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Glass, crystals, and brittle ceramics are subject to time and moisture related failures due to stress corrosion, or slow crack growth. Equations governing time to failure are well established, but presume knowledge of crack velocity, flaw shape factor, and initial stress intensity factors that are often unavailable. Alternative crack growth data, obtained from dynamic fatigue testing, is often used, but data of stressing rates and moist strength are necessary for calculation, and again are often unavailable. Programs that are available to calculate failure times presume such knowledge as well. A complication results in use of any available data in these programs, in that residual stress in the material of interest is not included. Tests show that residual stress is present even for polished surfaces, and such stresses greatly reduce failure times, often by several orders of magnitude. Further, such programs presume a completely moist environment, which is often not the case for spaced based or airborne systems. To overcome these difficulties, an approximate method of computing margin of safety in glass and other brittle ceramics under stress, time, and moisture is provided when only flaw growth fatigue resistance parameter and inert strength are known but details of crack velocity and initial crack depth are unknown. The method includes effects of relative humidity and residual stress, and is readily programmed to a spreadsheet to aid in quick computation. Using appropriate safety factors as herein defined, the method is compared to the exact formulation for several cases in which velocity data are known. Results compare with quite favorable accuracy, precluding the need for costly fracture mechanics data and unwieldy computation.
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