plies of thickness 0.275 mm each. The number of leaves is also the same for composite leaf spring. The design

A 3-D model of the leaf spring is used for the analysis in ANSYS 7.1, since the properties of the composite leaf spring vary with the directions of the ber. 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. The element allows up to 250 different material layers. To establish contact between the leaves, the interface

Individual leaves are fabricated using a filament-winding machine. A fiber volume fraction of 0.6 is used. All individual leaves are assembled together using a center bolt and four side

Table 3. Mechanical properties of E-glass/epoxy(Springer & Kollar,2003).

**3.3 Design and finite element analysis of composite leaf spring** 

(1)

S =

is assumed to be linearly elastic and orthotropic.

**3.2 Layup selection** 

process using filament-winding machine.

parameters selected are listed in Table 4.

elements CONTACT174 and TARGET170 are chosen.

Fig. 1. Electro-hydraulic leaf spring test rig.

For a light passenger vehicle with a camber height of 175 mm, 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).The load is gradually increased to obtain the deflection of steel spring first untill it becomes comepletely flat. Then, for similar deflection in composite leaf spring, the loads are measured for composite leaf spring. From Fig.2, it is understood that the deflection increases linearly as load increases in both steel and composite leaf springs. For a full bump load of 3250 N, composite leaf spring deflects to 94 mm only while steel leaf spring deflects 175 mm. The FEM results of longitudinal stress and deflection of CLS are shown in Figs.3&4. During full bump load test, experimental stress measurement (Fig. 5) is carried out to verify the results of FEM analysis (Figs 3 & 4). Fig.3 shows the variation of stress in CLS along the length of the spring. Fig.4 shows the deflection of CLS at various points along the length. It is found that CLS developes the maximum stress of about 215 MPa and it deflectes about 60 mm. E-glass/epoxy composite

Design, Manufacturing and Testing

Fig. 4. FEM results of deflection of composite spring.

Stress, MPa

Fig. 5. Variation of experimental stress of steel and composite springs.

Leaf spring is analyzed under transverse loading condition. The longitudinal compressive strength of composite used in this analysis is less than its longitudinal tensile strength. So failure criterion stress is longitudinal compressive stress. Maximum longitudinal compressive stress (Fig. 5) is 222 MPa for CLS. At a same loading, maximum stress developed in SLS is 680 MPa (Table 2, Fig. 5). When compared with stress developed in SLS, less stress (67.35%) is developed in CLS. Compressive strength of fiber glass/epoxy is 610

0 100 200 300 400 500 600 700

Spring Length, mm

Composite Leaf Spring Steel Leaf Spring

of Polymer Composite Multi-Leaf Spring for Light Passenger Automobiles - A Review 65

leaf spring has spring constants 34.57-53.59 N/mm. Thus, all the data of spring constants for CLSs are greater than the design value, 20 N/mm. The reason for increased stiffness is lower density of E-glass/epoxy composite combination.

Fig. 2. Load – deflection curves for steel and composite leaf springs.

Fig. 3. FEM results of longitudinal stress of composite spring.

leaf spring has spring constants 34.57-53.59 N/mm. Thus, all the data of spring constants for CLSs are greater than the design value, 20 N/mm. The reason for increased stiffness is lower

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Deflection ( x 10 mm)

Steel Leaf Spring Composite Leaf Spring

density of E-glass/epoxy composite combination.

Load ( x 10 N)

Fig. 2. Load – deflection curves for steel and composite leaf springs.

Fig. 3. FEM results of longitudinal stress of composite spring.

Fig. 4. FEM results of deflection of composite spring.

Fig. 5. Variation of experimental stress of steel and composite springs.

Leaf spring is analyzed under transverse loading condition. The longitudinal compressive strength of composite used in this analysis is less than its longitudinal tensile strength. So failure criterion stress is longitudinal compressive stress. Maximum longitudinal compressive stress (Fig. 5) is 222 MPa for CLS. At a same loading, maximum stress developed in SLS is 680 MPa (Table 2, Fig. 5). When compared with stress developed in SLS, less stress (67.35%) is developed in CLS. Compressive strength of fiber glass/epoxy is 610

Design, Manufacturing and Testing

Stress, MPa

of Polymer Composite Multi-Leaf Spring for Light Passenger Automobiles - A Review 67

Fig. 7. Variation of longitudinal stress of composite leaf spring.

