Fracture Resistance of Ceramics: Edge Fracture Method

Fracture resistance of macrohomogeneous linear elastic yttria, scandia, alumina, zirconia, and silicon nitride ceramics was studied upon flaking of the edges of rectangular specimens with the Rockwell indenter. The correlation between the obtained estimates and fracture toughness test results gained by the single-edge V-notched beam method was demonstrated. Loads giving rise to the flaking of a part of the edge and distances from this edge to the chip scar on the specimen surface were measured. The ratio of those values was considered as a flaking toughness characteristic. The data obtained were statististically reliable (based on more than a hundred determinations). This procedure termed the edge fracture method can be used along with other known fracture toughness test methods for ceramics, its application is especially advantageous when ceramic item sizes are comparable with those of the standard specimens or when expensive materials (e.g., nanoceramics) are tested. __________ Translated from Problemy Prochnosti, No. 5, pp. 84 – 92, September – October, 2005.     

Nondestructive method of evaluating the shock resistance of composite (polymer-polymer) materials

A study is made of the connection between the fracture toughness of shock-resistant polymeric composites based on polycarbonate, polybutyleneterephthalate, and an ABS-copolymer and parameters characterizing mechanical and electrical relaxation in the region of local molecular mobility. A method is proposed for determining shock characteristicsa priori on the basis of the temperature-frequency dependences of the elastic modulus and mechanical loss modulus. The difference between the theoretical and experimental values of fracture toughness, measured within a broad range of temperatures, is no greater than 10–15%. Translated from Izmeritel'naya Tekhnika, No. 2, pp. 39–42, February, 1996.     

Mechanical properties and fracture behavior of high-strength steels

Typical thicknesses of high-strength steels (HSS) sheets used in the car industry are inapplicable for standardized testing procedures. The aim of this study is to propose an appropriate methodology for testing and comparing of thin HSS sheets. Microstructures were observed by means of light and scanning electron microscopy. The modified Charpy impact tests and fracture toughness tests were used in order to compare the fracture properties of three different HSS sheets (Docol 1200 M, Multiphase 1200 and BTR 165). Ductile-to-brittle transition curves and tearing resistance (J − Δa) curves were measured. From the fracture toughness linked to the specimen thicknesses the value of fracture toughness K_Ic was estimated. Fractographic analysis of broken specimens has revealed that due to the fine microstructure of mixed ferrite-martensite fracture mechanism remains ductile even at low temperatures (down to −100°C). __________ Translated from Problemy Prochnosti, No. 1, pp. 155–158, January–February, 2008.     

Fracture toughness of multilayer pipes

Multilayer pipes composed of various materials improve partially the properties of a pipe system and are frequently used in service. To estimate the lifetime of these pipes the basic fracture parameters have to be measured. In the contribution a new approach to this estimation is presented. Special type of a C-shaped inhomogeneous fracture mechanics specimen machined directly from a pipe has been proposed, numerically analyzed and tested. The corresponding K values are calculated by finite-element method and fracture toughness values of polyethylene pipes material are obtained. __________ Translated from Problemy Prochnosti, No. 1, pp. 146–149, January–February, 2008.     

Effects of equal channel angular pressing on the strength and toughness of Al–Cu alloys

Tensile and impact tests were performed on Al–0.63 wt%Cu and Al–3.9 wt%Cu alloys subjected to equal channel angular pressing (ECAP) with different number of passes. Besides the tensile properties, data about the static toughness and the impact toughness were obtained. The strength and the toughness of the Al–Cu alloys were ameliorated and upgraded to a high level collectively. In addition, fracture surface observations show that the fracture behavior of the Al–Cu alloys changes from brittle mode to ductile mode after multi-pass ECAP.     

