Higher Fracture Toughness Values Research Articles

Effects of irradiation on mechanical properties of nuclear UO2 fuels evaluated by Vickers indentation at room temperature

Vickers indentation was used to evaluate the hardness and the apparent fracture toughness of unirradiated and irradiated UO2 fuels, with the objective to determine the effects of the irradiation on mechanical properties. Indentation tests were realized at room temperature on standard and doped UO2 fuels, irradiated in a pressurised water reactor up to a burnup of 81.6 GWd/tM. For the highest burnups, the fuels had restructured in a high burnup structure (HBS) on the periphery of the pellets.More than 100 valid impressions for the Vickers hardness determination were obtained on each fuel. The Vickers hardness of both UO2 increased with the local burnup, due to irradiation defects such as dislocations and fission products but decreased when the fuel restructured into the HBS.Valid crack patterns for the Vickers fracture toughness were only obtained for indentations inside the large grains of irradiated doped UO2 fuels or for both restructured fuels. The Vickers fracture toughness of the non-restructured doped UO2 was shown to increase quickly during the first irradiation cycle but was then constant with increasing local burnup. This increase of the apparent fracture toughness was mainly due to the decrease of the crack length from the corner of the indents, attributed to the interactions of the cracks with the defects such as bubbles or fission product precipitates and associated residual stresses. In the HBS, short cracks from the corner of the indents that followed grain boundaries of small grains led to high fracture toughness values. However, the use of the literature equations to calculate the apparent fracture toughness on porous samples such as HBS is questioned. All tests clearly illustrated the weakness of the grain boundaries in irradiated fuels.

Experimental investigation on Mode-I fracture toughness of Carbon-Carbon composites fabricated by preformed yarn method

Carbon-Carbon composites are one such material that gives designers significant importance for advanced applications over conventional materials. They are applied in applications at very high temperatures (up to 3000 °C), and under extreme conditions. They have a density which is much less than metals and ceramics and thus, make low part weight a significant factor for aerospace applications. Fracturing toughness is a measurable way to express a substance's resistance to fracture in the case of a break.This paper describes the experimental studies done to investigate the Mode-1 fracture toughness of carbon–carbon composites which was fabricated via the preformed yarn method. Fracture toughness was determined for four different (a/w) ratios i.e. 0.45, 0.47, 0.50 and 0.52 respectively. The results showed that with only two cycles of pitch impregnation, HIP and graphitizing, carbon–carbon composites were successfully produced. The specimen having an (a/w) ratio of 0.45 had a higher fracture toughness value in comparison with all values. As the (a/w) ratio was increasing, the fracture toughness value decreased and the fractured surface clearly shows a brittle fracture behavior.

Growth, characterization and thermo-mechanical analysis of Al/Al2O3 core/shell nanoparticles obtained under H2 atmosphere

In this work, growth, characterization and thermo-mechanical behavior of Al/Al2O3 core/shell nanoparticles (NP) is performed. The growth was carried out by gas condensation methods using H2 as carrier gas at a pressure of 100 Pa and with a temperature of the evaporation source of 1315 °C. The prepared NP were characterized by Energy Dispersive X-ray Spectroscopy for chemical information, transmission electron microscopy for morphological study, and electron diffraction patterns for structural information. The prepared Al NP exhibit a thin Al2O3 passivating oxide shell due to the air exposure when removed from the preparation vacuum chamber. Afterwards, the growth of the Al2O3 oxide shell of the Al NP and the thermo-mechanical interaction between this growing oxide shell and its Al core was studied by performing in-situ thermal X-ray diffraction from RT up to 505 °C. Coefficients of thermal expansion of both Al core and Al2O3 oxide shell were obtained by means of X-ray strain analysis. It was found that the thickness of the Al2O3 shell increases with temperature and the thermal stress induced in the system increases linearly with temperature. Our results highlight that these NP overcome higher values of fracture toughness compared with Al2O3 NP used in micro-nano composites, thus improving their mechanical properties for nanofluid applications.

