Mechanical properties of the solid Li-ion conducting electrolyte: Li0.33La0.57TiO3

Li_0.33La_0.57TiO₃ (LLTO) is a potential Li-ion conducting membrane for use in aqueous Li-air batteries. To be in this configuration its mechanical properties must be determined. Dense LLTO was prepared using a solid-state (SS) or sol–gel (SG) procedure and was hot-pressed to yield a high relative density material (>95 %). Young’s modulus, hardness, and fracture toughness of the LLTO-SS and sol–gel LLTO-SG materials was determined and compared to other solid Li-ion conducting electrolytes. The Young’s modulus for LLTO-SG and LLTO-SS was 186 ± 4 and 200 ± 3 GPa, respectively. The Vickers hardness of LLTO-SG and LLTO-SS was 9.7 ± 0.7 and 9.2 ± 0.2 GPa, respectively. The fracture toughness, K _IC, of both LLTO-SG and LLTO-SS was ~1 MPa m^1/2; the fracture toughness of LLTO-SG was slightly higher than that of LLTO-SS. Both LLTO-SG and LLTO-SS have a Young’s modulus and hardness greater than the other possible solid Li-ion conducting membranes; Li₇La₃Zr₂O₁₂ and Li_1+x+y Al _x Ti_2−x Si _y P_3−y O₁₂. Based on modulus and hardness hot-pressed LLTO exhibits sufficient mechanical integrity to be used as a solid Li-ion conducting membrane in aqueous Li-air batteries but, its fracture toughness needs to be improved without degrading its ionic conductivity.     

Micro- and macro-mechanical testing of transparent MgAl2O4 spinel

Advanced transparent ceramics with high chemical and thermal stability are gaining increasing interest as replacement of glass-based materials in technical window applications. The mechanical reliability and performance of transparent MgAl₂O₄ with a grain size of 5 μm has been characterized at ambient temperature using micro-mechanical indentation and macroscopic bending tests. The measurements focused on elastic modulus, fracture toughness, crack kinetics, and strength, the latter analyzed with Weibull statistics. The effect of slow crack growth is assessed using a strength–probability–time plot. Complementary fractography by optical, confocal and scanning electron microscopy provided a correlation between failure origin and fracture stress. The results and reliability aspects are discussed in terms of linear elastic fracture mechanics.     

Poly(ether ether ketone) with pendent methyl groups as a toughening agent for amine cured DGEBA epoxy resin

A diglycidyl ether of bisphenol-A (DGEBA) epoxy resin was modified with poly(ether ether ketone) with pendent methyl groups (PEEKM). PEEKM was synthesised from methyl hydroquinone and 4,4′-difluorobenzophenone and characterised. Blends of epoxy resin and PEEKM were prepared by melt blending. The blends were transparent in the uncured state and gave single composition dependent T _g. The T _g-composition behaviour of the uncured blends has been studied using Gordon–Taylor, Kelley–Bueche and Fox equations. The scanning electron micrographs of extracted fracture surfaces revealed that reaction induced phase separation occurred in the blends. Cocontinuous morphology was obtained in blends containing 15 phr PEEKM. Two glass transition peaks corresponding to epoxy rich and thermoplastic rich phases were observed in the dynamic mechanical spectrum of the blends. The crosslink density of the blends calculated from dynamic mechanical analysis was less than that of unmodified epoxy resin. The tensile strength, flexural strength and modulus were comparable to that of the unmodified epoxy resin. It was found from fracture toughness measurements that PEEKM is an effective toughener for DDS cured epoxy resin. Fifteen phr PEEKM having cocontinuous morphology exhibited maximum increase in fracture toughness. The increase in fracture toughness was due to crack path deflection, crack pinning, crack bridging by dispersed PEEKM and local plastic deformation of the matrix. The exceptional increase in fracture toughness of 15 phr blend was attributed to the cocontinuous morphology of the blend. Finally it was observed that the thermal stability of epoxy resin was not affected by the addition of PEEKM.     

Determination of mechanical properties of Al–Mg alloys dissimilar friction stir welded interface by indentation methods

A series of friction stir welds was produced between heat treated Al–Mg–Si and strain hardened Mg–Al–Zn alloy sheets. Weld evaluation by transverse tensile testing showed a wide range of strengths and all the failures occurred along the weld interface. The formation of intermetallic compounds in the weld joints was investigated by X-ray diffraction, scanning electron microscopy imaging, and elemental analysis techniques. Micro and nanoindentation characterization methods were used to evaluate the mechanical properties at the interface, including the fracture toughness. The fracture toughness measurements by a Vickers indenter introduced Palmqvist type cracks at all four corners of the indents and cube corner indenter resulted in the intermetallic chipping. The fracture toughness (K _IC) calculation by both the micro and nanoindentation methods showed very low values, which is the primary reason for the brittle failure of the dissimilar weld joints and concomitant low tensile strengths.     

