Ultrasonic Non Destructive Testing
Ultrasonic systems use high frequency sound waves. In most applications ultrasonic signals are used to probe a material or object. The signals pass through the material similar to an X-Ray but the ultrasonic waves are harmless.
The concept of ultrasonics sounds simple enough, just send a wave through something and see what happens. Of course just capturing a signal trace on an oscilloscope doesnít solve the problem. Appropriate cables and preamplifiers must be set up. Power amplifiers and digitizers must be employed and the signals need to be properly detected and analyzed. At every step the signals must be clean and linear. With the wrong equipment ultrasonics can become a science fair experiment.
With the right equipment ultrasonics can be made simple. Thatís why we developed the SIA-7. The SIA-7 is a self contained ultrasonics analysis tool. All aspects of the signal generation, transmission, reception and analysis are done using the SIA-7ís sophisticated and patented signal processing system.
There are at least three common ways of sending ultrasonics signals through a test object.
Ultrasonic Propagation Modes
There are a variety of ways that an ultrasonic signal can be applied to a test object or material. Some of the more common modes are as follows.
Contact ultrasonics systems cover a very wide frequency range. Simple pulser receiver units tend to be tuned for relatively modest ranges. In general itís possible to find contact ultrasonics equipment that operates from as low as 20kHz all the way into the GHZ range!
Water immersion ultrasonics tends to work in a more modest range, usually from 200kHz to a maximum of around 100MHz. More typical applications are in the 1MHz to 20Mhz range.
Air coupled ultrasonics applications have been demonstrated as high as 10MHz. For most practical applications the total air column involved determines the frequency range. Typical applications have been demonstrated at frequencies ranging from 20kHz up to about 5MHz as a practical limit.
How does ultrasonics fit in the general Non-Destructive Testing market? What other technologies are out there and what makes ultrasonics a good choice?
NDT technologies include X-ray, NMR, Infrared and spectroscopy to name a few.
No one technology will solve all applications.
A brief overview of each method and some
application related application areas helps put ultrasonics into context.
If the application is a good candidate for ultrasonics then how do we compare one ultrasonics system to another?
way to make a meaningful comparison of two different ultrasonics solutions is to
look at the signal quality and the rate at which the signals can be measured.
Detailed specifications can be misleading.
Ultimately itís necessary to measure the signal
strength relative to the noise floor and then decide if the ultrasonic
system can provide the needed signal quality.
If the system can deliver enough signal, then the performance
envelope helps decide if the system is Ďenoughí for a particular
Even among ultrasonic systems there seem to be different types. Which type of system is best and what are the advantages of each?
using air coupled, contact or immersion ultrasonics there are different
types of systems in use. The
most common is a simple pulser receiver. Another
long standing method is called tone burst (or the related continuous wave).
The SIA-7 supports both these modes and adds a more powerful method we
call Synthetic Impulseô.
does the air column affect an ultrasonic signal?
general terms as the frequency increases, so does attenuation.
Itís possible to calculate a good estimate of how much attenuation to
expect over some given distance.
Why is air coupled ultrasonics so difficult that conventional pulser receiver units just donít work?
easiest way to understand the problem is to consider the Ďtwo piece of
paperí problem. Which is more
difficult, getting an ultrasonic signal through 1 inch of titanium or 2 pieces
of ordinary paper? A block of
titanium will have just two boundaries, one at the entry point of the ultrasonic
wave and one at the exit point. Two
pieces of paper have a total of 4 boundaries so the net effect is a lot more
signal loss. The
boundaries between any solid or liquid and air cause most of the losses.
Some ultrasonics units have preamplifiers. What are these for and why should they be used?
are used to help match the ultrasonic transducer with the digitizers used to
record the signal data. Without a
properly constructed preamplifier the signals received can be adversely affected
by such things as the length of coax cable and the electrical noise pickup.
Copyright © 2007 VN Instruments Ltd