Self-similar solutions for stress driven material dissolution

During corrosive dissolution of metal ions from a body surface, an oxide compound is produced. This compound forms a protective film that reduces the dissolution rate. When a fraction of a millimetre depth is dissolved the dissolution rate become insignificant. However, repeated loading will damage the film with continued dissolution as a result. In connection with this a threshold strain is assumed to exist. This paper proposes a model where electro- chemical processes and the mechanical load work together in forming a corrosion pit. The ratio between the threshold strain and the remotely applied strain is shown to control the shape of the pit. For small applied strains cracks are formed. A crack evolving from a surface irregularity is studied. The growth rate of the crack is determined by the dissolution rate at the crack tip. No crack growth criterion is needed. The growing crack is itself creating conditions for strain concentration, which leads to a high crack growth rate. The model simulates how dissolution forms a pit that grows to become a crack in a single continuous process. For small loads the crack growth rate is independent of applied load

Corrosion cracks nucleation by deformation-induced passivity breakdown

A model for corrosion crack nucleation and growth is presented, where the corrosion forms the geometry of the crack tip, thus creating the conditions for strain concentration. The interaction between electro chemical processes and the deformation of the crack tip region is incorporated in a continuum mechanical theory. No crack growth criterion is used. The formation of a crack from a surface depression via a pits studied. Low frequency cyclic load is considered. At the end of a load cycle a metal oxide compound is growing on the crack surface. It is assumed that there is sufficient time for the chemical process to form a protective film that fully covers the crack surface. This temporarily interrupts the corrosion process. During the application of next load cycle the stretch of the surface breaks the protective film. This creates gaps in the film, which allow dissolution of the uncovered metal. The chemical environment of the crack tip is assumed to be constant and unaffected by the changing geometry as the crack is developing This leads to a lincar relationship between strain and corrosion rate, in the sense of removed material per unit of area during each load cycle. The model simulates how pits evolve to become cracks and how cracks then propagate in one continuous process. Mathematical and finite element analyses of stationary racks with appropriate geometry are involved to explain the behaviour predicted by the model

On dissolution driven crack growth

AbstractThe formation and growth of a crack in a body subjected to stress driven material dissolution is studied. The rate of material dissolution is proportional to strain energy and curvature of the body surface. The formation of a crack from a plane surface is preceded by an evolving surface roughness. The continued dissolution enhances roughness amplitude resulting in pit formation. As the pit grows deeper into the material, it assumes the shape of a crack. The sharpness of the crack reaches its maximum during this transition from a pit to a crack. As the crack grows, a self-similar state is gradually assumed. During this phase characteristic lengths of the crack shape scale with the crack length. In line with this the crack progressively becomes blunt. The widest part of the crack when unloaded is in the vicinity of the crack tip. A consequence of the model is that no criterion is needed for crack growth. Neither is a criterion needed for determination of the crack path. It also follows that the crack growth rate is almost independent of the remote load. Further, spontaneous crack branching is anticipated. A motivation for this is given

Influence of Hydrogen Content on Axial Fracture Toughness Parameters of Zr-2.5Nb Pressure Tube Alloy in the Temperature Range of 306-573 K

