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The effects of stress ratio (R) and maximum applied stress-intensity factor (Kmax) on fatigue crack growth rate (da/dN) have been investigated. Fatigue crack growth behavior was analyzed by plotting applied cyclic stress-intensity factor (ΔK) versus applied Kmax for an α+β titanium alloy (Ti 6-2-2-2-2) at several values of da/dN. This analysis revealed two distinct regions of fatigue crack growth behavior. At threshold da/dN (≈ 10-10 m/cycle) and for Kmax < 4.6 MPa√m the threshold cyclic stress-intensity factor (ΔKth) is greatly influenced by crack closure. For closure free fatigue crack growth (Kmax > 4.6 MPa√m), a subtle but distinct decrease in ΔKth is observed with increasing Kmax. Plots of ΔK versus Kmax were also generated for AA 2024 test data produced in two different studies. These data also revealed two distinct regions of fatigue crack growth behavior consistent with the Ti 6-2-2-2-2 results. This methodology can be used to determine closure free fatigue crack growth and to monitor subtle changes in ΔKth.

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The results of a fatigue crack growth life study of the service cracking in the dovetail notch roots of the turbine blades in the last stage of a low-pressure steam turbine are presented. After IS years or 91 518 service hours, the turbine was inspected for damage, and several cracked blades were found in the last stages of both spindles. Detailed stress analysis of the blade and rotor connection was accomplished by three-dimensional finite-element methods. Dynamic vibration analysis of the structural connection was also conducted. Environmental fatigue crack growth rate experiments were conducted using surface flawed specimens fabricated from the Type 403 stainless steel blades. The behavior in hotwell steam condensate showed acceleration over ambient laboratory air environment. Metallurgical and fractographic examination of several blade specimens showed that surface cracks emanated at the base of corrosion pits located at critically stressed locations in the dovetail notch roots. Fracture mechanics analysis showed that the applied ΔK level exceeded the threshold ΔKO level in hotwell steam condensate. Spectrum loading fatigue crack growth life analysis was conducted for the most critical location in the blade for a service spectrum consisting of hot start-up and shutdown cycles, small amplitude vibratory cycles due to steam flow, and an annual 20 percent overspeed. These results showed surface crack growth life to detectable size of 62 500 h and an inspection interval of 6750 h or 1 year of operation.

The results of an evaluation of the fracture tolerance of three components of the thrust vector control servo-actuator for the solid rocket booster of the space shuttle are described. These components were considered as being potentially fracture critical and therefore having the potential to fall short of a desired service life of 80 missions (that is, a service life factor of 4.0 on a basic service life of 20 missions). Detailed stress analysis of the rod end, cylinder, and feedback link components was accomplished by three-dimensional finite-element stress analysis methods. A dynamic structural model of the feedback system was used to determine the dynamic inertia loads and reactions to apply to the finite-element model of the feedback link. Twenty mission stress spectra consisting of lift-off, boost, re-entry, and water impact mission segments were developed for each component based on dynamic loadings. Experimental fracture response data da/dN, KIc, and Kc were used for the structural alloys of the actuator components. These alloys were Inconel 718, 17-4 PH (H1025), 6061-T651, 2024-T851 (T351), and A286. Linear fatigue crack growth life and residual strength analyses were performed on the component critical locations using the Forman da/dN equation and CRACKS III computer code. Most components were determined to have the potential of reaching a service life of 80 missions or service life factor of 4.0.