Ping hammering: how to choose peaks as modes/natural frequencies?

[davidd314152wind4]

I work in a lab that is performing ping hammer on automotive components in three directions: cross-car, fore-aft, and vertical.

We are looking for modes or natural frequencies in each of these directions.

Is there a specification or procedure detailing how to identify the first mode or natural frequency?

How does the first mode differ from the natural frequency?

In cases of natural frequency...

We often discount "insignificant" peaks with no clear way of determining what an insifnificant peak is, when measuring natural frequency.

In cases of modes...

We have had cases where a peak is not counted as the "first mode" because it was seen, in greater intensity, in a different direction. For example, if a large peak was present at 45Hz in the fore-aft direction a peak 1/4 the size was found in the cross-car direction at the same frequency, the peak was not counted as the first mode in the cross-car direction.

Can anyone shed some light on this and help clear this up for me?

Regrads,

windo4life

 

[Ali Kara]

As you said natural frequency is the same in all directions. But according to the nature, properties in different directions of a car are different. Because of this, although you reach the same NF(the car has one natural frequency in all directions), it is reasonable that you can't obtain the same peak value of the vibration.

To understand what is happening, you can study Plate and Panel books. Because a car can be modeled as a plate in vibration analysis. (You can use this book;

http://books.google.com.tr/books?id...ture&sig=Mq9e_sY5bLX_gSsp2SsJpx5ATlw#PPP1,M1)

The insignificant peaks may come from components of car. Because cencors that you use to measure vibration are so sensitive. For example, when you ping the car, the excitation frequency may coincide with the natural frequency of a seat, and you see this seat vibration in your graph.

I do not know very much about standartization.

 

[snoopnoon]

Saglamci, a ping hammering test is a broadband input frequency meaning that depending on the density of the tip, the more narrow/broad the range of excitation is. It sounded like you were descripting a single frequency impulse in your reply.

Forgive if assumption is wrong.

 

[GregLocock]

There is no such thing as a fore aft mode or a vertical mode or torsional or whatever, for parts with complex shapes. A mode shape is a 3 dimensional thing with motion in all directions, normally.

Things that are roughly beam shaped will have modes that can be described as vertical bending etc, but that is a conceptual description.

"How does the first mode differ from the natural frequency?"

I am not aware of any practical difference.

Cheers

Greg Locock

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