Fracture Toughness Kc Research Articles

Effect of Carbon Nanotubes on Strength Characteristics of Nanostructured Ceramic Composites for Biomedicine

The relationship between synthesis conditions, structure, and properties of the baddeleyite-based engineering nanostructured composite zirconia ceramic (natural zirconia mineral) with modifying alloying elements is studied. The elaborated composites possess high physical and mechanical properties at a level that is not only not inferior, but even superior to those of analogous ceramics prepared from precipitated zirconia (e.g., density is 0.95 of the theoretical value, the hardness reaches 12 GPa, the Young modulus is 220 ± 15 GPa, and the fracture toughness reaches 9 MPa m0.5). The embedding of carbon nanotubes (CNTs) is shown to alter the physical and mechanical properties of ceramics: the hardness is somewhat reduced, but fracture toughness KC gains more than 10%.

Enhancement of electro-optic and structural properties of TGS single crystals on doping with l-glutamic acid

Single crystals of pure and l-glutamic acid (LG) doped Triglycine Sulfate (TGS) crystals were grown by slow evaporation solution technique at ambient conditions. Effect of doping on various properties of the grown crystals was investigated. Powder X-ray diffraction (PXRD) studies confirmed the monoclinic system of crystal structure with lattice parameter a = ~ 9.28 A, b = ~ 12.7 A, and c = ~ 5.73 A, and space group of P21. PXRD and Fourier-Transform Raman (FT-Raman) analysis confirmed that there is no new phase formation due to doping except a systematic variation in the intensity of the peaks in correlation with the morphology due to LG doping. The Raman bands obtained in the spectrum corresponds to NH3 vibrations, the stretching vibrations of COO−, carboxyl vibrations, and vibrational modes of SO42−. Ultraviolet–Visible Spectroscopy (UV-Vis-NIR) analysis was carried out to see the changes in the optical transparency of pure TGS crystals due to LG doping. Optical band gaps (5.24 eV for PRTGS, and 5.07 eV for LGTGS) were calculated and found to decrease due to doping. The photoluminescence excitation and emission were studied. The thermal behavior of the grown crystal was investigated by Thermogravimetric analysis/Differential thermal analysis. Second harmonic generation (SHG) efficiency measurement showed the enhancement in the nonlinear optical characteristics of the as-grown pure and doped TGS single crystals. In the present study, the researchers found the good and comparable SHG efficiency with KDP in TGS crystals by LG doping for the first time. The surface morphology of the grown TGS single crystals was analyzed by using Scanning Electron Microscope. The mechanical studies showed the Mayer’s index (n) greater than 1.6 and thus predicting a soft-material nature of the as-grown crystals. The values of fracture toughness (Kc), brittleness indices (Bi), and yield strength (σν) were estimated for the crystals. The dielectric constant and the dielectric loss decreased with an increase in the value of frequency. Hysteresis loop showed a negligible change in the doped TGS. The above studies reveal the effect of incorporation of LG into the lattice of TGS crystals.

The Influence of Corundum Content and Sintering Temperature on the Mechanical Properties of CaO–ZrO2–Al2O3 Ceramic Composites

We have studied the mechanical properties (Vickers microhardness HV and fracture toughness KC) of nanostructured CaO–ZrO2–Al2O3 ceramic composites as dependent on the content of corundum (0 ≤ \(C_{Al_{2}O_{3}}\) ≤ 25%) and the temperature of sintering (1250°C ≤ T1 ≤ 1500°C). Optimum value of the corundum content (\(C_{Al_{2}O_{3}}\) = 5%) and optimum regime (T1 = 1300°C, 5 min; T2 = 1200°C, 4 h) of two-stage sintering are established, which favor attaining the best mechanical characteristics of ceramic composites (HV = 12.25 GPa, KC = 8.47 MPa m1/2).

