Fatigue crack growth in fiber-reinforced metal-matrix composites

Fatigue crack growth in fiber-reinforced metal-matrix composites is modeled based on a crack tip shielding analysis. The fiber/matrix interface is assumed to be weak, allowing interfacial debonding and sliding to occur readily during matrix cracking. The presence of intact fibers in the wake of the matrix crack shields the crack tip from the applied stresses and reduces the stress intensity factors and the matrix crack growth rate. Two regimes of fatigue cracking have been simulated. The first is the case where the applied load is low, so that all the fibers between the original notch tip and the current crack tip remain intact. The crack growth rate decreases markedly with crack extension, and approaches a “steady-state”. The second regime occurs if the fibers fail when the stress on them reaches a unique fiber strength. The fiber breakage reduces the shielding contribution, resulting in a significant acceleration in the crack growth rate. It is suggested that a criterion based on the onset of fiber failure may be used for a conservative lifetime prediction. The results of the calculations have been summarized in calibrated functions which represent the crack tip stress intensity factor and the applied load for fiber failure.     

Fatigue crack growth and stress redistribution at interfaces

The role of the interface in redistributing stress around cracks in multilayered ceramic/metal composites is investigated. The emphasis is on the different effects of interfacial debonding or of plastic slip in the metal phase adjacent to strongly bonded interfaces. The experiments are conducted on alumina/aluminum multilayered composites. Monotonic loading precracked test pieces causes plastic shear deformation within the aluminum layer at the tip of the notch without debonding. However, interfacial debonding can be induced by cyclic loading, in accordance with a classical fatigue mechanism. Measurements of the stress around the crack demonstrate that debonding is much more effective than slip at reducing the stress ahead of the crack.     

Fatigue crack propagation under varied mean stress conditions

Measurements of fatigue crack growth rates in copper monocrystalline and polycrystalline sheet specimens have been made at 295 K and 77 K to determine mean stress effects on growth rates. When load conditions remained unchanged throughout the period of crack growth, the rate of fatigue crack growth is independent of the level of mean stress and depends only on the cyclic stress amplitude. When the mean stress is changed during the crack growth period, a reduction of mean stress under plane strain conditions causes complete cessation of growth. A similar effect was not observed in plane stress crack growth, presumably due to reduced elastic constraint in narrow specimens containing large cracks. No change in growth rates occurs if the mean load is increased. In the event of crack growth stoppage, either restoration of the full previous mean load or crack re-nucleation under continued cycling at the reduced load levels is sufficient to restore the prior growth rate. A simple model is adapted to explain these observations which emphasizes the interaction of the growth rate with compressive residual stresses generated at the tip of the propagating crack. R. A. Yeske, formerly Research Assistant at Materials Science Department, Northwestern University, Evanston, III. This paper is based on a portion of a thesis submitted by R. A. Yeske in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University.     

Fatigue crack growth through

Fatigue cracks were grown at 25 °C and 800 °C in a titanium aluminide alloy heat-treated to give a γ+ α ₂ lamellar microstructure. These lamellae, having widths of =0.5 to 2 μm, were in colonies approximately 1.2 mm across. Crack growth was observed and photographed under high resolution conditions using a loading and heating cyclic stage for the scanning electron microscope. Stereoimaging was used to measure displacements around crack tips, from which crack opening displacements and strains were derived. Cracks were found to grow about 10 times faster at 25 °C than at 800 °C, and the threshold stress intensity for fatigue crack growth was lower at 25 °C. Strain to fracture the lamellae was determined as ≈0.08, while fatigue crack tips could sustain up to 0.3 strain at 25 °C and 0.5 strain at 800 °C. The lamellar micro- structure was found to have a strong influence on crack tip behavior.     

Fatigue crack growth resistance of unidirectional fiber-reinforced titanium metal-matrix composites under transverse loading

The transverse fatigue crack growth resistance of unidirectional 8 and 35 pct 1140+/Ti-6-4 fiber-reinforced composites has been investigated. It has been found that, at a low fiber volume fraction, the transverse fatigue crack growth resistance of metal-matrix composites (MMCs) is improved with respect to the monolithic matrix alloy. This occurs because “holes” from debonded interfaces can trap the crack and reduce the average fatigue crack growth rates by periodically increasing the effective crack-tip radius. However, an increase of fiber volume fraction from 8 to 35 pct decreases the fatigue crack growth resistance dramatically, due to the significant increase of the frequency of interaction and coalescence between the main crack, the debonded interfaces, and microcracks.     

