Materials characterization of silicon carbide reinforced titanium (Ti/SCS-6) metal matrix composites: Part I. Tensile and fatigue behavior

Flexural fatigue behavior was investigated on titanium (Ti-15V-3Cr) metal matrix composites reinforced with cross-ply, continuous silicon carbide (SiC) fibers. The titanium composites had an eightply (0, 90, +45, -45 deg) symmetric layup. Fatigue life was found to be sensitive to fiber layup sequence. Increasing the test temperature from 24 °C to 427 °C decreased fatigue life. Interface debonding and matrix and fiber fracture were characteristic of tensile behavior regardless of test temperature. In the tensile fracture process, interface debonding between SiC and the graphite coating and between the graphite coating and the carbon core could occur. A greater amount of coating degradation at 427 °C than at 24 °C reduced the Ti/SiC interface bonding integrity, which resulted in lower tensile properties at 427 °C. During tensile testing, a crack could initiate from the debonded Ti/SiC interface and extend to the debonded interface of the neighboring fiber. The crack tended to propagate through the matrix and the interface. Dimpled fracture was the prime mode of matrix fracture. During fatigue testing, four stages of flexural deflection behavior were observed. The deflection at stage I increased slightly with fatigue cycling, while that at stage II increased significantly with cycling. Interestingly, the deflection at stage III increased negligibly with fatigue cycling. Stage IV was associated with final failure, and the deflection increased abruptly. Interface debonding, matrix cracking, and fiber bridging were identified as the prime modes of fatigue mechanisms. To a lesser extent, fiber fracture was observed during fatigue. However, fiber fracture was believed to occur near the final stage of fatigue failure. In fatigued specimens, facet-type fracture appearance was characteristic of matrix fracture morphology. Theoretical modeling of the fatigue behavior of Ti/SCS-6 composites is presented in Part II of this series of articles. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Crack Growth Resistance of Hybrid Fiber-Reinforced Cement Matrix Composites

The effect of hybrid fiber reinforcement on fracture energy and crack propagation in cement matrix composites is examined. The crack in cement matrix composites is allowed to fracture under mode-I loading with three-point bending beam specimens. The influence of fiber types and their combination is quantified by using the toughness index and fracture energy. A proper hybrid combination of steel fibers and polyvinyl alcohol microfibers enhances the resistance to both the nucleation and growth of the crack. The micromechanical model of hybrid composites by using a fiber bridging law is emphasized, and the numerical model prediction closely matches the behavior obtained from the experiment. The influencing role of the material parameters in the fracture tests (e.g., the fracture toughness index and fracture energy) becomes more apparent than ones used in some conventional strength-based or fiber pullout tests, and these fracture parameters could screen the effect of fiber/microfiber reinforcement in enhancing the crack growth resistance of cementitious composites. This study demonstrates that fundamental fracture tests are effective to characterize and develop high-performance hybrid fiber–reinforced cement matrix composites.     

Environmental interactions in high-temperature fatigue crack growth of Ti-1100

The crack growth behavior of Ti-1100 is investigated for loading frequencies ranging from 30 to 0.0031 Hz at temperature levels extending from 23 °C to 650 °C in both air and vacuum environments. Two types of time-dependent damage mechanisms have been identified: oxidation and creep effects. It is concluded that the effect of oxidation on the crack growth acceleration is rapidly developed and only weakly dependent on total cycle time. Creep effects, on the other hand, are dominant at low frequencies in both air and vacuum and are loading rate dependent. The degree of contribution of each of these two damage modes during the steady state growth region has been phenomenologically determined by examining the frequency dependence on the exponent and coefficient parameters of the Paris-type crack growth equation. It is found that these parameters are largely determined by the extent of the viscoplastic response of the crack tip region below a specific, environment-sensitive transition loading frequency. Furthermore, the physical mechanisms involved in the environment-affected damage are identified with the nature of crack tip plastic work input as a function of loading frequency. The influence of frequency and environment on the anomalous appearance of pronounced stage I/stage II knee regions is also discussed with respect to closure levels and creep transient response.     

