PART 1. Liquid Properties of Liquid Penetrant

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PART 1. Liquid Properties of Liquid Penetrant Powered By Docstoc
					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
             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
             attraction of all the molecules within the                     2γ cos θ
                                                             (2)    P =
             liquid for each other. At the surface, with                       r
             no more liquid outside, the net force is

84   Liquid Penetrant Testing

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