[Company Logo Image]  

Home Feedback Contents Contact

Theory
Ultrasonics Applications and Solutions for Industry and Research

 

Home
NDT
Signal Quality
Signal Types
Performance
PreAmplifiers
Air Column
Boundaries

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.

Ultrasonic Modes

There are at least three common ways of sending ultrasonics signals through a test object.

  • Contact Ultrasonics:  A coupling liquid is applied to the test object, and then the transducer is pressed into the coupling liquid in order to transmit a strong and clear signal.  Without this couplant the ultrasonic signals are greatly attenuated by mismatch between the transducer and the sample.  Without the liquid couplant conventional units cannot detect or effectively measure the signals.  (This is why conventional pulser receiver units are not used for air coupled ultrasonics applications.)

  • Immersion Ultrasonics:  In this method the test object is placed in a liquid bath.  The ultrasonic transducers donít necessarily touch the object directly.  The liquid (usually water) couples ultrasonics into the test object well enough that conventional ultrasonics equipment works.

  • Air Coupled Ultrasonics:  Instead of using direct coupling or liquid coupling, as the name suggests, this method relies on a gas to allow the signals to pass through a test object.  This mode requires enormous dynamic range and a very sophisticated approach to signal processing.  The SIA-7 was specifically designed and built to handle air coupled ultrasonics.  Of course any system thatís good at air coupled ultrasonics must be great at immersion and contact ultrasound.

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.

  • Transmission Mode: Two transducers are used.  One transducer transmits a signal.  The signal propagates through the material and is received by another transducer.  The advantage of this mode is that the receiving transducer has an ideal environment for detecting faint signals.  This mode is also the easiest to understand and characterize.

  • Reflection Mode: Either one or two transducers are used but from just one side of a test sample.  Most contact ultrasonics is done using reflection mode.  A simple pulse is created by a transducer in contact with a surface.  The pulse propagates into the material and can then be reflected back if there are any boundaries or defects present.  The high amplitude pulse used to generate the sound wave tends to Ďringí inside the transducer for some time after the pulse.  This ringing can limit the dynamic range and the sensitivity of such applications.

  • Surface Mode: Itís often possible to set up waves that travel along the surface of a material.  If the object happens to be thin (compared to the wavelength) itís possible to create and detect waves traveling considerable distances along a surface.  The surface can even be curved or shaped.  This method is used to inspect sheet materials, cylinders and other curved shapes.  A wave is injected into the material and any reflections from distant defects and boundaries can be detected.

Ultrasonic Frequencies

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.

Questions

How does ultrasonics fit in the general Non-Destructive Testing market?  What other technologies are out there and what makes ultrasonics a good choice?

Existing 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?

The only 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 application.

Even among ultrasonic systems there seem to be different types.  Which type of system is best and what are the advantages of each?

Whether 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.

How does the air column affect an ultrasonic signal?

In 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?

The 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?

Preamplifiers 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.

 

 

Home ] NDT ] Signal Quality ] Signal Types ] Performance ] PreAmplifiers ] Air Column ] Boundaries ]

Copyright © 2007 VN Instruments Ltd
Last modified: April 02, 2007