On the Concentration Criterion of Fracture

Изучена кинетика накопления микротрещин на различных стадиях циклического и статического нагружения малоуглеродистой стали, а также сталей 45 и 12Х18Н9Т в процессе растяжения. Оценены концентрационный критерий (k-параметр), характеризующий начало процесса слияния микротрещин и формирования макротрещины, а также характеристики кумулятивных кривых распределения микротрещин по длине. Показано, что k-параметр связан степенной зависимостью с остаточной долговечностью материала. Рассмотрены некоторые общие закономерности процессов взаимодействия дефектов, проявляющиеся при разрушении металлических образцов.Вивчено кінетику накопичення мікротріщин на різних стадіях циклічного та статичного навантаження маловуглецевої сталі, а також сталей 45 і 12Х18Н9Т у процесі розтягання. Оцінено концентраційний критерій (k-параметр), що характеризує початок процесу злиття мікротріщин і формування макротріщини, а також характеристики кумулятивних кривих розподілу мікротріщин по довжині. Показано, що k-параметр пов’язаний степеневою залежністю із залишковою довговічністю матеріялу. Розглянуто деякі загальні закономірності процесів взаємодії дефектів, що проявляються при руйнуванні металічних зразків.The kinetics of microcracks’ accumulation at various stages of cyclic and static loading of specimens of the low-carbon grade 20, medium-carbon grade 45, and stainless 12Kh18N10T steels is studied. Concentration criterion of fracture (k-criterion) characterizing the initiation of process of microcracks’ coalescence and macrocrack formation as well as the characteristics of cumulative microcracks’ distribution over their lengths are estimated. As shown, the k-criterion is associated with residual lifetime of a material by means of the power-law dependence. Some general regularities of defect interaction manifesting themselves at fracture of metal specimens are considered

Spallation reactions. A successful interplay between modeling and applications

The spallation reactions are a type of nuclear reaction which occur in space by interaction of the cosmic rays with interstellar bodies. The first spallation reactions induced with an accelerator took place in 1947 at the Berkeley cyclotron (University of California) with 200 MeV deuterons and 400 MeV alpha beams. They highlighted the multiple emission of neutrons and charged particles and the production of a large number of residual nuclei far different from the target nuclei. The same year R. Serber describes the reaction in two steps: a first and fast one with high-energy particle emission leading to an excited remnant nucleus, and a second one, much slower, the de-excitation of the remnant. In 2010 IAEA organized a worskhop to present the results of the most widely used spallation codes within a benchmark of spallation models. If one of the goals was to understand the deficiencies, if any, in each code, one remarkable outcome points out the overall high-quality level of some models and so the great improvements achieved since Serber. Particle transport codes can then rely on such spallation models to treat the reactions between a light particle and an atomic nucleus with energies spanning from few tens of MeV up to some GeV. An overview of the spallation reactions modeling is presented in order to point out the incomparable contribution of models based on basic physics to numerous applications where such reactions occur. Validations or benchmarks, which are necessary steps in the improvement process, are also addressed, as well as the potential future domains of development. Spallation reactions modeling is a representative case of continuous studies aiming at understanding a reaction mechanism and which end up in a powerful tool.Comment: 59 pages, 54 figures, Revie

Dynamic fragmentation of shells: scale effects

AbstractThe effect of the diameter of the shells of structural steel (0.6% C) on the parameters of the cumulative number distribution of fragments by mass and the cumulative mass distribution on fragment length is studied. We tested geometrically similar closed cylinders of different diameters with different wall thicknesses, but maintaining a constant ratio of wall thickness to diameter of the shells equal to 0.175. Detonation velocity and pressure were 8300 m/s and 27 GPa, respectively.It was found that statistical fragment distributions by mass of the shells of different diameters are well described by the linear exponential relations, and with decreasing shell diameter, indices in these relations equal to reciprocal value of the characteristic mass increases. The diameter of the shell may affect the indices of cumulative mass distributions of fragments along their length described by a power function. With the increase in shell diameter, exponents in these equations are reduced, and the distributions are shifted to a shorter length fragments. These changes in the character and location of distributions are explained by change in the fragmentation mechanism. In the diagram, plotted using of the cumulated mass distributions on the fragment length for shells with different diameters, three parts corresponding to the three regimes of fragmentation is marked out.The initial stage of fragmentation of shells of small diameter (section I of the mass-fragment length distribution) is connected with the formation of small but numerous fragments limited by initial shear cracks on the inner surface of the shell. In the middle section II, which is linear in double logarithmic coordinates, the main spectrum fragments are formed, and this process develops steadily on the section II a, and with acceleration on the section II b, probably due to the transition from the shear to radial rupture fracture, accompanied by an increase in thickness of the fragments. On the last section III, corresponding to the fragment distribution of shell with the largest diameter, the length of the fragments is growing, but the cumulative mass remains almost constant, that is likely due to the formation of a relatively small number of longitudinal fragments of lesser thickness