Coupled phenomenological and fracture mechanics approach to assess the fracture behaviour of TWC piping component

The present study demonstrates the numerical prediction of experimental specimen J-R curve using Gurson-Tvergaard-Needleman phenomenologically based material model. The predicted specimen J-R curve is used to determine the geometric independent initiation fracture toughness (J_SZWc) value that compares well with experimental result. Using the experimentally determined and numerically predicted J_SZWc values and specimen J-R curves, the accuracy of predicting the fracture behaviour of the cracked component is judged. Thus the present study proposed a coupled phenomenological and fracture mechanics approach to predict the crack initiation and instability stages in cracked piping components using numerically predicted specimen J-R curve obtained from tensile specimens testing data.     

The effect of microstructure on the fracture toughness of 300 and 350 grade maraging steels

The fracture toughness of 300 and 350 grade maraging steel that has been given simple heat treatments has been studied. By heating into the austenite region at well-controlled rates and holding for short times, the toughness can be increased significantly above that which is obtained for the standard treatment. It is shown that the toughness increase is due to plastic deformation of the austenite for single-cycled material. When multiple cycles are used, the 300 grade steel undergoes a strain-induced transformation at the crack tip that contributes significantly to the toughness. When singly cycled, 300 grade material is tested at −196°C, its properties approach those obtained for conventionally treated material at room temperature. Models for the toughness improvements are developed that take crack tip phenomena into account, and they are in good agreement with experiment.     

Electrical potential technique for monitoring subcritical crack growth at elevated temperatures

The electrical potential technique has been successfully used to monitor crack extension under fatigue as well as sustained loading at elevated temperatures in the presence of gross creep deformation. Calibration curves for actual crack extension vs change in electrical potential were determined for two specimen geometries, namely the compact type (CT) and the center crack tension (CCT) type, for an ASTM, grade A470 class 8 steel at 538°C (1000°F) and for a type 304 stainless steel at 594°C (1100°F).

A normalizing factor for expressing crack extension has been derived for the CT specimen. This factor accounts for changes in calibration due to small differences in initial crack length and, it also makes the calibration curve independent of the test temperature and material. Hence, the calibration curves presented herein are applicable to other materials and temperatures provided the specimen geometry and size is the same and the current input and potential leads are also located at the same position.     

Consideration of primary creep at a stationary crack tip: Implications for the C t parameter

A finite element creep analysis of a center crack specimen has been carried out under small scale to extensive creep conditions. The crack was assumed to be stationary. Several constitutive models were used; these consisted of elastic, power-law creep with and without rate-independent plasticity, as well as one which also included primary creep. The mechanics basis of the C _t parameter, which has been proposed for correlating creep crack growth behavior under conditions ranging from small scale to extensive creep, is explored.For the aforementioned specimen geometry, consideration of primary creep seems to explain the differences between the measured and previously calculated load line deflection rates based on power-law creep only. It is also concluded that in small scale creep, C _t does not characterize the instantaneous crack tip singular stress field, but it accurately characterizes the rate of expansion of the crack tip creep zone regardless of whether primary or secondary creep is occurring. This result provides a rationale for using C _t to correlate creep crack growth rates even in the presence of significant primary creep deformation.

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Résumé On a procédé à une analyse par éléments finis du fluage d'une éprouvette comportant une fissure centrale, sous des conditions allant du fluage à petite échelle jusqu'à un fluage important. On suppose que la fissue est stationaire. Divers modèles ont été utilisés, à savoir la loi de fluage élastique ou parabolique, avec ou sans plasticité indépendante de la vitesse, ainsi qu'une loi incluant également le fluage primaire. On a étudié la base mécanistique du paramètre C _t, proposé pour relier le comportement de la croissance de la fissure de fluage sous des conditions entraînant un fluage depuis une petite échelle jusqu'à une grande échelle.Pour la géométrie citée, les différences entre les vitesses de variation de la courbe de la charge, mesurées et précédemment calculées en se basant sur la seule loi parabolique de fluage, semblent applicables par la prise en considération du fluage primaire. On conclut également que lors de fluage à petite échelle, C _t ne caractérise pas le champ singulier de contraintes instantanées à l'extrémité de la fissure, mais plutôt, et de maniére sûre, la vitesse d'extension de la zone de fluage à l'extrémité de la fissure, qu'il y ait fluage primaire ou fluage secondaire. Ce tésultat fournit une base d'utilisation de C _t pour connaître les vitesses de croissance d'une fissure de fluage, même en présence d'une déformation de fluage primaire significative.

