Lynn M. Powers

Slow crack growth and fracture toughness of sapphire for a window application

The slow crack growth parameters, fracture toughness, and inert strength of the r-and a-planes of sapphire were measured in water in order to perform a life prediction on a pressurized sapphire window. The window is being considered for use in a combustion chamber on the International Space Station. Sapphire is relatively susceptible to stress corrosion in water despite a large strength in the absence of humidity. Two life prediction approaches were considered: a deterministic fracture mechanics approach and a Weibull based reliability approach. Preliminary results indicate that the window is feasible if a short finish is avoided. Fractography and additional predictions are being performed.

Lifetime reliability evaluation of structural ceramic parts with the CARES/LIFE computer program

The computer program CARES/LIFE calculates the time-dependent reliability of monolithic ceramic components subjected to thermomechanical and/or proof test loading. This program is an extension of the CARES (Ceramics Analysis and Reliability Evaluation of Structures) computer program. CARES/LIFE accounts for the phenomenon of subcritical crack growth (SCG) by utilizing the power law, Paris law, or Walker equation. The two-parameter Weibull cumulative distribution function is used to characterize the variation in component strength. The effects of multiaxial stresses are modeled using either the principle of independent action (PIA), Weibulls normal stress averaging method (NSA), or Batdorfs theory. Inert strength and fatigue parameters are estimated from rupture strength data of naturally flawed specimens loaded in static, dynamic, or cyclic fatigue. Two example problems demonstrating cyclic fatigue parameter estimation and component reliability analysis with proof testing are included. 26 refs.

A Numerical Round Robin for the Reliability Prediction of Structural Ceramics

A round robin has been conducted on integrated fast fracture design programs for brittle materials. An informal working group (WELFEP-WEakest Link failure probability prediction by Finite Element Postprocessors) was formed to discuss and evaluate the implementation of the programs examined in the study. Results from the study have provided insight on the differences between the various programs examined. Conclusions from the study have shown that when brittle materials are used in design, analysis must understand how to apply the concepts presented herein to failure probability analysis.