As other captains have mentioned Devastating Strikes and Detonations are different situations.
Devastating Strikes are when you destroy an enemy ship with a single artillery salvo, torpedo salvo, or aircraft attack by causing damage over 50% of the destroyed ship's normal HP. These usually occur when a ship gets hit by multiple shells into a critical part of the ship (such as the Citadel) in quick succession. The only way to avoid a devastating strike is by careful positioning and being mindful of what is shooting you and from where.
A Detonation on the other hand is caused by the ignition of at least one of the Magazine modules of the ship. Historically, they are where the powder charges and ammunition for the main artillery are stored. In game, sufficient damage that causes one magazine to explode results in the destruction of the entire ship regardless of how much health it has remaining. As of patch 0.7.2, a ship must be under 75% of its health to have a chance of detonation. This is the situation in which a Juliet Charlie flag prevents.
I hope that helps you understand the game mechanics better. If you have any questions please don't hesitate to reach out on the forums again.