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Evolution of the eye-end design of a composite leaf spring for heavy axle loads
Composite Structures 78 (2007) 351–358 www.elsevier.com/locate/compstruct
Evolution of the eye-end design of a composite leaf spring for heavy axle loads J.P. Hou a
a,*
, J.Y. Cherruault b, I. Nairne b, G. Jeronimidis a, R.M. Mayer
b
School of Construction Management and Engineering, The University of Reading, Engineering Building, P.O. Box 225, Reading RG6 6AY, UK b Sciotech, c/o Engineering Building, School of Construction Management and Engineering, The University of Reading, P.O. Box 225, Reading RG6 6AY, UK Available online 18 April 2006
Abstract This paper presents the design evolution process of a composite leaf spring for freight rail applications. Three designs of eye-end attachment for composite leaf springs are described. The material used is glass fibre reinforced polyester. Static testing and finite element analysis have been carried out to obtain the characteristics of the spring. Load–deflection curves and strain measurement as a function of load for the three designs tested have been plotted for comparison with FEA predicted values. The main concern associated with the first design is the delamination failure at the interface of the fibres that have passed around the eye and the spring body, even though the design can withstand 150 kN static proof load and one million cycles fatigue load. FEA results confirmed that there is a high interlaminar shear stress concentration in that region. The second design feature is an additional transverse bandage around the region prone to delamination. Delamination was contained but not completely prevented. The third design overcomes the problem by ending the fibres at the end of the eye section. Ó 2005 Published by Elsevier Ltd. Keywords: Composite material; Double leaf spring; Eye end; Static testing; Design evolution; Finite element simulation
1. Introduction The low density and high elastic strain of glass fibre reinforced composites (GFRC) provide them with high specific strain energy capacity [1]. Thus, GFRC gives a more compliant suspension system that offers a more comfortable ride and minimises damage to road or track. A 60% reduction in suspension weight can be obtained by replacing a steel spring with a composite spring of the same function. The fatigue life of composite springs is about five times that of steel springs and they have excellent corrosion resistance. All these advantages make the glass fibre reinforced composite leaf springs for transportation an excellent substitution for steel springs. The design and testing of the GFRC springs have received a great amount of interest *
Corresponding author. Tel.: +44 0118 378 5227; fax: +44 0118 931 3327. E-mail address: [email protected] (J.P. Hou). 0263-8223/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.compstruct.2005.10.008
[1–14]. However, the application of composite materials is still limited by the design of the shackle which connects the leaf spring to the wagon. This requires the use of a shackle pin whose geometry is specified by UIC. Fig. 1 shows different types of the eye-end joint summarised by Shokrieh and Rezaei [12]. Type (a) consists of a steel eye that can be bolted or pinned to the glass reinforced plastic (GRP) body of the spring. Although bolted or riveted eye ends are fairly simple to manufacture for prototypes, they are not normally recommended for volume production [1]. That is because fasteners are relatively expensive to produce and assemble. Stress concentrations introduced by drilling are another concern for this type of joint. In joint type (b) the eye end and spring are manufactured simultaneously from the same material. There is no stress concentration in this type. Reinforcement of composites at the junction of the eye and spring is necessary to avoid the delamination of unidirectional fibres. This joint
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Fig. 1. Different designs of joint for attaching the leaf spring to the vehicle body—from Shokrieh and Rezaei [12].
The composite double leaf spring system has been designed to replace existing multi-leaf metal springs on freight wagons using the existing UIC attachments. So that the connection of the suspension to the wagon is unchanged. The spring system consists of two leaves with different curvatures, which introduces a gap between them at the ends. A dual stiffness rate can be achieved with different stiffness values before and after the two leaves come into
contact with each other. This is to satisfy suspension requirement between tare and full load. The designed distance between the two eye centres is 1165 mm when the spring is under no load. The leaf spring has a constant width of 114 mm throughout, compared with 120 mm for steel. For both top and bottom leaves, the maximum thickness is at the centre, 52 and 60 mm, respectively, tapering towards the ends. The total mass of the two GRP leaves is 26 kg giving a total suspension mass of 38 kg, compared with 150 kg for the UK 11 leaf steel suspension. The top spring is connected through shackles to the wagon via integral eye ends. From Fig. 3, it can be seen that the centre of the spring is clamped within a steel buckle which locates in a recess on top of the axle box. The mass of the wagon and payload is directly applied at the ends of the top leaf through rubber bushes inserted in the eye ends. The reinforcing E-glass rovings (Vetrotex) are assembled using knitting machinery to form a unidirectional glass tape of constant width (Culzean Textile Solutions). This consists of 97% glass in the longitudinal direction and 3% in the transverse direction. The parabolic thickness is achieved by cutting suitable core layers to length. These are then heat set to produce a preform glass tape pack. The glass fibre volume fraction of the springs is about 48% [8]. The material properties of the unidirectional composites measured at the Institute of Polymer Mechanics in Latvia and Risø National Laboratory in Denmark are given in Table 1 [16,17].
