Fatigue-crack propagation behavior of ductile/brittle laminated composites

A study has been made of the fatigue-crack propagation properties of a series of laminated Nb-reinforced Nb₃Al intermetallic-matrix composites with varying microstructural scale but nominally identical reinforcement volume fraction (20 pct Nb). It was found that resistance to fatigue-crack growth improved with increasing metallic layer thickness (in the range 50 to 250 μm) both in the crack-divider and crack-arrester orientations. For a given layer thickness, however, the properties in the crack-arrester orientation were superior to the crack-divider orientation. Indeed, the fatigue resistance of the crack arrester laminates was better than the fatigue properties of unreinforced Nb₃Al and pure Nb; both laminate orientations had significantly better fatigue properties than Nb-particulate reinforced Nb₃Al composites. Such enhanced fatigue performance was found to result from extrisic toughening in the form of bridging metal ligaments in the crack wake, which shielded the crack tip from the applied (far-field) driving force. Unlike particulate-reinforced composites, such bridging was quite resilient under cyclic loading conditions. The superior crack-growth resistance of the crack-arrester laminates was found to result from additional intrinsic toughening, specifically involving trapping of the entire crack front by the Nb layer, which necessitated crack renucleation across the layer.     

Ductile-reinforcement toughening in γ-TiAl intermetallic-matrix composites: Effects on fracture toughness and fatigue-crack propagation resistance

The influence of the type, volume fraction, thickness and orientation of ductile phase reinforcements on the room temperature fatigue and fracture resistance of γ-TiAl intermetallic alloys is investigated. Large improvements in toughness compared to monolithic γ-TiAl are observed in both the TiNb- and Nb-reinforced composites under monotonic loading. Toughness increases with increasing ductile phase content, reinforcement thickness and strength; orientation effects are minimal. Crack-growth behavior is characterized by steep resistance curves primarily due to crack trapping/renucleation and extensive crack bridging by the ductile-phase particles. In contrast, under cyclic loading the influence of ductile phases on fatigue resistance is strongly dependent upon reinforcement orientation. Compared to monolithic γ-TiAl, improvements in fatigue-crack growth resistance are observed in TiNb-reinforced composites only in the face (C-L) orientation; crack-growth rates for the edge (C-R) orientation are actually faster in the composite. In comparison, Nb-particle reinforcements offer less toughening under monotonic loading but enhance the fatigue properties compared to TiNb reinforcements under cyclic loading.     

Fatigue crack growth rates and fracture toughness of rapidly solidified Al-8.5 pct Fe-1.2 pct V-1.7 pct Si alloys

The room-temperature fatigue crack growth rates (FCGR) and fracture toughness were evaluated for different crack plane orientations of an Al-8.5 Pct Fe-1.2 Pct V-1.7 Pct Si alloy produced by planar flow casting (PFC) and atomized melt deposition (AMD) processes. For the alloy produced by the PFC process, properties were determined in six different orientations, including the short transverse directions S-T and S-L. Diffusion bonding and adhesive bonding methods were used to prepare specimens for determining FCGR and fracture toughness in the short transverse direction. Interparticle boundaries control fracture properties in the alloy produced by PFC. Fracture toughness of the PFC alloy varies from 13.4 MPa√m to 30.8 MPa√m, depending on the orientation of the crack plane relative to the interparticle boundaries. Fatigue crack growth resistance and fracture toughness are greater in the L-T, L-S, and T-S directions than in the T-L, S-T, and S-L orientations. The alloy produced by AMD does not exhibit anisotropy in fracture toughness and fatigue crack growth resistance in the as-deposited condition or in the extruded condition. The fracture toughness varies from 17.2 MPa√m to 18.5 MPa√m for the as-deposited condition and from 19.8 MPa√m to 21.0 MPa√m for the extruded condition. Fracture properties are controlled by intrinsic factors in the alloy produced by AMD. Fatigue crack growth rates of the AMD alloy are comparable to those of the PFC alloy in the L-T orientation. The crack propagation modes were studied by optical metallographic examination of crack-microstructure interactions and scanning electron microscopy of the fracture surfaces.     

