There are two issues here - 1st is that the prop/fan should never catastrophically fail. It's design and application should allow for sufficient safety margin for normal operation AND include a margin for "unforeseen" events (by that I mean a wave strike or minor debris impact (gravel, etc. that are KNOWN possibilities). The 2nd issues is that, once the safety limit has been established by design, the guarding should protect the prop/fan from ingesting objects above the design limit (that could cause catastrophic failure). Get those things right and the risk is negligble
If you want to reduce the energy of a blade then the single most effective way to do it is by reducing the speed it rotates at!
In the real world, it is impossible to protect from catastrophic blade failure using guarding. You would need to guard the intake making the device useless! A high speed/energy plastic fan/prop will cut through almost anything - including steel mesh. A stiff duct wall and mesh make very poor guards - ideally the energy in the blade should be gradually (over distance) absorbed by something "soft". On ducted fan craft, the addition of a few layers of loose (NO resin) glass or Kevlar cloth installed inside the duct cavity would probably be very effective at containment in that area (archery netting demonstrates the principle!).
A proper risk analysis should be carried out - question being what can cause catastrophic blade failure. First thing I'd look at is what is inside the guard? In this case, the hub, belt, frame and any other bits and pieces - should/can any of those go through the blade will it fail? If the answer is yes AND they can't be moved !outside the guarded volume, then they should all have secondary retention (belt retainers, etc.).