Fatigue life of automotive rubber jounce bumper

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Fatigue life of automotive rubber jounce bumperTo cite this article R S Sidhu and Aidy Ali 2010 IOP Conf Ser Mater Sci Eng 11 012008

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Fatigue life of automotive rubber jounce bumper

R S Sidhu1 and Aidy Ali2 1Automotive Engineering Unit Institute of Advanced Technology University Putra Malaysia 43400 UPM Serdang Selangor Malaysia

2Department of Mechanical and Manufacturing Engineering Faculty of Engineering Universiti Putra Malaysia 43400 UPM Serdang Selangor Malaysia E-mail aidyengupmedumy Abstract It is evident that most rubber components in the automotive industry are subjected to repetitive loading Vigorous research is needed towards improving the safety and reliability of the components The study was done on an automotive rubber jounce bumper with a rubber hardness of 60 IRHD The test was conducted in displacement-controlled environment under compressive load The existing models by Kim Harbour Woo and Li were adopted to predict the fatigue life The experimental results show strong similarities with the predicted models

1 Introduction Jounce bumper is a part of the McPherson strut assembly in chassis suspension system of light automotive The component is primarily made of rubber and is an active part of the suspension [1] Figure 1 shows a typical jounce bumper use in light vehicles Mars et al explained in his study that the crack nucleation method considers a material that has a natural life determined by the history of stresses or strains at a point This method is convenient because it is formulated in terms of stresses and strains and is commonly used by designers [2]The use of criterion to determine failure is essential in the experimental determination of fatigue life of a component Stiffness base approach is defined as the failure of a specimen at the point where the axial load amplitude reaches 85-80 of the respective amplitude for a specified reference cycle Kim and Woo et al defined failure as a 20 load drop while Harbour et al defined it at 15 [3-5]

Another important fact to take into consideration during the fatigue test is the presence of Mullinrsquos effect Mullin Effects can be described as an initial softening that occurs at the start of the fatigue test [6] It causes the jounce bumper to undergo an initial transient softening phase To eliminate the Mullinrsquos effect in this study the jounce bumper was cycle 30 times before counting the fatigue life

A range of fatigue models have been developed by researchers to predict the fatigue life of rubber [3-5 7-9] Fatigue models which are influenced by the use of strain include models by Harbour (Eq1) Kim (Eq2) Li (Eq3) and Woo (Eq4) The strain base models shown below are compared with the experimental results to determine how precise the fatigue models are in the case of predicting the fatigue life of jounce bumpers

Nf =77396(ε) -220 (1)

ε = 4989(Nf)-0115 (2)

Nf = 323072(ε) 6211 (3)

Nf = 495450(ε) -1324 (4)

9th National Symposium on Polymeric Materials (NSPM 2009) IOP PublishingIOP Conf Series Materials Science and Engineering 11 (2010) 012008 doi1010881757-899X111012008

ccopy 2010 Published under licence by IOP Publishing Ltd 1

Where ε is the strain and Nf is the fatigue life cycles

Figure 1 Jounce bumper

2 Jig Design The jig is made out of mild steel as shown in Figure 2 The jig is design to accommodate the loading condition where it allows the air to flow out while the jounce bumper is compressed The jig also mimics the piston rod where it keeps the jounce bumper in line and prevents slips

Figure 2 Jig

3 Experimental Procedures The Proton Saga P2-11A jounce bumpers were used as test samples in this study The jounce bumper consists of Natural rubber which contributes 90 of the buildup and Butadiene rubber 10 It has a hardness of 60 IRHD (International Rubber Hardness Degrees) Twelve sets of displacements were used for the fatigue compression test in an ambiance temperature of 20oC The fatigue tests were carried out using the Instron 8871 table model fatigue systems at a frequency of 2 Hz and a loading ratio of 0 Figure 3 shows the jounce bumper undergoing fatigue test