Fig. 2. Eye end of the first design.
Fig. 3. Geometrical set-up of the double GRP leaf spring for static test on shackles.
configuration has the disadvantages of high cost and manufacturing complexity [12]. Joint types (c) and (d) have a conical or concave width profile so that steel eye fittings with the same conical or concave profile can be mounted easily and reliably together with rubber pads. In these joints there is no stress concentration due to drilling, but the cost of manufacture of the conical or concave parts of the spring has to be considered. The original eye end of the double leaf considered in this work was moulded simultaneously with the composite spring itself, as shown in Fig. 2 [15]. This design reduces the complexity and cost of the composite leaf spring. One problem with this design is the delamination which occurs at the overlap of layers coming back from the eye end which may initiate extensive delamination in the top leaf. The aim of this investigation is to improve the eye-end design to overcome the problem of the delamination failure. 2. Current design and existing problem
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353
Table 1 Material properties of GRP used E11 (GPa)
E22 (GPa)
E33 (GPa)
G13 (GPa)
m12
m31
S11 (MPa)
S22 (MPa)
S13 (MPa)
q (kg/m3)
38
13
13
3.2
0.31
0.05
>1000
16
60
1850
To design a track friendly suspension, it is essential to have low friction between the leaves and to introduce a reliable and consistent source of damping. Steel parabolic leaf springs are better than flat leaf springs because they only make contact at their end points. For the glass fibre suspension, a low friction nylon wear-end is fitted to each end of the lower leaf and a steel wear plate is attached to the upper (eye end) leaf via the shackle pin. Additional sources of damping include the material internal damping and an external rubber-damping element. 2.1. Assembly and static test set-up
2.2. Testing results of the first design
Fig. 4. Delamination at the interface of the fibres coming back from the eye end to the fibres beneath.
160 140 120
Total load (kN)
The GRP suspensions are assembled as follows. Channels shaped resilient elastomeric pads are glued to central sections of both leaves to prevent any direct contact between steel buckle and GRP. A high temperature thermal barrier is used to restrict the flow of heat from the axle box to the suspension in case of bearing failure. A welded buckle is used and the assembly is clamped in place via sliding wedges. The assembly is then locked by driving a securing screw through the buckle into the wedge plates. The clamping pressure holding the centre sections of the springs together is 10 N/mm2. The static test set-up is shown in Fig. 3. The eyes of the top spring are connected by shackle pins to the shackle which allows the spring to flatten under load by moving around a radius of 125 mm from the fixed shackle pin centre. The fixed shackle pin centres are 1325 mm apart. Strain gauges were placed on the top and bottom surfaces of the leaf springs along the fibre direction. Deflection is measured at the centre of the suspension where the load is applied.
100 80 60 40 Test FEA
20 0
In the static test, the Euroleaf suspension was loaded in 10 kN steps, firstly up to 100 kN and then up to 150 kN. Strain gauge readings, load and deflection values were recorded during the process. The GRP spring with the first eye-end design survived the requirement of 150 kN maximum load. However, the delamination shown in Fig. 4, started at comparatively low loads and propagated until the fibres coming back from around the eye separated completely with the body of the top leaf. The load–deflection curve for the static test of the first design is shown in Fig. 5. It can be seen that the knee point occurs at a load of approximately 35 kN. The knee point is adjustable by changing the thickness of the pad inserted between the two leaf springs at the contact region towards the ends. The load–deflection curve itself
0
20
40
60
80
100
120
Deflection at the buckle (mm)
Fig. 5. FEA predicted deflection in comparison with the measured values for the first design.
is very smooth, showing no abrupt drop of load. This implies that the spring stiffness is not influenced by the delamination shown in Fig. 4. The measured axial strain values are plotted in Fig. 6. The maximum strain from the top leaf spring measured from gauge 2 is just under 1.3%. The strain values measured for the bottom leaf are lower than that for the top leaf. This is desired to ensure a safe failure mode. The bottom leaf can still carry the load for some considerable distance after the top leaf has failed.
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Strain readings (microstrain)
16000 14000
gauge 2
12000
gauge 2 FEA gauge 4
10000
gauge 4 FEA Gauge 2
8000 6000 Gauge 4
4000 2000 0 0
20
40
60
80
100
120
140
160
Vertical load (kN)
Fig. 6. FEA predicted strain values from the tension sides of the two leaves at 100 mm from the central line in comparison with the measured values for the first design.
