Larger Crack Sizes Research Articles

Experimental study on residual compressive bearing capacity of cracked reinforced concrete columns by SCDA

Abstract The static demolition of structures via soundless chemical demolition agent (SCDA) is attracting increasing attention because of its environmental friendliness and safety, especially in the dense urban areas. However, the research on the application of SCDA in demolishing the reinforced concrete is still scarce, seriously limiting the development of static demolition. In this work, with the combined efforts of experiments and theoretical analysis, we investigate the critical role of borehole layouts in the static demolition of reinforced concrete (RC) by determining the occurrence and expansion of cracks during static demolition, and the resulting residual mechanical properties of RC. The results show that the growth of cracks is significantly dependent on the reaction of SCDA. Additionally, the initiation time of cracking exhibits minimal sensitivity to variations in the layouts pattern of boreholes, while the progression of cracking is predominantly governed by the layouts of boreholes. We demonstrate that cracks formed in static demolition process can significantly reduce the strength of RC with about the remaining bearing capacity is only 10% to 33% of the original and change its failure mode. Further, it is point out that the key factor controlling the failure of RC under axial compression (i.e., the integrity of structures), and the RC with larger crack size and less structural integrity is more prone to fail. The results and findings in this work further clarify the role of borehole layouts and are of great significance in improving the efficiency of static demolition.

The effect of white etching layer patches on a larger rail defect

Manufacturing defects such as porosity that formed in the rail could result in catastrophic failure if it remain in service. In this study, a crack that initiated from a large void was modelled using boundary element model to investigate the effect of white etching layer patches on larger defects in rail. The configuration studied represents a large defect in the rail that was observed from a rail removed from service. These investigation has provided a better understanding of how a crack initiated from a larger defect grows under the effect of white etching layer. The results highlight that a continuous white etching layer region will dominates the growth of the crack compared to a patches white etching layer region for a smaller crack size while patches of white etching layer will accelerates the crack growth for a larger crack size. The crack growth predicted in this study for a small crack size is at least 6 times higher than the wear rate mentioned in the literature and will increase further if the expansion of a transformed layer is considered making it impossible to be removed by natural wear.

Restrained Shrinkage Cracking and Dry Shrinkage of Rapid-Set Prepackaged Cementitious Materials

AbstractThe dry shrinkage characteristics of 23 rapid-set prepackaged cementitious materials collected from the current market have been investigated in this study. The materials showed various trends and rates of free dry shrinkage strain development. Overall, most materials had the free dry shrinkage of less than 0.05% at 28 days; however, only materials with the 28-day free dry shrinkage of less than 0.025% were observed to exhibit a low risk of restrained shrinkage cracking. The restrained shrinkage cracking typically occurred within a week after casting primarily because of the rapid growth of dry shrinkage during this period. A higher free dry shrinkage strain or a higher rate of free dry shrinkage strain development at the early age normally resulted in an earlier formation of cracking and a larger crack size during the restrained shrinkage test. Coarse aggregate extension reduced the free dry shrinkage and the risk or the size of restrained shrinkage cracking or delayed the time of restrained shri...

Earthquake nucleation in intact or healed rocks

AbstractEarthquakes are generated because faults lose strength with increasing slip and slip rate. Among the simplest representations of slip‐dependent strength is the linear slip‐weakening model, characterized by a linear drop to a residual friction. However, healed fault rocks often exhibit some slip strengthening before the onset of weakening. Here we investigate the effect of such a slip‐hardening phase on the initial growth of a slip patch and on the nucleation of rupture instabilities. We assume a piecewise linear strength versus slip constitutive relation. We compute stress and slip distributions for in‐plane or antiplane rupture configurations in response to an increasing, locally peaked (parabolic with curvature κ) stress profile. In contrast with the strictly linear slip‐weakening case, our calculations show that the curvature of the loading profile and the level of background stress strongly influence the nucleation size. Even for small amounts of slip hardening, we find that the critical nucleation size scales with for κ→0, i.e., crack growth remains stable up to very large crack sizes for sufficiently smooth loading profiles. Likewise, when the background stress τb is very close to the initial strength τc, the critical crack size scales with . An eigenvalue analysis shows that the nucleation length increases as the proportion of the crack undergoing slip hardening increases, irrespective of the details of the loading profile. Overall, our results indicate that earthquake nucleation sizes can significantly increase due to slip hardening (e.g., in healed fault rocks), especially when the background loading is smooth.

