Energy dissipation via acoustic emission in ductile crack initiation

The final publication is available at Springer via http://dx.doi.org/10.1007/s10704-016-0096-8.This article presents a modeling approach to estimate the energy release due to ductile crack initiation in conjunction to the energy dissipation associated with the formation and propagation of transient stress waves typically referred to as acoustic emission. To achieve this goal, a ductile fracture problem is investigated computationally using the finite element method based on a compact tension geometry under Mode I loading conditions. To quantify the energy dissipation associated with acoustic emission, a crack increment is produced given a pre-determined notch size in a 3D cohesive-based extended finite element model. The computational modeling methodology consists of defining a damage initiation state from static simulations and linking such state to a dynamic formulation used to evaluate wave propagation and related energy redistribution effects. The model relies on a custom traction separation law constructed using full field deformation measurements obtained experimentally using the digital image correlation method. The amount of energy release due to the investigated first crack increment is evaluated through three different approaches both for verification purposes and to produce an estimate of the portion of the energy that radiates away from the crack source in the form of transient waves. The results presented herein propose an upper bound for the energy dissipation associated to acoustic emission, which could assist the interpretation and implementation of relevant nondestructive evaluation methods and the further enrichment of the understanding of effects associated with fracture

Application of Ligninolytic Enzymes in the Production of Biofuels from Cotton Wastes

The application of ligninolytic fungi and enzymes is an option to overcome the issues related with the production of biofuels using cotton wastes. In this dissertation, the ligninolytic fungus and enzymes were evaluated as pretreatment for the biochemical conversion of Cotton Gin Trash (CGT) in ethanol and as a treatment for the transformation of cotton wastes biochar in other substances. In biochemical conversion, seven combinations of three pretreatments (ultrasonication, liquid hot water and ligninolytic enzymes) were evaluated on CGT. The best results were achieved by the sequential combination of ultrasonication, hot water, and ligninolytic enzymes with an improvement of 10% in ethanol yield. To improve these results, alkaline-ultrasonication was evaluated. Additionally, Fourier Transform Infrared (FT-IR) and principal component analysis (PCA) were employed as fast methodology to identify structural differences in the biomass. The combination of ultrasonication-alkali hydrolysis, hot liquid water, and ligninolytic enzymes using 15% of NaOH improved 35% ethanol yield compared with the original treatment. Additionally, FT-IR and PCA identified modifications in the biomass structure after different types of pretreatments and conditions. In thermal conversion, this study evaluated the biodepolymerization of cotton wastes biochar using chemical and biological treatments. The chemical depolymerization evaluated three chemical agents (KMnO4, H2SO4, and NaOH), with three concentrations and two environmental conditions. The sulfuric acid treatments performed the largest transformations of the biochar solid phase; whereas, the KMnO4 treatments achieved the largest depolymerizations. The compounds released into the liquid phase were correlated with fulvic and humic acids and silicon compounds. The biological depolymerization utilized four ligninolytic fungi Phanerochaete chrysosporium, Ceriporiopsis subvermispora, Postia placenta, and Bjerkandera adusta. The greatest depolymerization was obtained by C. subvermispora. The depolymerization kinetics of C. subvermispora evidenced the production of laccase and manganese peroxidase and a correlation between depolymerization and production of ligninolytic enzymes. The modifications obtained in the liquid and solid phases showed the production of humic and fulvic acids from the cultures with C. subvermispora. The results of this research are the initial steps for the development of new processes using the ligninolytic fungus and their enzymes for the production of biofuels from cotton wastes

Acoustic-emission and dilatometric investigations of the influence of Cr on the austenite decomposition kinetics

The interest of small additions of Cr (< 3 wt%) in steels for automotive and bearing applications has been recently highlighted. The microstructure of these steels is primarily based on the bainitic phase. Therefore, in order to optimize their microstructure and mechanical properties, it is of primary importance to gain a better understanding of the influence of Cr on austenite decomposition in different temperature regimes with a particular attention for the transition from ferrite to bainite and martensite formation. In the present study, the acoustic emission technique is used together with dilatometry and optical microscopy in order to obtain a full view of the transformation processes and kinetics. Specifically, acoustic emission is a technique that is capable of distinguishing a displacive from a diffusional transformation

Acoustic-emission investigations on the austenite decomposition in a medium-carbon steel

Continuous cooling of the austenitic phase is important in controlling the microstructure and properties of steel. At high cooling rates, the initial formation of allotriomorphic ferrite undergoes a transition to bainite and martensite. Modelling and understanding the transformation process does therefore not only involve the ferrite formation, but also the bainite and martensite formation, and especially the mechanisms of phase formation and the conditions at which the transition occurs. In the present study the acoustic-emission technique is used to obtain more information on the varying transformationprocesses in different temperature regimes. The experimental results are combined with dilatometry and metallographic data in order to gain a full view of the transformation processes and kinetics

Acoustic-emission investigations on the ferrite and bainite formation in low-alloy steel during ultra-fast cooling

Ultra-fast cooling of the austenitic phase in low-alloy steel is rapidly gaining importance in controlling the microstructure and properties of the steel. At these high cooling rates, the initial formation of allotriomorphic ferrite undergoes a transition to bainite formation. Of course it is of paramount importance to control the presence of ferrite and/or bainite in the eventual microstructure. Modelling and understanding the transformation process for high cooling rates does therefore not only involve the ferrite formation, but also the bainite formation, and especially the conditions at which the transition occurs. In the present study the acoustic-emission technique is used to determine the point at which bainite formation takes over from the initial ferrite formation. The experimental results are combined with dilatometry data in order to gain a full view of the transformation process and kinetics. Moreover, the acoustic-emission technique provides essential information on the nature of the bainite formation; more specifically to what extent it can be considered displacive or diffusional