Design and performance of a microwave strain measuring system for materials tests

Abstract

Excellent sensitivity and accuracy in the measurement of deformation occurring in materials tests have been achieved with a newly developed microwave frequency sensor and instrumentation system with possible application for strain tests inside nuclear reactors. The strain sensor comprises a microwave cavity resonating in the circular TE 113 and TM 110 modes. Detection of axial strain occurs due to the changes of resonant frequency incurred by cavity length changes. Axial strain sensitivity for the TE 113 mode was 6 x 10$sup -6$ per MHz, whereas radial dimensions of the cavity were related to frequency of the TM 110 mode. Aperture coupling of the cavity to the end wall of K/sub a/ band waveguide provided signal excitation of the two monitored modes. Phase locked frequency stability enabled digital count/display of resonant frequencies to within 70 kHz at 35 GHz. Room temperature tension test results demonstrate a strain measuring sensitivity (+- 1 x 10$sup -6$) and accuracy (+ -1 percent of the measured value) equivalent to those of electrical resistance strain gages. The system yields accurate measurements of elastic strains as well as small departures from elastic response and hysteresis behavior during unloading and reloading. Creep test results confirm that measurement sensitivity and accuracy are retained in elevated temperature tests. Strain response on loading, subsequent creep deformation and creep recovery after unloading can all be measured in detail. Stability of microwave sensor calibration after exposure for 22 x 10$sup 6$ seconds at temperatures from 728$sup 0$K to 866$sup 0$K is shown to be excellent.