Viscoelasticity and fracture toughness of blended epoxy resins containing two monomers with different molecular weights

In the present work, we investigated the thermo-viscoelasticity and fracture toughness of various cured blends of two epoxy monomers with different molecular weights: 380 (Epikote 828) and 900 (Epikote 1001). The blended resins were easily prepared, and miscibility (no phase separation) in the blended resins was expected. The composition of the blended epoxy resins ranged from 0 to 100% by weight of the Epikote 1001. The measured damping factor and dynamic loss modulus in the glass-transition confirmed that each blended resin had a single phase, i.e., they were miscible. The fracture toughness at room temperature increased modestly with the Epikote 1001 content over the whole range (0–100 wt%). We found that below the glass-transition temperature, the macromolecular modifications enabled tailoring of the fracture toughness while maintaining the glassy bending modulus and with little change in the glass-transition temperature.     

A method for fracture toughness assessment by the scatter of hardness characteristics

A stable correlation between the material fracture toughness characteristics and statistical parameters of scatter of hardness values has been established. __________ Translated from Problemy Prochnosti, No. 6, pp. 5–12, November–December, 2007.     

Mechanical properties of rapidly solidified ribbons of some AI–Si based alloys

Continuous uniform ribbons of Al–16 Si, Al–12.5 Si–1 Ni and Al–12.5 Si–1 Mg were prepared by melt spinning. Microhardness was measured. The as-melt spun values were 1280, 1370 and 1500 MN m-2 which relax on thermal ageing to 700, 700 and 800 MN m-2 for Al–16 Si, Al–Si–Ni and Al–Si–Mg, respectively. The hardness values of the melt spun ribbons are higher than the as-cast rods from which the ribbons were produced by a factor ranging from 1.8–2.2 times. Tensile testing at room temperature shows that the load–elongation curves are linear with a change of slope occurring in some of the specimens. These curves also show serrations in the case of as-melt spun and the intermediately annealed Al–Si specimens, while no serration was observed in the fully annealed samples. No serration was observed in the Al–Si–Ni and Al–Si–Mg alloys. UTS values were 420, 270 and 100 MN m-2 for Al–16 Si, Al–Si–Ni and Al–Si–Mg, respectively. These values show that the rapid solidification process improved the tensile properties significantly in Al–16 Si and Al–Si–Ni alloys while no significant improvement can be detected for Al–Si–Mg alloy. A discussion is given on hardness relaxation and tensile testing results in terms of silicon precipitation. This revised version was published online in November 2006 with corrections to the Cover Date.     

North–South divide: contrasting impacts of climate change on crop yields in Scotland and England

Effects of climate change on productivity of agricultural crops in relation to diseases that attack them are difficult to predict because they are complex and nonlinear. To investigate these crop–disease–climate interactions, UKCIP02 scenarios predicting UK temperature and rainfall under high- and low-CO₂ emission scenarios for the 2020s and 2050s were combined with a crop-simulation model predicting yield of fungicide-treated winter oilseed rape and with a weather-based regression model predicting severity of phoma stem canker epidemics. The combination of climate scenarios and crop model predicted that climate change will increase yield of fungicide-treated oilseed rape crops in Scotland by up to 0.5 t ha⁻¹ (15%). In contrast, in southern England the combination of climate scenarios, crop, disease and yield loss models predicted that climate change will increase yield losses from phoma stem canker epidemics to up to 50 per cent (1.5 t ha⁻¹) and greatly decrease yield of untreated winter oilseed rape. The size of losses is predicted to be greater for winter oilseed rape cultivars that are susceptible than for those that are resistant to the phoma stem canker pathogen Leptosphaeria maculans. Such predictions illustrate the unexpected, contrasting impacts of aspects of climate change on crop–disease interactions in agricultural systems in different regions.     

Fracture toughness diagrams of a notched body

To describe fracture toughness diagrams of notched bodies, a model of the cohesion zone near the notch root and an averaging criterion of stresses in this zone were employed. The geometric stress concentration factor and biaxiality coefficient affect greatly the shape of fracture toughness diagram. The notch root critical stress intensity factor is a decreasing function of geometric stress concentration factor. __________ Translated from Problemy Prochnosti, No. 5, pp. 142–148, September–October, 2006. Report on International Conference “Dynamics, Strength, and Life of Machines and Structures” (1–4 November 2006, Kiev, Ukraine).     

