Identification of damage mechanism and validation of a fracture model based on mesoscale approach in spalling of titanium alloy

AbstractThe subject of this paper is identification of the physical mechanisms of spalling at low impact velocities for Ti–6Al–4V alloy and determination of the macroscopic stress of spalling via meso-macro approach. Spalling is a specific mode of fracture which depends on the loading history. The aspects of the initial microstructure and its evolution during plastic deformation are very important. In order to identify the spalling physical mechanisms in titanium alloy, numerous pictures by the optical microscopy of the spall surfaces created by plate impact technique have been taken. The scenario of failure observed is in complete agreement with known physical micro-mechanisms: namely nucleation, propagation and coalescence by adiabatic shearing of micro-voids. The most interesting point in spall fracture of Ti–6Al–4V alloy is the nucleation of micro-voids. A significant amount of small micro-voids in the region of the expected spall plane has been observed. It appears that microstructural effects are important due to dual α–β phase microstructure, called Widmanstätten structure. The orientation of microstructure has a direct influence on nucleation mechanism by means of distribution of nucleation sites and decohesion between the softer particles (α-phase lamellae) and the harder lattice (β-phase). According to these observations, a fracture model has been developed. This model is based on the numerous post-mortem microscopic observations of spall specimens. The goal is to determine the macroscopic stress of spalling in function of loading time and damage level via a meso-macro approach

Multiple Changes in Peptide and Lipid Expression Associated with Regeneration in the Nervous System of the Medicinal Leech

peer reviewe

Validation of a fracture model based on mesoscale approach for spalling of hard steel and aluminium alloy

A fracture model has been developed to determine the macroscopic stress of spalling as a function of loading time and damage level via meso-macro approach. This model is based on numerous post-mortem microscopic observations of spall specimens. The plate impact experiment was applied to estimate the macroscopic stress corresponding to the incipient stage of spalling for two different materials: hard martensitic steel Mars 190 and aluminum alloy 7020-T6. In order to verify the model, numerous scanning electron microscopy and optical microscopy pictures of the free surfaces created by spalling have been taken. The results of measurements in the form of statistical distribution of horizontal micro-segments of fractured surfaces of targets, corresponding to quasi-brittle fracture, and vertical micro-segments, corresponding to ductile and/or adiabatic shear banding, all along the entire specimen cross sections, are reported. It has been confirmed that the microstructure has a direct influence on the mechanism of nucleation, growth and coalescence of micro-cavities or micro-cracks by means of distribution of nucleation sites and decohesion between the harder particles and the softer lattice. The model allows for determination of the macroscopic normal stress necessary to obtain a specific level of damage from the modeling of fracture meso-mechanisms

The effect of renal replacement therapy and antibiotic dose on antibiotic concentrations in critically ill patients: data from the multinational sampling antibiotics in renal replacement therapy study

Background: the optimal dosing of antibiotics in critically ill patients receiving renal replacement therapy (RRT) remains unclear. In this study, we describe the variability in RRT techniques and antibiotic dosing in critically ill patients receiving RRT and relate observed trough antibiotic concentrations to optimal targets.Methods: we performed a prospective, observational, multinational, pharmacokinetic study in 29 intensive care units from 14 countries. We collected demographic, clinical, and RRT data. We measured trough antibiotic concentrations of meropenem, piperacillin-tazobactam, and vancomycin and related them to high- and low-target trough concentrations.Results: we studied 381 patients and obtained 508 trough antibiotic concentrations. There was wide variability (4–8-fold) in antibiotic dosing regimens, RRT prescription, and estimated endogenous renal function. The overall median estimated total renal clearance (eTRCL) was 50 mL/minute (interquartile range [IQR], 35–65) and higher eTRCL was associated with lower trough concentrations for all antibiotics (P < .05). The median (IQR) trough concentration for meropenem was 12.1 mg/L (7.9–18.8), piperacillin was 78.6 mg/L (49.5–127.3), tazobactam was 9.5 mg/L (6.3–14.2), and vancomycin was 14.3 mg/L (11.6–21.8). Trough concentrations failed to meet optimal higher limits in 26%, 36%, and 72% and optimal lower limits in 4%, 4%, and 55% of patients for meropenem, piperacillin, and vancomycin, respectively.Conclusions: in critically ill patients treated with RRT, antibiotic dosing regimens, RRT prescription, and eTRCL varied markedly and resulted in highly variable antibiotic concentrations that failed to meet therapeutic targets in many patients