Which of the following characteristics is NOT required of a good penetrant?

A good penetrant used in non-destructive testing, particularly in liquid penetrant testing, is expected to exhibit certain characteristics to be effective. One essential attribute is the ability to penetrate small cracks and voids in the material being examined. This ensures that any flaws, such as discontinuities or surface defects, can be accurately detected.

Another important quality is good wetting ability, which allows the penetrant to spread evenly over the surface of the material, ensuring complete and thorough coverage. This contributes to better visibility of any defects when the excess penetrant is removed and the developer is applied.

In contrast, a penetrant should not evaporate very rapidly. Rapid evaporation may lead to insufficient time for the penetrant to reach and fill small cracks effectively, compromising the testing process. Therefore, a penetrant with high viscosity is not a desirable characteristic for most tests as it can hinder flow and penetration into small defects.

Thus, the characteristic that is not required of a good penetrant is the ability to evaporate very rapidly. This ensures that the penetrant remains on the surface long enough to work effectively.