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.     

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.     

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.     

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 studies on V-notched ceramic specimens

The fracture toughness of silicon nitride, alumina, and zirconia ceramics was compared by the SEVNB method. This method, easily realized in practice both for compact and for composite ceramics, is shown to provide small scatter of data. The K_lc values determined by this method correspond to those obtained by the SEPB method. Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 120–127, January–February, 2000.     

Experimental and Numerical Investigation of Mixed-Mode Interlaminar Fracture of Carbon-Polyester Laminated Woven Composite by Using Arcan Set-up

Composite materials are widely used in marine, aerospace and automobile industries. These materials are often subjected to defects and damages from both in-service and manufacturing process. Delamination is the most important of these defects. This paper reports investigation of mixed-mode fracture toughness in carbon–polyester composite by using numerical and experimental methods. All tests were performed by Arcan set-up. By changing the loading angle, α, from 0° to 90° at 15° intervals, mode-I, mixed-mode and mode-II fracture data were obtained. Correction factors for various conditions were obtained by using ABAQUS software. Effects of the crack length and the loading angle on fracture were also studied. The interaction j-integral method was used to separate the mixed–mode stress intensity factors at the crack tip under different loading conditions. As the result, it can be seen that the shearing mode interlaminar fracture toughness is larger than the opening mode interlaminar fracture toughness. This means that interlaminar cracked specimen is tougher in shear loading condition and weaker in tensile loading condition.     

Synthesis, microstructure and mechanical properties of ceria stabilized tetragonal zirconia prepared by spray drying technique

Ceria stabilized zirconia powders with ceria concentration varying from 6 to 16 mol% were synthesized using spray drying technique. Powders were characterized for their particle size distribution and specific surface area. The dense sintered ceramics fabricated using these powders were characterized for their microstructure, crystallite size and phase composition. The flexural strength, fracture toughness and microhardness of sintered ceramics were measured. High fracture toughness and flexural strength were obtained for sintered bodies with 12 mol% of CeO₂. Flexural strength and fracture toughness were dependent on CeO₂ concentration, crystallite size and phase composition of sintered bodies. Correlation of data has indicated that the transformable tetragonal phase is the key factor in controlling the fracture toughness and strength of ceramics. It has been demonstrated that the synthesis method is effective to prepare nanocrystalline tetragonal ceria stabilized zirconia powders with improved mechanical properties. Ce-ZrO₂ with 20 wt% alumina was also prepared with flexural strength, 1200 MPa and fracture toughness, 9.2 MPa√m.     

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.     

Development and cytotoxicity evaluation of SiAlONs ceramics

SiAlONs are ceramics with high potential as biomaterials due to their chemical stability, associated with suitable mechanical properties, such as high fracture toughness and fracture resistance. The objective of this work was to investigate the mechanical properties and the cytotoxicity of these ceramic materials. Three different compositions were prepared, using silicon nitride, aluminum nitride and a rare earth oxide mixture as starting powders, yielding Si₃N₄–SiAlON composites or pure SiAlON ceramics, after hot-pressing at 1750 °C, for 30 min. The sintered samples were characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). Furthermore, hardness and fracture toughness were determined using the Vicker's indentation method. The biological compatibility was evaluated by in vitro cytotoxicity tests. Ceramic with elevated hardness, ranging between 17 and 21 GPa, and high fracture toughness of 5 to 6 MPa m^1/2 were obtained. Since a nontoxic behavior was observed in the cytotoxicity tests, it may be assumed that SiAlON-based ceramics are viable materials for clinical applications.     

Three-dimensional fractal analysis of fracture surfaces in titanium–iron particulate reinforced hydroxyapatite composites: relationship between fracture toughness and fractal dimension

Fractal dimension has been considered as a measure of fracture surface roughness of materials. Three-dimensional (3D) surface analysis is anticipated to provide a better evaluation of fracture surface toughness and fractal dimension. The objective of this study was to quantify the fracture surfaces and identify a potential relationship between fracture toughness and fractal dimension in a new type of core–shell titanium–iron particulate reinforced hydroxyapatite matrix composites using SEM stereoscopy coupled with a 3D surface analysis. The obtained results showed that both fracture surface roughness and fractal dimension increased with increasing amount of core–shell Ti–Fe reinforcing particles. The fractal dimension was observed to be a direct measure of fracture surface roughness. The fracture toughness of the composites increased linearly with the square root of fractal dimensional increment (i.e., followed the Mecholsky–Mackin equation well) due to the presence of Ti–Fe particles along with the effect of porosity in brittle materials. The 3D fractal analysis was suggested to be a proper tool for quantifying the fracture surfaces and linking the microstructural parameter to fracture toughness.     

