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Crack habits in metal/hydrogen interactive systems

Katz Y.

Journal of Achievements in Materials and Manufacturing Engineering

2015

Vol. 73, nr 2

144--150

Purpose: In terms of structural integrity aspects, interactive insights into metal/hydrogen systems become self-explanatory. In fact, ample of local critical events are involved namely, from sub-critical slow crack growth up to a total delayed failure. Design/methodology/approach: Mainly, crack-tip vicinity characterization became essential from both, chemical and mechanical factors, as related to the complicated aspects of crack stability. Note that the interactive situation has been currently investigated, imposing as such new local conditions. In fact, the current study adopted a highly comprehensive methodology in view of fracture physics. Thus, the material selection included iron-based (Fe-3%Si) and zinc single crystals. For the sake of background only, polycrystalline austenitic stainless steels were also investigated regarding hydrogen interaction affecting mechanical properties. Hydrogen charging has been performed either by electrolytic cathodic charging or, by high temperature/pressure gaseous methods. Fracture mechanics methodology was mainly implemented by utilizing external or internal hydrogen interaction procedures. Experimentally, research activities have been conducted by novel techniques, such as ultra-high visualization techniques and by using basic plasticity information like crack-tip dislocation emission and structures. Findings: Enhanced crack extension was established and the slow subcritical crack extension was traced in physically well-defined crack systems. Fracture mode transitions occurred due to the deformation/environment interaction. For example, in iron-based single crystals the unique cleavage mode emphasized the embrittlement impact. The broadness of the current study enabled a more local fundamental approach aimed to understand the crack-path habits. Here, the role of the crack-tip shielding beside the crack arrest potentials have been considered. Research limitations/implications: The dominating micro-mechanisms of hydrogen-related fracture have been thoroughly reviewed in the literature. Despite the remarkable research efforts involving coupled theory and physical findings, critical experiments still remain the key issue in order to establish more of any basic general concepts. Clearly, the hydrogen/deformation interactions have many facets involving broad service implications. Practical implications: The issue of hydrogen embrittlement or the possible decrease of fracture resistance causing severe mechanical degradation requires special attention. Originality/value: The current investigation includes nano-mechanical probes on top of surface probe microscopy. This technique offered additional critical experiments aimed to resolve the scaling relationship. Moreover, the study assisted to reveal local/global insights related to the interaction conditions involved. One of the values to be mentioned resulted even from the subcritical crack path input. The crack path habits with hydrogen interaction served as critical information. In this context, crack path findings supported the assessment of the possible viable micro-mechanical interactive embrittlement models. Basically, the crack path and the crack-tip front varied with the different crack systems. The important role of plasticity in the cleavage mode formation has been substantiated. Alluded to the above implies that critical experiments might eventually provide the building blocks for modeling efforts that can truly simulate and anticipate embrittlement events. It is demonstrated that huge gaps in knowledge exist preventing appropriate bridging of scales. This has often led to controversy when addressing multiple affected microstructures. This multiplicity occurs due to numerous second phases and interfaces that can interact in various modes with aggressive environment affecting both localized flow and fracture. Only by breaking the microstructure down to its individual building blocks can the scale bridging be appropriately dealt with.

Geometric constraints on fatigue crack paths in tubular welded joints.

Pook L.P.

Archive of Mechanical Engineering

1998

vol. XLV, nr 2

144-156

The complete solution of the problem of long Stage II fatigue crack growing in a metallic material includes determination of the crack path. Macroscopic aspects of the main features of the crack path are reasonably well understood in a qualitative sense. On a macroscopic scale it is a matter of observation that cracks tend to grow in Mode I. Geometric constraints may confine a crack to a particular path, and this sometimes leads to mixed mode fatigue crack growth. In a tubular welded joint fatigue crack growth is sometimes observed. Description of a crack path in three dimensions requires describtion of the surface followed by the crack (crack growth surface), and also of a family of lines on this surface which defines successive position of the crack front. In general Mode I crack growth surface are curved, and there are then geometric constraints on permissible crack from families. Theoretical prediction of crack paths in three dimensions is not well understood, and in practice fatigue crack paths in structures are usually determined by large scale structural tests. General features of fatigue crack paths in tubular welded joints are discussed from a theoretical viewpoint. It is concluded that further work is needed if the prediction of crack paths in tubular welded joints is to be put on a firm theoretical basis.

Pełne rozwiązanie problemu rozwoju pęknięć zmęczeniowych w metalach zawiera wyznaczenie ścieżki rozwoju pęknięcia. Względnie dobrze wyjaśnione są w sensie jakościowym makroskopowe aspekty głównych cech rozwoju ścieżki pęknięcia. W skali makroskopowej obserwacje wskazują, że pęknięcia mają tendencje do wzrostu wg Modu I. Więzy geometryczne mogą ograniczać swobodę rozwoju pęknięcia, co prowadzi do wzrostu wg modów mieszanych. W połączeniach spawanych rur, rozwój pęknięć zmęczeniowych ogranicza się zwykle do obszaru spoin i często są obserwowane przypadki rozwoju pęknięć wg modów mieszanych. Trójwymiarowy opis pęknięcia wymaga opisu powierzchni, w której następuje rozwój pęknięcia oraz rodziny linii na tej powierzchni, które określają kolejne pozycje frontu szczeliny. Pęknięcia rozwijane wg Modu I mają postać powierzchni zakrzywionych i istnieją geometryczne ograniczenia na dopuszczalne formy frontów pęknięć. Przewidywanie na drodze teoretycznej rozwoju szczelin w przestrzeni trójwymiarowej nie jest obecnie możliwe i w praktyce scieżki rozwoju szczelin w konstrukcjach są zwykle wyznaczone na drodze eksperymentów prowadzonych w dużej skali. Ogólne cechy rozwoju ścieżek zmęczeniowych w złączach spawanych rur są w pracy dyskutowane z punktu widzenia teorii. Konkluzja mówi, że jeśli przewidywanie ścieżek rozwoju pęknięć w złączach spawanych rur ma być oparte na mocnych teoretycznych zasadach, to niezbędne są dalsze prace w tym kierunku.