2.3. FEA analysis of the first design under static load Finite elements software MSC-Marc has been used to calculate the stresses in the GRP leaves and to predict the load–deflection curve. Plane strain 2D elements were used in the model. Half of the spring is modelled and symmetrical conditions about the x-axis are applied at the centre. Two pairs of contacts have been introduced. The first is the contact between the top and the bottom springs at the end. The other is the contact of the centre of the eye with the rigid arc centred at the pin point of the arm. With this, the centre of the eye is forced to move along an arc of 125 mm radius under vertical load.
The predicted load–deflection curve of the spring is plotted in Fig. 5. Reasonable agreement has been achieved between test and simulation. The predicted strain values at the position of strain gauges 2 and 4 are presented in Fig. 6, in comparison with the test data. Again, the model represented the leaf spring reasonably well. Plotted in Fig. 7 is the shear stress under 14 kN vertical load in the region of the fibres coming back from around the eye. There is a stress concentration at the tip of the fibre ends and the maximum ranges between 50 and 70 MPa. Test results from Risø National Laboratory in Denmark [17] show that the interlaminar shear strength is not higher than 60 MPa. This explains why the delamination shown in Fig. 4 happened after just 10% of the maximum vertical load was applied. The FEA predicted that the interlaminar shear stress at 150 kN vertical load is over 200 MPa. FEA also shows that there would be a high stress concentration in the fibre direction in the same region had the delamination not occurred. The next step of the work is to prevent delamination in the region of high shear stress. 3. Results of the design with transverse wrap The idea of the second design was to wrap the section where the high interlaminar shear stress occurs with a transverse bandage, as shown in Fig. 8. The material used for the wrapping was woven GRP. At the same time, the curvature of the top leaf spring is reduced to bring the knee point below 20 kN. The thickness of the leaf spring is kept the same.
Fig. 7. Interlaminar shear stress distribution under 14 kN vertical load.
J.P. Hou et al. / Composite Structures 78 (2007) 351–358
355 EL700_9
160 140
Load (kN)
120 100 80 60
test FEA
40 20 0 0
10
20
30
40
50
60
70
80
Deflection (mm)
Fig. 8. Wrap of the eye-end section.
The same static tests were carried out. It has been found that the wrap did not stop the delamination, and because of a bending stiffness mismatch [18] between the wrap and the spring surface, delamination occurs at the interface of the two materials, as shown in Fig. 9. The suspension was proof loaded up to 150 kN successfully. The delamination observed stopped at the thicker part of the transverse bandage and did not propagate to the eye region. Like the first design, there is no load drop during the process of load, as shown in Fig. 10. FEA results from 3D simulation predicted the load– deflection curve precisely. Plotted in Fig. 11 is the predicted interlaminar shear stress distribution in the same region as that shown in Fig. 7, under a vertical load of 150 kN. The maximum shear stress has dropped significantly but the
Fig. 10. Experimental and FEA predicted load–deflection curves for the second design.
level is still high enough to induce delamination in that region. A new approach was still required. 4. Results for the open eye end Results from the previous designs show that both the fibres coming back from around the eye and the transverse bandage introduce high interlaminar shear stresses at the interfaces with the leaf body. As suspensions of the first two designs survived over one million cycles fatigue load even with the delamination shown in Fig. 4, a third design was proposed to leave the eye-end open and thus avoid the local high interlaminar shear stress between the fibres coming from the eye end or the transverse wrap and the spring body, as shown in Fig. 12. As FEA simulations for the first two designs have been proven to represent the stiffness and stresses realistically, it was decided to use the FEA fully as a product development tool to overcome the problem of delamination. 4.1. FEA results of the third design
Fig. 9. Delamination of the wrapping layers under vertical load of 35 kN.
With the third design, FEA was carried out before the prototype was made to check that the suspension satisfied the stiffness and strength requirements. The load–deflection curves of the new design has been plotted in Fig. 13. The stiffness of the open eye-end spring is almost identical to that of the second design. The predicted strain levels along the fibre direction at the locations of strain gauges have been plotted in Fig. 14. It was noticed that the strain values in the fibre direction were lower than those of the first design. One reason for that is the knee point has moved from 35 to 15 kN. The interlaminar shear stresses close to the eye end under 150 kN load are plotted in Fig. 15. Because there is no interface between different materials, the highest value for the shear stress is about 23 MPa. These results suggest that the open eye-end design has stresses low enough to withstand the maximum shear stress, and no typical delamination should occur under the maximum load. It was then necessary to verify the design and FEA model with test results.
