Fracture toughness of selectively reinforced Al2124 alloy: Precrack tip in the composite side

In addition to the inherent fracture toughness of each bimaterial component, K _Q (5 pct) values of the Al2124/Al2124 + SiC bimaterials are largely affected by the thermal residual stresses, the elastic/plastic mismatch, and the precrack tip position. Regardless of the precrack tip distance to the interface, K _Q (5 pct) values are increased in general above “composite only” values. This is deduced to be due to the compressive residual stresses and despite the amplification of the crack driving force from the elastic/plastic mismatch. Additionally, K _Q (5 pct) values of the bimaterials increase if the precrack tip is positioned closer to the interface. When the crack propagates, it extends to the interface, bifurcates, and arrests. The load then has to be increased to promote further crack growth in the unreinforced Al2124 alloy side and the subsequent onset of plastic collapse. The crack tip blunting and deflection mechanism increase the toughness attained at the onset of plastic collapse of the Al2124 based bimaterials above both the composite only and “Al2124 only” values.     

Fracture toughness: A rationalization of the role of microstructure in an α-β titanium alloy

The role of microstructure in affecting fracture toughness is examined by considering how microstructure affects the formation of a critical increment of crack extension leading to catastrophic fracture. It is proposed that this critical increment of crack extension occurs by void formation ahead of the main crack and growth back to it. Factors affecting void nucleation and void growth are, therefore, examined in this connection. Published data on the Ti-5.25Al-5.5V-0.9Fe-0.5Cu alloy are used for this purpose. In equiaxed(E) α structure voids nucleate at Eα/agedβ matrix interfaces for both tensile and fracture toughness tests. Although the interparticle spacing, A, is four times more effective than priorβ grain size,D _β, in controlling void growth rate,G _L, in a tensile test,D _β is at least five times more effective in controlling fracture toughness. For Widmanstätten plus grain boundary (W + GB) α structures there are marked similarities betweenG _Lbehavior as a function of GBa thickness, J, and the contribution of J to fracture toughness. These similarities have led to the proposal that the increase in fracture toughness, ΔK_Q, with increasingl is due to blunting of the crack tip, and the plateau in ΔK_Q which follows, with increasingl, is due to a balance between blunting and sharpening processes. Blunting occurs by crack penetration into GBα. The sharpening occurs by void formation and growth along GBα/agedβ interfaces back to the main crack.     

Nonlinear Fracture Analysis of Delamination Crack Jumps in Laminated Composites

As part of the quest to add the infraply scale to high-fidelity simulations of damage evolution in composites, a model of the phenomenon of delamination jumping across transverse plies is formulated by using nonlinear cohesive fracture models in the augmented finite element method (A-FEM). The nonlinearity of the fracture process zone and the interaction between multiple cracks combines to determine the details of how the delamination jump occurs. Simulations reveal that the jumping process starts with the triggering of a sequence of kinking cracks branching from the propagating delamination crack into the transverse plies. The first few kinking cracks arrest within the transverse plies just above the further interface because of the crack-retarding effects of the nonlinear process zone and the effects of material heterogeneity. Eventually, one kinking crack reaches the interface and initiates a new delamination crack, a step that is accompanied by a significant load spike. The competition between delamination and kinking cracks shows global-local coupling: kinking cracks are triggered when the local stress satisfies a critical condition, but a kinking crack does not reach the second interface and initiate the new delamination crack until the global energy release rate reaches the kinking crack toughness. This suggests that the jumping process is controlled more by deterministic load and geometrical factors than by stochastic flaw populations.     

Crack closure and residual stress effects in fatigue of a particle-reinforced metal matrix composite

A study of the influence of macroscopic quenching stresses on long fatigue crack growth in an aluminium alloy-SiC composite has been made. Direct comparison between quenched plate, where high residual stresses are present, and quenched and stretched plate, where they have been eliminated, has highlighted their rôle in crack closure. Despite similar strength levels and identical crack growth mechanisms, the stretched composite displays faster crack growth rates over the complete range of ΔK, measured at R = 0.1, with threshold being displaced to a lower nominal ΔK value. Closure levels are dependent upon crack length, but are greater in the unstretched composite, due to the effect of surface compressive stresses acting to close the crack tip. These result in lower values of ΔK_eff in the unstretched material, explaining the slower crack growth rates. Effective ΔK_th values are measured at 1.7 MPa√m, confirmed by constant K_max testing. In the absence of residual stress, closure levels of approximately 2.5 MPa√m are measured and this is attributed to a roughness mechanism.     

