Decrease In KIC Research Articles

Pure mode I fracture specification of molybdenum disulfide nanoparticles (MDS)-reinforced hot mix asphalt (HMA) under long-term conditions

Low and intermediate temperature cracking are two key factors for fracture and failure of hot mix asphalt (HMA), which impose huge costs on transportation departments and related agencies around the world. These two kinds of cracking may apear in the short- or long-term owing to states such as freezeing-thawing damage (FTD) and the aging mechanism. On the other hand, due to its unique features, nanomaterials are a suitable option for improving the mechanical features of bituminous mixes, which can affect the short-term performance and long-term performance. In this study, low and intermediate temperature fracture specification of Nano-molybdenum disulfide (MDS)-reinforced hot mix asphalt (HMA) under long-term conditions was evaluated. For this goal, two semi-circular bend (SCB) geometries comprising 90 degree and angled cracks, which were called symmetric SCB and classical-reinforced SCB-2, were made. Then, two various conditions, i.e. three freeze–thaw cycles and six days of aging operation were performed on various mixes in order to simulate the long-term performance of the bituminous mixes. Finally, the HMA samples with and without MDS were subjected to the three-point SCB test at the final temperatures of −15 °C and + 15 °C. The findings demonstrated that mixes comprising 0.3 % and 0.6 % MDS had more increased fracture energy (FE), fracture toughness (KIC), and stiffness under pure tension at low and medium temperatures (short-term and long-term fracture specification). In fact, MDS caused an increase in the fracture indices by increasing the maximum force (Pmax) and displacement at the time of fracture. At ± 15 °C and for the geometry comprising the 90 degree and 35 degree cracks, the HMA and Nano MDS-reinforced HMA mixes had more suitable flexibility under 3 FTC compared to 0 FTC; while, the flexibility of the HMA and Nano MDS-reinforced HMA mixes decreased under aging mechanism. In discussing the long-term performance of HMA mixture (with and without modifiers), the findings showed that applying three FTD's cycles to the mixes caused a decrease in KIC, FE, and Pmax. Also, reducing the stiffness of various mixes under 3 FTC, and increasing the flexibility of the mixes were other findings of this study. In addition, the simultaneous use of both SCB geometries comprising 90 degree and 35 degree cracks was proposed to study the fracture specification of bituminous mixes owing to the more inferior average FE and KIC. Finally, the application of 3FTC on bituminous mixes can more suitable simulate the long-term performance of asphalt pavement (compared to aging mechanism).

Pressure effects on both fluorescent emission and charge transport properties of organic semiconductors: a computational study.

External pressure can regulate the photophysical property and charge transport performance of organic semiconductors, however, the underlying mechanism at the microscopic level is still elusive. Using thermal vibrational correlation function coupled quantum mechanics/molecular mechanics and full quantum charge transfer rate theory, we systematically explore the influence of pressure on fluorescence emission and charge transport behaviours of representative cyclooctatetrathiophene (COTh). It is found that, upon pressurization, the intramolecular configurations of COTh became more twisted, leading to the blue-shifted emission. The fluorescence quantum efficiency (FQE) of COTh crystals decreases monotonically in a wide pressure range of 0-4.38 GPa, because the increase of intermolecular electronic energy transfer rate constant (keet) is larger than the decrease of internal conversion rate constant (kic), and the variation of keet is dominant. The decrease in kic is attributed to the decreasing reorganization energy, reflecting the suppression of the low-frequency flipping vibrations of four thiophene rings and the high-frequency stretching vibrations of central cyclooctatetraene, while the keet increase is due to the simultaneous increase in exciton coupling and spectra overlap. Moreover, we predicted that the hole mobility of COTh increases monotonically by nearly an order of magnitude from 0.39 to 3.00 cm2 V-1 s-1 upon compression, because of the increase in transfer integral and the decrease of charged reorganization energy. Furthermore, its hole mobility exhibits obvious anisotropy. Our work systematically builds the external pressure, molecular packing, luminescence and transport properties relationships of organic semiconductors and provides theoretical guidance for the rational design of pressure responsive organic semiconductors with excellent photoelectric performance.

