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Stress rupture and crack velocity data must be used to predict the lifetime of structures. However, to use stress rupture data, the large amount of scatter characteristic of brittle materials must be treated, and to use crack velocity data the initial defect size must be known. Since the time-dependent failure of many brittle materials involves propagation of a crack from an initial defect to a critical size at which stress rupture occurs, crack velocity and stress rupture data can be combined to predict the initial defect size. This technique has been developed and used to provide (1) statistical interpretations of stress rupture data as a size distribution of critical defects and (2) probability of failure predictions of structures, including the size effect. An expression for the stress rupture curve which includes the initial defect size is produced by integrating an expression for the crack velocity curve that includes the threshold stress intensity factor. The scatter is treated along lines of constant initial stress intensity factor, so that the scatter is independent of rupture life. This approach is applied to literature data for soda lime glass in water. An initial defect size is inferred for 261 data points, and the Weibull distribution is used to display the data in terms of the inverse square root of the initial defect size. The defect size distribution for soda lime glass is predicted, and an example of the failure prediction of a structure is included where the size effect is considered.
fracture mechanics, defect sizes, stress rupture, crack growth rate, scatter, size effect, Weibull distribution, stress intensity factor, soda lime glass, structural reliability, brittle materials
Manager—CAD technology, General Electric Company, Plastics Business Group, Pittsfield, Mass.