PSI - Issue 60

P.A. Jadhav et al. / Procedia Structural Integrity 60 (2024) 631–654 P.A. Jadhav et. al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 2. A schematic of Round Bottom BPFF given in CSA Standard (2016)

Fig. 1. Bearing Pad Fretting Flaw (BPFF) in a Pressure Tube given by CSA Standard (2005)

During the manufacturing process for creating PTs, efforts are made to minimize the amount of hydrogen present in the alloy matrix. However, it is not possible to eliminate hydrogen completely. Hydrogen is picked up from the coolant, and the concentration of hydrogen increases over time. If a stress raiser, such as a volumetric or planar flaw, is present, the hydrogen will diffuse to the tip of the flaw. Under certain circumstances, this hydrogen can form brittle, plate-like structures called zirconium hydrides at the flaw tip. As more hydrogen migrates to the flaw, the hydrides grow in size. When they reach a critical size, they crack leading to the formation of a crack from the volumetric flaw. This crack however gets arrested by the surrounding ductile matrix. This creates a new stress raiser, starting the process over and leading to what is known as "delayed hydride cracking." This type of cracking is called "delayed" because it is time-dependent and requires an incubation period before hydrides of a critical size can form and crack as reported by Coleman (2003). The resulting fractures display striations, which indicate each cycle of hydride plate cracking and arrest. When the crack reaches a critical size, the material will fail by fracture. The movement of hydrogen in the Zirconium alloy is dependent on Coleman (2003):  Concentration: Hydrogen atoms move from a region of h igh concentration to low concentration following the Fick’s first law. The diffusivity is dependent on temperature.  Temperature: Hydrogen atoms preferable move to the cooler regions when subjected to a temperature gradient. This movement is opposite to the concentration gradient.  Stress level: In presence of a stress field, the hydrogen atoms move from a region of compressive stresses to tensile stresses. If the concentration of hydrogen atoms surpasses the maximum amount that can be dissolved in the alloy, the excess hydrogen will form platelets of zirconium hydride. The solubility of hydrogen in a zirconium alloy is dependent on the temperature of the alloy, and this relationship is represented by a curve that exhibits hysteresis. The solubility of hydrogen in the alloy will depend on whether the temperature is increasing or decreasing. The temperature at which hydrogen can dissolve in an alloy and the temperature at which it will precipitate out as a solid are different for the same concentration of hydrogen. The curve that represents the saturation point of hydrogen in a zirconium alloy when the material is being heated is called the Terminal Solid Solution Dissolution (TSSD) curve. The curve that represents the saturation point of hydrogen in a zirconium alloy when the material is being cooled is called the Terminal Solid Solution Precipitation (TSSP) curve.