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In the reliability analysis for a fail-safe structure, statistical information regarding service time until fatigue crack initiation as well as subsequent reduction in residual strength is indispensable. Safety during the service life requires that critical cracks be detected at inspections before the probability of the damaged structure meeting a load exceeding its residual strength has reached an unacceptable level. About 20 sheet panels of 2024-T3 aluminum have been fatigue tested until cracks of various lengths appeared, using a flight-by-flight load program. The fatigue panels had four rows, with four small strips of the same sheet material in each row. The strips were attached to the sheet by two rivets, forming 32 stress concentrations in each panel. After fatigue cycling, the strips were replaced by continuous stringers and the residual tensile strength of the panels was determined. The mean of the logarithm of the number of flights to crack initiation amounted to 4.43, that is, 27,000 flights, while the standard deviation was 0.17. The relationship between the residual strength of the stiffened panel and the critical crack length shows rather little stochastic variation. When the residual strength is plotted versus the crack propagation time, however, the scatter does not seem to be negligible.
fatigue(materials), reliability, probability theory, statistical analysis, distribution theory, Weibull density function, crack propagation, crack initiation, stress analysis, fatigue limit, tensile properties, fatigue tests
The Aeronautical Research Institute of Sweden, Stockholm,