Smear and Its Removal
Material smearing can have a very detrimental effect
on an LPI inspection as defects that are normally open to the surface
can be partially or completely covered over. Some of the processes
that can cause material to smear include machining, honing, lapping,
sanding, scraping, grit blasting, tumble de burring,
and preening operations. When high pressure is used, some cleaning
operations, such as vapor and steam cleaning, can also cause material
to smear in the softer materials. Softer materials, such as plastics
and aluminum alloys, are most prone to smearing but many other
materials, such as steel, titanium and Inconel alloys, have also
been shown to smear. To evaluate the effect of a process on liquid
penetrant inspection, cracked specimens are typically inspected
before and after performing the potential smearing operation
and a comparison between the inspection results are made. It must
be noted that under carefully controlled conditions, material
smear can be avoided. Whenever parts have been mechanically processed
prior to LPI, an evaluation should be performed to determine if
flaw detectability has been compromised. If material smearing
is a problem, an etching process can be used to remove the smeared
material prior to inspection. The curves below illustrate the
effect that metal smearing can have on the probability of detection
for a defect and how etching the sample surface improves detectability.
The top curve shows the probability of detecting
a crack versus crack length for as-machined aluminum specimens.
The bottom curve shows the POD for the same aluminum specimens
after their surfaces had been etched. Comparing the crack lengths
where the curves reach a POD level of 90%, it can be seen that
in the as-machined condition, the crack length would need to be
0.4 inches long. However, when the surface is etched, cracks under
0.10 inch can be detected with a 90 % probability.
Removal of Material Smearing
Etching of the specimens can return the flaw to the pre-mechanical
processing level of detectability. The amount of material that
must be removed by the etching process depends on the amount of
material that has been smeared and should be determined experimentally.
Volume two of the Nondestructive Testing Handbook provides
a great deal of information on material smearing and the amount
of etching required to remove a smeared metal. The handbook includes
a number of photographs such as the set below that graphically
show the effects of metal smearing.
Left Image: Original fluorescent penetrant inspection pattern
in a quench cracked aluminum sample.
Center Image: Fluorescent penetrant inspection pattern after
sanding with 240 grit paper.
Right Image: Fluorescent penetrant inspection pattern after
etching to remove 0.0003 inch of metal.
When an etchant is used, it must be properly removed from the
part before applying penetrant. Experts in the penetrant field
warn that acid and caustic entrapment from a prepenetrant etch
can have disastrous effects on the penetrant inspection. Careful
cleaning of both acid and caustic etches before penetrant inspection
is highly recommended. There are several other risks to the parts
being processed when an etchant is used. First, since the etching
process is removing metal from the surface of the part, the minimum
dimensional tolerances of the part must be considered. A second
possible risk is that the etching process could have an effect
on the material properties of the part. The chemical etchant used
should uniformly remove material from the surface and should not
etch microstructural features (such as grain boundaries) preferentially.
Ideally, a study should be conducted to evaluate the effects of
the etching process (or other chemical process) on the mechanical
properties and performance of the component.
Material Smearing Information
Rummel, W.D. and Matzkanin, G. A., Nondestructive Evaluation
(NDE) Capabilities Data Book, Published by the Nondestructive
Testing Information Analysis Center (NTIAC), NTIAC #DB-95-02,
Nondestructive Testing Handbook, Vol. 2, Liquid Penetrant
Tests, Robert McMaster, et al., American Society for Nondestructive