7 |
5. Multilayer-composite material
It is impossible to read out the thickness of uncoupled multilayer’s as the ultrasonic wave cannot go through
into the uncoupled space. Furthermore, the sonic wave cannot go through in to the composite material at an
even speed. So ultrasonic reflect principles cannot be applied for measuring multilayer’s and composite
material.
6. Influences from oxidized materials
For some metals such as aluminium, a layer of oxide can be generated on the surface. Although the oxidised
layer combines with the substrate tightly, the sonic wave does need to travel through 2 different materials so it
will lead to incorrect readings. With a thicker oxidised layer, more deviation will occur. Please calibrate the
instrument with a reference block of the same material as the sample. Obtain the thickness of the sample block
by using a micrometer/calliper.
7.
Any
abnormal readings
An operator should be capable to distinguish any abnormal readings as a result of erosive, recess surface,
incorrect calibration sample blocks or the inner flaw of materials.
8.
Coupling gel
Coupling gel causes the high frequency ultrasonic waves transmitting between the probe and material.
Choosing incorrect coupling or wrong operation may cause errors or pour coupling which results in failure to
measure. The coupling gel should be used in a proper way, normally, a small drop of gel is sufficient. It is
important to use proper coupling gel, low viscosity gel (as supplied) is suitable for smooth surfaces. For rough,
vertical, aluminium surfaces, high viscosity liquids like glycerine and lubrication grease is applicable. These
kinds of agents are normally available in the local market but you can buy them also from a local distributor.
Sound Velocity table:
Material Velocity
m/s Material Velocity
m/s
Aluminium 6320 Acetate
resin 2670
Zinc 4170
Phosphor
3530
Silver 3600
Turpentine
4430
Gold 3240 Glass 5440
Tin 3230
Incoloy
alloy
5720
Iron/steel 5900
Magnesium
6310
Brass
4640 Monel
alloy 6020
Copper 4700 Nickel 5630
SUS
5790
Steel 4330 (mid)
5850
Acrylic resin
2730
Steel 330
5660
Water
(20°C)
1480 Titanium 6070
Glycerin 1920 Zirconium 4650
Soluble glass
2350
Nylon
2620
