Residual stress induced fracture in glass-sapphire composites: planar geometry

Cracking resulting from thermal expansion mismatch generated residual stresses is investigated using a model system consisting of bonded layers of sapphire and borosilicate glass. Three planar geometries are employed; a bilayer configuration and two sandwich configurations. The bilayer configuration of a thin sapphire sheet bonded to a thick glass substrate models the cracking due to a thin film under residual tension. The cracks formed adopt a characteristic shape running parallel to the planar interface and at a depth consistent with recent predictions of Suo and Hutschinson for a K_II = 0 steady-state propagation trajectory. The same result is obtained with a novel test sample consisting of a sapphire sheet bonded to a triangular shaped substrate. The other two configurations, of a glass block sandwiched between two sapphire sheets and of a sapphire sheet between two glass blocks, enable the residual stress cracking to be explored for conditions under which no net bending moment exists.     

Small and large fatigue cracks in aluminum alloys

Crack opening loads and the displacements near the crack tip have been measured by stereoimaging for large (through) cracks and small (surface) cracks. These results have been published. The present paper combines these observations and considers them in a new way, showing the relationship between small and large cracks, and provides a method for computing an effective driving force (ΔK_eff) for small and large fatigue cracks.     

Low-Cycle fatigue properties of a SiC Whisker-reinforced 2124 aluminum alloy

Low-cycle fatigue microcracking leading to failure of smooth specimens of a powder metallurgy (PM) 2124 aluminum alloy reinforced with 20 vol pct SiC whiskers was studied. The crack size near the onset of unstable growth was inferred to be 50 to 70 μm in the stress amplitude range of the present study (400 to 600 MPa,R = −1) from observations of the fracture surfaces of the specimens. This corresponds to stress intensities between 1/3 to 1/2 typical values ofK _1c or 1/4 to 1/9 the critical length predicted fromK _1c values of 12 to 14 MPa√m. The microcrack size distributions and growth data were obtained from the low-cycle fatigue specimens at various stages of fatigue, using a surface replica technique. During continued cycling, microcracks formed and were lost through linkage with other cracks. At the same time, the fraction of small cracks (<5 μm) decreased, while that of larger cracks (>5 μm) increased. The total number of cracks increased with increasing numbers of cycles. Typical microcrack growth rates were determined to bedb/dn = (3.57 to 6.11) × 10⁻¹⁰ (Δ/K)^2.2to2.48 in the lateral direction of the crack, andda/dn = (5.83 to 13.0) × 10⁻¹¹ (ΔK)^{1.54 to 1.60} in the depth direction of the crack.     

Stress-corrosion cracking behavior of an 18 Pct Ni maraging steel

Stress-corrosion cracking of an 18 pct Ni maraging steel in aqueous solutions was studied using precracked cantilever beam specimens. By appropriate heat treatments, six different structures having the same yield strength were obtained. Although significantly different plane strain fracture toughness values (K _Ic ) resulted, it was found that the threshold plane strain stress intensity (K _Iscc ) was the same for all structures.K _Iscc had the same value in 3 pct NaCl at various pH values, in 1N H₂SO₄, and in distilled water. Specimens tested in 3 pct NaCl under both anodic and cathodic applied potentials also exhibited this sameK _Iscc value. Fractographic inspection of the crack surfaces revealed no apparent differences due to changes in solution, pH, or applied potential. The crack path was intergranular in all cases. However, specimens austenitized at 1500°F exhibited crack branching, whereas in specimens austenized at much higher temperatures branching no longer occurred. Aging time and temperature seemed to change only the time to failure. The mechanism most consistent with all observations appears to be hydrogen cracking.     

Cracking of brittle films on elastic substrates

An analysis is presented that relates the crack spacing in a brittle film on an elastic substrate to the stress in the film, its thickness and fracture toughness. This analysis differs from an earlier one presented by one of the authors in that it considers the effect of a sequential, rather than a concerted, propagation of cracks, and predicts a larger crack spacing. The validity of the present analysis was confirmed by experimental results from a model system consisting of epitaxial PrBa₂Cu₃O_7−x films on SrTiO₃ substrates. The experimentally observed relationship between the crack spacing and the film thickness was in excellent agreement with the theory.     