The effect of temperature on the

The tensile stress-strain behavior and failure mechanisms of Ti-24Al-11Nb and a SiC/ Ti-24Al-11Nb composite with continuous SCS-6 fibers oriented parallel to the loading direction have been examined over a range of temperatures from 23 °C to 815°C in air. Failure in Ti- 24Al-11Nb occurred at strains of approximately 4 pct soon after crack initiation at low tem- peratures. Ductility increased with temperature up to a maximum of 20 pct elongation at 600 °C, as surface-initiated cracks did not propagate readily at intermediate temperatures. At higher temperatures, the onset of grain boundary and interfacial void nucleation limited ductility. Com- posite failure appeared to be controlled by fiber fracture at all temperatures; for practical en- gineering purposes, composite failure occurred at 0.8 pct strain at all temperatures. At temperatures of 425 °C and less, fiber fractures occurred at intervals along the lengths of the fibers and appeared to be cumulative, while at temperatures of 650 °C and greater, fiber fractures were only observed locally to the fracture surfaces. The decreased radial residual stresses, interfacial strengths, and matrix properties at 650 °C and 815 °C allowed the composite to unload at 0.8 pct strain, due to fiber fractures, followed by a reloading in which fibers pulled out and the matrix failed, resulting in composite failure. The decreasing residual stresses with increasing temper- ature determined from an elastic-plastic concentric cylinder model were shown to affect the stress-strain response of the composite and were consistent with the measured decreasing inter- facial shear stresses, the increased fiber pullout with temperature, and the circumferential de- bonding observed around the fibers at higher temperatures.     

Micromechanism of accelerated near-threshold fatigue crack growth in Al2O3/Al-Cu composite at elevated temperature

Fatigue crack growth behavior of a peak-aged Al₂O₃/Al-Cu composite was examined at 150 °C and compared to the behavior at room temperature (RT). At 150 °C, fatigue crack growth rates showed strong dependence on loading time. At short loading time, when stress-intensity range was decreased to approach fatigue threshold, crack growth rates at 150 °C were comparable to those measured at RT. Prolonged fatigue testing at near-threshold crack growth rates resulted in oscillations of crack growth rate until the fatigue crack growth behavior was stabilized to become similar to that in an overaged composite. Measurement of the matrix hardness at different distances from the crack plane and transmission electron microscopy examination of the fatigue specimen have shown that the matrix microstructure at the tip of the fatigue crack underwent overaging during prolonged testing in the near-threshold regime. Consequently, the fatigue fracture mechanism was modified, a lower crack closure developed, and the fatigue threshold reduced to that of the overaged composite.     

Crack Bridging in Polymer Nanocomposites

Carbon nanotube reinforced composites offer enhancements in fracture properties since the reinforcing nanotubes provide a bridging mechanism to resist crack growth. In this paper, a study of crack bridging by nanotubes in a nanotube-reinforced polymer composite is presented. The process of crack bridging is idealized as normal pullout of the participating nanotubes from the polymer matrix. The resistance to crack growth due to bridging is taken as the aggregate of the resistance offered by all the nanotubes, ignoring any interaction among the nanotubes themselves. The pullout of a single nanotube from the polymer matrix is modeled as an axisymmetric, nearly one-dimensional problem. This is done by assuming that fracture along the nanotube–polymer interface is dominated by shear openings, and that the nanotube behaves as a rigid body. When the polymer is a linear elastic material, the force–displacement relation for pullout is obtained as a function of dimensionless variables representing the interfacial fracture energy and the pullout length scale. Applying the correspondence principle, the elastic results are extended to the case where the polymer is a linear viscoelastic material with a single relaxation time. The force–displacement relation is then a function of the viscoelastic properties of the polymer and the pullout velocity as well. Using these results, the apparent enhancement in the fracture energy of the composite is obtained. This provides a guideline to design these composites for desired fracture properties in terms of the interfacial strength of the nanotube–polymer interface and the volume fraction of the nanotubes. Results of numerical simulations of nanotube pullout are compared to the predictions of the analytical model.     

