Fracture processes and mechanisms of crack growth resistance in human enamel

Human enamel has a complex micro-structure that varies with distance from the tooth’s outer surface. But contributions from the microstructure to the fracture toughness and the mechanisms of crack growth resistance have not been explored in detail. In this investigation the apparent fracture toughness of human enamel and the mechanisms of crack growth resistance were evaluated using the indentation fracture approach and an incremental crack growth technique. Indentation cracks were introduced on polished surfaces of enamel at selected distances from the occlusal surface. In addition, an incremental crack growth approach using compact tension specimens was used to quantify the crack growth resistance as a Junction of distance from the occlusal surface. There were significant differences in the apparent toughness estimated using the two approaches, which was attributed to the active crack length and corresponding scale of the toughening mechanisms.     

Aging and fracture of human cortical bone and tooth dentin

Mineralized tissues, such as bone and tooth dentin, serve as structural materials in the human body and, as such, have evolved to resist fracture. In assessing their quantitative fracture resistance or toughness, it is important to distinguish between intrinsic toughening mechanisms, which function ahead of the crack tip, such as plasticity in metals, and extrinsic mechanisms, which function primarily behind the tip, such as crack bridging in ceramics. Bone and dentin derive their resistance to fracture principally from extrinsic toughening mechanisms, which have their origins in the hierarchical microstructure of these mineralized tissues. Experimentally, quantification of these toughening mechanisms requires a crack-growth resistance approach, which can be achieved by measuring the crack-driving force (e.g., the stress intensity) as a function of crack extension (“R-curve approach”). Here this methodology is used to study the effect of aging on the fracture properties of human cortical bone and human dentin in order to discern the microstructural origins of toughness in these materials.     

Metallurgical stability and the fracture behavior of ferritic stainless steels

The influence of aluminum on the thermal stability of ferritic stainless steels has been investigated using two commercial alloys— Armco Type 18SR and AISI430. Two reaction stages have been detected in these alloys during aging at 475 °; each stage is accompanied by changes in the hardness, yield strength, strain-hardening exponent, and elongation to fracture. The initial stage is attributed to the precipitation of carbide and nitride particles and the second stage to the precipitation of the chromium- rich a’ phase. The 430 alloy exhibits more pronounced changes than 18SR during the first stage due to the higher concentration of interstitials retained in solution after quenching. The effects of the second- stage aging reaction are detected after shorter aging times in the 18SR alloy and are more pronounced than in the 430 alloy, consistent with the influence of aluminum on the coherency strains associated with a’ precipitation. The fracture mechanism in both alloys changes from ductile dimples in the solution- treated and quenched condition to a mix of ductile dimples, intergranular fracture, and transgranular cleavage with increased aging times. Longitudinal cracking at the grain boundaries precedes failure of the aged alloys in tension; it is attributed to the combined effects of void initiation at fine grain boundary precipitates, a’ embrittlement that limits localized plasticity, and the transverse stress components resulting from triaxiality after the onset of necking.     

Modeling of stress ratio effect on Al alloy SAE AMS 7475-T7351: Influence of loading direction

The main goal of this work was to evaluate the effectiveness of Walker’s equation in collapsing the fatigue crack propagation data of a SAE AMS 7475-T7351 aluminum alloy loaded either longitudinally (L-T) or transversely (T-L) to the rolling direction. T-L orientation testpieces presented lower ductility and fracture toughness values than L-T orientation. As a consequence, during the fatigue crack propagation tests, T-L testpieces exhibited a stronger influence of monotonic modes of fracture, resulting in higher Paris exponent values,m. Walker’s model was able to collapse fatigue crack propagation data of L-T test pieces at different applied stress ratios,R. However, for the T-L orientation, due to theR ratio dependency onm andC, simply averaging ofm values for the calculations of Walker’s exponent proved to be inefficient. A simple analytical procedure was proposed by the authors to modify Walker’s model to take into account such effect. For T-L test pieces, when Walker’s model is modified by considering both Paris’s exponent as well the coefficient as a function of theR ratio, the fatigue crack growth data collapses within a narrow band, thus allowing predictions to be made satisfactorily. The collapsed band is even narrower if the empirical relationm=a+blogC is used instead of simple polynomial equations due to a better correlation coefficient.     