Examination of cryogenic durability in self-consolidating concretes through mechanical and fracture mechanics approaches

Thermal shocks under extreme conditions, such as leakage in concrete-enclosed tanks, which are a popular choice for cryogenic liquid storage, may lead to undesirable effects on the structures. In this study, the effects of cyclic cryogenic exposures have been investigated using both mechanical and fracture mechanics approaches. To ensure durable concrete, self-consolidating concretes have been chosen in the mix design. While previous studies have primarily investigated the effects of a single cryogenic exposure on conventional concrete, this study focuses on repetitive exposure to cryogenic effects on self-consolidating concretes. Furthermore, the self-consolidating concretes were subjected to two different curing conditions: air curing and water curing, before cryogenic processing, in order to consider real application conditions and laboratory circumstances. After the first and fifth cryogenic exposures, fundamental engineering properties and fracture mechanics parameters were evaluated. To determine the fracture mechanics parameters, beams of three different dimensions were produced, with three different notch lengths for each dimension. The dynamic modulus of elasticity, ultrasonic pulse velocity, compressive strength, and bending strength of self-consolidating concretes were examined after 0, 1, and 5 cycles of cryogenic exposure. As a result, the air-cured samples exhibited greater resistance to cryogenic cycles than the water-cured samples. The KIC and CTODC values increased after one cycle of cryogenic exposure, as noted in the literature. However, the results obtained in this study demonstrate a decrease in KIC and CTODC values after five cycles compared to one cycle of cryogenic exposure.

Conformational Flexibility-Controlled Piezochromic Behavior of Organic Fluorophores

The pressure-responsive behaviors of organic fluorophores are often irregular and highly system-dependent. Unraveling the pressure effects on them is the key to the precise design of piezochromic materials. Here, we demonstrate that the different conformational flexibilities of organic fluorophores can lead to their distinct piezochromic behaviors. By combining quantum mechanics/molecular mechanics models and thermal vibration correlation function formalism, we show that two organic fluorophores, dibenzo[b,d]thiophene 5,5-dioxide (DBTS) and carbazole (Cz), with similar chemical structures, display different conformations and conformational flexibilities, which determine the responsiveness of their piezochromic behaviors. It is found that pressurization continuously improves the planarity between the sulfonyl group and the aromatic skeleton in cross-shaped DBTS, leading to red-shifted emission. In addition, the fluorescence quantum efficiency (FQE) of DBTS increases monotonically in a wide range of 0–7 GPa and then shows a dramatic reduction upon further compression to 12 GPa. The non-monotonical FQE of DBTS is attributed to the continuously decreased radiative decay rate constant (kr) and non-monotonically changed non-radiative decay rate constant (kic). The initial decrease in kic is due to the restriction of aromatic skeleton vibrations in the high-frequency region, and the subsequent increase in kic is ascribed to the acceleration of the bending and rotational vibrations of the sulfonyl group in the low-frequency region. In contrast, pressurization hardly influences the conformation of planar Cz, resulting in a slight change in emission and FQE, which is well consistent with experimental results. This study opens an efficient shortcut to the rational design of advanced piezochromic luminescent materials.

O5 - Self-Adhesive Luting Composites: Effect of Curing-Mode and Storage on KIc

The aim of the present study was to investigate the influence of curing-mode (dual or self-curing) and storage time (24 h, 14 d, 60 d) on the fracture toughness (KIc) of self-adhesive luting composites (SALCs). The KIc of four different SALCs (RXU2, RelyX Unicem 2, 3 M ESPE; GCLA, G-Cem LinkAce; GC-Corporation; iCEM, iCem Self Adhesive, Kulzer; PAN, Panavia SA Universal Cement, Kuraray Noritake) was measured using the Chevron-Notched Beam method. From each material, 30 samples were prepared in dual-curing mode (20 s @ 1200 mW/cm2, 8 mm light-tip) and 30 samples in self-curing mode (curing-time according to manufacturer). All specimens were stored in artificial saliva in the dark at 37 C. KIc was measured after 24 h, 14 and 60 d (n=10 per material, curing-mode and storage time). Statistical analysis was performed with a t-test of independent samples and a single factor ANOVA (alpha=0.05). The curing mode had a significant influence on KIc, within the same material and storage time (p<.05), with exception of RXU2, GCLA and iCEM after 60 d. In general, the KIc of dual-cured specimens was significantly higher, except for RUX2 in self-cure mode after 24 h (p>.05). After 60 d of storage all materials, independently of the curing-mode, had a significant decrease in KIc (p<.05). Only PAN had any decrease in KIc. Under the same curing-mode and storage time, GCLA showed the significantly highest KIc (p<.05), whereas iCEM the lowest, with exception of the its 24 h dual-cure group. Within the limits of this study, it can be concluded that the mechanical stability of the SALCs under investigation, in terms of KIc, is influenced by curing-mode and storage time. The extent of mechanical deterioration is material dependent. The in-vitro measured decays in KIc could lead in-vivo to deterioration of the adhesive interface and in consequence to restoration loss or fracture. The long-term resilience to water degradation and chemical curing of SALCs show potential of improvement.

