Physical properties considered during LPT

• Surface tension

Atom inside the liquids experiences full pulling from all its neighboring atoms. The atoms present on the surface of the liquid has no liquid atoms outside the surface to exert the pulling force. But atoms inside liquid will pull the surface atoms and this results in a net force experienced by a surface atom.

• Contact angle

An angle that the fluid (penetrant drop) makes with the surface on which it is spread is called contact point. Draw a tangent to the fluid from the test piece droplet edge. The angle between the tangent and the test piece surface is known as the contact angle. In case the angle is low that implies the penetrant is wetting the surface better. In the event that the angle is near zero then its wettability is superb.

• Surface energy (surface wetting capability)

The adhesive forces are much stronger than the cohesive forces i.e. If the liquid molecules have a stronger attraction to the molecules of the solid surface than to each other wetting of the surface occurs. To evaluate a fluid’s surface wetting qualities is to quantify the contact edge of a drop of fluid put on the surface of an object. Fluids wet surfaces when the contact point is under 90 degrees

• Capillarity effect

Capillary is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. When the discontinuity dimension (width) is very small, capillarity effect will come into play. The capillarity effect helps the penetrant to penetrate in the discontinuity.

Meniscus: it is the top surface of the liquid in contact with air. Because of the shape of the meniscus, a net force is experienced by the liquid column either upward or downward depending on whether the liquid wets or does not wet at the surface

• Viscosity

Viscosity is related to the thickness of the fluid. The penetration rate of the penetrant is higher if the viscosity of the penetrant is lower. If the penetrant is highly viscous and the rate of penetration is slower, then one may have to increase the dwell time or penetration time. The time required to fill up a crack is;

• A to viscosity
• A to 1/surface tension
• A to 1/cos θ, where θ is the contact angle