A new mechanism in hydrogen-enhanced fatigue crack growth behavior of a 1900-MPa-class high-strength steel

This paper presents a new mechanism controlling the acceleration of fatigue crack growth of a hydrogen-charged high-strength steel (bearing steel SAE52100, ?? _ult?>?1, 900MPa, HV =?569). Three- dimensionally complicated shape of a primary crack and secondary cracks were observed in hydrogen- charged specimens. Marked acceleration of fatigue crack growth in the presence of hydrogen was observed particularly at low test frequency, and was attributed to the initiation and successive coalescence of secondary cracks formed ahead of primary crack. These secondary cracks were produced along prior-austenite grain boundaries and carbide boundaries, or by direct cracking of carbides. Surprisingly, secondary cracks were observed outside the ordinary plastic zone ahead of the crack tip. TEM observation elucidated that the secondary cracks outside the crack tip plastic zone were produced by hydrogen-induced deformation twins impinging on grain boundaries and carbides. These results suggest a new mechanism of the acceleration of fatigue crack growth rates in high-strength steels caused by hydrogen-induced deformation twins, rather than due to hydrogen- enhanced localized plasticity. The phenomena associated with time dependent fatigue crack growth are presumed to be correlated with the initiation and coalescence of secondary cracks in the presence of hydrogen.     

Fatigue crack growth behavior in weld-repaired high-strength low-alloy steel

Fatigue crack growth properties and Vickers micro-hardness of a weld-repaired high-strength low-alloy steel, known for high strength, low carbon, excellent notch toughness and good weldability and formability, have been studied under the following conditions: as-received high-strength low-alloy, weld-repaired high-strength low-alloy without buffer layer, and weld-repaired high-strength low-alloy with various thickness buffer layers. Those conditions are examined to determine the respective fatigue crack growth behaviors and Vickers hardness distribution, and the effects of different weld-repaired conditions on fatigue characterizations and microscopic features of the fracture and fatigue surface. The extended-compact tension specimen geometry is adopted in this study for all tests. Paris fatigue crack growth curves and the hardness distribution across weld metal, buffer layer and parent metal has been measured together with the relevant scanning electron microscope observations along the fatigue crack growth path, with special attention at and around the interfaces between the weld metal, buffer layer and parent metal. The results show the presence of the BL of a moderate thickness has a significant influence on the fatigue crack growth behavior in the heat-affected zone and around the interface between buffer layer and parent metal. The fatigue resistance of the selected high-strength low-alloy + buffer layer + weld metal tri-metal system is higher than that of the high-strength low-alloy + weld metal bi-metal system.     

Technical note High-cycle fatigue crack initiation and propagation behaviour of high-strength sprin steel wires

An attempt has been made to characterize high-cycle fatigue behaviour of high-strength spring steel wire by means of an ultrasonic fatigue test and analytical techniques. Two kinds of induction-tempered ultra-high-strength spring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa were used in this investigation.
The fatigue strength of the steel wires between 10⁶ and 10⁹ cycles was determined at a load ratio R = −1. The experimental results show that fatigue rupture can occur beyond 10⁷ cycles. For Cr–V spring wire, the stress–life ( S – N ) curve becomes horizontal at a maximum stress of 800 MPa after 10⁶ cycles, but the S – N curve of the Cr–Si steel continues to drop at a high number of cycles (>10⁶ cycles) and does not exhibit a fatigue limit, which is more correctly described by a fatigue strength at a given number of cycles. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviour have been examined. Experimental and analytical techniques were developed to better understand and predict high-cycle fatigue life in terms of crack initiation and propagation. The results show that the portion of fatigue life attributed to crack initiation is more than 90% in the high-cycle regime for the steels studied in this investigation.     

Fatigue dislocation structure and crack initiation in low carbon alloy steel

Abstract

The evolution of dislocation structure in a quenched and tempered low carbon alloy steel with increasing number of fatigue cycles has been investigated using transmission electron microscopy. The changes in dislocation structure can be divided into three stages: the production of the dislocation structure having fatigue features, the formation of microscopic fatigue slip bands, and the initiation and propagation of wide fatigue deformation bands. Microscopic fatigue cracks are formed during the propagation of these bands.

MST/1037     

Creep crack growth in low alloy steel weldments

Developments in the determination and analytical representation of creep crack growth property data during the past 30 years are reviewed. The testing and data analysis of weldments involve additional complexities, and these are appraised with respect to low alloy steel weldments. Creep crack initiation and growth properties are dependent on creep deformation and rupture ductility characteristics. Consideration is given to the relationship between these properties using data determined for a ½Cr½ Mo¼V/2½CrMo pipe joint.     

