A SYSTEM FOR MEASURING BIAXIAL CREEP STRAINS OVER SHORT GAUGE LENGTHS

This paper presents a new system for measuring biaxial strains over ga ge lengths of 150 micrometers at temperatures up to 300 degrees C unde r constant loading for times as long as 1300 hours. This system has be en used to measure axial and lateral strains at notch roots with a rad ius of 1 millimeter in zirconium specimens, A complete description of the techniques and procedures as well as an overview of the results is given in this paper. The principle of the measurement is the interfer ence of laser light reflected from three tiny indentations placed in a n orthogonal pattern at the notch root. The indentations, which are ap plied with a Vickers microhardness tester, produce four fringe pattern s; two are used for strain measurement in the axial direction and two for the lateral direction, The motions of these four fringe patterns a re monitored with linear diode arrays and converted via a microcompute r system into the two measured strains. The system operates on a real- time basis so it is possible to record the strains as the notched spec imen is loaded and then held at constant load.