Improving delamination resistance through tailored defects

Delamination is a well-known damage mode exhibited by laminated composites which has been extensively studied in unidirectional laminates (UD). In this work, a novel strategy consisting of the introduction of a small interlaminar defect to enhance the fracture resistance to delamination of UD composites is presented. This toughening concept is based on the simultaneous propagation of multiple interlaminar cracks along different interfaces. A preliminary numerical study showed that the minimum size required by the initial defect to trigger dual propagation was 4 times the ply thickness for intermediate and thin plies for any applied mixed mode, except for pure mode II. An experimental campaign to prove the toughening concept with 1 and 3 defects at adjacent interfaces was carried out and compared against a reference configuration under three different applied mode mixities (ϕglob=0.4,0.2 and 0). The single defect configuration reached an improvement in the fracture resistance of +300% under pure applied mode I, and +80% and +100% for applied mixed modes of 0.2 and 0.4, respectively. The configuration with 3 defects reached even higher fracture toughness values: +430% in pure applied mode I, and +115% and +100% for applied mixed modes of 0.2 and 0.4, respectively. A finite element model employing cohesive zones successfully predicted both, the phenomenon of multiple crack propagation, and the effective fracture resistance of the process. Interestingly, among the dissipation mechanisms reported experimentally, the extensive promotion of fiber bridging under applied pure mode I was observed, which was not reported in the baseline configuration.

Fabrication of mullite nano ceramic through addition of long-chain carbohydrates

Compared to common coarse-grained ceramics, nano-scaled mullite ceramics exhibit unique microstructures and mechanical properties. However, fabrication of homogeneous and ultra-fine ceramic crystals still remains a major challenge. In this study, fine-grain mullite nanocrystals with uniform particle distribution have been successfully synthesized by addition of agarose, which is a long-chain carbohydrate to the precursor. The effect of length of the carbohydrate chain on the size of mullite crystals has been investigated. The results show that long-chain carbohydrate agarose results in smaller grain size of mullite crystals as compared with glucose. Moreover, nano-scaled mullite ceramics with high hardness (18.36 GPa) and fracture toughness values (3.7 MPa m1/2) were obtained owing to the high relative density and small grain size. The present work highlights its potential application as long-chain carbohydrate in producing high-performance nanoceramics.

Antibacterial performance of glucose-fructose added MW based zirconia coatings – Possible treatment for bone infection

Use of ceramic coatings has increased dramatically in orthopedics by improving their wear resistance and consequent long-term stability. Such stability involves not only the strength of material but also its resistance toward bacterial attacks. Amongst all ceramics, zirconia is selected in the present study due to its white color and high value of hardness making it a potential candidate to be used as implants and their coatings. In the present study effect of varying microwave powers (i.e. 100W, 200W, 300W, 400W, 500W, 600W, 700W, 800W, 900W and 1000W) on sol–gel synthesized glucose and fructose added zirconia coatings has been investigated. Formation of mixed tetragonal – monoclinic phases has been observed at relatively low microwave powers, i.e. 100–500W. However, at 600–1000W phase pure tetragonal zirconia is observed without any post heat treatment. FTIR analysis confirms formation of tetragonal phase of zirconia at 600–1000W microwave power. XPS results confirm the binding energies of Zr 3d and O 1s of microwave assisted zirconia coatings. High value of transmittance, i.e. ~90%, is observed at higher microwave powers. Variation in microwave powers is observed to tune the energy band gap of zirconia coatings in the range of 4.2–5.1 eV. Dielectric constant of 8–10 at log f = 4 is observed. High value of hardness and fracture toughness i.e. 1231 HV and 24.85 MPam−1/2, respectively, is observed for stabilized tetragonal zirconia coatings. Stabilized glucose fructose added zirconia shows strong antioxidant activity. Zirconia coatings are tested against Staphylococcus aureus bacteria for their potential application to treat bone infection. Results suggest that stabilized tetragonal zirconia can be successfully employed for orthopedic coatings.

Nano-diamond reinforced ZrB2–SiC composites

The effect of adding various amounts of nano-diamond additive (0, 1, 2, and 3 wt%) on the densification behavior and mechanical properties of ZrB2-25 vol% SiC samples were investigated in this research. All samples were spark plasma sintered at 1900 °C under 40 MPa external pressure for 7 min. Relative density (RD) values higher than 99.9% were obtained for the samples with 0, 1, and 2 wt% nano-diamond, while adding 3 wt% diamond dropped the RD by ~1.2%. The XRD and microstructural evaluations revealed the formation of some in-situ phases, namely ZrC and B4C. The highest Vickers hardness (24.7 GPa) and fracture toughness (5.8 MPa m1/2) were achieved for the samples doped with 2 and 3 wt% nano-diamond, respectively. Ultimately, the SEM micrographs indicated the role of different toughening mechanisms on obtaining such a high value of fracture toughness.