Crack deflection during fatigue and monotonic fracture of a SiC whisker reinforced 2009 aluminium alloy

Fatigue crack growth under Mode I loading, and static fracture in both symmetrical and asymmetrical notched four point bend specimens, have been examined in SiC whisker reinforced 2009 aluminium alloy. In the fatigue tests a range of orientations of the starter notch, with respect to the extrusion direction, L, was examined. Slanted crack propagation was observed in all specimens except that in the T-L configuration. For the monotonic tests the specimen orientation (L-T) remained constant whilst the ratio of Mode I to Mode II loading was varied. Again crack deflection was observed in all cases apart from the L-T specimen under pure Mode I loading. Whisker debonding was found to be the dominant factor controlling crack deflection, independent of the mixity of the loading mode. Under mixed-mode static loading, the deflection angle was controlled by the average orientation of the whiskers subject to the asymmetrically distributed maximum principal stress. In fatigue loading, however, the crack tended to follow the most frequently observed whisker orientation. These contrasting observations are interpreted in terms of the different sampling volumes at the crack tip in monotonic fracture and in fatigue crack growth.     

Fracture and fatigue-crack growth behavior in ductile-phase toughened molybdenum disilicide: Effects of niobium wirevs particulate reinforcements

A study has been made of the fracture toughness/resistance-curve (R-curve) and cyclic fatigue-crack propagation behavior in a molybdenum disilicide composite, ductile-phase toughened with nominally 20 vol pct Nb-wire mesh reinforcements (Nb _m /MoSi₂); results are compared with monolithic MoSi₂ and MoSi₂ reinforced with 20 vol pct spherical Nb particles (Nb _p /MoSi₂). It is found that the high aspect ratio wire reinforcements induce significant toughening in MoSi₂, both under monotonic and cyclic fatigue loading conditions. Specifically, the Nb _m /MoSi₂ composite exhibits R-curve behavior with a steady-state fracture toughness of ∼13 MPa , compared to unstable fracture atK _c values below 5 MPa in unreinforced MoSi₂ or Nb _p /MoSi₂. Such behavior is seen to be associated with extensive crack deflection within the reaction layer between Nb and the matrix, which leads to crack bridging by the unbroken ductile phase. Similarly, resistance to fatigue-crack growth is found to be far superior in the wire-reinforced composite over pure MoSi₂ and Nb _p /MoSi₂. Although crack paths are again characterized by extensive deflection along the Nb/matrix reaction layer, the role of crack bridging is diminished under cyclic loading due to fatigue failure of the Nb. Instead, the superior fatigue properties of the Nb _m /MoSi₂ composite are found to be associated with high levels of crack closure that result from highly deflected crack paths along the (Nb,Mo)₅Si₃ reaction layer interface.     

Near-threshold fatigue crack growth behavior of 2195 aluminum-lithium-alloy—prediction of crack propagation direction and influence of stress ratio

Tensile properties and fatigue crack propagation behavior of a 2195-T8 Al-Li alloy were investigated at different stress ratios, with particular emphasis on their dependence on specimen orientation. Specimens with orientations of 0, 15, 30, 45, and 90 deg to the rolling direction were tested. The alloy contained a strong brass-type texture and a profuse distribution of platelike precipitates of T ₁ (Al₂CuLi) phase on {111} matrix planes. Both tensile strength and fatigue thresholds were found to be strongly dependent on the specimen orientation, with the lowest values observed along the direction at 45 deg to the rolling direction. The effect of stress ratio on fatigue threshold could generally be explained by a modified crack closure concept. The growth of fatigue crack in this alloy was found to exhibit a significant crystallographic cracking and especially macroscopic crack deflection. The specimens oriented in the L-T + 45 deg had the smallest deflection angle, while the specimens in the L-T and T-L orientations exhibited a large deflection angle. The dependence of the fatigue threshold on the specimen orientation could be rationalized by considering an equivalent fatigue threshold calculated from both mode I and mode II values due to the crack deflection. A four-step approach on the basis of Schmid’s law combined with specific crystallographic textures is proposed to predict the fatigue crack deflection angle. Good agreement between the theoretical prediction and experimental results was observed.     

