Determination of fracture initiation in hydride blisters using acoustic emission

An experimental study based on acoustic emission techniques was carried out to determine the conditions that lead to the initiation and growth of cracks in and from zirconium hydride blisters. The stress to initiate, at room temperature, a crack in a blister previously grown on a tensile speci-men could be accurately determined using an acoustic emission (AE) method based on linear event-location techniques. It was found that the applied stress to first form a crack in an unbroken blister decreases with blister depth but also that this value is statistically distributed. This is likely due to a size distribution of microcracks or incipient flaws in the blister. The propagation, by the delayed hy-dride cracking (DHC) mechanism, of a crack from a pregrown blister at 516 K seems to require both a critical applied stress, which decreases with blister depth, and approach of the testing temperature from above. However, DHC initiation was possible at 563 K (approached from below) for blisters grown under stress, provided the applied stress was sufficiently high. The stress intensity factor,K _IB , to initiate DHC ranged from 10.7 to 15.4 MPa √m. This is above the range forK _IH the thresholdK ₁ for DHC obtained in other experiments. The characteristics of AE generated by crack propagation from a blister due to DHC always follows the same pattern. It has a low event rate both at the beginning and at the end of DHC and a maximum value in between. The DHC initiation stage has a high proportion of high amplitude events. The peak in the distribution of events with peak am-plitude shifts to lower peak amplitudes as DHC progresses. An explanation for this trend is sought in terms of the maximum hydride size required near the crack tip to propagate the crack as a func-tion ofK ₁ .     

Anisotropic threshold stress intensity factor, KIH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressure tubes

The objectives of this study are to systematically investigate the delayed hydride cracking (DHC) velocity and the threshold-stress intensity factor, K _IH , of a Zr-2.5Nb pressure tube as a function of orientation and elucidate the cause of this anistropic DHC behavior. The DHC velocity as a function of orientation was determined using flattened cantilever beam specimens with 60 ppm H while the threshold-stress intensity factor K _IH , was evaluated as a function of orientation on the curved compact-tension (CT) and cantilever-beam (CB) specimens charged with hydrogen to 200 ppm H. To infer a difference in a stress gradient ahead of the crack tip as a function of orientation, tensile tests were conducted at temperatures ranging from room temperature (RT) to 560 °C using small tensile specimens of 2-mm-gage length taken from three directions of the tube. A textural change was investigated by comparing the inverse pole figures before and after DHC while the {10 7} pole figures were constructed to find out the growth pattern of the DHC crack as a function of orientation. Faster DHC velocity and lower K _IH were obtained over temperatures of 170 °C to 270 °C, when the DHC crack grew in the longitudinal direction of the Zr-2.5Nb pressure tube. The strain hardening after yielding and the extent of the textural change accompanied by DHC were higher in the longitudinal direction of the tube, suggesting a higher stress gradient ahead of the crack tip. Thus, the anisotropic DHC behavior of a Zr-2.5Nb pressure tube is discussed based on the stress gradient ahead of the crack tip governed by strain-hardening rate after yielding and a change in texture accompanied by DHC, and the distribution of the {10 7} hydride habit planes. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Hydrogen effects on brittle fracture of the titanium aluminide alloy Ti-24Al-11Nb

Varying amounts of hydrogen were dissolved in the titanium aluminide alloy Ti-24Al-llNb (atomic percent). Virtually all of this hydrogen probably precipitated as hydride on cooling because the terminal solubility in the dominant Ti₃Al phase is very low at room temperature. Although the yield strength (YS) increased, the ultimate tensile strength (UTS), ductility, fracture stress in notched bend bars, and fracture toughness decreased with increasing amounts of hydride. The strength and fracture properties, for all hydride contents, did not change with testing speed below about 5 to 50 mm/min but decreased steeply for speeds greater than that. The presence of hydride decreased the critical value of testing speed by about an order of magnitude. Brittle cracks in bluntly notched bend bars, with or without hydride, nucleated at the notch root or at a distance below the root which was less than one fifth of the distance to the peak stress location. This result suggests that the cleavagelike cracking in this material is not controlled by normal stress alone but has some dependence on the applied strain. The fracture surfaces of notched or precracked specimens, with or without hydride, consisted entirely of cleavagelike fracture, but these cracks exhibited stable crack propagation. This permitted both the measurement of crack resistance or R curves and also observation of the initiation and propagation of the crack with increasing K_I. The results showed that cracks initiated discontinuously at characteristic sites within the plastic zone and along the slip bands when the plastic deformation ahead of the precrack developed to a particular and reproducible extent. Literature cleavage models were compared to results for the present tests. WU-YANG CHU, Formerly Visiting Professor, Carnegie Mellon University,     

