What might block the openings of discontinuities if the test specimen surface is not properly cleaned?

The phenomenon of blockages in the openings of discontinuities primarily arises from the presence of materials or contaminants on the surface of a test specimen. When a specimen is not cleaned appropriately, substances like paint, scale, and core material can occupy these openings and interfere with inspection processes such as non-destructive testing.

Paint can create a barrier that conceals cracks or other flaws. Scale, which often forms due to oxidation or rust, can similarly mask problems beneath its surface. Core material, if present from manufacturing processes, can also obstruct the view of any discontinuities. Therefore, the presence of these specific materials directly affects the ability to detect and assess defects effectively, making them the primary concern when discussing blockage at discontinuity points.

The other aforementioned substances, while potentially problematic, do not align as directly with the issues of blocking openings inherent to the testing of discontinuities as paint, scale, and core material do. Each of those alternatives addresses different contaminants that may impact surface quality but not as specifically related to the presence of openings in discontinuities.