Finding impact force on Steel ball drop test

voltexx

I am doing a simple steel ball drop test on glass to see if the glass passes the standards.
We are dropping 1.18 inches (0.535kg) steel ball at the height of 51inches (1.295m).
Now, I can calculate final velocity at the impact, but cannot calculate the actual impact force.
One of the method is using F=(mgh)/s where s is the distance travelled after the impact as described in http://www.engineeringtoolbox.com/im...ce-d_1780.html
However since steel ball is non-elastic so the travelled distance after the impact on rigid glass will be nearly ZERO, then the impact force becomes infinite.
There must have been a report or analysis on this simple ball drop test.
Can anyone help me?
And please don't tell me about you need to know the elastic properties of materials or how long the bodies are in contact and others craps. It is straght forward situation I think anyone can assume assumptions and conditions. A Steel ball droping onto glass!

Simon Bridge

That is correct ... an absolutely rigid impact involves infinite forces. Well done.

There is no such thing though ... you'll find the glass bends a lot under the impact (unless it breaks).

We'd normally do impact tests with a pressure pad at the impact site measuring force vs time... or an accelerometer inside the ball.

voltexx

"Simon Bridge" Does that mean in this practical situation, evaluating impact force by mathematical model using only the known variables such as mass and height is invalid?

I do have a force guage (Mecmesin Advanced Force Gauge 500N) but not sure if this device is valid to measure the impact force. What do you normally use for pressure pad and when you say force vs time does time means overall time taken during the impact? - it will be in milliseconds :(

Simon Bridge

Yep - you don't have enough information to do the prediction. You could predict the result using listed Youngs modulus and brittleness for the materials.

For a pressure sensor I'd normally just use whatever my boss bought for the job - there are lots of different kinds.

For force vs time I plot a graph of force sampled at some small time interval. The specific impulse is the area under this graph. The force is not usually constant throughout the impact (the graph is usually a quadratic - but it needn't be) ... here's an example of containers subject to blast-impacts (the researchers shot them!)

... just to show you what I mean. Of course the collisions were nowhere near elastic.

As a force gauge - I have tended to rig them up from torsion bars and capacitors... stuff like that. You may be able to use the glass sheet as a torsion bar :)

You need to be able to record what your gauge says as something is in the process of pressing into it ... if it just gives a number on a screen then perhaps take a high-speed movie and play-back frame-by-frame? It also needs to be very responsive... you can't be hitting it and a few seconds later it tells you how hard you hit.

I've had to conduct this sort of test on playground soft-fall surfaces ... in that case I used a hollow steel ball a bit bigger than my fist with an accelerometer inside it. Knowing the mass of the ball and the sampled acceleration data with the sample rate gives my F vs t graph etc. i.e. there are commercial devices designed for this sort of testing.

Looking below I see a lot of discussions about impact testing - have you had a look to see how other people solved similar problems?

gmmunakaampe

Colleagues, I am trying to determine the Young's Modulus of a fibre reinforced polyester composite subjected to a drop impact test. I am trying to look for a relationship or formula to use but I am finding none. I have tried to use the relationship in the flat disc clamped around the edges with a central load. This seems to be wrong since the stead and impact load applications are different.

Any help please?

Simon Bridge

Measuring Youngs modulus normally goes like:
http://www.ehow.com/how_5147645_calc...s-modulus.html
... you should do it for a range of loads (stresses) and make a graph of stress vs strain ... Youngs modulus is the slope of the graph.

The replies above have help for the drop-impact test.

gmmunakaampe

Thanks, Simon. Will try that.

Cheers.

Grain.