of the Defect
The nature of the defect can have a large affect on sensitivity
of a liquid penetrant inspection. Sensitivity is defined as the
smallest defect that can be detected with a high degree of reliability.
Typically, the crack length at the sample surface is used to define
size of the defect. A survey of any probability-of-detection curve
for penetrant inspection will quickly lead one to the conclusion
that crack length has a definite affect on sensitivity. However,
the crack length alone does not determine whether a flaw will
be seen or go undetected. The volume of the defect is likely to
be the more important feature. The flaw must be of sufficient
volume so that enough penetrant will bleed back out to a size
that is detectable by the eye or that will satisfy the dimensional
thresholds of fluorescence.
Above is an example of fluorescent penetrant inspection probability
of detection (POD) curve from the Nondestructive Evaluation (NDE)
Capabilities Data Book. Please note that this curve is specific
to one set of inspection conditions and should not be interpreted
to apply to other inspection situations.
In general, penetrant inspections are
more effective at finding
- small round defects than small linear
defects. Small round defects are generally easier
to detect for several reasons. First, they are typically volumetric
defects that can trap significant amounts of penetrant. Second,
round defects fill with penetrant faster than linear defects.
One research effort found that elliptical flaw with length to
width ratio of 100, will take the penetrant nearly 10 times
longer to fill than a cylindrical flaw with the same volume.
- deeper flaws than shallow flaws.
Deeper flaws will trap more penetrant than shallow flaws, and
they are less prone to over washing.
- flaws with a narrow opening at the
surface than wide open flaws. Flaws with narrow surface
openings are less prone to over washing.
- flaws on smooth surfaces than on
rough surfaces. The surface roughness of the part
primarily affects the removability of a penetrant. Rough surfaces
tend to trap more penetrant in the various tool marks, scratches,
and pits that make up the surface. Removing the penetrant from
the surface of the part is more difficult and a higher level
of background fluorescence or over washing may occur.
- flaws with rough fracture surfaces
than smooth fracture surfaces. The surface roughness
that the fracture faces is a factor in the speed at which a
penetrant enters a defect. In general, the penetrant spreads
faster over a surface as the surface roughness increases. It
should be noted that a particular penetrant may spread slower
than others on a smooth surface but faster than the rest on
a rougher surface.
- flaws under tensile or no loading
than flaws under compression loading. In a 1987 study
at the University College London, the effect of crack closure
on detectability was evaluated. Researchers used a four-point
bend fixture to place tension and compression loads on specimens
that were fabricated to contain fatigue cracks. All cracks were
detected with no load and with tensile loads placed on the parts.
However, as compressive loads were placed on the parts, the
crack length steadily decreased as load increased until a load
was reached when the crack was no longer detectable.
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, May 1996.
Alburger, J.R., Dimensional Transition Effects in Visible Color
and Fluorescent Dye Liquids, Proceedings, 23rd Annual Conference,
Instrument Society of America, Vol. 23, Part I, Paper No. 564.
Deutsch, S. A, Preliminary Study of the Fluid Mechanics of Liquid
Penetrant Testing, Journal of Research of the National Bureau
of Standards, Vol. 84, No. 4, July-August 1979, pp. 287-291.
Kauppinen, P. and Sillanpaa, J., Reliability of Surface Inspection
Methods, Proceedings of the 12th World Conference on Nondestructive
Testing, Amsterdam, Netherlands, Vol. 2, Elsevier Science Publishing,
Amsterdam, 1989, pp. 1723-1728.
Vaerman, J. F., Fluorescent Penetrant Inspection Process, Automatic
Method for Sensitivity Quantification, Proceedings of 11th World
Conference on Nondestructive Testing, Volume III, Las Vegas, NV,
November 1985, pp. 1920-1927.
Thomas, W.E., An Analytic Approach to Penetrant Performance,
1963 Lester Honor Lecture, Nondestructive Testing, Vol. 21, No.
6, Nov.-Dec. 1963, pp. 354-368.
Clark, R., Dover, W.D., and Bond, L.J., The Effect of Crack Closure
on the Reliability of NDT Predictions of Crack Size, NDT International,
Vol. 20, No. 5, Guildford, United Kingdom, Butterworth Scientific
Limited, October 1987, pp. 269-275.