2

Figure 3 Jounce bumper undergoing fatigue testing

4 Results and Discussion The fatigue tests were conducted using various displacements Table 1 shows the fatigue life obtain in terms of number of cycles to failure The frequency was set at 2 Hz to eliminate the heat build-up which affects the fatigue life of the jounce bumper by further reducing it

Table 1 Jounce Bumper fatigue life

From the results in Table 1 a strain versus fatigue life graph (ε-N) is plotted to view the significant

difference of fatigue life at high strains compared to lower strains The fatigue curve shown in Figure 4 can be categorized into three zones Zone one has the highest strain range between 088 - 114 mmmm Strains in this zone should be avoided at all cost as it yields very low fatigue life Jounce bumper undergoing strains in zone two (053 - 079 mmmm) will have a much longer fatigue life However a regular replacement is necessary since it would not last for more than a 100000 cycles Zone three is the safest zone with strains below 043 mmmm Strain values in this zone have fatigue life ranging from 675000 cycles and goes above 15 million cycles

Displacement (mm) Strain No of Cycles 5 175E-01 gt1000000

75 263E-01 gt1000000 10 351E-01 gt1000000

125 439E-01 675000 15 526E-01 21000

175 614E-01 19000 20 702E-01 18000

225 789E-01 10000 25 877E-01 2000

275 965E-01 2000 30 105E+00 1000

325 114E+00 400


3

Figure 4 ε-N curve

Figure 5 shows the response of the jounce bumper towards the different forces acted upon it

Low displacement projects a normal curve on the graph however curves formed at higher displacements changes in shape and this phenomenon represents the viscoelsticity of the rubber jounce bumper

Figure 5 Jounce Bumper Response

Comparing the experimental fatigue life to the fatigue models can help verify how accurate the fatigue model is and determine whether the model can be applied to all elastomeric materials In this study four fatigue models based on strain parameter are compared with the experimental results

Zone 1 Zone 2 Zone 3


4

Figure 6 Experimental versus fatigue models

Figure 6 shows that the fatigue models have similar fatigue life at lower strains However at higher strains the experimental fatigue life drops dramatically This is due to the formation of cracks in the jounce bumper Severe cracks in on the jounce bumper as shown in Figure 7 causes the fatigue life to be reduce tremendously Furthermore Fatigue models are design primarly based on dogbone specimen testing Substituing components with complicated designd will not predict the right fatigue life due to its design complexicity Comparing the four models Harbourrsquos model was the closes to mimicking the fatigue life of the jounce bumper

Figure 7 Cracked jounce bumper

5 Conclusions The fatigue life of jounce bumper was successfully determined from the experiment conducted Higher displacement causes the jounce bumper to fail extremely fast However displacement below 15mm is considered save as the number of cycles exceeds 500000 The fatigue life via modelling was similar at lower strains compared to larger strains


5

Acknowledgements The authors would like to thank Sharon Khaw Pei Chin and Shamsul Kamaruddin from the Malaysian Rubber Board for their advice and support

References [1] Harza S Nallasamy 2007 Jounce Bumper Optimization - FE Approach Abaqus India Regional

Users Meet pp 1-10 [2] Fatemi A Mars W 2002 A literature survey on fatigue analysis approach for rubber

International Journal of Fatigue 24 949-961 [3] Harbour R J Fatemi A Mars W 2008 Fatigue life analysis and prediction for NR and SBR

under variable amplitude and multi-axial loading conditions International Journal of Fatigue 30 1231-47

[4] Jeong H Y Kim J H 2005 A study on the material properties and fatigue life of natural rubber with different carbon blacks International Journal of Fatigue 27 263-272

[5] Kim W D Lee H J Kim J Y 2004 Fatigue life estimation of an engine rubber mount International Journal of Fatigue 26 553-560

[6] Saintier N Cailletaud G Piques R 2006 Multiaxial fatigue life prediction for natural rubber International Journal of Fatigue 28 530-539

[7] Li Q Zhao J C Zhao B 2009 Fatigue life prediction of a rubber mount based on test of material properties and finite element analysis Engineering Failure Analysis 16 2304-2310