Fracture Toughness of Thick Boride Layers Estimated by the Cross-Sectioned Scratch Test

New results about the fracture toughness (Kc) of thick boride layers estimated by the cross-sectioned scratch test are presented in this study. The FeB-Fe2B layers developed at the surface of borided AISI 1018 and AISI 1045 steels and the Fe2B layer formed on the borided AISI 1045 steel exposed to a diffusion annealing process (DAP) were used for this purpose. The cross-sectioned scratch tests were performed with a Vickers diamond stylus drawn across the thick boride layer under a constant load to produce a half-cone-shaped fracture near to the top surface of the borided steels. The height of the half-cone-shaped fracture as a function of the cross-sectioned scratch loads was used to determine the fracture toughness of the FeB and Fe2B layers. The results showed a fracture resistance of $$\sim2.8\,{\text{MPa}}\sqrt m$$ for the FeB layer formed at the surface of borided AISI 1045 steel. Likewise, the effect of the DAP on the surface of the borided AISI 1045 steel promoted the formation of an exclusively Fe2B layer, with an increase in the fracture toughness of the whole boride layer around $$5\,{\text{MPa}}\sqrt m$$ . Finally, the principle of the technique can be used to minimize the influence of the anisotropic properties on the fracture toughness along the depth of boride layers.

Structural, Optical and Mechanical Studies on a Novel Nonlinear Optical Material: L-Arginine Potassium Penta Borate(LAKB5) Crystals

A new nonlinear optical material L-Arginine doped potassium penta borate (LAKB5) Single Crystals have been grown by slow evaporation solution growth technique from its aqueous solution. The synthesized compound was subjected to single crystal X-ray diffraction analysis to recognize the morphology and structure. An assortment of modes of vibrations there in the crystal was identified with FTIR. The linear and nonlinear optical investigations were recorded in identification of optical transmission spectrum and Second Harmonic Generation (SHG). The mechanical strength have been checked by Vicker’s microhardness tester and consequently various parameters such as yield strength (σv), elastic stiffness constant (C11), fracture toughness (Kc) and brittleness index (Bi) were calculated and reported for the first time.

Nanomechanical characterization and fracture toughness of FeB and Fe2B iron borides produced by gas boriding of Armco iron

FeB and Fe2B hard ceramic phases were produced in Armco iron using gas-boriding in N2–H2–BCl3 atmosphere. This process was carried out at 920°C (1193K) for 3h and caused acceleration in the diffusion of boron into the surface of a base material in comparison with other acceptable methods of diffusion boriding. FeB and Fe2B layers were characterized by a strong zonation, obtaining the average thickness of 32μm and 125μm, respectively. Young's moduli and hardness of iron borides were measured using the nanoindenter with a Berkovich diamond tip under load of 50mN. The higher average indentation hardness (HIT=20.95±0.93GPa) and Young's modulus (EIT=308.86±26.44GPa) were characteristic of FeB phase. Fe2B boride was characterized by HIT=17.42±0.80GPa, and EIT=252.96±15.57GPa. The fracture toughness of iron borides was measured by microindentation technique using a Vickers diamond indenter under load of 100gf (about 0.981N). The average fracture toughness (Kc), measured in FeB zone, was equal to 1.79±0.70MPa·m1/2. Fe2B phase was characterized by higher fracture toughness, obtaining Kc=2.42±0.66MPa·m1/2. However, at FeB/Fe2B interface the increase in brittleness was detected. Such a situation was caused by the differences in coefficients of thermal expansion of both iron borides and their mechanical properties. It could provide the preliminary cracks at this interface after cooling. During indentation, the value of shear stress probably exceeded the value of normal compressive stress. It could cause the failure at this interface, facilitating cracks' propagation.

Evaluation of size effect on mixed-mode fracture behavior of epoxy/silica nanocomposites

The aim of this study was to examine the effect of size on the mixed-mode fracture toughness of quasi-brittle nanocomposites with the help of modified maximum tangential stress criterion. The literature reveals that the effect of size on mixed-mode fracture behavior of brittle nanocomposites has not been well investigated previously using modified maximum tangential stress criterion. The studied nanocomposites were made of epoxy resin reinforced with 7 wt%, 20–30 nm nanosilica. The accuracy of the method was assessed by taking into account the high-order terms of Williams series expansion along with finite element over-deterministic method. To investigate the effect of size on fracture toughness, a number of three-point semi-circular bending tests with different radii and four angles of edge–crack orientation were conducted and subjected to mixed-mode loading. The size of fracture process zone and apparent fracture toughness ( Kc) were also evaluated as a function of sample size. Experimental results showed that the proposed approach can accurately predict the fracture behavior of studied nanocomposites.

