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To assist in developing a database of wood material properties for the finite element modeling of wood baseball bats, Charpy impact testing at strain rates comparable to those that a wood bat experiences during a bat/ball collision is completed to characterize the failure energy and strain-to-failure as a function of density and slope-of-grain (SoG) for northern white ash (Fraxinus americana) and sugar maple (Acer saccharum). Un-notched Charpy test specimens made from billets of ash and maple that span the range of densities and SoGs that are approved for making professional baseball bats are impacted on either the edge grain or face grain. High-speed video is used to capture each test event and image analysis techniques are used to determine the strain-to-failure for each test. Strain-to-failure as a function of density relations are derived and these relations are used to calculate inputs to the *MAT_WOOD (Material Model 143) and *MAT_EROSION material options in LS-DYNA for the subsequent finite element modeling of the ash and maple Charpy Impact tests and for a maple bat/ball impact. The Charpy test data show that the strain-to-failure increases with increasing density for maple but the strain-to-failure remains essentially constant over the range of densities considered in this study for ash. The flat response of the ash data suggests that ash-bat durability is less sensitive to wood density than maple-bat durability. The available SoG results suggest that density has a greater effect on the impact failure properties of the wood than SoG. However, once the wood begins to fracture, SoG plays a large role in the direction of crack propagation of the wood, thereby determining if the shape of the pieces breaking away from the bat are fairly blunt or spear-like. The finite element modeling results for the Charpy and bat/ball impacts show good correlation with the experimental data.

Foams are used in a variety of impact energy absorption applications because of their ability to engage in large deformations under steady load transfer during the cell collapse. Quantification of the energy absorption capabilities of foams, including those resulting from repeated loading and unloading, is critical to both modeling and prototype development of systems utilizing these important materials. This paper details a novel process of characterizing a cross-linked high-density polyethylene foam for its applicability within helmet liners designed for low-velocity blunt impact. The foams are characterized using various forms of compression testing and physical measurements. The analyses include examination of the tangent modulus, strain hardness, energy absorption ideality, and energy absorption efficiency. Together, these analyses identify the regions of changing behavior of the nonlinear impact absorption material system. A case study for the materials is presented, which reveals that the examined high-density polyethylene foam exhibits some of the most efficient impact properties during the first impact. However, this case study also identifies that those impact properties can reduce significantly, e.g. a 55% increase in stress in the case of a 0.50 strain-level deformation in the first impact, for a subsequent impact after only a 120 s rest period. The novel combination of testing and analysis presented within this paper enables the developer of a foam energy absorption system to advance their interrogation of foams for repeated large strain deformations and temperature variations.

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