Método do Menisco em Ensaios de Penetração

The meniscus method can be used to measure the concentration and grade the sensitivity of the penetrant. The fluorescent liquid meniscus test involves measuring the diameter of a contact point of a lens with a known curvature on a flat surface where a small amount of fluorescent penetrant liquid will be placed. The penetrant layer between the lens and the surface becomes progressively thinner at the central point of contact and between the convex lens and the surface it is increased. The term "thin film fluorescence" is associated with this test, as is the ability to measure fluorescence in a penetrant film. The smaller the non-fluorescent dot, the greater the ability to fluoresce in a thin film, and that, logically, is associated with the sensitivity of the penetrant liquid.

As the light constituents of a penetrant evaporate and it starts to become more and more concentrated, the diameter of the meniscus point is reduced, and so sensitivity increases. For example, a penetrant found in the trade initially had its viscosity and meniscus point diameter measured, was placed in a Petri dish and allowed to evaporate for 3 days. The viscosity increased from 6.05 cSt to 11.47 cSt. The meniscus diameter decreased from 4.4 mm to 3.5 mm. The meniscus point diameter was related to the sensitivity, evaporation and concentration of the penetrant exposed to air for 3 days, increasing its sensitivity.

Under real penetration conditions the loss by evaporation of the penetrant film on the test piece surface will be higher than on the Petri dish and the increase of the penetrant sensitivity will be substantial.

In another experiment, the penetrant base was prepared by dissolving a combination of 2.5% pigments in a "thick" liquid. The meniscus point was 3.5mm. This penetrant was diluted with 3 parts Stoddard solvent and its meniscus point increased to 7.5 mm. In other words, the meniscus point is a legitimate method for grading the sensitivity of the penetrant. Diluting the penetrant with a liquid solvent reduces its sensitivity. Theoretically, this suggests sufficient penetration time to allow the diluted solvent to evaporate completely should restore the higher sensitivity of the penetrant