Effects of hydrogen on notch ductility and fracture in spheroidized AISI 1090 steel

Notched bend specimens were tested after precharging or during dynamic charging with hydrogen. In the precharged case, hydrogen was shown to decrease markedly the critical strains for the onset of sh ear instability at the notch surface and for cracking in a mode II fracture. Dynamic charging was found to produce severe degradation in the form of a mode I fracture, independent of whether prior precharging was performed. The interactions of hydrogen with macroscopic plastic flow parameters were determined. Formerly Graduate Student at The Ohio State University.     

Effect of hydrogen on fracture of U-notched bend specimens of spheroidized AISI 1095 steel

The effect of hydrogen on fracture was studied in U-notch bend specimens of 1095 steel with three different notch acuities. The hydrogen was introduced by cathodic charging through the notch surfaces. Both precharging and dynamic charging effects on void initiation and growth, crack initiation and fracture were investigated. In prechargea specimens voids were initiated at lower strains and the void population increased more rapidly than in the uncharged case. The role of hydrogen is to promote plastic instability along characteristic slip lines and therefore accelerate void formation. In the dynamically charged specimens, cracks initiated at lower strains than for the other cases and propagated rapidly without accompanying void formation. The results are compared to those for the uncharged case and to previous theoretical and experimental results. formerly with Ohio State University, is now Metallurgist with Korea Institute of Science and Technology, Seoul, Korea, and formerly with Ohio State University, is now Metallurgist with Bell Telephone Laboratories, Allentown, PA 18103.     

Effect of hydrogen on fracture behavior of a quenched and tempered medium-carbon steel

This work examined the effects of hydrogen on fracture of quenched and tempered 1045 steel. Tests were made at room temperature on tensile, Charpy impact, and 4-point notched bend specimens. This steel exhibits tempered martensite embrittlement (TME) for tempering temperatures between 300 and 375 °C. Thus hydrogen in most cases affected fracture by increasing the amount of intergranular fracture. In bend specimens, hydrogen also induced quasicleavage (QC) fracture at points of maximum normal stress below the notch root, points which appeared to be the locations of crack initiation. Tear ridges on theseQC surfaces were at martensite lath packet boundaries. Crack orientations were largely mode I in uncharged specimens, with mode II appearing at the notch root in most hydrogen-charged specimens. These observations are in general agreement with earlier work on martensitic steel. Formerly graduate student, Carnegie-Mellon University     

Hydrogen degradation of spheroidized AISI 1090 steel

Spheroidized AISI 1090 steel was mechanically tested after precharging or dynamically charging with hydrogen at high fugacity. The degradation of properties was greater for plane-strain tensile tests or notched bend tests than for tensile tests of round bars. In precharged specimens the degradation was manifested as an early onset of plastic instability in the form of shear bands leading to premature fracture. In dynamically charged specimens, hydrogen caused brittle fracture in a mode I manner. The results are compared with those in other hydrogen embrittlement studies and discussed in relation to theories for the onset of plastic instability. Formerly Graduate Student, The Ohio State University     

Plastic instability in U-notched bend specimens of spheroidized AIS11090 steel

Studies on U-notched bend specimens of spheroidized AISI 1090 steel revealed that plastic instability initiated as a surface roughening in the notch root. The surface waviness preceded subsurface void formation and appeared at a strain in fair agreement with predictions from plasticity theory. With increasing plastic strain, surface microcracks, bulk instabilities in the form of shear localization along characteristic slip traces, and bulk microcracks along these traces appeared. Along with macrocracks, profuse void formation occurred along the slip traces in advance of the crack front. The results are analyzed on the basis of the shear bifurcation plasticity analysis for the case where a vertex appears on the yield surface. Formerly Graduate Student at The Ohio State University     

Ductile fracture processes in 7075 aluminum

The ductile fracture of 7075 aluminum was examined as a function of aging condition, from underaged to overaged. Notched specimens allowed the triaxiality of stress to be varied. In-creasing triaxiality, which was due to increasing notch acuity, decreased ductility and increased nucleation of voids, in accord with expectations from ductile fracture models. Overaged ma-terial, which is generally resistant to environmental damage, was not susceptible to hydrogen embrittlement even with high triaxiality, but void sizes were increased by hydrogen. That in-crease suggested that void growth rates may be assisted by hydrogen in overaged 7075 aluminum. Formerly with Carnegie Mellon University. Formerly with Carnegie Mellon. University Formerly with Carnegie Mellon University.     