Creep brack growth behavior of Two Al-Li alloys

Creep crack growth behavior of two Al-Li-Cu-Mg-Zr alloys with and without the addition of Ge was determined under static load at 150 °C and the results were analyzed using the linear elastic and elastic-plastic fracture mechanics methods. The alloys were produced through conventional ingot metallurgy processing and extruded into a plate form. Texture is more pronounced in the alloy that does not contain Ge. Because of the texture, crack growth rates in the two alloys differ significantly for the L-T and T-L orientations. The growth occurs by the nucleation of microcracks ahead of the main crack which grow and coalesce with the main crack. While this leads to rapid crack growth in the T-L orientation, it causes crack tip bifurcation and even complete crack arrest in the L-T orientation. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Strength,fracture, and fatigue behavior of advanced high-temperature intermetallics reinforced with ductile phases

The results of recent studies on the fatigue and fracture behavior of extruded Ti-48A1 + 20 vol pct TiNb and hot-isostatically pressed (“hipped”) MoSi₂ + 20 vol pct Nb are presented (compositions in atomic percent unless stated otherwise). The effects of ductile phase reinforcement of Ti-48A1 and MoSi₂ on the micromechanisms of fracture under monotonie and cyclic loading are elucidated. Micromechanics models are applied to the prediction of crack-tip shielding components, and the effects of temperature on tensile/compressive/flexure strengths are discussed. Ductile phase toughening under monotonie loading conditions is shown to be associated with lower fatigue crack growth resistance. The lower fatigue resistance is attributed to the absence of crack-tip shielding, higher crack opening displacements, and the effects of inelastic strains that are developed in ductile phase-reinforced composites under cyclic loading conditions. S.M.L. SASTRY, formerly Program Manager and Fellow, McDonnell Douglas Research Laboratories. This article is based on a presentation made in the symposium “Quasi-Brittle Fracture” presented during the TMS fall meeting, Cincinnati, OH, October 21–24, 1991, under the auspices of the TMS Mechanical Metallurgy Committee and the ASM/MSD Flow and Fracture Committee.     

Fracture toughness and R-Curve behavior of laminated brittle-matrix composites

The fracture toughness and resistance-curve behavior of relatively coarse-scale, niobium/niobium aluminide (Nb/Nb₃Al) laminated composites have been examined and compared to other Nb/Nb₃Al composites with (in situ) Nb particulate or microlaminate reinforcements. The addition of high aspectratio Nb reinforcements, in the form of 20 vol. pct of 50- to 250-μm-thick layers, was seen to improve the toughness of the Nb₃Al intermetallic matrix by well over an order of magnitude, with the toughness increasing with Nb layer thickness. The orientation of the laminate had a small effect on crack-growth resistance with optimal properties being found in the crack arrester, as compared to the crack divider, orientation. The high fracture toughness of these laminates was primarily attributed to large (∼1- to 6-mm) crack-bridging zones formed by intact Nb layers in the crack wake; these zones were of sufficient size that large-scale bridging (LSB) conditions generally prevailed in the samples tested. Resistance-curve modeling using weight function methods permitted the determination of simple approximations for the bridging tractions, which were then used to make smallscale bridging (SSB) predictions for the steady-state toughness of each laminate.     

Thermomechanical fatigue of particulate-reinforced aluminum 2xxx-T4

Thermomechanical fatigue (TMF) and isothermal fatigue of unreinforced and SiC_p-reinforced aluminum 2xxx-T4 alloy were examined. Thermomechanical fatigue experiments were conducted underT _min = 100 °C,T _max = 300 °C andT _min = 100 °C,T _max = 200 °C conditions, and isothermal experiments were conducted at 200 °C and 300 °C. Based on stress range, substantial improvements in fatigue life were observed with reinforcement under both isothermal and thermomechanical loading conditions. Based on strain range, the TMF lives of the reinforced material increased in out-of-phase (OP) loading and remained unchanged in in-phase (IP) loading. A decrease in isothermal fatigue lives of the reinforced material compared to those of unreinforced material was observed in both 3 × 10⁻³ s⁻¹ and 3 × 10⁻⁵ s⁻¹ experiments at 200 °C and in 3 × 10⁻³ s⁻¹ experiments at 300 °C. Crack growth mechanism maps were constructed to identify crack growth behavior of the unreinforced and the reinforced materials. The TMF OP conditions were more favorable to transgranular cracking. Mixed (transgranular and intergranular) crack growth occurred in TMF IP experiments. Evidence of void formation at grain boundaries, crack deflection due to particles, and oxide penetration at the crack tips is demonstrated using scanning electron microscopy (SEM) and Auger spectroscopy analysis.     