Corrosion fatigue crack initiation and growth in 18 Ni maraging steel

Nucleation of fatigue cracks in air and 3.5 wt% NaCl solution has been studied in an 18 wt% Ni maraging steel. Specimens tested on reverse bending fatigue machine showed a marked decrease in fatigue strength of the steel in NaCl solution reducing the 10⁷ cycles endurance limit from 410 MPa in air to 120 MPa. Microscopic studies revealed crack initiation to be predominantly associated with non-metallic silicate inclusions in both cases. In air, initiation is caused by decohesion of the inclusion/matrix interface, while in NaCl solution complete detachment of inclusions from the matrix results due to the dissolution of the interface. 70% more inclusions are quantitatively shown to be associated with cracks in NaCl solution than in air at the same stress levels. Experimental and theoreticalS-N curves and inclusion cracking sensitivity data are consistent with the mechanism suggested. The final fracture occurs by the main crack consuming the inclusions ahead of it by the “unzipping” of the shear band produced between the crack tip and the inclusion ahead.     

A model for predicting fatigue crack growth behaviour of a low alloy steel at low temperatures

In the present study, a model to predict the fatigue crack growth (FCG) behaviour at low temperatures is proposed for a low alloy steel (16 Mn). The experimental results indicate that fatigue ductile-brittle transition (FDBT) occurs in 16 Mn steel and the FDBT temperature (T_FDBT) is about 130 K. When T > T_FDBT, the FCG mechanism in the intermediate region is the formation of ductile striation and the FCG rates decrease with decreasing temperature. When T ≈ T_FDBT, the FCG mechanism changes into microcleavage and the fatigue fracture toughness K_fc of the steel decreases sharply. The FCG rates tend to increase as the temperature is further reduced. The test data of the FCG rates are well fitted by the formula developed by Zheng and Hirt. An approximate method to predict ΔK_th of the steel at low temperatures is proposed and then a general expression of the FCG rates is given at temperatures ranging from room temperature to T_FDBT. By means of the expressions proposed in this paper, the FCG rates at low temperatures can be predicted from the tensile properties if the endurance limit σ₋₁ and δk_th, at room temperature are known. Finally, a model for FDBT is tentatively proposed. Using this model, one can predict T_FDBT from the ductile-brittle transition curve determined from impact or slow bending tests of cracked Charpy specimens.     

Small fatigue crack growth behavior of titanium alloy TC4 at different stress ratios

In this paper, the small fatigue crack behavior of titanium alloy TC4 at different stress ratios was investigated. Single‐edge‐notch tension specimens were fatigued axially under a nominal maximum stress of 370 MPa at room temperature. Results indicate that fatigue cracks in TC4 initiate from the interface between α and β phases or within α phase. More than 90% of the total fatigue life is consumed in the small crack initiation and growth stages. The crack growth process of TC4 can be divided into three typical stages, ie, microstructurally small crack stage, physically small crack stage, and long crack stage. Although the stress ratio has a significant effect on the total fatigue life and crack initiation life at constant σ_max, its effect on crack growth rate is indistinguishable at R = ?0.1, 0.1, and 0.3 when crack growth rate is plotted as a function of ?K.     

Roles of precipitates on corrosion fatigue crack growth of high-strength steels in corrosive environments

The effect of microstructures on resistance to corrosion fatigue cracking and fracture surface morphology for age-hardened steels were investigated in a 3.5% NaCl aqueous solution under a cathodic potential of –0.85 V (Ag/AgCl). The free corrosion was about –0.63 V (Ag/AgCl). The resistance to corrosion fatigue cracking of materials containing coherent precipitates in the matrix (underaged conditions) was less than that of materials containing incoherent precipitates (reheated conditions) at equal strength levels. Accelerated fatigue crack growth rates of the underaged material in the aqueous solution were followed by cracking along prior-austenite grain boundaries, due to hydrogen embrittlement, while the overaged material did not show accelerated fatigue crack growth rates and had fracture surfaces similar to those in air. The difference in the fracture surfaces of both materials in air and in the aqueous solution was considered to depend on the ease of diffusion of hydrogen to the prior-austenite grain boundaries. It is concluded that incoherent precipitates in the matrix made hydrogen accumulation at prior-austenite grain boundaries much slower than for coherent precipitates.     

Small fatigue crack growth characteristics and fracture surface morphology of low carbon steel in hydrogen gas

Owing to energy conservation and environmental concerns, hydrogen has been suggested as a next-generation energy source. However, hydrogen known to seep into a metal, degrade its strength, and accelerate fatigue crack growth rates. We have investigated the effects of hydrogen gas on the small fatigue crack growth characteristics of low carbon steel JIS S10C by conducting bending fatigue tests on a specimen with a small blind hole and placed in a low-pressure hydrogen environment. The fatigue crack growth rate in hydrogen was higher than that in nitrogen. The fracture surface of the specimen in hydrogen showed intergranular facets in the low- growth-rate range and a quasi-cleavage fracture surface with brittle striations in the high-growth-rate range. The specimen only showed a ductile fracture surface for nitrogen. The small-fatigue-crack growth rate for nitrogen is given by ${dl/dN\propto \Delta \varepsilon_{p}^{n}l}$ , where l, N, and ${\Delta \varepsilon_{p}}$ represent the crack length, number of repetitions, and plastic strain range, respectively. This equation was also satisfied for hydrogen, but only over a short strain range from ${\Delta \varepsilon_t = 0.25}$ to 0.37?% in which the fracture surface exhibited intergranular facets and a ductile morphology, but no quasi-cleavage fracture. The exponent n of the equation was 1.22 in nitrogen and 0.66 in hydrogen environment. The small-fatigue-crack growth law can be used for safe material designs in hydrogen environments.     

