To perform a high temperature strain measurements, the sapphire extrinsic Fabry-Perot interferometer (EFPI) sensor head was attached to a silicon carbide rod and placed in a RF induction heater mounted on an MTS load frame. The high-temperature ceramic adhesive is designed to withstand temperatures up to 4000° F. As a basis for comparison, a gold-coated silica EFPI sensor was attached along side the sapphire EFPI.
Initially, strain data was recorded at room temperature as the silicon carbide rod was slowly loaded to 4500 lbs. When the temperature was stabilized at a maximum of 2000° F, the SiC rod was again loaded to determine the effects of temperature on strain. Although the sapphire sensor was capable of measuring strain at higher temperatures, die induction heater, being poorly insulated, was not able to further heat the test material.
The purpose of the second experiment was to determine the survivability and repeatability of tiie sapphire EFPI due to high-cycle fatigue loading. The long-term survivability of the sensor, in both static and fatigue loading, has been demonstrated. Although the load frame was shutoff after 200,000 cycles, one would expect to be able to fatigue load tiie sensor for millions of cycles since the components of the EFPI sensing head are free to move inside the alignment tube. This is consistent widi data obtained for similar silica opdcal EFPI sensors diat were fadgue loaded to over 10 million cycles as compared to resistive strain gages that typically can be cycled only to 60,000 at 2000°F.