Cleavage in beryllium monocrystals

A study has been made of the initiation and propagation of cleavage cracks in the basal (0001) plane of beryllium monocrystals. The double cantilever cleavage technique was used to determine the energy, φ _p , to propagate cleavage cracks, as a function of temperature, purity of material, and alloying. φ _p for 3-pass zone-refined Pechiney SR grade beryllium remains essentially constant at ≈2100±100 ergs per sq cm over the temperature range ?192° to ?72°C, and then increases with temperature due to plastic relaxation in the vicinity of the crak tip. This value is in good agreement with Gilman’s theoretical estimate of the surface free energy of the (0001) plane in beryllium. The value of φ _p determined from a limited number of specimens of beryllium containing 1.49 wt pct Cu, and also of higher-purity distilled beryllium, is somewhat greater than that for Pechiney material, namely ≈3000 ergs per sq cm. The energy to initiate cleavage fracture in beryllium, φ _I , also is 2100±400 ergs per sq cm. Since both crack initiation and propagation occur on the (0001) plane, φ _I should be, and is seen to be, essentially identical with φ _p .     

Effect of Annealing Temperature on the Thermoelectric Properties of the Bi0.5Sb1.5Te3 Thin Films Prepared by Radio-Frequency Sputtering

The effect of annealing temperature on the crystallinity, thermoelectric properties, and surface morphology of the Bi_0.5Sb_1.5Te₃ thin films prepared on SiO₂/Si substrate by radio-frequency (RF) magnetron sputtering was investigated using X-ray diffraction (XRD), the four-point probe method, and scanning electron microscopy (SEM). XRD results show that the crystallite structure of the Bi _x Sb_2–x Te₃ thin films belong to Bi_0.5Sb_1.5Te₃. When the Bi _x Sb_2–x Te₃ thin films were annealed between 423 K and 523 K (150 °C and 250 °C) for 10  minutes, the crystallinity of the thin films continuously increases with the temperature increase. In addition, the (015) reflection plane as the preferred orientation and the oxidation compound of Bi_3.73Sb_1.5O₃ first appeared when the Bi_0.5Sb_1.5Te₃ thin films were annealed at 523 K (250 °C) for 10 minutes. An activation energy of 51.66 kJ/mol for crystallite growth of Bi_0.5Sb_1.5Te₃ thin films annealed between 423 K and 523 K (150 °C and 250 °C) for 10 minutes was obtained. The resistivity was 2.69 × 10² and 5.93 × 10  μΩ·m, respectively, for the as-deposited Bi_0.5Sb_1.5Te₃ thin films and annealed at 523 K (250 °C) for 10 minutes. The maximum values of the Seebeck coefficient and power factor were 256.5 μV/K and 1.12 × 10³ μW/m·K², respectively, for the Bi_0.5Sb_1.5Te₃ thin films annealing treatment at 523 K (250 °C) for 10 minutes.     

A rationalisation of fatigue thresholds in pearlitic steels using a theoretical model

From a detailed re-examination of results in the literature, the effects of microstructure sizes, namely interlamellar spacing, pearlitic colony size and the prior austentitic grain size on the thresholds for fatigue crack growth (ΔK_th) and crack closure (K_{cl, th}) have been illustrated. It is shown that while interlamellar spacing explicitly controls yield strength, a similar effect on ΔK_th cannot be expected. On the other hand, the pearlitic colony size is shown to strongly influence ΔK_th and K_{cl, th} through the deflection and retardation of cracks at colony boundaries. Consequently, an increase in ΔK_th and K_{cl, th} with colony size has been found. The development of a theoretical model to illustrate the effects of colony size, shear flow stress in the slip band and macroscopic yield strength on K_{cl, th} and ΔK_th is presented. the model assumes colony boundaries as potential sites for slip band pile-up formation and subsequent crack deflection finally leading to zig-zag crack growth. Using the concepts of roughness induced crack closure, the magnitude of K_{cl, th} is quantified as a function of colony size. In deriving the model, the flow stress in the slip band has been considered to represent the work hardened state in pearlite. Comparison of the theoretically predicted trend with the experimental data demonstrates very good agreement. Further, the intrinsic or closure free component of the fatigue threshold, ΔK_{eff, th} is found to be insensitive to colony size and interlamellar spacing. Using a criterion for intrinsic fatigue threshold which considers the attainment of a critical fracture stress over a characteristic distance corresponding to interlamellar spacing, ΔK_th values at high R values can be estimated with reasonable accuracy. The magnitude of ΔK_th as a function of colony size is then obtained by summing up the average value of experimentally obtained ΔK_{eff, th} values and the predicted K_{cl, th} values as a function of colony size. Again, very good agreement of the theoretically predicted ΔK_th values with those experimentally obtained has been demonstrated.     