Tubes fabricated from dilute Zr-alloys serve as miniature pressure vessels in Pressurized Heavy Water Reactors and are subjected to stress, aqueous corrosion and intense irradiation during service. Hydrogen evolved during the corrosion reaction may enter into the material and precipitate as hydride phase, which acquire platelet shaped morphology in Zr-alloys and are known to embrittle the host matrix. Since hydride embrittlement is a major life limiting factor for the components made from these alloys, several theoretical and experimental studies have been carried out to understand the influence of hydrogen/hydride on the mechanical properties in general and micromechanisms assisting crack nucleation and its propagation in the presence of hydride, in particular. For ductile materials like Zr-alloys, crack initiation follows void nucleation and its growth in the plastic zone. Nucleation of voids is associated with fracture of second phase particle or separation of matrix-precipitate interface. Hydrides are suspected to be fracture initiating sites in Zr-alloys and the presence of hydride platelets normal to tensile load significantly influences crack propagation. However, the role of hydrides in crack nucleation and its propagation and influence of temperature on the same has not been delineated clearly. In this work, influence of hydrogen and temperature on the axial fracture toughness parameters of Zr-2.5Nb pressure tube alloys is reported. The fracture toughness tests were carried out using 17 mm width curved compact tension specimens machined from gaseously hydrogen charged tube-sections and tested in the temperature range of 306 to 573 K. Metallography of the samples revealed that hydrides were predominantly oriented along axial-circumferential plane of the tube. The fracture toughness parameters like JQ, J0.15, JMax, J1.5, dJ/da, KJC and KMax were determined as per the ASTM standard E-813, with the crack length measured using direct current potential drop technique. The plane strain K values were computed from the corresponding J values. The critical crack length for catastrophic failure was determined using a numerical method, which is widely used in literature. It is observed that for a given test temperature both the fracture toughness parameters representing crack initiation, such as JQ, J0.15 and KJC and crack propagation, such as JMax, J1.5, and KMax, decrease mildly with increase in hydrogen content whereas mean dJ/da is practically unaffected by hydrogen content. Also, for a given hydrogen content crack initiation fracture toughness parameters showed large scatter with a tendency to decrease with increase in test temperature whereas the crack propagation fracture toughness parameters increased with temperature to a saturation value

Master curve in upper region of ductile brittle transition: a modification based on local damage approach

AbstractThe fracture behaviour of ferritic/ferritic martensitic steels in Ductile to Brittle Transition (DBT) region is well captured by Master Curve approach, except in the upper region of transition due to ductile tearing prior to cleavage. The fracture toughness behavior in the upper region of DBT is generally censored by Master Curve. In this work the Master Curve approach is modified to extend its applicability to the upper region of transition. The ductile tearing in the upper region of transition, increased sampled volume and constraint increment are addressed in this work using constraint parameter Tstress and numerical analyses using GTN damage

Untersuchung von Katalysatoren auf Gold/TiO2-Basis und optoelektronischen Halbleitern mittels Transmissionselektronenmikroskopie und Tomographie

Gold, usually known for its inertness, can be prepared as a nanoporous bulk material showing catalytic properties. A particular advantage of this catalytic material is the stable monolithic structure, combining mechanical strength, thermal and electrical conductivity and a reproducible porosity due to self-organization during the preparation. Important structural properties are the pore size and the size of the gold ligaments. TEM (transmission electron microscopy) is a powerful tool to obtain information on these properties. Additionally, the functionalization of the nanoporous gold with TiO2 particles was investigated. For the application in catalysis of CO-oxidation, the gold can be coated with TiO2 in order to enhance catalytic activity. Furthermore semiconductor heterostructures were investigated. II-VI-based microcavities grown by molecular beam epitaxy designed for emission in the blue spectral region have been comprehensively characterized by transmission electron microscopy in bright field, dark field and scanning mode. Moreover the STEM signal for InP based semiconductors was simulated

A gene-centric approach to biomarker discovery identifies transglutaminase 1 as an epidermal autoantigen

Publisher Copyright: © 2021 National Academy of Sciences. All rights reserved.Autoantigen discovery is a critical challenge for the understanding and diagnosis of autoimmune diseases. While autoantibody markers in current clinical use have been identified through studies focused on individual disorders, we postulated that a reverse approach starting with a putative autoantigen to explore multiple disorders might hold promise. We here targeted the epidermal protein transglutaminase 1 (TGM1) as a member of a protein family prone to autoimmune attack. By screening sera from patients with various acquired skin disorders, we identified seropositive subjects with the blistering mucocutaneous disease paraneoplastic pemphigus. Validation in further subjects confirmed TGM1 autoantibodies as a 55% sensitive and 100% specific marker for paraneoplastic pemphigus. This gene-centric approach leverages the wealth of data available for human genes and may prove generally applicable for biomarker discovery in autoimmune diseases.Peer reviewe