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The mechanism of oxidation-assisted growth of surface cracks during fatigue with compressive holds has been studied experimentally and via a model that describes the role of oxide and substrate properties. The creep-based finite element model has been employed to examine the role of material parameters in the damage evolution in a Ni-base single-crystal superalloy Rene N5. Low-cycle fatigue experiments with compressive holds were conducted at 1255 K and 1366 K (982 °C and 1093 °C). Interrupted and failed specimens were characterized for crack depth and spacing, oxide thickness, and microstructural evolution. Comparison of experimental to modeled hysteresis loops indicates that transient creep drives the macroscopic stress–strain response. Crack penetration rates are strongly influenced by growth stresses in the oxide, structural evolution in the substrate, and the development of $$\gamma ^{\prime }$$ denuded zones. Implications for design of alloys resistant to this mode of degradation are discussed.

Growth stresses that develop in α-Al2O3 scale that form during isothermal oxidation of three Ni-based single crystal alloys have been studied to elucidate their role in coating and substrate degradation at elevated temperatures. Piezospectroscopy measurements at room temperature indicate large room temperature compressive stresses in the oxides formed at 1255 K or 1366 K (982 °C or 1093 °C) on the alloys, ranging from a high of 4.8 GPa for Rene N4 at 1366 K (1093 °C) to a low of 3.8 GPa for Rene N5 at 1255 K (982 °C). Finite element modeling of each of these systems to account for differences in coefficients of thermal expansion of the oxide and substrate indicates growth strains in the range from 0.21 to 0.44 pct at the oxidation temperature, which is an order of magnitude higher than the growth strains measured in the oxides on intermetallic coatings that are typically applied to these superalloys. The magnitudes of the growth strains do not scale with the parabolic oxidation rate constants measured for the alloys. Significant spatial inhomogeneities in the growth stresses were observed, due to (i) the presence of dendritic segregation and (ii) large carbides in the material that locally disrupts the structure of the oxide scale. The implications of these observations for failure during cyclic oxidation, fatigue cycling, and alloy design are considered.

The influence of acid strength was evaluated toward the selectivity to propene on conversion of 1-pentene. For the catalytic cracking of 1-pentene, the main reaction pathways and the molar ratio of propene to ethene (P/E ratio) were controlled by acid strength with the appropriate amount of total acid sites. The results showed that the P/E ratio increased with decreasing amounts of strong acid sites, since the activation energies of individual reaction pathways were influenced by acid strength to a different extent. Strong acid sites could promote pathway I′ (C52– → C22– + C32–) and pathway II′-1 (C62– → C22– + C42–), while weak acid sites preferred pathway II′ (2C52– → C10+ → C42– + C62–) and pathway II′-2 (C62– → 2C32–), since pathways II′ and II′-2 underwent some energetically favorable routes (tertiary–secondary, secondary–secondary) of carbenium ion intermediates. By manipulation of the acid strength distribution on ZSM-5, the P/E ratio and selectivity of propene could be significantly improved, sugg...

A flexural test for the in situ measurement of the delamination toughness of the interface between a thermally grown oxide and a bond coat in the presence of a thermal barrier coating (TBC) has been implemented. To accomplish the testing, a section of the substrate was removed by microelectro-discharge machining and a precrack introduced through the TBC by center point loading. This was followed by application of an asymmetric single-point load to extend a delamination along the interface. A displacement and strain mapping method was used to locate the delamination and to ascertain its extension. To relate the energy release rate and mode mixity to the crack extension and the loads, a finite-element method was implemented. The ensuing fracture resistance was found to vary along the interface with values in the range of 25-95 J/m(2).

The role of oxidation-induced layers in the failure process of aluminide-coated nickel base single crystals subject to high-temperature fatigue cycling has been investigated experimentally and via finite element analysis. Isothermal strain-controlled compressive fatigue experiments (R=−∞) with 120 s holds in compression were conducted at 982° and 1093°C. Surface-initiated cracks containing a layer of alumina progressively grew through the coating layers into the superalloy substrate, ultimately causing failure. Growth stresses in the oxide provided a driving force for extension of the oxide into the softer coating and substrate layers. Finite element modeling shows the rate of growth of the oxide-filled cracks is sensitive to the strength of the constituent layers and the magnitude of the oxide growth strains. Implications for design of failure-resistant coating–substrate systems are discussed.