The brittle-to-ductile transition in doped silicon as a model substance

In order to investigate the importance of the dislocation velocity for the brittle-to-ductile transition temperature, fracture experiments were performed on precleaved, dislocation-free silicon single crystals at elevated temperatures. The well known doping dependence of the dislocation velocity in silicon is used to obtain detailed information about the conditions in the plastic zone. At high temperatures, dislocations are generated at an applied stress intensity distinctly lower than the critical stress intensity for inducing cleavage in the dislocation-free samples at room temperature. As a consequence of this, the plastic zone which develops around the crack tip is saturated before the tensile stress at the crack tip reaches the cohesive stress. Because of saturation of the plastic zone, the brittle-to-ductile transition temperature is determined by the velocity at which dislocations move outward from the crack tip at low shear stresses existing in the vicinity of the shielded crack.     

Anisotropic threshold stress intensity factor,K IH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressure tubes

The objectives of this study are to systematically investigate the delayed hybride cracking (DHC) velocity and the threshold-stress intensity factor, K _IH, of a Zr-2.5Nb pressure tube as a function of orientation and elucidate the cause of this anistropic DHC behavior. The DHC velocity as a function of orientation was determined using flattened cantilever beam specimens with 60 ppm H while the threshold-stress intensity factor K _IH, was evaluated as a function of orientation on the curved compactension (CT) and cantilever-beam (CB) specimens charged with hydrogen to 200 ppm H. To infer a difference in a stress gradient ahead of the crack tip as a function of orientation, tensile tests were conducted at temperatures ranging from room temperature (RT) to 560°C using small tensile specimens of 2-mm-gage length taken from three directions of the tube. A textural change was investigated by comparing the inverse pole figures before and after DHC while the pole figures were constructed to find out the growth pattern of the DHC crack as a function of orientation. Faster DHC velocity and lower K _IH were obtained over temperatures of 170 °C to 270 °C, when the DHC crack grew in the longitudinal direction of the Zr-2.5Nb pressure tube. The strain hardening after yielding and the extent of the textural change accompanied by DHC were higher in the longitudinal direction of the tube, suggesting a higher stress gradient ahead of the crack tip. Thus, the anisotropic DHC behavior of a Zr-2.5Nb pressure tube is discussed based on the stress gradient ahead of the crack tip governed by strain-hardening rate after yielding and a change in texture accompanied by DHC, and the distribution of the hydride habit planes. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Hydrogen-induced slow crack growth in Ti-6Al-6V-2Sn

The effect of hydrogen and temperature on threshold stress intensity and crack growth kinetics was studied in Ti-6Al-6V-2Sn containing 38 ppm hydrogen. A slight decrease in threshold values occurred as temperature decreased from 300 K while they increased significantly above 300 K. For a given test temperature, crack growth rates exhibited an exponential dependence on stress intensity over a major portion of growth. At 300 K the rates reached a maximum. Slow crack growth occurred predominately by cleavage ofα grains which has been associated with hydride formation. The stress intensity required for hydride formation at a crack tip can be determined from hydrogen concentration and solubility considerations under stress. As these values differed from observed thresholds, a strong influence of microstructure was suggested and subsequently revealed by crack front examination. Quantification of this effect with a modified Dugdale-Barenblatt model relates the effective stress intensity at the crack tip to the applied stress intensity. Microstructure was also found to exert a strong influence on slow crack growth behavior when examined in terms of the effective stress intensity,K _eff. From Arrhenius plots of crack growth rates for variousK _eff, activation energies of 27.0 to 32.8 kJ/mol were obtained and related to the diffusion of hydrogen through theβ phase. The increase in crack growth rates with increasing temperatures up to 300 K is attributed to the temperature dependence of hydrogen diffusion. The decrease in crack growth rates above 300 K is related to a hydride nucleation problem.     