[8] Wang B Lu H Kim G H 2002 A damage model for the fatigue life of elastomeric materials Mechanics of Materials 34 475-478

[9] Woo C S Kim W D Kwon J D 2008 A study on the material properties and fatigue life prediction of natural rubber component Material Science and Engineering A 483-484 376-381


6

Fatigue life of automotive rubber jounce bumper

R S Sidhu1 and Aidy Ali2 1Automotive Engineering Unit Institute of Advanced Technology University Putra Malaysia 43400 UPM Serdang Selangor Malaysia

2Department of Mechanical and Manufacturing Engineering Faculty of Engineering Universiti Putra Malaysia 43400 UPM Serdang Selangor Malaysia E-mail aidyengupmedumy Abstract It is evident that most rubber components in the automotive industry are subjected to repetitive loading Vigorous research is needed towards improving the safety and reliability of the components The study was done on an automotive rubber jounce bumper with a rubber hardness of 60 IRHD The test was conducted in displacement-controlled environment under compressive load The existing models by Kim Harbour Woo and Li were adopted to predict the fatigue life The experimental results show strong similarities with the predicted models

1 Introduction Jounce bumper is a part of the McPherson strut assembly in chassis suspension system of light automotive The component is primarily made of rubber and is an active part of the suspension [1] Figure 1 shows a typical jounce bumper use in light vehicles Mars et al explained in his study that the crack nucleation method considers a material that has a natural life determined by the history of stresses or strains at a point This method is convenient because it is formulated in terms of stresses and strains and is commonly used by designers [2]The use of criterion to determine failure is essential in the experimental determination of fatigue life of a component Stiffness base approach is defined as the failure of a specimen at the point where the axial load amplitude reaches 85-80 of the respective amplitude for a specified reference cycle Kim and Woo et al defined failure as a 20 load drop while Harbour et al defined it at 15 [3-5]

Another important fact to take into consideration during the fatigue test is the presence of Mullinrsquos effect Mullin Effects can be described as an initial softening that occurs at the start of the fatigue test [6] It causes the jounce bumper to undergo an initial transient softening phase To eliminate the Mullinrsquos effect in this study the jounce bumper was cycle 30 times before counting the fatigue life

A range of fatigue models have been developed by researchers to predict the fatigue life of rubber [3-5 7-9] Fatigue models which are influenced by the use of strain include models by Harbour (Eq1) Kim (Eq2) Li (Eq3) and Woo (Eq4) The strain base models shown below are compared with the experimental results to determine how precise the fatigue models are in the case of predicting the fatigue life of jounce bumpers

Nf =77396(ε) -220 (1)

ε = 4989(Nf)-0115 (2)

Nf = 323072(ε) 6211 (3)

Nf = 495450(ε) -1324 (4)


ccopy 2010 Published under licence by IOP Publishing Ltd 1




Figure 2 Jig



2







75 263E-01 gt1000000 10 351E-01 gt1000000

125 439E-01 675000 15 526E-01 21000

175 614E-01 19000 20 702E-01 18000

225 789E-01 10000 25 877E-01 2000

275 965E-01 2000 30 105E+00 1000

325 114E+00 400


3

Figure 4 ε-N curve







4






5













6




Figure 2 Jig



2







75 263E-01 gt1000000 10 351E-01 gt1000000

125 439E-01 675000 15 526E-01 21000

175 614E-01 19000 20 702E-01 18000

225 789E-01 10000 25 877E-01 2000

275 965E-01 2000 30 105E+00 1000

325 114E+00 400


3

Figure 4 ε-N curve







4






5













6







75 263E-01 gt1000000 10 351E-01 gt1000000

125 439E-01 675000 15 526E-01 21000

175 614E-01 19000 20 702E-01 18000

225 789E-01 10000 25 877E-01 2000

275 965E-01 2000 30 105E+00 1000

325 114E+00 400


3

Figure 4 ε-N curve







4






5













6

Figure 4 ε-N curve







4






5













6






5













6













6

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Fatigue life of automotive rubber jounce bumper