L6-CRASH [Compatibility Mode]

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR CRASHWORTHINESS

- The main function of the body structure is to protect occupants in a collision- There are many standard crash tests and performance levels- For the USA, these standards are contained in Federal Motor Vehicle Safety

Standards (FMVSS)- They are FMVSS 208 (front), 214 (side), 301 (rear) and 216 (roof)



The insurance industry & consumer groups have their own test standard toevaluate vehicles beyond government standards For instance, the New Car Assessment Program (NCAP) It is based on the probability of injury; measured with a star rating where

the five stars indicate lower probability of injury and vice-versa



FRONT BARRIER Is a condition of a moving vehicle crashes onto a rigid barrier at a front end Lets model the frontal impact with a point mass

t = 0 t = 0, dx/dt = V0 t = 0, x = 0

dx/dt = 0

Resulting behavior of the point mass model The crush efficiency factor is used to consider non-uniform crash force properties



Example 1Consider a vehicle of mass 1580kg impacting a rigid barrier at 55 km/hand average crush load of 300 kN. Calculate crash acceleration, deformationand time.

Acceleration = -300000/1580 = 189.87 m/s^2 = -19.4g

time = 1580 x 55/(3.6 x 300000) = 0.0805s

Deformation = -300000 x (0.0805^2)/(2 x 1580) + (55/3.6) x 0.0805= 0.614m



The crush efficiency factor is used to consider non-uniform crash force properties

When crush factor approaching 1, it indicates the lower the head injury

When designing the collapsed structureof the motor compartment, it is desirableto have a square wave shape



Procedure for establishing Front body structural requirements: 1. Maximum allowable cabin decelerations based on occupant injury2. Consistent structural efficiency and crush space3. Average and maximum allowable crush forces 4. Total crush forces to be used in the structural elements

Crush force:10% - hood & fender20% - lower cradle50% - mid-rail20% - top of fender

The front end elements are sized to ensure that the cabin zone wont be intruded



Example 2

a) Determine the required crush space. The structure will be 80% efficient andthe allowable maximum deceleration is 20g. The impact speed is 48km/h.

b) Compute the average total crush force with a vehicle mass of 1200kg and impact speed of 35 km/h.

Solution:a) Crush space = (48/3.6)^2/(2*20*9.81*0.8) = 0.57m

b) Crush force = 0.5*1200*(48/3.6)^2/0.57 = 187N



Beam sizing

A thin-walled square section is subjected to an axial compressive load As the compressive load is gradually increased, the elastic buckling load

is reached and the walls buckle As the load increases further and past the ultimate load, the walls section crippleand the load drops

Beam section can be determined



Example 3

Each of motor compartment side rail must generate 25% of the crush forceF=300kN. A 100mm square section is used. Find the required thicknessfor yield stress of 207 MPa.

Solution:Pm = 0.25*300000 = 75000NPm = 386*t^1.86*100^0.14*207^0.57 = 75000t^1.86 = 4.88t = 2.34mm



Motor compartment packaging typicallyrequire flange location & section shapes



Limit load analysis Is the ultimate load-carrying ability for the structure Is used to determine the failure load that cause to initiation of permanentdeformation

Plastic hinge model



Model with small deflection



Vehicle pitch during impact

- Some vehicles rotate/pitch upon crash with a fixed barrier- It can increase the possibility of neck injuries- To reduce pitching, crushable beam is introduced



Example 4

Fup = 100000* 100/(400) = 25kN



Side impact- Plays an important role in sizing vehicle structure- FMVSS requires a minimum injury performance while NCAP uses star

scale- The injury criterion is TTI index where the larger values indicate a more

severe injury- TTI < 57 in desirable



Kinematic and load path analysis

Final velocity Acceleration & time

Distance traveled



Side impact model

Time at which the occupant hit the door



Rear impact- To minimize fuel system from leakage

Final speed Final speed Work of deformation

Equivalent impact velocity

Average rear crush force

Documents

L6-CRASH [Compatibility Mode]