Microstructure and mechanical properties of nucleant-free Li2O-CaO-SiO2 glass-ceramics

An attractive characteristic of the Li2O-CaO-SiO2 glass-forming system is the possibility of obtaining internally nucleated crystal phases – CaSiO3 and Li2SiO3 – having elongated morphologies that can potentially lead to tough glass-ceramics. Another great advantage of this system is that it does not need a nucleating agent to crystallize internally. In this research work, three glasses with systematic compositional variations within the range of 30–50 mol% CaSiO3/70-50 mol% Li2SiO3 were prepared, heat-treated and their microstructures and mechanical properties evaluated. Heating cycles were applied in two-stage treatments. The temperatures were selected based on previous DSC analyses. The crystallized samples were characterized by XRD and SEM to examine the nature, size and morphology of their crystals. The effect of microstructure on the Vickers hardness (Hv) and indentation fracture toughness (Kc) of the glass-ceramics was determined. Elastic modulus (E), biaxial strength (σm) and fracture toughness (KDTIC) were determined from nanoindentation, ball-on-three-balls and double torsion tests, respectively. The level of internal residual stresses was evaluated by X-rays diffraction. The best mechanical properties were exhibited by a composition containing 44 mol% CaSiO3 heat-treated at 498 °C for 24 h for nucleation and at 700 °C for 2 h for crystal growth. This composition resulted in a glass-ceramic with the following microstructural features and properties: average Li2SiO3 (LS) crystal size of 8.5 μm, wollastonite (CS) phase surrounding the LS crystals with approximately 50% LS crystallized volume fraction, KDTIC = 2.3 ± 0.5 MPa m1/2, σm = 270 ± 20 MPa, HV = 8.4 ± 0.7 GPa and E = 146 ± 8 GPa. These are exciting mechanical properties for glass-ceramics intended for load bearing applications.

Novel laser crystals in Ca9Y(VO4)7-x(PO4)x mixed system

The Ca9Y(VO4)7-x(PO4)x (x = 0, 1, 3, 4) solid solution whitlockite-like single crystals have been grown by the Czochralski method from mixtures of single phase Ca9Y(VO4)7 and Ca9Y(PO4)7 starting materials produced by solid-state synthesis. Real compositions of solid solution crystals has been determined and analyzed. It is found that crystals with x = 0, 1, 4 exist with polar structure at room temperature, space group R3c. At least one of them, Ca9Y(VO4)7, demonstrates ferroelectric behaviour with phase transition at 942 °C. Unlike the others, Ca9Y(VO4)3(PO4)4 crystallizes in centrosymmetric whitlockite-like structure with space group R3¯c confirmed by single-crystal X-ray structure analysis. Similar to other whitlockite-family phosphates and vanadates, its crystal structure is described as a disordered structural version of usual whitlockites, corresponding to their high-temperature paraelectric phases. The transmission and luminescence spectra of solid solution crystals are reported, the hardness Hv and fracture toughness K1c of solid solution single crystals are also determined.

Vickers Microindentation Hardness Studies of DL-methionine Doped Copper Sulphate (MCS) single crystal

A semi-organic single crystal of DL-methionine doped copper sulphate has been grown by slow evaporation solution growth technique. Lattice parameter of the grown crystal was confirmed by single crystal x-ray diffraction analysis. Vicker’s microhardness indentation tests were carriedout with the load ranging from 10-100gm. Mechanical parameters like fracture toughness (Kc), brittleness index (Bi), yield strength (σy) and elastic stiffness constant (C11) were calculated and reported.

