The effect of hydrogen on the fracture toughness of alloy X-750

The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 MPa√m. Charging with hydrogen decreased the fracture toughness, K _Ic , to 52 MPa√m at a rapid loading rate and further decreased the toughness to 42 MPa√m for a slow loading rate. This is consistent with the rate-limiting step for the embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.     

The yielding, plastic flow, and fracture behavior of ultra-high molecular weight polyethylene used in total joint replacements

The yielding, plastic flow, and fracture behavior of UHMWPE plays an important role in wear and failure mechanisms of total joint replacement components. The primary objective of this study was to compare the yielding, plastic flow, and fracture behavior of two implantable grades of UHMWPE (GUR 1120 vs 4150 HP). The first part of this work explored the hypothesis that up to the polymer yield point, the monotonic loading behavior of UHMWPE displays similar true stress strain behavior in tension and compression. Uniaxial tension and compression tests were conducted to compare the equivalent true stress vs strain response of UHMWPE up to 0.12 true strain. During monotonic loading, the equivalent true stress strain behavior was similar in tension and compression up to the yield point. However, investigation of the unloading behavior and permanent plastic deformations showed that classical deviatoric rate independent plasticity theory may dramatically overpredict the permanent strains in UHMWPE. A secondary goal of this study was to determine the ultimate true stress and strain for UHMWPE and to characterize the fracture surfaces after failure. Using a fracture mechanics approach, the critical flaw sizes were used in combination with the true ultimate stresses to predict the fracture toughness of the two resins. A custom video-based strain measurement system was developed and validated to characterize the true stress-strain behavior up to failure and to verify the accuracy of the incompressibility assumption in calculating the true stress-strains up to failure. In a detailed uncertainty analysis, theoretical expressions were derived for the relative uncertainty in digital video-based estimates of nominal strain, true strain, homogeneous stress, and true stress. Although the yielding behavior of the two UHMWPE resins was similar, the hardening and plastic flow behavior clearly discriminated between the GUR 1120 and 4150 HP. A statistically significant difference between the fracture toughness of the two resins was also evident. The long-term goal of this research is to provide detailed true stress strain data for UHMWPE under uniaxial tension and compression for future numerical simulations and comparison with more complex multiaxial loading conditions.     

The influence of hydride size and matrix strength on fracture initiation at hydrides in zirconium alloys

The effects of matrix strength (yield stress) on hydride fracture and alloy ductility have been studied as a function of stress state, hydride content, hydride size, and precipitation stress. Uniaxial and triaxial states of stress were investigated by using smooth and notched tensile specimens, respectively, containing 0.18 or 0.90 at. pct H, with the longest hydride platelet dimension varying from 5 to 400 μm. The majority of the hydrides in the specimens had their plate normals oriented parallel to the tensile axis direction. Crack initiation at hydrides was monitored using acoustic emission, finiteelement calculations were employed to determine the stresses and strains in the notched specimens, and metallographic and fractographic analyses were carried out to determine the state of fractured hydrides/voids near and on the fracture surface. These techniques showed that, up to a hydride platelet length of ∼50 to 100 μm and regardless of the stress state, a critical plastic strain, independent of matrix strength, controls the initiation of fracture in hydrides. The amount of plastic strain needed to fracture hydrides decreases as (a) the average hydride length increases and (b) the axiality of stress increases. The equivalent plastic strain to fracture small hydrides is ∼ 1 pct under a triaxial as opposed to ∼5 pct under a uniaxial state of stress. When the average hydride platelet lengths are longer than ∼50 to 100 μm, negligible plastic deformation is required to fracture hydrides. A critical applied stress then is the governing factor in all three materials, ranging from 750 to 850 MPa, depending on the stress state.     

