What are the common defects in timber structural element?

Broken in half?

Those irregularities appearing in or on timber that may effect its strength and durability are the timber defects..
1- blue stain
2- gray stain
3- twisting of timber fibers
4- attack of termites or fungus
5- rotting of wood during seasoning

Go look at a piece of dimensional lumber and see what is wrong with it. Google "lumber imperfections"