Impact Force and Dynamic Strain Measurement

[dzhoucam]

I am doing an experiment by using a steel ball (14 mm Diameter) to impact onto the center of a simply supported circular panel with a thickness of 1.2 mm at a velocity of 2~3 m/s. I am concerned with the impact force and strain in the panel. I have finished the numerical and analytical model but I have no idea how to measure the contact force during impact. Both the masses of impactor and circular panel are small (in the order of 10 g). Some people advised me to attach an accelerometer on the top of the ball and then calculate the force directly from the mass and acceleration of the impactor. Is this method acceptable?

Also, if I want to measure the strain of the panel, should I use three-element rosette gages?

Many thanks.

Daowu

 

[m777182]

I would try to attach a piece of a strong permanent magnet under the panel and put a coil around so that would be able to measure the acceleration (or damped vibration )of a impact panel throuhg a voltage response.Things like slightly bigger phone pick up or audio speaker.
m777182

 

[GregLocock]

And perhaps a high speed camera for the ball? You can get some very small accelerometers (

http://www.bksv.com/pdf/2222D.pdf

), but they have inconvenient cables sticking out of them. It is also rather insensitive.

Cheers

Greg Locock

 

[CoryPad]

Daowu,

Yes, using an accelerometer would be acceptable for determining the force. A three-element rosette strain gage should be acceptable for the panel. Make sure you work closely with the accelerometer/gage suppliers to get appropriate devices that provide suitable accuracy.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[bjpil]

I’ve never done anything like what you are trying to do and it sounds fun. You probably have already thought of the following but I thought I’d put my two cents in.

Anything you attach to the falling mass needs to be accounted for in your model such as the extra mass. As Greg indicated a cable attached to a free falling object is not going to help you and due to rotation of the ball you may need a three-axis accelerometer which will increase the mass. As for the strain gage, the area you are trying to capture is pretty small and a strain gages will give you the average strain under the entire gage(s). Due to impact the strains will be different on each side of the plate, point contact on one side and more spread out on the other. I like to use as many gages as possible if I have the time but of course a Rosette is larger than a single gage. A stacked Rosette may help.

Just to throw out some ideas that maybe you could get to work, could you make a pendulum set-up instead? This would reduce the cable problem and you would only need one accelerometer. Or, could you put the accelerometer on the backside of the plate to measure the response? Or maybe replace the ball with a small force transducer.

BJP