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Fig. 11. Interlaminar shear stress concentration at the interface of the layers from the eye end and body of the top leaf.
open eye 160 140
Vertical load (kN)
120 100 80 60 40 Test FEA
20 0 0
20
40
60
80
100
Deflection (mm)
Fig. 13. Predicted and measured load–deflection curves for the third design. Fig. 12. The design with the open eye end. 12000 gauge 1
9000
4.2. Testing results of the spring with open eye ends
gauge 2 gauge 1 FEA
Several springs with open eye ends have been made and tested. No eye movement relative to the axle was observed and the open section remained the same during the loading and unloading process. The spring with the open eye end was loaded statically up to 150 kN as required. The measured load–deflection curve for the new design is almost identical to that predicted by FEA, as shown in Fig. 13. Measured strain values shown in Fig. 14 are also in good agreement with the predicted values. There is no delamination or any kind of damage to the spring of the third design after static proof load.
Strain (microstrains)
6000
gauge 2 FEA
3000 0 0
20
40
60
80
100
120
140
160
-3000 -6000 -9000 -12000
Vertical load (kN)
Fig. 14. Comparison between predicted and measured strain values in the fibre direction. (Fig. 3 shows the positions of the strain gauges.)
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Fig. 15. Interlaminar shear stress distribution near the eye end predicted by FEA under a vertical load of 150 kN (eye to the left of picture).
5. Conclusions Three eye-end designs of a double GRP leaf suspension have been evaluated by finite element analysis and static and fatigue testing. The first two designs consisted of integral eye ends where the skin tape layers went around the eye and along the leaf body. These layers were then maintained in place via a transverse wrap using woven GRP tape. The third design consisted of open eye ends. FEA and static test results show that the stress concentration at the tip of the fibres coming back along the leaf body for the first two designs led to a local delamination. However, this did not have any effect on the static proof loading of the suspension nor on its fatigue life. The third eye-end design (open eye) showed that this option led to a reduction of shear stresses in the critical area and prevented the local delamination encountered with the first two designs. The open eye design survived the static proof loading and showed very good fatigue resistance and has been selected as the final design. Acknowledgements The authors would like to acknowledge that the work described here represents the collective endeavours of all the partners in the Eurobogie project (E!1841). This includes the materials suppliers and EM Fibreglass, who
moulded the leaf springs. Thanks for the UK Department of Trade and Industry for supporting the work of the UK partners. Our thanks also go to Mr. J.A. Frew and Mr. D.G. Keeley at the University of Reading for their invaluable technical support. References [1] Daugherty RL. Composite leaf springs in heavy truck applications. In: Composite materials. Proceedings of Japan–US conference, Tokyo, 1981. p. 529–38. [2] Kirkham BE, Sullivan LS, Bauerle RE. Development of the LiteflexTM suspension leaf spring. SAE Technical paper Series 820160, delivered at the International Congress and Exposition, Detroit, February 1982. [3] Tanabe K, Seino T, Kajio Y. Characteristics of carbon glass fiber reinforced plastic leaf spring, 1982. p. 1628–34 [SAE820403]. [4] Scowen GD. Transport allocations for fibre reinforced composites. IMechE C 1986;49/86:245–55. [5] Yu WJ, Kim HC. Double tapered FRP beam for automotive suspension leaf spring. Comp Struct 1988;9(4):279–300. [6] Harris LR. Composite leaf spring design. GKM technology report, August 1990. [7] Chaplin CR, Mayer RM, Rezakhanlou R. A new approach to composite leaf springs. Birmingham: Autotech; 1995. [8] Chianumba A, Jeronimidis G, Mayer RM. Advanced vehicle suspensions using glass reinforced plastics. In: Proceedings of the 6th European Congress on lightweight and small cars: the answer to future needs, Cernobbio, Italy; 1997. p. 549–58 [A2.11108]. [9] Al_Qureshi HA. Automobile leaf springs from composite materials. J Mater Proc Technol 2001;118:58–61.
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[10] Sancaktar E, Mathieu G. Design, analysis, and optimisation of composite leaf springs for light vehicle applications. Comp Struct 1999;44:195–204. [11] Rajendran I, Vijayarangan S. Optimal design of a composite leaf spring using genetic algorithms. Comput Struct 2001;79:1121–9. [12] Shokrieh MM, Rezaei D. Analysis and optimisation of a composite leaf spring. Comp Struct 2003;60:317–25. [13] Hou JP, Jeronimidis G. Static behaviour of composite leaf springs. Report no. EBG01, The University of Reading; 2002. [14] Hou JP, Jeronimidis G. Dynamic behaviour of composite leaf springs. Report no. EBG02, The University of Reading; 2002.
