Selection of a Penetrant Technique
The selection of a liquid penetrant system is not a straightforward task. There are a variety of penetrant systems and developer types that are available for use, and one set of penetrant materials will not work for all applications. Many factors must be considered when selecting the penetrant materials for a particular application. These factors include the sensitivity required, materials cost, number of parts, size of area requiring inspection, and portability.
When sensitivity is the primary consideration for choosing a penetrant system, the first decision that must be made is whether to use fluorescent penetrant or visible dye penetrant. Fluorescent penetrants are generally more capable of producing a detectable indication from a small defect. Also, the human eye is more sensitive to a light indication on a dark background and the eye is naturally drawn to a fluorescent indication.
The graph below presents a series of curves that show the contrast ratio required for a spot of a certain diameter to be seen. The ordinate is the spot diameter, which was viewed from one foot. The abscissa is the contrast ratio between the spot brightness and the background brightness. To the left of the contrast ratio of one, the spot is darker than the background (representative of visible dye penetrant testing); and to the right of one, the spot is brighter than the background (representative of fluorescent penetrant inspection). Each of the three curves right or left of the contrast ratio of one are for different background brightness (in foot-Lamberts), but simply consider the general trend of each group of curves right or left of the contrast ratio of one. The curves show that for indication larger than 0.076 mm (0.003 inch) in diameter, it does not really matter if it is a dark spot on a light background or a light spot on a dark background. However, when a dark indication on a light background is further reduced in size, it is no longer detectable even though contrast is increased. Furthermore, with a light indication on a dark background, indications down to 0.003 mm (0.0001 inch) were detectable when the contrast between the flaw and the background was high.
From this data, it can be seen why a fluorescent penetrant offers an advantage over a visible penetrant for finding very small defects. Data presented by De Graaf and De Rijk supports this statement. They inspected "identical" fatigue cracked specimens using a red dye penetrant and a fluorescent dye penetrant. The fluorescent penetrant found 60 defects while the visible dye was only able to find 39 of the defects.
Ref: De Graaf, E. and De Rijk, P., Comparison Between Reliability, Sensitivity, and Accuracy of Nondestructive Inspection Methods, 13th Symposium on Nondestructive Evaluation Proceedings, San Antonio, TX, published by NTIAC, Southwest Research Institute, San Antonio, TX, April 1981, pp. 311-322.
Ref: 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.
Under certain conditions, the visible penetrant may be a better choice. When fairly large defects are the subject of the inspection, a high sensitivity system may not be warranted and may result in a large number of irrelevant indications. Visible dye penetrants have also been found to give better results when surface roughness is high or when flaws are located in areas such as weldments.
Since visible dye penetrants do not require a darkened area for the use of an ultraviolet light, visible systems are more easy to use in the field. Solvent removable penetrants, when properly applied, can have the highest sensitivity and are very convenient to use. However, they are usually not practical for large area inspection or in high-volume production settings.
Another consideration in the selection of a penetrant system is whether water washable, post-emulsifiable or solvent removable penetrants will be used. Post-emulsifiable systems are designed to reduce the possibility of over-washing, which is one of the factors known to reduce sensitivity. However, these systems add another step, and thus cost, to the inspection process. Penetrants are evaluated by the US Air Force according to the requirements in MIL-I-25135 and each penetrant system is classified into one of five sensitivity levels. This procedure uses titanium and Inconel specimens with small surface cracks produced in low cycle fatigue bending to classify penetrant systems. The brightness of the indications produced after processing a set of specimens with a particular penetrant system is measured using a photometer. A procedure for producing and evaluating the penetrant qualification specimens was reported on by Moore and Larson at the 1997 ASNT Fall Conference. Most commercially available penetrant materials are listed in the Qualified Products List of MIL-I-25135 according to their type, method and sensitivity level. Visible dye and dual-purpose penetrants are not classified into sensitivity levels as fluorescent penetrants are. The sensitivity of a visible dye penetrant is regarded as level 1 and largely dependent on obtaining good contrast between the indication and the background.