Summary of Research on Metal Smearing

One of the earliest studies to publish results on the subject was published by McFaul in 1965. McFaul reports on the efforts of researchers at Douglas Aircraft Company. They developed thermal-fatigue-cracked block from 2024 aluminum alloy. These blocks had deeper cracks than the MIL-I-25135 quench-cracked blocks. The results are presented as a series of photographs and show that sanding, milling, hand scraping, shot peening, grit blasting, vapor blasting, and tumble deburring all reduced the sensitivity of penetrant inspection. They also found that, with the exception of shot peening, a mild etch to remove 0.0076 mm (0.0003 inch) removed the metal smear and returned the penetrant indications.

-- McFaul, H., Effects of Finishing Processes on Detectability of Surface Flaws by Penetrant Process, Materials Evaluation, Vol. 23, No. 12, December 1965, pp. 577-582.

In a similar study, Cook, Lord, and Roehrs investigated the effect that sanding has on the LPI detectability of quench cracks in aluminum specimens. They found that the sanding process adversely affected the LPI procedure and that a minimum of 0.0051 mm (0.0002 inch) must be chemically removed from the surface in order to restore detectability of the quench cracks.

-- Cook, J.F., Lord, R.J., and Roehrs, R.J., Quantitative Evaluation of the Effect of Mechanical Processing of the Effectiveness of Penetrant Inspection, Materials Evaluation, Vol. 32, No. 7, July 1974, pp. 133-141.

Perhaps the most quantitative data on this subject is presented by Rummel in his article on the use of probability of detection (POD) data to evaluate process capabilities. Two POD curves are presented which show the effect that metal smear and etching can have on crack detectability. One curve shows the POD of an as-machined, aluminum flat panel. A POD of 90-percent is not attained until the crack length reaches 11 mm (0.435 inch). The second curve shows that when the sample is etched, a 90-percent POD is possible with crack length around 2 mm (0.077 inch).

-- Rummel, W. D., Probability of Detection as a Quantitative Measure of Nondestructive Testing End-To-End Process Capabilities, Materials Evaluation, January 1998, pp. 35.

Volume 2, Liquid Penetrant Testing, of the ASNT Nondestructive Testing Handbook shows the effects of a number of mechanical processes on the sensitivity of penetrant inspection. Side-by-side comparison photographs of mainly quench cracked aluminum blocks show that penetrant indications are reduced or completely obscured when processes such as honing, lapping, hand sanding, hand scraping, shot peening, grit blasting, vapor blasting, and tumble deburring are employed prior to inspection. This same reference also presents some information relative to materials other than aluminum alloys, namely ANSI 1018 and 4340 steel, 300M steel and Ti-6Al-4V.

-- Nondestructive Testing Handbook, Vol. 2, Liquid Penetrant Tests, Robert McMaster, et al., American Society for Nondestructive Testing, 1982, pp. 283-319.

Henkener and Salkowski looked at the metal smear in 2024-T851 aluminum specimens with surface fatigue cracks. They studied end milling, fly cutting, grinding, polishing, and glass bead blasting. They concluded that with the exception of fly cutting, all the machining processes significantly degraded the penetrant inspection. Fly cutting slightly degraded about half of the indications and seem to improve the other half. Etching was reported to have invariably improved the dye penetrant inspections of both smeared and un-smeared cracks.

-- Nondestructive Testing Handbook, Vol. 2, Liquid Penetrant Tests, Robert McMaster, et al., American Society for Nondestructive Testing, 1982, pp. 283-319.

A study conducted in 1985, focused on the effect of blasting an aluminum alloy (with a hardness of 160 v.p.n.) with
lignocellulose media. Lignocellulose is a term used to describe wood-based blasting media which can be derived from almond or walnut shells or plum, peach and apricot pits. The author reported that at a blast pressure of 172 kN/m2 (25 pounds per square inch (psi)), some of the finer indications on the quench-cracked specimens either partially or totally disappeared. More serious indication losses were seen when the blast pressure was raised to 276 kN/m2 (40 psi).

-- Whitehorn, N.P., The Effect of Lignocellulose Abrasive Blasting on Subsequent Dye Penetrant Inspection, British Journal of NDT, January 1985, pp. 27-28.

The effect of plastic medium blasting on the LPI detection of cracks in aluminum alloys was the subject of a study by Conrad and Caudill. Plastic medium blasting is used to strip paint from aircraft to reduce the use of hazardous chemicals. Aluminum 2014-T61 and 7075-T7 specimens with both fatigue and stress corrosion cracks were used in the study. After subjecting the specimens to a ?worst case? plastic media blast, a reduction in crack detectability was observed in 24 of 33 samples. Microphotographs of the cracks showed the loss of detectability to be due to metal smearing and media entrapment. The researchers found that subsequent etching resulted in a gain in detectability that exceeded the baseline (prior to media blasting) values. Optical measurements were made to determine the lengths of the penetrant indications.

-- Conrad, D.A. and Caudill, G.R., Determination of Effects of Plastic Medium Blast on Surface-Crack Detection by Fluorescent Penetrant Inspection in Wrought Aluminum Alloys, Materials Evaluation, Vol. 48, No. 8, August 1990, pp. 985-990, 1000.

Burkle and Fraser conducted a study on the effect of metal smear on LPI using ASTM A-36 steel specimens. Copper-ferrite dilution cracking was induced in a V-groove butt weld. The study found that sandblasting to prepare a surface for painting or to remove paint, masked cracks so that penetrant inspection was not effective.

-- Burkle, W.S. and Fraser, B.K., The Effect of Mechanical Paint Removal on the Detectablility of Cracks by Visual, Magnetic Particle, and Liquid Dye Penetrant Testing, Materials Evaluation, Vol. 45, No. 8, August 1987, pp. 874-875.

Pratt & Whitney Aircraft under contract by the US Air Force studied the effects of the cleaning processes used on Inconel 718, Ti-6Al-4V, and Ti-6Al-2Sn-4Zr-6Mo materials in engine overhaul facilities. Using samples with low cycle fatigue cracks, they found that grit blasting increased surface roughness and caused metal smear that reduced LPI sensitivity. Alternately, they found that a light vapor blast (689 kN/m2 (100psi) at 406 to 457 mm (16 to 18 inches) for 30 seconds) did not degrade FPI sensitivity and actually enhanced crack indications by reducing background fluorescence and increasing the definition of the cracks. The authors cautioned that vapor blasting might cause metal smearing if applied too heavily. Chemically milling away between 0.0025 and 0.0038 mm (0.0001 and 0.00015-inch) was found to remove the smeared metal layer and restore flaw detectability.

-- Malpani, J.K. and Cargill, J.S., Methods Improvement of the Fluorescent Penetrant Inspection (FPI) Process, US Air Force Technical Report #AFWAL-TR-80-4161, October 1980.
-- Cargill, J.S. and Smith, K.D., Improved Penetrant Process Evaluation Criteria, US Air Force Report #AFWAL-TR-81-4124, October 1981.