Different analytical models of damage accumulation by cyclic plasticity have been developed to predict fatigue crack growth from monotonic, cyclic, fracture toughness and crack propagation threshold properties. The models' development logic is condensed as a flowchart, which emphasizes, in a clear and easily comprehensive way, all the required modeling steps. 1020 and API 5L X60 steels and 7075‐T6 aluminum alloy were used in the experimental verification of the models. Samples were extracted from materials of the same heat, in order to have a reliable comparison. The experimental results are better predicted by the models that use the plastic part of Coffin–Manson's equation to calculate the fatigue life of small volume elements ahead of the crack tip, and expressions of the HRR type to represent the elastic–plastic strain amplitude in the cyclic plastic zone.

The fatigue crack growth rates curves of engineering materials depend on two parameters. In addition to the dependence on the classical stress intensity factor (SIF) range ΔK, there is a dependence on the mean load (or mean SIF), mainly in the near‐threshold region. The present paper provides some useful suggestions and good practices for using three of the current available methods to reduce this second dependence through the use of tuning constants. The methods considered here are the Elber, Walker and Vasudevan (or unified approach). For each approach, multiple regression analyses are performed on experimental data from the literature, and the correlations in two and three dimensions are graphically analyzed. Numerical examples of crack growth analysis for cracks growing under nominal stresses of constant amplitude in single‐edge and notch/hole geometries are performed, assuming an identical material component to that of the available experimental data. The resulting curves of crack size versus number of cycles (a versus N) are then compared. All three models gave approximately the same (a versus N) curves in both geometries. Differences between the behaviors of the (a versus N) curves in both geometries are highlighted, and the reasons for these particular behaviors are discussed.

Fatigue assessments by the more robust strain-based approach demand the determination of the local strain history from nominal stresses. For notched members, a cyclic constitutive relation, the stress concentration factor SCF and a strain concentration rule are used with this aim in some approximate solutions. The plastic part of the cyclic constitutive relations for many materials is well adjusted by a Ramberg-Osgood RO type equation. The parameters in the RO equation are the cyclic strength coefficient and exponent H' and n' respectively. These parameters can be experimentally determined or estimated from the condition of strain compatibility between the RO and the Coffin-Manson-Basquin CMB equations. The present paper discusses the influence that the use of both types of parameters, independent or experimentally determined and compatible or estimated , has on the numerical stress-life curves of the AISI 4340 Aircraft Quality steel. By numerical stress-life curves we mean the stress amplitudes and the fatigue-life that result from the numerical solution of both, the strain-life CMB and the stress-strain RO relations, for the same strain amplitude. This would be equivalent to using a linear strain concentration rule notched members with two RO equations, one with independent parameters and the other with compatible parameters, for stress and life calculations. The effects of the stress state are also accounted for in the present investigation since both, stress-life and stressstrain equations are modified in accordance with the total deformation theory of plasticity and through the introduction of a plane stress biaxial ratio. The principal finding of the present paper is that, for the studied material, the numerical stress-life curves that result from the use of compatible and independent parameters are indistinguishable for the same stress state. Consequently, there are no important implications on life time calculations when the cyclic stress-strain curve is estimated in such a way that compatibility conditions for the AISI 4340 aircraft quality steel are ensured.

An experimental device was constructed with the aim of testing various cylindrical V-notched specimens until fracture and under variable amplitude torsional loads. The specimens had different notch depths resulting in the same number of values for the stress concentration factor. Strain gages directly bonded at the specimens’ surface and using a slip ring system for their communication with the conditioner, allowed the measurement of the actual applied loads. The well-known rain flow cycle counting procedure was then applied on the scaled signal for identifying the frequency of the 64 classes of stress amplitudes and means. The traditional nominal stress-based approach was then evaluated as the most widely used tool for fatigue lifetime calculations. As the occurrence of stress amplitudes above the endurance fatigue limit tends to lower it, the Miner elementary method was used. The results show damage sum ranges between 0.5 and 6.4 with a mean value of 2.0. Despite the small size of the sample used in the present paper (only 13 tests), these significant deviations are in agreement with previous results reported by different researchers.

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