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Types of Corrosion in the Offshore Environment
Published in Karan Sotoodeh, Coating Application for Piping, Valves and Actuators in Offshore Oil and Gas Industry, 2023
There are two types of cracks that can form in a material: ductile and brittle. Ductile cracks form more slowly and result in plastic deformation. A brittle fracture is a very rapid-forming crack that occurs without plastic deformation. A HISC crack is brittle and fast and occurs without any material deformation. Figure 1.49 illustrates hydrogen cracking or embrittlement in the form of a brittle crack in a piece of pipe. The reason why duplex and super duplex are susceptible to HISC is related to the ferritic phase inside these two materials. Duplex and super duplex have two phases: austenitic, which is ductile; and ferritic, which is brittle. Hydrogen makes the ferritic phase embrittle, while the austenitic phase acts as a crack barrier. The application of coating on the metal surface provides a barrier against hydrogen attack and prevents HISC. However, this method is not 100% effective in HISC mitigation due to possible defects in the coating. Even a small crevice in the coating where the metal is under high tensile stress can cause HISC. For this reason, the DNV standard does not accept coating as a means of preventing HISC. Instead, DNV proposes proper material selection and design with respect to stress and strain, even if the material is coated.
Static Failure Criteria and Reliability
Published in Ansel C. Ugural, Mechanical Engineering Design, 2022
Fracture is defined as the separation or fragmentation of a member into two or more pieces. It normally constitutes a pulling apart associated with tensile stress. A relatively brittle material fractures without yielding occurring throughout the fractured cross-section. Thus, a brittle fracture occurs with little or no deformation or reduction in area, and hence very little energy absorption. This type of failure usually takes place in some materials in an instant. The mechanisms of brittle fracture are the concern of fracture mechanics. It is based on a stress analysis in the vicinity of a crack, flaw, inclusion, or defect of unknown small radius in a part. A crack is a microscopic flaw that may exist under normal conditions on the surface or within the material.
Failure under Mechanical Loading
Published in Mahmoud M. Farag, Materials and Process Selection for Engineering Design, 2020
Generally, fatigue fractures occur as a result of cracks, which usually start at some discontinuity in the material or at other stress concentration locations, and then gradually grow under repeated application of load. As the crack grows, the stress on the load-bearing cross section increases until it reaches a high level enough to cause catastrophic fracture of the part. This sequence is reflected in the fracture surfaces that usually exhibit smooth areas that correspond to the gradual crack growth stage and rough areas that correspond to the catastrophic fracture stage, as shown in Figure 2.5. The smooth parts of the fracture surface usually exhibit beach marks, which occur as a result of changes in the magnitude of the fluctuating fatigue load. Another feature of fatigue fractures is that they lack macroscopic plastic deformation and, in this respect, they resemble brittle fractures. The following case studies are used to illustrate some of the frequent causes of fatigue fracture and to show some of the solutions that may be used to solve the problem.
Biopolymer composites: a review
Published in International Journal of Biobased Plastics, 2021
Basheer Aaliya, Kappat Valiyapeediyekkal Sunooj, Maximilian Lackner
Creep in a biopolymer composite is the tendency to deform slowly but permanently due to mechanical stress [128]. Even though creep occurs at high-stress levels, because of long-term stress exposure it will be below the yield strength of material. Unlike brittle fractures, creep fracture is not instantaneous and is a time-dependent deformation, where the accumulated strain is due to long-term stress [128]. The creep properties like creep strength and creep modulus are determined as time and temperature function, and most of the creep behavior is dependent on time, temperature, relative humidity (RH) and load [137]. Fatigue is the structural damage due to cyclic loading and unloading of a material. Similarly like creep failure, fatigue failure takes place within the yield stress limit. The structure or shape of the biopolymer composite influences the fatigue properties [128].
Fractal fatigue crack
Published in Philosophical Magazine, 2023
The fracture surfaces of materials are usually made up of a set of repeated patterns: e.g. ridges, cracks, pores, dimples, fissures, striations and intergranular defects, explained in detail elsewhere [41]. The quantitative analysis of fracture surfaces of materials is a common technique of researches working on engineering failure analysis and damage micro-mechanisms. Fractography investigators analysing loading conditions which generate the failure of elements manifest the respective characteristic regions/zones of fracture surfaces namely, crack nucleation/initiation, growth zone and eventual rapid fracture region. Quantitative fractography normally aims at simply translating such ‘fracture features’ to a parametrical shape, and hence, ‘fractal geometry’ comes into this research field [42]. The is an intensive property of any fractal object, explained elsewhere [43]. All the fractal objects are generally exhibiting a nature/character which means that the pattern of fractal object is repeated (ditto) at different length scales, which act as a quantitative marker in the microstructure [44]. The is a very useful tool in diverse scientific disciplines [45], including microstructure, fractography and crack branching. The correlation between fracture surface roughness and the corresponding fracture toughness property has been substantially researched from the clear of elsewhere [46–48].
Ductile fracture prediction of EH36 grade steel based on Hosford–Coulomb model
Published in Ships and Offshore Structures, 2019
Sung-Ju Park, Kangsu Lee, Burak Can Cerik, Joonmo Choung
Based on this background, in this paper, an experimental and numerical study that aims to characterise the strain hardening and fracture characteristics of EH36 grade shipbuilding steel is presented. The Hosford–Coulomb fracture model is employed to describe the effects of the stress state on the onset of ductile fracture. Finite element analysis of test specimens with different geometries are used to obtain the loading paths to the onset of fracture. The damage indicator framework is adopted to deal with non-proportional loading paths. The ductile fracture model parameters are identified using the numerically obtained loading paths. The calibrated fracture model is implemented in a user-defined material subroutine and applied in the simulations of the tests.