Simulated Composite Baseball Bat Impacts Using Numerical and Experimental Techniques

A study has been undertaken to develop techniques for assessing baseball bat durability and performance. Experimental investigations were conducted by constructing a testing device to simulate the ball-bat interaction occurring in play. A sensitivity study of the machine’s inertia and forces on the bat were found to have a small effect on hit ball speed, and a much larger effect on bat stress. A dynamic finite element analysis was employed to simulate the ball-bat interaction. The ball was modeled as a linear viscoelastic material. This provided a mechanism of energy loss during impact (coefficient of restitution) and accounted for its observed speed dependence. The model has found favorable correlation with experimental results of bat durability and performance. A bat reinforcement scheme was shown to significantly reduce bat stress with minimal accompanying changes in bat performance. INTRODUCTION Non-wood baseball bats were approved for collegiate play over 25 years ago. The primary motivation was to reduce the expense associated with replacing broken wood bats. There is growing concern recently, however, that non-wood bats (designed to hit the ball faster) have changed the historic balance of the game and may increase the risk of injury to players. Recent bat performance limits adopted by the NCAA have helped motivate interest in developing test methodologies and predictive capabilities regarding bat performance. The goal of these regulations is to make the hitting characteristics of non-wood bats similar to wood bats. The current study considers techniques to predict and measure the performance of baseball bats by examining the inverse problem: increasing the durability of wood bats. MATERIALS Key to modeling how components of a system, such as a bat and ball, interact is obtaining an understanding of the properties of the materials involved. The current study involves a ball and reinforced wood bat. The static properties of wood (Northern White Ash) and reinforcing composite (fiber reinforced polymer) are well understood and readily obtained from the literature [1,2]. However, the properties of a baseball are not available and are difficult to measure. In the current study two types of baseballs are considered. The first is a traditional baseball, produced from yarn wound around a cork and rubber pill, and covered with leather. The second baseball type is synthetic, and commonly used in batting cages. It is injection molded from an air filled rubber and designed to simulate the hitting characteristics of a traditional baseball. Several attempts were made to extract the elastic properties of the balls through quasi-static compression testing. These included compression loading a traditional baseball between flat plattens, and comparing the load displacement curve with a large deflection Hertzian type contact model [3]. For the case of the homogeneous synthetic ball, a uniaxial compression specimen was cut from the ball. The elastic modulus was then found from the compressive stress-strain response of this coupon. In both cases, however, the elastic modulus was apparently too low. This was determined by examination of ball deformation patterns from numerical impact simulations. It was postulated that the disparate strain rates achievable with a load frame and that occurring in an actual ball-bat impact (roughly three orders of magnitude) may be significant. Thus a time dependent material model and high load rate device were needed. A strategy was developed to determine the properties of the baseball at high strain rates indirectly by modeling an