what are the methods used commanly to detect cracks in Heavy duty crankshafts?

Answer 1

magnets and fine metal particals
they may also have a dye also

Answer 2

It's called Mag Flux, and it involves a magnet attached to your crank and then a fine powder sprinkled over the surface. The cracks disrupt the magnetic field and make them visible. Another cheap test would be to use dye penetrate test. A penetrate is sprayed over the surface, allowed to penetrate into any cracks, and then the surface is wiped clean. A dry developer is then sprayed over the surface, and any cracks that soaked up the penetrate show up against the white developer. This method is very good, but messy.