Life of composites under variable-amplitude fatigue using ST method
Seneviratne, Waruna P.
Tomblin, John S.
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Seneviratne, W., & Tomblin, J. (2021). Life of composites under variable-amplitude fatigue using st method. Paper presented at the AIAA Scitech 2021 Forum, 1-11. doi.org/10.2514/6.2021-0923
The primary goal in a damage-tolerance certification program is to avoid catastrophic failure due to fatigue, corrosion, or accidental damage throughout the operational life of the aircraft. The damage-tolerance philosophy is well established for metallic airframes, where proven methods (structural analysis and inspection procedures) and supporting databases exist to detect damage, as well as to predict crack growth and residual strength. Damage growth mechanics and load spectra for composite and metal structures have significant differences that make the certification of composite-metal hybrid structures challenging, costly and time consuming. Metallic damage growth under spectrum loading is approximated using Minor’s rule. Anisotropic heterogeneous characteristics and change in failure modes over the fatigue life, as well as multiple failure mechanisms that interact with each other make it challenging to predict damage growth in composite structures. Consequently, most of the damage mechanisms and wearout approaches discussed in the literature also depend on empirical data for refinement or calibration. Some approaches only discuss failure progression under certain loading configurations and often is specific to a material system. A semi-empirical fatigue analysis technique referred to as the strength tracking (ST) method is developed using the residual strength degradation or wearout. This model is developed using Sendeckyj analyses conducted on fatigue S-N curves with various stress ratios (R-ratios) expected in a load spectrum, so that the residual strength degradation during spectrum loading can be tracked. Unlike Minor’s rule, the presented approach is sensitive to load sequencing. Furthermore, this approach also accounts for data scatter observed in composite fatigue data for reliability analysis.
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