Composite bimaterial lattice structures which possess both low, tailorable thermal expansion and nearly optimal stiffness have been proposed for applications which require high structural stiffness in environments which include large temperature fluctuations, such as the surfaces of high-speed aerospace vehicles. An experimental validation of the thermal properties of these lattices when they are constructed of practical materials with easily manufactured bonded joints is contained herein. Bonded lattices, comprising aluminum and titanium alloys, have been manufactured with press-fit dovetail joints and tested in a variety of thermal environments. Results for equilibrium heating, rapid transient heating and thermal cycling leading to shakedown are presented and shown to be consistent with theoretically and numerically attained results.

Irradiation hardening produces increases in the cleavage transition fracture toughness reference temperature (Δ T 0 ). It is traditional to relate Δ T 0 to the corresponding changes in the yield stress, Δ σ y , as C 0 = Δ T 0 /Δ σ y . However, it is a strain-hardened flow stress, σ fl , in the fracture process zone that controls cleavage, rather than σ y . Thus irradiation induced decreases in the strain hardening Δ σ sh ( σ y (> 0) in evaluating Δ T 0 . The Δ σ sh in reactor pressure vessel (RPV) steels irradiated to low doses at around 300 °C are small, even for large Δ σ y . However, the Δ σ sh are much greater for high dose irradiations of tempered martensitic steels (TMS) that are candidates for fusion applications. As a result, for the TMS case, the C 0 are less, and in some instances much less, than for RPV steels and irradiation conditions. We address two key questions. First, how does Δ σ sh influence the C 0 = Δ T 0 /Δ σ y relation? Second, is it possible to derive a universal relation between Δ T 0 and Δ σ fl averaged over a pertinent range of e , 〈Δ σ fl 〉, such that a C 0′ = Δ T 0 /〈Δ σ fl 〉 is independent of the individual values of Δ σ y and Δ σ sh ? The results of this study suggest that 〈Δ σ fl 〉 averaged between 0 and 0.1 provides a similar C 0′ for various assumptions about the effect of irradiation on Δ σ sh . Notably, changes in indentation hardness, Δ H , are also directly related to this same 〈Δ σ fl 〉. Hence, measurements of Δ H should provide a good basis for assessing Δ T 0 for a wide range of alloys and irradiation conditions.

A new indentation hardness (H) approach to evaluating the true stress (σ)–true plastic strain (e) constitutive behavior of materials is described. Extensive elastic–plastic finite element (FE) simulations were carried out to assess the relation between H and σ(e). The analysis led to derivation of a universal relation between H and σ(e) given by H ≈ 4.05(1–34.6 σflow/E)σflow, where σflow = σy + 〈σsh〉, 〈σsh〉 is the average strain hardening between e = 0 and 0.1, and E is the elastic modulus. Experimental H–σflow data pairs for the large set of alloys are in reasonably good agreement with the model predictions, but the experimental σflow are slightly higher and more linearly related to H than predicted. The H–σflow relation provides insight into the large variation of the H/σy ratios that are observed for different materials, as well as the corresponding variation in the ΔH/Δσy ratios used to estimate Δσy due to changes in an alloy’s condition, such as that induced by irradiation, based on measurements of ΔH. Notably, combinations of tensile and hardness tests can be use to estimate the average strain hardening from e = 0 to 0.1 in irradiated alloys that have very small uniform strains.

Metallic sandwich panels with periodic lattice cores exhibit superior strength and blast resistance relative to monolithic plates of equivalent weight. Their implementation into engineering structures requires computationally-efficient analysis and design codes that properly account for the complex deformation modes within such cores. The objective of the present article is to devise and assess a continuum constitutive law and to demonstrate its application to the elastic–plastic response of one particular topology: the square honeycomb. The law is based on Hill's yield criterion for orthotropic materials, modified to account for the effects of mean stress and the associated compressibility upon plastic straining. Parameters characterizing initial yield are obtained from both approximate stress analyses and finite element calculations of unit cells. Finite element calculations are also used to calibrate the hardening. Once calibrated, the law is used to simulate the bending response of various sandwich pa...

