KIC Modelling for a critical strain criterion involving the stress triaxiality effect

An analytical model for predicting fracture toughness K_IC is proposed based on a stress-modified critical strain criterion, that reflects the effect of stress triaxiality on ductile fracture. For K_IC modelling, the notch-tip strain and stress state are given by introducing asingular field for the case of power-law-hardening materials. Notch fracture toughness is interpreted in terms of the notch-root radius (ϱ): K_IC is predicted to increase with increasing ϱ, but has a minimum at a small ϱ. The microstructuralls characteristics distance and the reference critical strain can be estimated by fitting the K_IC vs ϱ data on the model equation. Finally, previous notch fracture-toughness data are re-analyzed with the proposed model: the current analysis explains well the interaction effect between the notch-tip strain field and the local-fracture-controlling microstructure even in the small ϱ range.     

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.     

Effects of test temperature, grain size, and alloy additions on the low-temperature fracture toughness of polycrystalline niobium

The current work investigates the effects of test temperature (77 to 150 K), grain size (63 to 165 μm), and solid solution alloying additions of zirconium (Zr) on the fracture toughness (K _q , K _Ic ) of polycrystalline niobium (Nb). Extensive fracture surface analyses of the fractured specimens revealed the location of the apparent cleavage fracture nucleation sites. Comparisons have been made to models for cleavage fracture toughness as well as to predictions of the peak stress locations using existing finite element models for a crack loaded under plane strain conditions.     

The fracture toughness and toughening mechanisms of nickel-base wear materials

Nickel-base wear materials are typically used as weld hardfacing deposits, or as cast or hot isostatically pressed (HIP) inserts that provide the needed wear resistance to a base material with the desired mechanical properties. Most nickel-base wear materials contain high levels of chromium, silicon, carbon, and boron, which results in complex microstructures that are comprised of high volume fractions of silicide, carbide, and/or boride phases. The volume fraction of nickel-phase dendrite regions typically ranges from 40 to 70 pct, and these dendrite-phase particles are individually isolated by a matrix of silicide, carbide, and boride phases. The continuous matrix of brittle silicide, carbide, and boride phases results in a low damage tolerance for nickel-base wear materials, which is a concern in applications that involve high stresses, thermal transients, or shock loading. Fatigue crack growth (FCG) and fracture toughness (K _IC) testing in accordance with ASTM E399 methods has been used to quantify the damage tolerance of various nickel-base wear materials. Fractographic and microstructure examinations were used to define a generic toughening mechanism for nickel-base wear materials. The toughness of nickel-base wear materials is primarily controlled by the plastic deformation of the nickel-phase dendrites in the wake of a crack moving through the matrix of brittle silicide, carbide, and/or boride phases, i.e., crack bridging. Measured K _IC values are compared with calculated K _IC values based on the crack-bridging model. Microstructure examinations are used to define and confirm the important aspects of the crack-bridging model. This model can be used to predict the toughness values of nickel-base wear materials and direct processing methods to improve the K _IC values.     

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.     

THE APPLICATION OF FRACTURE MECHANICS TO CEMENTED TUNGSTEN CARBIDES

Abstract

The difficulties encountered in the measurement of the toughness of cemented tungsten carbides are discussed and the benefits that might be expected from an application of fracture mechanics to the problem are described. A simple method for the measurement of the fracture-toughness parameter, K_IC, for the more brittle grades of carbide is considered. The method involves indenting a beam-shaped specimen with a Knoop diamond to produce a crack, and loading the pre-cracked specimen to failure in four-point bending. Results from two grades of cemented carbide are presented and show that a standard error of the mean K_IC of ～3% can be obtained from a set of 10 measurements, with a minimum of specimen preparation and no special testing equipment. The results show also that the toughness of the cemented carbide can be affected by grain-size variations within the same batch of material and by the pressing direction during manufacture.     

Effects of test temperature and grain size on the charpy impact toughness and dynamic toughness (<Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="Italic">ID</Emphasis></Subscript>) of polycrystalline niobium

The effects of changes in test temperature (−196 °C to 25 °C) and grain size (40 to 165 μm) on the dynamic cleavage fracture toughness (K _ID ) and Charpy impact toughness of polycrystalline niobium (Nb) have been investigated. The ductile-to-brittle transition was found to be affected by both changes in grain size and the severity of stress concentration (i.e., notch vs fatigue-precrack). In addition to conducting impact tests on notched and fatigue-precracked Charpy specimens, extensive fracture surface analyses have been performed in order to determine the location of apparent cleavage nucleation sites and to rationalize the effects of changes in microstructure and experimental variables on fracture toughness. Existing finite element analyses and the stress field distributions ahead of stress concentrators are used to compare the experimental observations with the predictions of various fracture models. The dynamic cleavage fracture toughness, K _ID , was shown to be 37±4 MPa√m and relatively independent of grain size (i.e., 40 to 105 μm) and test temperature over the range −196 °C to 25 °C.     

