Graduate Thesis Or Dissertation
 

Fracture energies of Zircaloy and some zirconium alloys

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  • The fracture mechanics of Charpy V-notch specimens of Zircaloy-4 and phosphorus contaminated Zircaloy-4 were investigated. Three-point bend and impact tests with Charpy V-notch samples provided an efficient and economical method of fracture evaluation. Fracture failure was monitored by the force-displacement curve. Failure was initially analyzed using fracture toughness values as recommended by ASTM. Zirconium, a ductile material, failed to meet the validity criteria for determining KIC. The force-displacement curves were then dissected into four distinct segments (yield, plastic, crack, and tear) and the integrated energy values determined and analyzed. The combination of stress values, energy values, and surface analyses made it possible to explain the fracture mechanism. Zirconium fracture was studied as a function of (1) thickness (0.7 to 2.0cm), (2) orientation (TL, TS, LS, LT), (3) temperature (-200°C to +100°C), (4) loading rate (static and dynamic), (5) V-notch configuration (blunt, pre-cracked, and side-grooved), and (6) phosphorus concentration (10 to 360 ppm). Fracture toughness criteria could not be met for any orientation, for temperature as low as -200°C, for thickness as great as 2.0cm nor for phosphorus concentration as high as 360 ppm. As a substitute for fracture toughness values, energies were studied. Energy analysis showed that the tear energy (or crack arrest energy) dominated at high temperature. The energy required to initiate cracking was independent of temperature. Phosphorus concentration decreased the tear energy and crack propagation energy but did not affect the energy for crack initiation. Phosphorus did not seriously affect energy values or fracture toughness. Loading rate strongly affected fracture toughness but was unimportant to energy values. Sample orientation strongly affected energy values. SEM studies of different orientations as a function of temperature revealed several fracture mechanisms contributing to failure including micro-void formations microvoid coalescence, and serpentine glide. Charpy V-notch geometries strongly affected the strain constraints and the energy required for crack initiation, propagation, and tear. Sample thickness was independent of all energies except tear energy.
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