Influence of Porosity on Fracture Toughness and Fracture Behavior of Antibiotic-Loaded PMMA Bone Cement.

Aseptic loosening is the most common reason for the long-term revision of cemented arthroplasties with fracture of the cement being a postulated cause or contributing factor. In our previous studies we showed that adding an antibiotic to a polymethylmethacrylate (PMMA) bone cement led to detrimental effects on various mechanical properties of the cement such as bending strength, compressive strength and fracture toughness (KIC). This finding implied that the mechanical failure of antibiotic-loaded PMMA bone cement was influenced by its pore volume fraction. Up to now this aspect has not been studied. Hence the purposes of this study were to determine (1) the influence of antibiotic (telavancin) loading on the KIC of a widely used PMMA bone cement brand (Palacos®R) and (2) the influence of pore size and pore distribution on the fracture behavior of the KIC specimens. For (2) both experimental and numerical methods (extended finite element method [XFEM]) were used allowing a comparison between the two sets of results. We found that: (1) KIC decreased with increased porosity with the drop (relative to the value for the control cement) being significant when the telavancin loading was 4.8 wt/wt % (2 g of telavancin added to 40 g of control cement powder); (2) there was a critical pore size above which there was a significant decrease in KIC and is 1 mm; (3) crack propagation was strongly influenced by pore size and pore locations (pore-pore interactions); and, (4) there was good agreement between the experimental and XFEM results. The implications of these findings for the use of a telavancin-loaded PMMA bone cement in cemented total joint arthroplasties are commented upon.

Determination of Fracture Toughness and KIC-CVN Correlations for BM, HAZ, and WB in API 5L X60 Pipeline

This paper presents a study to determine the fracture toughness for the API 5L X60 pipeline in the base metal (BM), the heat-affected zone (HAZ), and the welding bead (WB) by SE(B) specimens in the short transverse direction in an API 5L X60 pipeline. Also, a comparison of the KIC results from SE(B) specimens and KIC-CVN (Charpy V-notch) correlation equations using CVN energy values was done. SE(B) specimens were machined and tested according to ASTM E339. Standard Charpy specimens were machined and tested according to ASTM E23. The notch remained within the evaluation zone to obtain KIC and Charpy specimens at the HAZ and WB. The KIC and CVN values are higher for the BM, followed by the HAZ and the WB. These KIC and CVN values are similar to those reported by other authors in API 5L pipelines. The decrease in KIC and CVN values is due to the microstructure present in the temperature transformations. For BM, the ferrite matrix with pearlite colonies could be presented, while for the WB zone, martensite and finer bainite are detected. In some regions of the HAZ temperatures are reached that promote the transformation of the ferritic/pearlitic structure of the BM into austenite. KIC-CVN correlation equations values should be used only to estimate KIC values when KIC data cannot be obtained from standard specimens.