Subcritical fatigue crack growth in alumina—II. Crack bridging and cyclic fatigue mechanisms

A crack re-notching procedure based on the “hinged straight crack” approximation is used to determine the distribution and magnitude of the bridging traction, σ(X), existing over the faces of the fatigue cracks grown in the experiments of Part I. From this distribution, the σ(u) relation between the bridging tractions and the crack opening is obtained and a simple method is tentatively proposed to measure the magnitude of the crack bridging stress intensity factor, K_b. The characteristics of the σ(X) and σ (u) relations are discussed in the light of the microscopical observations of crack profiles and in terms of the distribution of the frictional and elastic ligaments existing along the faces of the cracks. Crack growth rates and behaviour under different values of stress ratio R are compared and mechanisms of fatigue crack growth vs static crack growth are proposed.     

Dislocation nucleation at metal-ceramic interfaces

The ductile vs brittle behaviour of metal-ceramic interfaces is discussed within an atomistic framework, in which the mechanical response of an interfacial crack is assumed to be ultimately controlled by the competition between atomic decohesion and dislocation nucleation ahead of the crack tip. As in later versions of the Rice-Thomson model, this competition may be evaluated in terms of the parameters G_cleave, the energy release rate for cleavage of the metal-ceramic interface, and G_disl, the energy release rate associated with the emission of a single dislocation within the metal. The various models of dislocation nucleation are discussed, with emphasis on an approach which makes use of Peierls-like stress vs displacement relations on a slip plane ahead of a crack tip. A recent analytical result by Rice shows that for a mode II or III shear crack, with a slip plane parallel to the ack plane, a dislocation is emitted when G = γ_us (G is the energy release rate corresponding to the “screened” crack tip stress field and γ_us is the “unstable stacking” energy associated with the sliding of atomic planes past one another). This treatment permits the existence of an extended dislocation core, which eliminates the need for the core cutoff radii required by the Rice-Thomson model of emission. Results are presented here for the nucleation of dislocations under more realistic assumptions for metal-ceramic cracks, namely, the emission on inclined slip planes within a mixed-mode crack-tip field. The specific case of a copper crystal bonded on a {221} face to sapphire is analyzed, and the results are used to interpret the recent experimental observations of Beltz and Wang [Acta metall. mater.40, 1675 (1992)] on directional toughness along this type of interface.     

Magnetic properties of SmCo-based permanent magnetic films during 25 to 300 ℃

Magnetic properties of the SmCo-based permanent magnetic films prepared on hot substrate with Mo and Cr underlayer without subsequent annealing process were investigated by vibrating sample magnetometer （VSM）, X-ray diffraction （XRD）, and en- ergy dispersive X-ray spectroscopy （EDS）. The results showed that the film thickness of the SmCo-based films presented complex effect on the intrinsic coercivity Hci. Optimal Hc~ for the films with Mo underlayer, Cr underlayer, and without underlayer was ob- served with different film thicknesses. Furthermore, the monotonous temperature dependence of Hci was found to be strongly corre- lated with the magnetic parameters for the 3.0 μm thick SmCo7 films with Mo underlayer. From 25 to 300 ℃, the Hci decreased from 281.6 to 211.2 kA/m with a temperature coefficient of-0.091%/℃, exhibiting good temperature stability.     

Fatigue Strength and Crack Initiation Mechanism of Very-High-Cycle Fatigue for Low Alloy Steels

The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5?Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 10⁸ cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with ??fish-eye?? pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (K _I ) at different regions of fracture surface were calculated, indicating that the K _I value of fish-eye crack is close to the value of relevant fatigue threshold (??K _th ). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations.     

Effect of mean stress on hydrogen assisted fatique crack propagation in duplex stainless steel

Hydrogen assisted subcritical cleavage of the ferrite matrix occurs during fatigue of a duplex stainless steel in gaseous hydrogen. The ferrite fails by a cyclic cleavage mechanism and fatigue crack growth rates are independent of frequency between 0.1 and 5 Hz. Macroscopic crack growth rates are controlled by the fraction of ferrite grains cleaving along the crack front, which can be related to the maximum stress intensity, K_max. A superposition model is developed to predict simultaneously the effects of stress intensity range (ΔK) and K ratio (K_min/K_max). The effect of K_max is rationalised by a local cleavage criterion which requires a critical tensile stress, normal to the {001} cleavage plane, acting over a critical distance within an embrittled zone at the crack tip.     

