A micromechanical model for cleavage-crack reinitiation

A crack-tip screening analysis of cleavage fracture of steel is developed. The analysis incorporates evidence that reinitiation of an arrested cleavage crack requires less stress intensity than cleavage initiation from a fatigue precrack. Fractographic evidence as well as metallographic sectioning of arrested cracks have previously shown that the mechanism of rapid crack propagation by cleavage is affected strongly by partial crack-plane deflection which leaves unbroken ligaments in its wake. The tearing of these ligaments by dimple rupture is the dominant energy-absorbing mechanism. Earlier etch-pit experiments using an Fe-Si alloy showed that the crack-tip stress intensity based on plastic zone size is extremely low. These observations are incorporated into a model in which cleavage crack reinitiation is analyzed using a sharp crack that is shielded by a distribution of pinching forces along its faces. During reloading of the arrested crack, the ligaments restrict crack-tip blunting, leading to higher local stresses. As a result, lower stress intensities are needed for reinitiation than for initiation from a fatigue precrack.     

A micromechanical model for cleavage-crack reinitiation

A crack-tip screening analysis of cleavage fracture of steel is developed. The analysis incorporates evidence that reinitiation of an arrested cleavage crack requires less stress intensity than cleavage initiation from a fatigue precrack. Fractographic evidence as well as metallographic sectioning of arrested cracks have previously shown that the mechanism of rapid crack propagation by cleavage is affected strongly by partial crack-plane deflection which leaves unbroken ligaments in its wake. The tearing of these ligaments by dimple rupture is the dominant energy-absorbing mechanism. Earlier etch-pit experiments using an Fe-Si alloy showed that the crack-tip stress intensity based on plastic zone size is extremely low. These observations are incorporated into a model in which cleavage crack reinitiation is analyzed using a sharp crack that is shielded by a distribution of pinching forces along its faces. During reloading of the arrested crack, the ligaments restrict crack-tip blunting, leading to higher local stresses. As a result, lower stress intensities are needed for reinitiation than for initiation from a fatigue precrack.

     

Fundamental aspects of fatigue and fracture in a TiAl sheet alloy

The fatigue mechanisms in a TiAl sheet alloy, heat treated to the lamellar and equiaxed microstructures, were studied to determine the effects of microstructure on the initiation of microcracks and their subsequent growth into large cracks. The nucleation and growth history of individual microcracks were followed. For comparison, fatigue crack growth and fracture toughness were also characterized using specimens containing a machined notch with a fatigue precrack. The results indicated that microcracks initiated at grain/colony boundaries and at slip bands. Most microcracks were arrested after nucleation, but a few grew at stress intensity ranges below the large crack threshold. The populations of nonpropagating and propagating cracks varied with life fractions. Ligaments in the wake of a fatigue crack were more severely strained than the crack-tip region of the main crack, and, as a result, they were more prone to fatigue failure. The destruction of the crack-wake ligaments is expected to result in lower fracture resistance in materials under cyclic loading than those under monotonic loading. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Dependency of fracture toughness on the inhomogeneity of coarse TiN particle distribution in a low alloy steel

An investigation of the effect of the coarse TiN particle distribution on the fracture toughness of a steel, as determined by crack-tip opening displacement (CTOD), was carried out using a range of samples from a Ti-treated steel that had been thermally cycled to simulate a coarse grained heat-affected zone (CG HAZ) microstructure. Experimental results from tests carried out at room temperature showed that the inhomogeneous spatial distribution of the coarse TiN particles in the microstructure ahead of the fatigue precrack caused the samples to fail with significantly different CTOD values. Detailed fractographic investigation showed that, with an increased number of overall fracture initiation sites (FISs) and number density of local cleavage initiation sites (CISs) caused by coarse TiN particles, the fracture toughness CTOD values generally decreased. The increase in FIS number and CIS number density has been related to the inhomogeneous coarse TiN distribution ahead of the fatigue precrack and so the sampling of microstructural areas with a high number density of coarse TiN particles. The mechanism by which the coarse TiN particles cause cleavage fracture initiation is discussed.     

Fracture mechanics approach to hydrogen-assisted microdamage in eutectoid steel

A fracture mechanics approach to hydrogen-assisted microdamage in eutectoid steel is presented. Fractographic analysis revealed micromechanical effects of hydrogen in the form of tearing topography surface (TTS). The progress of this microdamage is modeled as a macroscopic crack that extends the original fatigue precrack and involves linear elastic fracture mechanics principles. In this case, the change from hydrogen-assisted microdamage (TTS) to cleavagelike topography takes place when a critical stress intensity factor (K _H) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue precrack plus the TTS area). It is shown that the value K _H depends on experimental variables—mainly on the fatigue precracking regime—and its value may be associated with a characteristic level of stress intensity factor in the crack growth kinetics curve.     

