Bumper Optimization

Jounce Bumper Optimization - FE ApproachNallasamy (FEA) & Sandip Hazra (Product Design)

Trelleborg Automotives

Abstract: This paper details the optimization of Jounce Bumper made of Microcellular polyurethane by Finite Element approach using ABAQUS. As we are demanded by the market to release our product with in a very short period of time to pick up the business, the conventional trial and error method in new product development will no more suit this rapidly changing environment. Therefore it is very important to ride on a well implemented analytical approach to win the race by accelerating our product development iterations without a physical product until the final prototype and hence a great saving in men, machine, material & time and thus the cost. The jounce bumper, that mostly accompany the McPherson strut in Automotive chassis suspension system is optimized by maintaining the given fitment dimensions and changing the free dimension & material properties, to get required bumper stiffness in iterative manner. By considering the geometric effects like height to diameter ratio, number of convolutes, their radius & placements on the overall performance of jounce bumper is explained by correlating the results to the laboratory test on the final prototypes. Quasi-static non-linear FEA with self contact is done to study the stiffness characteristics of this highly compressible Hyperelastic foam product.

Keywords: Jounce Bumper, Microcellular polyurethane, Hyperelastic foam, McPherson strut, stiffness, durability, polymer, self-contact.

1. Introduction

Jounce bumper is a part of strut (McPherson struts) assembly in chassis suspension system of light automotives. They are the foamy little progressive spring over the shock rod. In earlier cars, a rubber bumpstop was used to protect against metal-to-metal contact and now they are replaced by a supplemental, active foam spring, which are called micro-cellular progressive jounce bumpers. In recent days almost every car on the road has such Jounce bumper (JB) made of Microcellular polyurethane (MCU) in the front/rear struts.

Car manufacturers use this brilliant piece of high-tech engineering for many reasons. By using an almost zero-weight supplemental spring, they build the main (steel) spring with a reduced spring rate for a comfy ride and so their bumpstop have a specifically calculated spring rate and compressed height. This softer spring uses less weight and saving weight is the number one priority for a new car manufacturer.

2007 Abaqus India Regional Users’ Meet of 10

Figure 1. Location of Jounce bumper in the struts [2].

The foamy bumpstops are an active working part of the suspension. They work in harmony with the springs and the rest of the suspension components allowing the ride to be comfortable while cruising. When the car is in a handling situation, such as in a turn, they make the car feel firmer for better handling.

It works to make the suspension progressive, allowing a smooth transition to full compression. The compressed height is what limits the wheel travel and controls how far the wheel and tire move up into the fender wall. A Load Vs Deflection (L-d) curve for a Jounce Bumper (JB) can be divided into three regions, such as

1. Initial very soft entry region which will depend on the shape design of the JB and this region is responsible for rider feel by minimizing the harsh loading

2. The next gradual climbing middle portion is influenced by both the shape and density design of MCU and it is the decisive region with respect to vehicle handling and so the importance of JB lay over here.


3. Finally the very hard travel restriction path is sensitive to density of MCU and plays a major role in energy management

Figure 2. Importance of Jounce bumper in L-D curve [1]

2. MCU Material for Jounce Bumper (JB)

JB can be manufactured with wide range of MCU densities ranging from 0.3g/cc to 0.75g/cc this will decide the maximum jounce travel and thus responsible for handling performance of the suspension. With increase in density fatigue crack and lateral expansion will create problems in achieving overall performance and with lower density JB exhibits lower stiffness. So the optimum design will have a density range in between 0.45 to 0.55 g/cc

Many types of MCUs are available in the market based on the chemical formulation, but we use standard MDI formulation (Elastocell®M) for our production. Though the various grades have some differences in stiffness, yet the geometric design features play the most import role in product durability. For a particular MCU grade, the material chosen become irrelevant, if we don't consider the geometric design properly by taking care on both static load-deflection (L-d) plus fatigue factors.


Figure 3. Trelleborg sample MCU Jounce bumpers.