Influence of service-induced microstructural changes on the aging kinetics of rejuvenated Ni-based superalloy gas turbine blades

Rejuvenation of Ni-based superalloy gas turbine blades is widely and successfully employed in order to restore the material microstructure and properties after service at high temperature and stresses. Application of hot isostatic pressing (HIP) and re-heat treatment can restore even a severely overaged blade microstructure to practically “as-new” condition. However, certain service-induced microstructural changes might affect an alloy’s behavior after the rejuvenated blades are returned to service. It was found that advanced service-induced decomposition of primary MC carbides, and the consequent changes of the γ-matrix chemical composition during the rejuvenation, can cause a considerable acceleration of the aging process in the next service cycle. The paper will discuss the influence of the previous microstructural deterioration on the aging kinetics of rejuvenated gas turbine blades made from IN-738 and conventionally cast GTD-111 alloys.     

The Influence of Specimen Thickness on the High Temperature Corrosion Behavior of CMSX-4 during Thermal-Cycling Exposure

CMSX-4 is a single-crystalline Ni-base superalloy designed to be used at very high temperatures and high mechanical loadings. Its excellent corrosion resistance is due to external alumina-scale formation, which however can become less protective under thermal-cycling conditions. The metallic substrate in combination with its superficial oxide scale has to be considered as a composite suffering high stresses. Factors like different coefficients of thermal expansion between oxide and substrate during temperature changes or growing stresses affect the integrity of the oxide scale. This must also be strongly influenced by the thickness of the oxide scale and the substrate as well as the ability to relief such stresses, e.g., by creep deformation. In order to quantify these effects, thin-walled specimens of different thickness (t = 100−500 μm) were prepared. Discontinuous measurements of their mass changes were carried out under thermal-cycling conditions at a hot dwell temperature of 1100 °C up to 300 thermal cycles. Thin-walled specimens revealed a much lower oxide-spallation rate compared to thick-walled specimens, while thin-walled specimens might show a premature depletion of scale-forming elements. In order to determine which of these competetive factor is more detrimental in terms of a component’s lifetime, the degradation by internal precipitation was studied using scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDS). Additionally, a recently developed statistical spallation model was applied to experimental data [D. Poquillon and D. Monceau, Oxidation of Metals, 59, 409–431 (2003)]. The model describes the overall mass change by oxide scale spallation during thermal cycling exposure and is a useful simulation tool for oxide scale spallation processes accounting for variations in the specimen geometry. The evolution of the net-mass change vs. the number of thermal cycles seems to be strongly dependent on the sample thickness.     

Fracture of Open-Cell Nickel Foams Under Quasi-Static Tensile Loading

Open-cell nickel foams with average pore size of 600 μm have been subjected to room temperature tensile tests to explore their tensile properties. Using a state of the art extensometer of noncontact type, foam properties as ultimate tensile strength, yield strength, and the Young’s modulus (E) have been measured accurately. The reason behind the usage of this kind of extensometer is to avoid completely any minor deformation that might be caused by the attachment of conventional extensometer to the sample’s surface prior to testing. The function of this extensometer is based on the usage of a laser (CCD) camera that detects and records the dimensional changes as soon as the load is applied. A series of cyclic loading-unloading tests was performed to determine the foam’s Young’s modulus. The fracture behavior of foam cells was observed to be ductile. Complete separation of struts or cell walls took place successively by necking.     

The adhesion of alumina films to metallic alloys and coatings

The adherence of protective oxide scales to alloy substrates is governed by the stored elastic energy in the scale which drives delamination and the fracture resistance of the alloy oxide interface. Clearly, any modifications to the alloy or the exposure environment which decreases the former or increases the latter will improve the durability of a given system. The stored elastic energy is determined by the stress level in the scale and the scale thickness. The stress state in the scale is determined by stresses which arise during the oxide formation (Growth Stresses), stresses produced during temperature changes as the result of thermal expansion mismatch between the oxide and the alloy (Thermal Stresses), and any stress relaxation which occurs as the result of creep of the scale or alloy. The fracture energy of the interface is a function of the composition at the interface, the microstructure in the interfacial region, and the composition of the exposure environment. This paper focuses on the results of studies of a variety of alloys and coatings, all of which form continuous alumina scales, in which it has been attempted to evaluate the effects of various alloy and exposure parameters on the stress state in the scale, the microstructure of the alloy/oxide interface, and the fracture resistance of the interface. The alloy parameters include alloy type, sulfur content, and reactive element content. The exposure parameters include oxidation temperature, temperature profile during exposure, and water vapor and sulfur contents of the atmosphere.     