Transparent Nd-doped Ca1 − xYxF2 + x ceramics prepared by the ceramization of single crystals

Transparent Nd-doped Ca1−xYxF2+x ceramics were obtained through the ceramization of single crystals, and the morphology, microstructural characteristics, optical properties, and laser performance of the ceramics were examined. The results indicated that the microstructure of the transparent Nd-doped Ca1−xYxF2+x ceramics was similar to that of traditional hot-pressed CaF2 ceramics. In addition, the transparent Nd-doped Ca1−xYxF2+x ceramics exhibited the same optical properties as the precursor single crystals. Furthermore, the ceramization of the precursor single crystals produced transparent Nd-doped Ca1−xYxF2+x ceramics with mechanical properties (microhardness H=3.430GPa, fracture toughness K1c=0.61MPam1/2) that were superior to that of the Nd-doped Ca1−xYxF2+x single crystals. Continuous-wave laser operation with an output power of 35mW was obtained through the application of a fiber-coupled laser diode with a central wavelength of approximately 808nm. This suggests that the transparent Nd-doped Ca1−xYxF2+x ceramics fabricated in this study are promising candidates for diode-pumped solid-state lasers and laser amplifiers.

Environmentally Assisted Cracking in Silicon Nitride Barrier Films on Poly(ethylene terephthalate) Substrates.

A singular critical onset strain value has been used to characterize the strain limits of barrier films used in flexible electronics. However, such metrics do not account for time-dependent or environmentally assisted cracking, which can be critical in determining the overall reliability of these thin-film coatings. In this work, the time-dependent channel crack growth behavior of silicon nitride barrier films on poly(ethylene terephthalate) (PET) substrates is investigated in dry and humid environments by tensile tests with in situ optical microscopy and numerical models. The results reveal the occurrence of environmentally assisted crack growth at strains well below the critical onset crack strain and in the absence of polymer-relaxation-assisted, time-dependent crack growth. The crack growth rates in laboratory air are about 1 order of magnitude larger than those tested in dry environments (dry air or dry nitrogen). In laboratory air, crack growth rates increase from ∼200 nm/s to 60 μm/s for applied stress intensity factors, K, ranging from 1.0 to 1.4 MPa·m1/2, below the measured fracture toughness Kc of 1.8 MPa·m1/2. The crack growth rates in dry environments were also strongly dependent on the prior storage of the specimens, with larger rates for specimens exposed to laboratory air (and therefore moisture) prior to testing compared to specimens stored in a dry environment. This behavior is attributed to moisture-assisted cracking, with a measured power law exponent of ∼22 in laboratory air. This study also reveals that much larger densities of channel cracks develop in the humid environment, suggesting an easier initiation of channel cracks in the presence of water vapor. The results obtained in this work are critical to address the time-dependent and environmental reliability issues of thin brittle barriers on PET substrates for flexible electronics applications.

A REVIEW ON INVESTIGATION OF MIXED MODE FRACTURE IN BRITTLE MATERIALS

A series of mixed mode fracture tests were conducted on specimens made up of brittle materials to find out fracture parameters. Some of the parameters of interest are stress intensity factor (K), fracture toughness (Kc) and crack growth direction. In order to minimize catastrophic effects of mixed mode brittle fracture, investigation of mixed mode fracture is necessary. A number of researches have been carried out utilizing universal testing machine with suitable fixtures. Some of the experiments conducted used techniques like indentation techniques, tension tests, compression tests, bending and so on. Each one has its own advantages and disadvantages. The experimental results obtained were theoretically verified by many mixed mode fracture criteria. Some researchers also came up with empirical relations to approximate results which wouldn’t have been possible by existing famous fracture criteria. Even numerical methods like FEM analysis were carried out using commercially available softwares like ABACUS.