Silicon nitride with titania, calcia and silica additives for orthopaedic applications

Titanium, silicon and calcium ions have demonstrated positive effects in bone healing. Therefore, this paper investigates the densification, mechanical properties and in vitro biological behaviour of TiO2, together with SiO2 and CaO, on silicon nitride ceramics to be used in clinical applications. The results revealed that the sintered samples reached high values of relative density and fracture toughness, non-cytotoxicity as well as good ability to promote cell proliferation and form a layer of carbonated hydroxyapatite on their surfaces, after immersion in simulated body fluid. The sample with the highest amount of TiO2 reached the highest value of relative density and the best results of cell proliferation and fracture toughness, indicating the great potential to be explored by in vivo experiments for later application as intervertebral devices.

Effect of recycled poly(vinyl chloride) on the mechanical, thermal and rheological characteristics of recycled poly(methyl methacrylate)

The recycled poly(vinyl chloride) (r-PVC) collected from the data cable has been incorporated into the recycled poly(methyl methacrylate) (r-PMMA) matrix in different weight percentage through the melt blending technique. The obtained recycled blends were analysed for mechanical, rheological, thermal and morphological characteristics. Among the various recycled blends formulated, 70:30 wt% of r-PVC/r-PMMA exhibits higher fracture toughness value. The rheological analysis highlights that recycled blends have pseudo-plastic characteristics. Furthermore, the storage and loss modulus values are decreasing with the increment of r-PVC in the recycled blend. The scanning electron microscope (SEM) study reveals the transformation of recycled blend matrix from brittle to the ductile character with the higher concentrations of r-PVC. Similarly, the water absorption, chemical resistance behaviour of the developed blends was depended on the concentration of r-PVC in the recycled blend.

Effect of Preheating and Fatiguing on Mechanical Properties of Bulk-fill and Conventional Composite Resin.

Bulk-fill composite resins may have comparable mechanical properties to conventional composite resin. Preheating does not reduce the mechanical properties of composite resins.

Determination of fracture toughness and elastic module in materials based silicon nitride

The knowledge of the mechanical properties of any material subjected to loads is necessary for its use in structural applications. Silicon nitride (Si3N4) ceramics are well-known materials used in engineering applications due to their outstanding combination of high strength and fracture toughness. The most studied mechanical properties of Si3N4 are hardness, fracture toughness and mechanical resistance. Recent advances in the production processes that incorporate high purity rare earth elements as sintering additives have improved these mechanical properties. Using Vickers indentation method, the elastic module and fracture toughness of Si3N4 based materials modified with La2O3, Y2O3 and Al2O3 were determined as a function of the cracking system type that prevails under the effect of load. The results indicate that adding rare earth to the matrix increased the fracture toughness the Si3N4 base ceramic Samples containing La2O3+Y2O3 showed higher values of fracture toughness than the ones with Al2O3+La2O3, regardless of the equation used in the calculations. Meanwhile the elastic module decrease approximately 100 GPa for both types of nitrides by the effect of the temperature.

Effect of Initial Powders on Properties of FeAlSi Intermetallics.

FeAlSi intermetallics are materials with promising high-temperature mechanical properties and oxidation resistance. Nevertheless, their production by standard metallurgical processes is complicated. In this study, preparation of powders by mechanical alloying and properties of the samples compacted by spark plasma sintering was studied. Various initial feedstock materials were mixed to prepare the material with the same chemical composition. Time of mechanical alloying leading to complete homogenization of powders was estimated based on the microstructure observations, results of XRD and indentation tests. Microstructure, phase composition, hardness and fracture toughness of sintered samples was studied and compared with the properties of powders before the sintering process. It was found that independently of initial feedstock powder, the resulting phase composition was the same (Fe3Si + FeSi). The combination of hard initial powders required the longest milling time, but it led to the highest values of fracture toughness.

Nb and B effect on mechanical properties of Ti–Al based intermetallic materials

Ti–TiAl3 in-situ composites containing different percentages of Nb and B were effectively produced from Ti, Al, Nb and B powders by electric current assisted sintering (ECAS) technique which is a powder metallurgy processing method. Samples are sintered for 90 s with 2000 A current. The effect of B and Nb on the hardness, fracture toughness and wear resistance of samples were studied. The microstructure properties of the sintered samples were analysed with scanning electron microscopes (SEM), the phases in the samples were determined with XRD and their hardness and fracture toughness values were measured with a Vickers hardness tester with a load of 0.98 N and 98 N respectively. The highest fracture toughness value has been obtained with wt %10 Nb addition as 5.23 MPa m1/2, whereas the highest hardness was determined as 965 HV for wt%5 B reinforced in situ-Ti-TiAl3 composite. Best wear resistance was obtained in the 47.5Ti-47.5Al–5B sample. While Nb additive had a negatory effect on wear resistance, additive B had a positive effect on wear resistance.