     

Effect of long-term stress and temperature exposure on the fracture toughness of Ti-62222 alloy

Fracture toughness tests were conducted on a Ti-62222 (titanium alloy) sheet being considered for use in high temperature aircraft applications in the as received condition and after exposing the pre-cracked specimens to a sustained stress intensity, K, level between 55 and 60.5 MPa for 200 h at 350°C. It was concluded that the fracture toughness does not degrade as a result of exposure to high temperature and the K levels in this material. The tensile strength in the exposed condition also remained the same as in the as received condition.     

Kinetics of adsorption of sulfur mustard on Al2O3 nanoparticles with and without impregnants

BACKGROUND: Chemical warfare (CW) agents are highly toxic compounds and have been used in war to produce physical immobilization, so safe and effective ways to detoxify them without endangering human life or the environment is of great concern. One of the important ways to achieve protection against CW agents contaminating air is to utilize suitable adsorbent materials, e.g. activated carbon, nanoparticles, etc. In the present study nanoparticles, synthesized through sol–gel processes and loaded with reactive compounds have been used for the degradation of CW agents and to understand their adsorption kinetics using Fickian and linear driving force models. RESULTS: Nanoparticles of AP‐Al₂O₃ (aerogel‐produced alumina) in the size range 2–30 nm with high surface area (375 m² g^?1) were produced by an alkoxide‐based synthesis, and then characterized using N₂‐Brunauer–Emmet–Teller (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD) and thermogravimetric analysis (TGA) techniques. Thereafter, these were impregnated, and finally tested for kinetics of adsorption of sulfur mustard (HD) under static conditions. The kinetics was studied using linear driving force and Fickian diffusion models and the kinetics parameters determined. CONCLUSION: AP‐Al₂O₃ with 10% impregnation of 9‐molybdo‐3‐vanadophosphoric acid (10%, w/w) showed the maximum uptake (640 mg g^?1) of HD. The highest adsorption potential indicated that the adsorption was due not only to physisorption but also involved chemisorption. Values of the diffusional exponent indicated the mechanism to be Fickian and anomalous. Hydrolysis, dehydrochlorination and oxidation reactions (identified using gas chromatography‐mass spectrometry (GC/MS)) were found to be the route of degradation of HD over the prepared nanoparticle based adsorbents. Copyright © 2009 Society of Chemical Industry     

Synthesis and compressive behavior of Cr2GeC up to 48 GPa

M_n+1AX_n compounds have gathered huge momentum because of its exciting properties. In this paper we report the synthesis of ternary layered ceramic Cr₂GeC, a 211 M_n+1AX_n compound by hot-pressing. Scanning electron microscopy and X-ray diffraction have been employed to characterize the new synthesized phase. High-pressure compressibility of Cr₂GeC were measured using diamond anvil cell and synchrotron radiation at room temperature up to 48 GPa. No phase transformation was observed in the experimental pressure range. The bulk modulus of Cr₂GeC calculated using the Birch–Murnaghan equation of state is 169 ± 3 GPa, with K′ = 3.05 ± 0.15.     

Amyloid-β Inspired Short Peptide Amphiphile Facilitates Synthesis of Silver Nanoparticles as Potential Antibacterial Agents

An amyloid-β inspired biocompatible short peptide amphiphile (sPA) molecule was used for controlled and targeted delivery of bioactive silver nanoparticles via transforming sPA nanostructures. Such sPA-AgNPs hybrid structures can be further used to develop antibacterial materials to combat emerging bacterial resistance. Due to the excellent antibacterial activity of silver, the growth of clinically relevant bacteria was inhibited in the presence of AgNPs-sPA hybrids. Bacterial tests demonstrated that the high biocompatibility and low cytotoxicity of the designed sPA allow it to work as a model drug delivery agent. It therefore shows great potential in locally addressing bacterial infections. The results of our study suggest that these nanodevices have the potential to trap and then engage in the facile delivery of their chemical payload at the target site, thereby working as potential delivery materials. This system has potential therapeutic value for the treatment of microbiota triggered progression of neurodegenerative diseases.     

Experimental investigations on silicon carbide mixed electric discharge machining

Silicon - In this study, silicon carbide mixed electrical discharge machining (SCMEDM) process has been developed and later on modelled also using an artificial neural network (ANN) based technique...