Improving mesh stiffness calculation of cracked gears for the purpose of vibration-based fault analysis

For the purpose of vibration-based condition monitoring and to prevent occurrence of catastrophic gear failures it is important to improve the simulated dynamic response of the studied gear model. The time-varying gear mesh stiffness will contribute to the dynamic response of a geared system. Some previously applied methods for stiffness calculation, described in the literature, show good agreement with the results obtained with FEM simulation for smaller crack sizes. However, when larger crack sizes are reached, these methods show an increasing deviation from FEM simulation results. A reduction in the gear mesh stiffness can be considered to assess the status of tooth damage and, therefore, by increasing the accuracy of the calculated mesh stiffness, dynamic simulations of a gear can be improved. In this paper a new method is presented for calculating the gear mesh stiffness for a propagating crack in the tooth root. The influence of gear mesh stiffness on the vibration-based fault detection indicators, the RMS, kurtosis and the crest factor, is investigated. Different crack sizes are examined by using this new method for sizes up to around 50% of the total tooth root thickness. When compared to FEM simulations, the presented method shows more accurate results for calculations of the gear mesh stiffness (for the studied model) than the previously suggested methods.

RuO2-based anodes with tailored surface morphology for improved chlorine electro-activity

We report the preparation of Dimensionally Stable Anode® coatings with a special surface morphology, which is tailored for the chlorine gas evolution. A simple electro-deposition route is applied to create coatings with large crack sizes and three-dimensionally arranged catalyst spheres on top of the coating surface. This morphology shows an increase of the specific outer electro-catalyst surface area, which is the main working part during intensive chlorine gas evolution. By this means, a decrease of the chlorine overpotential and simultaneously a reduction of the noble metal content can be achieved, which is favorable in terms of energy and resource efficiency of the chlor-alkali industry. The influence of the deposition parameters on phase structure, phase composition, surface morphology and electrochemical properties were investigated using XRD, ICP-OES, SEM, CV and polarization techniques.

Effect of compressive dwell on low cycle fatigue behaviour of nickel based single crystal alloy at 1100°C

In this study, the low cycle fatigue behaviour of CMSX-2 oriented with the loading axis along [001] was investigated under zero compression (Rϵ = −∞) strain controlled loading at 1100°C. The experiments were performed with or without a 2 min dwell (hold) in compression at strain ranges of 0·7, 0·6 and 0·5%. Stress relaxation was observed to occur resulting in the development of tensile mean stress under dwell fatigue loading and zero mean stress under non-dwell fatigue loading conditions. Fatigue cracks were observed to originate from localised regions of oxide attack at the specimen surface. The surface fatigue cracks initially propagated perpendicular to the stress axis but transitioned to other crystallographic planes at larger crack sizes and after the coalescence of multiple fatigue cracks. Non-dwell fatigue loading promoted isotropic coarsening of the γ′ precipitates while compressive dwell fatigue loading resulted in γ′ rafting parallel to the loading direction. It was observed that CMSX-2 was compressive dwell sensitive and exhibited fatigue lives 3–6 times shorter than non-dwell tests under the same total strain ranges.Dans cette étude, on a étudié le comportement de fatigue oligocyclique de CMSX-2 orienté avec l’axe de chargement le long de [001] avec chargement contrôlé par la déformation sous compression zéro (Rϵ = −∞) à 1100°C. On a effectué les essais avec et sans un temps d’arrêt (pause) de deux minutes en compression à des gammes de déformation de 0·7%, 0·6% et 0·5%. On a observé qu’il se produisait une relaxation de la contrainte, résultant dans le développement d’une contrainte moyenne de traction sous le chargement de pause fatigue et d’une contrainte moyenne de zéro sous des conditions de chargement de fatigue sans pause. On a observé que les fissures de fatigue avaient pour origine des régions localisées d’attaque d’oxyde à la surface de l’échantillon. Initialement, les fissures de fatigue de la surface se propageaient perpendiculairement à l’axe de la contrainte, mais se déplaçaient vers d’autres plans cristallographiques à des tailles de fissures plus grandes et après la coalescence de multiples fissures de fatigue. Le chargement de fatigue sans pause favorisait le grossissement isotrope des précipités γ′ alors que le chargement en compression de pause fatigue résultait en flottage de γ′ parallèle à la direction de chargement. On a observé que CMSX-2 était sensible à la pause en compression et exhibait une endurance de 3 à 6 fois plus courte que les essais sans pause sous les mêmes gammes de déformation totale.