An investigation of the effects of interfacial microstructure on the fatigue behavior of a four-ply [75]4 continuous silicon carbide (SCS-6) fiber-reinforced titanium matrix composite

The effects of interfacial microstructure/thickness on the strength and fatigue behavior of a model four-ply [75]₄ Ti-15V-3Al-3Cr-3Sn/SiC (SCS-6) composite are examined in this article. Interfacial microstructure was controlled by annealing at 815 °C for 10, 50, or 100 hours. The reaction layer and coating thickness were observed to increase with increasing annealing duration. Damage initiation/propagation mechanisms were examined in as-received material and composites annealed at 815 °C for 10 and 100 hours. Fatigue behavior was observed to be dependent upon the stress amplitude. At high stress amplitudes, the failure was dominated by overload phenomena. However, at all stress levels, fatigue crack initiation occurred by early debonding and matrix deformation by stress-induced precipitation. This was followed by matrix crack growth and fiber fracture prior to the onset of catastrophic failure. Matrix shear failure modes were also observed on the fracture surfaces in addition to fatigue striations in the matrix. Correlations were also established between the observed damage modes and acoustic emission signals that were detected under monotonic and cyclic loading conditions.     

Micromechanics and fatigue crack growth in an alumina-fiber-reinforced magnesium alloy composite

A study has been made of fatigue crack growth through the magnesium alloy ZE41A and a composite of this alloy reinforced with alumina fibers. Crack growth rates were measured and failure mechanisms characterized for specimens with fibers parallel to the loading axis and for two off-axis orientations. Crack opening displacements and matrix and fiber strains in the vicinity of the crack tip were measured using the stereomaging technique. Crack growth rates through the composite were retarded by the fibers. For the composite with fibers at 22.5 deg to the loading axis, fibers were found to fracture in the composite at the same stress as measured for the fibers alone. Fiber fracture was the dominant growth-controlling mechanism for fibers oriented on and 22.5 deg to the loading axis, and little fiber pullout was observed. However, for crack growth through material with fibers oriented at 45 deg to the loading axis, crack growth was found to exist principally through the interface. Driving forces for cracks in interfaces were determined to be smaller than the applied δK. It was found that approximate fatigue crack growth rates through the composites could be predicted from those through the matrix by adjusting the tensile modulus. The upper and lower bounds of fatigue crack growth rate were also computed for the composite using a micromechanics-based model that incorporated observed failure mechanisms. A. McMINN, formerly with Southwest Research Institute, is with Failure Analysis Associates, Washington, D.C.     

Fatigue crack growth through partially stabilized zirconia at ambient and elevated temperatures

Fatigue crack growth through magnesia stabilized zirconia at 20, 450 and 650°C has been observed dynamically in a high temperature loading stage for the scanning electron microscope. Crack tip micromechanics parameters were measured using the stereoimaging technique. Fatigue crack growth at ambient temperature was found to be very similar to crack growth through metallic alloys. With increasing temperature, the stress intensity levels in which stable fatigue crack growth could be sustained were found to narrow significantly, until fatigue is expected to not be a valid mechanism of crack growth above about 750°C. Measured crack tip parameters were used to derive the low-cycle fatigue and the stress-cycles to failure characteristics. The latter agreed with measured SN curves. Deformation within the plastic zone was shown to account for the measured value of fracture toughness. The mechanisms of crack growth are discussed.     

Fatigue crack growth mechanisms in Ti6242 lamellar microstructures: Influence of loading frequency and temperature

Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The alloy was heat treated to provide three different lamella size; fine, coarse, and extra coarse. Tests were conducted at two temperatures, 520 °C and 595 °C, using two loading frequencies, 10 and 0.05 Hz. The latter frequency was examined with and without a 300-second hold time. All tests were performed in air environment and at a stress ratio of 0.1. This study shows that at 520 °C, the Fatigue crack growth rate (FCGR) is not significantly influenced by changes in the microstructure. For 0.05 Hz/low ΔK, however, the FCGR is higher in the fine lamellar microstructure and is accompanied by- the appearance of a plateau, which disappears in the extra large lamella microstructure. Furthermore, the addition of a 300-second hold time does not alter the crack growth rate. At 595 °C, while the general level of the FCGR is higher than that at 520 °C, the effects of loading frequency and hold time remain similar to those reported at the lower temperature. Unlike the results at 520 °C, however, the FCGR at low δK is not influenced by variations in lamellar microstructure. Under all test conditions, the fatigue process is predominantly controlled by one single mechanism associated with transcolony fracture and formation of quasi-cleavage facets. The fatigue crack growth results and the associated fracture behavior as obtained in this study are correlated to the crack-tip shear activity and transmission at the α/β interfaces. A general hypothesis accounting for the role of loading frequency, temperature, and microstructure on the observed cracking mechanisms is presented.     

The micromechanics of fatigue crack growth at 25 °C in Ti-6Al-4V reinforced with SCS-6 fibers

Micromechanics parameters for fatigue cracks growing perpendicular to fibers were measured for the center-notched specimen geometry. Fiber displacements, measured through small port holes in the matrix made by electropolishing, were used to determine fiber stresses, which ranged from 1.1 to 4 GPa. Crack opening displacements at maximum load and residual crack opening displacements at minimum load were measured. Matrix was removed along the crack flanks after completion of the tests to reveal the extent and nature of the fiber damage. Analyses were made of these parameters, and it was found possible to link the extent of fiber debonding to residual COD and the shear stress for fiber sliding to COD. Measured experimental parameters were used to compute crack growth rates using a well-known fracture mechanics model for fiber bridging tailored to these experiments.     

Moiré Interferometry Analysis of Fiber Debonding

Progressive fiber debonding in steel fiber∕cementitious matrix composites has been studied using a single fiber pullout test that permits simultaneous measurements of the load versus crack opening displacement relationship and Moiré interferometry fringe patterns. Analysis of Moiré interferometry patterns allows the fiber axial and interfacial shear stresses to be calculated along the entire fiber length. The interfacial shear stress distribution along the debonded length of the fiber indicates a steep decrease in shear stress with interfacial slip, from 6 to 1 MPa for 7 μm of fiber slip and a crack opening displacement of 22 μm. These results suggest that improvements in the toughness of cement-based composites could be achieved by developing materials in which the decrease in shear stress is less severe.     

High-Temperature Fatigue of a Hybrid Aluminum Metal Matrix Composite

An aluminum metal matrix composite (MMC) brake drum was tested in fatigue at room temperature and extreme service temperatures. At room temperature, the hybrid composite did not fail and exceeded estimated vehicle service times. At higher temperatures (62 and 73 pct of the matrix eutectic), fatigue of a hybrid particle/fiber MMC exhibited failure consistent with matrix overloading. Overaging of the A356 matrix coupled with progressive fracture of the SiC particles combined to create the matrix overload condition. No evidence of macro-fatigue crack initiation or growth was observed, and the matrix–particle interface appeared strong with no debonding, visible matrix phases, or porosity. An effective medium model was constructed to test the hypothesis that matrix overloading was the probable failure mode. The measured particle fracture rate was fit using realistic values of the SiC Weibull strength and modulus, which in turn predicted cycles to failure within the range observed in fatigue testing.     

High-cycle fatigue-life of the cast nickel base-superalloys in 738 LC and IN 939

The high cycle fatigue (HCF) properties of two cast nickel base-superalloys, IN 738 LC and IN 939, were investigated using both fracture mechanics samples and smooth specimens. The crack propagation behavior was studied in terms of linear fracture mechanics at RT and at 850 °C. In addition to the influence of temperature, the influences of frequency, mean stress, and environment (vacuum, air, sulfidizing atmosphere) were studied. At 850 °C, the fatigue thresholds were found to be higher in air than in vacuum. This could be explained by crack branching. The high scatter of fatigue crack propagation rates could be related also to this phenomenon. The S/N curves at 850 °C can be predicted treating crack growth from casting pores as the predominant failure mechanism. At RT the same method is not as successful. The reason for this may be that crack growth laws measured on long, branched cracks are not applicable to short, unbranched cracks. At RT, no significant influence of frequency on S/N-curves and fatigue crack growth rates was observed for frequencies up to 20 kHz.     