Visual simulation of fatigue crack growth

An attempt has been made to visually simulate fatigue crack propagation from a precrack. An integrated program was developed for this purpose. The crack-tip shape was determined at four load positions in the first load cycle. The final shape was a blunt front with an “ear” profile at the precrack tip. A more general model, schematically illustrating the mechanism of fatigue crack growth and striation formation in a ductile material, was proposed based on this simulation. According to the present model, fatigue crack growth is an intermittent process; cyclic plastic shear strain is the driving force applied to both state I and II crack growth. No fracture mode transition occurs between the two stages in the present study. The crack growth direction alternates, moving up and down successively, producing fatigue striations. A brief examination has been made of the crack growth path in a ductile two-phase material.     

Quantitative fractography and dislocation interpretations of the cyclic cleavage crack growth process

Fractographic features of cyclic cleavage crack growth in Fe and Fe-binary alloys as a function of applied stress intensity and test temperature are documented. Cyclic striations, cleavage rivers and twist angle misorientations are measured and discussed in terms of geometrical and local stress considerations. It is shown that the river height displacement must account for both twist angle misorientation between adjacent grains and the crack-tip opening displacement of an advancing fatigue crack. With regards to the rate of an advancing cleavage crack, a dislocation dynamics model is derived which qualitatively gives the correct ordering of power-law slopes (da/dN vs ΔK) and growth rate magnitudes as a function of test temperature.     

The Use of Atomic Force Microscopy to Study Crack Tips in Glass

This article presents a review of the application of atomic force microscopy (AFM) to crack-tip corrosion during subcritical crack growth in glass. The two principal experimental techniques used in this type of study are (1) the direct observation of crack motion by scanning the tip of a crack during crack growth and (2) the examination of fracture surfaces once the specimen has been fractured in two. The first technique has been used to demonstrate and quantify water condensation at crack tips during subcritical crack growth and is particularly useful at low crack velocities. The second technique has been used to quantify the crack-tip corrosion process and the shape of the crack tip during crack growth. In this article, we discuss experimental results showing that the environment that develops at the tips of freshly fractured glass surfaces in soda lime glass can corrode the glass surfaces near the crack tip. Soda lime silicate glass contains mobile alkali ions that will exchange with hydronium ions in solution at the crack tip, forming a highly basic solution that is corrosive to glass. Experimental evidence for such corrosion has been obtained by the atomic force microscope, which demonstrates a displacement of the two fracture surfaces near the crack tip that can be as much as 20 nm, depending on how long the crack is held open at the fatigue limit. Despite the corrosion and displacement of the crack surfaces, the crack tip itself appears to remain sharp, suggesting that the fatigue limit in soda lime silicate glass is not due to crack-tip blunting. Most likely, the fatigue limit is a consequence of ion exchange at the crack tip, in which hydronium ions in the crack-tip solution exchange with sodium ions in the glass. As hydronium ions are larger than sodium ions, this exchange process leaves a compressive stress within the fresh fracture surface of the glass that resists crack motion and results in a stress-corrosion fatigue limit, as first proposed by Bunker and Michalske. In agreement with this mechanism, no fatigue limit is observed for silica glass, which also exhibits no ion exchange. As the crack-tip solution in silica glass is only mildly acidic, pH ≈ 5, corrosion does not occur at crack tips of this glass as supported by the observation that no crack-tip displacements are observed in silica glass by AFM. As the proposed ion exchange mechanism used to explain the stress corrosion limit in glass is at variance with the belief that the fatigue limit in glass is the result of crack-tip blunting, we discuss the possibility of plastic deformation at crack tips in glass and conclude that the available experimental data does not support such a model. At the present time, chemical reaction based crack growth theories are most consistent with the body of crack growth data that is available on glass and are probably the best explanation for the phenomenon.     