The effect of hydrogen on the multiaxial stress-strain behavior of titanium tubing

The influence of internal hydrogen on the multiaxial stress-strain behavior of commercially pure titanium has been studied. Thin-walled tubing specimens containing either 20 or 1070 ppm hydrogen have been tested at constant stress ratios in combined tension and internal pressure. The addition of hydrogen lowers the yield strength for all loading paths but has no significant effect on the strain hardening behavior at strains ε ≥ 0.02. Thus, the hydrogen embrittlement of titanium under plain strain or equibiaxial loading is not a consequence of changes of flow behavior. The yielding behavior of this anisotropic material is described well by Hill’s quadratic yield criterion. As measured mechanically and by pole figure analysis, the plastic anisotropy changes with deformation in a manner which depends on stress state. Hill’s criterion and the associated flow rule do not describe the multiaxial flow behavior well because of their inability to account for changes of texture which depend on multiaxial stress path. Hence, a strain dependent, texture-induced strengthening effect in equibiaxial tension is observed, this effect having the form of an enhanced strain hardening rate. Formerly with Michigan Technological University     

The effect of the carbide phases on the crack grow of 0.5 pct Mo-B steels under impact,cyclic, and monotonically increasing loading

A study of the influence of carbide phases on the cracking resistance of as-quenched and of quenched and tempered 0.5 pct Mo-B steels was made using notched or notched and precracked specimens that were subjected to impact, cyclic, and monotonically increasing loading. The carbide influence on fracture, while limited in extent, was found to increase as load, loading rate, volume fraction, and particle size increase. The results for the asquenched condition showed that the susceptibility of these steels to crack initiation under impact loading at temperatures below - 100°F is greater when even a small amount of titanium carbide (less than 0.2 vol pet of 1 to 5 μm particles) is present than when none is present. At room temperature, this same carbide concentration has no influence on impact properties, fatigue-crack initiation (in the presence of a notch), fatigue-crack growth rate, or the ductile fracture resistance under monotonically increasing loading at slow strain rate. In the case of the quenched and tempered materials, the alloy containing a large amount of M₂₃C₆ (2 vol pct of 1 to 10 μm particles) exhibited behavior similar to that observed in the as-quenched materials containing titanium carbide. That is, the presence of M₂₃C₆ was associated with increased susceptibility to crack initiation for impact loading at low temperature. In addition, at room temperature this alloy had a reduced impact energy for crack propagation. For monotonically increasing loading at slow strain rate, this same carbide distribution had no influence on the net section stresses required to initiate stable or unstable crack growth. These stresses fall closely in line with, respectively, the yield stress and tensile strength of the material. The alloy containing M₂₃C₆ required less crack opening for a given crack extension—an effect most pronounced after maximum load. Finally, some attention is directed to the use of Charpy test data to assess fracture resistance for modes of loading other than impact.     

Further study on the scattering of the local fracture stress and allied toughness value

In this investigation, the values and their scatters of the local fracture stressof and the fracture load P_f and allied toughness values of base and weld metals were measured, and the main reasons for scattering were analyzed. The results showed that the scatters in σ _f ^* , in general, were the smallest. Measuring the fracture initiation sites with a scanning electron microscope and calculating the location of maximum stress and the range covered by the 95 pct maximum stress did not reveal a “characteristic distance” with a statistical feature, over which the local fracture stress must be exceeded by normal stress. Instead, a combined criterion of a critical strain for initiating a crack nucleus and a critical value and triaxiality of stress for its propagation must be satisfied. The present study discovered the phenomenon that the fracture initiation site could locate at the left side (shorter distance side) of the location of the maximum stress. The reasons for this phenomenon and for the minimum crack opening displacement (COD) value not being zero even though a characteristic distance did not exist were proposed.     

Further development of generalized constitutive relations for metal deformation

A set of constitutive equations is presented to describe the history dependent plastic deformation behavior of anisotropic metals under multiaxial loading conditions. The primary variables that characterize the material behavior with increasing deformation are the effective flow strength,k; the residual or back stress vectorα _i ; and the anisotropy matrix,M _tj . The strain rate is given in terms of an equivalent plastic strain rate, the gradient of a plastic potential, which incorporates key material variables and the stress state. All the material parameters have been determined for a recrystallized Zircaloy-2 from 25 to 450 °C and a solution treated 304 stainless steel from 25 to 650 °C based on experiments that included monotonic, load reversal, and load relaxation tests. The analytical model has been used to simulate the deformation behavior of both metals under a variety of testing conditions, including cyclic and plane strain loading conditions. The general form of the constitutive relation is shown to be consistent with experimental data obtained for various material orientations under different loading conditions.     