3. Geometric Design Parameters & their Effect on Performance

The following parameters have to be ensured for a durable JB Proper Folding pattern Height to diameter ratio Number of convolutes and its position Bend lip Seating cup and interference Effect of Retainer ring Shock Rod & interference Striker Cup Air escape channel and gate design

o Proper Folding Pattern:

The Jounce Bumper has to exhibit regular deformation under load with minimum lateral expansion for better durability. Figure-4 shows a properly designed bumper going for regular folding pattern with press fits. Maintaining the Height to diameter ratio will ensure the buckling pattern during compression and a conical shape/ draft angle will minimize the lateral expansion.

o Number of convolutes and its positions

Convolutes facilitate the jounce Bumper to fold uniformly by dividing the part into multiple segment with individual Height to Diameter ratio. Thus it is easy to control the folding pattern by adjusting the various dimensions of convolutes. There are some thumb rules to place and design convolutes. Figure-5 shown a typical Jounce Bumper design with External and internal convolutes


placed in proper position to have a good fatigue life. Total number of convolutes depends on the total height & density of the JB.

Figure 4. A typical JB folding pattern

o Bend lipThe lower end portion of JB is called Bend lip, its dimensions and shape are very sensitive

area in JB design. As this portion will hit the striker plate first and transfer the load. Some of the dimensions A1, A2, A3, A4, b1, b2, b3 & d1 shown in the Figure-5 & Figure-6 have certain level of values for which the Jounce Bumper performs best endurance characteristics.

Figure 5. Some design parameters


o Interference fit and retainer ring

Interference of bumper with seating cup and shock rod plays a vital role in rate build up and to increase the stiffness without compromising the fatigue life. To control the solid height of Jounce Bumper retainer rings are usually fitted on the outer surface of Jounce Bumper and also it restricts the lateral movement and increase axial stiffness. Thus it adds some more compaction during folding to increase the fatigue life. A retainer ring is generally made out of plastic material, which acts as a rate ring as well as a dirt shield.

Figure 6. Striker cup & Retainer ring

Many times small design changes will greatly guarantee the product life and mostly this geometric parameters lead to the conflict between load deflection character and fatigue life. Numerous times the needs for a durable design drive away the load-deflection requirement and vice versa. A typical fatigue cycle for jounce Bumper to operate is given below:

Maximum load @ 1Hz up to 2000 cycles

2/3 of Maximum Load @ 1.5 Hz up to 18000 cycles

1/6 th of Maximum Load @ 3 Hz up to 280000 cycles.

4. MCU Material behavior modeling in FEA

Foams are made up of polyhedral cells that pack in three dimensions. The foam cells can be either open or closed, common examples of elastomeric foam materials are cellular polymers such as cushions, padding, and packaging materials that utilize the excellent energy absorption properties of foams: the energy absorbed by foams is substantially greater than that absorbed by ordinary stiff elastic materials for a certain stress level.

Foams are commonly loaded in compression. There are three stages can be distinguished during compression:


1. At small strains (<5%) the foam deforms in a linear elastic manner due to cell wall bending.

2. The next stage is a plateau of deformation at almost constant stress, caused by the elastic buckling of the columns or plates that make up the cell edges or walls. In closed cells the enclosed gas pressure and membrane stretching increase the level and slope of the plateau.

3. Finally, a region of densification occurs, where the cell walls crush together, resulting in a rapid increase of compressive stress. Ultimate compressive nominal strains of 0.7 to 0.9 are typical.

Figure 7. Experimental material data’s.

Accurate hyperfoam modeling requires that the experimental data used to define the material parameters correspond to the dominant deformation modes of the problem being analyzed. For small strains (<5%) foams behave similarly for both compression and tension. However, at large strains the deformation mechanisms differ for compression (buckling and crushing) and tension (alignment and stretching). As compression dominates in JB the pertinent tests needed are:

1. Uniaxial compression2. Simple shear3. Planar compression (if Poisson's ratio ν ≠ 0)4. Volumetric compression (if Poisson's ratio ν ≠ 0)

As we have the lateral strain data to define the compressibility of the foam, which eliminates the need for a volumetric test to define the Poisson's ratio. Hence we use compression test data along with lateral strain and a simple shear for our FE analysis.


With these experimental data’s, Abaqus will calculate the material parameters µi, α i and β i in the elastomeric foam material model.

