Now we generalize our previous and new results. The efficiency of liquid penetrant inspection depends on two main factors: the velocity and completeness of liquid penetration inside defect's hollow and the developing time. As regards the first factor, the velocity of penetrant's flow inside the defects of simple geometry (cracks, cylindrical pores etc.) is described by Washburn equation under certain assumptions. Today we can take into account the dimensional viscosity effect (with the use of micropolar fluid theory), gas dissolution and diffusion in a liquid being in defects. As to the second basic factor, - development stage, - the hydrodynamic theory is based on three equations describing absorption of a penetrant from defect's hollow by porous developer layer.

For example, for leak testing the next three simple equations define the hydrodynamics of penetrant's blotting by the developer from through crack:

v= (H₂/ 12 µl₀) [P_a-P₁(t) ], dy_f / dt = (K /µy_f)[P₁ (t) +2/ R_e-p_a], v=(2h /H) (dy_f/ dt)

Here v - velocity; 2H - crack's width' µ- viscosity; - surface tension; R_e - effective pore's radius; h, and K - thickness, porosity and permeability of developer's layer; P_a - atmospheric pressure; y_f - coordinate of blotted zone. It is easy to derive the differential equation for kinetics of crack's trace forming. Similar equations have been derived for penetrant flaw detection for various types of defects.

The formulae for the calculation of the most important characteristics are obtained. For example, the formulae for the calculation of the sensitivity threshold of penetrant testing for prescribed family, maximum thickness of developer's layer hmax are derived. The exceeding in practice h_max makes impossible to reveal a defect.

Decisive importance has the determination of structural characteristics of powder developers because one needs the values K, , R_e and h to calculate both the sensitivity threshold and other important characteristics. Effective simple methods for the determination of these characteristics are worked out.

REFERENCE

1. P. Prokhorenko, N. Migoun, `Introduction in Theory of Penetrant Testing', Minsk: Science & Technology, 1988 (in Russian)