The effect of hydrogen on the fracture toughness of alloy X-750

The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 . Charging with hydrogen decreased the fracture toughness, K _lc, to 52 at a rapid loading rate and further decreased the toughness to 42 for a slow loading rate. This is consistent with the rate-limiting step forthe embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.     

Comparative Study on Failure Prediction in Warm Forming Processes of Mg Alloy Sheet by the FEM and Ductile Fracture Criteria

An important concern in metal forming is whether the desired deformation can be accomplished without any failure of the material, even at elevated temperatures. This paper describes the utilization of ductile fracture criteria in conjunction with the finite element (FE) method for predicting the onset of fracture in warm metal working processes of magnesium alloy sheets. The uniaxial tensile tests of AZ31 alloy sheets with a thickness of 3 mm and FE simulations were performed to calculate the critical damage values for three kinds of ductile fracture criteria. The critical damage values for each criterion were expressed as the function of strain rate at various temperatures. In order to find out the best criterion for failure prediction, Erichsen cupping tests under isothermal conditions were carried out at various temperatures and punch velocities. Based on the plastic deformation histories obtained from FE analysis of the Erichsen cupping tests and the critical damage value curves, the initiation time and location of fracture were predicted under bi-axial tensile conditions. As a result, Cockcroft–Latham’s criterion showed good agreement with the experiments.     

Wedge type creep damage in low cycle fatigue

A model is proposed to quantify the accumulation of wedge type creep damage in low cycle fatigue. It is proposed that such damage is produced primarily during the ramp periods of the cycle. Equations are developed for estimating incremental accumulation of damage per cycle in fully reversed, multiaxial loading. The rate of accumulation of damage depends on the strain-rate, the temperature, and the microstructure. The analysis is kept simple by making physically reasonable assumptions. Cycles to failure are predicted by invoking a fracture criterion. The model is applied to two sets of data; one set is a well characterized life test data on an aluminum alloy, and the other is phenomenological data on austenitic stainless steels. In both cases the predictions are good enough to prompt further experimental evaluation of the model. This paper deals with only one mechanism of creep-fatigue interaction. Other mechanisms of failure,e.g., ‘r’ type cavitation, or fatigue crack initiation and propagation, are also viable. The model described here may be expected to apply only under those conditions when wedge damage is the dominant failure mechanism.     

The influence of stress trlaxiality on the damage mechanisms in an equiaxedα/β Ti-6AI-4V alloy

The influence of the stress triaxiality on void formation, void growth, and fracture was investigated for an equiaxed Ti-6A1-4V alloy. Void nucleation in theα phase was found to occur for a critical value of macroscopic plastic strain, whereas void nucleation at theα/β interface also depends on triaxiality. Under low triaxiality and important plastic strain, voids appear and grow in the area where the microshear bands develop, with an angle close to 45 deg to the stress axis in theα particles. In contrast, with high triaxiality, voids nucleate preferably at theα/β interfaces and grow perpendicular to the stress axis by a cleavage mechanism. In a middle range of triaxiality and plastic strain, voids nucleate inα because of the sufficient plastic strain and also at theαβ interfaces because of the sufficient triaxiality(X). Void growth occurs with an angle of 60 deg to the stress axis, sinceX is not high enough to create cleavage andε _p is high enough to provide a ductile growth. Two types of fracture were identified and reported on a fracture map: under low triaxiality, failure appears by plastic instability, whereas for high triaxiality, the instability is induced by a void-growth process discussed with the help of Rice and Tracey’s approach.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Facture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Modeling and measurement of the notched strength of gamma titanium aluminides under monotonic loading