Mechanics and physics of quasibrittle fracture of polycrystalline metals under conditions of stress concentration Part 3. Toughness of metals and alloys

We consider new characteristics of toughness based on the fundamental ideas concerning the micromechanism of quasibrittle fracture of metals and taking into account the specific features of fracture under the conditions of stress concentration. Unlike the existing characteristics of toughness (e.g., impact toughness), these characteristics can be directly used in the numerical analysis of load-carrying structural elements. By analogy with the safety margin, we introduce the concept of the “safety margin of toughness” of a metal. We propose analytic relations for the evaluation of the safety factor of toughness of a metal in products containing structural stress concentrators and macrocracks. For these types of concentrators, we deduce the relationships between the ordinary safety factor and the safety factor of toughness. We analyze the values of toughness for low-, medium-, and high-strength steels and propose optimal combinations of the strength and toughness characteristics of metals. The relationships between the new characteristics of toughness and K_lc are also established. Institute of Physics of Metals, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 72–92, January–February, 2000     

High temperature flexural strength and fracture toughness of AlN with Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic

The effects of temperature on the fast fracture behavior of aluminum nitride with 5 wt% Y₂O₃ ceramic were investigated. Four-point flexural strength and fracture toughness were measured in air at several temperatures (30–1,300 °C). The flexural strength gradually decreased with the increase of temperature up to 1,000 °C due to the change in the fracture mode from transgranular to intergranular, and then became almost constant up to 1,300 °C. Two main flaw types as fracture origin were identified: small surface flaw and large pores. The volume fraction of the large pores was only 0.01%; however, they limited the strength on about 50% of the specimens. The fracture toughness decreased slightly up to 800 °C controlled by the elastic modulus change, and then decreased significantly at 1,000 °C due to the decrease in the grain-boundary toughness. Above 1,000 °C, the fracture toughness increased significantly, and at 1,300 °C, its value was close to that measured at room temperature.     

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.     

Improving the fracture toughness of MgO–Al2O3–SiO2 glass/molybdenum composites by the microdispersion of flaky molybdenum particles

The flake-forming behaviour of powders of molybdenum, niobium, nickel, BS 316 S 12, Ni–17Cr–6Al–0.6Y, iron, titanium and Ti–6Al–4V, using a wet ball mill, was investigated. MgO–Al₂O₃–SiO₂ (MAS) glass composites reinforced with these flaked particles were fabricated, and improvements in flexural strength evaluated. The MAS glass composites reinforced with flaky metallic particles such as molybdenum, niobium, iron, nickel and Ni–17Cr–6Al–0.6Y, showed an improvement. The effect of molybdenum particle size on the flake-forming behaviour of molybdenum, flexural strength and fracture toughness of MAS glass/molybdenum composites, were investigated. The flake-forming behaviour shows a high degree of dependence on molybdenum particle size and, upto a size of 32 μm, becomes conspicuous with increasing particle size. At 32 μm, the aspect ratio reaches a value of 17 and, above 32 μm, flake forming saturates. Fracture toughness is closely related to flake-forming behaviour and the more marked the flake forming, the greater is the increase in fracture toughness. A composite of MAS glass with flaky molybdenum particles has a greater improvement effect on fracture toughness than composites with SiC whiskers, SiC platelets or ZrO₂ particles. This is closely linked to plastic deformation of the flaky metallic particles at the crack tip at the time of fracture. This revised version was published online in November 2006 with corrections to the Cover Date.     