Fatigue crack growth behavior in niobium-hydrogen alloys

Near-threshold fatigue crack growth behavior has been investigated in niobium-hydrogen alloys. Compact tension specimens (CTS) with three hydrogen conditions are used: hydrogen-free, hydrogen in solid solution, and hydride alloy. The specimens are fatigued at a temperature of 296 K and load ratios of 0.05, 0.4, and 0.75. The results at load ratios of 0.05 and 0.4 show that the threshold stress intensity range (ΔK _th ) decreases as hydrogen is added to niobium. It reaches a minimum at the critical hydrogen concentration (C _cr ), where maximum embrittlement occurs. The critical hydrogen concentration is approximately equal to the solubility limit of hydrogen in niobium. As the hydrogen concentration exceeds C _cr , ΔK _th increases slowly as more hydrogen is added to the specimen. At load ratio 0.75, ΔK _th decreases continuously as the hydrogen concentration is increased. The results provide evidence that two mechanisms are responsible for fatigue crack growth behavior in niobium-hydrogen alloys. First, embrittlement is retarded by hydride transformation-induced and plasticity-induced crack closures. Second, embrittlement is enhanced by the presence of hydrogen and hydride.     

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.     

Fatigue crack growth in an as-rolled dual-phase steel

Fatigue crack propagation behaviour of an as-rolled dual-phase steel is investigated over a wide range from 10^?10 to 10^?6 m/cycle in a 3.5% NaCI solution and laboratory air under different load ratios. It was found that the as-rolled dual-phase steel studied in the present investigation shows good resistance to fatigue crack growth in laboratory air. The threshold values decrease with increasing stress ratios, which is consistent with the competition model proposed previously. The threshold value obtained using a 3.5% NaCI solution is higher than that in the laboratory air owing to the wedge effect of corrosion products within the crack. Fatigue crack growth rates in the higher ΔK range obey the Paris formula in each case within this study.     

Fatigue crack growth in MAR-M200 single crystals

The effects of crystallographic orientation on the fatigue crack growth behavior of MAR-M200* single crystals were examined. Using compact-tension specimens tested at 20 Hz, fatigue crack growth rates were determined at ambient temperature at minimum stress to maximum stress ratios,R, of 0.1 and 0.5. In most cases, subcritical crack growth occurred either along a single {111} slip plane or a combination of {111} planes. The mode of cracking was generally mixed and contained mode I, II, and III components. Considerable crack deflection and branching were also observed. Some fracture surfaces were found to contain a significant amount of asperities and, in some specimens, black debris. Based on Auger spectroscopic analyses and the fracture surface appearance, it appears that the black debris represented oxides formed due to rubbing of the fracture surfaces. Using stress intensity solutions obtained based on the Boundary-Integral-Equation technique, an effective ΔK was successfully used for correlating the crack growth rate data. The results indicate that the effect of crystallographic orientation on crack growth rate can be explained on the basis of crack deflection, branching, and roughness-induced crack closure. Formerly with Southwest Research Institute     

Fatigue crack growth behavior of 4340 steels

Fatigue crack growth behavior of 4340 steels was investigated in four gaseous environments; laboratory air, wet hydrogen, dry hydrogen and dry helium. Specimen orientation does not affect crack propagation rate results. The effects of R-ratio (load ratio) and environment on crack growth rate properties are interrelated. Increasing R -ratio increases the rates of near-threshold crack propagation. Nevertheless, the effect of R-ratio on crack growth rates in air is much more significant than that in the two dry environments. Interestingly, the R-ratio effect in wet hydrogen is comparable to that in dry environments. At an R-ratio of 0.1, the rates of crack propagation in air are slower than those in dry environments while crack growth rates are essentially identical in wet hydrogen and dry environments. Increasing R -ratio was found to decrease the environmental effect. Furthermore, increasing yield strength from 700 to 1040 MPa does not affect crack propagation behavior. While surface roughness-induced crack closure is thought to be minimal in affecting gaseous-environment near-threshold crack growth behavior of 4340 steels, oxide-induced crack closure governs crack propagation kinetics. It is suggested that in moisture-containing environments, thick oxide deposits measured on fracture surfaces may not result in high crack closure levels. Nevertheless, oxide-induced crack closure rationalized the effects of R-ratio and environment on near-threshold crack growth rate properties. Furthermore, hydrogen embrittlement is believed not to play an important role in influencing wet-hydrogen environment near-threshold crack propagation behavior. At higher ΔK levels (⩾ 12 MPa √m), an “intrinsic” dry hydrogen effect seems to be present, and crack closure, however, cannot account for the environmental effect.     

