crash attenuator calculations

[o0dreamer0o]

i'm looking for the formulas to do some calculations on my crash attenuator i just wanna know if the formulas i have are right and how to use them since i dont have all the numbers i need.
these are the formulas that i found:

V= u + at
V2 = u2 + 2aS
S = ut + (½) a2
(v-u) = a t

S = displacement , a = acceleration, t =time, v= velocity at finish, u = initial

Impact Attenuator Data Requirement
The team must submit calculations and/or test data to show that their Impact Attenuator, when mounted on the front of a vehicle with a total mass of 300 kgs(661 lbs) and run into a solid, non-yielding impact barrier with a velocity of impact of 7.0 metres/second (23.0 ft/sec), would give an average deceleration of the vehicle not to exceed 20 g.

so from where i will get the time (t) and the distance (s) and what about the force shouldn't i calculate that too.
can anyone please help me understand this.
thanks all

[o0dreamer0o]

i'm looking for the formulas to do some calculations on my crash attenuator i just wanna know if the formulas i have are right and how to use them since i dont have all the numbers i need.
these are the formulas that i found:

V= u + at
V2 = u2 + 2aS
S = ut + (½) a2
(v-u) = a t

S = displacement , a = acceleration, t =time, v= velocity at finish, u = initial

Impact Attenuator Data Requirement
The team must submit calculations and/or test data to show that their Impact Attenuator, when mounted on the front of a vehicle with a total mass of 300 kgs(661 lbs) and run into a solid, non-yielding impact barrier with a velocity of impact of 7.0 metres/second (23.0 ft/sec), would give an average deceleration of the vehicle not to exceed 20 g.

so from where i will get the time (t) and the distance (s) and what about the force shouldn't i calculate that too.
can anyone please help me understand this.
thanks all

[carlosm]

what data are you going to be dealing with? force displacement curves?

[o0dreamer0o]

these are the specification of my crash attenuator

Hexcel honey comb designation
Cell size:1/4
Alloy:5056
Foil Gauge : 0.0025
Nominal Density pcf : 5.2
Compressive
Bare-Strength psi : typ 790, min 600
Stabilized-Strength psi: typ 820, min 645, Modulus ksi: typ 230
Crush Strength psi: typ 410
Plate Shear
L Direction- Strength psi: typ 490, min 425, Modulus ksi: typ 84.0
W Direction- Strength psi: typ 300, min 245, Modulus ksi: 32.0

[carlosm]

thats all well and good but the rules this year state that you must have physical testing of the material. So what kind of data is your testing going to give you?

[DG]

You need to use the kinematics equations you have (which are correct) with Newton's Second Law to do your calculations.
However, as has been stated, they might not be of much use to you. The rules require physical testing. Furthermore, you may discover that your material properties are not as simple as the numbers you have.

[vandit]

http://fsae.com/eve/forums/a/t...607348/m/85510167411 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/85510167411)

[carlosm]

okay so i assume you are using a press and thus your data is going to be a force displacement curve.

just use F=ma
you have a force and a mass. simply solve for the acceleration...

[carlosm]

almost forgot.

calculate the amount of energy the impact attenuator absorbs during the crush and compare that to the amount of energy the rules define.

[o0dreamer0o]

thank you so much for your help guys. i will start working on the advices you gave me.
thanx again

[vandit]

you will get force vs displacement curve ... and the curve will be somewhat like in the link i posted earlier .... you will have a ride down force ... that force should be in accordance with your force required to keep average retardation of 20g ... and yes also calculate energy an compare...

[JamesCH13]

Also remember strain-rate sensitivity. If you do a "crush test" using a compression tester like an Instron than you have to quantify the strain rate sensitivity of the core you are using. Most materials get stronger the faster they are deformed. Off the top of my head the IA I designed was honeycomb core and had an average decel of around 14gs calculated from a crush test performed at ~20mm/min, the whole test took about 8 minutes.Obviously an impact starting at 7m/s with crush the IA in a fraction of a second. From the strain-rate sensitivity factor I calculated from empirical data the IA core would average a dangerous ~19.5gs. I would highly recommend testing two, two-layer samples at say 5mm/min and 350mm/min (or as fast as your compression tester can safely go) and account for strain-rate sensitivity in your design. I am assuming of course you're not doing dynamic testing.