A Replica Technique for Measuring Static Strains

Abstract

A new technique for determining static strains in engineering specimens and structures is described. It is basically a direct-measurement method in that relative displacements of surface scratches are measured; the novel feature of the technique is the recording of scratch patterns by means of accurate replicas taken with the structure in the unloaded and loaded states, or after loading, and their comparison for the measurement of strains. Measurements of relative displacements on a replica pair may be taken to an accuracy of at least one micron by visual microscope techniques; this accuracy permits good determinations of low elastic strains over gauge lengths from 1/2 cm to 1 cm, and measurements of plastic strain over very much shorter gauge lengths. Replication is a quick process, being complete in one minute or less and the replicas have permanence of the order of months. Measurement of the replicas may be carried out under laboratory conditions, remote in distance and time from the test, if that is necessary. The replicas are compensated for temperature within certain limits, permitting successive replicas to be taken over long intervals without correction for variations in the ambient temperature. A description of equipment for application and measurement of replicas is given; some results defining the limitations of the technique are presented, and a discussion on systems of measurement and computation of elastic strains and plastic elongations is included. Examples of application to fields of elastic and plastic strain are quoted.