EBSD analysis of fatigue crack initiation behavior in coarse-grained AZ31 magnesium alloy

Plane bending fatigue tests had been conducted to investigate fatigue crack initiation mechanism in coarse-grained magnesium alloy, AZ31, with hexagonal close-packed (hcp) crystallographic structure. The initial crystallographic structure was analyzed by an electron backscatter diffraction (EBSD) method. Subsequently, a fatigue test was periodically terminated and time-series EBSD analyses were performed. Basal slip and primary twin operated predominantly. In a twin band, secondary twin operated, and resulted in the fatigue crack initiation. The crack initiation was strongly affected by Schmid factors in the grains and twin bands.     

Finite element analysis of fretting fatigue behavior of steel wires and crack initiation characteristics

The effects of fretting parameters on stress distributions of contacting wires during the initial stage of fretting–fatigue of steel wires were investigated using the finite element method. The roles of fretting parameters on crack initiation characteristics were discussed employing the multiaxial fatigue criteria of Fatemi–Socie and Smith–Watson–Topper, and three-dimensional coordinate transformation. Non-uniform stress distributions on contact surfaces and ring-shaped stress distributions near the contact zone on the symmetric plane are observed. Different fretting parameters induce distinct fretting regimes, stress distributions and abrupt changes of stress near the trailing edge. Crack initiation becomes more difficult with increasing contact load as compared to the increased possibility of crack initiation with increasing relative displacement.     

Effects of particle size on fatigue crack initiation and small crack growth in SiC particulate-reinforced aluminium alloy composites

Fatigue crack initiation and small crack growth were studied under axial loading using powder metallurgy 2024 aluminum-matrix composites reinforced with SiC particles of three different sizes of 5, 20 and 60 μm. The 5 and 20 μm SiC_p/Al composites exhibited nearly the same fatigue strength as the unreinforced alloy, while the 60 μm SiC_p/Al composite showed a significantly lower fatigue strength due to its inferior crack initiation resistance that could be attributed to interface debonding between particles and the matrix. Small crack growth behaviour was different depending on stress level. At a low applied stress, the addition of SiC particles enhanced the growth resistance, particularly in the composites reinforced with coarser particles, while at a high applied stress, the 60 μm SiC_p/Al composite showed a considerably low growth resistance, which could be attributed to interaction and coalescence of multiple cracks. In the 5 μm SiC_p/Al composite, small cracks grew avoiding particles and thus few particles appearing on the fracture surfaces were seen, particularly in small crack size region. In the 20 and 60 μm SiC_p/Al composites, they grew along interfaces between particles and the matrix and the number of particles appearing on the fracture surfaces increased with increasing crack size or maximum stress intensity factor.     

Micro-defect effect on gigacycle fatigue S-N property and very slow crack growth of high strength low alloy steel

Abstract

Transformation induced plasticity (TRIP) assisted steels contain a small quantity of carbon enriched retained austenite, which transforms into martensite during the course of plastic deformation. Transformation of this kind can be induced by both stress and plastic strain. The detailed mechanism by which the martensite is induced is different for these two scenarios. An attempt is made here to discover the relative importance of these mechanisms and it is found that stress affected transformation can explain much of the variation in retained austenite content as a function of plastic strain.     

Vacuum crack growth behavior of austenitic stainless steel under fatigue loading

The fatigue crack growth behavior of an austenitic stainless steel in a vacuum environment has been studied. Fatigue crack growth tests were performed with the compact tension specimens in laboratory air and vacuum, and the environmental effect on the crack growth behavior was examined. The crack growth rate data were expressed in terms of the J-integral range during fatigue loading, while an elastic–plastic finite element analysis was employed to calculate the J-integral range. The fractographic examinations were also carried out to assess the crack growth mechanisms and correlate with fatigue characteristics. The results show an accelerated fatigue crack growth in air compared to that in vacuum and this environmental effect depends on the load ratio.     

Observation of small fatigue crack growth behavior in the extremely low growth rate region of low carbon steel in a hydrogen gas environment

To investigate the effects of hydrogen on crack propagation in the extremely low growth rate range, fully reversed bending fatigue tests were performed on low carbon steel (JIS S10C) in hydrogen and in nitrogen gas environments at a low pressure. A crack showed almost the same non-propagation behavior in nitrogen as that in air. However, a crack in hydrogen continued to propagate even near $10^{7}$ cycles in the same testing strain range as that in nitrogen. In hydrogen gas, a crack grew intermittently by coalescing with a new micro-crack generated by slip behavior. This implies that hydrogen could inhibit the action of any factor affecting non-propagation.