Microstructure and mechanical properties of mullite-zirconia reaction-sintered composites

The flexural strength, fracture toughness (K_IC), creep behaviour and thermal shock of mullite-zirconia and mullite-zirconia-alumina composites obtained by reaction-sintering of zircon + alumina mixtures have been studied in the temperature interval ranging from room temperature to 1400°C. The results are discussed in terms of the microstructural features of the reaction-sintered composites.     

Cyclic fatigue-crack propagation along ceramic/metal interfaces

The integrity of ceramic/metal joints is investigated under mechanically applied cyclic stresses using double-cantilever-beam, and compact-tension, sandwich test specimens. Specifically, fatigue-crack propagation rates for interfacial cracks are characterized over a range of velocities from 10⁻⁹ to 10⁻⁴m/s for glass/copper and alumina/aluminum-alloy interfaces tested in moist air. Compared to corresponding (stress-corrosion) results under sustained loading, it is found that true interfacial cracks in glass-copper joints are significantly accelerated under cyclic loads. In addition, crack extension force (G) thresholds for interfacial crack growth under cyclic loads are some 46% lower than under sustained loads and are typically over six times lower than the interface toughness (G_c). For the alumina/aluminum-alloy system, conversely, fracture never occurs in the interface; under monotonic loading cracking progresses near the interface in the ceramic layer whereas under cyclic loading failure may occur either in the ceramic or in the metal. Based on a comparison with fatigue-crack growth data in bulk alumina and bulk aluminum alloys, it is found that near interfacial crack-growth rates in the metal are much lower than those of the bulk ceramic and show a far higher dependency on the range of G than behavior in the bulk metal.     

A Comparative Study of Mechanical Properties,Thermal Conductivity,Residual Stresses,and Wear Resistance of Aluminum-Alumina Composites Obtained by Squeeze Casting and Powder Metallurgy

Squeeze casting and powder metallurgy techniques were employed to fabricate AlSi12/Al₂O₃ composites, which are lightweight structural materials with potential applications in the automotive industry. The impact of the processing route on the material properties was studied. Comparative analyses were conducted for the Vickers hardness, flexural strength, fracture toughness, thermal conductivity, thermal residual stresses, and frictional wear. Our results show that the squeeze cast composite exhibits superior properties to those obtained using powder metallurgy.

     

Ductile-phase toughening and Fatigue-Crack Growth in Nb-Reinforced Molybdenum Disilicide Intermetallic Composites

A study has been made of the role of ductile-phase toughening on the ambient temperature fracture toughness and fatigue-crack propagation behavior of a molybdenum disilicide intermetallicmatrix composite reinforced with 20 vol pct niobium spheres. Using disk-shaped compact DC(T) samples, only moderate improvements (∼24 pct) in fracture toughnessK _lcvalues were found for the composite compared to the unreinforced MoSi₂ matrix material. Moreover, (cyclic) fatigue- crack propagation was seen at stress intensities as low as 75 to 90 pct ofK _Ic, with growth rates displaying a high dependency (∼14) on the applied stress-intensity range. The lack of significant toughening due to the incorporation of ductile Nb particles is associated with an absence of crack/particle interactions. This is attributed to the formation of a weak reaction-layer interface and elastic mismatch stresses at the crack tip between the Nb and MoSi₂, both factors which favor interfacial debonding; moreover, the spherical morphology of Nb phase stabilizes cracking around the particle. Results suggest that increasing the aspect ratio of the distributed Nb rein- forcement phase with attendant interfacial debonding and eliminating possible Nb-phase em- brittlement due to interstitial impurity contamination are critical factors for the successful development of tougher Nb/MoSi₂ structural composites. Formerly with McDonnell Formerly with McDonnell     

