Clamping-force application for kolsky bar tension grips

Abstract

Dynamic testing using tension Kolsky bars presents a number of challenges, including the design of the grips to hold the specimen. Basic solid mechanics suggests that a grip can be designed such that the lateral clamping-force times the coefficient of friction exceeds the failure strength of the specimen. This approach is complicated by the specimen getting smaller—thereby releasing part of the clamping force—as it is stretched because of the Poisson strain. Therefore, a tension grip needs to apply an adequate lateral force plus it must have enough elastic recovery to maintain the force as the specimen shrinks. In applying the concept to a Kolsky bar, an impedance matched grip is ideal, which suggests a smaller grip body which requires a short clamping bolt. The effect of the bolt length on the clamping force is illustrated by analyzing a proposed design. Gripping of composites and polymer fibers for tension testing is especially challenging. The clamping force application for testing soft-material specimens using a Kolsky-bar grip is also discussed.