The tearing topography surface as the zone associated with hydrogen embrittlement processes in pearlitic steel

This article deals with a new nonconventional microscopic fracture mode with a characteristic feature: the tearing topography surface (TTS), associated with hydrogen embrittlement processes in pearlitic steel. The TTS mode appeared in fracture tests on precracked and notched specimens when tested under hydrogen charging. Experimental results showed phenomenological relations between the size of the TTS region and variables such as the electrochemical potential and the maximum stress intensity factor during fatigue precracking (for cracked samples) or the time to failure and the geometry (for notched samples). A hydrogen diffusion model is proposed which explains, from the theoretical point of view, the phenomenological relations between the TTS size and the test variables. According to this model, hydrogen diffuses not only toward the places of minimum concentration, but also to the sites of maximum hydrostatic stress. Formerly Associate Professor, Department of Materials Science, Polytechnical University of Madrid Formerly Associate Professor, Department of Materials Science, Polytechnical University of Madrid     

Crystallographic and fractographic studies of hydrogen- induced cracking in purified iron and iron- silicon alloys

Crystallographic and fractographic studies have been carried out on hydrogen charged purified iron and on iron-silicon alloys with silicon contents up to 3 pct. The specimens could be cracked by cathodically charging with hydrogen even without the application of an external stress. An experimental technique was developed which enabled the exposure of the fracture surface formed purely by hydrogen charging, and to contrast this with an adjacent mechanically induced fracture surface. In the case of purified iron, hydrogen induced cracks are found to occur on potential slip planes whereas in the case of iron-3 pct silicon, the crack follows the observed cleavage plane. Intermediate silicon content alloys showed transitional behavior. In agreement with the variation of crack plane in the alloys, the fracture surface appearances was also drastically different, reflecting the change in intrinsic toughness with alloy content. The observed transition from slip plane cracking to cleavage plane cracking was found to occur near a silicon content of 0.7 pct. The observed behavior is discussed in terms of how the intrinsic toughness of the ironbased lattice affects how hydrogen-induced cracks are formed. Formerly at Carnegie-Mellon University.     

Influence of nitrogen alloying on hydrogen embrittlement in AISI 304-type stainless steels

Hydrogen embrittlement of AISI 304-type austenitic stainless steels has been studied with special emphasis on the effects of the nitrogen content of the steels. Hydrogen charging was found to degrade the mechanical properties of all the steels studied, as measured by a tensile test. The fracture surfaces of hydrogen charged specimens were brittle cleavage-like whereas the uncharged specimens showed ductile, dimpled fracture. In sensitized materials transgranular cleavage mode of fracture was replaced by an intergranular mode of fracture and the losses of mechanical properties were higher. Nitrogen alloying decreased the hydrogen-induced losses of mechanical properties by increasing the stability of austenite. In sensitized steels the stability of austenite and nitrogen content were found to have only a minor effect on hydrogen embrittlement, except when sensitization had causedα′-martensite transformation at the grain boundaries. Formerly with Helsinki University of Technology, Laboratory of Physical Metallurgy, SF-02150 Espoo 15, Finland.     

Fatigue behavior in the potentiostatic passive corrosion regime of the iron-base superalloy A-286

Surface crack initiation and propagation behavior of the iron-base superalloy A-286 were tested using smooth hourglass specimens under passive corrosion conditions. In the very underaged (VUA) condition, cracks were initiated at slip steps (stage I) and later propagated in a stage II mode, resulting in a minimum or dip in crack growth rate. For the highly overaged condition (HOA), cracks initiated at inclusions, and only stage II was observed; therefore, anomalies in crack growth behavior were not observed. A planar slip distribution, observed in VUA, was associated with a reduced crack growth rate. In general, the VUA microstructure was superior to HOA, as it exhibited an impressive combination of excellent mechanical properties, decreased susceptibility to corrosion, good resistance against crack initiation, and low crack growth rate, under corrosion fatigue conditions. Possible reasons for such behavior are discussed. Formerly Postdoctoral Associate, Carnegie Mellon University Formerly Professor, Carnegie Mellon University     

Hydrogen-induced delayed fracture under mode II loading

Hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) of a high-strength steel 34CrNi₃Mo (T.S = 1700 MPa) under Mode II loading were investigated using notched specimens. The stress field around the notch tip was analyzed by means of finite element method. The result shows HIC and SCC under Mode II loading initiated at the back of the notch tip,i.e., θ = -110 deg, where hydrostatic stress has maximum value. However, cracking is oriented along the shear stress direction at the site, not normal to the direction of maximum principal stress component. On the contrary, if the specimens are loaded to fracture in air under Mode II loading, cracking at the maximum shear stress site around the notch tip and the cracking direction coincide with the direction of the maximum shear stress. The above facts indicate that hydrogen induced delayed plastic deformation is a necessary condition for HIC, and the nature of SCC for high-strength steel in 3.5 pct NaCl solution is HIC. The results show that HIC and SCC under Mode II loading can occur during dynamic charging with hydrogen and in 3.5 pct NaCl solution, respectively. The normalized threshold stress intensity factors under Mode II loading during dynamic charging in 1 N H₂SO₄ + 0.25 g As₂O₃/L solution and in 3.5 pct NaCl solution are K_IIH/K_IIX = 0.1 and K_IISCC/K_IIX = 0.45, respectively. The corresponding values under Mode I loading are K_IH/K_IX = 0.02 and K_ISCC/K_IX = 0.37, where K_IIX and K,_IX are critical values loaded to failure in air under Mode II and Mode I loading, respectively. Thus, (K_IIH/K_IIX)/ K_IH/K_IX) = 5 and (K_IISCC/K_IIX)/K,(_ISCC/K_IX) = 1.2. A typical intergranular fracture was observed during HIC and SCC under Modes II and I loading. But the fracture surfaces of specimens failed in air are composed of dimples for both kinds of loading. Formerly Student at Beijing University of Iron and Steel Technology     