The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel

The process of cleavage crack initiation and the character of the effective grain size which controls the fracture toughness of pearlitic eutectoid steel has been investigated using smooth tensile and precracked Charpy impact specimens. The results demonstrated that initial cracking in both specimens was largely the result of shear cracking of pearlite;i.e., localized slip bands in ferrite promoted cracking of the cementite plates, which was then followed by tearing of the adjacent ferrite laths. Such behavior initially results in a fibrous crack. In the tensile specimen, the initiation site was identified as a fibrous region which grew under the applied stress, eventually initiating an unstable cleavage crack. In precracked impact specimens, this critical crack size was much smaller due to the high state of stress near the precrack tip. Fracture mechanics analysis showed that the first one or two dimples formed by the shear cracking process can initiate a cleavage crack. Using thin foil transmission electron microscopy, a cleavage facet was found to be an orientation unit where the ferrites (and the cementites) of contiguous colonies share a common orientation. The size of this orientation unit, which is equal to the cleavage facet size, is controlled by the prior austenite grain size. The influence of austenite grain size on toughness is thus explained by the fact that the austenite grain structure can control the resultant orientation of ferrite and cementite in pearlitic structures. Y.J. PARK, formerly with Carnegie-Mellon University, Pittsburgh, PA.     

A model for creep-fatigue interaction in terms of crack-tip stress relaxation

A model was developed to explain the mechanism of the degradation of fatigue lives caused by the growth of transgranular crack without cavitational damage in spite of the creep-fatigue loading condition for some type 304L stainless steel and 1Cr-Mo-V steel. The model was developed by incorporating the stress relaxation effect during tensile hold time into the pure fatigue crack growth model based on the crack-tip shearing process. In the crack-tip region, the stress relaxation during hold time at the tensile peak stress reduces the maximum stress level but accumulates inelastic strain, which induces creep crack growth during hold time and enhances subsequent fatigue crack growth during subsequent loading by promoting the crack-tip shearing process. The predicted creep-fatigue lives by the model were in good agreement with the actual lives for type 304L stainless steel at 823 and 865 K and for 1Cr-Mo-V rotor steel at 823 K. The model was further expanded to explain the degradation of the life under the conditions of compressive hold cycling for 1Cr-Mo-V and 12Cr-Mo-V steels.     

Fatigue cracking characteristics of β-annealed large colony Ti-11 alloy

In order to better understand the large scatter in the fatigue results associated with β-annealed microstructures of α+ β titanium alloys, the fatigue crack initiation and propagation behavior of thin center notched Ti-11 specimens with a large colony α platelet microstructure was investigated. Colonies with a mean intercept diam of greater than 1 mm were grown in 2 mm thick specimens by means of a vacuum β-annealing process. This enabled crack path morphologies and crack propagation rates to be determined within single colonies by means of optical microscopy on the polished and etched surfaces. The results showed that the fracture is related to a shear mechanism across the colonies from the initiation stage through the overload fracture. Intense shear bands were observed ahead of and in the same direction as the propagating cracks. The density of the shear bands increased with increasing stress intensity. Since the colonies are randomly oriented, the fatigue cracks propagated at various angles with respect to the tensile axis. The crack propagation rate across a single colony is no faster than the propagation rate in the equiaxed α+ β microstructure of the same material. However, cracks were halted at the colony boundaries and forced to reinitiate through a cycle consuming process into the next colony. It is mainly this reinitiation process and microstructurally dependent growth which are responsible for the slower crack growth rates and large scatter band obtained for the β-annealed microstructures when compared to the α+ β microstructures. It is suggested that by reducing the colony size the crack growth rates will be reduced and the fatigue scatter band will be narrowed. Formerly with the Metals and Ceramics Division, Air Force Materials Laboratory, Wright-Patterson Air Force Base.     

Effect of mean stress on hydrogen assisted fatique crack propagation in duplex stainless steel

Hydrogen assisted subcritical cleavage of the ferrite matrix occurs during fatigue of a duplex stainless steel in gaseous hydrogen. The ferrite fails by a cyclic cleavage mechanism and fatigue crack growth rates are independent of frequency between 0.1 and 5 Hz. Macroscopic crack growth rates are controlled by the fraction of ferrite grains cleaving along the crack front, which can be related to the maximum stress intensity, K_max. A superposition model is developed to predict simultaneously the effects of stress intensity range (ΔK) and K ratio (K_min/K_max). The effect of K_max is rationalised by a local cleavage criterion which requires a critical tensile stress, normal to the {001} cleavage plane, acting over a critical distance within an embrittled zone at the crack tip.     

Failure Diagram and Chemical Driving Forces for Subcritical Crack Growth

Kitagawa-Takahashi diagram that is modified for fatigue is now extended to the subcritical crack growth behavior under stress-corrosion crack growth. The analogy with the fatigue helps us to identify several regimes of interest from both the point of understanding of the material behavior as well as quantification of the failure process for structural design of components that are subjected to stress-corrosion and corrosion fatigue crack growths and failure. In particular, the diagram provides a means of defining the mechanical equivalent of chemical stress concentration factor and the chemical crack-tip driving forces to crack growth or its arrest. In addition, threshold stresses, crack arrest, and nonpropagating crack growth conditions can be defined, which help in developing sound design methodology against stress corrosion and corrosion fatigue. Chemical crack driving forces under corrosion fatigue can be similarly defined using the inert behavior as a reference.     

