I need some help understanding principle stresses. I understand the formula i have problems calculating the stress in x and the stress in y to plug in to the formula. More simply how do i know what contributes to the stress in x and the stress in y for a given problem.
The first thing you should do is to draw a free body diagram of an element in the area you are interested in. The loads imposed will affect that FBD. That is what tells you what you need. Granted, there is a certain amount of practice and intuition involved as well.
Oh, come on, Brian. He has a legitimate question and your reply was a bit harsh. To answer your question, there are several steps you'll need to go through. Send me private message with your email address and I'll send you a .pdf of a good example. Good luck!