[15] Hou JP, Cherruault JY, Jeronimidis G, Mayer RM. Design, testing and simulation of fibre composite leaf spring for heavy axle loads. J Strain Anal 2005:497–504. [16] Valdamis V. Elastic properties of GFP for leaf spring FE analysis. IPM Eureka E!1841 Eurobogie project internal report; 2001. [17] Lystrup A, Mikkelsen LP. Interlaminar shear strength of glass fibre reinforced trailer spring. Risø National Laboratory report no. Risø-I1796(EN), December 2001. [18] Liu D. Impact-induced delamination—a view of bending stiffness mismatching. J Comp Mater 1988;22:674–92.
Evolution of the eye-end design of a composite leaf spring for heavy axle loads J.P. Hou a
a,*
, J.Y. Cherruault b, I. Nairne b, G. Jeronimidis a, R.M. Mayer
b
School of Construction Management and Engineering, The University of Reading, Engineering Building, P.O. Box 225, Reading RG6 6AY, UK b Sciotech, c/o Engineering Building, School of Construction Management and Engineering, The University of Reading, P.O. Box 225, Reading RG6 6AY, UK Available online 18 April 2006
Abstract This paper presents the design evolution process of a composite leaf spring for freight rail applications. Three designs of eye-end attachment for composite leaf springs are described. The material used is glass fibre reinforced polyester. Static testing and finite element analysis have been carried out to obtain the characteristics of the spring. Load–deflection curves and strain measurement as a function of load for the three designs tested have been plotted for comparison with FEA predicted values. The main concern associated with the first design is the delamination failure at the interface of the fibres that have passed around the eye and the spring body, even though the design can withstand 150 kN static proof load and one million cycles fatigue load. FEA results confirmed that there is a high interlaminar shear stress concentration in that region. The second design feature is an additional transverse bandage around the region prone to delamination. Delamination was contained but not completely prevented. The third design overcomes the problem by ending the fibres at the end of the eye section. Ó 2005 Published by Elsevier Ltd. Keywords: Composite material; Double leaf spring; Eye end; Static testing; Design evolution; Finite element simulation
1. Introduction The low density and high elastic strain of glass fibre reinforced composites (GFRC) provide them with high specific strain energy capacity [1]. Thus, GFRC gives a more compliant suspension system that offers a more comfortable ride and minimises damage to road or track. A 60% reduction in suspension weight can be obtained by replacing a steel spring with a composite spring of the same function. The fatigue life of composite springs is about five times that of steel springs and they have excellent corrosion resistance. All these advantages make the glass fibre reinforced composite leaf springs for transportation an excellent substitution for steel springs. The design and testing of the GFRC springs have received a great amount of interest *
Corresponding author. Tel.: +44 0118 378 5227; fax: +44 0118 931 3327. E-mail address: [email protected] (J.P. Hou). 0263-8223/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.compstruct.2005.10.008
[1–14]. However, the application of composite materials is still limited by the design of the shackle which connects the leaf spring to the wagon. This requires the use of a shackle pin whose geometry is specified by UIC. Fig. 1 shows different types of the eye-end joint summarised by Shokrieh and Rezaei [12]. Type (a) consists of a steel eye that can be bolted or pinned to the glass reinforced plastic (GRP) body of the spring. Although bolted or riveted eye ends are fairly simple to manufacture for prototypes, they are not normally recommended for volume production [1]. That is because fasteners are relatively expensive to produce and assemble. Stress concentrations introduced by drilling are another concern for this type of joint. In joint type (b) the eye end and spring are manufactured simultaneously from the same material. There is no stress concentration in this type. Reinforcement of composites at the junction of the eye and spring is necessary to avoid the delamination of unidirectional fibres. This joint
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Fig. 1. Different designs of joint for attaching the leaf spring to the vehicle body—from Shokrieh and Rezaei [12].
