Published: Jan 2004
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A new approach is introduced which can evaluate fracture toughness of aircraft and aerospace alloys by using static parameters that are obtainable from full stress-strain curves available in the MIL-HDBK. With this approach, the energy absorption rates related to the plastic deformation at the crack tip and near crack tip are estimated and used to extend the Griffith theory of brittle fracture to fracture mechanics of ductile metals. An equation has been established that can define the critical crack length as a function of fracture stress. Having fracture stress and critical crack length on hand, the plane strain and plane stress fracture toughness can be calculated by applying the stress intensity factor equation. The calculated fracture toughness for 2219-T8, 2014-T6 aluminums, and Ti-6Al-4V titanium alloy was compared against the experimental test data generated from reliable sources. Excellent agreement between test data and the theory was found. When fracture toughness was calculated by this method, fatigue crack growth curves for the above-mentioned alloys were then generated and compared with test data. The threshold stress intensity factor value, ΔKth, in region I of the da/dN curve, was approximated by establishing a point on the Kitagawa diagram associated with the region of linear elastic fracture mechanics. Results of ΔKth values estimated by this method were in fine agreement with values observed for many materials in the NASGRO database. Two additional points were estimated in region II that enable to establish the Paris region. Probabilistic evaluation of fracture toughness and fatigue crack growth analysis considered a 5 to 10% coefficient of variation of material Kc, and Kth random variables.
probabilistic, cumulative distribution function, density function, probability sensitive, material variation, fracture toughness, fatigue crack growth rate, life estimation, extended Griffith theory
Boeing Space & Communication, Huntington Beach, CA
Alpha Star Corporation, Long Beach, CA