Fracture analysis of Kevlar-49/epoxy and e-glass/epoxy doublers for reinforcement of cracked aluminum plates

Adhesively bonded composite patch repair is efficient means to regain load carrying capacity, alleviate the crack growth, and improve the service life of the damaged structure. In this paper, three dimensional finite element models are developed to examine the fracture behavior of a single edge V-notched Aluminum plate repaired with Kevlar-49/epoxy or e-glass/epoxy pre-preg patches on both sides. Contour integral method was used for evaluating the stress intensity factor (SIF), an indicator of the crack stability. The load transfer mechanisms, stress distribution, damage variable ( D), and crack mouth opening displacement (CMOD), were also presented to estimate the effectiveness of composite patch repair. The influence of the patch material, crack length and the adhesive thickness has been investigated. Results have shown that the crack induced damage increased nonlinearly with a larger crack size. With the composite patch repairs, SIF is reduced to 1/7–1/10 of that of the bare plate and CMOD decreased by 79%. The damage variable is reduced significantly and the load capacity is increased. A thinner adhesive layer results in a higher percentage of load shared by the composite patch.

Rail rolling contact fatigue dependence on friction, predicted using fracture mechanics with a three-dimensional boundary element model

Rolling contact fatigue crack growth continues to affect many railways worldwide. It is most often controlled through rail grinding in a preventive maintenance strategy, but to plan the required frequency and depth of grinding, prediction of crack growth rates has a vital role. This paper presents crack growth rate results from a new three-dimensional rail model containing an inclined surface breaking rolling contact fatigue crack. The calculations are based on a shear mode of crack growth, driven by the Hertzian contact pressure on the railhead and moderated by friction between the crack faces (“crack face friction”). The results from the model show good correlation with those from the previously published work in the area, with particularly good agreement at higher levels of surface friction coefficient. Applying the new model to a range of surface and crack face friction coefficients predicted that crack growth rate will rise with reduced internal crack face friction at all crack sizes. For small cracks (2 and 5 mm radius) rates were predicted to rise with increased surface traction, but this trend was reversed at larger crack sizes (10 and 19 mm radius). Identical trends were found when the modelling was repeated using the previously developed half-space based “2.5d” model, indicating that although this older model cannot represent the rail geometry its high speed means it remains a useful tool for investigating the effects of contact parameters on rail rolling contact fatigue. The next study in this area could therefore consider if there is a uniform or crack size related ratio for mapping 2.5d results to three-dimensional rail geometry to produce closer agreement in crack growth rates as well as trends. For the three-dimensional model, consideration of alternative crack morphologies and movement of the contact running band away from the rail centreline would provide additional data on the effect of rail grinding and re-profiling on rolling contact fatigue.

Modelling and full-scale trials to investigate fluid pressurisation of rolling contact fatigue cracks

Fluid penetration of cracks has been regarded as an important mechanism of crack growth for inclined surface breaking cracks under contact loading since the 1930s. However, there are only limited cases in which it is realistic to assume fluid is sealed in and pressurised by wheels crossing cracks of complex three-dimensional morphology in rolling contact fatigue affected rail. To investigate, modelling to predict crack growth rates was conducted following experimental verification that fluid penetration occurs for a full size rail-wheel contact. A fluid filled crack beneath a three-dimensional elliptic contact was modelled through development of an existing two-dimensional model. Stress intensity factor results from this model were in good agreement with available literature values, indicating very high growth rates for cracks containing pressurised fluid. At larger crack sizes, for which it was assumed the contact could not seal fluid inside the crack, a shear mode crack growth model was applied. This predicted much lower crack growth rates, of around ten times the rail wear rate, indicating a crack growing sufficiently fast to “keep ahead” of wear, but not unrealistically fast. Further work to correlate the predictions of the fluid pressurisation model with field measurements was suggested.

