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.

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