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.     

Development of damage in low-carbon steel under mode I and mode II loading conditions

The geometry of plastic deformation localization zones in low-carbon steel specimens under mode II and I conditions is estimated using replicas, and the evolution of microcrack patterns in these zones is studied by acoustic emission. It is shown that, under mode II loading, the plastic deformation zone area decreases and an additional system of microcracks appears; as a result, the number of microcracks and the number of acoustic emission signals increase. The following localized fracture criteria are discussed: the concentration criterion and parameters b _C and b _AE of the length and amplitude distributions of microcracks and acoustic signals, respectively. These parameters are used to characterize and predict the fracture probability.     

Fracture of a low-carbon steel under mode I,mode II,and mixed-mode loading conditions

The effect of the shear component of static loading on the evolution of the plastic deformation zones and the mechanical and acoustic properties (acoustic emission parameters, ultrasonic wave velocity and attenuation coefficient) of a low-carbon grade 20 steel is studied. It is found that an increase in the shear loading component leads to a change in the shape of a plastic deformation zone, the appearance of an additional system of microcracks, an increase in the total fracture energy, a decrease in the slopes of the cumulative distributions of the acoustic signal amplitude and the microcrack length, and a significant increase in the ultrasound attenuation coefficient.     

Hydrogen-induced cleavage fracture of Fe3Al-based intermetallics

Hydrogen-induced fracture of ductile Fe₃Al-based intermetallics was studied through mechanical testing, fracture surface observation, andin situ transmission electron microscopy (TEM) tests of tensile specimens. Mechanical properties of ordinary ductile X-80 pipeline steel (low-alloy steel) were tested and compared with Fe₃Al intermetallics. Elongations of the Fe₃Al alloy decreased from 14 to 10 pct, with increases in the strain rate from 10⁻⁶ to 10⁻³/s. The elongation reduction of Fe₃Al was caused by the hydrogen-induced fracture. There was no elongation reduction when the testing was done in mineral oil. Non-necking occurred near the fracture section, and the fracture surfaces mainly consist of cleavage and partial intergranular morphologies. Elongation near the fracture surface of the Fe₃Al intermetallics was about 14 pct, which is the same as the total elongation. For the pipeline steel, however, an elongation near the fracture cross section was greater than 130 pct, which was much higher than its total elongation of 17 pct.In situ TEM observation on a tensile test sample showed crack propagation accompanied by dislocation plasticity. When the Fe₃Al was precharged cathodically, the crack tip was sharp. Its radius was much less than that obtained without hydrogen charging. The crack propagated along the grain boundary for the charged specimens, but penetrated the grain boundary for the specimen without hydrogen charging. Effects of hydrogen on plastic deformation and grain-boundary cracking are discussed in this article.     

Environment-sensitive fracture of iron aluminides under monotonic tensile loading

In the present study, the environmental sensitivity under tensile loading of a high-performance Fe-40Al alloy prepared by mechanical alloying is investigated. Testing performed under various environmental conditions reveals a significant loss of ductility in the presence of a moist atmosphere without concomitant change in the stress-strain relationship and fracture mode. Furthermore, this embrittlement is essentially controlled by water vapor. In addition, it is shown that increasing the strain rate improves the final elongation in air up to the intrinsic ductility determined in an inert atmosphere. This embrittlement is attributed to the hydrogen released by the dissociation of water vapor molecules and subsequently swept into the material by mobile dislocations. Tests were designed and carried out specifically to assess the respective roles of exposure duration and straining in this process. Finally, a model is proposed to account for this phenomenon and the mechanisms are discussed.     

A Criterion for ductile fracture in sheets under biaxial loading

A criterion for ductile fracture is developed based on the statistical process of shear joining of voids and on the assumption that the voids responsible for fracture have experienced considerable growth prior to this stage of shearing. From the knowledge of uniaxial flow properties and fracture strain measurement, this model is capable of predicting the strain at fracture for other strain states. The predicted data are in good agreement with experiments. Although this model assumes spherical inclusions, some quantitative estimates for elongated inclusions can also be made.     

