VEHICLE ROOF CRUSH ANALYSIS USING HYPERMESH AND RADIOSS
Objective: To simulate a roof crush test on a Dodge Neon BIW, as per FMVSS 216 standards, and determine the roof crush resistance of the model.
Geometry Clean-up and Assembly: The meshed model of the BIW of Dodge Neon is used for quasi-static roof crush analysis.
With the material and property cards assigned, the free nodes and free masses as well as attached springs and beams are removed from the model.
Geometry Clean-up and Assembly: The meshed model of the BIW of Dodge Neon is used for quasi-static roof crush analysis.
With the material and property cards assigned, the free nodes and free masses, as well as unattached springs and beams are removed from the model.
Mass Balancing and Time Step Definition: The ADMAS card in the solver deck is used to add distributed masses on the floor as well as the b-pillar to obtain the global mass of 700kgs and a COG of:
x : 2.46e+2, y : 4.4e+l, z : 4.87e+2
A constant nodal time step of 0.5 microseconds is set for the run.
Interface Definition: The individual interfaces are removed, and a global TYPE 7 self-impact interface is defined. Besides, a TYPE 7 interface is defined between the impactor and the car. These interfaces are defined with the following properties:
Istf = 4(Initial stiffness (K0) = min (Km,Ks))
Igap = 3(Variable gap thickness with gap scale factor and mesh size factor)
Iform = 2(Sliding forces are calculated using stiffness and friction)
Gapmin = 1(Minimum gap around surfaces)
Stmin = 1(Minimum penalty stiffness in KN/mm)
FRIC = 0.2(Friction factor)
Idel = 2(Master segment gets deleted even if one master element fails.)
INACTI = 6(Gap is reduced and scaled if initial penetration is detected.)
Boundary Condition: Rigid bodies are defined in the shock tower. The master nodes of these shock towers are locked along the global Z-direction using a boundary collector.
The master node of the rigid body of the car is locked in all the global coordinate axes. The impactor assembly is provided with a spring, the free end of which is locked in all the global coordinate axes using a single point constraint.
The impactor is then imported, and a series of translation and rotation operations are carried out to position point A of the impactor at the node (2145.26, 584.822,1343.06).
A constant nodal time step of 0.5 micro-seconds is set for the run.
Interface Definition: The individual interfaces are removed, and a global TYPE 7 self-impact interface is defined. Besides, a TYPE 7 interface is defined between the impactor and the car. These interfaces are defined with the following properties:
Parameter & Description
Istf = 4
Initial stiffness (K0) = min (Km,Ks)
Igap = 3
Variable gap thickness with gap scale factor and mesh size factor
Iform = 2
Sliding forces are calculated using stiffness and friction
Gapmin = 1
The minimum gap around surfaces
Stmin = 1
Minimum penalty stiffness in KN/mm
FRIC = 0.2
Friction factor
Idel = 2
The master segment gets deleted even if one master element fails.
INACTI = 6
The gap is reduced and scaled if initial penetration is detected.
Boundary Condition: Rigid bodies are defined in the shock tower. The master nodes of these shock towers are locked along the global Z-direction using a boundary collector.
The master node of the rigid body of the car is locked in all the global coordinate axes. The impactor assembly is provided with a spring, the free end of which is locked in all the global coordinate axes using a single point constraint.
The impactor rigid body master node is also provided an SPC to limit its translation to only the direction normal to the impactor face.
The impactor rigid body master node is provided with an imposed displacement such that at T=0ms, the displacement is 0mm and at T=200ms the displacement is 200mm. The imposed direction is normal to the face of the impactor.
And the results are:
The internal energy is 755.7KN-M
The energy error is -12.2%
The mass error is 0.0058%
Internal Energy: The energy behavior in the roof crush is different from the other two crash cases. The total energy, kinetic energy, and internal energy all increase from zero and reach a maximum value by the end of the simulation.
The internal energy curve follows the total energy curve. Hourglass energy is 0KN-mm. Max Contact energy is 125.16 KN-mm. This contact energy, as well as the high friction generated between the impactor and the car, gives a negative energy error of -12.2%. Total Internal Energy absorbed is 755.526 KN-mm.
Mass Error: As indicated by the engine listing file, a mass error of 0.0052% is achieved. This is because of the simulation time step falling short of the target input time step fed by DT/NODA/CST.
Roof Crush Resistance: This is obtained through a force-displacement graph, which in turn is plotted in the following way:
The resultant normal force between the impactor and the roof is plotted. The time history of the contact interface between the impactor and the roof (imp_car) is requested in the output using the TH/INTER card. This gives the force-time curve. Maximum Force exerted is 8.35KN.
The displacement of the roof is measured in Hyperview using the contour option and plotted in Hypergraph. This gives the Displacement-Time curve. The maximum displacement of the impactor can be seen to be 201.775 mm.
The third and final step involves cross-plotting the Force-time and Displacement-time graph using the MATH option.
Inference:
As per FMVSS 216 standards, a force of 1.5-3 times the Gross Vehicle Weight is to be achieved within 130mm of the impactor stroke.
Assuming the full model mass to be 1400kgs:
Required force = (3*1400*9.81) N
= 41,202 N = 41.202 KN
From the graph, a maximum force of 8.35 KN or 8354 N has been achieved within 130mm displacement of the impactor, which is way below the FMVSS target load.
A part of the reason for such low roof crush resistance is the reduced model which does not include all the components of the BIW to offer proper geometric resistance to the impactor displacement.
Improving Roof Crush Resistance:
Geometric Approach: Increase the thickness of Roof, Roof Rail, Panel B-pillar inner rail, Side Panel, Inner Rocker panel, Floor channel, Front floor, Rear Floor, and Trunk Floor.
Material Approach: Use of Composite Body Solutions (CBS) such as Foams, Plastics, Alloys, and non-ferrous metals. CBS Materials like Foam have a high Strength to Weight Ratio which increases the Force absorption capacity of the structure hence the roof can withstand higher forces for a given deformation.
The material approach is preferred over the Geometric approach due to space constraints in the passenger compartment as well as weight limitations.
References -
https://www.technicaljournalsonline.com/ijaers/VOL%20III/IJAERS%20VOL%20III%20ISSUE%20I%20%20OCTBER%20DECEMBER%202013/371.pdf
https://www.researchgate.net/publication/330657766_Roof_Crush_Analysis_of_Passenger_Car_for_Vehicle_Safety_and_Crashworthiness
https://forum.altair.com/topic/19060-roof-crush-analysis/
https://altairhyperworks.com/solution/Crash-Safety-Impact
http://www.ijmerr.com/v3n3/ijmerr_v3n3_91.pdf