What is Penetrability?
Penetrability is a complex characteristic for a penetrant that is not as straightforward as comparing it to capillary tubes. What is certain is that it is affected by different factors, such as the condition and type of the surface being tested, the type of penetrant being used, the temperature, and whether there is any contamination present. The formula below has been developed to connect and consider all of these factors together.
P = (2S Cos θ)/D
- Where P is Capillary pressure, S is Surface tension of the liquid, θ is Contact angle and D is Width of the crack.
The formula reveals that high capillary pressure is achieved with high surface tension. However, it is important to note that a substance with high surface tension does not necessarily make it a good penetrant. For instance, water has high surface tension but is not effective as a penetrant.
In the formula, θ represents the angle formed between the interface of the liquid with air and the interface of the liquid with a solid. As mentioned earlier, the ability of a liquid to spread and adhere to the inner sides of a tube, known as wetting, is significant. A smaller contact angle indicates higher capillary pressure.
Penetrability of right Penetrant
To be considered a good penetrant, a substance should possess high wetting ability, which means it should have a low contact angle, usually below 5 degrees. However, it’s important to note that the wetting ability of a penetrant can vary depending on the type of surface it is applied to. Therefore, the dwell time, or the duration the penetrant is left on the surface, should be adjusted accordingly to optimize its effectiveness.
| Contact Angle | Wetting Ability | Droplet Shape |
| Less than 90o | High |  |
| 90o | Moderate |  |
| More than 90o | Low |  |
The formula further indicates that the width of a discontinuity or opening has an impact on capillary pressure. Specifically, the narrower the opening, the higher the capillary pressure will be.
Penetrants have demonstrated the ability to enter spaces as small as 0.3 microns in width, which is incredibly tiny (about one third of one thousandth of a millimeter). This is evident in capillary rise experiments, where liquids rise to greater heights in narrower tubes, although it may take longer for the rise to occur. In the context of penetrant inspections, smaller defects or openings will require longer dwell times for the penetrant to adequately fill and detect them.
Static Penetration Parameter (SPP)
SPP = SCos θ
Once again, S represents the surface tension of the liquid, and Cos θ represents the contact angle between the interface of the liquid with air and the interface of the liquid with a solid. The ability of a liquid to wet the inner sides of a tube is emphasized, and a smaller contact angle leads to a higher value of the Surface Penetrant Pressure (SPP).
Viscosity is another factor to consider. While it doesn’t significantly affect a liquid’s ability to enter a discontinuity (defect), it does influence the rate at which it can do so. Both experience and the formula for the Kinetic Penetration Parameter (KPP) demonstrate that a highly viscous penetrant takes much longer to enter a defect, therefore requiring a longer dwell time.
Since viscosity is greatly affected by temperature, penetrant inspections are also influenced by temperature. A viscous penetrant that has been dipped or sprayed will drain more slowly from a specimen, leading to excessive loss of penetrant due to drag out into the wash station.
KPP = (S Cos θ)/η
