Predicting crack patterns in SiC-based cladding for LWR applications using peridynamics

SiC continuous fibre reinforced SiC matrix (SiC-SiC) composites are a proposed material for accident tolerant fuel cladding. Thermomechanical models of SiC-based cladding under light water conditions indicate that microcracking in the radial direction of the tubing may lead to a loss of hermicity. SiC-based tubing is known to have anisotropic elastic properties but the effect of this anisotropy have not been incorporated into existing thermomechanical models of clad cracking. This work augments an existing isotropic 2D peridynamic model of cracking and damage in the r-θ plane of a SiC-based cladding to account for the orthotropic elastic properties of SiC-SiC composite tubing. Three SiC-based architectures are modelled under normal operating conditions of a UO2-fuelled pressurised water reactor (PWR). The results of the anisotropic SiC-cladding model are compared with the results of the isotropic model, and the sensitivity of results to material anisotropy, thermal conductivity, and applied linear power rating are analysed. The results of this analysis show that anisotropy has a significant effect on the damage and crack patterns observed in the r-θ plane of SiC-based cladding, if either an inner or outer monolith is present. The anisotropic model predicts more cracks in two layer clad with an inner monolith and higher levels of damage in a two layer clad with an outer monolith than the isotropic model. Under normal reactor conditions the outer monolith clad architecture was found to remain hermetic

Durability of high density polyethylene for potable hot water applications: crack propagation.

University of Minnesota M.S. thesis. September 2012. Major: Mechanical Engineering. Advisors:Dr. Susan Mantell, Dr. Jane Davidson. 1 computer file (PDF); viii, 70 pages, appendices A-C.Polyethylene (PE) pipes, are used for water delivery, are susceptible to oxidation. As a result of oxidation PE becomes brittle and brittle pipes/tubes crack under the influence of tensile loads. These cracks initially propagate slowly and later on grow quickly becoming unstable. The focus of this study is slow crack growth in high density polyethylene (HDPE). Crack propagation experiments were conducted to determine the dependence of crack growth on degradation and stress levels. HDPE samples, with 0.3mm thickness, were exposed to 80°C chlorinated water (5-8 ppm) for up to 65 days. Thin samples were selected to ensure uniform degradation through the thickness. Although the brittleness of the polymer can be evaluated using strain-at-failure, the drawback of this method is that it destroys the sample. The Carbonyl Index (CI) obtained by Fourier Transform Infrared (FTIR) spectroscopy was established as a nondestructive measure of the degradation level. CI ranged from 35 to 93. A higher value of carbonyl index represents a greater extent of degradation. The relationship between CI and loss of mechanical performance was validated by strain-at-failure. Crack propagation tests were conducted were conducted on degraded polymer samples at constant load. The load (stress level) ranged from 5.1 to 9.2 MPa. In all 5 samples were tested. It was found that the crack propagation rate ranged from 6.31 x 10-10 to 1.26 x 10-2 m/s while the stress intensity factor ranged from 0.91 to 4.07 MPa√m. For a single degradation level, regardless of stress, the data when converted to log scale, and fit with the linear elastic fracture mechanics (LEFM) relationship = CKn. As the degradation increased the crack propagation rate increased such that all data were fit by the relationship = C(CI)Kn such that the exponential parameter ‘n’ was a constant for all the samples regardless of the level of degradation. The LEFM model fit to the data was best for moderate and high levels of degradation corresponding to CI of 55 and 90. Scanning Electron Microscopy (SEM) images show minimal deformation in the region around the crack tip, and ductile fibril stretching in the process zone. While the polymer had become brittle upon oxidation, there is local ductility in the process zone. An LEFM approach is typically applied to brittle materials, while the SEM results show that crack propagation is a combination of brittle and ductile behavior. Future studies should consider other modeling approaches that allow for ductile behavior in the process zone