Resonance Raman Scattering Spectra of Co(II)- and Cu-5,10,15,20-Tetrakis[4-(N-methylpyridyl)]porphyrin in the dd Excited State and Mechanisms of Its Deactivation in a Solution and in Complexes with DNA

Resonance Raman scattering (RRS) spectra of the complexes of 5,10,15,20-tetrakis[4-(N-methylpyridyl)]porphyrin with Co(II) and Cu(II) (CoIITMpyP4 and CuTMpyP4) in various solvents and in complexes with DNA are studied. Under nanosecond pulsed excitation, additional lines have been found in the RRS spectra of CoIITMpyP4 in a dimethylformamide (DMF) solution containing formic acid as an impurity and in a complex with DNA. At the same time, such lines are absent in the spectra of CoIITMpyP4 in pure DMF, dimethyl sulfoxide, water, and alcohols under the same excitation conditions. To interpret the experimental data, we have calculated the structure and vibrations of the solvate complexes of CoIITMpyP4 and CuTMpyP4 with water, methanol, and formic acid in the ground and in excited states. Based on the obtained data, additional lines in the Raman spectra are assigned to the dd excited state corresponding to the d(z2) → d(x2 – y2) transition, the lifetime of which increases with the formation of complexes of CoIITMpyP4 with formic acid. According to the calculation results, this correlates with a decrease in rate constant kic of the internal conversion due to an increase in energy gap ΔE between the ground and dd states of CoIITMpyP4. A decrease in the kic value in the case of binding of CoIITMpyP4 and CuTMpyP4 to DNA is caused by the additional interaction of the extra-ligand water molecule with one of its bases directly or via several water molecules immobilized on DNA surface. This complicates the conformational rearrangement of the water molecule in the dd excited state, which gives rise to an increase in the ΔE value.

Спектры РКР Co(II)- и Cu-5,10,15,20-тетракис[4-(N-метилпиридил)]порфирина в возбужденном dd-состоянии и механизмы его дезактивации в растворе и комплексах с ДНК

A study of the resonance Raman scattering (RRS) spectra of Co(II)- and Cu-5,10,15,20-tetrakis [4-(N-methylpyridyl)]porphyrin (CoIITMpyP4, CuTMpyP4) in various solvents and in complex with DNA was carried out. Additional lines were found in the RRS spectra of CoIITMpyP4 in a dimethylformamide (DMF) solution containing formic acid as an impurity, as well as for its complex with DNA under nanosecond pulsed excitation. At the same time, under the same excitation conditions, such lines are absent in the spectra of CoIITMpyP4 solutions in pure DMF, dimethyl sulfoxide, water, and alcohols. To interpret the experimental data, we performed calculations of the structure and vibrations for the solvate complexes of CoII - and CuTMpyP4 with water, methanol and formic acid in the ground and in excited states. Based on the data obtained, additional lines in the Raman spectra were assigned to the excited dd-state corresponding to the d(z2) d(x2-y2) transition, the lifetime of which increases with the formation of complexes of CoIITMpyP4 with formic acid. According to the calculation results, this correlates with a decrease in the rate constant of the internal conversion kic due to an increase in the energy gap E between the ground and dd states of CoIITMpyP4. In the case of CoII and CuTMpyP4 binding to DNA, the decrease in kic is due to the additional interaction of the extra-liganding water molecule directly or with the participation of several immobilized DNA water molecules with one of its bases. This complicates the conformational rearrangement of the water molecule in the dd-state, which contributes to an increase in E.

Multi-parametric characterization of mode I fracture toughness of asphalt concrete: Influence of void and RA contents, binder and aggregate types

This study aims to evaluate the fracture toughness (KIc) in mode I cracking using semi-circular bending test (SCB). Experiment has been performed to investigate the influence of bitumen grade (using P15/25 and P50/70 bitumens), reclaimed asphalt (RA) content (using 0%, 20% and 40% RA contents) and temperature (using −20 °C, −5 °C, 10 °C test temperatures), through ANOVA. Additional investigations have been performed: (i) to evaluate the effect of the use of polymer modified bitumen (PMB), (ii) to evaluate the effect compactness using 5% and 8% air void contents, (iii) to evaluate the effect of aggregate type using siliceous-limestone and porphyry aggregates. The results show an important decrease in KIc when temperature increases from −5 °C to 10 °C and a slight decrease between −20 °C and −5 °C. The results also show that increasing RA content increases slightly the KIc. It was found from the ANOVA that the influent parameters can be ranked as follows: temperature, RA content and binder grade. The investigations show that PMB increases the KIc value than pure bitumens. Porphyry aggregates increase the KIc by about 16% than silica-limestone aggregates at low temperatures between −20 °C and −5 °C. However, this ranking is slightly inverted at 10 °C. In addition, KIc decreases by about 12% at 10 °C with an increase in air voids (by 5% to 8%). Void content effect is more significant at −5 °C and 10 °C, and negligible at −20 °C.

