X-ray diffraction study of microstructural changes during fatigue damage initiation in pipe steels: Role of the initial dislocation structure

The present work is the second part of an ongoing study of microstructural changes during fatigue damage initiation in pipe steels [B. Pinheiro et al., Mat. Sci. Eng., A 532 (2012) 158-166]. Microdeformations and residual stresses (macrostresses) are evaluated by X-ray diffraction during alternating bending fatigue tests on samples taken from an API 5L X60 grade steel pipe. Microdeformations are evaluated from the full width at half maximum (FWHM) of the diffraction peak and residual stresses are estimated from the peak shift. Here, to understand the role of the initial dislocation structure, annealed samples are considered. As previously found for as-machined samples, the evolution of microdeformations shows three regular successive stages, but now with an increase during the first stage. The amplitude of each stage is accentuated with increasing stress amplitude, while its duration is reduced. Residual stresses show a similar trend, with stages of the same durations than those observed for FWHM, respectively. Additionally, changes in density and distribution of dislocations are observed by transmission electron microscopy combined with the technique of focused ion beam. The results are very encouraging for the development of a future indicator of fatigue damage initiation for pipe steels based on microstructural changes measured by X-ray diffraction. © 2013 Elsevier B.V

Development of MWD and branching during peroxide modification of High-Density Polyethylene by SEC-MALS and Monte Carlo simulation

During the reactive processing of HD Polyethylene molecules are known to form branched structures, eventually resulting in a MFI decrease. This phenomenon was studied by experiments in a twin-screw extruder and measurement of the MWD by SEC-MALS. A Monte Carlo simulation model has been setup to calculate the MWD and branching distribution. The model includes both branching and random scission. Branch points are assumed to be created by coupling of vinyl ends of linear PE to secondary radical sites on PE backbones, yielding tertiary radical sites that undergo termination by disproportionation. This mechanism was recently proposed by Camara et al. [1]. Comparing the computed MWD to SEC-MALS data allowed determining the kinetic coefficients of the branching and scission reactions. Random scission turns out to be weak but still significant to prevent the formation of an extended MWD tail. For the found parameters perfect agreement was found between measured and computed MWD for various initial peroxide concentrations. These findings contribute to a better fundamental understanding of the simultaneous branching and scission during PE modification. It forms a firm support of the vinyl end to secondary radical coupling branching mechanism. Furthermore, it is shown that the MC simulations provide explicit branching topologies as a basis for predicting, for instance, complex rheological behavior of the modified polymer melt (Read et al. [2])