Effect of specimen size on fatigue crack growth rate in AISI 4340 steel

An investigation has been carried out to study the influence of specimen size parameters (thickness, with and aspect ratio) on fatigue crack growth rate. Compact tension specimens with a TL orientation, prepared from aircraft quality AISI 4340 steel and heat treated to a yield strength level of 1000 MPa, were used. All testing was done at a constant δK level. The investigation demonstrates that specimen thickness and width have no significant influence on fatigue crack growth rate for AISI 4340 steel. On the other hand, fatigue crack growth rate was found to increase marginally at high aspect ratios (a/W0.55). Paris constants C and m were also evaluated.     

Effect of high-temperature degradation of heat-resistant steel on the mechanical and fractographic characteristics of fatigue crack growth

We study the influence of degradation of 12Kh1MF heat-resistant steel under service and laboratory conditions on the variation of the threshold characteristics of its crack resistance and fractographic characteristics of the near-threshold growth of a fatigue crack. It is shown that hydrogen dissolved in the metal intensifies its cracking in the prefracture zone, first, along the grain boundaries and then, when the time of contact between the metal and hydrogenating media becomes sufficiently large, along the boundaries of subgrains, which leads to a stable decrease in the effective thresholdK _theff. The process of fragmentation of the damaged metal by secondary cracks is a typical feature of crack growth with the threshold rate. The longer the contact of the metal with the hydrogenating medium, the lower the growth rate for which the fracture surface contains fatigue grooves and the larger the opening displacement of crack lips decorating these grooves. Hydrogen also facilitates the shift of the regions of tunnel crack growth relative to the corresponding areas on the conjugated fracture surfaces and, thus, increases the level ofK _thcl. The actions of the factorsK _theff andK _thcl are mutually compensating and, therefore, it is impossible to determine the actual influence of hydrogen on the cyclic crack-growth resistance of the damaged metal by analyzing only the value ofK _th. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv; Wroclaw University of Technology, Wroclaw, Poland. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 4, pp. 49–58, July–August, 1999.     

Effect of plastic anisotropy on crack growth resistance under mode 1 loading

Crack growth in a solid with plastic anisotropy is modeled by representing the fracture process in terms of a traction-separation law specified on the crack plane, and crack growth resistance curves are calculated numerically. A phenomenological elastic-viscoplastic material model is applied, using one of two different anisotropic yield criteria to account for the plastic anisotropy. The analyses are carried out for conditions of small scale yielding, with mode I loading conditions far from the crack-tip. Different initial orientations of the principal axes relative to the crack plane are considered and it is found that the steady-state fracture toughness is quite sensitive to the type of anisotropy and to the angle of inclination of the principal axes relative to the crack plane.     

Double influence of hydrogen on fatigue crack growth in heat-resistant steels

We investigate fatigue crack growth in cast heat-resistant steel pipes of reforming furnaces in a vacuum, in air, and in gaseous hydrogen in the temperature range 20 – 800°C. It is shown that the character and intensity of hydrogen-induced effects depend on temperature and loading amplitude. For the crack resistance threshold, we discovered the phenomenon of temperature inversion of these effects. Namely, the value of K _th in hydrogen increases with temperature up to 400°C and then decreases. Under high-amplitude loading, the influence of hydrogen manifests itself only in the acceleration of crack growth. The ambiguity in the influence of hydrogen on the plastic strain resistance of the material at the crack tip is analyzed on the basis of well-known physical concepts of the influence of hydrogen on the processes of generation and displacement of dislocations. The effects discovered in this work are explained by the realization of different fracture mechanisms and different types of hydrogen-induced effects under different conditions. Thus, at low temperatures (up to 400°C) and high K, one observes a decrease in the tearing strength; the case of low temperatures and low K is characterized by the shear fracture mechanism and the strengthening effect of hydrogen; for high temperatures ( 400°C) and low K, the shear fracture mechanism is combined with a decrease in the plastic strain resistance under the influence of hydrogen.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No, 4, pp. 7–15, July – August, 1994.     

Effect of hydrogenation and plastic predeformation of steel on its crack resistance

We study the joint effect of hydrogenation and plastic predeformation of 40KhN steel of drill strings after heat treatment on its crack resistance under static and cyclic loads. In the nonhydrogenated state, plastic predeformation of steel only slightly decreases its short-term crack resistance and, owing to crack closure, significantly increases the crack-growth resistance in the near-threshold region. Hydrogenation of the material sharply decreases the short-term crack resistance of steel and intensifies the growth of fatigue cracks, especially in the middle-amplitude region of low-frequency loading. The joint action of plastic predeformation and hydrogenation results in a maximal decrease in the static and cyclic crack resistance of the steel under investigation. We also analyze possible consequences of such an effect from the viewpoint of the structural strength of a drill string. Ivano-Frankivs'k State Technical University of Oil and Gas, Ivano-Frankivs'k; Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 5, pp. 67–70, September–October, 1999.     