Effects of plasticity on the crack propagation resistance of a metal/ceramic interface

Fracture experiments have been conducted on a gold/sapphire interface. The interface is found to fail by interface separation in a nominally “brittle” manner with a critical strain energy release rate, G_c ≈ 50 Jm⁻², substantially larger than the work of adhesion, W_ad ≈ 0.5 Jm⁻². Evidence of plastic deformation on the gold fracture surface, such as blunting steps and slip steps, suggest that plastic dissipation is the primary contribution to the measured G_c. Calculations suggest that the majority effect occurs in the plastic zone through the crack wake. The interface is also found to be susceptible to slow crack growth.     

Role of matrix/reinforcement interfaces in the fracture toughness of brittle materials toughened by ductile reinforcements

Crack interactions with ductile reinforcements, especially behavior of a crack tip at the interface, have been studied using MoSi₂ composites reinforced with Nb foils. Effects of fracture energy of interfaces on toughness of the composites have also been investigated. Variation of interfacial bonding was achieved by depositing an oxide coating or by the development of a reaction prod- uct layer between the reinforcement and matrix. Toughness was measured using bend tests on chevron-notched specimens. It has been established that as a crack interacts with a ductile re- inforcement, three mechanisms compcte: interfacial debonding, multiple matrix fracture, and direct crack propagation through the reinforcement. Decohesion length at the matrix/reinforcement interface depends on the predominant mechanism. Furthermore, the results add to the evidence that the extent to which interfacial bonding is conducive to toughness of the composites depends on the criterion used to describe the toughness and that ductility of the ductile reinforcement is also an important factor in controlling toughness of the composites. Loss of ductility of the ductile reinforcement due to inappropriate processing could result in little improvement in tough- ness of the composites.     

Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys

A micromechanical model has been developed for predicting the time-dependent crack growth threshold and its variability by considering oxide formation or cavity formation ahead of an elastic crack subjected to a sustained load at a stress intensity factor, K, at elevated temperatures in air. It is demonstrated that stress relaxation associated with a volume-expansion process such as the formation of creep cavities or oxides with a positive transformation strain can induce residual stresses at the tip of the elastic crack. The near-tip residual stresses must be overcome by the external load, thereby instigating a growth threshold, K _th, for the onset of time-dependent crack growth. This micromechanical framework provides the basis for developing appropriate predictive models for the time-dependent crack growth thresholds associated with several damage processes, including (1) oxidation-assisted intergranular crack growth, (2) K-controlled creep crack growth along an intergranular path, and (3) stress corrosion cracking. The micromechanical threshold models have been utilized to predict the time-dependent crack growth thresholds of a variety of Ni-base superalloys. The material parameters that contribute to the variability of the time-dependent crack growth thresholds have been identified and related to variations of mixed oxides or creep cavities formed near the crack tip. A size scale effect is also predicted for the transformation toughening phenomenon, which is largest at or below K _th but diminishes at increasing K levels above the threshold. Finally, the micromechanical models are utilized to identify means for suppressing time-dependent crack growth in Ni-base alloys.     

Microstructure Development,Nanomechanical, and Dynamic Compression Properties of Spark Plasma Sintered TiB2-Ti-Based Homogeneous and Bi-layered Composites