The composite double leaf spring system has been designed to replace existing multi-leaf metal springs on freight wagons using the existing UIC attachments. So that the connection of the suspension to the wagon is unchanged. The spring system consists of two leaves with different curvatures, which introduces a gap between them at the ends. A dual stiffness rate can be achieved with different stiffness values before and after the two leaves come into
contact with each other. This is to satisfy suspension requirement between tare and full load. The designed distance between the two eye centres is 1165 mm when the spring is under no load. The leaf spring has a constant width of 114 mm throughout, compared with 120 mm for steel. For both top and bottom leaves, the maximum thickness is at the centre, 52 and 60 mm, respectively, tapering towards the ends. The total mass of the two GRP leaves is 26 kg giving a total suspension mass of 38 kg, compared with 150 kg for the UK 11 leaf steel suspension. The top spring is connected through shackles to the wagon via integral eye ends. From Fig. 3, it can be seen that the centre of the spring is clamped within a steel buckle which locates in a recess on top of the axle box. The mass of the wagon and payload is directly applied at the ends of the top leaf through rubber bushes inserted in the eye ends. The reinforcing E-glass rovings (Vetrotex) are assembled using knitting machinery to form a unidirectional glass tape of constant width (Culzean Textile Solutions). This consists of 97% glass in the longitudinal direction and 3% in the transverse direction. The parabolic thickness is achieved by cutting suitable core layers to length. These are then heat set to produce a preform glass tape pack. The glass fibre volume fraction of the springs is about 48% [8]. The material properties of the unidirectional composites measured at the Institute of Polymer Mechanics in Latvia and Risø National Laboratory in Denmark are given in Table 1 [16,17].
Fig. 2. Eye end of the first design.
Fig. 3. Geometrical set-up of the double GRP leaf spring for static test on shackles.
configuration has the disadvantages of high cost and manufacturing complexity [12]. Joint types (c) and (d) have a conical or concave width profile so that steel eye fittings with the same conical or concave profile can be mounted easily and reliably together with rubber pads. In these joints there is no stress concentration due to drilling, but the cost of manufacture of the conical or concave parts of the spring has to be considered. The original eye end of the double leaf considered in this work was moulded simultaneously with the composite spring itself, as shown in Fig. 2 [15]. This design reduces the complexity and cost of the composite leaf spring. One problem with this design is the delamination which occurs at the overlap of layers coming back from the eye end which may initiate extensive delamination in the top leaf. The aim of this investigation is to improve the eye-end design to overcome the problem of the delamination failure. 2. Current design and existing problem
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353
Table 1 Material properties of GRP used E11 (GPa)
E22 (GPa)
E33 (GPa)
G13 (GPa)
m12
m31
S11 (MPa)
S22 (MPa)
S13 (MPa)
q (kg/m3)
38
13
13
3.2
0.31
0.05
>1000
16
60
1850
To design a track friendly suspension, it is essential to have low friction between the leaves and to introduce a reliable and consistent source of damping. Steel parabolic leaf springs are better than flat leaf springs because they only make contact at their end points. For the glass fibre suspension, a low friction nylon wear-end is fitted to each end of the lower leaf and a steel wear plate is attached to the upper (eye end) leaf via the shackle pin. Additional sources of damping include the material internal damping and an external rubber-damping element. 2.1. Assembly and static test set-up
2.2. Testing results of the first design
Fig. 4. Delamination at the interface of the fibres coming back from the eye end to the fibres beneath.
160 140 120
Total load (kN)
The GRP suspensions are assembled as follows. Channels shaped resilient elastomeric pads are glued to central sections of both leaves to prevent any direct contact between steel buckle and GRP. A high temperature thermal barrier is used to restrict the flow of heat from the axle box to the suspension in case of bearing failure. A welded buckle is used and the assembly is clamped in place via sliding wedges. The assembly is then locked by driving a securing screw through the buckle into the wedge plates. The clamping pressure holding the centre sections of the springs together is 10 N/mm2. The static test set-up is shown in Fig. 3. The eyes of the top spring are connected by shackle pins to the shackle which allows the spring to flatten under load by moving around a radius of 125 mm from the fixed shackle pin centre. The fixed shackle pin centres are 1325 mm apart. Strain gauges were placed on the top and bottom surfaces of the leaf springs along the fibre direction. Deflection is measured at the centre of the suspension where the load is applied.
100 80 60 40 Test FEA
20 0
In the static test, the Euroleaf suspension was loaded in 10 kN steps, firstly up to 100 kN and then up to 150 kN. Strain gauge readings, load and deflection values were recorded during the process. The GRP spring with the first eye-end design survived the requirement of 150 kN maximum load. However, the delamination shown in Fig. 4, started at comparatively low loads and propagated until the fibres coming back from around the eye separated completely with the body of the top leaf. The load–deflection curve for the static test of the first design is shown in Fig. 5. It can be seen that the knee point occurs at a load of approximately 35 kN. The knee point is adjustable by changing the thickness of the pad inserted between the two leaf springs at the contact region towards the ends. The load–deflection curve itself
0
20
40
60
80
100
120
Deflection at the buckle (mm)
Fig. 5. FEA predicted deflection in comparison with the measured values for the first design.
is very smooth, showing no abrupt drop of load. This implies that the spring stiffness is not influenced by the delamination shown in Fig. 4. The measured axial strain values are plotted in Fig. 6. The maximum strain from the top leaf spring measured from gauge 2 is just under 1.3%. The strain values measured for the bottom leaf are lower than that for the top leaf. This is desired to ensure a safe failure mode. The bottom leaf can still carry the load for some considerable distance after the top leaf has failed.
