19:40 Aug-07-2009 Lewis Engineering USA, Joined Aug 2009 2
AE Impulse Generator
I was wondering if anyone knew of an acoustic emission generator that can produce a relative wide band impulse on to an aluminum plate. Until now, I've been using either the Hsu-Nielson lead break or a PZT transducer supplied with a square wave function. My aim is to try and get a reproducible emission that is representitve of that produced by crack growth in aluminum. I have heard of such a mechanical device, but I have no idea what it is or how to procure one. If someone could point me in the right direction it would be greatly appreciated.
In my opinion the wide band acoustic emission generator may not serve your purpose as this may not produce that characteristic frequency burst associated with the crack propagation in Aluminum.
Like you, I was very curious to watch the characteristic signals associated with the crack propagation in low alloy steels. I made a no. of long specimens (with square section), varying their size and heat treatment. A two mm deep pre-crack was introduced at the centre on one side by wire cutting method. A zig was designed where in these specimens can be held length wise (at either end) and can be loaded at the centre opposite to the crack side by tightening a screw.
Two AE probes with wide band capability were placed on either side of the specimen operating in linear location mode. When the specimens are gradually loaded you get two types of signals.
The first type is from the friction between specimen and the zig as well as the loading mandrel. These are of relatively low frequency and low energy with count rates raising and falling with each tightening of the loading screw.
The second type is the high energy high frequency bursts that are associated with the crack propagation. Since linear location mode is used you can see that the signals are coming from the centre of the specimen where the pre crack is located and from where crack propagates during loading. The count rate and energy release is pretty high just before the final failure. You get as many subtle variations in the monitored signals as the changes made in sizes and heat treatment of the specimens but you cannot miss the sharp raise in the count level and the high energy content associated with the crack propagtion.
I advise you to go for similar experiment with specimens made of Aluminum in different sizes and heat treatments and break them in a suitably designed zig. You may do well if you use a hydraulically loaded mandrel instead of the mechanical screw type loading that I used.
Coming back to your original question of wide band AE signal generator I have an idea that you may try. You may use wide band ultrasonic pulser with high energy and a shock wave probe that generates wide frequency band.
But the point is you will not get that characteristic signal bursts associated with the crack propagation in any of the signal generators. The closest you get is from the Neilson lead break that we all use.
Watching the signal from the actual crack growth in lab experiments makes it relatively easy to identify crack growth in the field when you get one.