Here's a quick post on how to create the crack in the office window tutorial. It's basically a quick use of the straight line and bezier curve tool and some moving around of nodes.
Combining the spiky shapes (Path/ Union) cleans it up a little bit more - as you won't see the overlaps. Keep in mind to combine two objects at a time in inkscape.
I hope this quick tutorial explains the creation of the crack well enough.
Get the source art (svg file) of this tutorial forUSD 1.00