Modeling the brittle–ductile transition in ferritic steels: dislocation simulations

We present a model for the brittle–ductile transition in ferritic steels based on two dimensional discrete dislocation simulations of crack-tip plasticity. The sum of elastic fields of the crack and the emitted dislocations defines an elasto–plastic crack field. Effects of crack-tip blunting of the macrocrack are included in the simulations. The plastic zone characteristics are found to be in agreement with continuum models, with the added advantage that the hardening behavior comes out naturally in our model. The present model is composed of a macrocrack with microcracks ahead of it in its crack-plane. These microcracks represent potential fracture sites at internal inhomogeneities, such as brittle precipitates. Dislocations that are emitted from the crack-tip account for plasticity. When the tensile stress along the crack plane attains a critical value σ _F over a distance fracture is assumed to take place. The brittle–ductile transition curve is obtained by determining the fracture toughness at various temperatures. Factors that contribute to the sharp upturn in fracture toughness with increasing temperature are found to be: the increase in dislocations mobility, and the decrease in tensile stress ahead of the macrocrack tip due to increase in blunting, and the slight increase in fracture stress of microcracks due to increase in plasticity at the microcrack. The model not only predicts the sharp increase in fracture toughness near the brittle–ductile transition temperature but also predicts the limiting temperature above which valid fracture toughness values cannot be estimated; which should correspond to the ductile regime. The obtained results are in reasonable agreement when compared with the existing experimental data.     

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.     

Comparative analysis of fracture toughness test methods for ceramics and crystals at room and lower temperatures

Flexural and indentation tests to estimate the fracture toughness of ceramics and crystals were compared. It was demonstrated that different methods can give reliable estimates in tests of homogeneous ceramics and quite ambiguous in tests of nonhomogeneous materials. Zirconia ceramics partially stabilized with magnesia were shown to exhibit a low-temperature inelastic-elastic transition in the temperature range of their fracture toughness variations. Translated from Problemy Prochnosti, No. 3, pp. 104–118, May–June, 1997.     

Effects of loading rate, notch geometry and loading mode on the local cleavage fracture stress of a C–Mn steel

In this work, notched specimens with two notch geometries were tested in two loading modes (four-point bending (4PB) and three-point bending (3PB)) at various loading rates at a temperature of − 110°C for a C–Mn steel. An elastic–plastic finite-element method (FEM) is used to determine the stress distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ _f is measured. The results obtained and combining with previous studies by the authors show that the local cleavage fracture stress σ _f is closely related to the cleavage fracture mechanism (critical events) in steels. The σ _f values do not change with loading rate, notch geometry and loading mode, as long as the critical event of cleavage fracture does not change at various testing conditions. The σ _f is mainly determined by the steel microstructure, and its scatter is mainly caused by the size distribution of the weakest constituent in steels (ferrite grain or pearlite colony with large sizes and large second phase particles) and the change of the critical events in cleavage process. The σ _f can characterize the intrinsic toughness of steels and may be used in a “local approach” model for assessing integrity of flawed structures. The σ _f values could be measured by both 4PB and 3PB tests.     

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.     

Prediction of crack resistance for heat-resistant steels taking into account the specimen size effect. Communication 1. Results of experimental investigations

We present a large volume of unique experimental data on local fracture parameters and crack resistance characteristics of different structural alloys, including steels 15Kh2MFA and 15Kh2NMFA and their welded joints, subjected to different heat treatments, simulating exposure to different doses of radiation bombardment and used in manufacturing VVéR-440 and VVéR-1000 atomic reactor pressure vessels. These experimental results allow us to generalize the scientific conclusions concerning the scale effect in fracture mechanics, since they were obtained by investigating many materials (σ_0.2 = 270–1500MPA) over a broad range of variation in specimen thickness (t=12.5–150 mm) and experimental temperature (T=77–573 K). The data on the fracture toughness characteristics of a broad class of structural alloys suggest that the scale effect is complex and contradictory, depending first of all on the range of variation in the specimen thickness and the physicomechanical properties of the material, including those determined by the effect of production and service factors. On the whole, the experimental data support the possibility noted in various literature sources that there are three different types of dependences of the fracture toughness on the specimen size (the fracture toughness can increase, decrease, or stay the same as the specimen size increases). It does not seem possible to predict and explain these dependences within approaches proposed earlier. Translated from Problemy Prochnosti, No. 1, pp. 5–25, January–February, 1997.