Calculating Impact Force of Freefalling Objects in Fenestration Testing

In summary: If I use the same mass for the Ver. setup and use the deceleration time from the Hor. setup, the impact energy should be the same. In summary, using a free-falling object in a drop tube can be used to approximate the kinetic energy of a missile fired by an air cannon. However, the mass of the free-falling object must be increased to equal the impact energy of a missile shot with an air cannon.
  • #1
Steve4446
3
0
There is a standard impact test that is used in the fenestration industry. An air cannon is used to fire a piece of lumber at the unit being tested. The mass of the 'missile' is 4.09 kg. The velocity of the missile is 15.26 m/s.

I am wondering if there is a way to approximate this test without an air cannon by using a free-falling object in a drop tube. I currently have a drop tube that would allow for a 2 meter freefall. I calculated that this should give the missile a velocity of 6.21 m/s in freefall. Can I compensate for this slower velocity by adding mass to the projectile to give an equivalent impact?

How much mass would be required at 6.21 m/s to equal the impact of a 4.09 kg projectile at 15.26 m/s? Or is there no way to equate the impacts?

Thank you for your help.
 
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  • #2


The kinetic energy of an object is 1/2mV^2
So to give the same impact energy with only 40% of the speed you would need about 6x ( 1/ (0.4)^2 ) as much mass.

That won't necessarily give the same effect on the object - it depends on the speed that the object deforms under impact. It won't be a million miles out - how accurate do you need?
 
  • #3


That is what we first tried. We based our initial calculations on the kinetic energy of the object, and therefore we increased the mass to 25 kg. However, the results of those tests seemed to suggest to us that the mass might be too large as the failures were much more catastrophic than we expected based on previous air cannon tests.

If the same object is being impacted in both the vertical and horizontal tests, is there any way to calculate what the relationship would be between the deformations in the two impact tests? [ie. Does (deformation upon impact of 4kg missile @ 15 m/s) = k * (deformation upon impact of 25kg missile @ 6 m/s) if all other factors in the test are the same?]
 
  • #4


It depends on the details of the material. If it's a plastic it migth be able to deform to absorb the impact but only upto a certain rate.

Did your two missiles have the same shape/area hitting the target?
The damage will depend on the pressure applied. A rifle bullet has a ke of around 150J, the same as an apple dropping from say 5feet - but the effect isn't quite the same !

In this case I would have thought it would be close if the area was the same.
 
  • #5


Steve4446 said:
There is a standard impact test that is used in the fenestration industry. An air cannon is used to fire a piece of lumber at the unit being tested. The mass of the 'missile' is 4.09 kg. The velocity of the missile is 15.26 m/s.

I am wondering if there is a way to approximate this test without an air cannon by using a free-falling object in a drop tube. I currently have a drop tube that would allow for a 2 meter freefall. I calculated that this should give the missile a velocity of 6.21 m/s in freefall. Can I compensate for this slower velocity by adding mass to the projectile to give an equivalent impact?

How much mass would be required at 6.21 m/s to equal the impact of a 4.09 kg projectile at 15.26 m/s? Or is there no way to equate the impacts?

Thank you for your help.

If you are limited to a 2m drop, could you use compressed springs to add energy to the standard projectile in that short distance? The energy of a compressed spring is 1/2 kx^2, where k is the spring constant, and x is the compression distance. You could look up the k values for some standard (large) springs, and consider adding that to your vertical "launch" apparatus...
 
  • #6


mgb,
The missiles have exactly the same shape/area on the contact surface. They are both 2x4's, but the vertical one has an iron weight on top of it. In this case, the damage I am talking about is not penetration into the wood, etc, but the actual breaking of structural components in the window. For example, a rail is struck on the unit, and the force transfers through the frame causing the jamb to break apart, etc.

berkemann,
I had considered using a linear spring, but the practicalities of the current setup make it difficult to implement without significant changes.

Considering the data I have...
Hor. setup: 4.09 kg, 15.24 m/s, ?unknown deceleration
Ver. setup: 24.5 kg, 6.21 m/s, deceleration 497.4 m/s^2

Working with F= ma, I know the deceleration of the vertical setup because I was able to obtain the deceleration time (.0125 s) from high speed video footage that we shot during the tests. Since I lack the ability to create the horizontal setup because I do not have an air cannon or a drop tube high enough to attain the required velocity, I do not know its deceleration time. However, is there a way to calculate it based on the deceleration time of the vertical setup and the constant change in variables? Something like this...

If x mass @ y velocity in given collision setup results in z deceleration
then 6x mass @ .4y velocity in same collision setup will result in k*z deceleration

As I see the situation, the fact that the setups are vertical or horizontal is almost immaterial to the calculation, and doesn't really affect the answer. The only changing variables are velocity and mass. However, I don't know whether it is possible to calculate k, with the information I have.

Thanks again for your help with this.
 

1. How is the impact force of freefalling objects calculated in fenestration testing?

The impact force of freefalling objects in fenestration testing is calculated by multiplying the mass of the object by the acceleration due to gravity, which is 9.8 m/s^2. This will give you the force, measured in Newtons (N), exerted on the fenestration system upon impact.

2. What is the purpose of calculating the impact force in fenestration testing?

The purpose of calculating the impact force in fenestration testing is to determine whether the fenestration system is strong enough to withstand potential impacts, such as from a person, furniture, or extreme weather conditions. This helps ensure the safety and durability of the fenestration system.

3. How is the mass of the object determined in fenestration testing?

The mass of the object is typically determined by weighing it on a scale. In order to accurately calculate the impact force, it is important to use the actual mass of the object rather than an estimated or rounded value.

4. Are there any other factors that can affect the impact force in fenestration testing?

Yes, there are other factors that can affect the impact force in fenestration testing. These include the angle and velocity of the object upon impact, as well as the material and design of the fenestration system. It is important to consider all of these factors in order to accurately assess the impact resistance of the fenestration system.

5. How can the results of the impact force calculation be used in fenestration testing?

The results of the impact force calculation can be used to compare the performance of different fenestration systems or to determine if a system meets certain safety standards. It can also be used to identify areas of weakness in the fenestration system and make necessary improvements to enhance its impact resistance.

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