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7. GAGING PRECAUTIONS
7.1 Very Thin Sections
As with any ultrasonic thickness gage, when the sectional thickness falls
below the minimum operating thickness for the specific probe in use,
erroneous readings will result. It is virtually impossible to precisely specify
the minimum thickness which can be gaged with a given probe/NOVA
TG950/material combination. The minimum thickness limit can be closely
approximated by experimentation on the test object or reference standards
of the same material, and using different probes.
One erroneous effect, called "thickness doubling", sometimes occurs when
gaging thicknesses fall below the minimum limit. Another effect “pulse-
envelope cycle-jumping" produces a reading somewhat larger than the
actual thickness. It is advisable to double check critical thinner sections by
using NDT Systems' NOVAScope, the Eclipse TG-2, or an ultrasonic flaw
detector such as Quantum TE.
7.2 Pitting Corrosion
Pitted areas on the test object’s back surface can cause unexpected erratic
changes in thickness readout, or, in extreme cases, no thickness readout.
Very small, or sharply pointed pits may go undetected, especially small,
isolated single pits. W hen pitting is either detected or suspected, the area
should be very carefully scanned while changing the orientation of the
probe's cross-talk barrier to enhance delectability of the thinnest pitted
locations. If positive results cannot be obtained, particularly on critical
structures, use NDT Systems' NOVAScope, the Eclipse TG-2, or an
ultrasonic flaw detector such as Quantum TE.
7.3 Material M isidentification
Always verify the type and anticipated thickness of material to be gaged.
Erroneous thickness readouts can result if an instrument is calibrated on a
material and/or thickness other than that of the test material.
7.4 Worn or Malfunctioning Probes
Immediately replace any probe that is malfunctioning, or showing excessive
or uneven wear.
7.5 M aterial Stacks
It is not possible to gage the stack thickness of uncoupled material sheets
piled upon one another. Ultrasound reflects totally from the back surface of
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the outer sheet. Likewise, it is not recommended to attempt to gage bonded
laminates layers of similar or dissimilar materials adhesively bonded.
Depending upon the adhesive properties, it may be possible for the
instrument to receive an adequate back-echo to be able to read the
thickness of the outer layer. However, for reliable ultrasonic testing of
laminates, it is recommended to use an instrument having an A-trace.
7.6 Plated or Painted M aterials
If a surface has plating of known and uniform thickness, it can be possible to
obtain accurate and reliable thickness measurement of the substrate
material. However, appropriate reference standards are necessary. If, for
any reason, the plating is separated from the basis material, the readout, if
any, will be erratic or unstable.
Painted surfaces may, or may not, create gaging problems. It is essential to
experiment with reference samples having the same condition as the test
material.
7.7 Abnormal or Unusual Thickness Readings
The operator should always qualify abnormal or questionable thickness
indications. W hile such indications may be legitimate, they should be
carefully evaluated. If an unexpected indication is seemingly random, or
cannot be reproduced, it was possibly caused by a couplant break or a small
foreign particle on the surface. If it can be reproduced, even if erratic, it
should be recorded for further evaluation.
Something
caused the strange or
erratic, but location-reproducible, indication. Sometimes, a small inclusion,
or unfavorably oriented crack, can cause such indications. Again, use an
instrument having an A-Trace to evaluate them.