Microstructure and toughness of coarse grain heat-affected zone of domestic X70 pipeline steel during in-service welding

The microstructures and mechanical properties of coarse grain heat-affected zone (CGHAZ) of domestic X70 pipeline were investigated. The weld CGHAZ thermal cycles having different cooling time Δt _8/5 were simulated with the Gleeble-1500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding fractographs, optical micrographs, and electron micrographs were systematically investigated to study the effect of cooling time on microstructure, impact toughness, and fracture morphology in the CGHAZ of domestic X70 pipeline steel during in-service welding. The results of simulated experiment show that the microstructure of CGHAZ of domestic X70 pipeline steel during in-service welding mainly consists of granular bainite and lath bainite. Martensite–austenite (M–A) constituents are observed at the lath boundaries. With increase in cooling time, the M–A constituents change from elongated shape to massive shape. The reduction of toughness may be affected by not only the M–A constituents but also the coarse bainite sheaves. Accelerating cooling with cooling time Δt _8/5 of 8 s can be chosen in the field in-service welding X70 pipeline to control microstructures and improve toughness.     

Mechanical properties of very thin cover slip glass disk

The biaxial flexural strength, Young’s modulus, Vicker’s microhardness and fracture toughness data for very thin, commercial, soda-lime-silica cover slip glass (diameter, D-18 mm, thickness, T-0.3 mm; T/D ≈ 0.02) are reported here. The ball on ring biaxial flexure tests were conducted at room temperature as a function of the support ring diameter (≈ 10–20 mm) and cross head speed (0.1–10 mm min⁻¹). In addition, the Weibull modulus data were also determined. The Young’s modulus data was measured using a linear variable differential transformer (LVDT) from biaxial flexure tests and was checked out to be comparable to the data obtained independently from the ultrasonic time of flight measurement using a 15 MHz transducer. The microhardness data was obtained for the applied load range of 0.1–20 N. The fracture toughnessK _IC data was obtained by the indentation technique at an applied load of 20 N.     

Relationship between fracture toughness and deformation ahead of the crack tip

An analytical relationship for the calculation of the intensity of elastoplastic strains at the front of a mode I crack is obtained on the basis of the suggested model. A strain criterion of fracture toughness has been developed which relates the stress intensity factor to the width of the stretch zone, the mechanical properties of the material, and its elastic constants. A comparison of experimental and numerical (obtained by using the proposed equations) data of the stretch zone width for 15Kh2NMFA steel in a wide temperature range demonstrates good agreement between the theory and the experiment. This proves the possibility of calculating fracture toughness via the stretch zone width and mechanical properties of the material. Translated from Problemy Prochnosti, No. 2, pp. 33–40, March–April, 1997.     

The mechanical properties of PMMA and its copolymers with ethyl methacrylate and butyl methacrylate

The mechanical properties of polymethyl methacrylate and copolymers formed with both ethyl methacrylate and butyl methacrylate were investigated. Six polymers were produced by bulk polymerization, measured for molecular weight and glass transition temperature, T _g and assessed for modulus of elasticity and fracture toughness. Increasing the concentration of ethyl methacrylate or butyl methacrylate resulted in a linear decrease in the glass transition temperature, modulus of elasticity, and fracture toughness. A comparison of testing environments revealed that the modulus of elasticity was reduced when conditioned and tested in water at 37 °C compared to ambient laboratory conditions for all polymers. Similar comparisons of the fracture toughness showed an increase for testing in water at 37 °C; however, this was not significant for the lower T _g compositions. Both modulus of elasticity and fracture toughness were strongly correlated with the glass transition temperature and composition.     

Enhancing mechanical and fracture properties of sandwich composites using nanoparticle reinforcement

Polyurethane (PU) foam is reinforced with SiC nanoparticles to develop core materials for sandwich composites. Isocyanate component (Part A) of PU foam was dispersed with SiC nanoparticles, and then mixed with polyol (Part B) to manufacture nanophased core materials. Nanoparticle reinforcement varied from 0.1 to 2.0 wt% of the total polymer. Both pristine and silane functionalized SiC nanoparticles were used in the investigation. Nanophased foams were tested in compression and flexure to determine the mechanical properties. Fracture toughnesses K _IC and G _IC were determined using the SENB test. Sandwich panels were fabricated and tested for face-core debond fracture toughness using the tilted sandwich debond test. The study has revealed that reinforcement of the foam by pristine nanoparticles substantially enhances mechanical properties but degrades fracture toughness. This loss in fracture toughness, however, may be recovered with the use of functionalized nanoparticles. Small concentrations (0.1–0.2 wt%) of functionalized nanoparticles provided large improvement in debond fracture toughness of sandwich specimens.     