Fatigue crack growth of SM-1240/TIMETAL-21S metal matrix composites at elevated temperatures

A series of high-temperature fatigue crack growth experiments was conducted on a continuous-fiberreinforced SM1240/TIMETAL-21S composite using three different temperatures, room temperature (24 °C), 500 °C, and 650 °C, and three loading frequencies, 10, 0.1, and 0.02 Hz. In all the tests, the cracking process concentrated along a single mode I crack for which the principal damage mechanism was crack bridging and fiber/matrix debonding. The matrix transgranular fracture mode was not significantly influenced by temperature or loading frequency. The fiber debonding length in the crack bridging region was estimated based on the knowledge of the fiber pullout lengths measured along the fracture surfaces of the test specimens. The average pullout length was correlated with both temperature and loading frequency. Furthermore, the increase in the temperature was found to lead to a decrease in the crack growth rate. The mechanism responsible for this behavior is discussed in relation to the interaction of a number of temperature-dependent factors acting along the bridged fiber/matrix debonded zone. These factors include the frictional stress, the radial stress, and the debonding length of the fiber/matrix interface. In addition, the crack growth speed was found to depend proportionally on the loading frequency. This relationship, particularly at low frequencies, is interpreted in terms of the development of a crack tip closure induced by the relaxation of the compressive residual stresses developed in the matrix phase in regions ahead of the crack tip during the time-dependent loading process.     

Time dependent crack growth in ceramic matrix composites with creeping fibers

Crack growth in ceramic matrix composites with creeping fibers has been investigated using a time dependent bridging law to describe the effect of fibers bridging a matrix crack. The fibers were assumed to creep linearly and the matrix was assumed to be elastic. Time dependent crack growth was predicted assuming that matrix crack growth occurs when the stress intensity factor at the matrix crack tip reaches a constant critical value. Crack growth rates are presented as a function of crack length and time. Domains of stable and unstable crack growth are outlined. The solutions illustrate that stable crack growth consists of a relatively brief period of decerelation followed by acceleration to large crack lengths, with crack velocity approaching constancy only at loads very near the matrix cracking stress and for very long cracks. Finally, the time needed to grow a long matrix crack is compared with a rough estimate for the time needed to rupture fibers. A transition is expected from life dominated by matrix crack growth at low stress to life dominated by fiber creep rupture after crack growth at higher stresses.     

Fatigue crack initiation and microcrack growth in 4140 steel

Initiation and growth of fatigue microcracks were studied in vacuum degassed 4140 steel in three conditions: as-quenched, tempered at 400°C, and tempered at 650°C. Micro-scopic examinations were made of specimens with metallographically polished notches using a 400 times long working distance microscope with an x-y micrometer base mounted directly on an MTS machine. Following Barsom and McNicol, the cycles to fatigue crack initiationN _i were plottedvs ΔK/√p and threshhold values of gDK√p were determined. The data of logN _i vs log [ΔK/√p-ΔK/√p|_th] fit on a straight line. Microcracks grew most rapidly in as-quenched specimens and least rapidly in 650°C tempered specimens at the same ΔK/√p. In as-quenched specimens, fatigue cracks initiated at grain boundaries but in the 400 and 650°C tempered specimens they initiated at intrusions-extrusions. They are also associated with Northwestern’s Materials Research Center.     