Finite element modeling and linear elastic fracture mechanics are used to model the residual stresses and failure stress of ceramic composites consisting of polyhedral alumina cores surrounded by thin alumina/mullite layers in residual compression. This type of composite architecture is expected to exhibit isotropic threshold strength behavior, in which the strength of the composite for a particular assumed flaw will be constant and independent of the orientation of tensile loading. The results of the modeling indicate that the strengths of such architectures will be higher than those of laminates of similar architectural dimensions that were previously found to exhibit threshold strength behavior for a particular flaw type. Flexural testing of the polyhedral architectures reveals that failure is dominated by processing defects found at junctions between the polyhedra. Fractography revealed the interaction of these defects with the residual stresses in the compressive layers that separate the polyhedra.

We describe the highly efficient master curves–shifts (MC–ΔT) method to measure and apply cleavage fracture toughness, KJc(T), data and show that it is applicable to 9Cr martensitic steels. A reference temperature, T0, indexes the invariant MC shape on an absolute temperature scale. Then, T0 shifts (ΔT) are used to account for various effects of size and geometry, loading rate and irradiation embrittlement (ΔTi). The paper outlines a multiscale model, relating atomic to structural scale fracture processes, that underpins the MC–ΔT method. At the atomic scale, we propose that the intrinsic microarrest toughness, Kμ(T), of the body-centered cubic ferrite lattice dictates an invariant shape of the macroscopic KJc(T) curve. KJc(T) can be modeled in terms of the true stress–strain (σ–e) constitutive law, σ (T,ϵ), combined with a temperature-dependent critical local stress, σ*(T) and stressed volume, V*. The local fracture properties, σ*(T)–V*, are governed by coarse-scale brittle trigger particles and Kμ(T). Irradiation (and high strain rate) induced increases in the yield stress, Δσy, lead to ΔTi, with typical ΔTi/Δσy≈0.6±0.15 °C/MPa. However, ΔTi associated with decreases in σ* and V* can result from a number of potential non-hardening embrittlement (NHE) mechanisms, including a large amount of He on grain boundaries. Estimates based on available data suggest that this occurs at >500–700 appm bulk He. Hardening and NHE are synergistic, and can lead to very large ΔTi. NHE is signaled by large (>1 °C/MPa), or even negative, values of ΔTi/Δσy (for Δσy 1 and Δc/Δy≫1. Indeed, in some circumstances, the benefits of irradiation due to increases in Pc may more than offset the liabilities of the decreases in Δc.

High-melting-point alkylammonium tetrafluoroborates melt with water or toluene to form biphasic mixtures at much lower temperatures than their normal melting points, thus could serve as green reaction media for Suzuki and Heck couplings. Advantages of using the aqueous-ammonium biphasic catalytic system include: (1) working well with a ligandless palladium catalyst, (2) solving the solubility problem of organic substrates and (3) simplifying work-up procedures for both separation of products and recycling of reaction media.

Factor Xa (fXa) is an important serine protease that holds the central position linking the intrinsic and extrinsic activation mechanisms in the blood coagulation cascade. Therefore, inhibition of fXa has potential therapeutic applications in the treatments of both arterial and venous thrombosis. Herein we describe a series of tetrazole fXa inhibitors containing benzamidine mimics as the P 1 substrate, of which the aminobenzisoxazole moiety was found to be the most potent benzamidine mimic. SR374 ( 12 ) inhibits fXa with a K i value of 0.35 nM and is very selective for fXa over thrombin and trypsin.

New innovative small specimen test techniques are described. Finite element simulations show that combinations of cone indentation pile-up geometry and load–penetration depth relations can be used to determine both the yield stress and strain-hardening behavior of a material. Techniques for pre-cracking and testing sub-miniaturized fracture toughness bend bars, with dimensions of 1.65×1.65×9 mm3, or less, are described. The corresponding toughness–temperature curves have a very steep transition slope, primarily due to rapid loss of constraint, which has advantages in some experiments to characterize the effects of specified irradiation variables. As one example of using composite specimens, an approach to evaluating helium effects is proposed, involving diffusion bonding small wires of a 54Fe-based ferritic–martensitic alloy to a surrounding fracture specimen composed of an elemental Fe-based alloy. Finally, we briefly outline some potential approaches to multipurpose specimens and test automation.

