PART 1. Liquid Properties of Liquid Penetrant

Penetrants and their ancillaries must have toward the liquid’s interior, and the physical properties that fall within fairly surface acts like a skin. It acts to minimize narrow ranges. None are difficult to attain the surface area of the liquid and it but properties cannot stray very far from requires effort (the surface tension) to optimum. The coverage of these topics is stretch this skin. essentially practical. Some physical Contact angle is the measured angle chemistry is included, particularly relating that a drop of liquid makes with a solid to capillarity, light absorption and surface. If contact angle θ is zero, cos θ = 1 scattering but not enough to require that and the liquid will wet and spread. If the the reader be a professional physical contact angle is 90 degrees or more, cos θ chemist. Discussions are limited to = 0 and the liquid will not wet but will descriptions of usable processes and how remain as a rounded drop. Intermediate they work. No effort is made to discuss contact angles indicate intermediate unproved processes or theories. degrees of wetting. Contact angles can be For readers who have interests in other measured on special sample surfaces with theories behind the mechanisms of special equipment. penetration, surface wetting, adsorption Energy of adhesion is a measure of the and development and other fundamental strength of attraction of a liquid to a solid physical chemistries of liquid penetrant surface and is of more theoretical than testing, additional readings1-7 are practical value. suggested. Penetration of a discontinuity is primarily a capillary effect. The forces involved are those associated with capillary action and are called capillary Penetration pressure or excess surface pressure. This The very name penetrant suggests that the pressure is given by Eq. 1: ability to penetrate into voids is the major 2γ feature of a penetrant. This is no doubt (1) P = true but it is not a critical feature. Very R nearly any liquid will wet a solid surface and penetrate into voids. In fact, it is not where γ is the surface tension of the liquid easy to find a liquid that will not and R is the radius of curvature of the penetrate. If it wets, it will penetrate; if liquid surface. The effect of this capillary not, it will not. pressure can best be shown by examining Wetting of smooth, chemically clean the two systems depicted in Figs. 1a surfaces has been studied extensively with and 1b. relationships worked out between surface In Fig. 1a the liquid wets the capillary tension, interfacial tension, wetting and the pressure P1 is up. In Fig. 1b the contact angle, energy of adhesion. None liquid does not wet the capillary and the are very appropriate because many of pressure P2 is down. The ability to wet or these quantities are not measurable on the not wet determines in which direction the kinds of surfaces that are tested with surface will curve, whereas the degree of liquid penetrant. Even a cleaned surface wetting determines to what extent the can pick up a molecular monolayer of oil surface will curve. Therefore, the first or oxide in a very short time. The requirement for penetration is its ability slightest taint of oil on a surface can to wet the surface of the discontinuity. change a surface and cause a penetrant The dimensions of the capillary are film to become less wet and to pull up also important and the radius of the into droplets. As a practical matter, this is capillary can be related to the capillary not too serious. Reapplication of pressure by examining Fig. 1c. It is found penetrant will usually dissolve this film that R = r/cos θ, i.e., cos θ = r/R, where r is and allow testing to proceed. the radius of the tube and θ is the angle Surface tension can be defined as the of contact of the liquid and the tube. force needed to expand (or pull apart) the Equation 1 becomes: surface of a liquid. It results from the 2γ cos θ attraction of all the molecules within the (2) P = liquid for each other. At the surface, with r no more liquid outside, the net force is

84 Liquid Penetrant Testing