Fatigue crack propagation in aluminum- lithium alloy 2090: Part I. long crack behavior

A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium alloy 2090-T8E41. In Part I, the crack growth and crack shielding behavior of long (≳5 mm) through-thickness cracks is examined as a function of plate orientation and load ratio, and results compared to traditional high strength aluminum alloys. It is shown that rates of fatigue crack extension in 2090 are, in general, significantly slower (at a given stress intensity range) than in traditional alloys, although behavior is strongly anisotropic. Differences in growth rates of up to 4 orders of magnitude are observed between the L-T, T-L, and T-S orientations, which show the best crack growth resistance, and the S-L, S-T, and L + 45, which show the worst. Such behavior is attributed to the development of significant crack tip shielding (i.e., a reduction in local crack driving force), primarily resulting from the role of the crack path morphology in inducing crack deflection and crack closure from the consequent asperity wedging. Whereas crack advance perpendicular to the rolling plane (e.g., L-T,etc.) involves marked crack path deflection and branching, thereby promoting very high levels of shielding to cause the slowest growth rates, fatigue fractures parallel to the rolling plane (e.g., S-L,etc.) occur by an intergranular, delamination-type separation, with much lower shielding levels to give the fastest growth rates. The implications of such “extrinsic toughening” effects on the fracture and fatigue properties of aluminum-lithium alloys are discussed in detail. R. O. RITCHIE, Professor and Director, Center for Advanced Materials, Lawrence Berkeley Laboratory     

Toughening mechanisms in ductile niobium-reinforced niobium aluminide (Nb/Nb3Al) in situ composites

Anin situ study has been performed in the scanning electron microscope (SEM) on a niobium ductilephase-toughened niobium aluminide (Nb/Nb₃Al) intermetallic composite to examine the crack-growth resistance-curve (R-curve) behavior over very small initial crack extensions, in particular over the first ~500 μm of quasi-static crack growth, from a fatigue precrack. The rationale behind this work was to evaluate the role of toughening mechanisms, specifically from crack bridging, in the immediate vicinity of the crack tip and to define the size and nature of bridging zones. Although conventional test methods, where crack advance is monitored typically over dimensions of millimeters using compliance or similar techniques, do not show rising R-curve behavior in this material,in situ microscopic observations reveal that bridging zones resulting from both uncracked Nb₃Al ligaments and intact Nb particles do exist, but primarily within ~300 to 400 μm of the crack tip. Accordingly, rising R-curve behavior in the form of an increase in fracture resistance with crack growth is observed for crack extensions of this magnitude; there is very little increase in toughness for crack extensions beyond these dimensions. Ductile-phase toughening induced by the addition of Nb particles, which enhances the toughness of Nb₃Al from ~1 to 6 MPa√m, can thus be attributed to crack-tip shielding from nonplanar matrix and coplanar particle bridging effects over dimensions of a few hundred microns in the crack wake. formerly Research Student, Department of Materials Science and Mineral Engineering, University of California-Berkeley     

Fatigue and creep crack growth in oxide dispersion strengthened INCONEL MA-754

The influence of temperature, orientation, and environment on fatigue and creep crack growth behavior in oxide dispersion strengthened INCONEL MA-754 was examined. With an increase in temperature, crack growth rates increase due largely to an increasing creep contribution. Environment also may influence crack growth behavior, its effect depending on orientation. Orientation has a marked effect on crack growth because of the propensity for creep void formation along particle stringers in the microstructure, which form in the processing. The rate of crack growth can be enhanced if the aligned voids are parallel to the main crack or retarded if these voids are normal to the direction of the crack. In the transverse-longitudinal (T-L) orimation crack growth is faster on a time basis in creep than in fatigue; the reverse of this is true in the longitudinal-transverse (L-T) orientation. Predicted fatigue crack growth rates based on a cumulative damage model agree with experimentally determined growth rates.     