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Strain readings (microstrain)
16000 14000
gauge 2
12000
gauge 2 FEA gauge 4
10000
gauge 4 FEA Gauge 2
8000 6000 Gauge 4
4000 2000 0 0
20
40
60
80
100
120
140
160
Vertical load (kN)
Fig. 6. FEA predicted strain values from the tension sides of the two leaves at 100 mm from the central line in comparison with the measured values for the first design.
2.3. FEA analysis of the first design under static load Finite elements software MSC-Marc has been used to calculate the stresses in the GRP leaves and to predict the load–deflection curve. Plane strain 2D elements were used in the model. Half of the spring is modelled and symmetrical conditions about the x-axis are applied at the centre. Two pairs of contacts have been introduced. The first is the contact between the top and the bottom springs at the end. The other is the contact of the centre of the eye with the rigid arc centred at the pin point of the arm. With this, the centre of the eye is forced to move along an arc of 125 mm radius under vertical load.
The predicted load–deflection curve of the spring is plotted in Fig. 5. Reasonable agreement has been achieved between test and simulation. The predicted strain values at the position of strain gauges 2 and 4 are presented in Fig. 6, in comparison with the test data. Again, the model represented the leaf spring reasonably well. Plotted in Fig. 7 is the shear stress under 14 kN vertical load in the region of the fibres coming back from around the eye. There is a stress concentration at the tip of the fibre ends and the maximum ranges between 50 and 70 MPa. Test results from Risø National Laboratory in Denmark [17] show that the interlaminar shear strength is not higher than 60 MPa. This explains why the delamination shown in Fig. 4 happened after just 10% of the maximum vertical load was applied. The FEA predicted that the interlaminar shear stress at 150 kN vertical load is over 200 MPa. FEA also shows that there would be a high stress concentration in the fibre direction in the same region had the delamination not occurred. The next step of the work is to prevent delamination in the region of high shear stress. 3. Results of the design with transverse wrap The idea of the second design was to wrap the section where the high interlaminar shear stress occurs with a transverse bandage, as shown in Fig. 8. The material used for the wrapping was woven GRP. At the same time, the curvature of the top leaf spring is reduced to bring the knee point below 20 kN. The thickness of the leaf spring is kept the same.
Fig. 7. Interlaminar shear stress distribution under 14 kN vertical load.
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The same static tests were carried out. It has been found that the wrap did not stop the delamination, and because of a bending stiffness mismatch [18] between the wrap and the spring surface, delamination occurs at the interface of the two materials, as shown in Fig. 9. The suspension was proof loaded up to 150 kN successfully. The delamination observed stopped at the thicker part of the transverse bandage and did not propagate to the eye region. Like the first design, there is no load drop during the process of load, as shown in Fig. 10. FEA results from 3D simulation predicted the load– deflection curve precisely. Plotted in Fig. 11 is the predicted interlaminar shear stress distribution in the same region as that shown in Fig. 7, under a vertical load of 150 kN. The maximum shear stress has dropped significantly but the
Fig. 10. Experimental and FEA predicted load–deflection curves for the second design.
level is still high enough to induce delamination in that region. A new approach was still required. 4. Results for the open eye end Results from the previous designs show that both the fibres coming back from around the eye and the transverse bandage introduce high interlaminar shear stresses at the interfaces with the leaf body. As suspensions of the first two designs survived over one million cycles fatigue load even with the delamination shown in Fig. 4, a third design was proposed to leave the eye-end open and thus avoid the local high interlaminar shear stress between the fibres coming from the eye end or the transverse wrap and the spring body, as shown in Fig. 12. As FEA simulations for the first two designs have been proven to represent the stiffness and stresses realistically, it was decided to use the FEA fully as a product development tool to overcome the problem of delamination. 4.1. FEA results of the third design
Fig. 9. Delamination of the wrapping layers under vertical load of 35 kN.