A substantial concern with gamma titanium aluminides is their limited ductility and, in particular, the consequences of limited ductility at stress concentrators. In this study, the notched strength of a cast Ti-47.9Al-2.0Cr-2.0Nb alloy is considered under monotonic tensile loading at room temperature. Efforts are further focused on the alloy’s behavior under conditions of plane stress and on cases where notch radii are large relative to grain size. Finite element predictions for notched tensile specimens and the Neuber design criterion are used to quantify relationships between tensile ductility, the ability to reduce local stress concentrations through plastic flow, and ultimate failure loads in notched components. Results from the testing of two modeled configurations are presented, and the numerical models are used to interpret the test results. Two major issues are addressed in this work. The first is how much plastic deformation is needed to blunt stress concentrations in gamma TiAl components under monotonic loading. For values of elastic stress concentrations commonly encountered in components, it is demonstrated that a total strain at failure as low as 1 pct (corresponding to a plastic strain at failure of 0.8 pct) is sufficient to essentially achieve the maximum reduction in stress concentration due to plastic flow. Second, the relationship between continuum-theory predictions of notched component failure (made using uniaxial tensile test strains at failure) and failure loads observed in notched specimens is explored. It is shown that, on average, continuum-theory predictions of notched strength based on unnotched specimen strains at failure act as conservative lower bounds on actual results. It is suspected that the notch strengthening observed in gamma TiAl is due to smaller volumes of highly strained material (a size effect) in the notched specimens.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Low-Cycle Fatigue Behavior of an As-Extruded AM50 Magnesium Alloy

The low-cycle fatigue behavior of an as-extruded AM50 magnesium alloy has been investigated. The cyclic stress response of the alloy strongly depends on the imposed strain amplitude. It is also noted that at the higher total strain amplitudes, the alloy exhibits a pronounced anisotropic deformation behavior in the direction of tension and compression, where the width of the σ-ε hysteresis loop in the compressive direction is greater than that in the tensile direction. At the total strain amplitude of 1.5 pct, a serrated flow can be observed in both tensile and compressive directions of the σ-ε hysteresis loop. This means that dynamic strain aging takes place during fatigue deformation. The relation between elastic and plastic strain amplitudes with reversals to failure shows a monotonic linear behavior and can be well described by the Basquin and Coffin–Manson equations, respectively. In addition, crack initiation and propagation modes are suggested, based on scanning electron microscopy observations on the fracture surfaces of fatigued specimens. This article is based on a presentation given in the symposium entitled “Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. Gerberich’s 70th Birthday,” which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS.     

Effects of the constraint parameter triaxial stress on the failure behaviour of steels

The constraint parameter triaxial stress which results from loading types and component geometries plays an important role in the failure behaviour of materials. Especially the local failure initiation of materials depends very strongly on triaxial stress. The failure behaviour of a material under different stress states (i.e. different constraint) differs. In this paper effects of triaxial stress on the failure behaviour of steels are presented by means of theoretical analysis, experiments and finite-element-calculations. A theoretical model describing the effects of triaxial stress on failure initiation of materials has been developed based on theoretical analysis. Corresponding experiments were conducted to determine the material constants in the failure model of materials. In order to obtain local damage parameters finite-element calculations have been conducted. According to the results from theoretical analysis, experiments and finite-element calculations the damage curves of materials, in which the critical equivalent plastic strain is plotted as a function of triaxial stress, have been obtained. Furthermore, the failure behaviour of materials under different triaxial stresses has been discussed.     

A comparison of the dislocation structures at small strains in copper deformed in tension and in torsion

OFHC copper specimens of 39 μm grain size were deformed in tension (to 8 pct tensile strain) and in pure torsion (to 8 pct shear strain) at 300 K and the resulting dislocation Burgers vectors, distributions and densities were determined using transmission electron microscopy. Employing the von Mises yield criterion and the total plastic-work hypothe-sis, good agreement was obtained between the tension and torsion test results for: (a) equivalent stress •σ versus equivalent strain •∈^P curves, (b) the dislocation Burgers vec-tors, distribution and density as a function of the equivalent strain and (c) the equivalent stress as a function of the square root of the dislocation density. These results imply that in the unidirectional straining of copper there results a constant dislocation struc-ture for a given amount of plastic work, irrespective of whether the deformation is in tension or torsion. However, equally good correlations were obtained on the basis of maximum shear stress and maximum shear strain and therefore a positive decision be-tween the two yield criteria could not be made.     

