Transcript Structural Requirements

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)
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
• The insurance industry & consumer groups have their own test standard to
evaluate 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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
DESIGN FOR CRASHWORTHINESS
SMC 4133 AUTOMOTIVE STRUCTURES
FRONT BARRIER
• Is a condition of a moving vehicle crashes onto a rigid barrier at a front end
• Let’s 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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
DESIGN FOR CRASHWORTHINESS
SMC 4133 AUTOMOTIVE STRUCTURES
Example 1
Consider a vehicle of mass 1580kg impacting a rigid barrier at 55 km/h
and average crush load of 300 kN. Calculate crash acceleration, deformation
and 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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
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 structure
of the motor compartment, it is desirable
to have a square wave shape
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Procedure for establishing Front body structural requirements:
1. Maximum allowable cabin decelerations based on occupant injury
2. Consistent structural efficiency and crush space
3. Average and maximum allowable crush forces
4. Total crush forces to be used in the structural elements
Crush force:
10% - hood & fender
20% - lower cradle
50% - mid-rail
20% - top of fender
The front end elements are sized to ensure that the cabin zone won’t be intruded
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Example 2
a) Determine the required crush space. The structure will be 80% efficient and
the 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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Beam sizing
Beam section can be determined
• 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 cripple
and the load drops
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Example 3
Each of motor compartment side rail must generate 25% of the crush force
F=300kN. A 100mm square section is used. Find the required thickness
for yield stress of 207 MPa.
Solution:
Pm = 0.25*300000 = 75000N
Pm = 386*t^1.86*100^0.14*207^0.57 = 75000
t^1.86 = 4.88
t = 2.34mm
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
• Motor compartment packaging typically
require flange location & section shapes
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Limit load analysis
• Is the ultimate load-carrying ability for the structure
• Is used to determine the failure load that cause to initiation of permanent
deformation
Plastic hinge model
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Model with small deflection
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Example 4
Fup = 100000* 100/(400) = 25kN
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Kinematic and load path analysis
Final velocity
Acceleration & time
Distance traveled
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Side – impact model
Time at which the occupant hit the door
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR CRASHWORTHINESS
Rear impact
- To minimize fuel system from leakage
Final speed
Work of deformation
Equivalent impact velocity
Average rear crush force
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.