Influence of nitriding on the fatigue behavior and fracture micromechanisms of nodular cast Iron

Surface modification processes are increasingly used to fully exploit material potential in fatigue critical applications because fatigue strength is sensitive to surface conditions. Nitriding is extensively adopted with ferrous materials because it forms a hard and strong surface layer and a system ofsuperficial compressive residual stresses. Fatigue, however, is strongly dependent also on defects and inhomogeneity. When nitriding is applied to nodular cast iron (NCI), the relatively thin hardened layer (about 300 μm) contains graphite nodules (diameter of the order of 30 μm), casting defects and a heterogeneous matrix structure. The paper presents and discusses the influence of nitriding on the fatigue response and fracture mechanisms of NCI. A ferritic NCI and a synthetic melt with different content of effective ferrite were initially gas-nitrided. Then, (i) structural analysis of nitrided layers, (ii) fatigue testing with rotating bending specimens, and (iii) fatigue fracture surface inspection were performed. Performance and scatter in fatigue performance is discussed by selective inspection of fracture surfaces and identification fracture micromechanisms. A semiempirical model explains observed trends in test results and is used for the process optimization.Процессы модификации поверхности все более широко применяются для полного раскрытия возможностей материала в условиях высоких усталостных нагрузок, поскольку на усталостную прочность влияет состояние поверхности. Азотирование широко используют для обработки железосодержащих материалов, поскольку оно создает прочный поверхностный слой и поверхностные остаточные сжимающие напряжения. Усталость также существенно зависит от дефектов и неоднородности. При азотировании чугуна с шаровидным графитом в относительно тонком упрочненном слое (примерно 300 мкм) присутствуют шаровидные включения графита (диаметр порядка 30 мкм), дефекты литья и неоднородная структура основы. Рассматривается влияние азотирования на характеристики усталости и механизмы разрушения чугуна. Сначала проводили газовое азотирование ферритного чугуна и синтетического расплава с различным содержанием активного феррита. Затем выполняли структурный анализ азотированных слоев; испытание на сопротивление усталости путем кругового изгиба образца; контроль поверхности усталостного разрушения. Эффективность и разброс усталостных характеристик оценивали путем выборочного контроля поверхностей разрушения и идентификации микромеханизмов разрушения. Полуэмпирическая модель используется для оценки результатов испытаний и оптимизации процесса

Constructions at work in foreign language learners\u2019 mind. A comparison between two sentence-sorting experiments with English and Italian learners.

This article reports empirical evidence of constructional priming effects in L2 learners of English and Italian. The well-known pioneering experiment carried out by Bencini and Goldberg (2000) with L1 speakers of English, pave the way for our investigation. We employed the same protocol to ascertain whether constructions have an ontological status also in the mind of L2 learners. We conducted experiments with four groups of learners whose language proficiency levels correspond to the B1 and B2 levels of the Common European Framework of Reference for Languages (CEFR). The results we obtained in our cross-linguistic experiments demonstrate that learners are reliant on constructional templates when they are required to produce linguistic generalizations

A methodology for the fatigue design of notched castings in gray cast iron

Fatigue failures of machine components remain a topic of relevant importance in the industrial world. They usually occur from geometrical features such as holes, notches, corners and grooves, whose actual influence is not well estimated in the design phase. Cast parts made in gray cast iron are typical examples of components difficult to design in fatigue because they are simultaneously characterized by complex geometries and microstructure. In this contribution the issue is discussed starting from the failure analysis of a cyclically pressurized hydraulic component. The work consists of an experimental procedure, i.e. the fatigue characterization of the material on specimens extracted from cast parts, and of a numerical design activity, i.e. the prediction of life time according to the critical distance method, [1]. The implication is that cracks and localized damage begin to appear in the microstructure of gray cast iron at sharp notches from the first cycles of loading. In order to obtain a correct prediction, the fatigue design should adopt fracture mechanics arguments to determine non-propagating conditions