Sure it’s rather a simple outcome. Forces acting on a metal...

Sure it’s rather a simple outcome. Forces acting on a metal structure cause a bending momentum, continue this bending and eventually a crack may appear. The stronger the forces the greater the structure will bend increasing the chance of metal fatigue to set in. A wing has a lifting force under it in flight so wing bends up, you can see this if you sit next to a window. Landing on the ground that lifting force disappears and gravity kicks in and the wing bends down under its own weight. Also the fuselage has these alternating forces from pressurisation, swells out at high altitude and compresses in on the ground.Carbon composites also can fail from this.So after so many flying cycles it’s deemed a crack check should be done to make sure a crack has not developed. Bend a piece of wire many times and the section at bending will get warmer and begin to crack then snap. Very low loading will mean the tested piece will hardly bend at all and the onset of a crack will never occur or it might after many years of the little wiggles.