A Criterion for ductile fracture in sheets under biaxial loading

A criterion for ductile fracture is developed based on the statistical process of shear joining of voids and on the assumption that the voids responsible for fracture have experienced considerable growth prior to this stage of shearing. From the knowledge of uniaxial flow properties and fracture strain measurement, this model is capable of predicting the strain at fracture for other strain states. The predicted data are in good agreement with experiments. Although this model assumes spherical inclusions, some quantitative estimates for elongated inclusions can also be made.     

Creep and fracture in model niobium-alumina laminates under shear loading

A simple experimental technique was developed to study the near-interface deformation and fracture behavior in ductile-phase-toughened brittle-matrix laminates subjected to elevated-temperature shear loading. In the study, specimens of Nb-foils bonded to Al₂O₃ blocks were subjected to shear loading parallel to the Nb/Al₂O₃ interfaces. The fracture path was controlled by the applied stress, the temperature and the thickness of the ductile Nb layers. At high shear stresses failure took place by brittle fracture within the Al₂O₃ phase with concurrent shear creep in the Nb, and multiple crack branching/arresting toward the interface. At lower stresses, shear-creep and ductile fracture within the Nb were the dominant damage modes. Shear deformation was found to localize along the mid-plane of the Nb, due to strengthening of the Nb adjacent to the interface via solid solution and precipitation resulting from interdiffusion. With thin 20 μm Nb-layers the fracture energy was low, similar to that found for pure Al₂O₃. Our findings suggest that the ductile-phase toughening of laminated brittle matrix composites depends critically on the thickness of the ductile phase. A concept of brittle-ductile transition to assist in the understanding of the toughness enhancement provided by ductile phase additions into a brittle matrix.     

Indomethacin-induced delayed fracture healing

The detection and measurement of antibodies in bloodstains represents a departure from the classical blood grouping used routinely in forensic serology. The antibodies in question are those associated with atopic allergy and infections by micro-organisms and other parasites. The production of these antibodies is not primarily under genetic control but reflects an individual's reaction to his environment. This paper describes the explicit information which may be obtained from an antibody profile, the high discriminating power and other advantages of the method.     

Stress corrosion cracking in high strength steel under mode III loading

Stress corrosion cracking (SCC) of high-strength steel in aqueous environment and hydrogen induced cracking (HIC) during dynamic charging under Mode III loading were investigated. The threshold stress intensities for SCC and HIC under Modes III and I were measured and compared. It was found that both SCC and HIC under Mode III loading initiated and propagated on the planes inclined at 45 deg to the notch plane, differing from that under Mode I loading. The fracture surfaces, however, revealed intergranular facets, similar to that under Mode I loading. The addition of thiourea decreased the threshold value for SCC under Mode III and Mode I loading, which was still higher than that for dynamic charging. The threshold values of both SCC and HIC under Mode III were larger than that under Mode I,i.e., K_IIIH> K_IH, K_IIISCC > K_ISCC. Based upon the fracture mechanics analysis, this difference is attributed to the different equilibrium hydrogen concentration between Modes III and I loading. These results give strong evidence that the SCC mechanism in high strength steel under Mode III loading is also related to hydrogen induced cracking. Formerly Student at Beijing University of Iron and Steel     

Mode I stress intensity factors induced by fracture surface roughness under pure mode III loading: Application to the effect of loading modes on stress corrosion crack growth

A model to estimate the reduction of effective crack tip Mode III stress intensity factors by frictional and asperity interaction of an idealized fracture surface is described. An extension of the model is used to calculate the Mode I stress intensity factors due to the crack tip opening displacement induced by the mismatch of the fracture surface asperities. The results of calculations based on a “reasonable” fracture surface profile are used to analyze experimental studies designed to determine the relative significance of hydrogen embrittlement and crack tip dissolution in stress corrosion crack growth in Al alloys by comparison of Mode I and Mode III stress corrosion cracking (SCC) resistance. It is concluded that a pure Mode III stress state is not possible for cracks with microscopically rough surfaces and that the magnitude of the induced Mode I stress intensity factor is sufficient to cause stress corrosion crack growth.     