The effects of artificial aging on the microstructure and fracture toughness of Al-Cu-Li alloy 2195

Aluminum-lithium alloys have shown promise for aerospace applications, and National Aeronautics and Space Administration (NASA) has selected the aluminum-lithium Alloy 2195 for the main structural alloy of the super light weight tank (SLWT) for the space shuttle. This alloy has significantly higher strength than conventional2xxx alloys (such as 2219) at both ambient and cryogenic temperatures. If properly processed and heat treated, this alloy can display higher fracture toughness at cryogenic temperature than at ambient temperature. However, the properties of production materials have shown greater variation than those of other established alloys, as is the case with any new alloy that is being transitioned to a demanding application. Recently, some commercial 2195 plates for the SLWT program were rejected, mostly due to low CFT or FTR at ambient and cryogenic temperatures. Investigation of the microstructure property relationships of Al-Cu-Li based alloys indicates that the poor fracture toughness properties can be attributed to excessive T₁ precipitation at subgrain boundaries. Lowering the aging temperature is one way to avoid excessive T₁ precipitation at subgrain boundaries. However, this approach results in a significant drop in yield strength. In addition, low-temperature aging is associated with sluggish aging kinetics, which are not desirable for industrial mass production. Therefore, the present study was undertaken to develop an aging process that can improve fracture toughness without sacrificing yield and tensile strength. A multistep heating-rate controlled (MSRC) aging treatment has been developed that can improve the cryogenic fracture toughness of aluminum-lithium Alloy 2195. At the same levels of yield strength (YS), this treatment results in considerably higher fracture toughness than that found in Alloy 2195, which has received conventional (isothermal) aging. Transmission electron microscopy revealed that the new treatment greatly reduces the size and density of subgrain-boundary T₁ precipitates. In addition, it promotes T₁ and θ" nucleation, resulting in a fine and dense distribution of precipitate particles in the matrix. The MSRC aging treatment consists of (a) aging at 127‡C (260‡F) for 5 h, (b) heating continuously from 127‡C (260‡F) to 135‡C (275‡F) at a rate of 0.556‡C/h (1‡F/h), (c) holding at 135‡C (275‡F) for 5 h, (d) heating continuously from 135 to 143‡C (275 to 290‡F) at a rate of 0.556‡C/h (1‡F/h), and (e) holding at 143‡C (290‡F) for 25 h to obtain a near peak-aged condition.     

The structure and mechanical behavior of ice

Since icebergs were first proposed as potential aircraft carriers in World War II, research has led to a better understanding of the mechanical behavior of ice. While work remains, especially in relating fracture on the small scale to that on the larger scale and to the appropriate structural features, the groundwork in materials science has been laid. This paper presents an overview of the structure and mechanical behavior of polycrystalline terrestrial ice. Editor’s Note: A hypertext-enhanced version of this paper can be found on JOM’s web site at www.tms.org/pubs/journals/JOM/9902/Schulson-9902.html. Author’s Note: The Ice Research Laboratory at Dartmouth College was founded by the author in 1983 through a development grant from Mobil Corporation. It was expanded in 1984 through an Army Research Office-URIP, expanded again in 1986 through an Office of Naval Research-URI, and expanded again in 1994 through a second Army Research Office-URIP. The IRL is a materials research facility housed within cold rooms. The laboratory currently consists of ten separate cold rooms, some capable of reaching below −40°C. Situated within are facilities for growing and characterizing ice of different kinds, preparing test specimens, and measuring mechanical and electrical properties. For more information, contact E.M. Schulson, Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755; (603) 646-2888; fax (603) 646-3856; e-mail erland.schulson@dartmouth.edu     

The Influence of Centerline Sigma (σ) Phase on the Through-Thickness Toughness and Tensile Properties of Alloy AL-6XN

The J-integral fracture toughness and tensile behavior of AL-6XN (ATI Properties Inc., Pittsburgh, PA, USA) plate material in the short-transverse (S-T) orientation was studied. The material was tested with respect to the presence or absence of microstructural ‘packets’ of brittle sigma phase (σ-phase) particles within the austenite matrix, located near the centerline of the plate. The J-integral-fracture resistance curve (J-R-curve) qualitatively indicated that the presence of the σ-phase packets is a detriment to the fracture toughness of the material in the S-T orientation. Tensile specimens containing the packets of σ-phase particles exhibited reduced yield and tensile strengths as well as pronounced reduction in ductility compared to specimens nominally free of σ-phase packets. Particle cracking was observed within the σ-phase packets, which lead to premature fracture. This limited the ability of the material to plastically deform and work-harden, thereby accounting for the observed reductions in ductility, toughness, and ultimate tensile strength.     

