superpostion

I am hoping someone has some historical knowledge of some
interesting mechanics, or just an interest to discuss it. A superposition principle is widely used to solve crack problems, with illustrations like these:

http://ars.sciencedirect.com/content/image/1-s2.0-S0142112308002211-gr1.jpg

http://ars.els-cdn.com/content/image/1-s2.0-S0142112399000778-gr2.gif