In small overlap frontal impact, the vehicle’s outer edges, which aren’t well protected by the crush-zone structures, are forming the main load-carrying path. Tires and wheels as well as the suspension system are usually subjected to the impact force. The tires hit by the rigid barrier usually burst with the rupture of tire and wheel as well as lose their air seals by de-beading. The tire burst affects the kinematics and deformation of the vehicle’s suspension and consequently affects the vehicle’s crash performance.
Accounting for the pressure decrease due to tire burst is important for accurate prediction of the vehicle’s crash response. Therefore modeling the tire burst which is caused by material damage and/or de-beading due to loss of seal between tire and rim is key to replicate the small overlap crash event. This article presents the finite element modeling of tires in Abaqus to account for tire burst in crash simulations.
Tire Modeling
The first step is to create a two-dimensional axisymmetric tire inflation model. This includes the modeling of tire tread and side wall using axisymmetric solid elements; the modeling of tire belts and carcass with embedded axisymmetric surface elements; and the wheel with axisymmetric shell elements.
The second step is to generate a symmetric three-dimensional tire model by revolving the two-dimensional model at the end of the inflation analysis by 360 degrees. With the “FILE NAME” parameter of the *SYMMETRIC MODEL GENERATION option in Abaqus/Standard, a three dimensional model with the extension name .axi will be created. This file includes the node, element and section definitions.
Modeling Tire Burst
The key feature to replicate the tire burst is to model the two failure mechanisms: material ruptures in tire and wheel rim and de-beading. Therefore, the following modifications must be made to the three-dimensional tire model: The tire side wall and the wheel rim must be modeled as two separate parts to allow separation in order to account for the de-beading. The steel bead needs to be modeled so that there is enough resistance and friction to keep the tire intact when inflated with suggested tire pressure. Here, we model the tire bead using 3D beam elements.
Material properties of the tire tread and side wall must be able to capture the destructive damage of the rubber material upon impact thus creating openings on the tire allowing air to escape outside the tire and burst. Depending on the conditions, such as impact angle and impact speed, either mechanism may trigger the tire burst. Whichever of the two mechanisms—material rupture and de-beading happens first—will be followed by tire burst and rapid drop in the tire pressure and loss of support to the vehicle’s suspension system.
The Abaqus fluid cavity feature is used to model the internal pressure of the tire. A closed volume is defined by using surface elements that cover the inside of the tire and rim by sharing nodes with the internal layer of nodes of the tire and rim. The pressure of the tire is related to the closed volume. To account for the material failure, we use the hyperelastic material together with damage criteria for the rubber material properties of the tread and side wall. Upon material damage and element removal, the surface elements underlying the rubber material will be free to be pushed outside of the tire by pressure difference.
The fluid cavity volume increase quickly and the tire pressure drops as the volume increases. To account for the de-beading, the area that the tire is contacting with the rim needs to be modeled as follows: The ring of surface elements on the rim, where one is attached to the tire side wall, are re-meshed with a finer mesh, thus creating two layers of free nodes on the closed volume of the fluid cavity. Two rings of shell elements with negligible material stiffness are added, sharing nodes with the two rings of surface elements. Contact is defined between the two rings of shell elements and the rim in order to keep the free nodes in place when tire and rim are not separated.
The free nodes are offset to avoid being in the same location as the rim for better contact conditions. When de-beading happens, the free nodes will be free to go through the opening thus making the volume larger and depressurizing the tire. The red lines correspond to the surface elements with underlying shell elements that close the gap between the tire and the rim, thus making a closed volume for the fluid cavity.
Simulation Results
Vertical impact, lateral impact and a 45-degree impact were simulated. It was found that in the vertical impact, when the impactor contacts the tire on the tire tread, the rubber material damages where the rim also contacts the tire tread. The tire burst follows with rapid pressure drop. In the lateral impact when the impactor contacts the tire side wall, the side wall separates from the rim. The tire burst follows with rapid pressure drop. In the 45-degree impact, both material damage and de-beading happens, resulting in tire depressurization.
Material failure happened first at about 4 msec and de-beading follows at about 4.5 msec at the other side of the tire where the rim moves faster than the tire wall due to impact. The fluid cavity pressure curve is obtained from the simulation. It is shown that the pressure keeps increasing after material damage due to air escaping speed is still lower than the pressure increase due to impact. But after 0.5 msec the pressure starts to drop. The tire entirely deflates at about 5.5 msec. The bursting process takes a total of 1.5 msec from first appearance of material failure to zero pressure.
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