The effects of constitutive and local fracture properties on constraint loss effects in fracture toughness tests of small specimens was carried out within the framework of the MC-Δ T method. Constraint loss (CL) decreases the temperature at a specified reference toughness. This temperature shift increases with decreasing: (a) specimen size, (b) ratio of the critical cleavage stress to yield stress and (c) strain hardening. The toughness–temperature curve shift due to CL increases with higher reference toughness and reference toughness–temperature. These results can guide the development and interpretation of small specimen fracture tests, as well as the use of even smaller specimens for particular applications, like comparative studies of irradiation variable effects on embrittlement. While they are only briefly noted in this work, additional consideration of both statistical and three dimensional size effects will be carried out in the future.

Multiscale processes control the true [σ(e)] and engineering [s(e)] stress–strain behavior of alloys. Strain hardening in unirradiated bcc alloys is modeled as a competition between production and annihilation of stored dislocations. Large increases in yield stress (σy) following irradiation are accompanied by loss of uniform engineering strain (eu). All major features of the tensile test, including the effect of irradiation, can be modeled using finite element (FE) methods and a self-consistent σ(e) that differs markedly from s(e). The irradiated σ(e) reflect large increases in σy and reductions in strain hardening. The very low tensile eu following irradiation is due to enhanced continuum necking instabilities as a consequence of the intrinsic property changes. However, large elevations of σ(e) persist up to very high strains. Homogeneous deformation constitutive and plasticity theory can be used in continuum FE modeling of irradiated alloys. On a mesoscopic scale, FE simulations indicate that the irradiated σ(e) may be linked to an array of severely strain softening shear bands embedded in an irradiation hardened and strain hardening matrix.

Previous analyses have shown that refractory metallic leading edges are feasible for Mach 6–8 airbreathing hypersonic vehicles when integrated with a structural heat pipe. This article investigates the effects of a coating on the exterior surface for potential thermal and environmental protection. Two conclusions are found: (i) Adding a thermal barrier coating does not allow operation at higher Mach number because the characteristic temperature of the system is unaffected by its presence. Instead, large (adverse) temperature gradients are induced through the coating. (ii) Coatings required to protect against oxidation can be safely applied provided that they are thin (≤0.1 mm) and have sufficiently high thermal conductivity (≥5 W/mK).

A finite element analysis was performed to determine the influence of microstructural inhomogeneities on the compressive and shear strength of 2-D carbon-carbon laminates. In particular, the effects of amplitude and distribution of the fiber bundle crimps, and lengths and distribution of the interply and transverse bundle cracks are determined. The results of the numerical study are successfully compared with experimental observations made in an earlier study under compression and shear loading. Under a warp-aligned compression load, the local material parameter which controls the overall instability in the form of multiple interply delaminations is the critical energy release rate for growth of pre-existing interply cracks, except for when the maximum length of these cracks is less than 0.6 mm when the failure is through bundle kink-induced shear faulting. For all pre-existing crack lengths, the same parameter was responsible for initiating a planar shear fault following the crimp boundaries under an interlaminar shear loading. Accordingly, the numerical cell-model considered here conforms to the above mechanism by allowing the participation of the energy release rate as the failure parameter.

Fiber-reinforced ceramic-matrix composites (CMCs) depend upon inelastic mechanisms to diffuse stress concentrations associated with holes, notches, and cracks. These mechanisms consist of fiber debonding and pullout, multiple matrix cracking, and shear band formation. In order to understand these effects, experiments have been conducted on several double-edge-notched CMCs that exhibit different stress redistribution mechanisms. Stresses have been measured and mechanisms identified by using a combination of methods including X-ray imaging, edge replication, and thermoelastic analysis. Multiple matrix cracking was found to be the most effective stress redistribution mechanism.