The growing threats due to increased use of small-caliber armor piercing projectiles demand the development of new light-weight body armor materials. In this context, TiB₂ appears to be a promising ceramic material. However, poor sinterability and low fracture toughness remain two major issues for TiB₂. In order to address these issues together, Ti as a sinter-aid is used to develop TiB₂-(x wt pct Ti), (x = 10, 20) homogeneous composites and a bi-layered composite (BLC) with each layer having Ti content of 10 and 20 wt pct. The present study uniquely demonstrates the efficacy of two-stage spark plasma sintering route to develop dense TiB₂-Ti composites with an excellent combination of nanoscale hardness (~36 GPa) and indentation fracture toughness (~12 MPa m^1/2). In case of BLC, these properties are not compromised w.r.t. homogeneous composites, suggesting the retention of baseline material properties even in the bi-layer design due to optimal relief of residual stresses. The better indentation toughness of TiB₂-(10 wt pct Ti) and TiB₂-(20 wt pct Ti) composites can be attributed to the observed crack deflection/arrest, indicating better damage tolerance. Transmission electron microscope investigation reveals the presence of dense dislocation networks and deformation twins in α-Ti at the grain boundaries and triple pockets, surrounded by TiB₂ grains. The dynamic strength of around 4 GPa has been measured using Split Hopkinson Pressure Bar tests in a reproducible manner at strain rates of the order of 600 s^?1. The damage progression under high strain rate has been investigated by acquiring real time images for the entire test duration using ultra-high speed imaging. An attempt has been made to establish microstructure-property correlation and a simple analysis based on Mohr–Coulomb theory is used to rationalize the measured strength properties.     

Evaluation of toughness in AISI 4340 alloy steel austenitized at low and high temperatures

It has been reported for as-quenched AISI 4340 steel that high temperature austenitizing treatments at 1200°C, instead of conventional heat-treatment at 870°C, result in a two-foldincrease in fracture toughness,K _Ic, but adecrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference betweenK _Ic and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of high temperature austenitizing be questioned as a practical heat-treatment procedure for ultrahigh strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely onK _Ic or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required.     

R-curve behaviour of Al2O3 ceramics

The fracture micromechanics and underlying physical processes of fracture in Al₂O₃-based ceramic specimens have been studied as a function of grain size by instrumented in situ dynamic scanning electron microscopy (SEM) using the double torsion technique. The toughness is found to increase with grain size. Crack bridging is found to extend over hundreds of grain diameters behind the crack tip, resulting in R-curve behaviour. Evidence is amassed which points to frictional energy dissipation, rather than distrubuted microcracking or crack-closure due to elastic ligaments, as the dominant contribution to toughening. The friction occurs at grains which bridge the crack faces and are pulled out as the faces separate. Restraining stresses, which constrain the bridging grains in their sockets, are believed to be the result of both grain morphology and the thermal expansion anisotropy of the material. Simple modelling indicates that only a few percent of the grains need be involved in the frictional process to account for the toughening. The conclusion is supported by hysteresis measurements.     

Fracture toughness and fatigue behavior of matrix II and M-2 high speed steels

The effects of heat treatment and of the presence of primary carbides on the fracture toughness,K _Ic and the fatigue crack growth rates,da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values ofK _Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix.     

Understanding the Interdependencies Between Composition,Microstructure, and Continuum Variables and Their Influence on the Fracture Toughness of α/β-Processed Ti-6Al-4V

The fracture toughness of a material depends upon the material’s composition and microstructure, as well as other material properties operating at the continuum level. The interrelationships between these variables are complex, and thus difficult to interpret, especially in multi-component, multi-phase ductile engineering alloys such as α/β-processed Ti-6Al-4V (nominal composition, wt pct). Neural networks have been used to elucidate how variables such as composition and microstructure influence the fracture toughness directly (i.e., via a crack initiation or propagation mechanism)—and independent of the influence of the same variables influence on the yield strength and plasticity of the material. The variables included in the models and analysis include (i) alloy composition, specifically, Al, V, O, and Fe; (ii) materials microstructure, including phase fractions and average sizes of key microstructural features; (iii) the yield strength and reduction in area obtained from uniaxial tensile tests; and (iv) an assessment of the degree to which plane strain conditions were satisfied by including a factor related to the plane strain thickness. Once trained, virtual experiments have been conducted which permit the determination of each variable’s functional dependency on the resulting fracture toughness. Given that the database includes both K _{1 C} and K _Q values, as well as the in-plane component of the stress state of the crack tip, it is possible to quantitatively assess the effect of sample thickness on K _Q and the degree to which the K _Q and K _{1 C} values may vary. These interpretations drawn by comparing multiple neural networks have a significant impact on the general understanding of how the microstructure influences the fracture toughness in ductile materials, as well as an ability to predict the fracture toughness of α/β-processed Ti-6Al-4V.     