The effect of fe on high temperature oxidation of NiAl

A study was conducted to observe the oxidation of NiAl+3.5at.%Fe alloy inβ-NiAl phase field at air temperatures of 1000, 1200 and 1400°C, and these results were compared with those of pure NiAl alloys. The primary effects of the Fe-addition in NiAl were found to be: 1) decrease in oxidation resistance and adherence of scales during both isothermal and cyclic oxidation tests, 2) enhancement of phase transformation rate from θ toα-AL₂O₃, 3) more rapid formation of characteristically ridgedα-A1₂O₃ scales during initial oxidation stages, and 4) partial sealing of voids formed at the scale-substrate interface and dissolution of Fe inside the alumina scale by the outward diffusion of Fe from the substrate alloy.     

Determination of Some Parameters for Fatigue Life in Welded Joints Using Fracture Mechanics Method

In this work, the parameters stress intensity factor (SIF), initial and final crack lengths (a _i and a _f), crack growth parameters (C and m), and fatigue strength (FAT) are investigated. The determination of initial crack length seems to be the most serious factor in fatigue life and strength calculations for welded joints. A fracture mechanics approach was used in these calculations based on SIF which was calculated with the finite element method (FEM). The weld toe crack was determined to be equal to 0.1 mm, whereas the weld root crack’s length was varied depending on the degree of the weld penetration. These initial crack length values are applicable for all types of joints which have the same crack phenomenon. As based on the above calculated parameters, the new limits of FAT for new geometries which are not listed yet in recommendations can be calculated according to the current approach.     

Structural properties in flame quenched Cu−9Al−4.5Ni−4.5Fe alloy

The flame quenching process has been employed to modify the surfaces of a commercial marine propeller material, aluminum bronze alloy (Cu−9Al−5Ni−5Fe), and the material’s microstructure and hardness properties have been studied. The thermal history was accurately monitored during the process at various surface temperatures and holding times. XRD and EDX analyses have shown that aboveT _β temperature the microstructure consisting of α and κ phases changes into α and β’ martensite due to an eutectoid reaction of α+β→κ and a martensitic transformation of β→β’. The β’ martensite phase formed has a face-centered cubic (FCC) crystal structure with typical twinned structure. The hardness of the flame-quenched layer having the α+β’ structure is similar to or lower than that of the α+κ structure, depending highly on the size and distribution of β’ and κ phases. It is noted that the sliding wear resistance of the flame-quenched layer is enhanced with the formation of β’ martensite.     

Effects of heat treatment on the mechanical properties of SiCp/6061 Al composite

Metal-matrix composites have been receiving considerable attention as light-weight materials for use in many advanced technology applications. Silicon carbide (SiC) particles and whiskers have several advantages over other discontinuous reinforcements. Studies have shown that heat treatment can change the mechanical properties of metal-matrix composites. Modified heat treatments were developed for SiCp/6061 Al composites through a series of heat treatment with varied solution temperatures and aging time. Mechanical tests were conducted to determine the mechanical properties of the composites in three conditions; as-received, annealed, and heat treated. The modified heat treatments resulted in increases in the yield strength of up to 12% over the manufacturer’s reported yield strength for the standard T6 heat treatment. The trends which occur during heat treatment of SiCp/6061 Al are simular to those which occur during heat treatment of aluminum alloys. In addition, the relationship between the mechanical properties and the heat treatment parameters was documented. Throughout this study, the values of elastic modules were rather erratic compared to the strength values. Scanning Electron Microscope fractographic analysis revealed various fracture initiation sites, such as particle clusters and iron inclusions.     

Oxidation Behavior of In Situ-Synthesized (TiB+TiC)/Ti6242 Composites

The oxidation behaviors of TiB and TiC particle-reinforced, titanium-matrix composites (TMCs) were studied in air at 550–650°C, The oxidation kinetics follow approximately a parabolic rate law. The oxidation rates, which were lower than those of Ti6242, decrease gradually as oxidation proceeds. The oxide scales formed on TMCs were predominantly rutile and α-Al₂O₃. No B₂O₃ and other oxides were observed within the oxide scale. The in situ-synthesized TiB and TiC reinforcements can increase the oxidation resistance of TMCs. The oxide scales that formed exhibited excellent spallation resistance under all testing conditions. No scale cracking or spallation could be observed, implying that growth and thermal stresses generated during heating and cooling have been effectively released. The mechanisms of the decrease in oxidation rate and the improvement on spallation resistance are discussed based on microstructure studies.     