Delayed hydride cracking of zirconium alloys: Appearance conditions and basic laws

The data on and concepts of delayed hydride cracking (DHC) in zirconium alloys are generalized. DHC appearance conditions and the general laws of DHC are considered. The DHC parameters (threshold stress intensity factor K _1H, crack growth rate) are shown to depend on both external (temperature, irradiation, loading scheme) and internal (strength, texture, microstructure) factors.     

Stress-corrosion cracking of Ti-8Al-lMo-lV in aqueous environments: 2. Plastic zones,crack morphology,and fractography

Optical and electron metallographic studies of stress-corrosion cracks in Ti-8Al-lMo-lV have verified that the principal crack extension mechanism is cleavage of theα grains. There are two distinct crack morphologies which correspond to the two regimes of subcritical crack velocity. At low stress intensities(a ∞ K _I) the microscopic crack front consists of small cleavage facets approximately 1 to 4α grain diameters in size, and ligaments of material which fracture by ductile rupture and corrosion. At high stress intensities (a ≅ constant), the crack front consists of large cleavage “fingers”, 20 to 50α grain diameters in length, separated by regions which fracture by a combination of cleavage (on a much smaller scale), ductile rupture, and corrosion. The transition from Stage I to Stage II crack propagation apparently occurs when the strain-energy release rate is sufficient to support two crack branches,i.e., K_I≥ √2K _Iscc. Thereafter, the diameter of the plastic zone at the crack tip remains constant, suggesting that the effective stress intensity at the tip of each branch is also invariant. The slip within the plastic zone is markedly nonhomogeneous, and trenches are often observed along the slip steps. Formerly with the Metal Science Group, Battelle Columbus Laboratories, Columbus, Ohio.     

Crack growth in a nearly fully-lamellar gamma TiAl alloy at 650 °C and 800 °C under constant load conditions

The crack growth behavior of a gamma titanium aluminide alloy, K5S, was investigated at 650 °C and 800 °C under constant load conditions in a nearly fully-lamellar microstructural form. Crack growth at both temperatures ensues at stress intensities (K) much higher than anticipated from the R curves. At 650 °C, creep crack extension occurs through the formation of microcracks (interlamellar (IL) separation) and their joining to the main crack tip through ligament fracture. This results in a mainly transgranular (TG) fracture with occasional IL separation. This process features a rapid initial crack growth but at decreasing growth rate, followed by a nearly no-growth stage. At 800 °C, crack extension is accompanied by extensive plastic deformation and consists of an initial rapid transition period and a subsequent steady state. For similar K’s, crack extension and growth rate are greater at 800 °C than at 650 °C, but even these are very slow processes for this alloy. The resistance to crack propagation at 650 °C is explained in terms of work hardening that arises during the extended primary creep deformation occurring ahead of the crack tip. Increased crack propagation at 800 °C is accredited to grain boundary and lamellar-interface weakening and extensive post primary creep damage in the plastic zone. The resulting fracture at 800 °C is mainly boundary fracture, which consists of IG fracture involving formation and coalescence of voids, and IL separation.     

Microcrack initiation and acoustic emission during fracture toughness tests of A533B steel

The initiation of microcracks at MnS inclusions during fracture toughness tests of ASTM A533B steel compact tension specimen was detected by an 8-channel acoustic emission recording system. The microcracks were located as far as 7 mm ahead of the precrack front with a large spread of 5 mm above and below the plane of the main crack. Most of the microcracks were found to form before they were engulfed by the plastic zone which was determined by a finite element analysis. Assuming that the material is homogeneous and elastic outside the plastic zone, we estimated that microcracks were formed when the normal tensile stress σ_z at each inclusion is in the range of 400 to 800 MPa. The estimated stress value exceeds the uniaxial yield strength, σ_ys = 477 MPa of the material because σ_z near the crack tip could be as large as3σ _ys due to the stress triaxiality under plane strain conditions. Formerly Visiting Scientist in the Department of Theoretical and Applied Mechanics, Cornell University (on leave from Institute of Industrial Science, University of Tokyo, Tokyo, Japan).     