Interfacial Fracture Toughness of Adhesive Resin Cement-Lithium-Disilicate/Resin-Composite Blocks.

Resin composite blocks (RCB) are advocated as alternative to ceramic blocks (CB). Prior to use, adherence to these materials should characterized. This study aimed to test the null hypothesis (H0 ) that material and surface treatment combinations do not influence interfacial fracture toughness (KIC ) of a self-cured adhesive resin cement [RelyX Ultimate (RXU)] to RCB or CB, under nonaged and aged conditions. Two RCB, Lava Ultimate (LU) and Enamic (EN), and one CB, IPS e.max Press (EMP) were used. Half-size [(6 × 6 × 6 × 6 mm)] specimens were prepared for EMP (n = 30), EN (n = 30), and LU (n = 60). RCB specimens were prepared by wet cutting/grinding, while CB specimens were pressed. Surfaces of EMP and EN were preconditioned with hydrofluoric acid (5%); surfaces of LU were sandblasted with either 27 μm alumina (LUS) or 30 μm silica-modified alumina Rocatec soft (LUR). All specimens were bonded with Scotchbond Universal adhesive and RXU. Additionally, twenty (4 × 4 × 4 × 8 mm) RXU specimens were prepared. All specimens were stored in water at 37°C and tested after 1 and 60 days. Interfacial KIC was determined with the notchless triangular prism specimen KIC test. Results were analyzed with two-way ANOVA and Scheffé multiple means comparisons (α = 0.05). Preconditioned and selected fractured surfaces were characterized with scanning electron microscopy. At 24 hours, LUS-RXU and LUR-RXU had significantly higher interfacial KIC than EN-RXU and EMP-RXU and were not different from KIC of RXU. Aging lead to a significant decrease in KIC of RXU and interfacial KIC of LUS-RXU, LUR-RXU, and EMP-RXU; interfacial KIC of EN-RXU was not affected. Based on the results, H0 was rejected. Under the conditions of this study, at 24 hours, interfacial KIC of LUS-RXU and LUR-RXU was superior to EMP-RXU and EN-RXU. Aging in water at 37°C did not affect interfacial KIC of EN-RXU but adversely affected KIC of RXU and the other interfacial KIC . The results suggest that RXU and its adherence to LU and EMP deteriorates upon exposure to water at 37°C. In making clinical decisions related to material selection, practitioners should consider in vitro results.

High-Temperature Fracture Toughness of Chromium-Doped Sapphire Fibers

Pure and chromium (Cr)-doped sapphire microfibers (100–160 μm in diameter) have been tested under constant tensile stress rates. KIC has been evaluated by means of fractographic measurements to determine the size of the critical flaw and the size of the subcritical crack growth (SCG) region surrounding the originating flaw. For high-temperature conditions a simple relationship between the size of the mirror and the size of the SCG zones is also presented. This information allows KIC to be determined by simply measuring the size of the mirror zone. The failure stress of sapphire fibers is highest at room temperature, 2.6 GPa for pure sapphire, diminishing to 0.5 GPa at 1400°C. A decrease in strength is also observed in Cr-doped fibers, 1.7 GPa at room temperature to 0.4 GPa at 1400°C. A similar decrease in KIC is also observed for Cr-doped fibers, 0.38 MPa·m0.5 at room temperature to 1.4 MPa·m0.5 at 1400°C. Advantages of the procedure suggested for KIC determination include no need for artificially introduced precracks, repeatability, applicability to high-temperature conditions, and good agreement with literature results. Additionally, the short evaluation times and simple measurements and calculations involved are well suited for industrial quality control settings.

A specific feature in the fracture of polycrystalline zirconia ceramic

The effect of indentor load F and V-notch tip radius r on the crack resistance factor K Ic of ZrO2 + 3 mol.% Y2O3 ceramic is studied by microindentation and bending methods. Materials composed of 100% tetragonal phase (T-phase) and 70 – 80% T-phase were used. In both materials, an increase in F and a decrease in r cause a decrease in K Ic . The higher sensitivity of K Ic to F and r is observed in 100% T-phase specimens. This effect is explained by the involvement of martensite-type phase transformations.

