There have been many accidents involving metal fatigue. How do the engineers, maintenance team, etc. know if an aircraft has metal fatigue? What is done to "minimize" metal fatigue or repair it?
How do the engineers, maintenance team, etc know if an aircraft has metal fatigue?
First of all, is not an "aircraft" that has fatigue, but a component.
To the naked eye, there is no way to detect metal fatigue until it starts being too late: you can only see cracks that are already forming and/or propagating.
There are tools available to allow for early detection, my bachelor's thesis was about one of these: the photoelasticity of certain materials is used. A light coat of one such material is applied on the new component and the photoelastic properties and exams done during the lifetime of the component tell the maintenance people what is the current status of the inner layers of the piece.
Another method uses electric resistance, as metal fatigue, by cracking the metal, increases the resistance.
Depending on the scenario, X-rays could also be used.
What do they do to "minimize" metal fatigue or repair it?
You cannot minimize fatigue, once it starts setting in, it is an irreversible process. You can delay the likely catastrophic failure by applying strengthening elements around the affected one, but the safest course of action is to replace the component and recycle the old one.
Engineers have a variety of ways to detect metal fatigue in aircraft:
- Visual inspection: sometimes cracks are visible to the naked eye, or can be seen under magnification
- Auditory inspection: sometimes a thump sounding wrong can give a clue that there's an issue. It's not a primary means of checking for fatigue, but something that may clue in an astute engineer that something needs to be looked into
- Ultrasound: Ultrasound uses very high pitched sound waves to image materials and can be used to find cracks inside materials
- Radiology: xrays and other types of radio imaging can be used to find sub-surface cracks
- Visible dyes: flourescent dyes in penetrating oils show up cracks which make it to the surface
- Magnetic powders: these only work on iron-based parts which are rare on an aircraft but are used
Here's a good article that goes into some details on non-destructive fatigue testing.
Some metal fatigue can be spotted visually as an visible crack or as a collection of tiny cracks that can give the metal a 'frosty' look but most is done via non-visual non destructive testing techniques such as radiography (basically X-raying) or Ultrasonic testing to search for microfractures before they reach the problem stage. Visual checks are no use for platform critical damage as by the time you see it it is well into the failed zone which is why NDT techniques are used to detect an impending failure well before it causes a problem.
There are other techniques such as Dye Penetrant tests where a dye in a carrier fluid is painted into the surface and then wiped off. The fluid is light enough to penetrate and 'wick' into any fine cracks and will highlight cracks that are invisible to the eye. The dye is often fluorescent.
As for what can be done it very much depends in the extent and location of the cracking. Extensive damage and/or damage to critical area will be repaired by replacement, minor damage can be gouged out and welded up. The best method of minimising fatigue it to prevent it and so most manufacturers will feed back fatiguing data from surveys to the design sections who will design out the weak point if one exists. Real life data trumps even the most extensive FEA predictions.
The pilot listens and notify the technicians that the plane is giving out strange noises.
And now seriously: For each plane type there are cards which parts need to be replaced or inspected after certain flight hours, start count or time. Together with required quality control at production and maintenance it should prevent metal fatigue incidents as much as possible.
And for the rest - @AndyW already mentioned techniques used during the inspections