What is the minimum allowable radius for the test cap used in SFRM bond strength testing?

• A. 0.5 inch
• B. 1 inch
• C. 1.5 inch
• D. 2 inches

Answer: (B)

In SFRM bond strength testing, the cap that is bonded to the sprayed material must provide a well-defined contact area for loading. The radius of that test cap controls how the load is distributed across the bond interface. A cap with a radius that’s too small concentrates stress around the edge, which can cause edge debonding or cap/adhesive failure before the bond itself is truly tested. Setting a minimum radius ensures enough surface area to spread the load evenly and reduces edge effects, leading to more reliable, repeatable results that reflect the actual bond strength. The required minimum radius is one inch, which ensures a proper balance between adequate contact area and compatibility with standard testing fixtures. Larger radii can be used if needed, but the one-inch minimum is the standard threshold.