I think this is more a comment than an answer, but I have too small reputation to comment (poor me!).
Summarizing, you are asking: how dynamic loads propagate inside a structure?
I would underline that, in my perspective, telling that you are not concerned about vibrations, is a wrong starting point (fatigue crack derives exactly from them); for a vibration, you have a variable strain and consequently a variable stress and consequently you face fatigue problems.
For static cases, a very interesting technique is the so called "photoelasticity", that correlate the stress to the "pictorial" view by means of the so called "birefringence" (I am not really an expert, do not ask me more, but it is really impressing seeing it!).
For dynamic cases, situation becomes more and more complex and further important with the development of composite materials, more prone to fatigue problems.
Several attempts are numerical, and they fundamentally obtain operating loads from modal analysis, operational modal analysis and general in-situ tests. Then, this loads are traduced into a refined model for stress analyses.
Several techniques, depending on materials, boundary conditions and loads type, can be used for transforming them in stresses.
Moreover, fatigue tests, in particular for standard "metal" airplanes start very soon with the project, starting from parts to the entire mockup model of the aircraft.
Moreover, there are several projects for deducing the state of stress during operating condition. Consider that, i.e., if you mount an extensometer in one part of you airplane, you measure the strain state and consequently the stress.
Several attempt are made with FBG and OF in general for deriving stress state during operating conditions.
Finally, there are also technique for damage detection based on something similar to the one you suggest, but they are still at they early stages: they seem to be promising, but till now they worked with very simple geometries and boundary conditions (nothing similar to your aircraft brackets...)
I hope I gave you some interesting references...