Mode I Fracture Toughness of a Small‐Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects

The Mode I fracture toughness (KIC) of a small‐grained Si3N4 was determined as a function of hot‐pressing orientation, temperature, testing atmosphere, and crack length using the single‐edge precracked beam method. The diameter of the Si3N4 grains was &lt;0.4 µm, with aspect ratios of 2–8. KIC at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in KIC was observed through 1200°C, to ∼4.1 MPa·m1/2, before increasing rapidly to 7.5–8 MPa·m1/2 at 1300°C. The decrease in KIC through 1200°C was a result of grain‐boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in KIC. Crack healing was observed in specimens annealed in air. No R‐curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C.

Mechanical Properties of Silicon Nitride with Anisotropic Microstructure

Silicon nitride with anisotropic microstructure was fabricated by tape-casting of raw powder slurries with 0-10vol% seed particles of morphologically regulated β-Si3N4 single crystal, followed by gas pressure sintering of stacked sheets. The effect of orientation degree and area fraction of large elongated grains on mechanical properties was studied. Fracture toughness (KIC) and bending strength (σf) measured in the direction perpendicular to grain alignment increased with an increase in seeds amount, which was associated with the fact that both orientation degree and area fraction of large elongated grains increased with increasing amount of seeds. The specimen with 5vol% seeds, in which large elongated grains occupy a cross-sectional area of about 46%, exhibited a dominant orientation parallel to the casting direction. The specimen showed high KIC of 11.7MPa·m1/2 and high σf of 1200MPa. Further addition of seeds resulted in a slight decrease in KIC and σf owing to pore generation and coalescense of large elongated grains. On the contrray, KIC and σf measured in the direction parallel to the grain alignment decreased monotonously with an increase in seeds amount.

Fracture toughness of single crystal alumina in air and a simulated body environment

The critical intensity factor KIC of a single crystal alumina (1100)-plane in air and in a simulated body environment was measured. Based on these measurements, the estimated durability of a single crystal alumina disk in a ceramic heart valve was determined. Standard single-edge notched beam specimens were prepared. The specimens were classified into three groups: a) stored in air (n = 10), b) immersed in Ringer's solution for 2 months (n = 11), and c) immersed in Ringer's solution for 4 months (n = 6). KIC was 2.345 +/- 0.072 MPam1/2 for specimens stored in air, 2.141 +/- 0.147 in Ringer's solution for 2 months, and 2.005 +/- 0.153 in Ringer's solution for 4 months. The KIC decreased for specimens stored in Ringer's solution (P < .01), but it did not differ significantly between 2 and 4 months. The decrease of KIC is the stress corrosion effect of Ringer's solution at the crack tip, and is not associated with an overall deterioration of strength. The stress intensity factor for possible cracks (length: 0.4 mm) in the single crystal alumina was calculated. Maximum stress was assumed 2.668 MPa (= 40 x 500 mm Hg). The stress intensity factor calculated was far less than the KIC. The single crystal alumina disk can endure a static pressure of 500 mm Hg.

Fracture Toughness and SCC Propagation Properties of Al-Li Alloys.

Fracture toughness and stress-corrosion cracking (SCC) behavior of Al-Li 2090-T8 and 2091-T8 alloys containing small amounts of Zn were studied and compared with conventional high-strength alloys 7075-T6 and 2024-T3. The fracture toughness value (KIC) and SCC extension rate (da/dt) as functions of stress intensity factor KI were evaluated using precracked DCB specimens with S-L orientation of the extruded plate. With Zn addition of 0.7%, 2090 and 2091 alloys showed little decrease of KIC and also superior SCC resistance as compared to conventional alloys in terms of the threshold value KISCC and (da/dt)II in region II. Fract graphic and electrochemical observations suggest that SCC of these Al-Li alloys is a result of local anodic dissolution along grain boundaries.