Fatigue crack growth in Si−Fe

Fatigue crack growth rates were measured in vacuum in the temperature range from 100° to 250°C. At 200°C and above, crack growth occurs by a ductile mechanism. The ductile crack growth rate is proportional to the stress intensity factor amplitude raised to the 5/2 power, is equal to about one-tenth of the crack opening displacement per cycle, and is not measurably dependent on the peak stress intensity factor in the range measured. Local crack growth occasionally becomes arrested by large scale blunting of the crack tip. The fracture surface has many “river markings” and often adheres to specific growth planes on a scale of the grain size. The overall crack growth is approximately in the plane of maximum tensile stress. Inclusions and grain boundaries are not of importance in the growth mechanism in the range of crack growth rates measured. Growth rates in air at 200°C are at most twice those in vacuum. Although a mechanism, using continuum mechanical concepts, can explain the external features of the ductile crack growth rate, the local features showing a combination of all three modes of straining suggests that the actual process of crack growth must be far more complicated than hitherto realized. At 150°C and below, the ductile growth mechanism is augmented by the wholesale cleavage of single grains or grain clusters. The growth rates at these temperatures are higher and more sensitive to stress intensity factor amplitude.     

Corrosion fatigue crack growth behavior of a squeeze-cast Al-Si-Mg-Cu alloy with different precrack histories

Fatigue experiments have been performed on a squeeze-cast Al-Si-Mg-Cu alloy as a function of precrack history. The precracked conditions were that the compact tension specimen was precracked with a relatively long through-thickness crack (about 6 mm) in air, in aqueous 3 pct NaCl solution, and in air followed by hydrogen precharging. It was found that a relatively long through-thickness crack can grow more rapidly than would be predicted by a traditional ΔK involving three stages under either a corrosion fatigue test after precracking in air or a hydrogen precharging experiment followed by fatigue testing in air. The experimental evidence confirms that a hydrogen-assisted damage mechanism is mainly responsible for the rapid growth phenomenon of a relatively long crack in a corrosive environment compared to the result of fatigue testing in air after hydrogen precharging. The amount of hydrogen production in chemical-microstructure interaction processes in a corrosion fatigue experiment and the effectiveness of hydrogen transport to the region ahead of the crack tip determine the degree of hydrogen-assisted fatigue crack growth, which is controlled by the microstructure of the alloy and the chemical attack on a sharp and fresh crack tip.     

XCQ16和20Mn2车轴用钢疲劳失效的微观机理

利用扫描电镜(SEM)对XCQ16和20Mn2两种车轴用钢疲劳断口和裂纹扩展断口进行分析,研究疲劳失效过程中的裂纹萌生及扩展机理.结果表明:裂纹均是从试样表面萌生,非金属夹杂和位错是主要的萌生机制;XCQ16钢在疲劳断口中部区的裂纹闭合效应大于20Mn2钢,裂纹扩展较慢;两者在裂纹扩展区的扩展机理不同,XCQ16钢属于塑性断裂,20Mn2钢属于解理断裂;在非金属夹杂上,XCQ16钢多含氧化物和硫化物的混合夹杂,20Mn2钢主要是氧化物夹杂,混合夹杂对材料的疲劳性能影响较小.     

Fatigue behavior of a 2XXX series aluminum alloy reinforced with 15 vol Pct SiCp

The fatigue behavior of a naturally aged powder metallurgy 2xxx series aluminum alloy (Alcoa MB85) and a composite made of this alloy with 15 vol pct SiC_p, has been investigated. Fatigue lives were determined using load-controlled axial testing of unnotched cylindrical samples. The influence of mean stress was determined at stress ratios of −1, 0.1, and 0.7. Mean stress had a significant influence on fatigue life, and this influence was consistent with that normally observed in metals. At each stress ratio, the incorporation of SiC reinforcement led to an increase in fatigue life at low and intermediate stresses. When considered on a strain-life basis, however, the composite materials had a somewhat inferior resistance to fatigue. Fatigue cracks initiated from several different microstructural features or defect types, but fatigue life did not vary significantly with the specific initiation site. As the fatigue crack advanced away from the fatigue crack initiation site, increasing numbers of SiC particles were fractured, in agreement with crack-tip process zone models. Formerly Graduate Student, Department of Materials Science and Engineering, The University of Michigan