Experimental and analytic evaluation of the influence of high-pressure hydrogen on the low-cycle fatigue of steels

We have established correlations between the effect of hydrogen under a pressure of 35 MPa on the low-cycle fatigue of 15 corrosion-resistant steels, on the one hand, and their initial mechanical characteristics and the parameter of austenite stability A _γ on the other. Materials with A _γ<1 are subjected to catastrophic degradation in hydrogen. We have proposed relations that enable one to range adequately steels with different structures according to their hydrogen resistance and to regulate this property of steels by means of alloying. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 110–112, September–October, 2007.     

Flexural and low-cycle fatigue behavior of atmospheric ice

Accretion of atmospheric ice on power transmission lines may have detrimental effects, sometimes with major socio-economical consequences. The mechanical behavior of this type of ice as an important aspect in the understanding of that issue is still unclear. In the present study, more than 70 tests were conducted using cantilever beams under gradually increasing cyclic load to measure the bending strength of various types of atmospheric ice. Atmospheric ice was accumulated in a closed-loop wind tunnel at −6, −10, and −20 °C, with a liquid water content of 2.5 g m⁻³. Ice samples accumulated at each temperature level were tested at the accumulation temperature, but the ice accumulated at −10 °C was also tested at −3 and −20 °C. Compared to the bending strength results for atmospheric ice under static load, the ice showed less resistance against fracture under cyclic load. It was also revealed that bending strength of atmospheric ice decreases with the test temperature. Another 60 samples of atmospheric ice were also tested under cyclic loads with constant amplitude. The tests revealed that the samples of atmospheric ice accumulated at −10 and −6 °C do not fail under stresses less than 1 MPa after 2000 cycles. At stress levels close to the bending strength of atmospheric ice, however, sometimes the specimen fails after a few hundred cycles. In comparison with the ice accumulated at −10 and −6 °C, atmospheric ice accumulated at −20 °C fails at stresses less than its bending strength. This can be attributed to the colder test temperature and the presence of cavities and cracks in this ice that reduce its bending strength during cyclic stresses.

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Comparative study on biaxial low-cycle fatigue behaviour of three structural steels

In this study the uniaxial/biaxial low‐cycle fatigue behaviour of three structural steels (Ck45 normalized steel, 42CrMo4 quenched and tempered steel and AISI 303 stainless steel) are studied, evaluated and compared. Two parameters are considered for estimating non‐proportional fatigue lives: the coefficient of additional hardening and the factor of non‐proportionality. A series of tests of uniaxial/biaxial low‐cycle fatigue composed of tension/compression with cyclic torsion were carried out on a biaxial servo‐hydraulic testing machine. Several loading paths were carried out, including proportional and non‐proportional ones, in order to verify the additional hardening caused by different loading paths. The experiments showed that the three materials studied have very different additional hardening behaviour. Generally, the transient process from the initial loading cycle to stabilized loading cycle occurs in a few cycles. The stabilized cyclic stress/strain parameters are controlling parameters for fatigue damage. A factor of non‐proportionality of the loading paths is evaluated based on the Minimum Circumscribed Ellipse approach. It is shown that the microstructure has a great influence on the additional hardening and the hardening effect is dependent on the loading path and also the intensity of the loading.     

On effect of hydrogen on very high cycle fatigue behaviours of high strength steels

Abstract

In order to analyse the effect of hydrogen on very high cycle fatigue properties, hydrogen was precharged into two high strength steels. The applied stress intensity factor range at the periphery of inclusions before and after being precharged is approximately proportional to the cubic root of inclusion size. In addition, the applied stress intensity factor range at the periphery of inclusions after being precharged was lower compared with uncharged specimens. The additional stress intensity factor range generated by hydrogen ΔK_H is raised after the hydrogen was precharged. A simple prediction equation of S–N curve was proposed by introducing the hydrogen influence factor. The proposed prediction equation can reasonably describe the S–N curves for precharged specimens.     

The effect of two-frequency low-cycle loading on the fatigue life of steels and welded joints

Some parts of nuclear power equipment (NPE) are subjected mostly to two-frequency loading during their service. Oscillations with a lower stress amplitude are superimposed on the basic slow loading with high stress (or strain) amplitude. The damage cumulation law, which is valid relatively well for random loading, seems to be less suitable for two-frequency loading. According to the design specifications [1] the service life of parts at two-frequency loading may be 10 to 20 times lower than that at single-frequency loading. The decrease in life depends on both the ratio of loading frequencies and amplitudes, and the material characteristics.Published inProblemy Prochnosti, Nos. 1–2, pp. 118–125, January–February, 1995.     

The characteristics of granular-bright facet in hydrogen pre-charged and uncharged high strength steels in the very high cycle fatigue regime

In this study, the effect of hydrogen on fatigue strength of high strength steels in the very high cycle fatigue regime was further discussed. It is found that the calculated results of fatigue strength by modified Murakami’s expression are in good accordance with the experimental results in ±15% error band. The relationship between fatigue life (N _f) and the ratio of granular-bright facet (GBF) to inclusion size (\frac?{A_\textGBF } ?{A_\textinc } ) \left({\frac{{\sqrt {A_{\text{GBF}} } }}{{\sqrt {A_{\text{inc}} } }}}\right) for quenching and tempering (QT) specimens and pre-charged specimens by soaking (SK) and cathodic (CD) charging can be approximately expressed by \frac?{A_\textGBF } ?{A_\textinc } = \fracR_\textGBF R_\textinc = 0. 2 5N_\textf ^{0. 1 2 5} {\frac{{\sqrt {A_{\text{GBF}} } }}{{\sqrt {A_{\text{inc}} } }}} = {\frac{{R_{\text{GBF}} }}{{R_{\text{inc}} }}} = 0. 2 5N_{\text{f}}^{ 0. 1 2 5} ; however, the value of \frac?{A_\textGBF } ?{A_\textinc } {\frac{{\sqrt {A_{\text{GBF}} } }}{{\sqrt {A_{\text{inc}} } }}} for specimens pre-charged by high-pressure thermal hydrogen charging is obviously greater than that for QT specimens and pre-charged specimens by SK and CD charging at an identical N _f. The stress intensity factor range at the periphery of the GBF, ΔK _GBF, was calculated in this work. It is found that the value of ΔK _GBF is not a constant but approximately proportional to (?{A_\textGBF } ) ^{1/ 3} (\sqrt {A_{\text{GBF}} } )^{ 1/ 3} . Besides it is also found that ΔK _GBF decreases with the increase of hydrogen content.