How much force can bolt cutters exert? What's the mechanical advantage of an ordinary, let's say, 3 feet long bolt cutters? 
How many pounds can they exert? 
I'm asking because I have a lock which is apparently immune to over 9 tons of cutting force ... I'm curious if 18,000 pounds of force can be exerted by an ordinary person using bolt cutters. 
 A: Ok I used ForceEffect by AutoDesk to do a static analysis. I placed a $18000\mbox{ lb}$ load on the cutter and measured the reaction force on the handle. The result was $12\mbox{ lb}$ only.

Considering that most humans can squeeze at about $30 \mbox{ lbs}$ it is entirely feasible you can cut the lock.
A: This depends on the design and measurements of the bolt cutters.  Some bolt cutters have a single fulcrum, some have two.
If you have a one-fulcrum bolt cutter, then you have a pair of first-class levers working together.  So, to get the mechanical advantage, you need to measure the distance from the point where you apply the force on the handle to the fulcrum ($d_1$), and the distance from the fulcrum to the point where the blades contact the lock ($d_2$).
In that case, the mechanical advantage is $\frac{d_1}{d_2}$, meaning that if you apply force $F_{in}$, the output force $F_{out}$ is given by
$$F_{out} = \frac{d_1}{d_2} F_{in}$$.
If you have a two-fulcrum bolt cutter, then you've actually got four first class levers working together, and your mechanical advantage is different.  For that type of bolt cutter, there is some trigonometry involved related to the angles at which the two levers are attached; the mechanical advantage actually changes over the course of the cut, so it's difficult to say here what it would be exactly.