Kitagawa‐Takahashi diagrams define the limiting conditions for cyclic fatigue failure in human dentin

As cyclic fatigue is considered to be a major cause of clinical tooth fractures, achieving a comprehensive understanding of the fatigue behavior of dentin is of importance. In this note, the fatigue behavior of human dentin is examined in the context of the Kitagawa-Takahashi diagram to define the limiting conditions for fatigue failure. Specifically, this approach incorporates two limiting threshold criteria for fatigue: (i) a threshold stress for fatigue failure, specifically the smooth-bar (unnotched) fatigue endurance strength, at small crack sizes and (ii) a threshold stress-intensity range for fatigue-crack growth at larger crack sizes. The approach provides a "bridge" between the traditional fatigue life and fracture mechanics based damage-tolerant approaches to fatigue-life estimation, and as such defines a "failure envelope" of applied stresses and flaw sizes where fatigue failure is likely in dentin This approach may also be applied to fatigue failure in human cortical bone (i.e. clinical "stress fractures"), which exhibits similar fatigue behavior characteristics, and in principle may aid clinicians in making quantitative evaluations of the risk of fractures in mineralized tissues.

Indentation fracture mechanics toughness dependence on grain size and crack size: Application to alumina and WC–Co

Measurements, for alumina, of the indentation fracture mechanics stress intensity, K, at different grain sizes show a reversed trend in the material toughness property: at small crack sizes, although larger than the material grain size, K increases with decrease in grain size in accordance with a Hall–Petch (H–P) type dependence of being linearly dependent on the inverse square root of the grain diameter; whereas, at larger crack sizes, K reverses itself and increases with increase in grain size. The experimental results are interpreted here in terms of approximate equations obtained from the Bilby/Cottrell/Swinden (BCS) model for critical crack growth with an associated plastic zone size, s, at the crack tip. The particular variations in s associated with the two types of K dependencies, nevertheless, give a positive H–P grain size dependence, at sensibly constant crack size, for computed pre-crack fracture stresses, though of lesser H–P slope than the reference crack-free fracture stress. By comparison, K measurements for WC–Co cemented carbide systems show lower values both at smaller WC particle sizes and, especially, at smaller mean free path, λ, of the fracture-prone Co binder phase. Such result is predicted to occur if s is proportional to λ. The WC–Co system is of special interest, also, because H–P dependencies for the WC and Co constituent components in different alloy compositions make an important contribution, along with the contiguity of the WC particles, to determining the corresponding material hardness properties.

Validity of bolt‐removal and gap‐insertion techniques to experimentally simulate the vibration of a cracked rotor

ABSTRACTExperimental studies of the vibration of a cracked rotor are essential in the development of vibration monitoring techniques. The greatest difficulty in conducting experimental parameter studies is the extensive amount of work required when placing a fatigue crack (FC) of known size and location in the structure. Two alternative techniques, the bolt‐removal method (BRM) and the gap‐insertion method (GIM), have been suggested to provide an easy mechanism to experimentally simulate the behaviour of a breathing crack. This study aims to qualitatively and quantitatively evaluate these methods and compare them with the results from a grown FC in a slow rotating drum specimen. Deflection results show good agreement, particularly in the vertical direction. For the horizontal direction the two simulation techniques overestimate the deflection from the FC for large crack sizes. However, overall GIM and BRM provide close enough results to support the usage in experimental parameter studies at significantly lower cost.

On the origin of the toughness of mineralized tissue: microcracking or crack bridging?

Two major mechanisms that could potentially be responsible for toughening in mineralized tissues, such as bone and dentin, have been identified—microcracking and crack bridging. While evidence has been reported for both mechanisms, there has been no consensus thus far on which mechanism plays the dominant role in toughening these materials. In the present study, we seek to present definitive experimental evidence supporting crack bridging, rather than microcracking, as the most significant mechanism of toughening in cortical bone and dentin. In vitro fracture toughness experiments were conducted to measure the variation of the fracture resistance with crack extension [resistance–curve (R-curve) behavior] for both materials with special attention paid to changes in the sample compliance. Because these two toughening mechanisms induce opposite effects on the sample compliance, such experiments allow for the definitive determination of the dominant toughening mechanism, which in the present study was found to be crack bridging for microstructurally large crack sizes. The results of this work are of relevance from the perspective of developing a micromechanistic framework for understanding fracture behavior of mineralized tissue and in predicting failure in vivo.

