Which property of penetrant materials is NOT listed as needing control?

• A. Thermal expansion
• B. Surface wetting
• C. Viscosity
• D. Removability

Answer: (A)

Penetrant inspection works because the liquid must enter tiny surface openings and then be revealed against the background. The factors that directly control how well this happens are how well the liquid wets the surface (wetting), how easily the liquid flows into defects (viscosity), and how easily it can be removed after inspection without taking the indications with it (removability). Thermal expansion of the penetrant is not a property typically treated as needing control for LPI, since small changes in volume with temperature don’t directly affect the formation or visibility of indications as much as wetting, viscosity, and removability do.