Materials that seem homogeneous often have internal strains, or voids, or even inclusions. Under stress, rather than uniform deformation (bending), those flaws may undergo brittle fracture, or stretch excessively, or become chemically active.
A cosmic ray can create internal damage, a particle decay track.
So, after some kinds of handling (bending, hammering, stretching, twisting, rapid heating/cooling) it is normal to find some cracks have propogated from any and all flaws. Since those cracks weaken the material, they RELIEVE the local stress, and that means the cracks may remain microscopic. Dye penetration is one way to detect
small cracks at a surface. Usually, such cracks pick up oxygen, and do not close and 'heal', but will grow in strain and corrosion conditions.
A thin aluminum sheet, annealed, can be nearly crack-free (as aluminum foil can stretch without tearing), or can have impurities that, even in small quantities,
cause hardening or internal strains. The exact aluminum alloy and its history of mechanical working and chemical surface treatment will determine its behavior when worked.