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Aim: To analyze the front impact on the vehicle using HyperCrash and RADIOSS. Objective: To set up the pre-processed FEA model in HyperCrash to recommended parameters in property cards and engine files To analyze how the deformation takes place when the vehicle experience a frontal crash. To compare the forces…
Rakesh Mulgund
updated on 23 Mar 2021
Aim: To analyze the front impact on the vehicle using HyperCrash and RADIOSS.
Objective:
Theory:
Crashworthiness:
First used in the aerospace industry in the early 1950s, the term “crashworthiness” provided a measure of the ability of a structure and any of its components to protect the occupants in survivable crashes. Similarly, in the automotive industry, crashworthiness connotes a measure of the vehicle’s structural ability to plastically deform and yet maintain a sufficient survival space for its occupants in crashes involving reasonable deceleration loads. Restraint systems and occupant packaging can provide additional protection to reduce severe injuries and fatalities. Crashworthiness evaluation is ascertained by a combination of tests and analytical methods. One of such tests includes Frontal Crash.
Frontal Crash:
The front and the back of a vehicle absorbs impact, and a durable cabin helps protect passenger space, helping to reduce damage from a collision.
Impact Absorption and Load Dispersion in Frontal Crash is shown below:
In case of a frontal collision, the crumple zone at the front of the vehicle will effectively absorb the impact. The load dispersion is following 3 paths here, i.e., it is distributed in 3 different directions/paths:
(i) Bumper, side members, crash boxes
(ii) Upper members, A-pillar, beltline
(iii) Torque box, Locker
The components in three paths are deformable and can absorb the impact energy. Especially, the first path affords more than 50% of total crash energy in most frontal crashes. For this reason, the components in the first path are highly considered by engineers in vehicle crashworthiness design. The features and functions of these components are discussed as follow:
Bumper: The bumpers are usually reinforcement bars made of steel, aluminum, plastic, or composite material and can absorb crash energy to a certain extent. The main purpose of the bumper is to minimize the cost of repair after low-speed crashes. It can also benefit the protection of pedestrians.
Crash boxes: The crash boxes are generally thin-walled tubes with well-designed cross-section shapes and crumple points (e.g. ditches and crash beads). They may collapse in a particular pattern to absorb energy efficiently.
Side Members: The longitudinal beams are also thin-walled structure, but longer and stronger than crash boxes. The deformation modes of longitudinal beams include folding, tearing, and bending. Some reinforcing components may be used to strengthen the beams and optimize energy absorption.
Besides the deformable parts, some components in the vehicle frontal structure should be strong enough. In most crashworthiness studies, engine and firewall are generally considered as rigid bodies. Especially, the firewall refers to the rigid wall between the engine room and passenger cabin. If a vehicle crashes, the firewall can prevent the intrusion of the vehicle cabin and therefore ensure enough living space for driver and passenger.
Preparations of Car For Crash Analysis using HyperCrash
The unit system of the model is checked by opening the starter file in the notepad and it is as follows:
The unit system is in [Kg mm ms].
The software used for this simulation is a student version, so the complete FE model with a high volume of elements is not supported. To overcome this problem only part of the model i.e., the frontal structure of the car is used for simulation.
The model is loaded in the HyperCrash. The FE model of the Frontal structure is as follows:
The model is checked for connectivity before applying any recommended parameters as follows:
The result of the quality check for connectivity shows (image below), 2 free parts (spherical joints) that are not used in this simulation therefore they are deleted.
The model contact interface is checked and is found as below:
The contact interfaces available are not really useful for this particular simulation and the entire model should be in self contact. So the current contact interfaces were deleted and a new self contact interface was created with TYPE7, friction 0.2, and recommended parameters.
The model with the self contact interface TYPE7.
TYPE7 parameters:
Quality check for penetrations and intersection after the contact interface is applied.
Creation of RIGID WALL
The rigid wall is created with sliding friction of 0.1. The search distance is the area under which the slave nodes are captured for the impact. The search distance is 1900, the shaded box in the image below shows, the area covered in that distance and the yellow color represents slave nodes in that search distance.
The COG of the model is as shown below:
The red box shows the coordinates of the CG and the dot in the red circle shows the CG point of the model. It is not at the appropriate location of the CG of the vehicle. The exact location is shown with an arrow in the above image. To achieve that mass is added on the rear side of the model. The target mass is 700kg.
The mass added is shown below and the target mass is achieved:
The red circle shows the CG of the model at the right location and its coordinates are given in the red box.
Applying Initial velocity to the model
The model is given an initial velocity of 35mph (15.6464mps). The entire model is selected to apply the velocity in x-direction as the vehicle approaches the rigid wall in the x-direction.
Applying Timestep and Run time
The timestep of 0.1 microseconds is applied by adding the DT card in the engine file which available in the Control Card tab. The scale factor is 0.9 (shown below in image 1). The run time is set at 80 microseconds in the RUN card in the engine file (shown below in image 2).
Output Requests:
1. Sectional force in the rails
To determine the sectional force, the section is created at a particular location on the side rails which has the normal same as the rigid wall.