**4.1 Fatigue life of Steel Leaf Spring (SLS)** 

**4.2 Fatigue life of Composite Leaf Spring (CLS)** 

This leads to high amplitude low frequency fatigue test.

**4. Fatigue analysis (Senthilkumar & Vijayarangan, 2007)** 

10,00,000 cycles (Fig. 8) by the procedure outlined in (SAE manual,1990).

Main factors that contribute to fatigue failures include number of load cycles experienced, range of stress and mean stress experienced in each load cycle and presence of local stress concentrations. Testing of leaf springs using the regular procedure consumes a lot of time. Hence (SAE manual,1990) suggests a procedure for accelerated tests, which give quick results, particularly for SLSs. As per the outlined procedure(SAE manual,1990 & Yasushi,1997), fatigue tests are conducted on SLSs and CLSs. Fatigue life(Yasushi,1997) is expressed as the number of deflection cycles a spring will withstand without failure (Fig. 8).

0 100 200 300 400 500 600 700

Experimental FEM

Spring Length, mm

Fatigue life calculation of SLS is given as follows: stroke available in fatigue testing machine, 0-200 mm; initial deflection of SLS, 100 mm; initial stress (measured by experiment), 420 MPa; final deflection of SLS (camber), 175 mm; maximum stress in the final position (measured by experiment), 805 MPa. Fatigue life cycles predicted for SLS is less than

A load is applied further from the static load to maximum load with the help of the electrohydraulic test rig, up to 3250 N, which is already obtained in static analysis. Test rig is set to operate for a deflection of 75 mm. This is the amplitude of loading cycle, which is very high. Frequency of load cycle is fixed at 33 mHz, as only 20 strokes/min is available in the test rig.

MPa and yielding stress of steel is 1175 MPa. So, factor of safety obtained in SLS is 1.73, while in CLS it is 2.75. Experimental deflection of CLS under full bump loading is 94 mm (Fig. 2), which is less than the maximum value (175 mm). It shows that CLS is stiffer (64.95 %) than SLS. Table 5 gives the results of analysis of CLS using experimental, analytical and finite element methods. The variation of longitudinal stress of SLS and CLS is also presented in Figs.6&7. Fig. 6 shows the variation of longitudinal stress of steel leaf spring in FEM and experimental analysis. There is about 8% higher stress value obtained in the FEM than experiments. This is due to the fact that the constraints given at the ends of leaf spring are stiffer than the actual stiffness due the cantilever configuration in FEM. There could also be deviation from the material properties used in FEM analysis.


Table 5. Stress analysis of composite leaf spring using experimental, analytical and FEM.

Fig. 6. Variation of longitudinal stress of steel leaf spring.

MPa and yielding stress of steel is 1175 MPa. So, factor of safety obtained in SLS is 1.73, while in CLS it is 2.75. Experimental deflection of CLS under full bump loading is 94 mm (Fig. 2), which is less than the maximum value (175 mm). It shows that CLS is stiffer (64.95 %) than SLS. Table 5 gives the results of analysis of CLS using experimental, analytical and finite element methods. The variation of longitudinal stress of SLS and CLS is also presented in Figs.6&7. Fig. 6 shows the variation of longitudinal stress of steel leaf spring in FEM and experimental analysis. There is about 8% higher stress value obtained in the FEM than experiments. This is due to the fact that the constraints given at the ends of leaf spring are stiffer than the actual stiffness due the cantilever configuration in FEM. There could also be

**Parameters Experiment Analytical FEM**  Load, N 3250 3250 3250 Maximum stress, MPa 222 310.82 215.46 Maximum deflection, mm 94 59.20 60.65 Maximum stiffness, N/mm 34.57 54.89 53.59

0 100 200 300 400 500 600 700

Spring Length, mm

FEM Experimental

Table 5. Stress analysis of composite leaf spring using experimental, analytical and FEM.

deviation from the material properties used in FEM analysis.

Fig. 6. Variation of longitudinal stress of steel leaf spring.

Stress, MPa

Fig. 7. Variation of longitudinal stress of composite leaf spring.
