Which factor is crucial when selecting a penetrant?

When selecting a penetrant for liquid penetrant testing, the type of material being tested is crucial because different materials can exhibit varying behaviors when exposed to penetrants. For instance, certain metals may require a specific type of penetrant that can effectively seep into surface-breaking defects without being adversely affected by the material's properties, such as porosity, surface texture, or the presence of coatings. Additionally, some materials might react with specific penetrants, leading to false indications or inadequate detection of flaws. Therefore, understanding the characteristics of the material helps ensure that the chosen penetrant will be effective in revealing potential defects, ultimately leading to accurate testing results.

Considerations like cost, color, and facility size are secondary factors that, while relevant, do not influence the fundamental compatibility and efficacy of the penetrant in relation to the material being tested.