Of the following, which is NOT an advantage of the visual method of non-destructive testing?

The visual method of non-destructive testing (NDT) is a widely used technique that helps in assessing the integrity of materials and structures without causing damage. The significant advantages of this method include its quick application, ease of use, and cost-effectiveness.

However, the ability to detect sub-surface discontinuities is not one of its strengths. The visual method primarily relies on direct observation, making it ideal for identifying surface flaws such as cracks, corrosion, and other superficial issues. Sub-surface discontinuities, such as voids or delaminations lying beneath the surface, typically require more advanced techniques like ultrasonic testing or radiographic testing, which can penetrate beyond the surface layer. Hence, the option indicating that the visual method can detect sub-surface discontinuities correctly recognizes a limitation of this approach.