Measurement of Impact Strains by a Carbon-Strip Extensometer

Abstract

Abstract This paper describes a method of experimentally ascertaining the actual stress-time curve in a mechanical part subjected to an impact blow. The apparatus consists of a resistance strain gage in conjunction with a high-speed recording oscillograph, the combination being sufficiently rapid in response to record strain variations occurring within a few microseconds. Consequently, the strains during impact are recorded with sufficient accuracy for detailed analysis. Longitudinal strain waves in long bars striking end to end have been subjected to theoretical analysis and have served as a subject for testing of the apparatus. The theory is reviewed in this paper, and computed results based thereon are compared with experimental data. The agreement is shown to be satisfactory, the form being as predicted and the magnitude within a few per cent of the computed value. The strain waves and reflections occurring in this simple case are surprisingly complex. An investigation of other impact problems by this method, such as the correlation of standard impact tests of materials, might well yield information of considerable value.