Two bats were considered for study, a wood and an aluminum bat. Both bats have similar strength or resistance to bending. The main difference between the two bats is that the aluminum bat is hollow having a shell thickness less than 0.10 inches. The high stress regions for each mode shape are indicated in red. With ball contact, all of these modes are excited, but some are excited more than others and contribute more to the stress in the bat.
Stress is important because it is stress that causes bats to break.
It turns out that the aluminum bat responds much differently than the wood bat after impact. In the aluminum bat, the shell modes are excited more than the bending modes. The wood bat, being solid, does not have the shell modes and responds by bending. The maximum stress in the aluminum bat occurs in the barrel and the maximum stress in the wood bat occurs in the handle.