The Echo Images Examples are a selection of Echo Signals. The examples picks up common situations of Echo Signal conditions and displays them with their RF Signal, as it will appear at an Oscilloscope or an Ultrasonic System screen as a so called "one shoot". The conditions will be mostly based on plastic pipes and will relate to in-line wall thickness measurement.The principles are also adaptable to other materials and to flaw detection contitions. The Database starts on this page with a text list, and a description of the selectable Image case. With a click on the Image symbol the related RF Signal, together again with the description, will be displayed.
For evaluation of Echoes it is important to analyse them for both of the half-waves (RF). For Wall Thickness Measurement it is common to use the time of flight (TOF) method within the Interface and Backwall Echo. The release (trigger), for the Start and Stop Function, is due to a threshold exceeding of the echo half-wave. The thresholds are adjustable for both echoes separately. The adjust of the position, that means where (distance), the echo is expected. The of the amplitude, that means according to the certain echo amplitude and noise level. The first wavephase that will exceeds the threshold, will release the time measurement function. For more details refer to the chapter Ultrasonic Basics at this www site.
Good signal condition, with low noise,
high damped, good wavecurves, and with a dominant trigger half-wave.
Good signal condition. Interface echo amplitude is higher than backwall echo amplitude.
This amplitude relation rise due to: Acoustic Impedance differences between water and specimen or
high material sound attenuation. It's even much higher interface echo possible as it is here displayed (e.g. steel)
Thin wall thickness condition with a limit of evaluation possibilities.
The limitation is due to the last interface echo wavecurves. The
backwall echo evaluation-threshold can be positioned very close
to the interface echo. Even the interface echo waves are within
the backwall echo threshold, but not exceeding. Enough signal/noise
ratio! This is to take care in automatic testing, should not be
less than 6-10 dB.
Thin wall thickness condition with a limitation of evaluation possibilities.
The limitation is due to the last interface echo wavecurves.
Small outer diameter, interface echo amplitude is smaller than
backwall. Condition is acceptable but interface echo can be improved
with use of focused transducers.
Small outer diameter and very small wall thickness. The thin measurement
capability is highly improved due to small interface echo amplitude.
Good signal condition. Interface echo amplitude is higher than backwall echo
amplitude. This amplitude relation rise due to: Acoustic Impedance differences
between water and specimen or high material sound attenuation. It's even much
higher interface echo possible as it is here displayed (e.g. steel) .
Backwall echo attenuation and frequency shift to lower frequency. This situation
accrue from material properties, the material works like a filter.
For instance there is for rubber a frequency shift, from original 5 MHz to low of 2 MHz possible.
The exact value depends on the thickness.
Multi-(3)Layer wall and the small first Layer Interface echo. The Thin EVOH Layer
is here not possible to separate. The condition
is acceptable but still difficult for a reliable automatic In
Line test. The necessary signal/noise must be achieved. If to
evaluate the positive or the negative half-wave of the Layer echo, this has to
be calculated empirically.
Flaw within the wall. In Plastic Pipe Extrusion it can sometimes also rise
due to solid and melt Interface within the wall, appears during
the cooling section.
Flaw (irregular) within the wall, the flaw's echo wavecurves point to a large
and irregular or oblique flaw.
Will be continued soon
Rolf Diederichs 02. April 1996,info@ndt.net