Effect of high-temperature annealing on the valence state of Ce ions and the mechanical properties of (ZrO2)0.972 (Y2O3)0.02 (CeO2)0.008 crystals

The effect of high-temperature annealing in various atmospheres on the change in the valence state of Ce ions as well as on the phase composition and mechanical properties of (ZrO2)0.972(Y2O3)0.02(CeO2)0.008 crystals has been studied. The crystals have been grown by directional crystallization of the melt in a cold crucible. Electron paramagnetic resonance method has been used to identify Ce3+ ions and their relative amount in crystals. The phase composition and structure of the crystals have been studied by X-ray diffraction and transmission electron microscopy. Hardness and fracture toughness of crystals have been studied by indentation. We show that (ZrO2)0.972(Y2O3)0.02(CeO2)0.008 crystals have high values of hardness and fracture toughness. Anisotropy of fracture toughness values has been detected. Thus the observed increase in the fracture toughness of crystals after vacuum annealing is caused by the increasing transformable t phase content and Ce3+ ion concentration. Sequential annealing in oxidizing and reducing atmospheres has led to reversible changes in the valence state of Ce ions as well as in the phase composition and fracture toughness of (ZrO2)0.972(Y2O3)0.02(CeO2)0.008 crystals.

Electromagnetic-wave sintering of alumina ceramics from nano-sized particles: possible material for high-pressure cell for millimeter-wave electron spin resonance

Electromagnetic-wave sintering of alumina ceramics using 28 GHz gyrotron has been performed aiming for a high fracture toughness value in order for use as pistons for pressure cells of high-frequency electron spin resonance (ESR) measurements. We have tried to improve the fracture toughness by using nano-sized alumina powder (140 nm in average particle size) having smaller particle size than our previous work (400 nm in average particle size). Rapid densification was observed around the sintering temperature of 1200 °C. We obtained the relative density over 99 % above 1400 °C. It was found that alumina ceramics made in this work at sintering temperatures have smaller grain size and higher density simultaneously as compared to the previous work. These results suggest that alumina ceramics made from powder of smaller particle size sintered by electromagnetic-wave sintering possibly have high fracture toughness which can be used as materials for the pressure cell for ESR.

Influence of in-situ Al2O3 content on mechanical properties of Al2O3 reinforced Fe–Cr–Ni alloys

AbstractIn-situ Al2O3 (14.8–31.3 wt.%) reinforced Fe–Cr–Ni alloys were prepared using reactive hot pressing at a temperature of 700°C and pressure of 30 MPa for 2 h. X-ray diffraction showed that the obtained metal matrix composites (MMCs) mainly comprised Al2O3, Fe–Cr and Cr0.7Fe0.36Ni2.9 phases. Vickers hardness values of the composites with varying Al2O3 contents were all around 4.28 GPa. Scanning electron microscopy images showed that the Al2O3 particles in the MMCs (with Al2O3 content of 14.8 wt.%) were ultrafine and uniformly distributed. Moreover, the metal matrix composite with 14.8 wt.% Al2O3 exhibited the highest values of fracture toughness and bending strength, which were 15.2 MPa · m1/2 and 529 MPa, respectively.

Mo‐doped CeO2 Synthesized by the Combustion Method for Carbon‐Air Solid Oxide Fuel Cell (CA‐SOFC) Applications

AbstractMolybdenum(Mo)‐doped ceria (CMO) nanoparticles were synthesized by the combustion method with three different Mo compositions: 5 wt.%, 7 wt.%, and 10 wt.%. The catalytic activity of CMO for wet gasification of carbon was studied in a fluidized bed reactor, while the mechanical and electrical properties of this material were characterized using dense sintered CMO pellets.The Young's modulus was found to increase with the Mo content; the higher value measured was 289.4 GPa for CMO with 10 wt.% Mo. Measurements of Vickers microhardness demonstrated that an increase in the Mo content produces a decrease in the microhardness of the material, suggesting that Mo confers semi‐metallic characteristics to CMO. The higher fracture toughness value, determined by the Niihara equation, was 4.39 MPa m0.5 for CMO with 10 wt.% Mo. In addition, an increase in the molybdenum content produced an increase in the electrical conductivity under air and H2 atmospheres. The maximum electrical conductivities under air and H2 were found for CMO with 10 wt.% Mo at 800 °C: 1.87 × 10−3 S cm−1 and 9.37 × 10−1 S cm−1, correspondingly. Finally, CMO with 10 wt.% Mo exhibited good catalytic activity for carbon gasification, which renders it a promising material for a combined fluidized bed‐SOFC system.

