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Dynamic compression response and material characteristics of a foam EPDM seal and its effect on vehicle door closure.

It is widely recognized that the door closing event in a vehicle plays a crucial role in the customer's perception of overall vehicle quality. Customers have come to expect good door sealability without having to slam the door hard when closing it. Door closure has, therefore, become a major consideration in the design of a door sealing system.

There are usually complex components made of multiple profiles co-extruded together as a combination of low-density sponge bulb for the sealing function and a dense material which covers a metal carrier for the attachment either to the car door or the car body sheet metal. In the literature, the research on the seal is mainly focused on sound transmission (ref. 1) and closure sound quality (refs. 2 and 3), while the door closure dynamics have received very little attention.

There is, however, some research work carried out on the development of models of the damping effect of the air flow through the air pressure relief holes of the seal bulb when the door is closed (ref. 4). The rate at which the seal is compressed can affect the door closing effort. A door seal has multiple purposes, one of which is acting as a damper for the closing door while trying to balance the desire for low closing efforts. The seal must, therefore, be characterized correctly to provide adequate resistance for the management of the kinetic energy of the closing door.

Currently, the only available method for driving the seal design is the static CLD curve (compression load deflection). The CLD curve represents the stiffness of the seal and is usually given as the static force versus the displacement for 100 mm of seal. The CLD of the seal gives insight on the order of magnitude of the static force needed to compress the seal as a function of the door closure gap. This test is usually carried out at a velocity of 1.7 x [10.sup.-3] m/s. Ideally, the area under this curve, which represents an energy term, needs to be balanced against the kinetic energy of the closing door. However, the static curve net area is usually well below the kinetic energy of the door. The apparent low stiffness of the seal is due to the fact that the CLD is characterized at very low speed compared to the real speed of the door (static stiffness is well below the dynamic stiffness) (ref. 5). In fact, during a door closure event, the seal is dynamically compressed due to the impact of the door closing at high velocity. The velocity at the latch area is generally within the range of 0.6 to 1.5 m/s. The dynamic stiffness response of the seal is seen to be higher than the static stiffness. The rate at which the seal is compressed has a major effect on the CLD characteristics of the seal.

Cooper-Standard Automotive has been very active for some time now in trying to develop a reliable method capable of characterizing the force due to the nonlinear elastic compression load deflection behavior of the seal under real door closing conditions, including the effect of seal damping. The apparent higher stiffness is certainly due to a combination of the foam material's microstructure under large deformations (ref. 6) and the shape of the sealing bulb. The foam material is a viscoelastic material; its mechanical properties are strain, frequency and temperature dependent. However, in this investigation, only the effect of strain was accounted for by measuring the damping of the seal on a pendulum system under normal door closing conditions. A finite element analysis (FEA) technique was then developed to calculate the dynamic stiffness of the seal. In addition, an attempt was made to set up a case study on a production vehicle to quantify the seal stiffness contribution to door closing energy, and demonstrate the ability of the new CLD measurement method to capture the real stiffness.

Experimental section

Pendulum system

An impact pendulum (figure 1) was developed to carry out load deflection measurements under appropriate door and body-in-white sheet metal conditions (terminologies used to describe the body side and the door side sheet metal of the vehicle). The force-displacement data are acquired at impact while the seal is compressed using two force transducers mounted behind the seal sample and a displacement transducer mounted around the hinge axis of the pendulum. The objective of the test is to measure in a continuous manner the resistive force due to the compression of the seal under comparable door inertia and velocity.

The pendulum package with its acquisition system has been thoughtfully created with the emphasis on being able to replicate the entire door setting variables, such as gap conditions, door-in-white shapes, and equivalent mass and closing velocity. The final optimum package was arrived at after extensive experience was logged on the first prototype.

Different types &fixtures can be mounted to accommodate any seal design. Figure 2 shows the positions of the force and displacement transducers. The transducers are connected to an acquisition hardware system driven by in-house developed software for measuring load deflection. Since the compression occurs in less than 0.03 s, the sampling rate can be as high as 5 KHz, which would allow about 150 data points to be collected during the test.

The sectional shapes of the door-in-white at various sections around the door aperture are created as fixtures which are then utilized as the striker of the pendulum, and a typical example is shown in figure 3a. A simple method is also used to set up the vehicle door gap conditions on the pendulum (figure 3b). A section molded on the vehicle is used to set the pendulum striker to the correct gaps "H" and "W". The pendulum is designed with vertical and horizontal adjustments to be able to vary the interference between the seal and the striker, as well as the relative angle between these elements. A high resolution camera is used to observe the seal under deflection when the striker is released.