Fracture and fatigue behavior of sintered steel at elevated temperatures: Part I. Fracture toughness

This study investigates fracture resistance of a sintered steel in the temperature range from 25 °C to 300 °C. The temperature-dependent fracture resistance is experimentally determined by fracture toughness tests. The fracture toughness, K _IC , decreases from 28.8 at room temperature to 23 MPa√m at 300 °C. The finite element analysis shows an insight of the rationale of using K _IC as the parameter to characterize the fracture resistance of porous sintered steel in which the stress intensity (K) field has been severely distorted at the porous crack tip. The analysis indicates that crack onset of sintered steel is controlled by a critical stress mechanism.     

Further study on the mechanism of the ductile-to-brittle fracture transition in C-Mn base and weld steel

In the present study, the crack opening displacement (COD) tests of specimens of C-Mn base and weld steel were carried out in the ductile-brittle transition temperature region. The majority of the specimens were fractured and others were unloaded prior to fracture after ductile fracture initiated and extended. The cavities and cleavage microcracks located in the vicinities of tips of fibrous cracks of the unloaded specimens were observed in detail. The finite element method (FEM) calculations of the stress and strain distribution ahead of the tip of an extending fibrous crack were completed. The mechanism of the ductile-to-brittle fracture transition was further investigated. It was revealed that in the ductile-brittle transition temperature region, the ductile fracture process was independent of temperature. The ductile-to-brittle fracture transition was triggered by initiating a catastrophic extension of a cleavage crack ahead of the fibrous crack tip, which occurred in a condition satisfying a combined criterion composed of three items, i.e., ε _p ≥ ε _pc for initiating a crack nucleus; σ _m √σ ≥ T _c for preventing the crack nucleus from blunting; and σ _yy ≥ σ _f for propagating the crack nucleus. For a specimen in which a fibrous crack occurred and propagated, the critical event for initiating a brittle cleavage fracture was the propagation of a ferrite grain-sized crack into neighboring grains. With extension of a fibrous crack, the behavior of the ductile-to-brittle fracture transition could be analyzed by the effect of the size of an “active zone” on the initiation of the brittle cleavage fracture.     

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10 °C to 80 °C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from ~150 nm at room temperature to ~400 nm at 313 K (40 °C). A re-evaluation of published data reveals the apparent activation energy, E _a for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently ~35 kJ/mol for temperatures above ~313 K (40 °C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al₂CuMg where E _a is ~80 kJ/mol. For temperatures below ~313 K (40 °C) E _a is independent of copper content for stress intensity factors below ~14 MNm^−3/2, with a value ~80 kJ/mol but is sensitive to copper content for stress intensity factors above ~14 MNm^−3/2, with E _a , ranging from ~35 kJ/mol for copper-free alloys to ~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently ~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested, depending on the circumstances, that intergranular crack propagation in humid air and distilled water can be enhanced by the generation of aluminum hydride, AlH_3, ahead of a propagating crack and/or its decomposition after formation within the confines of the nanoscale volumes available after increments of crack growth, defined by the crack arrest markings on intergranular fracture surfaces.     

Dislocation emission and fatigue crack growth threshold

The stress intensity range ΔK below which no cyclic plastic deformation at the crack tip, and hence, no fatigue crack propagation occurs is investigated. The emission of dislocations from the crack tip is assumed as mechanism for the dislocation generation. For a mode III crack, a computer simulation is carried out to study the influence of the number of dislocations, the friction stress and the critical stress intensity, k_e, to emit a dislocation. If during loading only one dislocation is emitted, the return of this dislocation to the crack tip and the emission of a dislocation with an opposite sign and the recombination with the first dislocation are possible during unloading. The ΔK necessary for both mechanisms is about 2k_e. If during loading more than one dislocation is emitted, during unloading at first a certain number of disclocations return to the crack tip before a dislocation of opposite sign is emitted. The necessary ΔK to move one dislocation back to the crack tip during unloading decreases with increasing number of dislocations and reaches a constant values of about 1.1k_e. This value of ΔK then is roughly independent of the friction stress and k_e.     