Size distribution of creep cracks in materials showing a mixed mode fracture simulated by one-dimensional crack growth model

The size distribution of creep cracks has been simulated by a logarithmic normal distribution [1, 2] or a normal distribution [3]. Evans [4] summarized the experimental data and concluded that the crack size distribution can be approximated quite well by a logarithmic normal distribution. In the previous study [5], the growth and linkage of grain-boundary cracks was simulated by a two-dimensional multicrack growth model similar to the random walk model proposed by Nishiuma et al. [6, 7]. In the simulation, the crack size distribution could be approximated by a logarithmic normal distribution, and the number of the cracks (N) of the size (x ′) equal to or larger than a given size (X) (the ranking of the crack size) could be approximated by a power law relationship (N (x ′ ≥ X ) ∝ X−a, a: scaling exponent) at the larger crack sizes [5]. The results of the simulation could explain the crack size distribution characteristics on the crept specimens of the austenitic 21Cr-4Ni-9Mn steel, in which creep fracture of specimens is governed by grain-boundary fracture [8, 9]. However, a mixed mode of grain-boundary fracture and transgranular fracture is often observed on the ruptured specimens of materials such as the austenitic SUS304 steel [10]. The difference in the fracture mechanisms may be correlated with the difference in the growth and linkage of cracks and may affect the size distribution of creep cracks in materials. In this study, the simulation of growth and linkage of cracks was made using one-dimensional crack growth model that is similar to the (two-dimensional) multicrack growth model in the previous study [5]. The principal rules in the simulation using the present model are as follows:

K Variations and Anisotropy: Microstructure Effect and Numerical Predictions

Computer experiments were performed on simulated polycrystalline material samples that possess locally anisotropic microstructures to investigate stress intensity factor (K) variations and anisotropy along fronts of microcracks of different sizes. The anisotropic K, arising from inhomogeneous stresses in broken grains, was determined for planar microcracks by using a weight function-based numerical technique. It has been found that the grain-orientation-geometry-induced local anisotropy produces large variations in K along front of microcracks, when the crack size is of the order of few grain diameters. Synergetic effect of grain orientation and geometry of broken grains control K variations and evolution along the microcrack front. The K variations may diminish at large crack sizes, signifying a shift of K calculation to bulk stress dependence from local stress dependence. Local grain geometry and texture may lead to K anisotropy, producing unusually higher/lower K at a segment of the crack front. Either K variation or anisotropy cannot be ignored when assessing a microcrack.

On the application of the Kitagawa–Takahashi diagram to foreign-object damage and high-cycle fatigue

The role of foreign-object damage (FOD) and its effect on high-cycle fatigue (HGF) failures in a turbine engine Ti–6Al–4V alloy is examined in the context of the use of the Kitagawa–Takahashi diagram to describe the limiting conditions for such failures. Experimentally, FOD is simulated by firing 1 and 3.2 mm diameter steel spheres onto the flat specimen surface of tensile fatigue specimens at velocities of 200 and 300 m/s. Such damage was found to markedly reduce the fatigue strength of the alloy, primarily due to four factors: stress concentration, microcrack formation, impact-induced plasticity and tensile residual stresses associated with the impact damage. Two groups of fatigue failures could be identified. The first group initiated directly at the impact site, and can be readily described through the use of a fatigue-crack growth threshold concept. Specifically, a Kitagawa–Takahashi approach is presented where the limiting threshold conditions are defined by the stress-concentration corrected smooth-bar fatigue limit (at microstructurally small crack sizes) and a “worst-case” fatigue-crack growth threshold (at larger crack sizes). The second group of failures was caused by fatigue cracks that initiated at locations far from the impact site in regions of high tensile residual stresses, the magnitude of which was computed numerically and measured experimentally using synchrotron X-ray diffraction. Specifically, these failures could be rationalized due to the superposition of the residual stresses on the far-field applied mean stress, leading to a locally elevated load ratio (ratio of minimum to maximum loads). The effects of residual stress, stress concentration, and microstructurally small cracks are combined in a modified Kitagawa–Takahashi approach to provide a mechanistic basis for evaluating the detrimental effect of FOD on HCF failures in Ti–6Al–4V blade alloys.

Size distribution of surface cracks and crack pattern in austenitic SUS316 steel plates fatigued by cyclic bending

The size distribution of surface cracks and the crack pattern were examined on the specimens of the SUS316 steel plates fatigued by cyclic bending. The size distribution of the cracks could be approximated to a logarithmic normal distribution, irrespective of the maximum total strain range or the number of fatigue cycles. The number of the cracks (Nu) of the length (x′) equal to or larger than a given size (X) could be approximated to a power law, Nu∝ X−a, with a scaling exponent a at the larger crack sizes in the fatigued specimens of the SUS316 steel. The value of a decreased with increasing the number of fatigue cycles because of the increase in the number and size of fatigue cracks, and was larger in the specimens tested at the smaller total strain range. Effects of experimental variables on the scaling exponent (a) were also shown in this study. The fractal dimension of spatial crack distribution (the fractal dimension of crack pattern) (D) increased in the range from about 0.9 to about 1.2 with increasing the number of fatigue cycles, and was larger in the specimens fatigued at the larger total strain range. There was a negative correlation between the value of a and the value of D on fatigue cracks, although there was no unique relationship between these two values.