Nonlinear Behavior in Compression and Tension of Thermally Sprayed Ceramic Coatings

Mechanical properties of thermally sprayed coatings, especially of ceramics, are strongly influenced by a high density of mesoscopic defects, microcracks of dimensions between fractions of μm up to tens of μm. The anisotropic linear elastic stress–strain relations are valid only at very low deformations, e.g., |e| < 0.05%, with small values of Young’s moduli due to elastic openings and elastic partial closings of microcracks. At higher deformations, e.g., 0.05% < |e| < 0.4%, the stress–strain relations are strongly nonlinear. Under compressive stresses, elastic closing of microcracks leads to a gradual decrease of the microcrack density and to an increase of Young’s modulus in compression. Under tensile stresses, the microcracks slightly grow by inelastic processes; the microcrack density gradually increases and effective Young’s modulus in tension decreases. A two-parametric equation containing linear and quadratic terms is used to describe the nonlinear stress–strain curves of plasma-sprayed ceramic coatings. The effect of nonlinearity on the bending of beams with coatings and the nonlinear combination of external and residual stresses are discussed. The fracture of coatings at higher tensile stresses due to coalescence of the microcracks is mentioned.     

A cantilever beam method for evaluating Young’s modulus and Poisson’s ratio of thermal spray coatings

Young’s modulus and Poisson’s ratio for thermal spray coatings are needed to evaluate properties and characteristics of thermal spray coatings such as residual stresses, fracture toughness, and fatigue crack growth rates. It is difficult to evaluate Young’s modulus and Poisson’s ratio of thermal spray coatings be-cause coatings are usually thin and attached to a thicker and much stiffer substrate. Under loading, the substrate restricts the coating from deforming. Since coatings are used while bonded to a substrate, it is desirable to have a procedure to evaluate Young’s modulus and Poisson’s ratio in situ. The cantilever beam method to evaluate the Young’s modulus and Poisson’s ratio of thermal spray coat-ings is presented. The method uses strain gages located on the coating and substrate surfaces. A series of increasing loads is applied to the end of the cantilever beam. The moment at the gaged section is calcu-lated. Using a laminated plate bending theory, the Young’s modulus and Poisson’s ratio are inferred based on a least squares fit of the equilibrium equations. The method is verified by comparing predicted values of Young’s modulus and Poisson’s ratio with reference values from a three-dimensional finite ele-ment analysis of the thermal spray coated cantilever beam. The sensitivity of the method is examined with respect to the accuracy of measured quantities such as strain gage readings, specimen dimensions, ap-plied bending moment, and substrate mechanical properties. The method is applied to evaluate the Young’s modulus and Poisson’s ratio of four thermal spray coatings of industrial importance.     

Stress corrosion cracking of A588 steel weldments in flue gas related environments

This study investigated stress corrosion cracking (SCC) of A588 steel welds as determined by U-bend immersion tests and slow strain rate tensile (SSRT) tests to evaluate the steel’s cracking susceptibility in various regions of the weldments. The immersion test results indicated that the fusion zone (FZ) had better corrosion resistance than the other regions in the weld. It was also demonstrated that the columnar grain boundaries exhibited a higher resistance to corrosion than the grain interior of the FZ. However, the coarse elongated ferrite in the FZ is susceptible to hydrogen embrittlement (HE), which results in the formation of microcracks. As a result, a severe degradation of the weld’s tensile properties in the saturated H₂S solution was observed. Scanning electron microscope (SEM) fractographs of tensile specimens reveal a cleavage fracture in the coarse-grained heat-affected zone (CGHAZ) and featherlike rupture in the FZ, both indicating a high sensitivity to HE.     

超细晶粒高强度钢的延迟断裂行为

对于微合金化处理的42CrMoVNb钢，通过快速循环热处理的方法获得最小2μm的超细奥氏体晶粒，采用缺口拉伸延迟断裂实验研究了超细晶粒试样的延迟断裂行为。结果表明，随着晶粒细化，42CrMoVNb钢的强度和缺口拉伸延迟断裂抗力逐渐提高；但当晶粒细化到2μm时，强度和延迟断裂抗力均不再提高，在高温回火态，当晶粒尺寸在20—4μm范围时，断裂机制主要为穿晶断裂；但当晶粒进一步细化到2μm时，断裂机制转变为沿晶断裂，在低温回火态，不同晶粒尺寸的试样均主要为沿晶断裂，从降低应力集中和夹杂元素晶界偏聚等角度对超细晶粒高强度钢的延迟断裂行为进行了探讨。