When you tear or rip a thin paper, you just rip it easily "by its surface" (I am not sure I am saying correctly).
Now, you have a very thick cardboard, and it is not easy to rip the cardboard "by its surface" like you often do to a thin paper.
But a cardboard is made by sticking many layers together, and you can put your fingers in the between layers of the cardboard and rib it apart as shown in the above picture.
Is it correct to say "I ripped the cardboard apart by its side" in this specific situation?