Embrittlement of amorphous Fe40Ni38Mo4B18 alloy by electrolytic hydrogen

The effects of hydrogen on the tensile properties and fracture processes at room temperature were investigated. Specimens were tested at various strain rates in air or under different cathodic charging-current densities. The slopes of the stress-strain curves were essentially identical for all the specimens, except that the fracture points varied under different test conditions. Macroscopically, hydrogen only affected the elastic deformation behavior, but microscopically, the embrittlement was caused by the heterogeneous nucleation of localized plastic deformation. The degree of hydrogen embrittlement increased as the charging current increased or as the strain rate decreased. With the same charging current and time, longer dynamic charging resulted in more severe embrittlement. Before fracture took place, the strength of the alloy could be completely restored if hydrogen had been removed. Hydrogen diffusivity and solubility were used to draw the time-dependent hydrogen concentration profiles for the specimens under different charging conditions. The difference in the mechanical properties was correlated with the hydrogen concentration within the specimen. Formerly Graduate Student, Department of Materials Science and Engineering, National Tsing Hua University.     

Relationship between hydrogen-assisted crack propagation rate and the corresponding crack path in AISI 4340 steel

In the present investigation, relationship between hydrogen-assisted (HA) crack propagation rate and the corresponding fracture mode in AISI 4340 steel has been elucidated with critical hydrogen concentration concept. Hydrogen assisted crack-propagation rate and the corresponding fracture surface morphology were determined from double cantilever beam (DCB) specimens as a function of hydrogen pressure and temperature. As hydrogen pressure decreased, threshold stress intensity factor necessary for the onset of hydrogen-assisted crack propagation increased and the stage II (plateau) crack-propagation rate decreased. The kinetics of stage II crack propagation indicated substantial difference, i.e., positive and negative responses in the two investigated low and high temperature regions, respectively. Fractographic analysis showed that increased amounts of the microvoid coalescence mode resulted in slower crack-propagation rates. The observed changes in crack-propagation rate and the corresponding fracture mode with hydrogen pressure and temperature are discussed in terms of critical stress or strain and critical hydrogen concentration concepts.     

Fatigue crack growth in MAR-M200 single crystals

The effects of crystallographic orientation on the fatigue crack growth behavior of MAR-M200* single crystals were examined. Using compact-tension specimens tested at 20 Hz, fatigue crack growth rates were determined at ambient temperature at minimum stress to maximum stress ratios,R, of 0.1 and 0.5. In most cases, subcritical crack growth occurred either along a single {111} slip plane or a combination of {111} planes. The mode of cracking was generally mixed and contained mode I, II, and III components. Considerable crack deflection and branching were also observed. Some fracture surfaces were found to contain a significant amount of asperities and, in some specimens, black debris. Based on Auger spectroscopic analyses and the fracture surface appearance, it appears that the black debris represented oxides formed due to rubbing of the fracture surfaces. Using stress intensity solutions obtained based on the Boundary-Integral-Equation technique, an effective ΔK was successfully used for correlating the crack growth rate data. The results indicate that the effect of crystallographic orientation on crack growth rate can be explained on the basis of crack deflection, branching, and roughness-induced crack closure. Formerly with Southwest Research Institute     

Combined mode I-mode III fracture of a high strength low-alloy steel

Special formulations of theJ integral were used to quantify the combined mode I-mode III fracture behavior of ASTM A710 Grade A steel, a tough material which displays elastic-plastic behavior at room temperature. Experimental fracture initiation loci for two heat-treated conditions were linear inJ _i-J_iii space, agreeing with analytical predictions based on linear elasticity. As commonly observed in mode I fracture behavior, large tearing modulus values accompanied largeJ values for mode I and mode III components, individually as well as for totalJ values. There was a pronounced tendency toward antiplane shear flow in the modified compact tension specimens tested. This may result from the concentration of mode III shear strains in the plane of the propagating crack as predicted by slip line theory. Shear fractures with the crack propagating at inclined angles to both load line and plate free surfaces are favored energetically over pure mode I cracks even though the total surface area formed by the latter is much smaller. formerly Graduate Research Assistant, The Ohio State University     

