Car crumple zone deformation physics

In summary, the conversation discusses the concept of cause and effect in relation to collisions and the deformation of cars. It is noted that the longer the time of a collision, the smaller the average force applied, which is achieved through crumple zone deformation. However, there is a circular argumentation present in this concept. The conversation also touches on the design of a special forces landing device and the use of numerical methods to solve problems involving force and deformation. Finally, the conversation concludes with a discussion about the relationship between average force and deformation in a head-on collision.
  • #1
skazis
8
0
Hi,

I got confused thinking about cause and effect. If force is applied to a car due to collision, it deforms car. The longer is the time of a collision, the smaller is average force applied. Longer time is achieved by crumple zone deformation, which is affected by force. I find here circular argumentation. Could someone clarify for me this?

Thanks.
 
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  • #2
The key is its is a zone, i.e extensive in space. The impactor hitting the car is not accelerating the whole of the car at the same rate so it is imparting less force on the driver.


Start with you me and a third person named "Mr. Headlight" in a race.
  • You take a running start.
  • "Mr. Headlight being very light and quick is able to get up to the same speed as you almost the instant you cross the starting line He experiences an intense bit of acceleration at that point. It's as if you hit him as you cross the start line and carry him with you.
  • I being old and heavy and fragile and weak need a longer time to accelerate to your speed and so to make it fair you give me enough of a head start so that by the time you and Mr. Headlight catch up with me I'm up to speed and we're all running even.
Can you visualize such a race start?

Now imagine if your car (made of concrete so it won't crumple, but very massive) hits the front my stationary car with a front crumple zone, the headlights and bumper of my car almost instantaneously accelerate to your speed. I in the driver's seat accelerate up to your speed but over a longer period of time. Eventually you I and my headlights are moving at the same speed. The headlights of my car are closer to me and the space in between them and me has to have crumpled.

Imagine you're designing a new special forces landing device to replace the parachute. It consists of a very long telescoping pogo stick (pneumatically driven). Some aerodynamic fins keeps the faller oriented so the stick lands upright. The pogo stick compresses but locks before the rider can bounce and the hot compressed air inside is released via vents.
He steps off and begin his intended combat mayhem.
 
  • #3
It gets better. Thanks.

Energy absorbed by car equals its kinetic energy (assuming head-on collision and that both cars stop after), so work done by average force on the car is

[itex]F_{aver}d = -\frac{1}{2}mv^2[/itex]

where d is the deformation of the car.
From 2nd Newton's law the average force is equal to

[itex]F_{aver} = m\frac{\Delta v}{\Delta t}[/itex]

Are these 2 average forces the same?
 
Last edited:
  • #4
skazis said:
I got confused thinking about cause and effect. If force is applied to a car due to collision, it deforms car. The longer is the time of a collision, the smaller is average force applied. Longer time is achieved by crumple zone deformation, which is affected by force. I find here circular argumentation. Could someone clarify for me this?


Yes, the deformation depends on the force and the force depends on the deformation. Many problems are like this in structural engineering - even static problems can usually only be solved if one knows the properties of the materials involved. It would be particularly difficult to find a closed-form solution to your crumple zone problem since it has the additional complexities of being 1) dynamic and 2) inelastic. That's why such problems are not solved analytically. Rather, they are solved using methods that march forward in time in incremental steps, numerically satisfying the equation of motion along with the applicable force-deformation relationships (dependent on material properties) at each increment in time. Such methods can be terminated once a steady-state is reached (i.e. the collision is complete), for example.

As mentioned by jambaugh, the acceleration of the driver would be the key parameter. One would seek to design the crumple zone so as to minimize the acceleration that the driver experiences, at the expense of the "headlights" and hood, etc.


skazis said:
Energy absorbed by car equals its kinetic energy (assuming head-on collision and that both cars stop after), so work done by average force on the car is

[itex]F_{aver}d = -\frac{1}{2}mv^2[/itex]

where d is the deformation of the car.
From 2nd Newton's law the average force is equal to

[itex]F_{aver} = m\frac{\Delta v}{\Delta t}[/itex]

Are these 2 average forces the same?


Looks fine to me
 
  • #5


Hello,

I understand your confusion about the cause and effect relationship in car crumple zone deformation. Let me try to clarify it for you.

When a car is involved in a collision, there is a transfer of energy between the car and the object it collides with. This transfer of energy results in a force being applied to the car. The amount of force depends on the speed and mass of the objects involved, as well as the duration of the collision.

The crumple zone in a car is designed to deform upon impact, increasing the duration of the collision. This increased duration means that the force is spread out over a longer period of time, leading to a smaller average force being applied to the car. In this way, the crumple zone helps to reduce the impact force on the car and its passengers.

So, the force applied to the car causes the crumple zone to deform, which in turn helps to reduce the force applied to the car. This is not circular reasoning, but rather a cause and effect relationship where one action leads to another.

I hope this helps to clarify the concept of crumple zone deformation in car physics. Please let me know if you have any further questions.
 

1. How do car crumple zones work?

Car crumple zones are designed to absorb the impact of a collision by deforming and crushing. This deformation helps to slow down the car and reduce the force of impact on the passengers inside. The crumple zones are typically located in the front and rear of the car.

2. What materials are used in car crumple zones?

Most car manufacturers use high-strength steel or aluminum alloys in their crumple zones. These materials are able to absorb a large amount of energy before failing, making them ideal for protecting passengers in a collision.

3. How do car crumple zones affect passenger safety?

Crumple zones are specifically designed to protect passengers in the event of a collision. By absorbing the impact, they reduce the force experienced by passengers, which can help prevent serious injuries. However, the effectiveness of crumple zones also depends on the overall design and safety features of the car.

4. Can crumple zones be repaired after an accident?

In most cases, crumple zones cannot be repaired after a collision. They are designed to deform and absorb impact, so attempting to repair them would compromise their effectiveness in future accidents. If a car has been in a collision, it is important to have the crumple zones inspected and potentially replaced by a qualified technician.

5. Do all cars have crumple zones?

Not all cars have crumple zones, but they have become increasingly common in modern vehicles. Many governments around the world have implemented regulations requiring car manufacturers to include crumple zones in their designs. It is always important to check the safety features of a car before purchasing it.

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