The relationship between fracture toughness and microstructure of fully lamellar TiAl alloy

Fully lamellar (FL) Ti–46.5Al–2Cr–1.5Nb–1V (at%) alloy is used to study the relationship between microstructure and fracture toughness. A heat treatment process is adopted to control the microstructural parameters of the studied alloy. Fracture toughness experiments and scanning electron microscope (SEM) in-situ straining experiments are carried out to determine the influence of lamellar spacing and grain size on the fracture toughness of FL TiAl alloys. It is found that ligament length depends on the lamellar spacing, and fracture toughness varies non-monotonously with the increase of grain size. The results are ascribed to the competition between the microcrack nucleation and microcrack propagation. Finally a semi-empirical relationship between the fracture toughness and microstructure parameters was established.     

Mechanical characterization of microwave sintered zinc oxide

The mechanical characterization of microwave sintered zinc oxide disks is reported. The microwave sintering was done with a specially designed applicator placed in a domestic microwave oven operating at a frequency of 2.45 GHz to a maximum power output of 800 Watt. These samples with a wide variation of density and hence, of open pore volume percentage, were characterized in terms of its elastic modulus determination by ultrasonic time of flight measurement using a 15 MHz transducer. In addition, the load dependence of the microhardness was examined for the range of loads 0.1–20 N. Finally, the fracture toughness data (K _IC) was obtained using the indentation technique.     

Physical properties of low-<Emphasis Type="Italic">k</Emphasis> films based on the co-condensation of methyltrimethoxysilane with a bridged silsesquioxane

Though spin-on organosilicates are considered as the promising candidates of low dielectric constant materials, it is necessary for a successful integration to improve mechanical strength such as modulus and fracture toughness. In this study, five sets of MTMS–BTMSE copolymers were synthesized and characterized while the monomer content of BTMSE (bis(trimethoxysilyl)ethane) was varied from 9.1 to 47.3 mol% vs. MTMS (methyltrimethoxysilane). In parallel with the measurement of dielectric constant, four different tests were carried out to evaluate mechanical properties of the MTMS–BTMSE copolymers. Modulus was measured by the nanoindentation technique and the modified edge lift-off test (MELT) was employed to extract adhesive fracture toughness quantitatively. In addition, residual stress was calculated by sensing the change in radius of curvature of the substrate. The chemical structure of the copolymers was also analyzed with FTIR and NMR. Network formation was enhanced as the amount of BTMSE increased, which led to improvement of modulus and the increase in refractive index and dielectric constant. However, an increasing rate of fracture toughness by the MELT was not proportional to the increase in the amount of BTMSE, which implied that it was necessary to optimize the composition of the copolymers since adhesion strength was conjectured the most critical factor for a successful integration.     

Mechanical properties of sodium and potassium activated metakaolin-based geopolymers

In this study, a set of mechanical properties of geopolymers, synthesized by alkali (NaOH or KOH) activation of metakaolin and SiO₂ mixture, were characterized at ambient temperature. Samples with K/Al or Na/Al atomic ratios equal to 1, Si/Al atomic ratios in the 1.25–2.5 range and H₂O/Al₂O₃ molar ratios of 11 or 13 are cured at 80 °C for 24 and 48 h before characterization, to determine effect of Si/Al ratio and curing time on the structure and mechanical properties of geopolymers. The structure of synthesized geopolymers characterized using XRD, NMR, SEM, and density measurements was correlated to their mechanical properties, including compressive strength, Young’s modulus, hardness, and fracture toughness. The results of this study suggest a strong effect of Si/Al ratios (in the 1.5–2 range), density, and microstructure on the maximum strength, Young’s modulus, and hardness of geopolymers. There were also notable differences in strength between samples cured for 24 and 48 h, suggesting that the degree of geopolymerization reaction also plays important role in mechanical properties of this new class of inorganic polymers.     

Modeling the brittle–ductile transition in ferritic steels. Part II: analysis of scatter in fracture toughness

A dislocation simulation model has been proposed to predict the brittle–ductile transition in ferritic steels in Part I. Here we extend the model to address the problem of inherent scatter in fracture toughness measurements. We carried out a series of Monte Carlo simulations using distributions of microcracks situated on the plane of a main macrocrack. Detailed statistical analysis of the simulation results showed the following: (a) fracture is initiated at one of the microcracks whose size is at the tail of the size distribution function, and (b) the inherent scatter arises from the distribution in the size of the critical microcrack that initiates the fracture and not from the variation of the location of the critical microcrack. Utilizing the weakest-link theory, Weibull analysis shows good agreement with the Weibull modulus values obtained from fracture toughness measurements.     