Dynamic fracture toughness of a Ti-45Al-1.6Mn alloy at high temperature

Dynamic fracture toughness of a reactive sintered γ-base TiAl alloy is studied in the temperature range from 298 to 1073 K. The stop block method is employed in order to observe the crack paths and microcrack distribution ahead of a main crack tip under dynamic loading conditions at high temperature. Fracture surface, crack path, and microcrack observations are carried out using a scanning electron microscope (SEM). Microcrack initiation criteria and crack-tip stress shielding effect caused by crack deflection are discussed. The experimental results demonstrate that the dynamic fracture toughness, J _Id , increases with increasing temperature, and after attaining the maximum value at 873 K, the toughness decreases. Crack path morphology varies with temperature. The stress shielding effect at the crack tip caused by main crack deflection was found to affect the difference in crack extension energy for each temperature. The number of microcracks ahead of a main crack varies with temperature. The stress shielding effect at the crack tip caused by microcracking was found to contribute to toughening around 873 K.     

Effects of loading rate on fracture behavior of low-alloy steel with different grain sizes

Four-point bend (4PB) tests of notched specimens loaded at various loading rates, for low alloy steel with different grain sizes, were done, and the microscopic observation and finite-element method (FEM) calculations were carried out. It was found that for the coarse-grained (CG) microstructure, an appreciable drop in notch toughness with a loading rate of around 60 mm/min appeared, and further increasing the loading rate leads to a slight additional decrease in notch toughness. For the fine-grained (FG) microstructure, the effect of loading rate was not apparent. The change in toughness resulted from a change of the critical event controlling the cleavage fracture with increasing loading rate. For the CG microstructure with a lower cleavage-fracture stress (σ _f ), with an increasing loading rate, the critical event of cleavage fracture can be changed from the propagation of a pearlite colony-sized crack or a ferrite grain-sized crack, through the mixed critical events of crack propagation and crack nucleation, then to crack nucleation. This change deteriorates the toughness. For the FG microstructure with a higher cleavage-fracture stress, the critical event of cleavage fracture is the crack propagation and does not change in the loading-rate range from 120 to 500 mm/min. The measured σ _f values do not change with loading rate, as long as the critical event of cleavage fracture does not change. The higher notch toughness of the FG microstructure arises from its higher σ _f and the critical plastic strain (ε _pc ) for initiating a crack nucleus, and the fracture behavior of this FG steel is not sensitive to loading rate in the range of this work.     

Effects of tensile prestrain on the notch toughness of low-alloy steel

Tensile prestrains of various levels were applied to blank steel specimens. Four-point bend tests of notched specimens at various temperatures revealed an appreciable drop in the notch toughness of the specimens, which experienced 3 pct tensile prestrain. Further prestrains of up to 20 pct had a negligible effect on the notch toughness despite additional increases in the yield strength. Microscopic analyses combined with finite element method (FEM) calculations revealed that the decrease in toughness resulted from a change of the critical event controlling the cleavage fracture. The increase in yield strength provided by prestraining allowed the normal tensile stress at the notch tip to exceed the local fracture stress σ _f for propagating a just-nucleated microcrack. As a result, for the coarsegrained steel with low σ _f tested presently, the critical event was changed from tensile stress-controlled propagation of a nucleated microcrack to plastic strain-controlled nucleation of the microcrack at the notch tip. A reduction of toughness was induced as a result of this. The increase in yield strength provided by decreasing the test temperature acted in the same way.     

Effect of heat treatment on delayed hydride cracking in Zr-2.5 Wt Pct Nb

The effect of heat treatments on delayed hydride cracking (DHC) in Zr-2.5 wt pct Nb has been studied. Crack velocities were measured in hydrided specimens, which were cooled from solution-treatment temperatures at different rates by water-quenching, oil-quenching, liquid-nitrogen quenching, and furnace cooling. The resulting hydride size, morphology, and distributions were examined by optical metallography. It was found that fast cooling rates, which produce very finely dispersed hydrides, result in higher crack growth rates and a stronger dependence of the crack velocity on the applied-stress intensity factor. Also, the incubation period before cracking commences was found to be relatively short for specimens with fine hydrides, whereas specimens with coarse hydrides required considerably longer incubation periods. These results can be explained by rapid growth of preexisting hydrides within the crack-tip plastic zone. In addition, different solution temperatures were used to investigate the effect of the continuity of the grain-boundary phase(β-phase) on the crack velocity. Transmission electron microscopy was used to examine the structure of this grain-boundary phase. It was found that for heat treatments, which destroyed theβ-phase continuity, the crack velocity was significantly reduced, as would be expected from the theory of enhanced diffusion through grain boundaries.