An investigation of fracture and fatigue in a metal/polymer composite

This article presents the results of a combined experimental and analytical study of the fatigue and fracture behavior of a polymer/metal composite which was developed recently for self-lubricating applications in automotive engines that utilize liquefied natural gas as fuel. For comparison, the microstructure and the fatigue and fracture behavior of a nonpolymer-containing “matrix” material are also presented. Since the crack profiles observed in both systems under monotonic or cyclic loading reveal significant components of ligament bridging, micromechanics models are presented for the modeling of crack bridging. The resulting predictions of resistance-curve behavior are compared with measured resistance curves. The shielding effects of ligament bridging are also quantified under cyclic loading. The implications of the work are also discussed for the modeling of fatigue damage and fracture in polymer/metal coatings.     

Fatigue crack growth in a particulate TiB<Subscript>2</Subscript>-reinforced powder metallurgy iron-based composite

Fatigue crack growth behavior has been examined in a particulate titanium diboride (TiB₂)-reinforced iron-based composite that had been produced via a mechanical alloying process. Comparison with equivalent unreinforced material indicated that fatigue crack growth resistance in the composite was superior to monolithic matrix material in the near-threshold regime. The composite exhibited relatively low crack closure levels at threshold, indicative of a high intrinsic (effective) threshold growth resistance compared to the unreinforced iron. The lower closure levels of the composite were consistent with reduced fracture surface asperity sizes, attributable to the reinforcement particles limiting the effective slip distance for stage I-type facet formation. The observed shielding behavior was rationalized in terms of recent finite-element analysis of crack closure in relation to the size of crack wake asperities and the crack-tip plastic zone. The different intrinsic fatigue thresholds of the composite and unreinforced iron were closely consistent with the influences of stiffness and yield strength on cyclic crack-tip opening displacements. Cracks in the composite were generally seen to avoid direct crack-tip-particle interaction.     

Role of foreign-object damage on thresholds for high-cycle fatigue in Ti-6Al-4V

The increasing incidence of military aircraft engine failures that can be traced to high-cycle fatigue (HCF) has prompted a reassessment of the design methodologies for HCF-critical components, such as turbine blades and disks. Because of the high-frequency vibratory loading involved, damagetolerant design methodologies based on a threshold for no crack growth offer a preferred approach. As impact damage from ingested debris is a prime source of HCF-related failures, the current study is focused on the role of such foreign-object damage (FOD) in influencing fatigue crack-growth thresholds and early crack growth of both large and small cracks in a fan blade alloy, Ti-6Al-4V. FOD, which was simulated by the high-velocity (200 to 300 m/s) impact of steel spheres on a flat surface, was found to reduce markedly the fatigue strength, primarily due to earlier crack initiation. This is discussed in terms of four salient factors: (1) the stress concentration associated with the FOD indentation, (2) the presence of small microcracks in the damaged zone, (3) the localized presence of tensile residual hoop stresses at the base and rim of the indent sites, and (4) microstructural damage from FOD-induced plastic deformation. It was found that no crack growth occurred from FOD impact sites in this alloy at ΔK values below ∼ 2.9 MPa √m, i.e., over 50 pct higher than the “closure-free”, worst-case threshold value of ΔK _TH = 1.9 MPa √m, defined for large cracks in bimodal Ti-6Al-4V alloys at the highest possible load ratio. It is, therefore, concluded that such worst-case, large fatigue crack thresholds can, thus, be used as a practical lower-bound to FOD-initiated cracking in this alloy.     

Fracture of Ti-Al<Subscript>3</Subscript>Ti metal-intermetallic laminate composites: Effects of lamination on resistance-curve behavior