With the third design, FEA was carried out before the prototype was made to check that the suspension satisfied the stiffness and strength requirements. The load–deflection curves of the new design has been plotted in Fig. 13. The stiffness of the open eye-end spring is almost identical to that of the second design. The predicted strain levels along the fibre direction at the locations of strain gauges have been plotted in Fig. 14. It was noticed that the strain values in the fibre direction were lower than those of the first design. One reason for that is the knee point has moved from 35 to 15 kN. The interlaminar shear stresses close to the eye end under 150 kN load are plotted in Fig. 15. Because there is no interface between different materials, the highest value for the shear stress is about 23 MPa. These results suggest that the open eye-end design has stresses low enough to withstand the maximum shear stress, and no typical delamination should occur under the maximum load. It was then necessary to verify the design and FEA model with test results.
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Fig. 11. Interlaminar shear stress concentration at the interface of the layers from the eye end and body of the top leaf.
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Fig. 13. Predicted and measured load–deflection curves for the third design. Fig. 12. The design with the open eye end. 12000 gauge 1
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Several springs with open eye ends have been made and tested. No eye movement relative to the axle was observed and the open section remained the same during the loading and unloading process. The spring with the open eye end was loaded statically up to 150 kN as required. The measured load–deflection curve for the new design is almost identical to that predicted by FEA, as shown in Fig. 13. Measured strain values shown in Fig. 14 are also in good agreement with the predicted values. There is no delamination or any kind of damage to the spring of the third design after static proof load.
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Fig. 14. Comparison between predicted and measured strain values in the fibre direction. (Fig. 3 shows the positions of the strain gauges.)
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Fig. 15. Interlaminar shear stress distribution near the eye end predicted by FEA under a vertical load of 150 kN (eye to the left of picture).
5. Conclusions Three eye-end designs of a double GRP leaf suspension have been evaluated by finite element analysis and static and fatigue testing. The first two designs consisted of integral eye ends where the skin tape layers went around the eye and along the leaf body. These layers were then maintained in place via a transverse wrap using woven GRP tape. The third design consisted of open eye ends. FEA and static test results show that the stress concentration at the tip of the fibres coming back along the leaf body for the first two designs led to a local delamination. However, this did not have any effect on the static proof loading of the suspension nor on its fatigue life. The third eye-end design (open eye) showed that this option led to a reduction of shear stresses in the critical area and prevented the local delamination encountered with the first two designs. The open eye design survived the static proof loading and showed very good fatigue resistance and has been selected as the final design. Acknowledgements The authors would like to acknowledge that the work described here represents the collective endeavours of all the partners in the Eurobogie project (E!1841). This includes the materials suppliers and EM Fibreglass, who
moulded the leaf springs. Thanks for the UK Department of Trade and Industry for supporting the work of the UK partners. Our thanks also go to Mr. J.A. Frew and Mr. D.G. Keeley at the University of Reading for their invaluable technical support. References [1] Daugherty RL. Composite leaf springs in heavy truck applications. In: Composite materials. Proceedings of Japan–US conference, Tokyo, 1981. p. 529–38. [2] Kirkham BE, Sullivan LS, Bauerle RE. Development of the LiteflexTM suspension leaf spring. SAE Technical paper Series 820160, delivered at the International Congress and Exposition, Detroit, February 1982. [3] Tanabe K, Seino T, Kajio Y. Characteristics of carbon glass fiber reinforced plastic leaf spring, 1982. p. 1628–34 [SAE820403]. [4] Scowen GD. Transport allocations for fibre reinforced composites. IMechE C 1986;49/86:245–55. [5] Yu WJ, Kim HC. Double tapered FRP beam for automotive suspension leaf spring. Comp Struct 1988;9(4):279–300. [6] Harris LR. Composite leaf spring design. GKM technology report, August 1990. [7] Chaplin CR, Mayer RM, Rezakhanlou R. A new approach to composite leaf springs. Birmingham: Autotech; 1995. [8] Chianumba A, Jeronimidis G, Mayer RM. Advanced vehicle suspensions using glass reinforced plastics. In: Proceedings of the 6th European Congress on lightweight and small cars: the answer to future needs, Cernobbio, Italy; 1997. p. 549–58 [A2.11108]. [9] Al_Qureshi HA. Automobile leaf springs from composite materials. J Mater Proc Technol 2001;118:58–61.
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[15] Hou JP, Cherruault JY, Jeronimidis G, Mayer RM. Design, testing and simulation of fibre composite leaf spring for heavy axle loads. J Strain Anal 2005:497–504. [16] Valdamis V. Elastic properties of GFP for leaf spring FE analysis. IPM Eureka E!1841 Eurobogie project internal report; 2001. [17] Lystrup A, Mikkelsen LP. Interlaminar shear strength of glass fibre reinforced trailer spring. Risø National Laboratory report no. Risø-I1796(EN), December 2001. [18] Liu D. Impact-induced delamination—a view of bending stiffness mismatching. J Comp Mater 1988;22:674–92.