Conventional Kolsky bars

Abstract

A Kolsky bar, also widely known as a split Hopkinson pressure bar (SHPB), is a characterization tool for the mechanical response of materials deforming at high strain rates (10 2-10 4 s-1). This chapter presents the brief history, general working principles, considerations in design, and data reduction process of a Kolsky bar, illustrated by its compression version. 1.1 Background Generally, material properties such as yield stress and ultimate strength, listed in handbooks and design manuals, are obtained under quasi-static loading conditions using common testing load frames with the guidance of standardized testing procedures. To ensure product quality and reliability under impact conditions such as those encountered in the drop of personal electronic devices, vehicle collision, and sports impact, the mechanical responses of materials under such loading conditions must be characterized accurately. However, high-rate loading conditions are beyond the scope of conventional material testing machines. For example, if one end of a 10-mm long specimen is deforming at speeds of 1-100 m/s, the strain rate in the specimen is 10 2-10 4 s-1. This strain-rate range is commonly faced in collision-related loading situations. However, this range is difficult for most testing machines to reach in a well-controlled manner. On the other hand, the strain-rate range produced by hammer impact corresponds to dynamic events commonly encountered in engineering applications, such as club impact on golf balls, helmet impact on hard surfaces, and bird impact on aircraft engine components. Therefore, it is desired to determine the material properties under hammer-blowing conditions. A hammer can deform a specimen to failure; however, there are two major issues if the purpose of such a hammer impact is to characterize material properties. The first issue is that there is little detailed information that can be recorded. The second issue is that the conditions on the specimen are not well controlled. To obtain dynamic response of materials under laboratory controlled conditions, Kolsky (1949) solved these problems with a very cleaver solution. Instead of direct impact on the specimen, he placed two elastic rods on both sides of the specimen and then struck one of the rods with an explosive blast. This concept is schematically shown in Fig. 1.1, where the elastic rod between the external impact and the specimen is called the incident bar and the rod on the 1 Mechanical Engineering Series,