New water-soluble acrylic bonding agents for shaping ceramic materials

Preliminary results are reported on the synthesis and use of new water-soluble polymer binders for making ceramic films from Al ₂O₃ by casting. The binders produce cast materials having high contents of the solid phase and with rheological properties that provide for good flow of the cast mass and for easy separation of the film from the substrate. The emulsions wet Al₂O₃ well, so one can make ceramic films with appropriate densities, elasticities, and mechanical strength in the raw state, and these can be fired to make planeparallel specimens free from cracks and having high density, good mechanical strength, and low roughness. Warsaw Polytechnic. Translated from Poroskhovaya Metallurgiya, Nos. 5–6(407), pp. 15–20, May–June, 1999.     

Mass transfer characteristics of cobalt and nickel in di(2-ethylhexyl)Phosphoric acid under steady-state extraction conditions

The mass-transfer characteristics of cobalt and nickel in di(2-ethylhexyl)phosphoric acid have been studied using a gradientless or steady-state stirred cell contactor. The effects of contacting conditions, stirrer Reynolds number, extractant concentration and organic phase loading were investigated. The results showed that under the conditions studied overall transfer rates could be considered diffusion or mass-transfer controlled. There was, however, evidence of an interfacial or chemical resistance as shown by the effect of contacting conditions on values of the overall mass-transfer coefficient, K_org. This was confirmed when experimental K_org values were compared with individual phase mass-transfer coefficient values predicted from correlations proposed in the literature by Lewis and by McManamey et al.     

Structure and Electrochemical Activity of C60 Fullerite Films

Results are presented from structural and electrochemical researches on C₆₀ films. The fullerite films were made by thermal vacuum evaporation and deposition on NaCl crystals. The substrate temperatures were 293-473 K. The examinations were made in a transmission electron microscope at 100 kV and by x-ray diffraction. Dark-field images were obtained from the individual fullerite particles and particularly from grains in continuous thin films, which showed stacking faults or twin boundaries. The numbers of these defects increase with the substrate temperature. The x-ray diffraction patterns of the fullerite films show extremely diffuse reflections together with a weak reflection around the FCC (111), which may be assigned to a hexagonal close packed HCP modification of fullerite. Rietveld's method was used in processing the x-ray patterns. The best fit between the experimental and theoretical diffraction lines was obtained with the following structure parameters: ratio of FCC phase (a = 1.4117 nm) and HCP phase (a = 0.9756 nm and c = 1.7084 nm) was 46/54 mass%. The electrochemical data indicate that a palladium-activated fullerite film shows prominent hysteresis, which confirms that certain hydride phases are formed at the surface of the C₆₀ film.     

Correlation of microstructure and tempered martensite embrittlement in two 4340 steels

This study is concerned with a correlation between the microstructure and fracture behavior of two AISI 4340 steels which were vacuum induction melted and then deoxidized with aluminum and titanium additions. This allowed a comparison between microstructures that underwent large increases in grain size and those that did not. When the steels were tempered at 350°C,K _Ic and Charpy impact energy plots showed troughs which indicated tempered martensite embrittlement (TME). The TME results of plane strain fracture toughness are interpreted using a simple ductile fracture initiation model based on large strain deformation fields ahead of cracks, suggesting thatK _Icscales roughly with the square root of the spacing of cementite particles precipitated during the tempering treatment. The trough in Charpy impact energy is found to coincide well with the amount of intergranular fracture and the effect of segregation of phosphorus on the austenite grain boundaries. In addition, cementite particles are of primary importance in initiating the intergranular cracks and, consequently, reducing the Charpy energy. These findings suggest that TME in the two 4340 steels studied can be explained quantitatively using different fracture models.     

The growth of small fatigue cracks in A286 steel

Fatigue crack propagation in high-strength A286 steel was studied by comparing crack growth rates determined from: (1) conventional long-crack propagation tests, (2) closure-free long-crack tests at constant K_max, and (3) small-crack propagation tests. Small-crack growth rates were measured by following the growth of surface cracks in samples cycled from near-zero stress to 0.5 or 0.8σ_y. While most of the surface cracks became dormant shortly after nucleation, some grew into long cracks, and some of these propagated at cyclic stress intensities below the long-crack threshold, ΔK_th (or ΔK _th ^eff , the threshold cyclic stress intensity after crack closure effects have been removed). Surface cracks grew more rapidly than long cracks at the same ΔKor ΔK_eff. The small-crack effect disappeared when the crack-tip plastic zone size became greater than the grain size. The results show that the absence of crack closure is only one of several factors that influence short-crack growth in A286 steel. Both peak stress and microstructural effects are important. Microstructural effects are apparently responsible for subthreshold crack growth; the cracks that grow at ΔK < ΔK _th ^eff form and grow in statistically weak regions of the microstructure.