Over years of flying small airplanes I have seen fatigue cracks and frayed brackets and connections.
I wonder if there is an easy way to search the propagation of stress wave in machinery and connection brackets and parts. I am not concerned here with vibrations so much.

Let's say we have a bracket holding the block of an engine to another part and at short intervals of time it undergoes huge stresses of impact of rotation of the engine cylinders, shafts, etc.

This stress wave propagates inside the bracket and reflects and interfere with its refractions and echo until it dies out by dispensing its energy into friction or transferring to other parts!
I know there is a large volume of empirical tests involved but I am concerned with a mathematical modeling scheme. Thank you!

  • $\begingroup$ The key word in your question is "easy". It won't be. $\endgroup$
    – DLS3141
    Aug 3 '16 at 14:37
  • $\begingroup$ So your question is, "What options exist to model the effects of stress on metal parts?" $\endgroup$
    – feetwet
    Aug 3 '16 at 18:54
  • $\begingroup$ Yes. I guess stress wave travels in metals with a speed near sound pressure waves, much faster than speed of sound in the air. So could we study the destructive behavior of these waves, how they compound the damage when they interfere with their reflection around the edges of the metal media? I see how rivets have to be replaced after fatigue removes their ductility, brackets which are fused by heat to bolts! e g. If you hammer a piece of metal hard to leave dents on it, how the shock spreads in it? does it travel too fast to give metal enough time for elastic behavior, leaving a pass of ruin? $\endgroup$
    – kamran
    Aug 3 '16 at 19:20
  • $\begingroup$ I'm not sure what your specific question is, but you seem to be describing what is referred to as 'Structural health monitoring'. You may find the following (quite readable) MEng thesis reviewing Structural Health Monitoring in Commercial Aviation useful: dspace.mit.edu/handle/1721.1/73846 $\endgroup$
    – atom44
    Aug 5 '16 at 11:11

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. https://www.youtube.com/watch?v=_CdULw4-j_o

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...

  • $\begingroup$ thank you @Fabio. I had checked before and hove come across some FEM analysis of large and small parts of engines and individual parts and cranks in jet and piston engines. So far I have not seen much on the stress load trajectory and analytical way of modeling the behavior of different alloys. The reason I did not want to get into dynamics part of it is because there has been extensive research already done on it and there are ways of balancing the centripetal forces and using better quality bearings and in small planes detailing the airfoils to damp the vibrations or flutter. $\endgroup$
    – kamran
    Aug 5 '16 at 18:46

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