Got sidetracked with some cloth tearing while I was trying to do some cloth shading :P
On the left is a SOP solver with some SOP tearing, on the right is the default tearing from the cloth solver. Trying to get a more consistent tear going on as well as weakening along an existing tear (which is why the left one tears easily once it has started)
In Houdini 12.5 ... tearing wasn't working for me in 13 ....
you can get the hip file and an explanation from here: http://ihoudini.blogspot.com.au/2013/11/sop-solver-cloth-tearing.html