铸造奥氏体不锈钢的疲劳裂纹扩展

用紧凑拉伸试样研究了载荷比、单峰过载和两步高-低幅加载对Z3CN20-09M铸造奥氏体不锈钢疲劳裂纹扩展速率的影响.当应力强度因子范围相同时,疲劳裂纹扩展速率随载荷比的增大而增大.单峰过载使裂纹扩展速率先有短暂的增加后长距离的减速扩展,出现裂纹扩展迟滞现象.两步高-低幅加载时,若两步的最大载荷不同,第二步裂纹扩展也会出现迟滞现象.用两参数模型和Wheeler模型能够预测恒幅载荷和变幅载荷下的疲劳裂纹扩展行为.     

Fatigue crack growth in a low-alloy steel in gaseous environments

The growth of fatigue cracks in notched and pre-fatigued DCB-specimen of low-alloy steel 90 MnV 8 with 0.89% C, 2.06 Mn, 0.32 Cr, tempered to average strength, was studied in UHV and in nitrogen, hydrogen, oxygen and water vapour, respectively. Conditions were chosen such that linear-elastic fracture mechanics were applicable, and that static-load crack extension (i.e. H-induced dry SCC) was excluded. The gas pressure was varied between 10^?9 and 1 000 hPa, at temperatures of 298, 358 and 423 K. Hydrogen, oxygen and water vapour all increase the crack growth rate. While the influence of oxygen levelled off at ca. 0.1 hPa, both hydrogen and water vapour caused a monotonous increase of the crack growth rate over the whole range of pressures investigated. Oxygen is assumed to accelerate crack growth by surface adsorption (or surface oxidation), enhancing the production of dislocations at the strained crack tip, thus enhancing the rate of plastic deformation in the plastic zone preceeding the crack, with a saturation of the effect due to saturation of either adsorption or oxidation. Both hydrogen and water vapour are assumed to act via the ingress of atomic hydrogen into the plastic zone essentially by dislocation transport, thus enhancing the local concentration of hydrogen stripped off dislocations at carbide/matrix-interfaces. Crack extension occurs by coagulation of microcracks thus formed. The concept can be developed quantitatively.     

Thermoelastic analysis of matrix crack growth in particulate composites

We examine the conditions under which differences in thermal expansion between a particle and the matrix lead to crack growth within the matrix. Using linear elasticity fracture machanics, we obtain closed-form, analytical results for the case of a penny shaped crack present in the matrix interacting with a spherical inclusion which is misfitting with respect to the matrix. A simple and direct relationship is established between the strain energy release rate, the crack size, the crack orientation with respect to the inclusion, the crack/inclusion separation, the degree of thermal expansion mismatch and the elastic properties of the medium. We also analyze the size to which these cracks can grow and find that for a given misfit strain and material properties, crack growth is inhibited beyond a certain critical crack size. We find that beyond this critical size, the elastic strain energy released upon crack growth is no longer sufficient to compensate for the energy expended in extending the crack, since the crack is growing into the rapidly decreasing stress field. The modification of the above conditions for crack growth due to the superposition of an external stress field has also been analyzed. The preferred orientation of these cracks as a function of misfit strain is predicted. The implication of these results for thermal cycling are analyzed.     

Fatigue crack growth through alloyed niobium, Nb-Cr2Nb, and Nb-Nb5Si3 in situ composites

Fatigue cracks were grown through several niobium-based materials. For Nb-Cr-Ti composition materials, the single-phase alloy represented the matrix of two in situ composites with about 22 and 38 vol pct Cr₂Nb. Grain boundaries were coated with intermetallic in the lower-volume fraction material, while the 38 vol pct Cr₂Nb composite consisted of mainly spherical, dispersed intermetallic. The Nb-10Si composite was composed of about 28 vol pct primary Nb₅Si₃, with most of the matrix alloy in “fiberlike” shapes due to extrusion. Crack growth rates through the composites were generally faster than for unalloyed Nb, roughly in proportion to the volume fraction of intermetallic, although differences in microstructure make this comparison difficult. The presence of intermetallic greatly alters deformation of material near the crack tip. Particles of Cr₂Nb were broken during the crack growth process, leading to increased crack growth rates. These results suggest microstructural modifications that could be expected to enhance fatigue crack growth resistance.