Dependence of dynamic fracture resistance on crack velocity in tungsten: Part 1. single crystals

The dependence of dynamic fracture resistance on crack propagation velocity on (100) in tungsten has been examined. A correlation is obtained between the measured local crack velocity with the surface and subsurface deformations. Based on the experimental results on one pass, two passes, and prestrained, electron beam zone refined single crystals, a discussion is given on the slip modes activated at the crack tip, the contributions to the dynamic fracture resistance from dislocations and surface features, and from the preexisting deformed microstructure. Formerly with the State University of New York at Stony Brook Formerly with the State University of New York at Stony Brook     

Ductile fracture in HY100 steel under mixed mode I/Mode II loading

A number of criteria have been proposed which predict the direction of cracking under mixed Mode I/Mode II loading. All have been evaluated for brittle materials, in which a crack subjected to tension and shear propagates normal to the maximum tensile stress (i.e. fracture is of the Mode I type). In a ductile material, however, a notch subjected to mixed Mode I/Mode II loading may initiate a crack in the direction of maximum shear. This paper shows that the profile of the notch tip changes with increasing mixed mode load in such a way that one side of the tip blunts while the other sharpens. Various specimens, subjected to the same mixed mode ratio, were unloaded from different points on the load-displacement curves to study the change in notch-tip profile. Studies under the Scanning Electron Microscope (SEM) have shown that cracks initiate at the sharpened end, along a microscopic shear band. Using a dislocation pile-up model for decohesion of the carbide-matrix interface, a micromechanical model has been proposed for crack initiation in the shear band. It is shown that a theoretical prediction of the shear strain required for decohesion gives a result that is, of magnitude similar to that of the shear strain at crack initiation measured in the experiments.     

The cooperative relation between temper embrittlement and hydrogen embrittlement in a high strength steel

A sample plate of HY 130 steel (5 pet Ni-0.5 pet Cr-0.5 pet Mo-0.1 pet V-0.1 pet C) was found to be quite susceptible to temper embrittlement. Step-cooling produced a shift in transition temperature of 583 K (310°C). In the step-cooled condition the plane strain stress intensity threshold for crack growth in 0.1 N H₂SO₂ was about 22 MNm^-3/2 (20 ksi √in. ) and the fracture mode was inter granular, whereas in the unembrittled condition the threshold for a 1.27 cm (1/2 in.) plate (not fully plane strain) was around 104.5 MNm^-3/2 (95 ksi Vin. ) and the fracture mode was mixed cleavage and microvoid coalescence. The interaction between the impurity-induced and the hydrogen embrittlement is discussed in terms of Oriani’s theory of hydrogen embrittlement. Formerly Research Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania     

Fatigue crack propagation in Ni-base superalloy single crystals under multiaxial cyclic loads

The effects of crystallographic orientation and stress state on the multiaxial fatigue behavior of MAR-M200* single crystals were examined. Using notched tubular specimens subjected to combined tension/torsion cyclic loads, crack growth rates were determined at ambient temperature as functions of stress intensity range, the shear stress range-to-normal stress range ratio, and crystallographic orientation. Comparison of crack growth data at the same effective ΔK reveals a weak dependence of the crack growth rate on both the tube axis and the notch orientation. For a given set of tube axis and notch orientation, the crack growth rate might or might not vary with the applied stress state, depending on whether roughness-induced crack closure is present. In most cases, subcritical cracking occurs either along a single 111 slip plane or on ridges formed with two 111 slip planes. Neither fracture mode is altered by a change in the applied stress state. This complex crack growth behavior will be discussed in terms of the crack-tip stress field, slip morphology, and crack closure. Formerly with Southwest Research Institute     

Corrosion Fatigue Crack Initiation in a Mode II Notch Specimen

The stress intensityK _II of a Mode II specimen was calculated using a finite element methodvia theJ integral. The site, direction, and the threshold value for crack initiation from the notch under cyclic Mode II loading in air, in water, and under dynamic charging with hydrogen were investigated. The results showed that the Mode II fatigue crack in a high strength steel initiated at or close to the site of the maximum principal stress, rather than at the site of the maximum shear stress, and the subsequent crack growth was oriented approximately normal to the direction of the maximum principal stress. The site and direction of crack initiation in water and under dynamic charging with hydrogen were similar but different from that in air. The threshold values for crack initiation in air, in water, and under dynamic charging were 28.8, 12.3, and 10.2 MPa m^1/2, respectively. The fracture surface of a corrosion fatigue crack in water and under dynamic charging consisted of intergranular facets at low ΔK _II values but of quasi-cleavage at higher ΔK _II values and were different from those in air.