Elevated-temperature fatigue crack growth

The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally occurred normal to the principal stress direction and in a direction along which ΔK _II vanished. Consequently, the effective ΔK was reduced to ΔK_I and the rate of propagation was controlled by ΔK _I only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and 982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model.     

Effects of temperature,rate, and cyclic loading on the strength and toughness of monolithic ceramics

The concept of grain bridging and pullout is applied to monolithic ceramics to understand the effects of temperature, displacement rate, and load cycling on crack wake shielding. At low temperature, the pullout of completely debonded grains accounts for all the toughening. The importance of this process diminishes with temperature. This is because of the more uniform stress distribution along the crack plane and the softening of grain boundary glassy phases, both of which tend to reduce the incidence of complete grain boundary decohesion. At sufficiently high temperature, however, the softening of grain boundary phases may allow the sliding zone to extend to the grain's end, increasing the incidence of intergranular fracture. This “high temperature” pullout triggers a sudden increase in toughness. Our pullout model successfully explains the high temperature peak and the dependence of peak position on displacement rate in fracture toughness and strength observations for some monolithic and whisker-reinforced ceramics. Degradation of interfacial friction, as by cyclic loading, is seen to decrease the frictional work for low temperature pullout but increases the frictional work for high temperature pullout. Thus, this model also provides a rationale for the opposite effect of stress cycling on crack resistance at low and high temperatures reported recently for ceramics.     

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.     

Fracture and toughening mechanisms in an α2 titanium aluminide alloy

The deformation and fracture behaviors of the Ti-24Al-11Nb alloy with an equiaxed α₂ + β microstructure have been characterized as a function of temperature by performing uniaxial tension andJ _IC fracture toughness tests. The micromechanisms of crack initiation and growth have been studied bypost mortem fractographic and metallographic examinations of fractured specimens, as well as byin situ observation of the fracture events in a scanning electron microscope (SEM) equipped with a high-temperature loading stage. The results indicate that quasistatic crack growth in the Ti-24Al-11Nb alloy occurs by nucleation and linkage of the microcracks with the main crack, with the latter frequently bridged by ductile β ligaments. Three microcrack initiation mechanisms have been identified: (1) decohesion of planar slipbands in the α₂ matrix, (2) formation of voids and microcracks in β, and (3) cracking at or near the α₂ + β interface due to strain incompatibility resulting from impinging planar slip originated in α₂. The sources of fracture toughness in the 25 °C to 450 °C range have been attributed to crack tip blunting, crack deflection, and a bridging mechanism provided by the ductile β phase. At 600 °C, a change of toughening mechanisms leads to a lowering of the initiation toughness (theK _IC value) but a drastic increase in the crack growth toughness and the tearing modulus.     

<Emphasis Type="Italic">In-Situ</Emphasis> Fracture Observation and Fracture Toughness Analysis of Ni-Mn-Ga-Fe Ferromagnetic Shape Memory Alloys

The fracture property improvement of Ni-Mn-Ga-Fe ferromagnetic shape memory alloys containing ductile γ particles was explained by direct observation of microfracture processes using an in-situ loading stage installed inside a scanning electron microscope (SEM) chamber. The Ni-Mn-Ga-Fe alloys contained a considerable amount of γ particles in β grains after the homogenization treatment at 1073 K to 1373 K (800 °C to 1100 °C). With increasing homogenization temperature, γ particles were coarsened and distributed homogeneously along β grain boundaries as well as inside β grains. According to the in-situ microfracture observation, γ particles effectively acted as blocking sites of crack propagation and provided the stable crack growth, which could be confirmed by the R-curve analysis. The increase in fracture resistance with increasing crack length improved overall fracture properties of the Ni-Mn-Ga-Fe alloys. This improvement could be explained by mechanisms of blocking of crack propagation and crack blunting and bridging.