Fatigue crack propagation and cryogenic fracture toughness behavior in powder metallurgy aluminum-lithium alloys

Fatigue crack propagation and cryogenic fracture toughness properties of powder metallurgy (P/M) aluminum-lithium alloys have been examined by studying the behavior in mechanically alloyed (MA) Al−4.0Mg−1.5Li−1.1C−0.8O2 (IN-905XL) and rapid solidification processed (RSP) Al−2.6Li−1.0Cu−0.5Mg−0.5Zr (Allied 644-B) extrusions. Results are presented as a function of microstructure, mean stress, and specimen orientation and are compared with previous data on equivalent high-strength aluminum alloys fabricated by both ingot metallurgy (I/M) and P/M methods. It is found that the fatigue crack propagation resistance of the RSP Al−Li alloy is superior to traditional RSP aluminum alloys without lithium and even comparable to I/M Al−Li alloys, particularly at near-threshold and intermediate stress intensity levels. In contrast, crack growth rates in MA 905XL P/M extrusions are nearly three orders of magnitude faster and do not show benefits of alloying with lithium. Growth rate behavior in both alloys, however, is anisotropic; for example, crack growth rates in RSP 644-B alloy are up to three orders of magnitude faster in theT-L, compared toL-T, orientation. However, when characterized in terms of a closure-corrected near-tip “driving force,” ΔKff such differences are reduced. With respect to toughness, plane strainKIc values (L-T orientation) in the RSP alloy are observed to increase with decrease in temperature from 298 to 77 K; conversely, the MA alloy shows a small decrease inKIc at 77 K. Such results are interpreted in terms of the micromechanisms influencing fatigue and fracture behavior in Al−Li alloys, specifically involving the microstructural role of hardening mechanism, slip mode, grain structure, and texture on the development of crack tip shielding (crack path deflection and crack closure) and short-transverse delamination cracking.

Electronic Relaxation Processes from Single Vibronic Levels in the S1(n, π*) State of Pyridine Vapor

Abstract The rate constants of radiative decay (kF), internal conversion (kIC), and intersystem crossing (kISC) for single vibronic levels in the S1(n, π*) state of pyridine-d0 and -d5 vapors have been determined from measured fluorescence quantum yields (ΦF), intersystem crossing quantum yields (ΦICS), and picosecond fluorescence lifetimes. The ΦISC values are fairly constant in the region of lower excess vibrational energy (&amp;lt;1000 cm−1), while the ΦF values are particularly large for vibronic levels involving non-totally symmetric vibrations, ν16b and ν10a. The latter observation is attributed to both an increase of kF and a decrease of kIC and kISC (relative to the zero-point level) for the ν16b levels (16b2 and l6b26a1), and to an increase of kF for the ν10a levels. The relative values of kF are discussed using the Herzberg-Teller theory of vibronic coupling. The values of kF for 16b1 and 16b2 are significantly larger than that for the zero-point level, though no absorption band corresponding to 16b01 can be observed. In both pyridine-d0 and -d5, the value of kIC is as large as 1.7×1010s−1 even for the zero-point level, which is more than twice the kISC value for the same level.

TiC-Ni合金および他種炭化物を添加したTiC-Ni系合金の破壊靭性と限界強度

The fractre toughness KIC and limiting strength of TiC-base cermets were determined by means of analysis of the effect of fracture-initiating defect-size on fracture stress. The KIC value of TiC-16-44 mass%Ni alloys increases in the range of 6.0-9.5 MPa\sqrtm with increase in nickel content. The value of KIC (8.2 MPa\sqrtm) of a TiC-28 mass%Ni alloy decreases monotonically to 5.8-6.4 MPa\sqrtm with increase in the content of the added carbides (Mo2C, NbC, TaC, VC) up to 18.5 mol% (added carbides in carbide phase), except for WC addition. The decrease of KIC by adding different carbides is due to the embrittlement of binder phase which is caused by dissolution of titanium and other elements be contained in the added carbides.By comparing the KIC of TiC-Ni alloys with that of WC-Co alloys, it was found that the KIC for a cermet composed of carbide and metal phases depends mainly on the elastic moduli (E & ν) of the alloy, and also on the critical strain energy release rate (GIC(B.P.)) and volume fraction (f) of metal phase, according to the following equation.(This article is not displayable. Please see full text pdf.) The limiting strength of TiC-Ni alloys, which is independent of the size of fracture-initiating defect, is raised from 1.98 to 2.57 GPa as the nickel content inceases from 16 to 44 mass%, and is lowered slightly by addition of carbides except tungsten carbide.