8 R. W. L. Fong and J. Patrick (extensive hoop strain) before the tube bursts. If the tube is highly stressed axially, in a certain biaxial stress state, contractile deformation (inverse of ballooning) can result as the tube elongates in length and breaks apart, axially. A strong anisotropy in creep behavior is evident in zirconium-based alloy materials that is highly correlated to the preferred orientation of αphase grains in the microstructure. The deformation behavior (e.g., creep rates) is also affected by oxidation or hydriding. For pressure tube materials made from Zr-2.5Nb alloy (a dual-phase material), the high-temperature deformation behavior is consistent with that of fuel sheath single-phase materials made from dilute zirconium alloys (e.g., Zircaloy-2, Zircaloy-4, or Zr-1Nb). At Canadian Nuclear Laboratories (CNL), we have developed a specially designed facility for thermo-mechanical testing of uniaxial tensile and fuel cladding burst specimen for test inside a gas-tight environmental chamber. The facility is equipped with a uniaxial tensile/compressive loading system, a pressurizing gas delivery system, and an AC power supply for resistive heating of the specimen. In particular, this test facility is specially equipped with a four-laser scanning system for noncontact strain measurement. Techniques such as using video extensometer [1], mechanical LVDT extensometer [2, 3], optical telescope [4], photographs [5], high-powered digital cameras [6, 7], bonded strain gauges [8, 9], and laser extensometer [10– 14] have been used, but all these methods can only measure a one-directional (azimuthal) diameter of the tube which may not be its maximum diameter. Therefore, we have developed a 4-laser measurement technique that allows for online scanning of the fuel sheath specimen during the test. This technique provides a means to determine the maximum deformation rate of change in the specimen. In biaxial burst testing of fuel sheath, the changes in maximum creep rate during creep and ballooning usually corresponds to the tube maximum bulge (diameter) location, where metal instability would first occur. As such, representative time events of the deformation data can now be extracted from the online scanning measurement. The representative data of the tube’s deformation behavior can be used for modelling or testing existing deformation equations. This paper provides a brief description of design features of CNL’s biaxial burst test facility developed for thermomechanical testing of fuel sheath and uniaxial tensile specimens. Two examples of non-contact strain measurements made on these two types of specimens are presented. 2.2 Experimental Design 2.2.1 Design of CNL’s Online Biaxial Burst Test Facility Figure 2.1 shows a schematic diagram of the design of CNL’s biaxial burst test facility. A specially designed gas-tight chamber is mounted on a commercial tensile testing machine, which provides a controlled uniaxial tensile or compressive axial stress loading on a test specimen. Although not illustrated in the figure, the chamber has three mutually accessible compartments (top, middle, and bottom boxes), where the top and bottom boxes housed the feedthroughs for electrical buss bars for connections of an AC power supply to the specimen. The sample is joule-heated at high heating rates with alternating current. A spot-welded thermocouple on the specimen and a PID controller are used for temperature control. The bottom box is equipped with access ports for gas evacuation and for purging (back filling) argon and a port for the pressuring gas inlet line connection to the specimen for burst testing. The gas lines are electrically isolated using ceramic spacers. A rupture disk is equipped in the bottom box for rapid exhaustion of excess pressurized gas from the chamber into an expansion tank immediately when a tube sample bursts. The top box is equipped with a flexible gas-tight (bellows) connection to the middle box. It has an access port for the pressurizing gas outlet line connected to the top end of the burst specimen that is used for argon purging before test and for bleeding the pressurizing gas for control of internal pressure in the burst specimen during the test. A pressure transducer installed at the outlet line is used to measure the pressure inside the fuel sheath burst sample. The top of this box is connected to an additional compartment equipped with feedthroughs for thermocouple wire sensors, pressure transducer, piping lines for cooling-jackets, and a flexible gas-tight (bellows) connected to the pull bar of the tensile testing machine. The middle box surrounding the test specimen has eight-sided walls, which is equipped with two demountable covers secured on opposite sides of the chamber. The two covers both have two gas-tight quartz windows that provide four cardinal directions for viewing with the four lasers used to scan the sample. One of the demountable covers is used to allow access for changing the test specimen on the tensile machine, installing spot-welded thermocouples on the specimen, connection of the pressuring gas inlet and outlet lines (for bursting the sample), and sample rotation alignment to the laser beam (mainly for a flat specimen) before closing the cover. Four-laser displacement sensors (KEYENCE model LJ-V7200, “blue” line laser), mounted on two mechanical slides, are positioned outside of the octagonal chamber. The slides are programmed to move the lasers up and down to allow axial
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