Synthesis, growth, structure, mechanical and optical properties of a new semi-organic 2-methyl imidazolium dihydrogen phosphate single crystal

A new semi-organic compound, 2-methyl imidazolium dihydrogen phosphate (2MIDP), was prepared and good quality single crystals of 2MIDP were grown by slow evaporation solution growth technique. Crystal structure elucidated using Single crystal XRD showed that 2MIDP crystallizes in monoclinic system with P21/c space group. FT-IR, UV-Vis-NIR, Fluorescence and FT-NMR spectra confirm the molecular structure of 2MIDP. The UV-Vis-NIR spectra established the suitability of the compound for NLO applications. TG-DSC showed that 2MIDP is thermally stable up to 200°C. Mechanical characteristics like hardness number (Hv), stiffness constant (C11), yield strength (σv), fracture toughness (Kc) and brittleness index (Bi) were assessed using Vicker’s microhardness tester. Third order nonlinear optical properties determined from Z-scan measurement using femto and picosecond lasers showed two photon reverse saturable absorption. The enhancement of nonlinear optical properties in femto second laser, revealed the suitability of 2MIDP for optical limiting applications.

Effect of Ni2+ doping on structural, optical, mechanical and dielectric properties of ammonium dihydrogen phosphate (ADP) single crystals: A novel NLO material

Owing to its exceptional applications in nonlinear optical devices, ammonium dihydrogen phosphate (ADP) is considered as potential candidate in modern era of hi-tech applications. Therefore, in the present work we have grown good quality single crystals of pure and Ni2+ doped (1, 2.5% and 5%), ADP using slow evaporation technique. X-ray study has been carried out in order to see the effect of dopant on the structural parameter of ADP. The UV–vis–NIR studies confirmed that 1.0mol%, 2.5mol% doped crystals have high transmittance compare to 5.0mol% Ni2+ doped ADP crystals. Surface studies have been carried out on pure and doped crystals. Typical striations are observed on the (100) faces of pure and doped crystals, suggest that the growth occurs by 2D nucleation. The microhardness studies of pure and doped crystals carried out at different loads. The hardness of 5.0mol% Ni2+ doped crystals shows higher hardness value compare to 1.0mol%, 2.5mol% doped crystals. The mechanical properties such as fracture toughness (Kc), brittleness index (Bi) and yield strength (σy) are presented. The dielectric studies were carried out in the high frequency range (1–6MHz) and all the results indicate that the grown crystals are of good quality.

Simulation of the influence of the hot-pressing parameters and the mixture compositions in the ZrO2(m)–Al–C system on the hardness and the crack resistance of ceramics

The work is aimed at studying the change in physical and mechanical properties (Vickers hardness HV15 and the fracture toughness K1C) of ceramic tool material obtained by hot pressing of ZrO2–Al–C mixtures depending on the concentration of the initial components and the parameters of the synthesis process (temperature, pressure, and isothermal holding time). On the basis of the data obtained in the course of the five-factor experiment, the relationships that allow establishing the optimum level of baseline are derived for the synthesis of the material with an optimal combination of properties that predetermine the use of the ceramic as the basis of the blade tool (as a cutting insert). In the considered intervals of factors, the derived empirical models can also be used as an interpolation formula for determining the value of the Vickers hardness (HV15) and the fracture toughness (K1C) of materials synthesized in the ZrO2–Al–C system.

A Hydrogen-Induced Decohesion Model for Treating Cold Dwell Fatigue in Titanium-Based Alloys

Cold dwell fatigue in near-alpha Ti alloys is a time-dependent fracture process at ambient temperature that involves fatigue in the presence of creep to produce cracking on low-energy fracture (e.g., cleavage) facets in hard alpha grains. In this article, cold dwell fatigue is treated as a hydrogen-induced decohesion process by using a nonlinear cohesive stress-strain relation to describe the decrease in the cohesive strength with increasing local hydrogen contents. It is postulated that during cold dwell fatigue, time-dependent deformation occurs by 〈a〉 slip that results in dislocation pileups in soft alpha grains. The stress and dilatational fields of the dislocation pileups assist the transport of internal hydrogen atoms from soft grains to neighboring hard grains. The accumulation of internal hydrogen atoms at the trap sites leads to decohesion along crystallographic planes, which can be slip or hydride habit planes. The decohesion model is applied to treat cold dwell fatigue in Ti-6Al-4V with a basal-transverse texture by modeling the effects of hydrogen-induced decohesion on the stress-fatigue life (S-N f) response, the time-dependent crack growth response (da/dt), and the fracture toughness (K c) as functions of grain orientation. A probabilistic time-dependent fatigue crack growth analysis is then performed to assess the influence of microtexture on the dwell fatigue life of a Ti-6Al-4V ring disk subjected to a long-duration hold at the peak stress of the loading cycle. The results of the probabilistic life computations indicate that dwell fatigue resistance in Ti-6Al-4V may be improved and the risk of disk fracture may be reduced significantly by controlling the microtexture or reducing the size and volume fraction of hard alpha grains in the microstructure.