Effects of be and fe content on plane strain fracture toughness in A357 alloys

The effect of Be and Fe content on the plane strain fracture toughnessK _IC of aluminum-based A357 alloys is investigated. The fracture behavior of A357 alloys has been evaluated as a function of both the magnitude and morphology of iron-bearing compounds and silicon particles. Addition of Be is beneficial for tensile properties and fracture toughness in the case of alloys containing intermediate (0.07 pct) and higher (0.15 pct) Fe levels. On the other hand, Be added to alloys containing the lower Fe (0.01 pct) level appears detrimental to tensile strength, but the quality index, notch-yield ratio (NYR), and plane strain fracture toughness were improved. Fractographic analysis reveals that crack extension of A357 alloys occurs mainly in an intergranular fracture mode. The fracture processes are initiated by void nucleation at iron-bearing compounds or irregularly shaped eutectic silicon particles as a result of their cracking and decohesion from the matrix. Then, void growth and coalescence result in growth of the main crack by shear-linkage-induced breakdown of submicronstrengthening particles. The effect of Be on increasingK _IC is more apparent in the higher Fe alloys than in the lower Fe alloys. Superior toughness obtained by microstructural control has also been achieved in the intermediate and higher Fe levels of Be-containing alloys, with values equal to those obtained in alloys of lower Fe content.     

Analysis of fracture toughness in the transition-temperature region of an Mn-Mo-Ni low-alloy steel

This study is concerned with the analysis of fracture toughness in the transition region of an Mn-Mo-Ni low-alloy steel, in accordance with the ASTM E1921 standard test method. Elastic-plastic cleavage fracture toughness (K _Jc ) was determined by three-point bend tests, using precracked Charpy V-notch (PCVN) specimens, and relationships between K _Jc , the critical component of J (J _c ), critical distance (X _c ), stretch-zone width (SZW), local fracture stress, and plane-strain fracture toughness (K _Ic were discussed on the basis of the cleavage fracture behavior in the transition region. The master curve and the 95 pct confidence curves well explained the variation in the measured K _Jc , and the Weibull slope measured on the Weibull plots was consistent with the theoretical slope of 4. Fractographic observation indicated that X _c linearly increased with increasing J _c , and that the SZW had a good correlation with K _Jc , irrespective of the test temperature. In addition, the local fracture stress was independent of the test temperature, because the tempered bainitic steel used in this study showed a propagation-controlled cleavage fracture behavior.     

Tempered martensite embrittlement in SAE 4340 steel

The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K _1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K _IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).     

Fine carbide-strengthened 3Cr-2WVTa bainitic steel

Both the 3Cr-3WV and the 3Cr-2WVTa steels exhibit an acicular bainite microstructure under the normalized and the normalized-and-tempered condition. The addition of Ta to the 3Cr-3WV steel substantially decreases the prior austenite grain size, but it has little effect on the bainite packet size. Fine TaC precipitates are formed in the normalized 3Cr-3WVTa specimen. After further tempering of 3Cr-3WVTa steel, fine TaC particles are further precipitated and dispersed within grains. The carbides at the prior austenite grain boundaries in the Ta-containing steel are much smaller than those in the steel without Ta. Tensile tests and fracture toughness (K _IC ) tests have been performed on both the 3Cr-3WV and 3Cr-3WVTa steels at room temperature. The 0.2 pct yield strength of the Ta-containing steel is higher than that of the steel without Ta, especially under the normalized-and-tempered condition. The 3Cr-3WVTa steel is primarily strengthened by a secondary-phase precipitation mechanism represented by the formation of fine carbides after tempering. The 3Cr-3WVTa steel exhibits higher fracture toughness than the 3Cr-3WV steel. The toughening mechanism is also discussed based on the dependence of the calculated fracture stress upon the carbide size and the bainite packet size.     