Effect of loading mode on the fracture toughness of a ferritic/martensitic stainless steel

The critical J integrals of mode I (J_IC), mixed-mode I/III (J_TC), and mode III (J_IIIC) were examined for a ferritic stainless steel (F-82H) at ambient temperature. A determination of J_TC was made using modified compact-tension specimens. Different ratios of tension/shear stress were achieved by varying the principal axis of the crack plane between 0 and 55 deg from the load line. The value for J_IC was determined by means of specially designed specimens. The results showed that F-82H steel had high fracture toughness. Both J_IC and J_IIIC were about 500 kJ/m₂, and the mode I tearing modulus J_Ida) was about 360 (kJ/m²)/mm. However, J_TC and mixed-mode tearing modulus (dJ_T/da) values varied with the crack angles and were lower than their mode I and mode III counterparts. Both the minimum J_TC and dJ_T/da values occurred at a crack angle between 40 and 50 deg, at which the load ratio of σ_iii/σ, was 0.84 to 1.2. The J_min was 240 kJ/m², and ratios of J_lC/J_min and J_IIICJ_min were 2.1 and 1.9, respectively. The morphology of the fracture surfaces was consistent with the change in J_TC and dJ_T/da values. While the upper shelf-fracture toughness of F-82H depended on loading mode, the J_min value remained high. Other important considerations include the effect of mixed-mode loading on the ductile-brittle-transition temperature and effects of hydrogen and irradiation on J^.     

Fracture behavior of quaternary Al-Li-Cu-Mg alloy under mixed mode I/III loading

The fracture toughness under mixed-mode I/III loading conditions was evaluated for a quaternary 8090 Al-Li-Cu-Mg alloy in underaged and peak-aged conditions. The mixed-mode fracture behavior was found to be significantly different for the two aging conditions. Super-imposed mode III component lowered the fracture resistance of the alloy in underaged condition, whereas it had no significant effect in peak-aged condition. The results obtained have been discussed in the light of the prevalent fracture processes, namely, transgranular shear and ductile intergranular fracture mechanisms. Further, these results are analyzed in terms of different frac-ture criteria and they were found to deviate significantly from those predicted by the energy release rate criterion. On leave from the Defense Metallurgical Research Laboratory, Hyderbad     

GH4169合金高温多轴非比例加载下疲劳断口特征

研究了总应变控制下比例加载和不同相位差(45°和90°)的非比例加载条件下GH4169镍基高温合金650℃双轴疲劳的断口特性.结果表明,比例加载时,裂纹在试样表面均匀萌生;随非比例度提高,裂纹萌生的数量明显减少.比例加载的断口表面有明显的撕裂条带,且沿径向扩展;而非比例加载时这些撕裂条带减少,相应地撕裂面增多,在相位差为90°时断口表面完全为撕裂面,且沿圆周方向扩展.在两裂纹扩展的交界处,发现有疲劳条纹,沿径向分布.在瞬间断裂区,随着非比例度的增加,韧窝逐渐加深,且撕裂面逐渐加大.     

Component specific influences on the fracture behaviour of high speed steels under static loading

This paper presents the first results of a series of investigations into the fracture behaviour of high speed steels with regard to component specific influences. The aim of the investigations was the analysis of the interaction of internal and external notches during the fracture process. Hardened and tempered high speed steel S 6-5-2 of various degrees of hot forming, which was produced by electroslag remelting, as well as powder-metallurgically produced high speed steel ASP 23 were tested to investigate the influence of different carbide sizes and distribution. In-situ bend tests showed the role of the carbides as fracture initiating defects, which form subcritical cracks as a result of the higher stiffness and lower failure stress at loads smaller than the global failure load of the specimen. The observed fracture process was simulated by FEM using the observations and results of the in-situ bend tests. The interaction of internal flaws and external notches was proven using notched and coated specimens as well as specimens of various surface roughness. The fracture initiating defect is the largest defect in the loaded volume regardless of whether it is an internal flaw (carbide) or an external notch (surface roughness, surface layer). Information can be derived from the investigations allowing the optimisation of the manufacture of high speed steel tools with regard to their fracture behaviour.     

