Intrinsic, closure-free, threshold stress intensity, ΔKth,i, is uniquely related to a certain computable near-tip residual stress, σ*; σ* is sensitive to applied stress ratio at given Kmax under constant-amplitude loading and, particularly so, to load history under variable-amplitude loading. ΔKth,i will vary significantly, depending on load conditions, rendering its characterization under controlled σ* crucial. Experiments on additive manufactured (AM) PH1 steel and 18Ni300 maraging steel confirm the possibility of characterizing the relationship between ΔKth,i and σ* using small-size compact tension, C(T), specimens. This opens the way for material characterization from miniature specimens cut out from smaller components. Application of the relationship between ΔKth,i and σ* leads to improved assessment of the structural integrity of AM components through residual crack growth analysis as well as specification of allowable defect size. It can serve as an input for optimization of AM process parameters.

Procedures for obtaining da/dN-ΔK curve on the basis of empirical a-N data were shown. Estimating approaches of crack growth rates for constant, block, variable amplitude, and similar loading were given. Computational methods of equivalent load range ΔPeqv, required for ΔK approximation in case of variable amplitude loading and plotting of a da/dN-ΔK chart, were presented. Advantages and disadvantages of suggested methods were also described. The application of approximation methods for da/dN-ΔK curve determination was imposed in case of constant amplitude loading and for modified SAESUS loading.

Closure-free intrinsic threshold stress intensity, ΔKth,i, can vary significantly depending on near-tip residual stress, σ*. σ* is a computed parameter that is sensitive to hysteretic stress–strain crack-tip response to applied load history. The test procedure developed in earlier work to characterize ΔKth,i as a function of σ* was optimized and validated on three different materials, demonstrating both consistent estimates of ΔKth,i as well as its unique relationship with σ*. A detailed description of the improved test procedure is provided to enable implementation on a target test system. Establishment of the ΔKth,i versus σ* relationship permits improved estimates of extended duration spectrum load fatigue crack growth associated with high cycle fatigue (HCF) and very high cycle fatigue (VHCF). The test procedure to characterize this relationship merits adoption as a standard test practice given the promise of bridging a longstanding gap between laboratory test techniques and requirements of residual crack growth life estimates under long-duration spectrum loading.

The coefficients of the Paris equation for crack growth rates represent fundamental material properties describing the resistance against crack propagation. Up to now, there is no unified procedure to identify these parameters. The current paper shows an approach for testing and analytical processing of its results which makes it possible to obtain the intermediate region of fatigue crack growth rates on a da/dN-ΔK curve as a result of each test. Also, ways to estimate the crack closure effect on fatigue crack growth and computation of effective ranges of stress intensity factor are shown. The approach was validated by the example of an aluminum alloy 2024-T3. Further development trends for the described method are proposed.

This a preprint and has not been peer reviewed. Data may be preliminary.