High-Temperature Extensometers < 950 °C Capacitive
There are many different types of high temperature extensometers available. The type of instrument is dependent on the type of testing (hot tensile, creep, LCF etc) and the type of heating system used (split tube furnace, straight tube furnace, side entry port furnace or environmental chamber).
The PSH8MS is typically used in hot tensile applications with tube type furnaces and consist of a separate measuring unit and suspension arm assembly. The measuring unit uses an LVDT with a fixed travel that remains outside of the furnace, the arms are located mostly inside of the heating system and attach the specimen to the measuring unit.
The 3448, 3548 and 2632 Series of instruments are strain gauge instruments designed for high temperature tension , compression and fatigue testing using a side entry port furnace. The arms are ceramic rods that pass through the furnace and contact the specimen. Depending on the model type, the extensometer is attached to the specimen with ceramic cord, or spring-loaded / counter-balanced mounting.
Several models of creep extensometers are available, each using different materials, head designs, and measuring units, though there are many similarities across the instruments. Most employ a replaceable insert system to not only match the specimen size, but to also allow user replacement of the specimen contacting portion of the instrument. Most instruments also employ a rod and tube design to contain the fixed and movable portions of the instrument. Some instruments are only compatible with LVDTs to produce an electrical output proportional to the measured creep, while others are compatible with LVDTs, SLVCs, strain gauge extensometers or dial indicators for use as the measuring device.
Principle Of Operation
The PSH8MS suspension arms transfer the elongation of the specimen to the measuring unit, they also determine the gauge length and maximum strain value of the unit. The measuring unit has a lower temperature tolerance than the suspension arms, so the arms are rated to a significantly higher value than the measuring unit.
The side-entry models employ heat shields, cooling fins, and optional air or water cooling to protect the strain gauges from excessive temperatures. The ceramic rods pass through the furnace wall, and the length of the rods are matched to the distance from the extensometer body to the specimen surface so there is minimal heat loss from the heating system. Side-entry types require a compatible furnace system which typically features a cut-out and mounting surfaces for the cooling blocks.
The double telescoping rod and tube design used on most of the creep extensometers allows the gauge length to be adjusted over a wide range, rather than being fixed at one point.
The separately available measuring unit has a fixed amount of travel that is selected based on the material being tested and the maximum strain value required. The measuring unit is mounted to a saddle at the lower portion of the instrument, and there is typically a micrometer adjusting knob to zero the instrument prior to starting a test.
- Various models to cover high temperature tests on metallic and non-metallic materials in creep, tension, compression, and fatigue