The tensile deformation and fracture of Al-“Saffil” metal-matrix composites

In this paper we have studied the tensile deformation and fracture of aluminium alloy composites containing “Saffil” δ-Al₂O₃ fibres. The Bauschinger effect has been used to measure internal stress and shows that the tensile behaviour of these materials is determined by the development of these stresses due to the plastic flow of the matrix, differences in elastic constants of the two phases and residual thermal stresses developed during fabrication. Good agreement is obtained with the theoretical predictions derived in an earlier paper. Fracture is observed to occur by the growth of cracks from failed fibres until a crack large enough to nucleate catastrophic failure is formed.     

The decohesion of thin films from brittle substrates

Experimental observations have been made concerning the decohesion of Cr films from glass substrates. The observations reveal that the films first split, the cracks then extend into the substrate and eventually acquire a steady-state trajectory parallel to the interface. A comparison of fracture mesurements with mechanics solutions for substrate cracks affirms a prior postulate that steady-state cracks grow along the plane for which K_II = 0. Evaluations of K_I, on that plane, deduced from measurements of crack velocities (in a controlled humidity environment) are also in good agreement with predicted values.     

Ceramic/metal interfacial crack growth: Toughening by controlled microcracks and interfacial geometries

Toughening of ceramic/metal interfaces through the use of controlled interfacial geometries and non-coplanar microcrack-like pores is examined with respect to both critical and subcritical crack growth. Patterned uniform arrays of inclined interfacial steps and of “microcracks/voids” (with width 22 μm and spacing 10 μm), out-of-plane to the main interfacial crack, were produced for glass/copper interfaces by photo-lithographic techniques combined with evaporation and diffusion bonding processes. Significant toughening and improved stress corrosion crack-growth resistance is achieved through the promotion of crack-tip shielding primarily from crack bridging. Specifically, plastic void growth within the copper is seen to generate bridged ligaments of metal film between the glass substrates; the resulting mechanical crack bridging leads to plastic stretching of the film and provides the dominant toughening mechanism, with a smaller contribution from crack deflection. Correspondingly, subcritical (pre-instability) crack-growth rates with the patterned arrays in “wet” and “dry” gaseous atmospheres are retarded by orders of magnitude compared to rates for plain interfaces. The toughness with the various patterned interfaces exhibits marked resistance-curve (R-curve) behavior with fracture toughness values increased by factors of 4–9 compared to intrinsic fracture toughness, G₀, values of ∼2 J/m² for these plain glass/copper interfaces. Surface roughness of the glass substrate is reasoned to be a controlling parameter for the shape and magnitude of such crack-resistance curves.     

Microstructure and fracture of 52100 steel

The fracture behavior of 52100 steel hardened and tempered to R_C62 has been investigated as a function of austenitizing over the temperature range from 800 to 1100°C. Specimens were homogenized at 1150°C and either furnace cooled or isothermally transformed at 580°C to produce a pearlitic microstructure prior to austenitizing for hardening. Furnace-cooled specimens developed a proeutectoid carbide network that did not dissolve during subsequent austenitizing below A_cm . The residual proeutectoid carbides and the carbide-free martensite-austenite structure between them controlled fracture and produced K_IC of 19 MPa \ m^1/2, the highest determined in this investigation. The specimens isothermally transformed prior to austenitizing below A_cm produced a microstructure of fine spherical carbides dispersed throughout a fine martensitic matrix and did not contain residual proeutectoid carbides. The transgranular fracture of the latter specimens by microvoid coalescence around the closely spaced spherical carbides resulted in the lowest values of fracture toughness, 14 to 16 MPa\ m^1/2, determined in these experiments. Austenitizing above A_cm caused solution of all carbides, a gradual coarsening of the austenitic grain size, a transition to plate martensite, and an increase in retained austenite. Fracture toughness increased slightly with increasing austenitizing temperature above A_cm despite the fact that fracture propagated primarily along the austenitic grain boundaries. The improved fracture toughness, verified by scanning electron microscopy of the fatigue crack-overload fracture interface, is believed to be caused in part by transgranular crack propagation during the first stages of crack extension that are most important in determining K_1C.