2 Non-contact Measurement of Strains Using Two Orthogonal Sets of Twin “Blue” Lasers 9 Load cell Electrical buss-bar Axial load Fuel sheath sample Pressurizing gas delivery system Pressure transducer Bleed valve Electrical buss-bar Test chamber Rupture Disk Vacuum pump Thermocouples Laser 1 Laser 4 Laser 3 Laser 2 AC power supply Temperature controller (via thermocouple) Load controller (via load cell) Laser (2 & 4) controllers Laser (1 & 3) controllers Slide 2 controller (for laser 3 & 4) Slide 1 controller (for laser 1 & 2) Quartz window LabView data acquisition and control system Fig. 2.1 Schematic diagram of CNL’s online biaxial burst test facility using four lasers for non-contact strain measurement scanning of the specimen. Both slides are equipped with LVDTs to obtain the axial (vertical) positions of the lasers. The use of “blue” lasers (with a UV wavelength outside of the infrared regime) allows displacement measurements to be made on a sample heated to high temperatures up to 1000◦C. Each laser is aligned using a U-joint and X–Y rotation stage to be set along a targeted cardinal direction. All four lasers are used to scan the specimen for strain measurements, each laser measuring more than one quadrant of the specimen. With the use of two orthogonal sets of twin lasers, the total circumference of the fuel sheath can be measured during creep and ballooning continuously without contacting the specimen. Similarly, the width and thickness dimensions of a flat specimen are measured without contact. Two video cameras, positioned at the front and back quartz windows, are used to record the sample ballooning and burst during the test. A LABVIEW data acquisition and control system is used to regulate the alternating trigger duty cycle of the two sets of twin lasers and collect all the laser displacement data. Also simultaneously, data are collected from the pressure transducers (for internal pressure in the specimen and test chamber), thermocouples (for sample temperatures), tensile machine (for axial load and displacement on the specimen), and the two slides (LVDTs positions). Figure 2.2 shows photographs of the complete online biaxial burst test facility for thermo-mechanical testing of fuel sheath and flat tensile samples. 2.2.2 Test Specimens Figure 2.3 shows two types of test specimens: (a) burst specimen and (b) flat tensile specimen. The burst specimen (Fig. 2.3a) is a tube, with the typical application being a sample of fuel sheath, which is sealed with two laser-welded end plugs that have a hollow spigot at the ends equipped with Swagelok fittings for connection to the pressurizing gas inlet and outlet lines. Specimens 130 mm in length and 9–15 mm outside diameter, typical of fuel sheath samples, can be investigated. Changes in the average tube circumference displacement as small as 0.015 mm can be resolved with this method. Flat (dog-bone) tensile specimen (Fig. 2.3b) can be tested. One typical application is a transverse specimen, cut from the transverse direction of a pressure tube material. The specimen has a gauge length of 10 mm, 6 mm in width, and 3 mm in wall thickness. Changes in the average displacements of the width and thickness dimensions as small as 0.005 mm can be resolved.
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