Hypersonic flight with hydrocarbon-fueled airbreathing propulsion requires sharp leading edges. This generates high temperatures at the leading edge surface, which cannot be sustained by most materials. By integrating a planar heat pipe into the structure of the leading edge, the heat can be conducted to large flat surfaces from which it can be radiated out to the environment, significantly reducing the temperatures at the leading edge and making metals feasible materials. This paper describes a method by which the leading edge thermal boundary conditions can be ascertained from standard hypersonic correlations, and then uses these boundary conditions along with a set of analytical approximations to predict the behavior of a planar leading edge heat pipe. The analytical predictions of the thermostructural performance are verified by finite element calculations. Given the results of the analysis, possible heat pipe fluid systems are assessed, and their applicability to the relevant conditions determined. The results indicate that the niobium alloy Cb-752, with lithium as the working fluid, is a feasible combination for Mach 6–8 flight with a 3 mm leading edge radius.

A new series of pyrazole-based factor Xa inhibitors have been identified as part of our ongoing efforts to optimize previously reported clinical candidate razaxaban. Concern over the possible formation of primary aniline metabolites via amide hydrolysis led to the replacement of the primary amide linker between the pyrazole and phenyl moieties with secondary amides. This was accomplished by replacing the aniline with a variety of heterobicycles, of which indolines were the most potent. The indoline series demonstrated subnanomolar factor Xa binding Kis, modest to high selectivity versus other serine proteases, and good in vitro clotting activity. A small number of indoline fXa inhibitors were profiled in a dog pharmacokinetic model, one of which demonstrated pharmacokinetic parameters similar to that of clinical candidate razaxaban.

Sharp leading edges on hypersonic vehicles experience very large heating loads and consequent high temperatures. One strategy for for accommodating these effects is to provide very high effectively thermal conductivity which allows heat to be transferred from the hot leading edge to large cool surfaces for radiation into space. Heat pipes integrated within metallic leading edges provide this function, as well as being easy to manufacture and highly robust compared to other material choices. This paper will examine the feasibility of metallic leading edge heat pipes for hypersonic vehicles in Mach 7 flight. Using temperatures and heat fluxes calculated elsewhere, analytic approximations of the temperature distributions and stresses in a prototypical system are analyzed. The analysis is supplemented and confirmed by finite element calculations. Feasibility of the system is assessed by simple calculations on the operational limits of heat pipes.Copyright © 2007 by ASME

Attempts to further optimize the pyrazole factor Xa inhibitors centered on masking the aryl aniline P4 moiety. Scaffold optimization resulted in the identification of a novel bicyclic pyrazolo-pyridinone scaffold which retained fXa potency. The novel bicyclic scaffold preserved all binding interactions observed with the monocyclic counterpart and importantly the carboxamido moiety was integrated within the scaffold making it less susceptible to hydrolysis. These efforts led to the identification of 1-[3-aminobenzisoxazol-5'-yl]-3-trifluoromethyl-6-[2'-(3-(R)-hydroxy-N-pyrrolidinyl)methyl-[1,1']-biphen-4-yl]-1,4,5,6-tetrahydropyrazolo-[3,4-c]-pyridin-7-one 6f (BMS-740808), a highly potent (fXa Ki=30 pM) with a rapid onset of inhibition (2.7x10(7) M-1 s-1) in vitro, selective (>1000-fold over other proteases), efficacious in the AVShunt thrombosis model, and orally bioavailable inhibitor of blood coagulation factor Xa.

Modification of a series of pyrazole factor Xa inhibitors to incorporate an aminobenzisoxazole as the P(1) ligand resulted in compounds with improved selectivity for factor Xa relative to trypsin and plasma kallikrein. Further optimization of the P(4) moiety led to compounds with enhanced permeability and reduced protein binding. The SAR and pharmacokinetic profile of this series of compounds is described herein. These efforts culminated in 1-(3'-aminobenzisoxazol-5'-yl)-3-trifluoromethyl-N-[2-fluoro-4-[(2'-dimethylaminomethyl)imidazol-1-yl]phenyl]-1H-pyrazole-5-carboxyamide (11d), a potent, selective, and orally bioavailable inhibitor of factor Xa. On the basis of its excellent in vitro potency and selectivity profile, high free fraction in human plasma, good oral bioavailability, and in vivo efficacy in antithrombotic models, the HCl salt of this compound was selected for clinical development as razaxaban (DPC 906, BMS-561389).