The elastic behavior of the foams is defined through the strain energy function as

U=

−−+−++∑

=

)1)((132 ^

3

^

2

^

11

2iiel

N

i i

i Ji

βα

βλλλ

αµ

ααα

The quality of this hyperfoam model behavior in Abaqus is assessed by doing single-element test cases to calculate the nominal stress–nominal strain response of the material model. Thus prediction of material behavior under different deformation modes are checked to judge the elastomeric foam constants, based on the correlation between the Abaqus predictions and the experimental data.

In order to account for the exponential growth of stiffness, we use second order Strain energy potential function (N=2), As with higher order functions the material behavior models at higher strains is strongly sensitive to the material parameters (µi, α i & νi), hence the stability of this material model is checked for different forms of loading.

5. FE Modeling:

JB geometry can be created by “solid of revolution method” and deformation (folding pattern) to axial compressive loading also follows symmetry about the axis of the part, Hence the classical 2-D Axisymmetric modeling approach is exploited to reduce the computational time (both FE modeling and solver) drastically.

Normally for Hyperelastic Elastomer analysis in Abaqus/Standard, it is advisable to use “hybrid” (mixed formulation) elements than pure displacement formulation elements. But since elastomeric foams are assumed to be highly compressible, the use of hybrid elements will not yield any advantage over the use of normal (purely displacement-based) elements. Hence to reduce the computational time normal continuum Axisymmetric solid elements CAX4 (4-node bilinear) in Abaqus/Standard and CAX4R (4-node bilinear, reduced integration with hourglass control elements) in Abaqus/Explicit were used.

Appropriate assumptions are made to meet the purpose JB design with out loosing the essence of underlying physics of MCU behaviors and they are

1. JB is Isotropic and Homogeneous (direction independent and void of blow holes/ air entrapments etc...)

2. Material is non-linear as per dominant deformation mode and capable of 90% compression, the required geometric nonlinearity are accounted during the analysis through non-linear options with friction activated for self contacts.


3. Temperature effects are eliminated due to quasi-static analysis and also material relaxation time is enough to neglect the visco-elastic nature of this polymer.

4. Modeled as 2D Axisymmetric problem, due to the existence of geometric, loading and boundary conditions symmetries with respect to JB axis.

6. Critical points while Discretization & their effects on Accuracy

Very smooth folding behavior without self pinching on compression can ensure a better quality JB. Hence the self-contact, Interference fit and other contacts modeling with friction will become a critical factor assuring the accuracy of FEA results during our virtual product development

Mesh are fine enough to capture all the features of the geometry, since small curves, minor angles and fillets are most critical design parameters in ensuring seamless deformations with uniform folding on the convolutes without damaging itself. Therefore mesh size and element shape are very important throughout the deformation. But MCU are highly compressible in nature, element failure may not be avoidable, but we can ensure this at critical points with small mesh modification. Although the severe maximum load that will be applied during Lab test can’t be easily reachable in Abaqus with implicit solver, yet we can go for higher load by using the explicit solver.

Figure 8. Jounce Bumper with full compression @ 7 KN load.


7. Reliability of FEMost times we are able to get a comparable FE static stiffness curves within 5% deviation

with that of test results on prototypes. As complete test load may not be simulated easily and accurately, the nearest feasible loadings are done to get quick computation. Thus lead time for new JB development is reduced considerably

Figure 9. FEA Vs Lab Test -Stiffness comparison

8. ConclusionIn Jounce Bumper, a smart geometric design can guarantees both load deflection

characteristics as well as fatigue life. As FEA can readily suit to solve these geometric issues by iterating the parameters to establish a better design before a physical prototype. Hence with the good agreement exists between FEA and lab tests, we can easily satisfy the customer driven parameters such as design space, load deflection characteristics and product life with sound engineering judgments to fetch the business.

9. References

1. A Primer on Jounce Bumper Design Using Microcellular Polyurethane, Daniel G.Dickson, SAE Technical paper Series,2004-01-1541, 2004

2. System Modeling of A Damper Module, Janson J.Tai and Todd A. Bishop, SAE Technical paper Series,2000-01-0727, 2000

3. ABAQUS Documentations.


Bumper Optimization

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