High Load Ratio Fatigue Strength and Mean Stress Evolution of Quenched and Tempered 42CrMo4 Steel

Abstract

The fatigue strength at a high number of cycles with initial elastic–plastic behavior was experimentally investigated on quenched and tempered 42CrMo4 steel. Fatigue tests on unnotched specimens were performed both under load and strain controls, by imposing various levels of amplitude and with several high load ratios. Different ratcheting and relaxation trends, with significant effects on fatigue, are observed and discussed, and then reported in the Haigh diagram, highlighting a clear correlation with the Smith–Watson–Topper model. High load ratio tests were also conducted on notched specimens with C (blunt) and V (sharp) geometries. A Chaboche model with three parameter couples was proposed by fitting plain specimen cyclic and relaxation tests, and then finite element analyses were performed to simulate the notched specimen test results. A significant stress relaxation at the notch root became clearly evident by reporting the numerical results in the Haigh diagram, thus explaining the low mean stress sensitivity of the notched specimens

Fatigue resonant tests on S140 and S150 grade corroded drill pipe connections and pipe bodies

“…Indeed, the high tensile axial load on the drillstring introduces a mean stress effect [18], and the make-up torque preload also leads to a tensile static load, especially at the Pin FET root. Although the FE analysis accurately provides the thread root stresses, any ductile steel such as the quenched and tempered 42CrMo4 [19] similar to drill pipe steels, can intrinsically experience a significant mean stress relaxation, thus reducing the detrimental effect of the tensile load. Although analyses are usually focused on the connection, the upset can also be a critical region for fatigue initiation, especially when a fatigue crack grows and then degenerates into a wash-out [3,6].…”

A Numerically Efficient Method to Assess the Elastic–Plastic Strain Energy Density of Notched and Imperfective Cast Steel Components

“…The elastic SED ∆W (e) ij,k,R=0 represents the difference between the total and the plastic SED, as given by Equation (29).…”

“…In order to consider non-linear deformations during finite element analysis, the constitutive material equation provides the connection between strains and stresses [27]. Several elastic-plastic models are reported in the literature to include plasticity effects during cyclic loading such as Bauschinger's effect or ratcheting [28,29]. A common constitutive model to describe the inelastic behaviour of metallic materials is given by Chaboche [30].…”

Mean stress and plasticity effect prediction on notch fatigue and crack growth threshold, combining the theory of critical distances and multiaxial fatigue criteria

“…The hardening rule is given by the superposition of three Chaboche kinematic hardening submodels . An optimization procedure, described in detail in Bertini et al, has been devised to tune the coefficients of the kinematic submodels so as to minimize a penalty function defined as the cumulative absolute deviation of the numerical from the experimental values taken from the stabilized hysteresis loop measured at the largest strain amplitude (corresponding to a total axial strain amplitude ε a = 0.015, and a plastic axial strain amplitude ε a,pl = 0.012). This choice is motivated by the fact that this strain amplitude level is comparable with those experienced at the tip of the notched samples investigated in the present paper (see the following discussion) and that, at this strain amplitude, the near‐symmetric behavior of the material can be more easily captured by the hardening model than that at the lowest strain levels.…”