Ash, maple and birch are three popular wood types for baseball bats (Figure 1). The ash wood is said to be wood-porous as it has very distictive hard and soft layers. Birch and maple are said to be diffuse-porous as they have less distictive layers. Layers in wood are growth rings consisting of Earlywood and Latewood (Figure 2). Ash has Earlywood with large pores called vessel elements. These large pores make the Earlywood less dense and weaker than the Latewood. Maple and birch have small pores that are distributed evenly through-out resulting in a more uniform structure and a harder wood overall. The Earlywood for ash, or soft layer, can disrupt a crack from propegating through the diameter of the bat. This often results in a "leaf sping" like fracture as the soft layers will fail leaving seperated hard layers. Maple on the otherhand is hard through-out making easy for a crack to propegate through the diameter of the bat. This is why maple bats tend to "explode" as they fracture cleanly absorbing little of the bats kinetic energy. Here is a good article that reinforces these points.
Figure 1 shows cross-sections of birch (left), ash (middle) and maple (right)
Figure 2 shows a close-up of the ash cross-section clearly showing the different layers.
Figure 3 Ash magnified at 35X. Sample size is approximately 0.124in x 0.093in.
Figure 3 Maple magnified at 35X. Sample size is approximately 0.124in x 0.093in.
Figure 3 Birch magnified at 35X. Sample size is approximately 0.124in x 0.093in.