The effect of temperature and loading rate on the crack initiation and propagation energy in carbon steel charpy specimens

The Charpy impact tests were carried out at different temperatures and loading rates. The temperature dependences of crack initiation and propagation in carbon steels 45 and St. 3 under impact testing were determined from the obtained force variation plots. The effect of the impact velocity in the range from 1 to 4.4 m/s on the fracture toughness temperature dependence is estimated. __________ Translated from Problemy Prochnosti, No. 5, pp. 120–127, September–October, 2006.     

Synthesis and characteristics of monticellite bioactive ceramic

Mono-phase ceramics of monticellite (CaMgSiO₄) were successfully synthesized by sintering sol–gel-derived monticellite powder compacts at 1,480 °C for 6 h. The mechanical properties and the coefficient of thermal expansion (CTE) of the monticellite ceramics were tested. In addition, the bioactivity in vitro of the monticellite ceramics was evaluated by investigating their bone-like apatite-formation ability in simulated body fluid (SBF), and the biocompatibility in vitro was detected by osteoblast adhesion and proliferation assay. The results showed that the bending strength, fracture toughness and Young’s modulus of the monticellite ceramics were about 159.7 MPa, 1.63 MPa m^1/2 and 51 GPa, respectively. The CTE was 10.76 × 10⁻⁶ °C⁻¹ and close to that of Ti-6Al-4V alloy (10.03 × 10⁻⁶ °C⁻¹). Furthermore, the monticellite ceramics possessed bone-like apatite-formation ability in SBF and could release soluble ionic products to significantly stimulate cell growth and proliferation. In addition, osteoblasts adhered and spread well on the monticellite ceramics, which indicated good bioactivity and biocompatibility.     

Mechanical properties and microstructure of reaction sintered β′-sialon ceramics prepared by a slip casting method

Reaction sintered β′-sialon ceramics Si_6-zAl_zO_zN_8-z, were prepared by slip casting from α-Si₃N₄, Al₂O₃, and AlN starting powders. The mechanical properties and microstructures of sintered bodies were investigated as a function of composition (varying the z value). The maximum value of the flexural strength, ∼ 600 MPa, and fracture toughness, ∼ 4.1 MPa m^1/2 were observed in the z range of 0.5–1. In the z value range of 2–4, the mechanical properties decreased drastically. This phenomenon is attributed to the variation of fracture energy, which is greatly affected by the sintered crystallite size. This revised version was published online in November 2006 with corrections to the Cover Date.     

A new method of fracture toughness determination in brittle ceramics by open-crack shape analysis

Improvement of the fracture toughness of high-quality ceramics remains one of the most important goals in materials development. An associated problem is the accurate measurement of fracture toughness in such brittle or semi-brittle ceramics, particularly in small samples encountered in material development. Previously used methods relying on measurement of the size of fracture mirrors, the indentation load and crack length in Vickers hardness-induced cracking, and a variant of similar techniques, have all been less than satisfactory in discriminating quantitative differences among materials. A hitherto unused technique of inferring the fracture toughness in samples from measurements of open-crack flank displacements, which we have developed, avoids most of the theoretical and experimental difficulties of other methods. While it is somewhat intensive in terms of evaluation and requires high resolution of open cracks, the technique is fundamentally the soundest of all techniques and is capable of furnishing discriminating results. We present results of its application to the measurement of some model materials such as soda–lime glass, single-crystal silicon, alumina, and a reaction-bonded silicon nitride whose porosity would ordinarily present difficulties with other methods. This revised version was published online in November 2006 with corrections to the Cover Date.     

Improvement in the mechanical properties of polycrystalline beta-alumina via the use of zirconia particles containing stabilizing oxide additions

Polycrystalline beta-alumina ceramics containing yttria-doped zirconia particles have been produced by hot-pressing and “two-peak” sintering schedules. With the former fabrication process, both a chemical reaction involving sodium metazirconate and α-alumina, and a direct mixing route were employed. The mechanical properties of the ceramics produced by the direct mixing route were superior to those produced by the chemical route. The maximum amount of tetragonal zirconia retention, and thus fracture toughness, obtained using direct mixing occurred for additions of 4wt% yttria-doped zirconia. An increase of ∼ 124% in the fracture toughness was obtained compared with the pure beta-alumina ceramic. Transfer of this fabrication route to a pressureless sintering schedule was less successful owing to difficulties in attaining full densification. The increases in strength observed with introduction of second phase zirconia could be attributed to an improvement in the degree of densification achieved, and the maximum increase in toughness was only ∼27%.     

