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Tight vehicle crash regulations and high quality and comfort standards continuously raise the bar in vehicle body and door design. The inclusion of additional crash safety structures, as well as the use of softer rubber elements, increases the loading of components when doors are being closed. Also changes made to trim packages for increased passenger comfort, such as integrating new acoustic isolation materials, automotive body electronics, and audio speakers, potentially impact the durability of doors and associated body components.
When these components are not properly designed for durability, intensive use of the doors may cause interference when they are opened or closed. Such occurrences may generate undesired noise and rust, and may even lead to malfunctioning of the door lock mechanism.
In order to avoid these problems and maintain reputation for extremely high quality, BMW performs tests on physical prototypes to guarantee the durability performance of door and body components. This test approach delivers reliable results, although a number of disadvantages are associated with it.
PHOTO: David Noels
Expensive prototypes are required to perform the verification tests, and the evaluation of numerous door slam events consumes a considerable amount of time. When durability problems are discovered, the design needs to be adapted, prototypes modified, and tests rerun—adding both time and cost to the vehicle development process.
When the door hits the closing hook
BMW's goal was to explore the possibility of evaluating the durability of door and body components in relation to door slam events by "electronic" means of virtual durability simulation. The durability test that was simulated consists of repeated door slam events with a predefined mixture of three different door-closing velocities. The simulations and physical tests involved were carried out on two different designs. The first design exhibited early failures in the metal sheets close to the reinforcement spot welds. In the second design, the reinforcement was revisited to obtain a more satisfactory load distribution.
When studying a door slam sequence in greater detail, three phases can be distinguished. First, the door contacts the rubber grommet on the body, which undergoes large deformations. The kinetic energy absorbed by the rubber is converted into heat and elastic energy. Then, the door hits the closing hook and is completely stopped. In the final phase, the previously stored elastic energy tries to swing back the door, but is stopped by the closed hook.
The precise capturing of the time-dependent contact conditions between the different components of the closing mechanism is essential in simulating the load transfer between the door and the frame. The body-door contact sequence brings about a variable deformation of the rubber grommet, due to the curbed movement of the door and the resulting modal oscillations of the door itself.
Advanced FE modeling and analysis
The finite-element (FE) model that was used to simulate the door slam sequences consists of the door, body frame, grommet, and lock. The basic FE mesh for the body frame components was taken from an existing full-vehicle model that was previously used for stiffness calculations. The engineers "cut out" the area around the front-left front door to reduce the number of elements, and refined the mesh density in the critical area around the lock hook, as seen here.
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From previous side-impact simulations, they retrieved a detailed model of the door (below) and realized the connection between the body frame and the door models by means of joint elements.
Altogether, the resulting model consisted of approximately 60,000 elements and 65,000 nodes. The rubber grommet was modeled by a single row of solid elements with a skin of contact shells on top. To take into account the non-linear behavior of the rubber, a low-density foam material model was used for the solids. The material parameters of the rubber have been calibrated by comparing calculated and experimental data from a simple test setup. The lock was modeled by using a mixture of solid and shell elements (see below). The surfaces involved in the sliding contact were modeled in detail to avoid unrealistic peak loads when the contacting component is sliding over the mesh edges. The plastic buffers, coating and bump stops were modeled as low-density foam. Prestressed spring elements were included as well as a damper element to account for the friction of the locking mechanism.
Instead of using simple node-to-node beam connections, the beams were connected to the sheet elements by so-called rigid spiders. This modification simplifies the placement of the beams, especially when dissimilar meshes need to be connected. The application of rigid spiders also improves the overall stiffness behavior of the model. The engineers validated the FE model by comparing simulation and experimental results, and this showed an acceptable level of correlation.
It became very clear that the force acting on the closing hook is significant. The validation showed a high sensitivity of the force histories with respect to the lateral positioning of the hook, the local stiffness around the lock mount, and the material properties of the rubber. The transient FE simulations for a single door slam for three different closing velocities and two different designs were performed using the explicit ANSYS LS-DYNA FE analysis package. In these simulations, the modal superposition technique accounted for the rigid-body motion of the door, the extensive deformations of the rubber sealing, and the modal oscillations of the door. In the simulations performed, global as well as Rayleigh damping were used.
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