Fracture Microindentation on boride layers on AISI 1020 steel

In this paper, an attempt has been made to enhance the fracture toughness (Kc) of boride layer using multi-component (Ni, Cr and B) laser bonding. The fracture toughness of continuously pack borided, interrupted pack borided and multi-component (Ni, Cr and B) laser borided steel specimens was measured using Vickers microindentation fracture toughness test as per ASTM E384 standard. The fracture toughness of continuously pack borided layer was - 3.3 MPa.m1/2. The fracture toughness of interrupted boride layer was in the range of - 4.9 MPa.m1/2. The fracture toughness of multi-component (Ni, Cr and B) laser borided layer was in the range of 13.8 - 18.3 MPa.m1/2. A significant improvement in fracture toughness of laser treated specimens was observed from the experimental results. This may be due to better distribution of boron, nickel, chromium and other alloying elements due to laser treatment and relatively more uniform boride layer as compared with continuously pack borided layer and interrupted pack borided layer.

Property and Oxidation Behaviours of (Mo,Cr)Si2 + ZrO2 Composite Produced by Pressure-Less Sintering

A composite of (Mo0.9Cr0.1)Si2+15vol% ZrO2 was prepared with powder metallurgy and pres-sure-less sintering, aiming at applications of high temperature structural materials. Mechanical properties of the composites were assessed with hardness, fracture toughness Kc and KIC tested using indentation fracture and SEVNB methods, flexure strength at room temperature and 1200˚C, and isothermal oxidation at 1400˚C. The results showed that the native silica oxide on the silicide feedstock surface, and in turn, the SiO2 in the sintered bulks was significantly re-duced in terms of Cr-alloying. (Mo0.9Cr0.1)Si2 and (Mo0.9Cr0.1)Si2+15vol% ZrO2 composite exhib-ited enhanced sinterability, owing to the formation of (Cr, Mo)5Si3. Fracture toughness of the composite was increased by a factor of 1.6 of that of the monolithic silicide. Mechanical prop-erty of the composite was not affected by Cr addition. However, the high temperature oxidation resistance was greatly improved in the (Mo0.9Cr0.1)Si2+15vol%ZrO2 composite compared with the as-sintered MoSi2+15vol%ZrO2. The Cr-alloying effects on feedstock native oxides, micro-structure, mechanical property and high temperature oxidation resistance were discussed.

Fracture Toughness of Explosively Welded Al/Ti Layered Material in Cryogenic Conditions

In the paper results of comparative analysis of the fracture toughness of explosively welded Al/Ti composite determined for the ambient temperature and for the cryogenic conditions were presented. Testing of mechanical properties of inhomogeneous materials very often presents a lot of difficulties connected with the experimental procedures as well as with the interpretation of study results. It is also in the case of layered Al/Ti material, for which cracking history has different nature for both layers: aluminium and titanium alloy. However, from the viewpoint of practical application, the knowledge of ‘global’ material properties is important. Due to differences of properties of both materials constituting the Al/Ti bimetal, it is hard to talk about determination of material property, which is plane strain fracture toughness K1C. In such case more adequate is fracture toughness KQ value, which can refers to the system of two materials. In investigations compact tension (CT) specimens were used. For specimens used in the tests the ratio of width to thickness W/B was 4. The initial pre-cracks were made by cyclic axial loading of specimens. The experimental tests were made with the use of the axial servohydraulic test system equipped with an original cryogenic chamber. During the tests all parts including specimen, clevis grips, extensometer were immersed in liquid nitrogen. The influence of cryogenic temperature on the fracture toughness was observed as a result of the tests.