Mixed mode interface fracture criteria

The shielding of the full effect of mode II and III for interface cracks in brittle materials is demonstrated to be very important such that proper interface fracture criteria need to be modelled. A method for comparison of different phenomenological fracture criteria is discussed and applied to an experiment for a polyimide/glass interface. Finally, micromechanical models describing shielding for interface geometries with kinks are set up, allowing the phenomenological criteria to be linked up with measurable quantities such as the interface roughness.     

Role of large-scale slip in mode II fracture of bimaterial interface produced by diffusion bonding

Bimaterial interfaces present in diffusion-bonded (and in-situ) composites are often not flat interfaces. The unevenness of the interface can result not only from interface reaction products but also from long-range waviness associated with the surfaces of the component phases bonded together. Experimental studies aimed at determining interface mechanical properties generally ignore the departure in the local stress due to waviness and assume a theoretically flat interface. Furthermore, the commonly used testing methods involving superimposed tension often renders the interface so extremely brittle that if microplastic effects were present it becomes impossible to perceive them. This article examines the role of waviness of the interface and microplastic effects on crack initiation. To do this, a test was selected that provides significant stability against crack growth by superimposing compressive stresses. Mode II interface fracture was studied for NiAl/Mo model laminates using a recently developed asymmetrically loaded shear (ALS) interface shear test. The ALS test may be viewed as opposite of the laminate bend test. In the bend test, shear at the interface is created via tension on one surface of the bend, while in the ALS test, shear is created by compression on one side of the interface relative to the other. Normal to the interface, near the crack tip, an initially compressive state is replaced by slight tension due to Poisson’s expansion of the unbonded part of the compressed beam.     

石墨球对系列冲击温度下球墨铸铁断口作用机制

 为了明确低温用球墨铸铁材料断裂微观机理,针对石墨球对系列温度球墨铸铁冲击断口演变过程的作用机制进行研究。采用SEM、激光共聚焦显微镜等手段系统分析了不同温度下石墨球对球墨铸铁冲击断裂过程的影响。定量断口分析结果表明,与冲击功随温度的变化一致,断口表面粗糙度Sa和空穴扩张比Rc/R0(韧窝与石墨球半径之比)均随温度的下降呈明显降低趋势。随着冲击试验温度的降低,由韧性断裂逐渐过渡到脆性断裂,这种断裂机制的变化导致断面粗糙度指数剧烈下降,空穴扩张比值趋近于1。冲击过程中裂纹总是在石墨 基体界面处发生开裂并沿着石墨 基体界面不断扩展,因此实际生产过程中应该注重改善石墨球与基体界面处组织状态。     

The effect of loading mode on hydrogen embrittlement

Hydrogen embrittlement is shown to occur very easily in notched-round bars under opening modeI (tension) but not under antiplane shear modeIII (torsion). The stress tensor invariants under modeI,II, andIII loadings and how these affect interstitial diffusion are discussed. It is suggested that long range diffusion of hydrogen down orthogonal trajectories to the vicinity of the crack tip, which can occur under modeI but not modeIII, is a key part of any hydrogen embrittlement mechanism. This premise was evaluated with AISI 4340 steel heat treated to ultrahigh strength levels. It was found that an initial modeI stress intensity level of 17,000 psi-in.^1/2 produced failure in several minutes. ModeIII stress intensity levels three times this produced no crack initiation in 300 min. Further analysis of the time-dependent hydrogen concentrating effect utilized a stress wave emission technique. This produced plausible critical hydrogen concentrations even though the present elastic analysis is a first order approximation of the stress field. Formerly Graduate Student and Lecturer, respectively, University of California, Berkeley