Recycling of WC-TiC-TaC-NbC-Co by zinc melt method to manufacture new cutting tools

Abstract This paper discusses manufacturing of tungsten carbide cutting materials from both fresh powders and recycled ones. As received powder with nominal chemical composition of WC-9% Co-6% TiC-1% TaC-1%NbC was used for preparing cutting tool. Various compaction pressures from 250 to 730 MPa and sintering temperatures from 1350 to 1450 °C were used in the study. The density, hardness and fracture toughness were estimated for all sintered samples. Statistical analysis showed that the most suitable compaction pressure and sintering temperature are 460 MPa and 1400 °C, respectively. The highest relative density and hardness values are 98% and 1710 HV30, respectively for the sample compacted at 730 MPa and sintered at 1450 °C, while the highest value of fracture toughness is 11.657 MPa m1/2 for the sample compacted at 460 MPa and sintered at 1350 °C. The prepared samples were recycled by zinc melt method and the best ratio of zinc to carbide is 1.4:1, which has the highest efficiency of recycled powders (91.55–97.28%). The recycled WC powder was mixed with the fresh one by 10, 20, 30… and 100 wt% and was used to manufacture new samples. Results have shown that 70 wt% recycled powder is the optimal mixing composition.

Fracture toughness of cemented carbides obtained by electrical resistance sintering

The unique combination of hardness, toughness and wear resistance exhibited by WC-Co cemented carbides (hardmetals) has made them a preeminent material choice for extremely demanding applications, such as metal cutting/forming tools or mining bits, in which improved and consistent performance together with high reliability are required. The high fracture toughness values exhibited by hardmetals are mainly due to ductile ligament bridging and crack deflection (intrinsic to carbides). In this work two WC-Co grades obtained by using the electric resistance sintering technique are studied. The relationships between the process parameters (cobalt volume fraction, sintering current and time, die materials, etc.), the microstructural characteristics (porosity, cobalt volume fraction, carbide grain size, binder thickness and carbide contiguity) and mechanical properties (Vickers hardness and fracture toughness) are established and discussed. Also the presence of microstructural anisotropy and residual stresses is studied. The sintering process at 7 kA, 600 ms and 100 MPa, in an alumina die, followed by a treatment of residual stress relief (800 °C, 2 h in high vacuum), allows to obtain WC-Co pellets with the best balance between an homogeneous microstructure and mechanical behaviour.

Enhanced thermo-mechanical damage tolerance of functionally graded ZrB2-20SiC ceramic reinforced with carbon nanotubes

The demand of a new generation spacecraft component has led to the development of a novel structure, which combines the traits of both functionally graded material (FGM) as well as an ultra-high temperature ceramics (UHTC). Herein, FGM based on the well-established ZrB2-20SiC (vol%) UHTC with the reinforcement of carbon nanotubes (CNT) is processed via powder metallurgy and spark plasma sintering (SPS). A densely graded microstructure with a crack-free seamless interface is witnessed along the cross-section of the FGM. The CNT in the optimized FGM is found to be efficaciously embedded in the matrix retaining its structure during sintering (at 1850 °C), shown microstructurally and supplemented by Raman spectroscopy. Enhanced mechanical and wear resistance of the FGM when compared to that of the conventional ZrB2 UHTC is attributed to the dense, crack-free microstructure and CNT pull-out mechanism. Oxidation test revealed an enhancement in the thermal stability as the onset of oxide formation starts above 886 °C (i.e. > 30% when compared to that of monolithic ZrB2). The thermo-structural performance of the processed FGM simulated using finite element method (FEM) indicated a reduction in the thermal residual tensile stresses (~ 8.1 times) which mitigates the crack propagation. The FEM analysis complements the high indentation fracture toughness values of the FGM even at the interfaces (similar to that found in each layer), establishing FGM approach superior to that of the conventional UHTC composites. Thus, addition of CNT in ZrB2-20SiC based FGM are conducive enough to be engineered as a potential material, poised to shape the future of re-entry vehicles.