Olympus 45MG User Manual page 72

DMTA-10022-01EN, Rev. C, January 2015
couplant than larger-diameter transducers. In all modes, tilting the transducer
distorts echoes, and causes inaccurate readings, as noted below.
Curvature of the test piece
A related issue involves the alignment of the transducer with respect to the test
piece. When measuring on curved surfaces, it is important that the transducer be
placed approximately on the centerline of the part, and held steadily as close to
the surface as possible. In some cases, a spring-loaded V-block holder may be
helpful in maintaining this alignment. In general, as the radius of curvature
decreases, the size of the transducer should be reduced, and transducer alignment
becomes more critical. For very small radii, an immersion approach is necessary.
In some cases, it may be useful to observe the waveform display as an aid to
maintaining optimum alignment. Observe the best method for holding a
transducer with the aid of a waveform display. On curved surfaces, it is important
to use only enough couplant to obtain a reading. Excess couplant forms a fillet
between the transducer and the test surface where sound reverberates, and
possibly creates spurious signals that may trigger false readings.
Taper or eccentricity
If the contact surface or back surface of the test piece is tapered or eccentric with
respect to the other, the return echo is distorted due to the variation of the sound
path across the width of the beam. The accuracy of the measurement is therefore
reduced. In severe cases, no measurement is possible.
Acoustic properties of the test material
There are several conditions in certain engineering materials that can potentially
limit the accuracy and range of ultrasonic thickness measurements:
• Sound scattering:
In materials such as cast stainless steel, cast iron, fiberglass, and composites,
sound energy scatters from individual crystallites in the casting, or from
boundaries of dissimilar materials within the fiberglass or composite.
Porosity in any material can have the same effect. Adjust the instrument
sensitivity to prevent detection of these spurious scatter echoes. This
compensation can in turn limit the ability to discriminate a valid return echo
from the back wall of the material, thereby restricting the measurement range.
• Sound attenuation or absorption:
In many organic materials, such as low-density plastics and rubbers, sound
energy is attenuated very rapidly at the frequencies used for ultrasonic
gaging. This attenuation typically increases with temperature. The maximum
thickness that can be measured in these materials is often limited by
attenuation.
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