Hint: At which location along the beam will the cantilever fail?

On a wall-mounted cantilever beam, the maximum bending moment, hence bending stress occurs at the fixed side of the beam where it’s mounted. This is visually illustrated in the bending moment diagram below.

For the cantilever beam, the top and bottom sides are subjected to two difference forces: compressive and tensile forces subjectively.

- Top Side: Tension (-> away from wall)
- Bottom Side: Compression (<- toward wall)

If the cantilever beam material has a lower compressive yield strength than a tensile yield strength, the beam will fail at the bottom. Alternatively, if the tensile yield strength is lower, the beam will fail at the bottom.

Check out this Youtube Video for a good visual reference.

Correct me if I’m wrong, but won’t the top side be under tension if the load is being applied downwards as shown?

Why the beam will fail at the top if the cantilever beam material has a lower compressive yield strength than a tensile yield strength?

In my mind, if the material has a lower compressive yield strength < tensile yield strength, the cantilever beam should fail at the compressive area, which is the bottom point (assume a point load’s direction is downward)

Hey @Weijun_Zhang, good catch! The original solution didn’t have that updated. Different materials have varying values for compressive and tensile yield strength, so your logic is correct.