Strain gauging for accurate determination of K and G in impact tests

Abstract

Impact testing of materials is becoming increasingly important as a wide range of new materials are being developed for demanding high loading-rate working conditions. Charpy pendulum and many other impact-testing machines are being better instrumented to provide more information about the forces acting on a specimen up to and during fracture. Mostly, the force sensors are near the points of contact on the striker or the support and these can provide well for recording the overall forces acting on the specimen to be monitored. Of increasing interest is the distribution of stress and strain within the specimen during the initiation and propagation of fracture. This paper reports research using on-specimen strain-gauge sensors for impact testing of non-metallic specimens. Comparisons are made between force-time traces from sensors on the specimen and those located on the striker. Observations are made as to how the stresses relate to the fast crack in the core of the material specimen and those acting on the surface of the material about the crack, and also those acting on the plastic hinge formed on the compression side of the specimen. Optical and scanning electron microscopic studies are made of the crack surfaces and high-speed photography is used to observe the crack propagation in specimens with and without side-grooves to guide the crack and increase constraint.