Prop Strikes

Incidentally, what's a "stress riser?"

A stress riser is any flaw in a structural piece that concentrates
the stresses through that area to the point that failure might occur. A
common example is cutting glass. The "cutter" doesn't cut; its small
roller causes a shallow crack in the glass that will allow you to break
the glass cleanly when it's bent. On a propeller a nick intereferes
with the lines of force in the blade, causing them to have to bend
around the nick and so concentrating them below the damage. Their
concentration can start the propeller cracking. The blade undergoes
huge G forces outward, thrust forces forward, and drag forces
chordwise; a prop is often the most heavily stressed part on the whole
airplane, and I often see chewed-up props on otherwise cared-for
airplanes.
Owners don't understand the risks. A prop that throws a foot or so
of blade is liable to tear the engine out of the mounts before the
pilot can get it stopped, and guess what happens to the CG when about
300 pounds of engine and prop leave a 172? The airplane can't even
glide.
I demonstrate the stress riser phenomenon to my class using
strips of light aluminum flashing. The students try (unsuccessfully) to
tear a piece of the flashing. Then I file a tiny nick in the edge, and
it tears easily. A second piece with a nick dressed out becomes
impossible to tear.

Dan