The fracture toughness and resistance-curve (R-curve) behavior of Ti-Al₃Ti metal-intermetallic laminate (MIL) composites have been studied in the crack-divider orientation, by examining the effect of ductile-laminate-layer thickness and volume fraction. The MIL composites were fabricated in open air by a novel one-step process, and the final structure consists of alternating layers of ductile Ti and brittle Al₃Ti. Such a laminate architecture in conjunction with a relatively low volume fraction of tougher Ti (18 to 40 pct) was seen to augment the fracture toughness of the inherently brittle intermetallic by over an order of magnitude. Additionally, as a result of their low density, MIL composites exhibit a specific fracture toughness (K/ρ) on par with tougher but relatively denser ductile metals such as high-strength steel. Such vast improvements may be rationalized through the toughening provided by the ductile Ti layers. Specifically, toughening was obtained through plastically stretching the intact ductile Ti layers that formed bridging zones in the crack wake, thus reducing the crack driving force. Such toughening resulted in R-curve behavior, and the toughness values increased with an increase in the volume percentage of Ti. Weight-function methods were used to model the bridging behavior, and they indicated that large bridging zones (∼2 to 3 mm) were responsible for the observed increase in toughness. The role of large-scale bridging (LSB) conditions on the resistance curves is explored, and steady-state toughness (K _SS ) values are estimated using small-scale bridging (SSB) approximations. A new approach to gage the potential of laminate composites in terms of their true fracture-toughness values as determined from a cyclic crack-growth fatigue test is proposed, wherein small-scale specimens can be utilized to obtain fracture-toughness values.     

High cycle fatigue and fatigue crack growth of the oxide dispersion strengthened alloy MA 754

The high cycle fatigue (HCF) behavior of the oxide dispersion strengthened (ODS) MA 754 alloy has been determined as a function of specimen orientation. The fatigue life showed anisotropic behavior with the longest and shortest lives in the longitudinal and short transverse directions, respectively. Surface porosity, due to oxidation, was found to affect fatigue life in the long transverse orientation more than in the longitudinal orientation. The fatigue crack growth behavior in MA 754 exhibited a directional dependence. In general, the crack growth rates in the longitudinal direction were lower than those in the long transverse direction. The ΔK _th was ∼11 MN ·^-3/2 and 9 MN · m^-3/2 for the longitudinal and the long transverse orientation, respectively. This behavior was explained on the basis of the unusual grain structure and the texture exhibited by this alloy as well as different crack closure effects. It was found that a consideration based on the crack growth rates results, obtained from fracture mechanics specimens, could not explain the anisotropic behavior of the HCF properties of MA 754. However, the anisotropic HCF properties could be rationalized on the basis of the differences in the modes of crack initiation.     

The influence of ageing on fatigue crack growth in SiC-particulate reinforced 8090

A study of the influence of SiC-particulate reinforcement on ageing and subsequent fatigue crack growth resistance in a powder metallurgy 8090 aluminium alloy-SiC composite has been made. Macroscopic hardness measurements revealed that ageing at 170°C in the composite is accelerated with respect to the unreinforced alloy, though TEM studies indicate that this is not due to the enhanced precipitation of S'. Fatigue crack growth rates in the naturally aged condition of the composite and unreinforced matrix are similar at low to medium values of δK, but diverge above ≈ 8 MPa√m owing to the lower fracture toughness of the composite. As a result of the presence of the reinforcement, planar slip in the composite is suppressed and facetted crack growth is not observed. Ageing at or above 170°C has a deleterious effect on fatigue crack growth. Increased ageing time decreases the roughness of the fracture path at higher growth rates. These effect are though to be due to microstructural changes occurring at or near to the SiC/matrix interfaces, providing sites for static mode failure mechanisms to operate. This suggestion is supported by the observation that as δK increases, crack growth rates become K_max dependent, implying the crack growth rate is strongly influenced by static modes.     

Environmental fatigue crack propagation in Ti-6Al-4V sheet

A study was made of environmental fatigue crack propagation in 2.5 mm thick Ti-6A1-4V sheet conforming to AMS 4911, and 2.2 mm thick IMI 318 conforming to BS TA 10. The environments were dry argon, normal air, distilled water and 3.5 pct aqueous NaCl. There were three alloy/orientation combinations: Ti-6A1-4V L-T, IMI 318 L-T and IMI 318 T-L. Test frequencies were 30 and 50 Hz, at which there was a general trend of higher crack growth rates in the order: argon, air, distilled water, 3.5 pct aqueous NaCl. For both dry argon and 3.5 pct aqueous NaCl there were large differences in crack growth rates at low δK values between the three types of specimen. There was a correlation between the texture and cleavage fracture and crack growth rates in 3.5 pct aqueous NaCl. This result is of considerable practical importance. For dry argon the ranking of specimen types could be explained by the relative importance of mechanical and environmental crack growth, using the structure-sensitive to structure-insensitive transition concept of Irving and Beevers.