Correlations between fracture surface appearance and fracture mechanics parameters for stage II fatigue crack propagation in TÏ-6AI-4V

Stage fatigue crack propagation in Ti-6A1-4V has been studied as a function of various fracture mechanics parameters, including the stress intensity range (ΔK) and both positive and negative ratios of the minimum to maximum stress (R). It was found that the fracture surface appearance undergoes a transition from cyclic cleavage to striations at a ΔK_eff of approximately 13 MNm^-3/2 (11.8 ksi√in.). It was also observed that the measured striation spacings are generally within a factor of two of the optically measured crack growth rates. Both of these results can be particularly useful for determining unknown component cyclic loadings during failure analysis. The criterion for the cyclic cleavage to striation transition is considered to be a change from primarily single to multiple slip within the individual grains at the crack tip. This occurs when the cyclic plastic zone size becomes approximately equal to the α grain size.     

Internal hydrogen embrittlement of ultrahigh-strength AERMET 100 steel

Near-peak-aged AERMET 100 is susceptible to severe internal hydrogen embrittlement (IHE) at 23 °C, if a sufficient diffusible hydrogen content is present, compromising the high toughness of this ultrahigh-strength steel (UHSS). Evidence includes the threshold stress intensity for subcritical IHE (K _TH ) as low as 10 pct of the plane-strain fracture toughness (K _IC ) and a fracture-mode transition from microvoid coalescence to brittle transgranular (TG) cracking, apparently along martensite lath interfaces and cleavage planes. The K _TH value decreases from a K _IC value of 132 to 143 MPa√m to 12 MPa√m, and the amount of brittle TG fracture increases to nearly 100 pct as the concentration of diffusible H increases from essentially 0 to 8 wppm, with severe embrittlement in the 0 to 2 wppm H regime. The IHE is time dependent, as evidenced by increasing K _TH values with increasing dK/dt and K-independent subcritical crack growth rates, and is attributed to diffusional H repartition from reversible trap sites to the stressed crack tip. The partition distance is ∼1 μm, consistent with the fine-scale microstructure of AERMET 100. The causes of the susceptibility of AERMET 100 to TG IHE are very high crack-tip stresses and a reservoir of mobile H trapped reversibly at (Fe,Cr,Mo)₂C precipitates. These factors enable repartition of H to misoriented martensite lath interfaces and interstitial sites near cleavage planes, with each prone to decohesion along a connected path. Predissolved H also reduces the ductile fracture toughness of AERMET 100 at high loading rates, perhaps due to reduced void growth caused by H trapped strongly at undissolved metal carbides.     

The effect of the austenitizing heat-treatment variables on the fracture toughness of high-speed steel

The influence of austenitizing treatment and tempering on the fracture behavior of high-speed steel (DIN 1.3333) has been investigated. The fracture behavior has been characterized by determining the K _IC and J _IC values via the performance of modified compact tension (CT) and single edge notched (SEN) tests. The micromechanisms of crack initiation and propagation have been studied by metallographic examination of the fractured specimens. The results indicate that austenitizing conditions of temperature range 1050 °C to 1190 °C and time 0.25 to 6 minutes and tempering at 550 °C to 650 °C up to 150 minutes alter the microstructure and, subsequently, the fracture toughness. It was found that cracking occurs by nucleation at the interface of matrix/vanadium-enriched large carbides, where sulfur is segregated and where linkage of the microcracks bridges ductile ligament of voids at small Mo + W enriched carbides. The improvements of the fracture toughness and hardness by short austenitizing time of 15 to 75 seconds at 1190 °C are attributed to (1) the optimum distribution of a dense network of small carbides, (2) the lack of grain growth as the boundaries are pinned down by these small carbides, and (3) the retained austenite at a level up to 16 vol pct transformed to martensite.     

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.     

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.     

Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.     

Effects of texture and microstructure on the propagation of iodine stress corrosion cracks in zircaloy

An investigation of stress corrosion crack propagation in Zircaloy is performed at 300 °C in four Pa flowing iodine environment. By varying the orientation of fracture mechanics specimens, the effect of crystallographic texture, heat treatment, and microstructure onK _ISCC is studied. Texture is found to have a strong effect on bothK _ISCC and the fracture path. As the resolved fraction of basal poles parallel to the direction of crack opening decreases,K _ISCC in stress-relieved material increases from 4 MPa√m atf = 0.70 to 17 MPa√m atf = 0.19. The same trend is observed in recrystallized material. However, theK _ISCC values are somewhat greater. Transgranular cleavage is the preferred mode of crack propagation. Several ductile modes of separation complement the cleavage process. At high crack velocity, tearing between facets is promoted. At lowK, nearK _ISCC, very little tearing is observed and cleavage zones larger than the grain size are common. Fluting is preferred in the low regime. In recrystallized material a transition to completely intergranular failure is observed nearK _ISCC.