Comparison of the fatigue and fracture of α+β and β titanium alloys

The present study compares the fatigue and fracture properties of the high-strength β titanium alloy β-Cez with the conventional α+β titanium alloy Ti-6Al-4V, because of increasing interest in replacing α+β titanium alloys with β titanium alloys for highly stressed airframe and jet engine components. This comparison study includes the Ti-6Al-4V alloy in an α+ β-processed condition (for a typical turbine blade application) and the β-Cez alloy in two distinctly different α+β-processed and β-processed conditions (optimized for a combination of superior strength, ductility, and fracture toughness). The comparison principally showed a much lower yield stress for Ti-6Al-4V (915 MPa) than for both β-Cez conditions (1200 MPa). The Ti-6Al-4V material also showed the significantly lower high-cycle fatigue strength (resistance against crack initiation) of 375 MPa (R=−1) as compared to the β-Cez alloy (∼600 MPa, R=−1). Particularly in the presence of large cracks (>5 mm), the fatigue crack growth resistance and fracture toughness of the Ti-6Al-4V material is superior when compared to both β-Cez conditions. However, for small crack sizes, the conditions of both the alloys under study show equivalent resistance against fatigue crack growth. For the β-Cez material, where microstructures were optimized for high fracture toughness (conventional large crack sizes) by thermomechanical processing, maximum K Ic-values of 68 MPa√m of the β-processed β-Cez condition (tested in the longitudinal direction) decreased by ∼50 pct in the presence of small cracks (1 mm). A similar decrease in fracture toughness was obtained by loading the β-processed β-Cez condition perpendicular to the flat surfaces of the pancake-shaped β grain structure (tested in the short transverse direction). These results were discussed in terms of the effectiveness of the crack front geometry in hindering crack propagation. Further, the results of this study were considered for alloy selection and optimized microstructures for fatigue and fracture critical applications. Finally, the advantage of the α+β-processed β-Cez condition in highly stressed engineering components is pointed out because of its overall superior combination of fatigue crack initiation and propagation resistance (especially against small fatigue cracks).

Mixed-mode, high-cycle fatigue-crack-growth thresholds in Ti-6Al-4V: Role of bimodal and lamellar microstructures

Mixed-mode, high-cycle fatigue-crack growth thresholds are reported for through-thickness cracks (large compared to microstructural dimensions) in a Ti-6Al-4V turbine blade alloy in both lamellar and bimodal microstructural conditions. Specifically, the effect of combined mode I and mode II loading, over a range of phase angles (β=tan −1 (ΔK II/ΔK I) from 0 to 62 deg (ΔK II/ΔK I∼0 to 1.9), is examined at a load ratio (ratio of minimum to maximum loads) of R = 0.1 and a cyclic loading frequency of 1000 Hz in ambient-temperature air. When the mixed-mode, crack-driving force is characterized in terms of the strain-energy release rate (ΔG), incorporating contributions from both the applied tensile and shear loading, the threshold for fatigue-crack growth is observed to increase significantly with the applied mode-mixity (ΔK II/ΔK I) for both microstructures, an effect attributed to enhanced crack-tip shielding. The pure mode I threshold, in terms of ΔG TH, is observed to be a lower bound (worst case) with respect to mixed-mode (I + II) behavior. For large crack sizes, the threshold fatigue-crack growth resistance of the lamellar structure is observed to be superior to that of the bimodal material for all phase angles investigated. Consideration of mode I fatigue-crack growth thresholds for small cracks in these same microstructures suggests that this rank ordering of mixed-mode fatigue resistance may not hold for crack lengths that are comparable to microstructural size scales. Examination of the fatigue-crack wake indicates that, for the lamellar microstructure, the path of crack extension is significantly influenced by the local microstructure over length scales on the order of the relatively coarse lamellar colonies (∼500 µm). Comparatively, the crack path in the bimodal material is more strongly influenced by the applied crack-driving force. This disparity in behavior is attributed primarily to the relatively heterogeneous crack-growth resistance of the coarse lamellar microstructure.