Bat hitting a ball When a bat hits a ball, consider two cases:
1) The batsman goes for a defense, and stonewalls it, to reduce its speed.
2) the batsman goes for a shot, e.g. a home-run, etc.
in which case will the bat have the highest chance of breaking due to the impact?
And am I right in assuming that the force on the bat due to the reaction force of the ball is the only thing that causes it to break?
 A: The chances of breaking your bat will increase with wanting to change the momentum of the ball. That includes not only a change in the speed of the ball, but also the change in its direction/angle. Hitting a straight drive will increase the chance of breaking, whereas playing a leg glance or a late cut will decrease the chance of breaking. A block stroke will be somewhere in between.
(Note that a block stroke, although usually played with a light grip, is not intended to "catch" the ball and reduce its speed. Rather, it is intended to change the direction away from the stumps and downwards.)
To hit a "six" (a cricket home run), you actually don't have to change the momentum of the ball too much (and risk breaking your bat). Youtube shows you how it's done with the Dilscoop: "That's using the pace to your advantage."
(Taken from here.)
In practice, the best ways to break your bat are to play with a dried-out bat, jamming the ball between your bat and the ground whilst attempting to hit a fast yorker, and hitting a rigid metal object (in the nets). I've done all.
A: It depends on where the ball hits the bat. Bats aren't broken only by tremendous force, but also by the kinetic energy they receive from the ball--they vibrate and bend and then break. In this way, stonewalling or stopping the ball could be worse than just hitting it, since this forces the bat to absorb most of the ball's kinetic energy. However, if you hit with the bat and the ball collides with an anti-node (i.e. not in the "sweet spot", this could be close to the tip or lower by the handle), that could make the bat vibrate a lot, too. Doing this, the ball won't gain as much energy as it should, and the bat is more likely to break. So according to a rep from Louisville Sluggers, most breaks are due to hitting with the wrong part of the bat. 
A: A good swift swing of the bat changes the ball's momentum the most.  Recall that force F = change in momentum.  According to plain simple Newton's laws, this puts the greatest force on the bat at the point of contact.  As fast as interatomic forces can convey stresses and changes of motion, that impact along with other forces such as the batsman's grip, create the most torque and strain at the most vulnerable place of the bat.  
A: Every one might have studied about material science . When a body (bat)undergoes repeated stress due transfer of kinetic energy by the ball.this in turn develop micro structural crack in the bat,which keeps on developing when a ball travels with a kinetic energy which is required to develop and break the bat is strikes by the bat.It eventually breaks 