Fracture Toughness Test with a Sharp Notch Introduced by Focussed Ion Beam

The validity of fracture toughness data obtained from tests with V-notched bending bars is affected by the notch root radius and the presence of R-curve behavior. A macroscopic test specimen has been developed that contains a notch introduced by focused ion beam machining. This produces a notch root radius of less than 0.1 μm, so that notch effects can be ignored for most ceramics. Also, due to the very small notch depths the influence of a rising R-curve should be very close to that of natural cracks. First tests, carried out on a Ce-doped zirconia ceramic resulted in a toughness of K _Ic ≈ 5.9 MPa√m.     

Effects of organic additives on microstructure and mechanical properties of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Green bodies of porous Si₃N₄ ceramics were shaped by extrusion technique using different organic additives as binder during extrusion molding. Different porosity, microstructures and mechanical properties after the extrusion, drying, debinding and sintering stages were investigated. The solid slurry content of 70–75% and extrusion pressure of 0.5–1.0 MPa had played a decisive role in the smooth realization of extrusion molding. The porous Si₃N₄ ceramics were obtained with excellent properties using 4% hydroxypropyl methyl cellulose (HPMC) as binder and polyethylene glycol (PEG) of molecular weight, 1000, as plasticizer with a density of 1.91 g cm⁻³, porosity of 41.70%, three-point bending strength of 166.53 ± 20 MPa, fracture toughness of 2.45 ± 0.2 MPa m^1/2 and Weibull modulus (m) of 20.75.     

Fracture mechanics of oilseed rape pods

The basic theory of linear elastic fracture mechanics was applied to the fracture of pods from six genetic lines of oilseed rape (Brassica napus). An experiment was designed to allow the energetics of the fracture process to be accurately determined. The work of fracture, toughness and fracture toughness of five experimental varieties and one common commercial variety (Apex) were measured. The values for the toughness (0.006–0.271 kJ m-2) and fracture toughness (0.026–0.233 MN m-3/2) obtained from each line were distinct from each other but broadly similar to those of other brittle materials. The toughness and fracture toughness of Apex were approximately midway between the lowest and highest values measured. This result indicates that there is scope for improving the fracture resistance of oilseed rape crops so as to reduce seed loss before and during harvest. The approach described would be useful in selecting fracture-resistant genetic lines to help to develop such crops. This revised version was published online in November 2006 with corrections to the Cover Date.     

Hardness properties and microscopic investigation of crack–crystal interaction in SiO<Subscript>2</Subscript>–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–K<Subscript>2</Subscript>O–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–F glass ceramic system

In view of the potential engineering applications requiring machinability and wear resistance, the present work focuses to evaluate hardness property and to understand the damage behavior of some selected glass–ceramics having different crystal morphologies with SiO₂–MgO–Al₂O₃–K₂O–B₂O₃–F composition, using static micro-indentation tests as well as dynamic scratch tests, respectively. Vickers hardness of up to 5.5 GPa has been measured in glass–ceramics containing plate like mica crystals. Scratch tests at a high load of 50 N in artificial saliva were carried out in order to simulate the crack–microstructure interaction during real-time abrasion wear and machining operation. The experimental observations indicate that the novel “spherulitic-dendritic” shaped crystals, similar to the plate like crystals, have the potential to hinder the scratching induced crack propagation. In particular, such potential of the ‘spherulitic-dendritic’ crystals become more effective due to the larger interfacial area with the glass matrix as well as the dendritic structure of each mica plate, which helps in crack deflection and crack blunting, to a larger extent. While modest damage tolerant behavior is observed in case of ‘spherulitic-dendritic’ crystal containing material, severe brittle fracture of plate like crystals were noted, when both were scratched at 50 N load.     

Effect of annealing on the microstructure, room-temperature strength, and fracture toughness of Al2O3−ZrO2−TiN ceramics

The microstructure, phase composition, room-temperature flexural strength, and fracture toughness of Al₂O₃−ZrO₂−TiN (AZT) ceramics were studied on specimens annealed in air at 1000, 1200, and 1400°C. The strength of the ceramics decreased with annealing temperature. The degradation in strength was caused by defects formed on or near the surface of the ceramics during oxidation of TiN which started at 600–700°C. The surface defects after annealing are influenced by the formation of rutile (TiO₂) at 1000 and 1200°C, aluminum titanate (Al₂TiO₅), and titanium suboxide Ti₅O₉ at 1400°C as well as by diffusion processes associated with ZrO₂. If the annealing of smooth AZT specimens in air resulted in lower strength, specimens in the form of single-edge notched beam (SENB) exhibited a considerable increase in fracture toughness (K _Ic) with annealing temperature. Such behavior was caused by the formation of an oxide layer which hindered the propagation of the main crack from the notch base. Thermal treatment of the smooth AZT specimens and further edge notching and testing did not result in a change of K _Ic values. The Al₂O₃ and Al₂O₃−ZrO₂ ceramics were also tested for comparison. Translated from Problemy Prochnosti, No. 1, pp. 132–138, January–February, 1999.