Part I-the effects of pre-dissolved hydrogen on cleavage and grain boundary fracture initiation in metastable beta Ti-3Al-8V-6Cr-4Mo-4Zr

The effects of electrochemically pre-dissolved hydrogen on room-temperature fracture initiation in Beta-C titanium (Ti-3Al-8V-6Cr-4Mo-4Zr wt pct) have been investigated using circumferentially notched tensile specimens. Finite element-based analysis of notch stress fields was used to define relationships between the local threshold stress for crack initiation vs total internal hydrogen concentration. The as-received, solution heat treated (ST, σ_y.2 pct=865 MPa) and the ST + peak-aged conditions (STA, σ _y.2% pct=1260 MPa) were compared after defining the relationships between the fracture process zone hydrogen concentration, hydrogen-metal interactions (i.e., hydrostatic stress field occlusion, trapping, hydriding), and the resulting fracture initiation behavior of each. Solutionized + peak-aged (β+α) Beta-C fractured intergranularly above total hydrogen concentrations of ∼1000 wt ppm. (5.1 at. pct). A fracture mode consistent with cleavage occurred at ∼2100 wt ppm. (10.7 at. pct). Solutionized Beta-C resisted hydrogen-assisted cracking (e.g., did not crack intergranularly) but was not immune; cleavage cracking was provoked at ∼4000 wt ppm. (20.4 at. pct). Coldworked ST Beta-C (CW, σ _y.2 pct=1107 MPa) did not crack intergranularly; fracture initiation behavior was similar to the ST condition regardless of specimen orientation. This suggests that high yield strength alone does not account for the susceptibility to intergranular cracking observed in the STA β+α condition. Stroke-rate studies and X-ray diffraction investigation of H partitioning suggests that equilibrium hydriding and/or irreversible trapping does not singularly control intergranular fracture initiation of the STA condition. Fractographic evidence and finite element results show that a finite plastic zone exists prior to intergranular fracture of the STA condition. This suggests that a criterion for fracture that incorporates plastic strain and stress should be considered.     

Weibull statistical fracture theory for the fracture of ceramics

The Weibull statistical fracture theory is widely applied to the fracture of ceramic materials. The foundations of the Weibull theory for brittle fracture are reviewed. This theory predicts that brittle fracture strength is a function of size, stress distribution, and stress state. Experimental multiaxial loading results for A1₂O₃ tubes are compared to the stress state predictions of the Weibull theory. For the most part, the Weibull theory yields reasonable predictions, although there may be some difficulties in dealing with shear stress effects on fracture. This paper is based on a presentation made at the symposium “Stochastic Aspects of Fracture” held at the 1986 annual AIME meeting in New Orleans, LA, on March 2-6, 1986, under the auspices of the ASM/MSD Flow and Fracture Committee.     

Mechanical behavior of Al−Li−SiC composites: Part I. Microstructure and tensile deformation

The microstructure and tensile properties of an 8090 Al−Li alloy reinforced with 15 vol pet SiC particles were investigated, together with those of the unreinforced alloy processed following the same route. Two different heat treatments (naturally aged at ambient temperature and artificially aged at elevated temperature to the peak strength) were chosen because they lead to very different behaviors. Special emphasis was given to the analysis of the differences and similarities in the microstructure and in the deformation and failure mechanisms between the composite and the unreinforced alloy. It was found that the dispersion of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al₃Li at ambient temperature. The deformation processes in the peak-aged materials were controlled by the S′ precipitates, which acted as barriers for dislocation motion and homogenized the slip. Homogeneous slip was also observed in the naturally aged composite, but not in the unreinforced alloy, where plastic deformation was concentrated in slip bands. The most notorious differences between the alloy and the composite were found in the fracture mechanisms. The naturally aged unreinforced alloy failed by transgranular shear, while the failure of the peak-aged alloy was induced by grain-boundary fracture. The fracture of the composite in both tempers was, however, precipitated by the progressive fracture of the SiC reinforcements during deformation, which led to the early failure at the onset of plastic instability.