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 electroslag refining on the fracture toughness and fatigue crack propagation rates in heat treated AISI 4340 steel

The AISI 4340 steel has been electroslag refined and the improvement in mechanical properties has been assessed. Electroslag refining (ESR) has improved tensile ductility, plane strain fracture toughness, Charpy fracture energy, and has decreased fatigue crack growth rates. The K_IC values for the ESR steel are nearly twice those estimated in the unrefined steel and higher than those obtained in the vacuum arc remelted steel. Fatigue crack growth rates in region I and in region III are found to be decreased considerably in the ESR steel, while they are unaffected in region II. Measurements on heat treated samples have shown that the ESR steel has a better response to heat treatment. Both the suggested heat treatments namely austenitizing at 1140–1470 K as well as the conventional heat treatment of austenitizing at 1140 K have been followed. The improvement in the mechanical properties of ESR steel has been explained on the basis of removal of nonmetallic inclusions and reduction in sulfur content in the steel.     

Effect of corrosive medium on fatigue crack growth behaviour and fracture in high martensite dual phase steel

Fatigue crack growth (FCG) behaviour in both near-threshold and higher stress intensity range (ΔK) in intercritically annealed dual-phase (DP) steel containing martensite between 32% and 76% in ferrite has been studied in 3·5% NaCl solution. It is shown that the amount of martensite content in dual phase steel has a significant effect on threshold (ΔK _th) values and FCG rates. Higher content of martensite in ferrite leads to higher threshold values and lower FCG rates. Further, ΔK _th is much higher in 3·5% NaCl solution as compared to that in laboratory air. Fractography studies reveal that in the near-threshold region, fracture surfaces are characterized mainly by intergranular cracking in corrosive (3·5% NaCl solution) environment. Higher threshold values in 3·5% NaCl solution is attributed to the higher crack closure induced by rougher fracture surface and by the strong wedge effects of corrosion products.     

A study on fatigue crack growth in dual phase martensitic steel in air environment

Dual phase (DP) steel was intercritically annealed at different temperatures from fully martensitic state to achieve martensite plus ferrite, microstructures with martensite contents in the range of 32 to 76%. Fatigue crack growth (FCG) and fracture toughness tests were carried out as per ASTM standards E 647 and E 399, respectively to evaluate the potential of DP steels. The crack growth rates (da/dN) at different stress intensity ranges (ΔK) were determined to obtain the threshold value of stress intensity range (ΔK_th). Crack path morphology was studied to determine the influence of microstructure on crack growth characteristics. After the examination of crack tortuosity, the compact tension (CT) specimens were pulled in static mode to determine fracture toughness values. FCG rates decreased and threshold values increased with increase in vol.% martensite in the DP steel. This is attributed to the lower carbon content in the martensite formed at higher intercritical annealing (ICA) temperatures, causing retardation of crack growth rate by crack tip blunting and/or deflection. Roughness induced crack closure was also found to contribute to the improved crack growth resistance at higher levels of martensite content. Scanning electron fractography of DP steel in the near threshold region revealed transgranular cleavage fracture with secondary cracking. Results indicate the possibility that the DP steels may be treated to obtain an excellent combination of strength and fatigue properties.     

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.     

Effect of sudden load decrease on the fatigue crack growth in cold drawn prestressing steel

This paper analyzes the overload retardation effect (ORE) on the fatigue crack growth (FCG) of cold drawn prestressing steel when different loading sequences are used. The ORE is more intense for elevated load decrease or for low initial stress intensity factor (SIF) range ΔK₀. A transient stage can be observed in the Paris curve (da/dN–ΔK) when the K_maxΔK value suddenly decreases, associated with the ORE and with the evolution of the plastic zone and compressive residual stresses near the crack tip. In tests with K_max decrease, a small zone appears related to FCG initiation, with a fatigue fractography resembling the tearing topography surface (TTS) mode, and associated with a decrease of crack tip opening displacement (CTOD).     