Factor Xa is an important serine protease in the blood coagulation cascade. It generates thrombin and holds the central position that links the intrinsic and extrinsic activation mechanism in the final common pathway of coagulation. Therefore, inhibition of factor Xa has potential therapeutic applications in the treatment of both arterial and venous thrombosis. We have designed and synthesized a series of bisbenzamidine isoxazoline derivatives as factor Xa inhibitors. The most potent compound in this series has a Ki of 18 nM against factor Xa.

Finite element simulations of ball indentation tests were performed and analyzed using the automated ball indentation method. The accuracy and reliability of this methods were assessed.

The strain energy release rates for the converging decohesion crack in a multilayered film on a substrate have been calculated using the finite element method. The results for the energy release rate as a function of the intrinsic stress, the thickness of the superlayer and the modulus ratio will be presented. A simple functional form for the results will be shown. The effects of plasticity of the thin metal layer on the energy release rate have been examined. The results show that the effect of plastic deformation is not significant for the converging decohesion crack. The effects of the line width have also been addressed. The results show that for two-layer films the energy release rate for steady-state decohesion cracks decreases dramatically as B/h decreases, in the range B/h

One of the key factors affecting the long-term reliability of thin film structures in microelectronics devices is the fracture resistance of their interfaces. Of the many different interfaces formed during device fabrication, the SiO2/TiN interface is amongst the most common. In order to measure the fracture energy of this interface, we have been utilizing a test in which a narrow strip of a material, having a large intrinsic tensile stress, is deposited and lithographically defined onto the SiO2/TiN/Al thin film structure. The elastic strain energy in the micro-strip provides an additional driving force to debond the weakest interface in the structure. Decohesion occurs when the thickness of the micro-strip, and hence the total elastic strain energy, exceeds a critical value. Another measure of the fracture resistance in the same test is the length of the strip that remains intact after debonding at both ends. This is a function of the intrinsic stress, the thickness of the superlayer, and the modulus ratios. In addition to presenting results on the measured fracture energy of the SiO2/TiN interface, evidence for stress-corrosion will be described.

In this paper the compression and shear failure of laminated carbon-carbon composites is numerically analyzed using a cell model. The effect of microstructural inhomogeneities in the form of cracks and fiber bundle misalignments is investigated by calculating the energy release rates and the local stress strain distributions.

By using the elastic singular field as a prescribed loading condition, small scale yielding solutions are obtained for a crack normal to the interface between a brittle and a ductile material. Results for both a crack in the brittle material and one in the ductile material are obtained by finite element analysis. The crack tip fields obtained by the finite element analysis are compared with the asymptotic solutions. It is found that near the tip the stress fields approach the asymptotic solutions. If the crack is in the brittle material, the high triaxial stresses are developed near the interface ahead of the crack tip.

Investigation of the flow properties of materials containing rigid reinforcements has been prompted by various technological challenges concerned with high specific strength composites. The materials of interest range from metallic materials such as Al alloys, to ceramic materials such as Si3N4 and also include the intermetallics: TiAl and MoSi2. The reinforcements are typically A12O3 and SiC. The strengths of interest may be the flow strength at ambient temperature or the creep strength. The present analysis constitutes an attempt to provide estimates of either the flow or creep strength for matrices that exhibit power law deformation.

Analyses of debonding along interfaces and of the kinking of interface cracks into a fiber have been used to define the role of debonding in fiber-reinforced, brittle matrix composites. The results reveal that, for fibers aligned with the tensile stress axis, debonding requires an interface fracture energy, Γi, less than about one-fourth that for the fiber, Γf. Further-more, once this condition is satisfied, it is shown that fiber failure does not normally occur by deflection of the debond through the fiber. Instead, fiber failure is governed by weakest-link statistics. The debonding of fibers inclined to the stress axis occurs more readily, such that debonds at acutely inclined fibers can deflect into the fiber, whereupon the failure of fibers is dominated by their toughness.