Vehicle measurements

Figure 4 shows the standard test configuration used on the vehicle for quantifying the individual contributions to door closing energy (sealing, mechanical and air pressure contributions). The actual closing velocity is measured when the door just latches without any residual energy. The minimum closing energy is then calculated from the measurements using equation 1 :

[E.sub.c] [approximately equal to] 1/6 [m.sub.door] x [V.sup.2] (1)

Where:

[E.sub.c] is the minimum door closing energy (J)

[m.sub.door] is the mass of the door (kg)

V is the door closing velocity (m/s).

An appropriate number of test combinations is required to identify seal stiffness contributions to door closing energy. The door aperture is split into a number of sections. The measured energy data obtained using the pendulum are then compared to the vehicle data.

The door-in-white of each area is then defined and fixtured. Measurements are undertaken using the pendulum system under similar door set conditions (see "areas" in color in figure 4).

To identify and rate the contribution of each area and combination of areas around the door, the minimum door closure velocity is measured using the set-ups shown above. The measurements and the energy contributions are reported in table 1 and figure 5.

The results show good agreement, except for the area 1, which would indicate that the door set conditions were not captured properly. This could be the result of the door seal path curvature which distorts the seal position with respect to the door sealing surface.

Seal damping measurement

In seal design, most often a quasi-static approach using a nonlinear FE analysis is used which neglects the viscoelastic effects and the inertia effect resulting from the mass density. Here, a compression study was performed taking into consideration the damping properties of the seal to account for the dynamic stiffening during compression. A specific measurement procedure was developed to provide a perspective of how the damping might be evaluated to provide information for FEA.

The non-linear finite element program MSC-Marc from MSC Software allows the damping matrix D to be specified as Rayleigh damping and is equal to the following (equation 2):

[n.summation over (i=1)] {[alpha]iMi + ([beta]i + [gamma]i[DELTA]t/[pi])Ki} (2)

Where,

M is the mass matrix

K is the stiffness matrix

[DELTA]t is the time step

[alpha] is the mass damping coefficient

[beta] is the stiffness damping coefficient, and

[gamma] is the numerical damping coefficient.

This is a linear combination of the mass and the stiffness matrices, where [alpha] is the mass damping coefficient, [beta] is the stiffness damping coefficient and [gamma] is the numerical damping coefficient. The stiffness damping will be used to characterize the dissipation of the energy of the pendulum movement.

The set-up shown in figure 6 was developed and built with the aim of obtaining, with a single measurement, the required data for characterizing the damping. The set-up consists of a fixture on which the elastomer sample is mounted, which in turn is mounted on two force transducers capable of measuring the reaction force of the striker head. The material sample is obtained by cutting a strip of 19 mm section from the seal. It is placed as shown in figure 6a. The displacement of the pendulum head ([theta](t)) is evaluated as a function of time with a damping constant [lamda] using equation 3.

[theta](t) = [[theta].sub.0] x [e.sub.-[lambda].[omega].t] x cos ([square root of 1 - [[lambda].sup.2]] x [omega] x t + [phi]) (3)

The stiffness damping [beta] is given as a function of the natural frequency of the pendulum oscillations [omega] and the damping

constant:

[beta] = [lambda]/[omega] (4)

Different impact speeds can be tested by adjusting the angle of the pendulum. Results for an impact speed of 0.5 m/s are shown in figure 7. Table 2 gives the corresponding stiffness damping value of the damping constant.

Numerical implementation

The MSC-Marc non-linear finite element solver is used to model the dynamic compression of the car door seal to replicate the case study on the pendulum. This is a highly nonlinear problem with geometrical non-linearities due to large displacement and large strains, and material non-linearities due to non-linear stress-strain and viscoelastic behavior. The pendulum head is modeled as a rigid body with an equivalent mass derived from the inertia of the pendulum set-up. The seal is modeled via full integration Hex eight-node brick elements. The foam hyper-elastic material model is used to characterize the behavior of the seal material with material damping properties. The theory of large displacement dynamic transient analysis is implemented in the MSC-Marc (ref. 7). The modeling approach is a two phase approach.

Material model validation

Figure 8 displays a quasi-static analysis performed on a two-dimensional section of a door seal component assuming a plain strain condition to allow for model validation, to check that the material properties used in the hyper-elastic model are valid. The compression load deflection (CLD) of the component results from mounting the seal to a rigid body and applying a static displacement pre-load compression at the top by another rigid body.

The comparison of simulation and test results, given in table 3, shows a good agreement, which indicates that the hyper-elastic model used does describe the static compression behavior of the foam material correctly.

Damping parameters validation

The dynamic stretch experiments carried out on the sample strips of the material on the pendulum were simulated using the finite element method to validate the damping constants and ensure the consistency of the data.

