Industrial Visit to USL

Mechanics

To enable the application of ultrasound testing it was desired by industry to have a fast rate of examination per object. Take the example of the fan blades for jet engines, before companies like USL developed machines to mount the transducers on, it took a man 2-3 hours to cover the entire surface of a fan blade, but now the same area can be covered in just 25 minuets, thanks to machine operated arms supporting the transducer. However industry is still demanding faster and faster rates.

These arms involve movement on 3 axis, left to right, forwards and backwards and up and down.

The Tray and Arms

The old mechanical arms supporting the transducers were extremely large relative to the mass they were moving, this also impeded the acceleration that they could reach according to the equation:

F=MA

A=F

M

A= a constant/ Mass

USL's innovation has been to decrease the mass of the mechanical arm as opposed to the traditional method of increasing the mass of the base table, usually by mounting the entire machine on a large slab of granite. They have done this by introducing a counter balance system and introducing the method of moving the tray containing the object towards the transducer rather than having a heavy machine for lowering the transducer towards the object.

Reducing the mass there for will increase the acceleration as long as the force remains a constant. Thereafter increasing the force will increase the acceleration. This has allowed the arm to move at its maximum acceleration of 4m/s/s, however this would never be used in practise as the movement causes so much turbulence in the water tray containing the object that the sound waves get distorted, the common top acceleration used is only 1m/s/s.

Ultrasonics

The ultrasound is used when the piezoelectric crystal in the transducer turns electrical energy into mechanical energy in the form of vibrations. This is caused by its rapid expansion and contraction as an alternating current is passed across it. The practical upshot of this is that it produces ultrasonic sound waves.

A Transducer

The most common use of the transducers are for the testing technique called ''Acoustic microscopy'' where the object for testing is placed in a tray of water and the transducer is passed over the top of it. As it travels the transducer is emitting and detecting ultrasonic waves, these penetrate the object to a depth, which depends upon the frequency being used. Where the wave hits any boundaries it partially reflects, the level to which it reflects depends upon the relative difference between the medium it is travelling from to the medium it is travelling to. This is why the object is tested under water because the difference between air and metal is such that over 99.9% of the wave is reflected and therefore there are insufficient subsequent reflections to be of any use.

The intensity of the reflected wave can be determined using the equation:

Ir=(Z1-Z2)2

Ii (Z1+Z2)2

I=Incident and refraction intensity

Z=Acoustic Impedances of the two mediums (kg m-2s-1)

(Z=pc p=density, c=speed of ultasonics)

Therefore using this equation and the data of:

Air has a Z= 410 m-2s-1

Water has a Z= 1.5x106 m-2s-1

Steel has a Z= 5x106 m-2s-1

I can now work out the refracted intensity for air to steel and the refractive intensity of water to steel to clearly show the benefit of using water.

Ir for air to steel =0.9998

Ir for water to steel =0.2899

Ultrasound is defined as sound waves, which are above the audible range for the human ear, this is usually quoted at about 20KHz, this is the frequency, number of oscillations per second. Once the reflections have been detected, amplified and analysed to produce the computer image, you can then interpret the data. On a simple object you will get 2 peaks on the graph, one for the boundary and one exit boundary or the fault in the object, depending on what you are testing for.

A typical readout

If you know the wave speed and the time difference between the two peaks you can determine the distance between the two boundaries using the simple equation:

S=Distance

V=Velocity

t=Time

S= V*t

The velocity of the wave can be determined using the equation:

ƒ=frequency

?=wavelength

V= ?*ƒ

One of the limitations of this method is that to achieve a higher degree of accuracy, a clearer picture from the results you need to use a higher frequency transistor, however this limits the penetrative power of the ultrasound. For example if you where to use a frequency of 100MHz, then effective penetration could only be about 1mm.

The ultrasound technology is also used for medical examination, for example it can produce pictures of unborn babies. In this case the transducer is passed over the mothers stomach with a gel layer to eliminate any travel through air. Ultrasound in this case has the advantage of being completely harmless for the patient as opposed to the alternative technique, using X-rays, which eventually can lead to cancer with high doses of radiation.

The future

Ultrasonics have a secure future as a safe and reliable method for testing, but as I have already stated corporate industry is always looking to increase prophets, one way to do this is to decrease the time taken. Water has reached its maximum use as the transmitting medium as if the transducer moves any faster turbulence alters the results. So in the future decreasing the mass of the supporting arm and experimenting with new mediums may increase the acceleration.

Another area for improvement would be to try and improve the accuracy of deeper penetration, this would involve finding ways to increase the strength of high frequencies, or improve the accuracy of the lower frequencies.

Andrew Cook 1 Industrial Visit