Strength characteristics of Mg–Li alloys

We study the mechanical properties of Mg–Li alloys obtained by high-pressure die casting in a cold pressing chamber. A procedure of evaluation of the principal strength characteristics obtained under the conditions of high plastic strains is presented. The fracture toughness of materials under quasistatic and dynamic concentrated loads is investigated. Translated from Problemy Prochnosti, No. 3, pp. 78–88, May–June, 2009.     

Epoxy composites with 200 nm thick alumina platelets as reinforcements

Composites were prepared by dispersing Alumina platelets of polygonal shape having a thickness of 200 nm and size of 5–10 μm in epoxy (LY 556) matrix using sonication. Good dispersion of the platelets was observed through scanning electron microscopy (SEM). The quasi-static plane-strain fracture toughness and tensile properties of the composites were determined for platelet volume fraction varying from 0% to 10%. The results indicated that addition of the platelets give considerable improvement in fracture toughness and good improvement in the elastic modulus of epoxy. For 10% volume fraction of the platelets, the fracture toughness improved by 110% where as the improvement in elastic modulus was 78%. However there was an associated reduction of 53% in tensile strength and 73% in failure strain. SEM of fractured surface was carried out to understand the various mechanisms responsible for the improvement in fracture toughness. By appropriately accounting for the orientation and stacking effects of the platelets, the applicability of predictive models, such as the Halpin-Tsai and Mori-Tanaka, for estimating the composite modulus is demonstrated.     

Degradation of steels used in gas main pipelines during their 40-year operation

The degradation of the mechanical and corrosion properties of grade 17G1S steel in gas main pipelines after their 28–40-year operation is studied. A simultaneous decrease in their strength and hardness on the one hand, and impact toughness, fracture toughness and resistance to hydrogeninduced cracking, on the other, is observed. The influence of damages to the steel on the degradation of its properties is analyzed.     

Ab initio study on fracture toughness of Ti<Subscript>0.75</Subscript>X<Subscript>0.25</Subscript>C ceramics

Ab initio density functional theory calculations have been performed to evaluate the fracture toughness for selected Ti_0.75X_0.25C ceramics (X = Ta, W, Mo, Nb and V). The calculated Young’s modulus E, surface energy γ and fracture toughness K _IC of pure TiC are in a good agreement with experimental data and other theoretical calculations. The results for Ti_0.75X_0.25C system show that alloying additions increase Young’s modulus, and all but vanadium increase surface energy. It was observed that tungsten has the most significant effect on increasing Young’s modulus, while tantalum on increasing surface energy of the Ti_0.75X_0.25C system. Surface energy plays a dominated role in determining the trend of fracture toughness. Overall, tantalum and tungsten are the most effective alloying elements in increasing the fracture toughness of Ti_0.75X_0.25C ceramics.     

Abnormal manifestation of the high-temperature degradation of the weld metal of a low-alloy steel welded joint

We have shown that hardness, impact toughness, mechanical properties in tension, and the local parameters of fracture mechanics (static and cyclic crack resistance) are sensitive to the operating degradation of weld metal of steam pipelines of thermal power plants made of 15Kh1M1F steel. The simultaneous decrease in the resistance to brittle and plastic fracture (hardness, strength, and impact toughness) represents a phenomenon of the operating degradation of weld metal. We have established a specific correlation between the characteristics of plasticity and other mechanical parameters of operated metal: the increase in °₅ of operated weld metal is in good agreement with the decrease in its strength, whereas the reduction of Ψ correlates with the lowering of resistance to brittle fracture. Electrolytic hydrogenation decreases the characteristics of strength and plasticity of operated weld metal much stronger than in the initial state. The absence of ferritic edgings on the boundaries of primary austenitic grains makes for a low resistance to brittle fracture, and the change in acicular ferrite deteriorates the mechanical properties. The ductile fracture of nonoperated metal is replaced by brittle intercrystalline failure in operated metal. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 73–79, January–February, 2007.     

Gd–Ni–Al bulk glasses with great glass-forming ability and better mechanical properties

A series of Gd–Ni–Al ternary glassy alloys with the maximum diameter of 4 mm were obtained by common copper mold casting. The maximum values of the reduce glass transformation temperature (T _g/T _m) and the distance of supercooling region ΔT _x of these alloys in this study were 0.648 and 50 K, respectively. The compressive fracture strength (σ _f) and Young’s modulus (E) of Gd–Ni–Al glassy alloys were 1,240–1,330 MPa and 63–67 GPa, respectively. The magnetic properties of these BMGs were investigated. The Gd–Ni–Al bulk glassy alloys with great glass forming ability and good mechanical properties are promising for the future development as a new type of function materials.