The red arrow shows the Frame which is used for reference for the section at the rail and has its normal towards the vehicle. The shell elements involved in the section are shown with the blue circle. The nodes determining the XY plane are the same as that of the frame (yellow boxes). Similarly on the other side of the rail section is taken.
2. Axial force received on the rails from bumper
The bumper is in contact with the headlight mounting bracket which is in contact with the rail. So the axial force received is the same as what is found on the headlight brackets. Therefore a contact is established between headlight brackets and the bumper. The bumper is assigned as master surface and headlight nodes as slave nodes with the TYPE7 contact interface.
3. Shotgun cross-sectional forces
The shotgun is a part of the BIW structure on which the front fenders are mounted. The cross-section is taken in a similar manner as that was done previously in rails.
4. A-pillar cross-section
The process for the A-pillar cross-section is the same as done for previous cases.
5. Acceleration curve received on the accelerometer at the base of B pillar
The acceleration curve can be received by using an accelerometer at a particular node on the B pillar rocker. In HyperCrash accelerometer can be accessed in the Data History tab. The nodes of either B pillar are selected and the accelerometer is assigned to both the nodes.
6. Intrusions on the dash wall nodes 66695, 66244
The intrusion of the nodes can be measured using the TH of Nodes option under the Time History menu in the Data History tab. The variables to be measured here is kept DEF as it has all the vectors inclusive. Follow the image below for details related to intrusion.
Orange color annotations represent node 66695 and red color annotations represent node 66244.
After setting up all the output data required for the simulation, the model quality is checked.
Model Check
The model quality is checked using Model Checker in Quality Tab.
The quality check shows no errors only warnings. The model is good to run for simulation. The file is exported and run for simulation in RADIOSS.
Simulation Results:
Animation of the crash simulation is as follows:
The Von-Mises stress experienced maximum by the model is 0.375GPa at shell 272683.
The maximum displacement is 1199mm by node 79695.
The axial force received from Bumper:
The graph shows force exerted by the bumper on to the rails. It starts to increase from the point of impact at 8ms and goes on increasing to a max value of 28.9252kN. After reaching the peak value due to impact the force reduces rapidly to 15kN and gradually decreases with time. This force is due to deformation post-impact and the transfer of energies to other parts that are coming in contact with the bumper throughout the simulation.
Sectional Forces at Rails:
The graph shows forces acting at the cross-section of both the rails (RH and LH). It is observed that it starts gaining some force after 8ms as it's the time when the crash occurs. But starts dropping down to a minimum value of 33kN. This happens because the forces acting at the section are in the direction of crash and the forces turn negative in the direction as the section normal is in opposite direction. The animation shows left side of the structure is having a larger displacement in the z-direction, this can be justified by the sectional force plot. It's clearly seen that the LH rail experiences a larger amount of force compared to the RH rail.
Sectional Forces at Shotgun:
The graph shows forces acting at the cross-section of both shotguns (RH and LH). It is observed that the graph has a negative slope for both the sections from the time of impact. It is because of the forces acting at those sections is in the direction opposite to the normal of the section. Post 25ms of simulation it's seen that the graph moves upward direction as the deformation starts taking place at the place and forces are moving the direction of normal.
Sectional Forces at A-Pillars:
The graph shows forces acting at the cross-section of both the A-pillars (RH and LH). It is observed that forces are negative for the entire simulation time and have some gain in the positive direction but still under zero. The negative direction indicates that the forces are acting in the direction of the rigid wall which is opposite to the normal of the section at the A-pillars. The forces acting here are comparatively lower than what was observed in the rails. This suggests that impact energy is absorbed more by the rails than the A-pillars.
Acceleration curve received on the accelerometer at the base of B pillar
The graph shows acceleration on both the B pillars base (RH and LH). Its seen that the LH accelerometer is having more acceleration compared to RH. This is can be justified with forces acting on the RH side and LH side of the vehicle which was discussed in previous cases i.e., in sectional forces. As the LH side also gets deformed a lot compared to the RH side this also added up some more acceleration to the nodes.
Intrusions on the dash wall 66695,66244
The graph shows the displacement of the nodes 66695 and 66244. The slope is negative because the skew associated with the nodes for intrusions is in the direction opposite to the motion of the vehicle. The nodes move towards the rigid wall. It is seen that nodes 66695 and 66244 have displacements of 899mm and 960mm respectively.
Energy Plot
The internal energy and the kinetic energy curves are mirror images of each other. It is because the internal is gain during deformation is from the kinetic energy and conserve the energies. But it's seen that there is a drop in energy as the simulation progress, this is because of the loss of energy in the form of contact energy. The gain in contact energy is the same as the loss in total energy. The deformation increases the intersection of the shell elements and to avoid the intersection the contact energy is absorbed by the nodes which move out of the intersected shell. The hourglass energy is zero because of the shell formulation used in the model. QEPH shell formulation is used in this model which eliminates the hourglass effect in the shells during deformation.
Conclusion:
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