The objective of the test is to stretch a strip (119 x 19 x 3 mm) of rubber under various strain rates and measure the resistive force of the strip. Figure 9 illustrates the test stretch set-up on the pendulum, and figure 10 shows the corresponding simulation. The tests were done at room temperature. Various stretch ratios of the samples were obtained by increasing the impact velocity from 0.5 m/s to 1.3 m/s to cover the range of the door closing speeds. Multiple samples are measured, typically three, with the average value plotted. Table 4 shows the derived values of the corresponding damping constants from the stretch tests. The results of the tests on the strip bands subjected to three impact velocities (0.5, 1.0 and 1.3 m/s) are then compared with those observed in simulation for the first cycle of the pendulum (figure 11). In table 5, a comparison of the dissipated energy for each velocity is given. This validation is a key element to figure out the seal compression energy. It can be considered that there is a fairly good correlation between the results.

Application to a door seal

During a door closing event, kinetic energy of the motion of the door is partly absorbed by the seal around the aperture of the door. A typical section of the seal under a pendulum dynamic load deflection test is shown in figure 12. The dynamic compression test using the pendulum was shown to give a good insight into vehicle door closing energy. As an essential step in the modeling approach, the compression test is simulated under the same testing conditions, taking the damping into consideration. Figure 12 displays the deformed shape of the seal at the nominal gap condition (15mm gap). The compression force results obtained for an initial velocity of 0.5 m/s are given in figure 13. The graph displays three cycles of the pendulum on the seal. The reaction force delay due to damping can also be observed.

The variation of the dissipated compression energy is given in table 6. It can be seen that the validity of the dynamic stiffness force and the energy dissipation due to the seal damping predicted through the simulation is proved. The results show a good correlation.

Conclusions

In this article, we have addressed the damping parameter influencing the compression response of a door seal.

The discussion of the presented testing and simulation results has shown that the whole developed methodology for the compression load deflection of a seal under a dynamic door condition is capable of predicting the dynamic compression response of the seal. Moreover, the required damping characteristics of the foam rubber of the seal could be characterized using a new pendulum testing method. This method provides a sufficiently accurate way of defining the seal material damping property.

We have shown that the impact velocity (deformation rate) has a large effect on the reaction of the seal; therefore, it is important to take into account the dynamic effects to evaluate the seal design performance.

The intent of this study was to provide a better and more realistic approach to testing and analysis when designing door seal systems for door closure performance.

References

(1.) A. Stenti, D. Moens, P. Sas and W. Desmet, "Development of a numerical modeling methodology for the NVH behavior of elastomeric line connections," K.U. Leuven, Department of Mechanical Engineering, Belgium.

(2.) A. Petniunas, N.C. Otto, S. Amman and R. Simpson, "Door system design for improved closure sound quality," Society of Automotive Engineers, 1999.

(3.) D. Hamilton, "Sound quality of impulsive noises: An applied study of automotive door closing sounds, " Society of Automotive Engineers, 1999.

(4.) Y. Gur and K.N. Morman, "Modeling the dissipative effect of seal air hole spacing and size on door closing effort, "Society of Automotive Engineers, 1997.

(5.) M. Oumohand, J.M. Veille, K. Louaisil and V. Guizouarn, "Automotive door closing efforts: Static versus dynamic compression load deflection measurements," Automotive Elastomers Conference, June 2005.

(6.) B. Wang, Z. Peng, Y. Zhang and Y. Zhang, "Compressive response and energy absorption of foam EPDM," published online in Wiley InterScience, May 2007.

(7.) Marc User's Manual, Volume A, Version 2011.1.0, MSC Software Corporation.

Mahmoud Oumohand, Cooper Standard France, and Moulay Sakaly, ENSIAME


Table 1--evaluated data

Configuration       1        2       3    1+2     1+3     2+3    1+2+3
V (m/s)          0.36      0.5    0.27    0.6    0.42    0.54     0.63
E (mJ)            521    1,143     228   1,711    762   1,357    1,901

Table 2--stiffness damping evaluation

Velocity      [lambda]    [omega]    [beta]

0.5 m/s        0.056       11.43    43.3[E.sup.-3]

Table 3--results correlation at nominal gap

                        Test--measured    FEA--prediction

F, [N/100 mm]                 4.1                4
(Nominal gap, 15 mm)

Table 4--data of damping constant

Velocity           [beta]
0.5 m/s      4.3 x [10.sup.-3]
1.0 m/s      3.6 x [10.sup.-3]
1.3 m/s      3.3 x [10.sup.-3]

Table 5--dissipated energy

Velocity (m/s)        Stretch tests      Simulated tests
                  Dissipated energy    Dissipated energy
                  (J x [10.sup.-3])    (J x [10.sup.-3])

1.3                            65.2                 67.1
1.0                            40.1                 41.5
0.5                            24.1                 25.1

Table 6--seal dissipated energy

               Energy (mJ)    Energy (mJ)
                      Test            FEA

1st impact            28.56          29.83
2nd impact            13.52          12.83
3rd impact             7.59            7.5
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Author:Oumohand, Mahmoud; Sakaly, Moulay
Publication:Rubber World
Date:Jan 1, 2013
Words:2855
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