Effects of a single tensile overload on fatigue crack growth in a 316L steel

The aim of this study was to investigate the effects of a single tensile overload on subsequent fatigue crack growth in a 316L stainless steel. Fatigue tests were conducted under the plane stress condition, and further supplemented with compliance measurements and field emission scanning electron microscopy (FESEM) observations. Effects of a tensile overload, e.g. initial acceleration and subsequent retardation of fatigue crack growth, were explained and quantified by FESEM and compliance measurements. The FESEM observations suggest that the initial crack growth acceleration stems from void and quasi-cleavage fracture within the fatigue damage zone in the vicinity of the crack tip. Systematic compliance measurements taken during fatigue crack growth suggest that the overall crack growth retardation is related to strain hardening and residual compressive stress produced by the plastic deformation associated with the tensile overload.     

The early stage of fatigue crack growth in martensitic steel

In low ductility and high strength steels, the early stage fatigue behavior associated with non-metallic inclusions is a highly localized phenomenon near the inclusions. However, the nature of the fatigue crack initiation process is not clear. In this paper, a special emphasis is placed on the possible differences in the mechanism of initiation and the early growth of fatigue cracks between a martensitic steel and ordinary ductile materials.The poor adhesion between matrix and aluminum oxide inclusion leads to the formation of an inclusion pit which serves as a simple stress raiser. The fatigue crack originates at the periphery of this inclusion pit at an angle of 45 degrees to the principal stress direction. Metallurgical and micro-fractographical observations revealed that the initiation and early growth of fatigue cracks from non-metallic inclusion are of the shear rather than the tensile mode. Thus, it is concluded that, though the fatigue process is quite localized in the vicinity of inclusion, the mechanism for an initiation and early growth of fatigue cracks is essentially the same as that for ductile materials. The effects of metallurgical heterogeneities in the martensite, such as the prior austenite grain boundaries, packets, and plates on the initiation and early growth of the cracks, are also discussed from the mechanical-metallurgical viewpoint.

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Résumé Dans les aciers à faible ductilité et à haute résistance, le comportement à la fatigue au cours des premiers stades du phénomène est associé aux inclusions non métalliques et présente un caractère hautement localisé au voisinage de l'inclusion. Cependant, la nature du processus d'initiation d'une fissure de fatigue n'est pas claire. Dans ce mémoire, l'accent à été placé sur des différences que l'on peut rencontrer dans le mécanisme d'amorçage et dans le début de la croissance des fissures de fatigue entre un acier martensitique et des matériaux de ductilité normale.La faible liaison existant entre la matrice et une inclusion d'oxyde d'aluminium conduit à la formation d'une singularité associée à l'inclusion qui détermine une croissance de la contrainte. La fissure de fatigue prend naissance à la périphérie de cette singularité à un angle de 45° par rapport à la direction de la contrainte principale. Les observations métallurgiques et micro-fractographiques ont révélé que l'amorçage et le début de croissance des fissures de fatigue à partir d'inclusions métalliques relèvent du cisaillement plutôt que de la traction simple. On conclut donc que, bien que le processus de fatigue est entièrement localisé au voisinage d'une inclusion, le mécanisme d'amorçage et de début de croissance d'une fissure de fatigue est essentiellement le même que dans un matériau ductile. L'effet d'hétérogénéités métallurgiques dans la martensite, tel que les frontières du grain austénitique primaire, les empilements et les plaquettes, peuvent être considérés sur l'amorçage et le début de croissance des fissures de fatigue; ce point est également discuté au point de vue mécanique et métallurgique.

     

Random hydrogen-assisted fatigue crack growth in steel plates

A stochastic analysis of hydrogen-assisted fatigue crack growth in steel plates is presented. First, a simplified deterministic model of the process is proposed. It captures the basic empirical property that the influence of hydrogen diminishes, as the crack growth rate increases. However, it only applies to cases, when diffusion is rate limiting. Next, the model parameters are randomized to reflect the uncertainty inherent in the physical situation. On the basis of the obtained stochastic equation, probabilistic moments of the time, in which the crack reaches a critical length, are computed. Theoretical results are illustrated by a numerical example.