Effect of tensile overloads on fatigue crack growth of high strength steel wires

Fatigue of the tensile armor wires is the main failure mode of flexible risers. Techniques to increase the life of these components are required to improve the processes safety on oil exploration. This work evaluates the crack growth retardation of high strength steel wires used in flexible pipelines. Fracture toughness tests were performed to establish the level of stress intensity factor wherein the wires present significant plastic deformation at the crack tip. The effect of tensile overload on fatigue behavior was assessed by fatigue crack growth testing under constant ΔK control and different overload ratios with two different load ratios. The outcomes show that the application of controlled overloads provides crack retardation and increases the fatigue life of the wires more than 31%. This behavior is also evident at stress ratio of 0.5, in spite of the crack closure effect being minimized by increasing the applied mean stress.     

Effect of laser shock processing on fatigue crack growth of duplex stainless steel

Duplex stainless steels have wide application in different fields like the ship, petrochemical and chemical industries that is due to their high strength and excellent toughness properties as well as their high corrosion resistance. In this work an investigation is performed to evaluate the effect of laser shock processing on some mechanical properties of 2205 duplex stainless steel. Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. A convergent lens is used to deliver 2.5 J, 8 ns laser pulses by a Q-switched Nd:YAG laser, operating at 10 Hz with infrared (1064 nm) radiation. The pulses are focused to a diameter of 1.5 mm. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is determined by the contour method. It is observed that the higher the pulse density the greater the compressive residual stress. Pulse densities of 900, 1600 and 2500 pul/cm² are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness if this steel.     

Effect of a second phase particle on the rate of growth of fatigue cracks

The effect of the shear modulus, size and dispersion of second phase particles on rate of growth of fatigue cracks in two-phase materials is determined using the kinetic theory approach. The results obtained explain the experimental observations in pearlitic steels.

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Résumé L'étude du module de sélectivité de la dimension et de la disposition des particules de face secondaire sur la croissance de fissure de fatigue dans des matériaux comportant 2 faces a été déterminée en utilisant une théorie cynétique. Les résultats obtenus rendent compte des observations expérimentales effectuées dans les aciers perlitiques.

     

Effect of aging on fatigue crack growth behaviour of Cu bearing HSLA-100 steel

Abstract

The effect of aging on fatigue crack growth rate (FCGR) of Cu bearing HSLA-100 steel has been studied. The steel was solution treated, water quenched and aged at various temperatures in the range of 350–700°C. The fatigue crack growth resistance of the steel decreased for the initial stages of aging from 350–500°C. Further aging up to 650°C resulted in an improvement in the crack growth resistance. Beyond 650°C, once again an inferior crack growth resistance was observed. This nature of variation of FCGR behaviour was similar to the trend portrayed by the strength properties with aging treatment. The results are related to the changes in the microstructural constituents owing to the aging treatment.     

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.

     

Inclusion induced anisotropy of short fatigue crack growth in steel

Abstract

In this study, the growth behaviour of short fatigue cracks in En7A steel with a high content of elongated MnS inclusions was investigated by generating and evaluating the short crack data under different loading conditions for the six principal specimen orientations. Short cracks showed a strong anisotropic behaviour in which they grew muchfaster in the transverse and short transverse orientations than in the longitudinal orientations in all fatigue processes. This was due to the relatively high content of the elongated inclusions. Inclusion induced anisotropy is also observed in the percentages of fatigue life of the initiation and growth periods of short cracks which may result in inaccurate fatigue life predictions. Inclusion induced anisotropy was the result of strain intensification at the matrix/inclusion interface.

MST/3250A     

Influence of texture on the short fatigue crack growth in austenitic stainless steel

We study the influence of texture on the propagation of short cracks on the basis of the evolution of a family of microcracks in 316L austenitic stainless steel under the conditions of symmetric tension-compression. It is shown that the process of crack initiation is concentrated on grain boundaries and strongly depends on the mesostructure. In the process of fatigue fracture, the transcrystalline propagation of the largest crack (affected mainly by the global texture) is predominant. We discuss two approaches to the prediction of the influence of texture on the fatigue life. According to the concept of equivalent crack, this influence can be estimated depending on the size of the largest crack. On the contrary, the Zhurkov criterion deals with the state of the defect characterized by the presence of a family of microcracks. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 4, pp. 84–94, July–August, 2006.